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Organosilicon compounds as fluorescent chemosensors for fluoride anion recognition
Accepted Manuscript Title: Organosilicon Compounds as Fluorescent Chemosensors for Fluoride Anion Recognition Author: Lizhi Gai John Mack Hua Lu Tebello Nyokong Zhifang Li Nagao Kobayashi Zhen Shen PII: DOI: Reference:
S0010-8545(14)00282-3 http://dx.doi.org/doi:10.1016/j.ccr.2014.10.009 CCR 111947
To appear in:
Coordination Chemistry Reviews
Received date: Revised date: Accepted date:
3-9-2014 22-10-2014 24-10-2014
Please cite this article as: L. Gai, J. Mack, H. Lu, T. Nyokong, Z. Li, N. Kobayashi, Z. Shen, Organosilicon Compounds as Fluorescent Chemosensors for Fluoride Anion Recognition, Coordination Chemistry Reviews (2014), http://dx.doi.org/10.1016/j.ccr.2014.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract Various fluoride chemosensors based on organosilicons, the associated photophysical studies and their molecular
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modeling calculations, are summarized.
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Highlights Organosilicon-based chemosensors for F¯ recognition are summarized. The design strategies organosilicon-based chemsensors are elaborated.
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The photophysical properties and electronic structures are analyzed.
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Molecular modeling calculations are carried out.
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Organosilicon Compounds as Fluorescent Chemosensors for Fluoride Anion Recognition Lizhi Gai,a,b John Mack,c Hua Lu,a,d,* Tebello Nyokong,c Zhifang Li,a Nagao Kobayashid,* and Zhen
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Shenb,*
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ADDRESS: a Key Laboratory of Organosilicon Chemistry and Material Technology, Ministry of Education, Hangzhou Normal University, Hangzhou, 311121, P. R. China
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b State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing, 210093, P. R China
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c Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
d Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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EMAIL: [email protected] (HL); [email protected] (NK); [email protected] (ZS)
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Table of Contents: Introduction
2.
Sensing F based on organosilicon compounds
2.1
F triggered silicon-oxygen bond cleavage
2.1.1
BODIPY-Based Sensors
2.1.2
Coumarin-Based Sensors
2.1.3
BODIPY-Coumarin Based Sensors
2.1.4
Fluorescein-Based Sensors
2.1.5
Naphthalimide-Based Sensors
2.1.6
Cyanine-Based Sensors
2.1.7
Thiazole-Based Sensors
2.1.8
Quinoline-Based Sensors
2.1.9
Azo-Based Sensors
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1.
2.2.10 Pyrene-Based Sensors
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2.1.11 Rhodamine-Based Sensors 2.1.12 Other Fluorochrome-Based Sensors 2.1.13 F triggered Cyclization Reaction
F triggered silicon-carbon bond cleavage
2.2.1
BODIPY-Based Sensors
2.2.2
Pyrene-Based Sensors
2.2.3
Diketopyrrolopyrrole (DPP)-Based Sensors
2.2.4
Naphthalene-Diimide-Based Sensors
2.2.5
Other Systems
2.3
Sensors Based on F-Si Interactions
3.
Conclusions and Perspectives
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Fluoride anion recognition; Organosilicons; SiO bond cleavage; Chemosensors.
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Keywords:
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2.2
Abstract: Recent developments in organosilicon-based chemosensors for F recognition are reviewed. The design
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strategies for improving the photophysical properties of organosilicon-based chemosensors are elaborated, with an
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emphasis placed on their utility for biological applications. The photophysical properties and electronic structures
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are analyzed in depth with reference made to the results of molecular modeling calculation and possible future research directions are assessed.
1. Introduction
In recent years, there has been a strong focus on the development of recognition sensors for environmentally and biologically important anions [1-15], because of the vital role played by these anions in a wide range of chemical and biological processes [16-34]. The fluoride anion (F) is one of the most significant in this regard due to its mild toxicity and important role in bone-growth and its presence in various environmental, clinical and food samples [3539]. It is widely used as an additive in toothpastes and water to prevent dental caries and enamel demineralization resulting from wearing orthodontic appliances [40-44]. Fluorosis is caused by the elevated intake of fluoride over prolonged periods of time. An abnormal intake of a large dose or the chronic ingestion of lower doses of fluoride 4 Page 4 of 79
ions also produce ecological damage and cause dental or skeletal fluorosis, urolithiasis and nephrotoxic changes in humans, and even death [42, 45-48]. Tdemostrtehe presence of fluoride in water at about 50 μM prevents tooth decay but at around 250 μM it causes mottled teeth and bone damage [49]. The United States Environmental Protection Agency (USEPA) mandates a drinking water standard for fluoride of 200 μM to prevent osteofluorosis
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and a secondary fluoride standard of 100 μM to protect against dental fluorosis [36]. Drinking water is the largest single contributor to daily fluoride intake [50]. Fluoride permeates naturally into the
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water supply due to solubilization of minerals that contain fluor, such as fluorapatite, cryolite and fluorite [51]. It is
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estimated that more than 100 million people regularly drink water which contains fluoride, of geogenic origin in concentrations that lie over the limit suggested by the World Health Organization guidelines [36]. Concerns have
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been raised about the manner in which the fluoride anion is introduced into the environment by anthropogenic activities, such as the use of phosphate containing fertilizers and by the aluminium processing industries. Different
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diseases are caused based on differing amounts of fluoride ingestion and the duration of intake. Dental fluorosis is caused by excessive fluoride levels and can cause white spots, yellowing of teeth and pitting or mottling of enamel.
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Skeletal fluorosis is a bone disease exclusively caused by excessive consumption of fluoride [52]. The detection
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and recognition of F has, therefore, become the focus of considerable research interest. Until relatively recently, the methods that tended to be used for fluoride determination, such as fluoride-ion
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spectrophotometry [53-54], ion-selective electrodes [55-56],
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F NMR analysis [57], ion chromatography [58],
reverse-phase HPLC [59] and fluorimetry [60-61], were broadly similar to those used for other inorganic ions. However, all these methods have significant disadvantages, such as complicated procedures, high expense, or their unsuitability for use in the field on environmental samples [62-64]. Recently, there has been a strong focus on the development of simple and effective fluorescent or colorimetric sensors for the fluoride anion. Chemosensors enable the visual detection of the presence of fluoride anions and have been used for F recognition and detection based on noncovalent or covalent interactions with the sensor molecules, including hydrogen bonding between F and the NH protons of amides [65-66], indoles [67], pyrroles [68], urea and thiourea [69-70], Lewis acid coordination [71-73], and anion-π interactions [74-75]. This type of approach provides poor selectivity, however, because it is prone to interference by oxygen-containing basic anions, such as H2PO4, CO32 and AcO, so these 5 Page 5 of 79
chemosensors can only be used in organic solvents to detect tetrabutylammonium (TBA+) fluoride and are not suitable for inorganic fluoride salts such as NaF [76-78]. Molecular recognition based on a specific chemical reaction provides higher selectivity and has attracted more attention from researchers in recent years. Over the last fifteen years, two different types of fluoride selective
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reaction-based detection strategies have been developed. Fluoride is a strong hydrogen-bond acceptor, and has a high affinity for silicon. Facile cleavage of either a C−Si or O−Si bond (bond-dissociation energies: 69 and 103
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Kcal/mol) by Fˉ has been the key to designing the reaction sites for these probes [79]. Various chemosensors have
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been reported based on these strategies. The aim of this review is to highlight and summarize the various approaches that have been used to design fluoride chemosensors based on organosilicons, the associated
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photophysical studies, and their biological applications since the year 2000. A critical assessment of research in the field is provided, which should inspire further research in the design of new organosilicon based chemosensors.
in using chemosensors for biological imaging.
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This will form the basis for an overview of possible future directions in the analysis of environmental samples and
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2. Sensing F based on high affinity between silicon and fluoride
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2.1 F triggered Silicon-Oxygen bond cleavage Silyl groups such as TMS (trimethylsilyl), TIPS (triisopropysilyl), TBS (tert-butyldimethylsilyl), TBDPS (tertbutyldiphenylsilyl) are typically used to protect hydroxyl groups during organic synthesis. These protection groups render the corresponding dye inactive to interfering compounds, but there is easy cleavage upon attacking by F. Upon reaction with F, the protection group is removed to form an unprotected O anion. This results in significant differences in the electronic structure and hence also in the photophysical properties, including the UV-visible absorption spectra and the fluorescence quantum yield (F) values.
As a result, numerous organosilicon
compounds with SiO bonds have been reported to act as chemosensors for the detection and recognition of F. In this review, they are classified according to their organic fluorophore platforms into several different categories including boron-dipyrromethenes (BODIPYs), coumarins, cyanines, fluoresceins, naphthalimides, pyrenes, and other related dyes. 6 Page 6 of 79
2.1.1 BODIPY-Based Sensors Over the last two decades, BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorescent dyes have been the focus of considerable research interest due to their advantageous photophysical properties such as their excellent photochemical stabilities, large molar absorption coefficients, and F values [80-85]. BODIPYs have been widely
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used in a variety of organic functional materials, such as labeling reagents, chemosensors and laser dyes, and have
O
O
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Si
B
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F N
N
N
B
N
F F
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F F 1
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been studied for use in applications such as photodynamic therapy [86-94].
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N
Si O
B
F
N
N
F F
B
N
F F
2 O
Scheme 1. Cleavage of the Si˗O bonds of 1 and 2 by F [95].
In 2010, Akkaya prepared two BODIPY derivatives 1 and 2 that can be used as turn-off sensors for the F ion based on the photoinduced electron transfer (PET) and the intramolecular charge transfer (ICT) quenching mechanism that results when a silyl-protected phenolic group is deprotected both in solution and in a poly(methylmethacrylate) (PMMA) matrix [95]. The HOMO and LUMO of the protected forms of 1 and 2 are associated primarily with the core BODIPY fluorophore (Figure 1). When the silyl groups are removed, the LUMOs are directly comparable to 7 Page 7 of 79
those of the protected forms. This is not the case with the HOMOs of the deprotected species, however, because MOs that are associated primarily with the meso-phenyl and styryl substituents are destabilized relative to those which have angular nodal patterns that are comparable to the HOMOs of 1 and 2. This results in significant ICT within the S1 excited states. Upon addition of TBAF, the emission bands are quenched due to the formation of non-
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emissive phenolate ions. The main absorption band of 1 at 498 nm shifts slightly to the blue and there is a decrease in its intensity. There is a red shift of the absorption band of the styryl-conjugated derivative 2 from 560 to 682 nm,
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however, and the color of the solution changes from purple to green.
Figure. 1. The frontier MOs of the protected and deprotected forms of 1, 2 and 3 calculated on the B3LYP optimized geometries using the CAM-B3LYP functional with 6-31G(d) [96] basis sets at an isosurface of 0.02 a.u. Occupied MOs are highlighted with black diamonds. MOs with angular nodal patterns similar to those of the HOMO and LUMO of the BODIPY chromophore [95] are highlighted with gray lines.
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F N
B
N
N
F F
B
N
F F
Si O
O
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3
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Scheme 2. Cleavage of the SiO bond of 3 by F [97].
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Zhu developed a novel highly selective colorimetric and ratiometric near-infrared (NIR) region chemosensor, 3, for F based on a 6-hydroxyindole-based BODIPY. Si–O bond cleavage and phenol/phenolate interconversion results
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in a fluorescence quenching mechanism [97]. Upon titration with TBAF in CH2Cl2, the main absorption band of 3 at 546 nm decreases in intensity and there is a concomitant increase in the intensity of a new band at 644 nm. An
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isosbestic point is observed at 567 nm and there is a visible color change from pink to indigo, since the ratio of the absorbance at 644 and 546 nm increases over 3000-fold. This enables the colorimetric and ratiometric detection of
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F even with the naked eye. The main emission band of 3 at 573 nm decreases gradually in intensity and there is a
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simultaneous appearance of a new red-shifted emission band at 676 nm. The color of the fluorescence changes
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from yellow to pink when viewed under an UV lamp. The ratio of the emission intensities at 676 and 573 nm increases from 0.13 to 9.3, a 71-fold ratiometric enhancement, which indicates that there is an efficient ratiometric response for F. The lower detection limit was 0.12 μM with a response time of under 30 s.
N
N
B
F
N N
N
F
F
F
B
N F
4 Si O
O Si
O
O
Scheme 3. Cleavage of the SiO bond of 4 by F [98].
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Lu and coworkers developed a novel, highly specific rapid colorimetric and ‘turn off’ fluorescent fluoride NIR region sensor for F, 4 [98], by using an aza-boron dipyrromethene (aza-BODIPY) moiety as the fluorophore. The introduction of the aza-nitrogen atom shifts the main BODIPY absorption and emission bands significantly to the red [80]. When F is added, a significant decrease is observed in the intensity of the absorption band at 718 nm
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along with a concomitant increase in the intensity of a new band at 780 nm resulting in a visible color change from blue to indigo. In addition, the emission peak of 4 at 750 nm gradually decreases in intensity. An analysis of the
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TD-DFT calculation results (Figure. 2) demonstrated that the S1 excited state of [4-O2]2− has significant ICT character from the electron donating phenoxy moiety to the BODIPY core, and this enhances the rate of
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nonradiative decay and hence results in fluorescence quenching. The detection limit towards F was 2.1μM.
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Confocal fluorescence microscopy experiments established the utility of 4 for monitoring the presence of F in
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HeLa cells.
Figure. 2 The frontier π-MOs of aza-BODIPY 4 and [4-O2]2− at an isosurface value of 0.04 a.u., calculated during TD-DFT calculations using the CAM-B3LYP functional with 6-31G(d) basis sets (Reprinted with permission from ref. 98. Copyright © 2014 Royal Society of Chemistry.).
2.1.2 Coumarin-Based Sensors Coumarin is a natural flavoring substance found in cinnamon and many other plants, which possesses numerous biological activities including anticancer, anticoagulant, anti-inflammatory, anti-HIV, antimicrobial, analgesic, 10 Page 10 of 79
antioxidant, antiviral, and antimalarial properties, due to their low toxicity [99-103]. Coumarin dyes have found widespread use, since they exhibit excellent spectroscopic properties, such as high extinction coefficients, high F values, and large Stokes shifts, and are widely used as dopants for OLEDs, sensors for ions, and fluorescent markers
F Si
O
O
O
acetone/H 2O(7:3,v/v)
HO
O
O
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5
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in the biochemical determination of enzymes [104-109].
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Scheme 4. Cleavage of the SiO bond of 5 by F [110].
Yang developed a novel highly sensitive and selective coumarin-based fluorogenic chemosensor, 5 [110]. When 5
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was treated with fluoride, the main UV-visible absorption band undergoes a marked shift from 318 to 361 nm and there is a red shift of the emission band from 394 to 459 nm. The fluorescence increase is linear over a fluoride
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concentration range of 508000 nM, with a lower detection limit of 19 nM. 5 shows excellent selectivity for the F,
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due to the high affinity of F toward the silicon atom. This approach has been successfully applied to the fluoride
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determination in toothpaste and in tap water samples. CF3
CF3
CF3
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F
Si
O
O
6
H 2O
O
CH3CN O
O
O
HO
O
O
Scheme 5. Cleavage of the SiO bond of 6 by F [111].
Lee designed a similar selective fluorogenic coumarin-based chemosensor, 6, with colorimetric and fluorescent ‘turn-on’ modes in acetonitrile and water [111]. In acetontitrile, there is a moderately strong absorption band at 330 nm in the UV-visible absorption spectrum. The addition of F (as its TBA salt) results in a dramatic color change from colorless to yellow, due to the gradual decrease in the intensity of the absorption band at 330 nm and the simultaneous appearance of an intense absorption band at 434 nm. A chromenolate is formed due to the fluorideinduced cleavage of the TIPS group. Upon exposure to fluoride, a strong fluorescence emission band appears at 11 Page 11 of 79
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500 nm. In contrast, 6 is non-fluorescent due to its non-emissive S1 state. The lower detection limit was ca. 50 nM,
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with a reasonable response time of ca. 10 min in aqueous solution. Test paper was successfully prepared through
Si
O
O
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immersion in a solution of 6.
O
NaF
O
O
O
O
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7
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O
O O
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Scheme 6. Cleavage of the SiO bond of 7 by F [112].
Park designed a non-cytotoxic fluorescence chemosensor, 7, with a coumarin used as the signaling chromophore and a TBDPS group as the protecting group. The addition of a methyl ester group increases the water solubility and enhances cell permeability [112]. Upon treatment with 1 mM NaF for 3 h in phosphate-buffered solution, there is a more than 4-fold enhancement in the fluorescence intensity at 461 nm. 7 has been used for bioimaging and for the detection of NaF in A549 human epithelial lung cancer cells under physiological conditions. Since the excitation wavelength lies in the UV region and there is a long assay time of ca. 4 h, further improvements are needed before this chemosensor will be suitable for practical applications.
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Si
O
O
O
O
O
O
NaF H N
H N 8
C 12H 25
C 12H 25
O
O
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Scheme 7. Cleavage of the SiO bond of 8 by F [113].
Hong reported a chemosensor, 8, with a fluorescent signaling chromphore and a silyl ether protecting group, which
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can detect F through the formation of fluorescent gels in aqueous media [113]. Although the turbid solution of 8
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shows little fluorescence emission, upon the addition of F, the intensity of the fluorescence emission at 468 nm increases dramatically due to the removal of the silyl ethers and a sol-to-gel transition at pH 7.40 in 1:1
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methanol/HEPES buffer (v/v). The gels formed after complete removal of the tert-butyldimethylsilyl (TBDMS) protecting groups are only weakly fluorescent, due to the formation of densely stacked coumarin moieties, resulting
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in a self-quenching effect. More intense fluorescence is observed for the gels of 8+F when the protecting groups
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Si
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are not completely removed.
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O
O
O F
O
O
9
Si
O
O
O
O
O
O
Si
O
O
O
F O
O
O
O
10
Scheme 8. Cleavage of the SiO bonds of 9 and 10 by F [114]. Ghosh reported the anion binding behavior of the silyl protected 4-hydroxycoumarin 9 and dicoumarol 10 in organic and aqueous organic solutions [114]. 10 acts as a selective sensor for F in CHCl3 and aq. CH3CN solutions, 13 Page 13 of 79
due to a large change in the emission properties. Upon addition of fluoride, a dianionic species is formed which is yellow in color. The responses of both 9 and 10 are less selective for F in CH3CN due to its higher polarity. Only F solutions with concentrations between 103 to 104 M result in a significant change in the emission intensity of both 9 and 10. Both dyes exhibit cell permeability and can be used for detecting F in living cells.
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2.1.3 BODIPY-Coumarin Based Sensors
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Among the various interactions within electronic excited states, ICT is one of the most promising for sensing the fluoride ion with a ratiometric response [115]. Donor-π-acceptor dyads with coumarins and other similar dyes as
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the acceptor and BODIPYs as the donor moieties, have been studied in a wide variety of fields, including their possible use as chemosensors for ions, in optical materials, in photosynthetic model systems, and in biotechnology,
B
N
F
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N
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since their absorption and emission properties can be readily fine-tuned [116-120].
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F F O
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11
O
N
B
N
F F O O
O Si
O
Scheme 9. Cleavage of the SiO bond of 11 by F [121].
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Figure. 3. The HOMO and LUMO energy levels and angular nodal patters for 11 and its deprotected product. (Reprinted with permission from ref. 121. Copyright © 2011 American Chemical Society.)
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Lin designed a new ratiometric fluorescent fluoride chemosensor based on a coumarin-BODIPY dyad, 11, by making use of an ICT quenching mechanism [121]. The addition of F to a DMSO solution causes an 88 nm red
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shift in the absorption maximum from 592 to 680 nm, in conjunction with a change in the color of the solution from pink to red. In the presence of other anions, only F triggers a blue shift of the emission band of 11 from 606 to 472
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nm, resulting in a color change from orange to blue. Density functional theory (DFT) and time-dependent DFT
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(TDDFT) calculations, predict that both the highest occupied molecular orbital (HOMO) and lowest unoccupied
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molecular orbital (LUMO) have large MO coefficients on the coumarin, ethenyl bridge and BODIPY moieties (Figure 3). Upon cleavage of the SiO bond, however, the HOMO is located primarily on the coumarin moiety and the ethenyl bridge, while the LUMO is located mainly on the BODIPY unit and the ethenyl bridge, so ICT is induced from the coumarin to the BODIPY moiety resulting in significant changes in the photophysical properties. The lower detection limit for F of 11 was 0.12 µM. 2.1.4 Fluorescein-Based Sensors
Fluorescein is one of the most commonly-used fluorescent chemosensors due to the high molar absorptivity of its main absorption band and its high F value [5, 122-124]. Fluoresceins are used as fluorescent labels due to their rich and diverse properties, and prototropism properties [125-127]. Fluorescein derivatives in their lactone form are
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always non-fluorescent, but in their ring-opened form there are significant color changes and an enhancement of the fluorescence intensity [128-129].
TBSO
O
O
OTBS
O
O
OTBS
O
OH
F
F O
COO
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COO O moderately fluorescent
highly fluorescent
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non-fluorescent 12
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Scheme 10. Cleavage of the SiO bond of 12 by F [130].
Yang designed a fluorescein-based fluorogenic chemosensor to detect the fluoride ion in aqueous media. The
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fluorescein di-tert-butyldimethylsilyl ether, 12, is a colorless, non-fluorescent compound [130]. Upon incubation with the fluoride ion in DMF-water solution (7:3, v/v, pH 7.0), the SiO bond is cleaved, and the fluorescein
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absorption band appears at 496 nm along with a large increase in fluorescence emission intensity at 532 nm. The increase in fluorescence intensity is linear for fluoride concentrations between 0.1–2.0 μM and there is a lower
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detection limit of 0.041 μM. This method has been successfully applied to fluoride determination in multi-trace
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element injection and toothpaste samples, and the results agree well with those obtained from other selective
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fluoride-ion sensors. It is of particular interest to identify color changes of 12 from colorless to yellow green that would allow the “naked-eye” detection of F.
OH HO
OH
O
N
OH
N
O
OTBDPS
N
F
O
OH
OH O
HO
N OH
N
O
O
N COO
O
13
Scheme 11. Cleavage of the SiO bond of 13 by F [131]. Du developed the first example of a sugar-functionalized fluorescent chemosensor, 13, for the recognition and detection of F by using a desilylation-triggered chromogenic reaction in pure water [131]. The introduction of a 16 Page 16 of 79
sugar group to 13 can improve both the water solubility and bio-compatibility, which increases the colorimetric and fluorescent “turn-on” selectivity and sensitivity toward fluoride ions in fully aqueous and living cells. When fluoride (TBA+ and Na+ salts) is added into the solution, the emission fluorescence intensity of 13 at 520 nm increases more than 160-fold as the concentration of F ions is increased from 0 to 1.4 mM, and a quinone-type
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intermediate is generated with a strong fluorescence signal and a yellow color visible to the naked eye. 13 exhibits an excellent selectivity for the F ions and a lower detection limit of 10.5 µM in pure water. The color change is
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visible to the naked eye at concentrations as low as 75 µM. 13 was successfully used for imaging F in HepG2 cells.
O
nO
NH
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S O
N H
O
O
nO
S
NH N H
O
O
O
Si
14
M
Si
an
NaF
COO
O
O
O
d
Scheme 12. Cleavage of the SiO bond of 14 by F [132].
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Wu designed a highly sensitive, responsive, and visible region fluorescent chemosensor, 14, for F detection in
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totally aqueous solutions and for fluoride imaging in living cells [132]. To ensure solubility in water, cell membrane permeability and low-cytotoxicity, a hydrophilic polymer, polyethylene glycol (PEG), was conjugated into the system. PEGs are biocompatible, and have non-toxic, non-antigenic and non-immunogenic properties in a wide range of biomedical applications [need a ref here?]. Upon incubation with F in pH 7.4 HEPES-buffered water, the emission intensity at 526 nm increases gradually and reaches its maximum value within ten minutes. The lower detection limit of 14 was 1 µM. Sensor 14 can be used to detect fluoride levels in real samples such as running water, human urine, and serum, and can also be used for imaging F in both HeLa and L929 cells.
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COOMe
MeO
O
COOMe
COOMe Oxidation
NaF
O Si
MeO
O
O
MeO
O
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15
O
cr
Scheme 13. Cleavage of the SiO bond of 15 by F [133].
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Li reported a new chemical reaction-based “turn-on” fluoride chemosensor for NaF, 15, which contains a TBDPS group as a trigger and a fluorescein moiety as the optical signaling chromophore [133]. Upon the addition of NaF,
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the emission intensity observed at 520 nm in the green region of the spectrum increases about 2.7-fold at F concentrations as low as 50 µM, due to SiO bond cleavage and the readily air-oxidized highly fluorescent anionic
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species that is formed. The fluorescence emission of 15 is significantly enhanced in intensity and the F value increases to about 0.37. 15 provides a specific response for F with high sensitivity. A lower detection limit of 18
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µM in HEPES-THF (9/1, v/v, pH 7.2) has been reported. Furthermore, the use of 15 has been successfully applied
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Si O
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to the detection of F in HeLa cells. A “turn-on” fluorescence response was observed.
(Me) 3C
O
O
O
O
OOC
F
+
O
O
16
O MeO
O
O
OMe
Scheme 14. Cleavage of the SiO bond of 16 by F [134]. Talukdar developed a cascade reaction-based colorimetric and fluorescent “turn-on” chemosensor, 16, for selective and relatively rapid (t½ = 2.41 min) F detection [134]. In DMSO, none of the characteristic UV-visible fluorescein absorption bands are observed between 300-700 nm and there is no fluorescence emission intensity. Upon addition 18 Page 18 of 79
of F, a fluorescence emission band was observed cat 523 nm due to a cascade reaction in the presence of the F ion released by the lactone phthalide. A carboxyfluorescein derivative is formed which is a strongly fluorescent species and the color of the solution changes from colorless to yellow under ambient light. During treatment with 300 eq. of TBAF at room temperature, a 550-fold enhancement of the fluorescence emission intensity of 16 is observed at
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523 nm within 7 minutes. 16 has a lower detection limit of F of 1.03 μM. Sensor 16 can be used to detect F in
N
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live HeLa cells by confocal microscopy.
O N
H HN N N
O
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N
S O
O
O
Si
17
N
S O
HO
M
O
F
an
HOOC
O Si
H HN N N
O
O
O
te
d
Scheme 15. Cleavage of the SiO bond of 17 by F [135]. Thennarasu reported a novel fluorescein-rhodamine dyad, 17, for the detection of F and Hg2+ [135]. Upon addition
Ac ce p
of F, new bands are observed at 500 nm and 523 nm, respectively, in the absorption and fluorescene emission spectra. The calculated detection limits of the probe are 5.17 x 10-8 M (~1.0 ppb). 17 can be used for the detection and imaging of W138 normal lung fibroblast cells contaminated with F and Hg2+ ions. 2.1.5 Naphthalimide-Based Sensors
1,8-Naphthalimides (NIs) have been used extensively to design fluorescent fluoride chemosensors using various quenching mechanisms such as ICT, PET, and Förster resonance energy transfer [136-140]. NIs have also attracted considerable attention due to their tendency to form n-type semiconductor materials and their easy functionalization through the imide nitrogen or via core substitution [141-142].
19 Page 19 of 79
O
N
O
O
N
O O
N
O
F O
O
O
NH 2
O
O
Si
Si
F
us
O
NH
ip t
NH
cr
O
+
18
an
Scheme 16. Mechanism for the reaction of 18 with F [143].
M
Cho developed a highly selective NI-based ratiometric and colorimetric fluorescent chemosensor, 18, which displays both one- and two-photon ratiometric changes [143]. Upon reaction with F in MeCN as well as in
d
aqueous buffer solutions, the absorption band of 18 at 365 nm red shifts to 487 nm, since the NH bond is
te
deprotonated by the F. On addition of 5 eq. of F the absorbance at 487 nm decreases in intensity as a new band
Ac ce p
appears at 421 nm, and this results in a color change from colorless to jade-green, due to the cleavage of the SiO bond and the release of a green fluorescent 4-aminonaphalimide. The addition of higher concentrations of F results in a new emission band at 508 nm and a 220-fold ratiometric intensity enhancement. One- and two-photon ratiometric changes can be obtained via two-photon microscopy, which can offer intrinsic three-dimensional (3D) resolution combined with reduced phototoxicity, increased specimen penetration, and negligible background fluorescence.
20 Page 20 of 79
O
N
O
O
N
O
F
+
CH 3CN OTBDPS
O
cr
19
ip t
O
O
Ac ce p
te
d
M
an
us
Scheme 17. Cleavage of the SiO bond of 19 by F [144].
Figure. 4. The HOMO and LUMO energy levels and angular nodal patterns of 19 and its deprotected product. (Adapted with permission from ref. 144. Copyright © 2013 Elsevier.)
Zhang reported a dual-channel F chemosensor comprised of a 4-hydroxy-1,8-naphthalimide derivative which generates a “turn-off” response in the fluorescence intensity due to an ICT mechanism in both aqueous and organic solutions [144]. Upon addition of F, the intensity of the main absorption band at 365 nm is significantly reduced and there is the concomitant appearance of a new band at 475 nm.
This is consistent with the smaller
HOMOLUMO gap predicted for the deprotected product (Figure 3). In the presence of other anions, only F exhibits a large enhancement of the intensity at 560 nm with the solution changing color from colorless to yellow. The lower detection limits were 18 μM and 0.1 μM in MeCN/H2O (v/v, 50:50) and MeCN solution, respectively. TDDFT calculations (Figure 4) predict that the LUMO is predominantly associated with the imide ring and the 21 Page 21 of 79
oxygen atom of the carbonyl group in the deprotected structure, while the HOMO is associated primarily with the amino group resulting in strong ICT from the naphtholate to the imide moiety. Sensor 19 can be used for imaging
O
N
O
O
N
O
CH 3CN O
Si
us
O
cr
F
ip t
fluoride in A549 cells with high selectivity and sensitivity.
20
NA
an
TBS-NA
Scheme 18. Cleavage of the SiO bond of 20 by F [145].
M
Ren developed a colorimetric fluoride chemosensor, 20, based on a fluorescent naphthalimide [145]. In MeCN solution, the addition of TBAF triggers a decrease in the intensity of the main absorption band at 362 nm along with
d
an increase in the intensity of a new band at 474 nm resulting in an obvious and relatively rapid color change from
Ac ce p
te
colorless to yellow in ca. 3 min. The lower detection limit for F was 0.59 µM.
Figure 5 The HOMO and LUMO energy levels and the angular nodal patterns for 20
and NA at an isosurface value of 0.02 a.u. (Adapted
with permission from ref. 146. Copyright © 2011 Elsevier.)
Chu assessed the sensor mechanism of 20 by using DFT and TDDFT calculations to analyze the frontier molecular orbitals (MOs), and the predicted electronic transition energies and oscillator strengths [146]. Structures were 22 Page 22 of 79
optimized for the groundstates and first-excited-states of 20 and the desilylation product (NA) (Figure 5). Although the groundstates of 20 and NA have similar structures, there is a large red-shift in the main absorption band of NA due to the strong ICT character of the S0–S1 transition. The calculated fluorescence emission spectrum exhibited a large red-shift of ca. 100 nm and this was also ascribed to the relatively strong ICT character of the S1 state. The
N
O
O F CH 3CN
O
O
an
OTBDPS
21
N
us
O
cr
ip t
large red-shift of both the absorption and emission bands is what makes 20 suitable for use as a F chemosensor.
M
Scheme 19. Cleavage of the SiO bond of 21 by F [147]. Hu designed a highly selective colorimetric and ratiometric fluorescent chemosensor, 21, by using a 4-silylhydroxy-
d
1,8-naphthalimide to detect F [147]. When F was added gradually, the maximum absorption peak at 361 nm
te
decreased in intensity and a new absorption band was observed at 471 nm, resulting in a color change from
Ac ce p
colorless to yellow due to the cleavage of the Si–O bond to form the fluorescent 4-hydroxy-N-butyl-1,8naphthalimide. 21 can therefore serve as a naked-eye chemodosimeter for F. The emission peak undergoes a red shift of 123 nm from 430 to 553 nm and a 510-fold enhancement (F553/F430) in intensity. O N
O Si
N
O
O F
O O
N O Si
O
O
22
Scheme 20. Cleavage of the SiO bonds of 20 and 22 by F [148]. Cao reported a fluoride chemosensor, 22, comprised of an N-aryl-1,8-naphthalimide bearing a trimethylsilyl ether that is cleaved in the presence of F to give a ratiometric fluorescence response for an excimer-monomer conversion 23 Page 23 of 79
[148]. Upon addition of F in DCM, the main emission band shifted to the blue from 524 to 378 nm, because the excimer is disrupted by the presence of F, since desilylation results in the formation of a phenoxide ion. The excimer emission at 524 nm is completely quenched within 2 min resulting in a 2.7-fold enhancement in the
F
OMe
O O
N
OMe
an
23
O
cr
N
us
O
OTMS O
ip t
monomer emission intensity at 378 nm. The lower detection limit was 7 µM.
Ac ce p
te
d
M
Scheme 21. Cleavage of the SiO bond of 23 by F [149].
Figure. 6 The postulated mechanism for PET between the naphthalene and aryl moieties, which results in the fluorescence quenching observed in the presence of the fluoride ion. (Reprinted with permission from ref. 149. Copyright © 2014 Royal Society of Chemistry.)
Cao also developed a chemosensor, 23, based on the 1,8-naphthalimide fluorophore by using a trimethylsilyl ether as the recognition unit for the detection of F in its inorganic forms (e.g. KF and NaF). The response time of ca. 5 min in MeCN/H2O media is relatively rapid [149]. The main absorption band lies at 366 nm in MeCN/H2O (v/v = 7:3), while the emission band lies at 442 nm. Upon addition of increasing concentrations of F, the emission band is quenched drastically due to what was described as a strong PET effect related to the formation of the phenolate ion that is formed by the fluoride-triggered desilylation reaction. The overall rate constants for the pseudo-first-order 24 Page 24 of 79
reactions of 10 M solutions of TBAF and KF and a 30 mM solution of NaF were calculated to be 7.8 × 103, 7.1 × 103, and 4.2 × 103 s1, respectively. The authors used calculation results (Figure 6) to postulate that the electron of the HOMO of the aryl moiety can transfer to that of the naphthalene moiety after the formation of a phenolate due to the desilylation of 23 and that this results in significant fluorescence quenching. The lower detection limit was
O
O
an
F
Si
N O
Bu N
cr
O
O
us
Bu N
ip t
2.1 µM.
O
HO
M
24
Scheme 22. Cleavage of the SiO bond of 24 by F [150].
d
Liu reported a completely selective and highly sensitive chemodosimeter for F, 24, by exploiting a dual reaction-
te
based site for F-triggered Si–O bond cleavage and the deprotonation/autoxidation of the secondary nitrile group
Ac ce p
[150]. Upon addition of 0.0–2.0 equiv of F, two new bands were observed at 370 and 625 nm in the UV–visible absorption spectrum, due to formation of [C–H…F] complexes, resulting in a blue solution. On further addition of F, the intensity of the band at 625 nm decreased and there was a slight red-shifted to 660 nm. This was ascribed to the formation of a phenolate anion with a strong negatively charged donor oxygen atom, accompanied by a strong increase in the intensity of the band centered at 367 nm. Upon the addition of F, a significant decrease in the fluorescence intensity of 24 was observed at 500 nm, which was ascribed to enhanced PET. The lower detection limit during the fluorescence studies was 0.105 μM. O
O N
N O Si
O
N
O
N
F O
O
O
25
25 Page 25 of 79
Scheme 23. Cleavage of the SiO bond of 25 by F [151]. Cao developed another chemosensor for F, 25, using fluoride-triggered desilylation to provide a quantitative measurement of the F concentration [151]. Upon addition of F, new absorbance peaks were observed at 423 and 552 nm resulting in a color change from colorless to red in MeCN. In addition, the intensity of the emission
cr
ip t
observed at 486 nm decreased. The detection limit to F was 2.6 μM.
2.1.6 Cyanine dye -Based Sensors
us
The structure of cyanine dyes are comprised of two aromatic or heterocyclic rings, which are linked by a
an
polymethine chain [152-153]. Cyanines are highly fluorescent and have found use as fluorescent chemosensors,
d
M
photographic sensitizers and biomolecular labels [154-157].
N
O
Ac ce p
26
N
THF/H 2O 7:3 (v:v) TBDMS
te
O
F
Scheme 24. Cleavage of the SiO bond of 26 by F [158].
Xu reported a novel highly sensitive colorimetric method based on selective attack by the fluoride ion on the SiO bond of cyanine dye 26 [158]. On addition of F, in THF/water (70:30, v/v) 26 undergoes a dramatic color change from yellow to green accompanied with a blue shift of the main absorption band from 600 to 440 nm, due to the increased ICT character of the S1 state. The lower detection limit was 0.1 μM in THF/water (70:30, v/v) with a long reaction time of 1.5 h. The analytical approach has been successfully applied to the determination of F concentrations in rainwater samples.
26 Page 26 of 79
I N
I N
I N F
excess F
CH 3CN
CH 3CN
O Si O
ip t
OH 28
cr
27
us
Scheme 25. Cleavage of the SiO bond of 27 by F [159].
Zhang designed a dual-channel “off-on-off” chemosensor, 27, comprised of a phenylpyridylvinylene fluorophore
an
and a Si–O bond receptor, by making use of the ICT quenching mechanism [159]. Upon addition of F, the main absorption band undergoes a drastic red-shift from 362 to 545 nm, accompanied by a striking color change from
M
colorless to purple. The Si–O bond is cleaved and a highly fluorescent molecule, 28, is released, which has enhanced fluorescent intensity at 476 nm. 28 is subsequently converted into a phenolate ion and the emission
te
d
intensity gradually decreases. The lower detection limit was 0.1 μM.
Si
Ac ce p
O
OH
O PBS pH 7.4
NaF
S
N
S
N
S
N I
I
I
29
Scheme 26. Cleavage of the SiO bond of 29 by F [160]. Ma developed a new chemosensor, 29, with an ICT quenching mechanism, which provides colorimetric and ratiometric detection of F in buffered aqueous solutions and living matrices [160]. A conjugated benzothiazolium 27 Page 27 of 79
hemicyanine dye provides the latent fluorophore. Upon addition of F, a red shift of the absorption band from 407 to 517 nm is observed in an ethanol/water mixture (3:7, v/v, phosphate buffered saline (PBS), pH 7.4) and this provides a color change from light yellow to orange, making 29 suitable for use as a naked-eye indicator. Similarly, the reaction of 29 with F triggers the cleavage of the SiO bond resulting in a ratiometric red shift in the emission
ip t
maximum from 500 to 558 nm. The lower detection limit was 0.08 mM. Sensor 29 can be used as a fluorescent
cr
ratiometric biosensor for F in live RAW 264.7 macrophage cells.
OH
O
us
Si O
F
F
H 3C
H 3C
30
I
H 3C
N
I
M
I
N
H 3C
d
N
H 3C
PBS pH 7.4
an
Ethanol/H 2O 3/7 PBS pH 7.4 H 3C
te
Scheme 27. Cleavage of the SiO bond of 30 by F [161].
Ac ce p
Feng have reported a fluorine chemosensor, 30, consisting of a 1,3,3-trimethyl-3H-indolium hemicyanine dye as the chromophore and a TBDPS group as the reaction site. Promising colorimetric and fluorescent turn-on properties were observed in aqueous solution with reaction times of only a few minutes [161]. Upon addition of F, there is a large bathochromic shift of the main absorption band from 420 to 535 nm, resulting in a color change from light yellow to red, which is easily discernable by the naked-eye. The fluorescence emission intensity at 558 nm increases by about 25-fold and there is a lower detection limit of 6.6 µM. The optimal pH range for the desilylation process lies between 7–9. Test papers can be readily prepared and were used to demonstrate the potential applicability of 30 for F determination in aqueous samples. 2.1.7 Thiazoles-Based Sensors
28 Page 28 of 79
Thiazoles are an important class of fluorescent heterocyclic compounds, which provide many potent biologically active molecules for medical applications [162]. They contain an electron-deficient core and hence have a strong electron-accepting ability. Thiazoles have found applications as drugs for the treatment of HIV and bacterial
HO N
NaF
N
S S
O
cr
Si O
O
HN
us
HN
31
ip t
infections, inflammation, as inhibitors of bacterial DNA gyrase B, and as fluorescent sensors for ions [163-165].
31-BTTPB
an
BTTPB
Scheme 28. Cleavage of the SiO bond of 31 by F [166].
M
Yang have described a thiazole fluoride chemosensor, 31, with a rapid reaction time in aqueous cetyltrimethylammounium bromide (CTABr) micellar media, which has two independent modes of signal
d
transduction based on fluoride-induced changes in the fluorescence color and intensity [166]. In the absence of F,
te
31 has a blue-violet emission band at 418 nm. Upon addition of F, a new band appears at 560 nm due to the
Ac ce p
formation of a benzothiazole derivative and there is a gradual increase in the intensity of the bright yellow emission. The lower detection limit was 5.3 µM. Test papers were prepared by immersing filter paper into a THF solution of 31 and were successfully used to detect the presence of F in aqueous solutions.
29 Page 29 of 79
Figure. 7 The angular nodal patterns calculated for the HOMOs and LUMOs of 31 and 31-BTHPB at the DFT/B3LYP/TZVP level of theory and an isosurface value of 0.02 a.u. (Reprinted with permission from ref. 167 Copyright © 2014 Royal Society of Chemistry.)
Chu investigated the F sensing mechanism of 31, with DFT and TDDFT calculations to examine the electronic structures of the ground and excited states of the molecules that are involved in the sensor process [167]. Figure 7
ip t
contains the frontier MOs of the optimized structures of 31 and its desilylated product (31-BTHPB). In each case, it is obvious that that there are larger MO coefficients on the thiazole moiety in the LUMO than in the HOMO. The
cr
excited states of both BTTPB and 31-BTHPB, therefore, have significant ICT character. In the 31-BTHPB structure, the proton of the hydroxyl hroup can transfer to the nitrogen atom of the thiazole moiety, and this excited-
us
state intramolecular proton transfer (ESIPT) process results a significant red shift of the main fluorescence emission
an
band and hence also a very large Stokes shift value.
Si O
S
O
N S
O HN
d
HN
te
32
M
F
N
HO
Ac ce p
Scheme 29. Cleavage of the SiO bond of 32 by F [168]. Yang used hydrogel as the support for a F-specific reaction between a water insoluble sensor molecule, 32, and F in aqueous environments [168]. 32/polyvinylpyrrolidone (PVP) composites were prepared by adding a methanol solution of 32 (2.0 μL, 2.0 mM) into a xerogel film (1.0 mg) followed by drying to remove the solvent. When NaF solution was added, the maximum absorption band shifted to the red from 345 to 370 nm, and the main fluorescence emission band maximum shifted from 406 to 494 nm with a color change from blue−violet to green. The 32/PVP system is sufficiently sensitive to provide naked-eye detection for F in pure water media with a detection time of only 15 s at concentrations relevant to the USEPA’s drinking water standard.
30 Page 30 of 79
S
N F
S
O Si
N OH
ip t
33
Scheme 30. Cleavage of the SiO bond of 33 by F [169].
cr
Goswami reported a ratiometric and colorimetric chemosensor, 33, based on an ESIPT process [169]. Upon
us
treatment with F, the main absorption band decreases in intensity and a weak band at 330 nm is formed along with a large increase in the intensity of a new absorption band at 440 nm, resulting in a color change from colorless to
an
green in CH3CN-H2O (8/2, v/v). When F is added, the fluorescence emission is red-shifted from 407 to 477 nm and intense green fluorescence is observed. Test strips were formed by immersion in 33 solution and provided
M
convenient and efficient detection of F. The lower detection limit was 6.5 μM. Sensor 33 can be used as a fluoride
N
O
Si
te
N
d
sensor in bioimaging applications.
S
F
N
N
O
S
Ac ce p
34
Scheme 31. Cleavage of the SiO bond of 34 by F [170].
Beckert reported another novel O-silyloxy-1,3-thiazole sensor with a fast-response ratiometric fluorescent response, 34, based on a desilylation-triggered chromogenic reaction in polar aprotic solvents and in aqueous CTABr micellar media [170]. After the addition of TBAF to a DMSO solution, the O-silyloxy-1,3-thiazole is converted into an enolate and there is a red shift of the absorption band from 344 to 481 nm. The fluoride-triggered deprotection of these silyl ethers leads to a bathochromic-shift of the main fluorescence emission band from 418 to 602 nm in DMSO from the blue end of the visible region to the red. The lower detection limit was 0.1 µM.
31 Page 31 of 79
OH O
HO HO S
TBSO
OH
NH
N
OH
F
O
HO HO S
O
NH
OH
PBS, pH 7.4
O
N
O
ip t
35
Scheme 32. Cleavage of the SiO bond of 35 by F [171].
cr
Wang successfully designed a stable sugar-functionalized fluorescent fluoride chemosensor, 35, based on a desilylation reaction in aqueous media [171]. The sugar moiety was incorporated to improve the solubility and has
us
no measurable effect on the fluoride sensing. In the presence of other ions, such as Cl, Br, I, F, H2PO4, N3,
an
NO2, NO3, AcO, and SO42, only F triggers significant fluorescence intensity changes. After the addition of 0.1 M NaF, the fluorescence emission intensity at 508 nm increases 30 fold in 10 min in PBS (10 mM, DMSO 0.5%,
M
pH = 7.4), due to the high reactivity of TBS and F. Sensor 35 is non-cytotoxic and can be used to monitor the
d
presence of F in KB human carcinoma cell lines by fluorescence imaging.
S
O
N
NH
te
O
Ac ce p
Si
O
S
N
37
F
S
O
N
NH
N
36
Si
O
N Cl
Cl
O
S
O NH
N
Cl
F
O S
S
N
N
NH
N
N Cl
Scheme 33. Cleavage of the SiO bonds of 36 and 37 by F [172]. Tang reported the rapid detection of fluoride ion (NaF) in aqueous solution and living cells by sensitive fluorescence thiazole chemosensors with quaternary ammonium moieties, 36 and 37, [172]. The quaternary 32 Page 32 of 79
ammonium not only improves the solubility of the probes, but also enhances their sensitivity and selectivity. Upon addition of F, the TBDPS group of 36 is desilylated and the fluorescence intensity at 520 nm increases more than 121- and 143-fold, respectively, for NaF and TBAF solutions, with detection limits as low as 30 μM.. In a similar manner, only F
induces a significant increase in fluorescent intensity of 37 at 550 nm along with a decrease in
ip t
intensity at 450 nm. 37 can be used to detect fluoride ion in cells by both green channel and ratiometric imaging.
cr
2.1.8 Quinoline-Based Sensors
us
Quinoline derivatives are pharmacologically active synthetic compounds that are widely used in applications such as their use in ion detection and as antitumor DNA intercalating agents. [173-177].
an
Br
Br
N
Br O
N
Br OH
d
38
Si
M
F
te
Scheme 34. Cleavage of the SiO bond of 38 by F [178].
Ac ce p
Bai designed a ratiometric and colorimetric fluorescent fluoride chemosensor, 38, based on the combination of a desilylation reaction and an excited-state proton transfer (ESPT) from the product [178]. After the addition of TBAF in THF, the absorption peak at 313 nm gradually decreases in intensity and two new peaks are observed at 363 and 410 nm. The color of the solution changes from colorless to yellowish-green in a manner that can be readily observed by the naked-eye within 10 s. Blue fluorescence is observed in THF, which on addition of F changes color to strong yellowish-green, since the band maximum shifts from 403 to 520 nm due to the ESPT process and the formation of an OH···F hydrogen bond. The lower detection limit was below 1.0 µM. When increasing amounts of water are added to the solvent mixtures, the intensity of the emission peak decreases significantly.
33 Page 33 of 79
TBSO
CHO
O
CHO
F
N
N 39
ip t
Scheme 35. Cleavage of the SiO bond of 39 by F [179]. Bai designed another new strategy for the highly selective and sensitive detection of both F and Zn2+ based on an
cr
F-induced desilylation reaction and a Zn2+ chelation-enhanced fluorescence effect [179]. Upon addition of F, the
us
bright blue fluorescence that is associated with an emission band at 458 nm is quenched due to efficient radiationless relaxation to the ground state via intra- and intermolecular ESPT. In THF, however, 39 exhibits a
an
different set of photophysical changes. The emission intensity at 418 nm gradually decreases, and a new peak is observed at 520 nm with the color of the solution and fluorescence changing from blue-violet to yellowish-green.
M
This can be attributed to the combination of a desilylation reaction and an excited state proton transfer from the product to the fluoride ion. The lower detection limit was 1 µM.
d
2.1.9 Azo-Based Sensors
te
Azo dyes contain a N=Nbond. When an N=N bond is attached to two aryl groups there are photochromic
Ac ce p
properties, since the relative orientations of the chromophores have a significant effect on the energies of the main π-π* absorption bands [180-181]. The light- or heat-induced isomerization reactions result in the formation of trans and cis isomers. Many fluorescent sensors have been reported based on the orientation of anchored azo dye systems [182-183].
Si
O O F N N
N
N
N N 40
34 Page 34 of 79
Si
O O F N
N
N
ip t
N
N
MeO MeO
Si Si
OMe
OMe
us
MeO
MeO
cr
N
41
an
Scheme 36. Cleavage of the SiO bonds of 40 and 41 by F [184].
M
Martínez-Máñez reported a fluoride chemosensor, 40, in which a pyridine azo dye is functionalized with a protecting TBDMS moiety. A selective phenolate fluoride response is observed in acetonitrile/water mixtures [184].
d
Upon the addition of F‾ to 9:1 (v/v) solutions, the main absorption band is red-shifted from 350 to 470 nm and there
te
is a visible color change from colorless to orange-red due to the cleavage of the SiO bond and the formation of a phenolate anion. The lower detection limits are 0.04, 0.09, 0.14 ppm, respectively, for 15, 40, 60 min reaction times
Ac ce p
in 90:10, 75:25, 50:50 (v/v) acetonitrile/water mixtures.
41 can be anchored onto a silica support with a
trialkoxysilane moiety. Treatment with TLC silica foils functionalized with 41 by immersion at concentrations of 104, 103, 102 M, results in red, green, and blue coloration, respectively in a manner that can also be used to detect F‾ in solution.
35 Page 35 of 79
O O F N
N
N
N
N
N
ip t
Si
OH
NO2
NO2
us
cr
NO2 42
Scheme 37. Mechanism of the reaction of 42 with F. [185].
an
Li reported a novel ratiometric and colorimetric fluoride chemosensor, 42, by taking advantage of both the azobenzene structure and the selective reaction site provided by the SiO bond [185]. Upon addition of F‾, a
M
significant color change from colorless to deep blue is observed, with a dramatic red-shift of the absorption wavelength maximum from 365 to 595 nm. The lower detection limit was 15 µM. Test strips were successfully
d
used to detect F. There is an obvious color change from light yellow to blue based on the fluoride-initiated
te
cleavage of the SiO bond and the resulting formation of an azobenzene compound. It is noteworthy that 42 can be
Ac ce p
used as colorimetric indicator for convenient analyses even in the absence of a spectrometer. 2.2.10 Pyrene-Based Sensors
Pyrene molecules are widely-used as fluorophores, since they possess excellent properties, such as large fluorescence quantum yields, long lifetimes, and ratiometric signals for the monomer and excimer emission [186187]. Pyrenes have been successfully used in the selective detection of ions, DNA and small molecules [188-190].
O
Si
Si
O
O F
43
Scheme 38. Cleavage of the SiO bond of 43 by F [184]. 36 Page 36 of 79
ip t cr
Figure 8. The use ratiometric pyrene fluorescence probes as chemodosimeters. (Reprinted with permission from ref. 191. Copyright
us
© 2012 Royal Society of Chemistry.)
Shen designed the first example of the use of a pyrene as a ratiometric fluorescent chemosensor, 43. An –O–Si–Si–
an
O– or –O–Si–O– bridging moiety is used to link two pyrene rings. Fluoride-induced Si–O bond cleavage results in a ratiometric excimer/monomer conversion and significant changes in the emission spectrum [191] (Figure 8).
M
Upon incubation with F in THF/H2O (v/v, 50/50), the excimer emission band at 470 nm is significantly quenched, and there is a large increase in the intensity of monomer emission bands at 378 and 396 nm, which can be attributed
d
to the ion-specific cleavage of Si–O bonds by F, leading to the breakage of the stacked conformation of the two
te
pyrene moieties. The lower detection limit was in the micromolar range. Confocal fluorescence microscopy
Ac ce p
experiments have established the utility of water soluble nanoparticles with encapsulated molecules of 43 as a ratiometric biosensor in living cells. 2.1.11 Rhodamine-Based Sensors
Rhodamine has been widely used in many research areas, including use as fluorescent markers in structural microscopic studies, and as laser dyes and photosensitizers, due to their excellent photophysical properties, such as their high photostability, excellent quantum yields, and high extinction coefficients [192-197].
37 Page 37 of 79
(-O)H
R O
O
Si O
O F
N N
O
N H
N H
O
44 , R=H, O Si
cr
45, R=
N H
ip t
N H
N N
us
Scheme 39. Cleavage of the SiO bonds of 44 and 45 by F [198].
an
Bhattacharya designed two dual ‘turn-on’ chemosensors 44 and 45 for the detection of Hg2+ and F‾ ions using the rhodamine fluorophore. At physiological pH in aqueous media, the sensor mechanism is based on the ring-opening The addition of F‾ results in the
M
reaction of the spirolactam and the cleavage of the O-silyl bond [198].
instantaneous appearance of a yellow color, since the absorption band shifts from 300 to 385 nm for 44, and from
d
300 and 350 nm to 395 nm for 45. Bright yellow emission is observed with a maximum at 490 nm, which increases
te
in intensity by 10-fold. The lower detection limit was estimated to be 0.03 µM and 2.3 µM for 44 in MeCN and DMF/water (9/1, v:v), respectively, and 47 nM and 7 µM for 45 in MeCN and DMF/water (8/2, v:v). These sensors
Ac ce p
could be used to analyze the fluoride content in toothpaste.
O
O Si O
F
NH SO2
Et2N
O 46
F Si F
NH SO2
F
SO3
NEt2
SO3
Et2N
O
NEt2
Scheme 40. Cleavage of the SiO bond of 46 by F [199].
38 Page 38 of 79
NH
O O Si O
O
N N
48
ip t
47
N H
cr
O O Si O
NEt2
us
Scheme 41. Structures of 47 and 48 [199].
an
Martínez-Máñez designed a series of sensors for use in aqueous samples, 46-48, based on a fluoride-specific reaction between hydrofluoric acid and MCM-41 mesoporous nanoparticles functionalized with fluorescent or
M
colorimetric signaling units [199]. Upon treatment with F‾, 46-48 is freed from the surface and the colorless solution of 46 in MeCN–H2O (7:3, v:v) (buffered to pH = 2.5 with 0.1 M potassium hydrogenphthalate and HCl)
d
turns pink. Sensor 46 can also be used for the quantitative determination of the fluoride content in commercial
Ac ce p
te
toothpaste.
2.1.12 Other Fluorochrome-Based Sensors
39 Page 39 of 79
MeO
O O MeO
O
O
O
O
O
O
O O
O
CH 3
O
ip t
F
CH 3
O
O
O
O
O
O MeO
O O
MeO
O O
us
49
cr
TBDMSO
O O
Scheme 42. Mechanism of the reaction of 49 with F. [200]
an
Akkaya reported a new chemosensor, 49, which incorporates a self-immolative linker to trigger two simultaneous chemiluminescence processes [200]. When F was added to a DMSO/PBS buffer solution (90/10, v:v, pH 7.2), an
M
emission band was observed at 466 nm with bright blue chemiluminescence due to fluoride-mediated deprotection of the silyl-protecting group and an electronically triggered dioxetane cleavage, followed by self-immolation via a
te
d
1,4-quinone-methide rearrangement. PMMA polymer strips impregnated with 49 can be used to detect F in aqueous solutions at low micromolar concentrations.
Ac ce p
NO2
N
O
N
HN
O
O
NO2
OTBDMS F pH= 7.4
N O N NH2
50
Scheme 43. Cleavage of the SiO bond of 50 by F [201].
Talukdar successfully designed a 7-nitro-2,1,3-benzoxadiazole-based off–on chemosensor, 50, through the use of molecular modelling [201]. In the presence of other ions, only F triggered SiO cleavage, which results in a dramatic color change from colorless to yellow in 9:1 ethanol-HEPES buffer (v:v, 10 mM, pH = 7.4) solution, due to a decrease in the intensity of the main absorption band at 399 nm and the emergence of a new band at 460 nm. 40 Page 40 of 79
Upon addition of 300 equiv of F, a significant “turn-on” response is observed for strongly green fluorescence at 535 nm with a 110-fold enhancement in the intensity and a relatively slow response time of 80 min. Sensor 50 can be used to detect intracellular F ions in live cells. O
O
O
F
O
O
+
Si
N
cr
O
O
ip t
N
O
us
51
an
Scheme 44. Cleavage of the SiO bond of 51 by F [202]. Hong developed a novel chromogenic and fluorescent chemosensor, 51, based on the formation of a highly Upon treatment with fluoride in
M
fluorescent resorufin upon the addition of F at the silicon center [202].
acetonitrile–water (1:1, v/v) and acetonitrile solution, the main absorption band undergoes a large red shift from 445
d
to 586 nm resulting in a change in the color of the solution from yellow to pink. The main fluorescence emission
te
band of the product increases in intensity at 589 nm. In MeCN/H2O (1:1), after the addition of 3000 equiv of F, there is a 200-fold enhancement of the fluorescence intensity. Neither enhanced fluorescence intensity nor a color
Ac ce p
change, was observed in the presence of other ions. NC
CN
O
O
NC
CN
F DMSO/H2O 95:5(v:v)
O
Si
O
52
Scheme 45. Cleavage of the SiO bond of 52 by F [203]. Zhu devised a novel NIR region chemosensor, 52, with a selective “turn-on” fluorescence response for F by combining a tert-butyldiphenylsiloxy group with a dicyanomethylene-4H-chromene moiety [203]. In the presence 41 Page 41 of 79
of other ions, only the F‾ ion triggers the cleavage of the SiO bond resulting in a dramatic color change from pale yellow to blue. In DMSO/H2O (95:5, v/v) solution, there is a decrease in the intensity of the absorption band at 447 nm within only 30s, and a new band is observed at 645 nm. Upon addition of 400 equiv of F‾, a “turn-on” fluorescence emission response is observed at 718 nm with a 1000-fold enhancement in intensity. The lower
Si
F
an
H 2O/CTABr
cr
O
us
O
ip t
detection limit for F was 85 nM.
N
N
M
53
te
d
Scheme 46. Cleavage of the SiO bond of 53 by F [204].
Ac ce p
Martínez‐Máñez designed a novel pyridine-based fluoride chemosensor, 53, with a silyl ether group that enables the
selective recognition of F in pure water at pH 7.4 containing the cationic surfactant cetyltrimethylammonium bromide (CTABr) solutions [204]. The addition of F triggers a change in the color of the solution from colorless to yellow due to a decrease in the intensity of the absorption band at 325 nm, which has been ascribed to the fluorideinduced hydrolysis of the silyl ether moiety and the formation of a phenolate ion. A new absorption band is observed at 445 nm, with an emission band at 540 nm. The lower detection limit was 76 µM.
42 Page 42 of 79
NO2
NO2
F O
OTBDPS
H N CONH(CH2)5 CH3
NHCONH(CH2)5 CH3
ip t
54
Scheme 47. Mechanism of the reaction of 54 with F. [205]
cr
Cho reported a urea-based chemosensor, 54, for fluoride detection in mixed aqueous media. Upon addition of F in
us
HEPES buffered MeCN/water solution (9/1, v/v, 10 mM HEPES buffer, pH = 7.0), produces a readily observed colorimetric response from colorless to yellow due to the formation of a new absorption band at 420 nm. The F‾-
an
induced Si–O bond cleavage leads to the deprotection of a phenolic OH group and the formation of hydrogen bonds with the urea NH protons or carbonyl group, due to its free rotation along the acetylene axis [205]. In DFT
M
calculations, the distance between the oxygen atoms is predicted to be 2.73 Å which is consistent with the presence of a strong hydrogen bond. The lower limit of detection was less than 1 µM when the colorimetric approach is
te
d
taken into consideration. OTBS
Ac ce p
TBSO
N
O 2N
OH
O
F
N
55
HO
O
N NO2
O 2N
OH
N HO
NO2
Scheme 48. Cleavage of the SiO bond of 55 by F [206].
Bhalla designed a novel terphenyl fluorescent chemosensor, 55, for the selective recognition of Cu2+ and F‾ ions in two contrasting modes by making use of an excited state intramolecular proton transfer (ESIPT) process [206]. Upon addition of F‾, there is a decrease in the absorption intensity at 265 and 342 nm and a new red shifted band is formed at 447 nm resulting in a color change from colorless to yellow. The addition of F‾ to a THF solution leads 43 Page 43 of 79
to a significant decrease in the intensity of the emission band at 517 nm due to the inhibition of the ESIPT interaction. Upon further addition of F‾, a new blue-shifted band was observed at 478 nm due to the cleavage of the SiO bond. This results in increased negative charge on the oxygen atoms, which interacts strongly with the -
HO
TBSO N
F
N H
56
cr
N H
N
ip t
conjugation system. The lower detection limit was 10 nM when 55 was used as a fluorescence sensor.
us
Scheme 49. Cleavage of the SiO bond of 56 by F [207].
an
Yang designed a ratiometric fluorescent chemosensor, 56, based on the modulation of the ESIPT process associated with the protection/deprotection reaction of the hydroxyl group [207]. Upon addition of F, the absorption band at
reaction time.
M
293 nm decreases in intensity, while new bands at 315 and 324 nm gradually intensify over the course of a 40-min In aqueous DMF solution, the emission band at 360 nm decreases in intensity with a new
d
fluorescence peak appearing at 454 nm. This was ascribed to the restoration of the ESIPT process. The lower
Ac ce p
toothpaste and tap water samples.
te
detection limit was 0.19 µM. This analytical method has been successfully applied to fluoride determination in
Si
HO
O
N
N
F
N
N
57
Scheme 50. Cleavage of the SiO bond of 57 by F [208]. Lu reported a new highly selective and sensitive probe with a rapid response to the fluoride anion, 57 [208]. At low fluoride concentrations, the main absorption band of 57 was red shifted from 280 to 320 nm. With the further addition of F, the emission band was red-shifted from 384 to 475 nm. The detection limit was 0.43 µM. 44 Page 44 of 79
O
O F
Si O
O
ip t
58
Scheme 51. Cleavage of the SiO bond of 58 by F [209].
cr
Song used the naphthol fluorophore to design a colorimetric and “turn-on” chemosensor, 58, for fluoride detection
us
in acetonitrile [209]. Upon addition of F‾, the main absorption band of 58 gradually decreases in intensity at 304 nm and a new band is formed at 406 nm, resulting in a color change from colorless to yellow. Upon treatment with
an
10 eq. of F‾, the fluorescence emission at 488 nm is enhanced 9663-fold within 30s due to an ICT process. The
O
O
H
N
H
O
te
N
N
O H
Ac ce p
O Si
O H O
d
Si
M
lower detection limit was 40 nM.
O
H
N
N
F
H
Si
O
O
H
N
O H
H O
59
Scheme 52. Mechanism of the reaction of 59 with F. [210]
Lee reported that 59 can be used as a “turn-on” chemosensor for the detection of F based on a conformational switch of a covalently modifiable fluorophore [210]. Upon addition of F‾ in CH2Cl2 solution, there is a 3-fold increase in the emission band intensity at 458 nm, due to F‾-triggered SiO bond cleavage and the formation of a rigid molecular structure under mild conditions (spring-loaded conformational transitions).
45 Page 45 of 79
NC
NC NC
CN
CN
NaF
O
Si O
NC
O
O 60
ip t
Scheme 53. Cleavage of the SiO bond of 60 by F [211].
cr
Zhu developed a colormetric and red-emitting fluorescent dual-channel chemosensor, 60, for imaging F‾ in living cells, by using the Si–O bond as a highly selective recognition receptor [211]. When F‾ is added to a solution of 60,
us
there is a red shift in the maximum of the main absorption band of 158 nm from 438 to 596 nm and the color of the solution changes from yellow to blue. In the fluorescence spectrum, the continuous addition of F‾ results in a
an
gradual increase in emission intensity at 612 nm, due to a strong ICT effect. Sensor 60 can be used to determine F‾ concentrations in the 0–6 mM range with a lower detection limit of 0.07 mM and can also be used for the bio-
M
imaging of F‾ in living HeLa cells.
d
O Si O
O
Ac ce p
61
N
te
N O
F
Scheme 54. Cleavage of the SiO bond of 61 by F [212].
Yang reported a new chemosensor, 61, for the rapid and sensitive colorimetric detection of F‾ in aqueous solution, coupling the specific molecular recognition ability of silylated spiropyran (SPS) dyes with the unique features of graphene oxide to develop a nanocomposite [212]. During the reaction with F‾ in the presence of graphene oxide, a very strong absorption band maximum centered at 490 nm appears within 30 min and there is a rapid color change from colorless to orange-yellow due to the cleavage of the SiO bond that is triggered by the conversion of the closed spiropyran to an open mercyanine. 61 acts as a sensor for the F‾ ion between 710 pH. The SPS/graphene oxide nanocomposite has been successfully used to monitor F‾ levels in serum. 2.1.13 F triggered cyclization reaction 46 Page 46 of 79
COOEt
COOEt F
COOEt O Si(t-Bu)Me 2
S S
O S S
62
COOEt
ip t
COOEt
COOEt OTIPS
O
cr
O
F
S
S
OC 12H 25
C 12H25 O
OC 12H 25
us
C 12H25O
O
n
n
an
63
M
Scheme 55. Cleavage of the SiO bond and cyclization of 62 and 63 by F [213]. Swager reported a new strategy for the detection of F‾ based on the formation of highly fluorescent coumarins, 62
d
and 63, through fluoride-triggered Si−O bond cleavage and cyclization reactions. The response is successfully
te
amplified using exciton migration in a semiconducting organic polymer [213]. Upon the gradual addition of F‾, the absorbance maximum of 62 exhibits a red shift from 384 to 412 nm with a 46-fold enhancement in fluorescence
Ac ce p
emission intensity at 450 nm due to the formation of a coumarin. Upon addition of F‾, the emission band maximum of the electric coupling polymer 63 was red shifted from 482 to 517 nm with a 100-fold intensity enhancement since the structure of the polymer enables dexter energy transfer. COOEt
COOEt
COOEt OTBS OTBS
F
O O
O
64
Scheme 56. Cleavage of the SiO bond and cyclization of 64 by F [214]
47 Page 47 of 79
Wang reported a new sensor, 64, for sensing F‾ that makes use of a rational reaction-based relay recognition strategy [214]. In the presence of other ions, only F‾ generates a significant quenching in the intensity of the fluorescence band at 360 nm and an increase in the intensity of the characteristic monomer peak of a coumarin derivative at 460 nm. The color of the fluorescence changes gradually from blue to yellowish green. The lower
O O
O 113
x O O
y O O
O 113
z O O
O
O
O
O O
O
an
8-9 O
O
O
65 O
M
O
y O O
z O O
us
F
x O O
cr
O O
ip t
detection limit was 1.86 μM.
O
O O
8-9 O O O
O
OTBS
O
COOEt
COOEt
d te
O
COOEt
Ac ce p
Scheme 57. Cleavage of the SiO bond and cyclization of 65 by F [215]
Liu reported a highly selective and sensitive fluorescence “turn-on” chemosensor, 65, based on the preparation of a novel type of responsive double hydrophilic block copolymer in purely aqueous media by exploiting the F‾-induced cyclization reaction of non-fluorescent moieties to induce the formation of fluorescent coumarins [215]. Upon treatment with 100 equiv of TBAF, the emission intensity at 402 nm exhibits a monotonic 88-fold increase in intensity, leading to a fluorometric transition from a non-emissive solution to an intense blue color, due to the generation of coumarin moieties. The lower detection limits were 3.4 and 2.6 µM for diblock monomers and micelle nanoparticles, respectively.
48 Page 48 of 79
S
S N
F
N
CN
Et2 N 66
O TBS
Et2 N
O
NH
ip t
Scheme 58. Cleavage of the SiO bond and cyclization of 66 by F [216] Peng reported that a novel chemosensor, 66, has unprecedented sensing properties for fluoride and copper (II) ions
cr
[216]. Upon addition of F‾, the fluorescence intensity at 523 nm significantly increases by 164-fold in ethanol-
us
water(1:1; v/v), and there is a change in the fluorescence color from almost colorless to bright jade green, since the fluoride ion triggers Si−O bond cleavage and an intramolecular cyclization to form a highly fluorescent
an
iminocoumarin−benzothiazole. In pure ethanol solution there is a dramatic 833-fold enhancement in the emission intensity at 510 nm. The color of the solution changes from yellow to yellowish green. The lower detection limit
M
was calculated to be 19.6 nM.
S NC
te
d
N
O Si
F
N N
O
NH
Ac ce p
N
S
67
Scheme 59. Cleavage of the SiO bond and cyclization of 67 by F [217]
Venkatesan reported a coumarin-based fluorescent F chemosensor, 67,[217]. Upon addition of F, the main absorption band at 465 nm shifts to 483 nm with an obvious color change from yellow to green. A new emission band centered at 521 nm is also observed. 67 can be used as an efficient fluorescent sensor for F both in vitro and in vivo. The lower limit of detection was 1.45 μM.
49 Page 49 of 79
N
O Si N
F N
HEPES pH 7.4 68
NC
O
NH
N
CN
CN
ip t
Scheme 60. Cleavage of the SiO bond and cyclization of 68 by F [218] Songdesigned a novel red emitting fluorescent chemosensor, 68, for the detection of F‾ in aqueous solution (35 mM,
cr
pH 7.4, HEPES buffer with 30% CH3CN) that is based on a F‾ triggered Si–O bond cleavage and a subsequent
us
rigidizing, cyclization reaction [218]. A blue shift from 492 to 473 nm is observed in the main absorption band within 15 min. 68 is essentially non-fluorescent due to fast non-radiative decay of the singlet excited state mediated
an
by internal bond rotations. An intense fluorescence emission band is observed for the product at 616 nm in the red region of the spectrum, with a large Stokes shift of 143 nm. The lower detection limit was calculated to be 5.4 µM.
CN
d
NC
M
68 can penetrate cell membranes and has been successfully used in the imaging of F‾ in living HaCaT cells.
H
O Me Si t-Bu
H 2O
Me 2N
O
NH
Me
Ac ce p
69
te
Me 2N
CN
F
Scheme 61. Cleavage of the SiO bond and cyclization of 69 by F [219]
Ahn reported a novel molecular chemosensor, 69, which exhibits excellent analyte selectivity and sensitivity based on fluoride-mediated desilylation to form an iminocoumarin [219]. A red shift of the absorption band center from 460 to 585 nm is observed upon addition of fluoride ions and there is a significant two-photon absorption crosssection value (GM =180). 69 initially exhibits almost no fluorescence in HEPES buffer solution (10 mM, pH 7.4) containing acetonitrile (20% by volume). Upon the addition of F‾, however, there is a dramatic increase in the fluorescence intensity at 595 nm within 10 min. The lower limit of detection was below 210 µM. Sensor 69 can be used to monitor F levels in living cells and in zebrafish by confocal and two-photon fluorescence microscopy.
50 Page 50 of 79
Si O
O
F
NH
CN CN
CN 70
ip t
Scheme 62. Cleavage of the SiO bond and cyclization of 70 by F [220] Peng reported a highly selective and sensitive organosilicon fluoride ion probe, 70, with intense green fluorescence,
cr
which provided the first example of fluoride ion bioimaging in mitochondria [220]. In the absence of F‾ in CH3CN solution, the major absorption band of 70 lies at 463 nm and no fluorescence is observed. Intriguingly, upon adding
us
F‾ to the solution within 10 min the absorption band is blue shifted to 441 nm and strong fluorescence intensity is
an
observed at 485 nm with detection limits as low as 0.16 μM.. Fluoride paper test strips can easily be prepared with lower detection limits as low as 19 ppb in aqueous solutions.
TBSO
M
I
O
OTBS
O
te
71
I
d
F
Ac ce p
Scheme 63. Cleavage of the SiO bond and cyclization of 71 by F [221]
Kim reported a fluoride-induced aromatic cyclization sensor method which produces ladder-type conjugated molecules [221]. An analysis of the absorption and emission spectroscopy confirmed that their rigid planar structures result in an extension of the -conjugation system. 71 is non-fluorescent. Upon addition of F‾, strong fluorescence is observed at 336 nm due to fluoride-induced cyclization. When only two equiv of fluoride are added in THF, there is complete rapid conversion to the ladder-type product in only 2 min.
51 Page 51 of 79
TBSO
OTBS
HO
OH
F
TBSO
HO
OTBS
72
OH
HO
OH
ip t
TBSO
OTBS
Scheme 64. Cleavage of the SiO bond and cyclization of 72 by F [222]
cr
Bhalla developed a novel terphenyl based chemosensor, 72, based on the fluoride ion induced cyclization of On
us
symmetrically/unsymmetrically-substituted triphenylenes in the absence of any oxidizing reagent [222].
addition of F‾, the main absorption band at 295 nm undergoes a red-shift to 342 nm, due to cleavage of the SiO
an
bond. An extra band is observed at 663 nm resulting in a dramatic color change from colorless to violet. On the further addition of F‾, the absorption band at 663 nm disappears and new bands are observed at 526 and 879 nm
M
which can be ascribed to cyclization of 72 to form a triphenylene. Upon addition of F‾, the fluorescence emission band shifts from 393 to 425 nm with a 32% enhancement in intensity, due to the increased negative charge on the
d
phenolate oxygens and the subsequent cyclization. The lower detection limit for F‾ was 2 μM.
O
Ac ce p
TBSO
OTBS
te
OTBS TBSO
O
OTBS
73
O
OTBS
OTBS OTBS
O
O O
O
F
O
O O
O O
Scheme 65. Cleavage of the SiO bond and cyclization of 73 by F [223]
Bhalla have also reported that 73 provides a novel fluoride sensor, since the oxy-tert-butyldimethylsilyl (OTBS) groups undergo irreversible cyclization to substituted higher quinones in the presence of TBAF in dry THF [223]. Upon the addition of 2 equiv of F‾, the absorption band of 73 shifts from 357 nm to 400 and 299 nm. When a further 3 equiv of F‾ are added, the absorption bands at 400 and 299 nm reach their maximum intensities and the band at 400 nm is further red-shifted to 411 nm, while the band at 299 nm shifts slightly to the blue to 296 nm. This is accompanied with a gradual change in the color of the solution from yellow to brown as fluoride-induced 52 Page 52 of 79
desilylation and the extension of the -conjugation system leads to an enhanced negative charge on the phenolate oxygens and hence a red shift of the spectral bands. Upon addition of 10 equiv of F, the fluorescence band at 551 nm was efficiently quenched, due to charge transfer from the phenolate oxygens to the quinone moiety. OTBS
OH
TBSO OTBS
OTBS
OH HO
OTBS
us
F
OTBS
TBSO
OH
an
OTBS TBSO
OH
HO OH
M
OTBS 74
OH
OH
HO OTBS
OH
cr
TBSO
ip t
HO
Scheme 66. Cleavage of the SiO bond and cyclization of 74 by F [224]
d
Bhalla developed a highly selective triphenylene-based chemosensor, 74, with twelve OTBS groups, which
te
undergoes cyclization to a supertriphenylene [224]. In the absence of F‾, the main absorption band of 74 lies at 306
Ac ce p
nm. Upon treatment with F, this band undergoes a red shift to 325 nm and the color of the solution changes from colorless to green. When 12 µM of F‾ was added to a THF solution of 74, the intense fluorescence emission band at 395 nm was completely quenched. 74 has a lower detection limit of 0.8 μM for F‾. These TLC strips coated with 74 can be used for the instant detection of the fluoride ion in aqueous media.
2.2 F triggered Silicon-Carbon bond cleavage The fluoride ion can also trigger silicon-carbon bond cleavage and this has been used to form fluoride chemosensors. There is a more rapid bond cleavage than is the case with the OSi bond, possibly due to the low bond-dissociation energy of the CSi bond.
In contrast with the silicon-oxygen systems, there is usually no ICT quenching
mechanism available to create a “turn-off” sensor. Colorimetric changes are observed instead based on the changes in the HOMOLUMO gaps of the sensors. 53 Page 53 of 79
2.2.1 BODIPY-Based Sensors TMS
CH3 CH3 F
B
N N
F F TMS
75
B
N
N
F F
B
N
F F 76
TMS
N
B
N
F F
cr
N
ip t
F
Ac ce p
te
d
M
an
us
Scheme 67. Cleavage of the SiC bond of 75 and 76 by F [225].
Figure. 9. The frontier MOs of the protected and deprotected forms of 75 and 76 calculated for the B3LYP optimized geometries by using the CAM-B3LYP functional with 6-31G(d) basis sets at an isosurface of 0.02 a.u. Occupied MOs are highlighted with black diamonds. MOs with angular nodal patterns similar to those of the HOMO and LUMO of the BODIPY chromophore are highlighted with gray lines.
54 Page 54 of 79
Ravikanth developed a novel BODIPY derivative, 75, for use as a fluoride chemosensor by introducing trimethylsilylethynyl groups at the 3,5-positions [225]. On addition of TBAF in CH2Cl2, the main absorption band shifts from 571 to 551 nm and the fluorescence emission band shifts from 584 to 564 nm with a color change from bright orange to green due to a F‾-induced transformation of the electron-rich trimethylsilylethyne group to an
ip t
electron-deficient ethyne group. MO calculations demonstrate that this is due to the change in the HOMOLUMO gap (Figure 8). In contrast, there is no obvious change in the absorption and emission spectra of 76, since the
cr
phenyl group at the 4-position is not conjugated with the main BODIPY -system. Mes
N
Si(C 6H13) 3
N
B
F Acetone
N
77
Si(C6 H13) 3
N
B
F F
an
F F
us
(C 6H13) 3 Si
Mes
M
Scheme 68. Cleavage of the SiC bond of 77 by F [226]. Jiang reported an innovative reaction-based 2,6-trihexylsilylacetylene-conjugated BODIPY dye, 77 [226]. In the
d
presence of other anions in acetone, only the interaction with F‾ results in a decrease in the intensity of the
te
absorption band at 555 nm and the appearance of a new band at 538 nm. At the same time, the fluorescence
Ac ce p
emission band of 77 exhibits a blue shift from 571 to 554 nm and the color is observed to change from orange to green in less than 5 min. The lower detection limit for 77 was 67.4 nM. Mes
THS
N
N
Mes THS
B F F 78
F
N
B
N
F F O2 N O O S O
NO2
O H
Scheme 69. Cleavage of the SiC bond of 78 by F [227].
55 Page 55 of 79
Jiang
designed
a
second
2,6-trihexylsilylacetylene
BODIPY
chemosensor,
78,
containing
2,4-
dinitrobenzenesulfonyl and trihexylsilylacetylene (THS) as a “two-in-one” chemosensor for F‾ and HS‾ [227]. Upon addition of F‾, the fluorescence band shifts from 613 to 553 nm with a marked increase in intensity in ca. 18 min resulting in a color change from purple to yellow. The lower detection limit was 14.5 nM, which is nearly 5
ip t
times lower than that of 77 (67.4 nM). 2.2.2 Pyrene-Based Sensors
TMS
an
F
cr
TMS
us
TMS
TMS
79
Ac ce p
te
d
M
Scheme 70. Cleavage of the SiC bond of 79 by F [228].
Figure. 10 The HOMO and LUMO energy levels and angular nodal patterns of 79 (left) and the product formed after the addition of F‾ (right) at an isosurface value of 0.02. (Reprinted with permission from ref. 228. Copyright © 2011 Royal Society of Chemistry.)
Shen developed a new pyrene-based fluorescent chemosensor, 79 [228]. Upon addition of F‾, the main absorption bands undergo a blue shift from 436 and 410 nm to 417 and 392 nm, resulting in a significant colour change from light green to colorless, which can be attributed to the elimination of the TMS substituents due to the strong interaction between the fluoride ion and the silicon atoms. This results in an increase in the HOMOLUMO gap 56 Page 56 of 79
due to the absence of δ–π interactions between the silicon atoms and the pyrene -system (Figure 10). Upon addition of F‾, the fluorescence emission is blue shifted within a few seconds and the emission changes color from blue to purple.
The detection limit was approximately 53 µM, which approaches standard drinking water
concentrations. Test papers prepared by immersion in THF solutions can be used to detect the presence of F in
ip t
aqueous samples. 2.2.3 Diketopyrrolopyrrole (DPP)-Based Sensors C12H 25
cr
C12H 25
C10H21 S
O TMS
S
O
N
S
O
N
an
TMS
C10 H21
F
us
N
C 12H25
O S
N
C 12H25
C 10H 21
M
80
C 10H 21
Ac ce p
te
d
Scheme 71. Cleavage of the SiC bond of 80 by F [236].
Figure 11. The HOMO and LUMO energy levels and angular nodal patterns of 80 (left) and the desilyl product formed after the addition of F (right) at an isosurface value of 0.02. (Adapted with permission from ref. 236. Copyright © 2014 Elsevier.)
Diketopyrrolopyrrole (DPP) dyes are a synthetically versatile class of visible light-harvesting chromophores with high fluorescence quantum yields, which have been exploited in both small molecule and polymeric formats in areas as diverse as photorefractive materials, fluorescence sensors, thin film transistors, and light-emitting 57 Page 57 of 79
diodes[229]. DPP-based sensitizers have also emerged as promising candidates for use as low cost photovolatics (PVs) based on either organic bulk heterojunctions or dye-sensitized solar cells [230-235]. Choi reported that DPP derivative 80 can act as a highly sensitive, selective and rapid recognition sensor for F‾ in THF/HEPES (8:2, v/v; pH 7.4) by using a chemodosimeter approach [236]. Upon addition of F‾, an absorption
ip t
band at 319 nm increases in intensity and there is a significant decrease in the intensity of bands at 385, 408, 549 and 589 nm. There is also a blue shift of the fluorescence emission maximum from 617 to 604 nm and a slight
cr
change in emission intensity. A red shift is observed for the major bands in both the absorption and emission spectra of 80, relative to those of the desilyl compound, because of a destabilization of the HOMO (Figure 11). The
us
reaction occurs in less than 5 minutes at 25 ˚C at low reactant concentrations. The lower detection limit of 80 was 0.2 µM. 80 can also be used to detect F‾ in the solid state such as in glass films and test strips, opening the door to
an
practical applications such as solid optical sensors.
M
2.2.4 Naphthalene-Diimide-Based Sensors
Naphthalene-diimide derivatives are planar molecules with high electron mobilities, which have been used in
d
artificial photosynthetic reaction centers, in conducting materials, and as chemosensors for ions and small molecules,
te
because of their electron acceptor properties [237-242]. (CH2)7 CH3 N O
Ac ce p
O
TMS
O
N O (CH2)7 CH3
O
TMS
(CH2)7 CH3 N O
F
O
N O (CH2)7 CH3
81
Scheme 72. Cleavage of the SiC bond of 81 by F [243]. Bhosale have reported a naphthalene diimide based colorimetric and fluorescent chemosensor, 81 [243]. Upon addition of TBAF, absorption bands at 289, 382 and 439 nm are blue shifted to 275, 376 and 419 nm and there is a significant visible change in the color of the solution from yellow to dark brown. The emission intensity at 329 and
58 Page 58 of 79
455 nm is enhanced due to the deprotection of both the trimethylsilyl moieties of 81. The analytical process is rapid and can be carried out within tens of seconds or a few minutes. 2.2.5 Other Systems
N
ip t
N
N
N
O
O
F TIPS
O
N
N
us
O
cr
TIPS
N
an
N 82
M
Scheme 73. Cleavage of the SiC bond of 82 by F [244]. Miljanić reported the F‾ detection properties of a silylethynyl-substituted benzobisoxazole cruciform compound, 82,
d
that are caused by the desilylation of silylethynyl and the formation of an ethynyl moiety [244]. Stabilization of the
te
HOMO upon desilylation results in a wider HOMOLUMO gap. When 82 is treated with F, the intensity of the
Ac ce p
emission band at 381 nm increases by more than 50% and the color of the solution changes from cyan to purple. The lower detection limit for F‾ was 50 μM.
2.3 Sensors Based on F-Si Interactions
The addition of a fluoride ion to tetrahedral organosilicon compounds results in a structural change to a trigonal bipyramidal geometry due to pentacoordination. This causes changes in the interligand through-space interactions of the substituent -systems leading to changes in the photophysical properties, which can be used to form a sensor system. F KF Si Ant Ant Ant [2.2.2]cryptand 83
F Ant Ant Si Ant F
K /cryptand
84
59 Page 59 of 79
Scheme 74. Possible structure of the 83−F complex [245]. Yamaguchi reported the use of 83 as a fluorescent chemosensor for F, based on the perturbation in the throughspace interaction between the anthracene moieties of 84 [245]. 83 has red-shifted absorption and emission band maxima relative to anthracene, for this reason. The silicate 84•K+/cryptand was isolated by using KF/[2.2.2]
ip t
cryptand as a source of F. Upon addition of TBAF, there is a blue shift in the absorption band of 83 from 401 nm to 392 nm and the emission band shifts from 416 nm to 396 nm. These photophysical properties can mainly be
cr
ascribed to the electronic perturbation associated with the hypercoordination. A decrease in the degree of the
F
Ant
Si
us
through-space interaction between the anthryl groups would be anticipated based on the crystal structures.
Ant
Ant
Si
Ant F SMe2
an
SMe2
F
F
OTf
M
85
d
Scheme 75. Possible structure of the 85−F complex [246].
te
Gabbaï developed a novel proximal third row onium ion based chemosensor, 85, by making use of the cooperative
effects between the fluoride atom and fluorosilanes [246]. Upon treatment with TBAF, the absorption bands were
Ac ce p
blue shifted due to decreased intramolecular anthryl-anthryl π-stacking interactions that are related to the change in the coordination geometry at the silicon center when 85-F is formed. The trigonal bipyramidal geometry around the Si atom that is formed by the Si–F–S bridge was demonstrated by X-ray crystallography. F3C CF 3 O
Si
O CF3 CF3 86
F F
CF3 CF3 O Si O F3C CF3 87
Scheme 76. Possible structure of the 86−F complex [247].
60 Page 60 of 79
F
F
Si F
Si
89
88
90
ip t
NO2
Na
F
NO2
NO2 92
us
91
cr
Si
an
Scheme 77. Mechanisms of the reactions of 88 and 91 with F [247]. Fensterbank developed three silicon-based sensors, 86, 88, and 91 to detect the presence of fluoride ions in solution
M
[247]. Upon capture of F‾, 86 forms a three-center four-electron hypervalent bond at the apical position to form 87 and this results in a red shift of the emission band from 290 to 311 nm in DCM with a detection limit of 10 μM. 86
d
exhibits high selectivity for F‾ in the presence of other naturally abundant anions. Although 90 is formed from 88
te
in a similar manner, in protic solvents or in the presence of water, the emission signal at 348 nm disappears upon
Ac ce p
addition of 1 equiv of TBAF and is replaced within 17 min by a band at 301 nm which was assigned to the formation of biphenyl, 89. When 91 is treated with TBAF in the presence of traces of water, the emission is blue shifted from 336 to 305 nm with detection limits as low as 5 μM due to the formation of 92.
61 Page 61 of 79
Table 1. Summaries of physiochemical properties of F‾ chemosensors. F¯ type
1 2
TBAF TBAF
498→488 560→682
λemA →λemP (nm) 506→ 575→
3
TBAF
546→644
573→676
4
TBAF
718→780
750→
↓ ↓ 71-fold ↑ ↓
5
TBAF
318→361
394→459
↑
19 nM
6
TBAF
330→434
→500
↑
50 nM 16 μM
7
NaF
－
→461
4-fold ↑
－
8
NaF
－
→468
↑
－
9 10
TBAF TBAF
－ －
→387 →374
－ －
11
TBAF
592→680
606→472
↑ ↑ 17.4fold ↑
12
NaF
－
→532
↑
13
TBAF
－
→520
160-fold ↑
10.5 μM
14
F¯
－
→526
↑
1 μM
15
NaF
－
→520
16
TBAF
－
→523
17
TBAF
→500
→523
18
TBAF
365→421
449→508
220-fold ↑
19
TBAF
365→475
→560
↑
20
TBAF
362→474
－
21
F¯
361→471
430→553
22
TBAF
－
524→410
－ 510-fold ↑ 2.7-fold ↑
23
TBAF/ NaF/KF
366→
442→
24
TBAF
→370, 625
25
TBAF
26
Limit
Media
Probe type
Ref.
－ －
CH3CN CH3CN
turn off turn off
95 95
0.12 μM
DCM
ratiometric
97
2.1 μM
CH3CN
turn off
98
turn on
110
turn on
111
turn on
112
turn on
113
turn on turn on
114 114
us
cr
C3H6O-H2O (7:3, v:v, pH 9.1 ) CH3CN H2O PBS pH 7.4 CH3OH-HEPES buffer (1:1, v:v, pH 7.4 ) CHCl3 CHCl3
0.12 μM
DMSO
ratiometric
121
0.041 μM
DMF-buffer (7:3, v:v, pH 7.0 )
turn on
130
H2O(PBS, pH 7.4 )
turn on
131
turn on
132
turn on
133
M
an
φ ratio
ip t
Dye
λabsA →λabsP (nm)
2.7-fold ↑ 550-fold ↑ ↑
18 μM
HEPES buffer pH 7.4 HEPES-THF (9:1, v:v, pH 7.2 ) DMSO
turn on
134
0.05 μM
CH3CN
turn on
135
－
MeCN
ratiometric
143
18 μM 0.1 μM 0.59 μM
MeCN-H2O (1:1, v:v) MeCN MeCN DMSO-buffer (9:1, v:v, pH 7.5 )
turn on
144
colorimetric
145
ratiometric
147
7 μM
DCM
turn on
148
↓
2.1 μM
MeCN-H2O (7:3, v:v)
turn off
149
500→
↓
0.105 μM
MeCN
turn-off
150
→423, 552
486→
↓
2.6 μM
MeCN
colorimetric
151
NaF
600→440
－
－
0.1 μM
colorimetric
158
27
TBAF
362→545
→476→
↑↓
0.1 μM
off-on-off
159
29
F¯
407→517
500→558
↑
80 μM
ratiometric
160
30
NaF
420→535
→558
colorimetric
161
31
NaF
320→340
418→560
32
NaF
345→370
406→494
33
TBAF
293→440
407→477
Ac ce p
te
d
1.03 μM
25-fold ↑ 6.6-fold ↑ ↑ 55-fold ↑
－
6.6 μM
THF-H2O (7:3, v:v) MeCN Ethanol-H2O (3:7, v:v, pH 7.4,PBS) Ethanol-H2O (3:7, v:v, pH 7.4,PBS)
5.3 μM
H2O
ratiometric
166
－
PVP MeCN-H2O (8:2, v:v, pH
ratiometric
168
turn on
169
6.5 μM
62 Page 62 of 79
0.1 μM
7.4,HEPES) DMSO
ratiometric
170
－
PBS, pH 7.4
turn on
171
30 μM
PBS buffer (10 mM, pH = 7.2)
turn on
172
↑
－
PBS buffer (10 mM, pH = 7.2)
turn on
172
1.0 μM
THF
ratiometric
178
1.0 μM
THF
TBAF
344→481
418→602
35
NaF
－
→508
36
NaF/TBAF
－
→520
37
NaF/TBAF
－
450→550
38
TBAF
313→410
403→520
39
TBAF
－
418→520
40
TBAF
350→470
－
↑
－
41
TBAF
－
－
↑
－
42
TBAF
365→595
－
43
TBAF
－
470→396
44
TBAF
300→385
→490
45
TBAF
350→395
→490
49
TBAF
－
→466
50
TBAF
399→460
→535
51
TBAF
445→586
→589
52
TBAF
447→645
→718
53
F¯
325→445
→540
54
NaF
→420
55 56 57
F¯ NaF TBAF
342→447 293→324 280→320
58
TBAF
304→406
→488
59
TBAF
－
→458
9663fold ↑ 3-fold ↑
60
NaF
438→596
→612
↑
62
TBAF
384→412
489→529
63
TBAF
378→396
482→517
64
TBAF
303→335
360→460
46-fold ↑ 100-fold ↑ ↑
65
NaF
－
→402
↑
179
colorimetric
184
ratiometric
185
THF-H2O (1:1, v:v) MeCN DMF-H2O (9:1, v:v) MeCN DMF-H2O (9:1, v:v) DMSO-PBS (9:1, v:v, pH 7.2 ) Ethanol-HEPES (9:1, v:v, pH 7.4)
ratiometric colorimetric turn on
191
turn on
198
turn on
200
turn on
201
－
MeCN-H2O (1:1, v:v)
turn on
202
85 nM
DMSO-H2O (95:5, v:v)
turn on
203
↑
76 μM
H2O (pH 7.4)
turn on
204
－
↑
1.0 μM
colorimetric
205
517→478 360→454 384→475
↑ ↑ ↑
10 nM 0.19 μM 0.43 μM
MeCN-H2O (9:1, v:v, pH 7.0) THF DMF (pH 7.0) THF
turn on ratiometric turn on
206 207 208
40 nM
MeCN
turn on
209
－
DCM Ethanol-H2O (7:3, v:v, pH 7.4, PBS)
turn on
210
colorimetric
211
290-fold ↑ ↑ 10-fold ↑ 10-fold ↑
1.0 μM 0.03 μM 2.3 μM 47 nM 7 μM
↑
1.0 mM
→510
THF
an
us
15 μM
－
M
110-fold ↑ 200-fold ↑ 1000fold ↑
d
te
－
cr
184
Ac ce p
TBAF
MeCN-H2O (9:1, v:v) MeCN-H2O (9:1, v:v)
turn off
colorimetric
→523 66
2.1-fold ↑ 15-fold ↓
ip t
↑ 30-fold ↑ 121-fold ↑
34
0.07 mM
198
－
DCM
turn on
213
－
DCM
turn on
213
1.86 μM 3.4 μM 2.6 μM
THF Diblock unimers Micelles nanoparticles
ratiometric
214
turn on
215
turn on
216
turn on
217
turn on
218
turn on
219
164-fold ↑ 833-fold ↑
19.6 nM
146-fold ↑
1.45 μM
Ethanol-H2O (1:1, v:v) Ethanol
67
TBAF
465→483
→521
68
NaF
492→473
→616
↑
5.4 μM
69
NaF
460→585
→595
↑
210 μM
MeCN-H2O (1:1, v:v, pH 7.0) HEPES (30% MeCN, pH 7.4) Buffer
63 Page 63 of 79
(20% MeCN, pH 7.4) 463→441
→485
0.16 μM
MeCN
turn on
220
71
TBAF
255→256
→336
72 73 74 75 77
TBAF TBAF TBAF KF TBAF
295→663→879 357→411 306→325 571→551 555→538
393→425 551→ 395→ 584→564 571→554
78
TBAF
－
613→553
79
TBAF
436→417
→423
4600fold ↑ ↑ ↓ ↓ ↑ ↑ 50-fold ↑ ↑
－
THF
turn on
221
2 μM － 0.8 μM 67.4 nM
THF THF THF DCM Acetone
turn on turn off turn off ratiometric colorimetric
222 223 224 225 226
14.5 nM
Acetone
ratiometric
227
53 μM
colorimetric
228
turn on
236
－
THF THF-H2O (8:2, v:v, pH 7.0) DCM
80
TBAF
588,549,408,388→319
617→604
↑
0.2 μM
81
TBAF
289→275
→329,455
82
TBAF
－
→381
83
KF
401→392
416→396
86 88 91
TBAF TBAF TBAF
－ － －
290→311 348→301 336→305
↑ 1.5-fold ↑ 19-fold ↑ ↑ ↑ ↑
turn on
243
50 μM
THF
turn on
244
THF
turn on
245
DCM Ethanol Acetone-H2O(1:1, v:v)
turn on turn on turn on
247 247 247
－
ip t
TBAF
us
70
cr
1049fold ↑
an
10 μM － 5 μM
M
λabsA: the maximum absorption wavelength of the chemodosimeter in the absence of F‾. λabsP: the maximum absorption wavelength of the chemodosimeter in the presence of F‾. λemA: the maximum emission wavelength of the chemodosimeter in the absence of F‾. λemP: the maximum emission wavelength of the chemodosimeter in the presence of F‾. φ ratio: the change
te
detecting F‾.
d
ratio of the emission intensity of the chemodosimeter upon addition of F‾. Limit: the detection limit of the chemodosimeter for
Ac ce p
3. Conclusions and perspectives
A comprehensive account of the research that has been carried out to develop organosilicon compounds for use as chemosensors for the fluoride anion (Table 1) has been provided. Organosilicon compounds, which take advantage of the affinity between fluoride and silicon, have faster response times, better selectivity, higher sensitivity and stability both in organic solvents and in aqueous solution than the other chemosensors that have been used for F‾ detection. There are three main sensor reactions between the F‾ ion and silicon: (i) F triggered silicon-oxygen bond cleavage; (ii) F triggered silicon-carbon bond cleavage; and (iii) direct FSi interactions. Ideally, fluoride chemosensors should meet the requirements for cellular and biological systems, such as easy detection of F in aqueous environments, permeability through cell membranes and nontoxicity to cells. However, most of organosilicon chemosensors that have been reported have been for the recognition of fluoride in organic 64 Page 64 of 79
medium. To date, only a few organosilicon chemosensors have been developed that can be used to detect F in living cells, with high detection sensitivity and short response times, and this limits the wider applicability of fluoride anion fluorescence sensor research. Despite the relative lack of progress in this regard, fluorine sensor research remains a highly active emerging field. In order to promote the further development of fluoride ion
ip t
fluorescent chemosensors, an obvious approach will be to carry out the rational design of sensors using DFT/TDDFT calculations to model trends in the optical properties and electronic structures of organosilicon
cr
compounds. And the development of sensors that are suitable for use in the clinical detection of fluoride ions remains the ultimate goal. Most of the chemosensors that have been reported do not absorb in the NIR region, so
us
the incident light cannot penetrate the cell membrane, limiting their suitability for in vitro and in vivo imaging. Given the progress that has been made in the context of BODIPY dyes, which retain excellent photophysical
an
properties even after structural modification [84], it appears probable that rational strategies for forming reactionbased F‾ chemosensors that are soluble in aqueous solution and absorb and emit strongly in the NIR region will be
M
reported in due course.
d
Acknowledgment
te
Financial support was provided by the National Natural Science Foundation of China (no. 21471042) and Zhejiang
Ac ce p
Provincial Natural Science Foundation of China (Grant No. LY14B010003). Theoretical calculations were carried out at the Centre for High Performance Computing in Cape Town.
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[227] L. Fu, F.F. Tian, L. Lai, Y. Liu, P.D. Harvey, F.L. Jiang, Sens. Actuat. B 193 (2014) 701. [228] H. Lu, Q. Wang, Z. Li, G. Lai, J. Jiang, Z. Shen, Org. Biomol. Chem. 9 (2011) 4558. [229] Z. Hao, A. Iqbal, Chem. Soc. Rev. 26 (1997) 203. [230] Y. Qu, J. Hua, H. Tian, Org. Lett. 12 (2010) 3320. [231] W.K. Chan, Y. Chen, Z. Peng, L. Yu, J. Am. Chem. Soc. 115 (1993) 11735. [232] S. Qu, W. Wu, J. Hua, C. Kong, Y. Long, H. Tian, J. Phys. Chem. C, 114 (2009) 1343. [233] S. Qu, H. Tian, Chem. Commun. 48 (2012) 3039. [234] Y. Qu, S. Qu, L. Yang, J. Hua, D. Qu, Sens. Actuat. B 173 (2012) 225. [235] G. Zhang, L. Wang, X. Cai, L. Zhang, J. Yu, A. Wang, Dyes Pigm. 98 (2013) 232. [236] M. Kaur, M.J. Cho, D.H. Choi, Dyes Pigm. 103 (2014) 154.
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[237] S. Setayesh, A.C. Grimsdale, T. Weil, V. Enkelmann, K. Müllen, F. Meghdadi, E.J.W. List, G. Leising, J. Am. Chem. Soc. 123 (2001) 946. [238] R. Beavington, M.J. Frampton, J.M. Lupton, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 13 (2003) 211. [239] S. Kola, J.H. Kim, R. Ireland, M.L. Yeh, K. Smith, W. Guo, H.E. Katz, ACS Macro Lett. 2 (2013) 664.
[241] X. Guo, M.D. Watson, Macromolecules, 44 (2011) 6711.
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[242] H. Li, F.S. Kim, G. Ren, S.A. Jenekhe, J. Am. Chem. Soc. 135 (2013) 14920.
cr
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[240] J. Qu, J. Zhang, A.C. Grimsdale, K. Müllen, F. Jaiser, X.H. Yang, D. Neher, Macromolecules, 37 (2004) 8297.
[243] D. Buckland, S.V. Bhosale, S.J. Langford, Tetrahedron Lett. 52 (2011) 1990.
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[244] M. Jo, J. Lim, O.Š. Miljanić, Org. Lett. 15 (2013) 3518.
[245] S. Yamaguchi, S. Akiyama, K. Tamao, J. Am. Chem. Soc. 122 (2000) 6793.
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[246] Y. Kim, M. Kim, F.P. Gabbaï, Org. Lett. 12 (2010) 600.
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[247] H. Lenormand, J.P. Goddard, L. Fensterbank, Org. Lett. 15 (2013) 748.
BDE: Bond-dissociation energy
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Glossary
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BODIPY: Boron dipyrromethene difluoride
B3LYP: Becke 3-Parameter (Exchange), Lee, Yang and Parr CTABr: Cetyltrimethylammonium bromide DCM: Dichloromethane
DFT: Density functional theory DMF: Dimethylformamide
DMSO: Dimethylsulfoxide DNA: Deoxyribonucleic acid DPP: Diketopyrrolopyrrole ESPT: Excited-state proton transfer ESIPT: Excited-state intramolecular proton transfer F: fluorescence quantum yield
77 Page 77 of 79
HaCaT: Cultured human keratinocyte HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid HIV: Human immunodeficiency virus HOMO: Highest Occupied Molecular Orbital ICT: Intramolecular charge transfer
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LUMO: Lowest Unoccupied Molecular Orbital NI: 1,8-naphthalimides
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NIR: Near-infrared region NMR: Nuclear magnetic resonance
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OTBS: Oxy-tert-butyldimethylsilyl OLED: Organic light-emitting diode
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PBS: Phosphate buffered saline PEG: Polyethylene glycol
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PET: Photoinduced electron transfer PMMA: Poly(methyl methacrylate)
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PVP: Polyvinylpyrrolidone
SPS: Silylated spiropyran
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TBA: Tetrabutylammonium
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RAW: Mouse leukaemic monocyte macrophage cell line
TBAF: Tetrabutylammonium fluoride TBDMS: Tert-butyldimethylsilyl TBDPS: Tert-butyldiphenylsilyl TBS: Tert-butyldimethylsilyl
TDDFT: Time dependent density functional theory calculations THF: Tetrahydrofuran THS: trihexylsilylacetylene TIPS: Triisopropysilyl TMS: Trimethylsilyl TZVP: Triple zeta valence plus polarization USEPA: United States Environmental Protection Agency
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UV: ultraviolet
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S0010-8545(14)00282-3 http://dx.doi.org/doi:10.1016/j.ccr.2014.10.009 CCR 111947
To appear in:
Coordination Chemistry Reviews
Received date: Revised date: Accepted date:
3-9-2014 22-10-2014 24-10-2014
Please cite this article as: L. Gai, J. Mack, H. Lu, T. Nyokong, Z. Li, N. Kobayashi, Z. Shen, Organosilicon Compounds as Fluorescent Chemosensors for Fluoride Anion Recognition, Coordination Chemistry Reviews (2014), http://dx.doi.org/10.1016/j.ccr.2014.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract Various fluoride chemosensors based on organosilicons, the associated photophysical studies and their molecular
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modeling calculations, are summarized.
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Highlights Organosilicon-based chemosensors for F¯ recognition are summarized. The design strategies organosilicon-based chemsensors are elaborated.
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The photophysical properties and electronic structures are analyzed.
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Molecular modeling calculations are carried out.
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Organosilicon Compounds as Fluorescent Chemosensors for Fluoride Anion Recognition Lizhi Gai,a,b John Mack,c Hua Lu,a,d,* Tebello Nyokong,c Zhifang Li,a Nagao Kobayashid,* and Zhen
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Shenb,*
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ADDRESS: a Key Laboratory of Organosilicon Chemistry and Material Technology, Ministry of Education, Hangzhou Normal University, Hangzhou, 311121, P. R. China
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b State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing, 210093, P. R China
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c Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
d Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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EMAIL: [email protected] (HL); [email protected] (NK); [email protected] (ZS)
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Table of Contents: Introduction
2.
Sensing F based on organosilicon compounds
2.1
F triggered silicon-oxygen bond cleavage
2.1.1
BODIPY-Based Sensors
2.1.2
Coumarin-Based Sensors
2.1.3
BODIPY-Coumarin Based Sensors
2.1.4
Fluorescein-Based Sensors
2.1.5
Naphthalimide-Based Sensors
2.1.6
Cyanine-Based Sensors
2.1.7
Thiazole-Based Sensors
2.1.8
Quinoline-Based Sensors
2.1.9
Azo-Based Sensors
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1.
2.2.10 Pyrene-Based Sensors
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2.1.11 Rhodamine-Based Sensors 2.1.12 Other Fluorochrome-Based Sensors 2.1.13 F triggered Cyclization Reaction
F triggered silicon-carbon bond cleavage
2.2.1
BODIPY-Based Sensors
2.2.2
Pyrene-Based Sensors
2.2.3
Diketopyrrolopyrrole (DPP)-Based Sensors
2.2.4
Naphthalene-Diimide-Based Sensors
2.2.5
Other Systems
2.3
Sensors Based on F-Si Interactions
3.
Conclusions and Perspectives
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Fluoride anion recognition; Organosilicons; SiO bond cleavage; Chemosensors.
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Keywords:
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2.2
Abstract: Recent developments in organosilicon-based chemosensors for F recognition are reviewed. The design
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strategies for improving the photophysical properties of organosilicon-based chemosensors are elaborated, with an
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emphasis placed on their utility for biological applications. The photophysical properties and electronic structures
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are analyzed in depth with reference made to the results of molecular modeling calculation and possible future research directions are assessed.
1. Introduction
In recent years, there has been a strong focus on the development of recognition sensors for environmentally and biologically important anions [1-15], because of the vital role played by these anions in a wide range of chemical and biological processes [16-34]. The fluoride anion (F) is one of the most significant in this regard due to its mild toxicity and important role in bone-growth and its presence in various environmental, clinical and food samples [3539]. It is widely used as an additive in toothpastes and water to prevent dental caries and enamel demineralization resulting from wearing orthodontic appliances [40-44]. Fluorosis is caused by the elevated intake of fluoride over prolonged periods of time. An abnormal intake of a large dose or the chronic ingestion of lower doses of fluoride 4 Page 4 of 79
ions also produce ecological damage and cause dental or skeletal fluorosis, urolithiasis and nephrotoxic changes in humans, and even death [42, 45-48]. Tdemostrtehe presence of fluoride in water at about 50 μM prevents tooth decay but at around 250 μM it causes mottled teeth and bone damage [49]. The United States Environmental Protection Agency (USEPA) mandates a drinking water standard for fluoride of 200 μM to prevent osteofluorosis
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and a secondary fluoride standard of 100 μM to protect against dental fluorosis [36]. Drinking water is the largest single contributor to daily fluoride intake [50]. Fluoride permeates naturally into the
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water supply due to solubilization of minerals that contain fluor, such as fluorapatite, cryolite and fluorite [51]. It is
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estimated that more than 100 million people regularly drink water which contains fluoride, of geogenic origin in concentrations that lie over the limit suggested by the World Health Organization guidelines [36]. Concerns have
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been raised about the manner in which the fluoride anion is introduced into the environment by anthropogenic activities, such as the use of phosphate containing fertilizers and by the aluminium processing industries. Different
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diseases are caused based on differing amounts of fluoride ingestion and the duration of intake. Dental fluorosis is caused by excessive fluoride levels and can cause white spots, yellowing of teeth and pitting or mottling of enamel.
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Skeletal fluorosis is a bone disease exclusively caused by excessive consumption of fluoride [52]. The detection
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and recognition of F has, therefore, become the focus of considerable research interest. Until relatively recently, the methods that tended to be used for fluoride determination, such as fluoride-ion
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spectrophotometry [53-54], ion-selective electrodes [55-56],
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F NMR analysis [57], ion chromatography [58],
reverse-phase HPLC [59] and fluorimetry [60-61], were broadly similar to those used for other inorganic ions. However, all these methods have significant disadvantages, such as complicated procedures, high expense, or their unsuitability for use in the field on environmental samples [62-64]. Recently, there has been a strong focus on the development of simple and effective fluorescent or colorimetric sensors for the fluoride anion. Chemosensors enable the visual detection of the presence of fluoride anions and have been used for F recognition and detection based on noncovalent or covalent interactions with the sensor molecules, including hydrogen bonding between F and the NH protons of amides [65-66], indoles [67], pyrroles [68], urea and thiourea [69-70], Lewis acid coordination [71-73], and anion-π interactions [74-75]. This type of approach provides poor selectivity, however, because it is prone to interference by oxygen-containing basic anions, such as H2PO4, CO32 and AcO, so these 5 Page 5 of 79
chemosensors can only be used in organic solvents to detect tetrabutylammonium (TBA+) fluoride and are not suitable for inorganic fluoride salts such as NaF [76-78]. Molecular recognition based on a specific chemical reaction provides higher selectivity and has attracted more attention from researchers in recent years. Over the last fifteen years, two different types of fluoride selective
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reaction-based detection strategies have been developed. Fluoride is a strong hydrogen-bond acceptor, and has a high affinity for silicon. Facile cleavage of either a C−Si or O−Si bond (bond-dissociation energies: 69 and 103
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Kcal/mol) by Fˉ has been the key to designing the reaction sites for these probes [79]. Various chemosensors have
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been reported based on these strategies. The aim of this review is to highlight and summarize the various approaches that have been used to design fluoride chemosensors based on organosilicons, the associated
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photophysical studies, and their biological applications since the year 2000. A critical assessment of research in the field is provided, which should inspire further research in the design of new organosilicon based chemosensors.
in using chemosensors for biological imaging.
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This will form the basis for an overview of possible future directions in the analysis of environmental samples and
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2. Sensing F based on high affinity between silicon and fluoride
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2.1 F triggered Silicon-Oxygen bond cleavage Silyl groups such as TMS (trimethylsilyl), TIPS (triisopropysilyl), TBS (tert-butyldimethylsilyl), TBDPS (tertbutyldiphenylsilyl) are typically used to protect hydroxyl groups during organic synthesis. These protection groups render the corresponding dye inactive to interfering compounds, but there is easy cleavage upon attacking by F. Upon reaction with F, the protection group is removed to form an unprotected O anion. This results in significant differences in the electronic structure and hence also in the photophysical properties, including the UV-visible absorption spectra and the fluorescence quantum yield (F) values.
As a result, numerous organosilicon
compounds with SiO bonds have been reported to act as chemosensors for the detection and recognition of F. In this review, they are classified according to their organic fluorophore platforms into several different categories including boron-dipyrromethenes (BODIPYs), coumarins, cyanines, fluoresceins, naphthalimides, pyrenes, and other related dyes. 6 Page 6 of 79
2.1.1 BODIPY-Based Sensors Over the last two decades, BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorescent dyes have been the focus of considerable research interest due to their advantageous photophysical properties such as their excellent photochemical stabilities, large molar absorption coefficients, and F values [80-85]. BODIPYs have been widely
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used in a variety of organic functional materials, such as labeling reagents, chemosensors and laser dyes, and have
O
O
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Si
B
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F N
N
N
B
N
F F
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F F 1
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been studied for use in applications such as photodynamic therapy [86-94].
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N
Si O
B
F
N
N
F F
B
N
F F
2 O
Scheme 1. Cleavage of the Si˗O bonds of 1 and 2 by F [95].
In 2010, Akkaya prepared two BODIPY derivatives 1 and 2 that can be used as turn-off sensors for the F ion based on the photoinduced electron transfer (PET) and the intramolecular charge transfer (ICT) quenching mechanism that results when a silyl-protected phenolic group is deprotected both in solution and in a poly(methylmethacrylate) (PMMA) matrix [95]. The HOMO and LUMO of the protected forms of 1 and 2 are associated primarily with the core BODIPY fluorophore (Figure 1). When the silyl groups are removed, the LUMOs are directly comparable to 7 Page 7 of 79
those of the protected forms. This is not the case with the HOMOs of the deprotected species, however, because MOs that are associated primarily with the meso-phenyl and styryl substituents are destabilized relative to those which have angular nodal patterns that are comparable to the HOMOs of 1 and 2. This results in significant ICT within the S1 excited states. Upon addition of TBAF, the emission bands are quenched due to the formation of non-
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emissive phenolate ions. The main absorption band of 1 at 498 nm shifts slightly to the blue and there is a decrease in its intensity. There is a red shift of the absorption band of the styryl-conjugated derivative 2 from 560 to 682 nm,
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however, and the color of the solution changes from purple to green.
Figure. 1. The frontier MOs of the protected and deprotected forms of 1, 2 and 3 calculated on the B3LYP optimized geometries using the CAM-B3LYP functional with 6-31G(d) [96] basis sets at an isosurface of 0.02 a.u. Occupied MOs are highlighted with black diamonds. MOs with angular nodal patterns similar to those of the HOMO and LUMO of the BODIPY chromophore [95] are highlighted with gray lines.
8 Page 8 of 79
F N
B
N
N
F F
B
N
F F
Si O
O
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3
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Scheme 2. Cleavage of the SiO bond of 3 by F [97].
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Zhu developed a novel highly selective colorimetric and ratiometric near-infrared (NIR) region chemosensor, 3, for F based on a 6-hydroxyindole-based BODIPY. Si–O bond cleavage and phenol/phenolate interconversion results
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in a fluorescence quenching mechanism [97]. Upon titration with TBAF in CH2Cl2, the main absorption band of 3 at 546 nm decreases in intensity and there is a concomitant increase in the intensity of a new band at 644 nm. An
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isosbestic point is observed at 567 nm and there is a visible color change from pink to indigo, since the ratio of the absorbance at 644 and 546 nm increases over 3000-fold. This enables the colorimetric and ratiometric detection of
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F even with the naked eye. The main emission band of 3 at 573 nm decreases gradually in intensity and there is a
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simultaneous appearance of a new red-shifted emission band at 676 nm. The color of the fluorescence changes
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from yellow to pink when viewed under an UV lamp. The ratio of the emission intensities at 676 and 573 nm increases from 0.13 to 9.3, a 71-fold ratiometric enhancement, which indicates that there is an efficient ratiometric response for F. The lower detection limit was 0.12 μM with a response time of under 30 s.
N
N
B
F
N N
N
F
F
F
B
N F
4 Si O
O Si
O
O
Scheme 3. Cleavage of the SiO bond of 4 by F [98].
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Lu and coworkers developed a novel, highly specific rapid colorimetric and ‘turn off’ fluorescent fluoride NIR region sensor for F, 4 [98], by using an aza-boron dipyrromethene (aza-BODIPY) moiety as the fluorophore. The introduction of the aza-nitrogen atom shifts the main BODIPY absorption and emission bands significantly to the red [80]. When F is added, a significant decrease is observed in the intensity of the absorption band at 718 nm
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along with a concomitant increase in the intensity of a new band at 780 nm resulting in a visible color change from blue to indigo. In addition, the emission peak of 4 at 750 nm gradually decreases in intensity. An analysis of the
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TD-DFT calculation results (Figure. 2) demonstrated that the S1 excited state of [4-O2]2− has significant ICT character from the electron donating phenoxy moiety to the BODIPY core, and this enhances the rate of
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nonradiative decay and hence results in fluorescence quenching. The detection limit towards F was 2.1μM.
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Confocal fluorescence microscopy experiments established the utility of 4 for monitoring the presence of F in
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HeLa cells.
Figure. 2 The frontier π-MOs of aza-BODIPY 4 and [4-O2]2− at an isosurface value of 0.04 a.u., calculated during TD-DFT calculations using the CAM-B3LYP functional with 6-31G(d) basis sets (Reprinted with permission from ref. 98. Copyright © 2014 Royal Society of Chemistry.).
2.1.2 Coumarin-Based Sensors Coumarin is a natural flavoring substance found in cinnamon and many other plants, which possesses numerous biological activities including anticancer, anticoagulant, anti-inflammatory, anti-HIV, antimicrobial, analgesic, 10 Page 10 of 79
antioxidant, antiviral, and antimalarial properties, due to their low toxicity [99-103]. Coumarin dyes have found widespread use, since they exhibit excellent spectroscopic properties, such as high extinction coefficients, high F values, and large Stokes shifts, and are widely used as dopants for OLEDs, sensors for ions, and fluorescent markers
F Si
O
O
O
acetone/H 2O(7:3,v/v)
HO
O
O
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5
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in the biochemical determination of enzymes [104-109].
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Scheme 4. Cleavage of the SiO bond of 5 by F [110].
Yang developed a novel highly sensitive and selective coumarin-based fluorogenic chemosensor, 5 [110]. When 5
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was treated with fluoride, the main UV-visible absorption band undergoes a marked shift from 318 to 361 nm and there is a red shift of the emission band from 394 to 459 nm. The fluorescence increase is linear over a fluoride
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concentration range of 508000 nM, with a lower detection limit of 19 nM. 5 shows excellent selectivity for the F,
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due to the high affinity of F toward the silicon atom. This approach has been successfully applied to the fluoride
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determination in toothpaste and in tap water samples. CF3
CF3
CF3
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F
Si
O
O
6
H 2O
O
CH3CN O
O
O
HO
O
O
Scheme 5. Cleavage of the SiO bond of 6 by F [111].
Lee designed a similar selective fluorogenic coumarin-based chemosensor, 6, with colorimetric and fluorescent ‘turn-on’ modes in acetonitrile and water [111]. In acetontitrile, there is a moderately strong absorption band at 330 nm in the UV-visible absorption spectrum. The addition of F (as its TBA salt) results in a dramatic color change from colorless to yellow, due to the gradual decrease in the intensity of the absorption band at 330 nm and the simultaneous appearance of an intense absorption band at 434 nm. A chromenolate is formed due to the fluorideinduced cleavage of the TIPS group. Upon exposure to fluoride, a strong fluorescence emission band appears at 11 Page 11 of 79
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500 nm. In contrast, 6 is non-fluorescent due to its non-emissive S1 state. The lower detection limit was ca. 50 nM,
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with a reasonable response time of ca. 10 min in aqueous solution. Test paper was successfully prepared through
Si
O
O
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immersion in a solution of 6.
O
NaF
O
O
O
O
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7
d
O
O O
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Scheme 6. Cleavage of the SiO bond of 7 by F [112].
Park designed a non-cytotoxic fluorescence chemosensor, 7, with a coumarin used as the signaling chromophore and a TBDPS group as the protecting group. The addition of a methyl ester group increases the water solubility and enhances cell permeability [112]. Upon treatment with 1 mM NaF for 3 h in phosphate-buffered solution, there is a more than 4-fold enhancement in the fluorescence intensity at 461 nm. 7 has been used for bioimaging and for the detection of NaF in A549 human epithelial lung cancer cells under physiological conditions. Since the excitation wavelength lies in the UV region and there is a long assay time of ca. 4 h, further improvements are needed before this chemosensor will be suitable for practical applications.
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Si
O
O
O
O
O
O
NaF H N
H N 8
C 12H 25
C 12H 25
O
O
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Scheme 7. Cleavage of the SiO bond of 8 by F [113].
Hong reported a chemosensor, 8, with a fluorescent signaling chromphore and a silyl ether protecting group, which
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can detect F through the formation of fluorescent gels in aqueous media [113]. Although the turbid solution of 8
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shows little fluorescence emission, upon the addition of F, the intensity of the fluorescence emission at 468 nm increases dramatically due to the removal of the silyl ethers and a sol-to-gel transition at pH 7.40 in 1:1
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methanol/HEPES buffer (v/v). The gels formed after complete removal of the tert-butyldimethylsilyl (TBDMS) protecting groups are only weakly fluorescent, due to the formation of densely stacked coumarin moieties, resulting
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in a self-quenching effect. More intense fluorescence is observed for the gels of 8+F when the protecting groups
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Si
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are not completely removed.
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O
O
O F
O
O
9
Si
O
O
O
O
O
O
Si
O
O
O
F O
O
O
O
10
Scheme 8. Cleavage of the SiO bonds of 9 and 10 by F [114]. Ghosh reported the anion binding behavior of the silyl protected 4-hydroxycoumarin 9 and dicoumarol 10 in organic and aqueous organic solutions [114]. 10 acts as a selective sensor for F in CHCl3 and aq. CH3CN solutions, 13 Page 13 of 79
due to a large change in the emission properties. Upon addition of fluoride, a dianionic species is formed which is yellow in color. The responses of both 9 and 10 are less selective for F in CH3CN due to its higher polarity. Only F solutions with concentrations between 103 to 104 M result in a significant change in the emission intensity of both 9 and 10. Both dyes exhibit cell permeability and can be used for detecting F in living cells.
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2.1.3 BODIPY-Coumarin Based Sensors
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Among the various interactions within electronic excited states, ICT is one of the most promising for sensing the fluoride ion with a ratiometric response [115]. Donor-π-acceptor dyads with coumarins and other similar dyes as
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the acceptor and BODIPYs as the donor moieties, have been studied in a wide variety of fields, including their possible use as chemosensors for ions, in optical materials, in photosynthetic model systems, and in biotechnology,
B
N
F
d
N
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since their absorption and emission properties can be readily fine-tuned [116-120].
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F F O
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11
O
N
B
N
F F O O
O Si
O
Scheme 9. Cleavage of the SiO bond of 11 by F [121].
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Figure. 3. The HOMO and LUMO energy levels and angular nodal patters for 11 and its deprotected product. (Reprinted with permission from ref. 121. Copyright © 2011 American Chemical Society.)
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Lin designed a new ratiometric fluorescent fluoride chemosensor based on a coumarin-BODIPY dyad, 11, by making use of an ICT quenching mechanism [121]. The addition of F to a DMSO solution causes an 88 nm red
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shift in the absorption maximum from 592 to 680 nm, in conjunction with a change in the color of the solution from pink to red. In the presence of other anions, only F triggers a blue shift of the emission band of 11 from 606 to 472
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nm, resulting in a color change from orange to blue. Density functional theory (DFT) and time-dependent DFT
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(TDDFT) calculations, predict that both the highest occupied molecular orbital (HOMO) and lowest unoccupied
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molecular orbital (LUMO) have large MO coefficients on the coumarin, ethenyl bridge and BODIPY moieties (Figure 3). Upon cleavage of the SiO bond, however, the HOMO is located primarily on the coumarin moiety and the ethenyl bridge, while the LUMO is located mainly on the BODIPY unit and the ethenyl bridge, so ICT is induced from the coumarin to the BODIPY moiety resulting in significant changes in the photophysical properties. The lower detection limit for F of 11 was 0.12 µM. 2.1.4 Fluorescein-Based Sensors
Fluorescein is one of the most commonly-used fluorescent chemosensors due to the high molar absorptivity of its main absorption band and its high F value [5, 122-124]. Fluoresceins are used as fluorescent labels due to their rich and diverse properties, and prototropism properties [125-127]. Fluorescein derivatives in their lactone form are
15 Page 15 of 79
always non-fluorescent, but in their ring-opened form there are significant color changes and an enhancement of the fluorescence intensity [128-129].
TBSO
O
O
OTBS
O
O
OTBS
O
OH
F
F O
COO
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COO O moderately fluorescent
highly fluorescent
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non-fluorescent 12
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Scheme 10. Cleavage of the SiO bond of 12 by F [130].
Yang designed a fluorescein-based fluorogenic chemosensor to detect the fluoride ion in aqueous media. The
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fluorescein di-tert-butyldimethylsilyl ether, 12, is a colorless, non-fluorescent compound [130]. Upon incubation with the fluoride ion in DMF-water solution (7:3, v/v, pH 7.0), the SiO bond is cleaved, and the fluorescein
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absorption band appears at 496 nm along with a large increase in fluorescence emission intensity at 532 nm. The increase in fluorescence intensity is linear for fluoride concentrations between 0.1–2.0 μM and there is a lower
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detection limit of 0.041 μM. This method has been successfully applied to fluoride determination in multi-trace
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element injection and toothpaste samples, and the results agree well with those obtained from other selective
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fluoride-ion sensors. It is of particular interest to identify color changes of 12 from colorless to yellow green that would allow the “naked-eye” detection of F.
OH HO
OH
O
N
OH
N
O
OTBDPS
N
F
O
OH
OH O
HO
N OH
N
O
O
N COO
O
13
Scheme 11. Cleavage of the SiO bond of 13 by F [131]. Du developed the first example of a sugar-functionalized fluorescent chemosensor, 13, for the recognition and detection of F by using a desilylation-triggered chromogenic reaction in pure water [131]. The introduction of a 16 Page 16 of 79
sugar group to 13 can improve both the water solubility and bio-compatibility, which increases the colorimetric and fluorescent “turn-on” selectivity and sensitivity toward fluoride ions in fully aqueous and living cells. When fluoride (TBA+ and Na+ salts) is added into the solution, the emission fluorescence intensity of 13 at 520 nm increases more than 160-fold as the concentration of F ions is increased from 0 to 1.4 mM, and a quinone-type
ip t
intermediate is generated with a strong fluorescence signal and a yellow color visible to the naked eye. 13 exhibits an excellent selectivity for the F ions and a lower detection limit of 10.5 µM in pure water. The color change is
cr
visible to the naked eye at concentrations as low as 75 µM. 13 was successfully used for imaging F in HepG2 cells.
O
nO
NH
us
S O
N H
O
O
nO
S
NH N H
O
O
O
Si
14
M
Si
an
NaF
COO
O
O
O
d
Scheme 12. Cleavage of the SiO bond of 14 by F [132].
te
Wu designed a highly sensitive, responsive, and visible region fluorescent chemosensor, 14, for F detection in
Ac ce p
totally aqueous solutions and for fluoride imaging in living cells [132]. To ensure solubility in water, cell membrane permeability and low-cytotoxicity, a hydrophilic polymer, polyethylene glycol (PEG), was conjugated into the system. PEGs are biocompatible, and have non-toxic, non-antigenic and non-immunogenic properties in a wide range of biomedical applications [need a ref here?]. Upon incubation with F in pH 7.4 HEPES-buffered water, the emission intensity at 526 nm increases gradually and reaches its maximum value within ten minutes. The lower detection limit of 14 was 1 µM. Sensor 14 can be used to detect fluoride levels in real samples such as running water, human urine, and serum, and can also be used for imaging F in both HeLa and L929 cells.
17 Page 17 of 79
COOMe
MeO
O
COOMe
COOMe Oxidation
NaF
O Si
MeO
O
O
MeO
O
ip t
15
O
cr
Scheme 13. Cleavage of the SiO bond of 15 by F [133].
us
Li reported a new chemical reaction-based “turn-on” fluoride chemosensor for NaF, 15, which contains a TBDPS group as a trigger and a fluorescein moiety as the optical signaling chromophore [133]. Upon the addition of NaF,
an
the emission intensity observed at 520 nm in the green region of the spectrum increases about 2.7-fold at F concentrations as low as 50 µM, due to SiO bond cleavage and the readily air-oxidized highly fluorescent anionic
M
species that is formed. The fluorescence emission of 15 is significantly enhanced in intensity and the F value increases to about 0.37. 15 provides a specific response for F with high sensitivity. A lower detection limit of 18
d
µM in HEPES-THF (9/1, v/v, pH 7.2) has been reported. Furthermore, the use of 15 has been successfully applied
Ac ce p
Si O
te
to the detection of F in HeLa cells. A “turn-on” fluorescence response was observed.
(Me) 3C
O
O
O
O
OOC
F
+
O
O
16
O MeO
O
O
OMe
Scheme 14. Cleavage of the SiO bond of 16 by F [134]. Talukdar developed a cascade reaction-based colorimetric and fluorescent “turn-on” chemosensor, 16, for selective and relatively rapid (t½ = 2.41 min) F detection [134]. In DMSO, none of the characteristic UV-visible fluorescein absorption bands are observed between 300-700 nm and there is no fluorescence emission intensity. Upon addition 18 Page 18 of 79
of F, a fluorescence emission band was observed cat 523 nm due to a cascade reaction in the presence of the F ion released by the lactone phthalide. A carboxyfluorescein derivative is formed which is a strongly fluorescent species and the color of the solution changes from colorless to yellow under ambient light. During treatment with 300 eq. of TBAF at room temperature, a 550-fold enhancement of the fluorescence emission intensity of 16 is observed at
ip t
523 nm within 7 minutes. 16 has a lower detection limit of F of 1.03 μM. Sensor 16 can be used to detect F in
N
cr
live HeLa cells by confocal microscopy.
O N
H HN N N
O
us
N
S O
O
O
Si
17
N
S O
HO
M
O
F
an
HOOC
O Si
H HN N N
O
O
O
te
d
Scheme 15. Cleavage of the SiO bond of 17 by F [135]. Thennarasu reported a novel fluorescein-rhodamine dyad, 17, for the detection of F and Hg2+ [135]. Upon addition
Ac ce p
of F, new bands are observed at 500 nm and 523 nm, respectively, in the absorption and fluorescene emission spectra. The calculated detection limits of the probe are 5.17 x 10-8 M (~1.0 ppb). 17 can be used for the detection and imaging of W138 normal lung fibroblast cells contaminated with F and Hg2+ ions. 2.1.5 Naphthalimide-Based Sensors
1,8-Naphthalimides (NIs) have been used extensively to design fluorescent fluoride chemosensors using various quenching mechanisms such as ICT, PET, and Förster resonance energy transfer [136-140]. NIs have also attracted considerable attention due to their tendency to form n-type semiconductor materials and their easy functionalization through the imide nitrogen or via core substitution [141-142].
19 Page 19 of 79
O
N
O
O
N
O O
N
O
F O
O
O
NH 2
O
O
Si
Si
F
us
O
NH
ip t
NH
cr
O
+
18
an
Scheme 16. Mechanism for the reaction of 18 with F [143].
M
Cho developed a highly selective NI-based ratiometric and colorimetric fluorescent chemosensor, 18, which displays both one- and two-photon ratiometric changes [143]. Upon reaction with F in MeCN as well as in
d
aqueous buffer solutions, the absorption band of 18 at 365 nm red shifts to 487 nm, since the NH bond is
te
deprotonated by the F. On addition of 5 eq. of F the absorbance at 487 nm decreases in intensity as a new band
Ac ce p
appears at 421 nm, and this results in a color change from colorless to jade-green, due to the cleavage of the SiO bond and the release of a green fluorescent 4-aminonaphalimide. The addition of higher concentrations of F results in a new emission band at 508 nm and a 220-fold ratiometric intensity enhancement. One- and two-photon ratiometric changes can be obtained via two-photon microscopy, which can offer intrinsic three-dimensional (3D) resolution combined with reduced phototoxicity, increased specimen penetration, and negligible background fluorescence.
20 Page 20 of 79
O
N
O
O
N
O
F
+
CH 3CN OTBDPS
O
cr
19
ip t
O
O
Ac ce p
te
d
M
an
us
Scheme 17. Cleavage of the SiO bond of 19 by F [144].
Figure. 4. The HOMO and LUMO energy levels and angular nodal patterns of 19 and its deprotected product. (Adapted with permission from ref. 144. Copyright © 2013 Elsevier.)
Zhang reported a dual-channel F chemosensor comprised of a 4-hydroxy-1,8-naphthalimide derivative which generates a “turn-off” response in the fluorescence intensity due to an ICT mechanism in both aqueous and organic solutions [144]. Upon addition of F, the intensity of the main absorption band at 365 nm is significantly reduced and there is the concomitant appearance of a new band at 475 nm.
This is consistent with the smaller
HOMOLUMO gap predicted for the deprotected product (Figure 3). In the presence of other anions, only F exhibits a large enhancement of the intensity at 560 nm with the solution changing color from colorless to yellow. The lower detection limits were 18 μM and 0.1 μM in MeCN/H2O (v/v, 50:50) and MeCN solution, respectively. TDDFT calculations (Figure 4) predict that the LUMO is predominantly associated with the imide ring and the 21 Page 21 of 79
oxygen atom of the carbonyl group in the deprotected structure, while the HOMO is associated primarily with the amino group resulting in strong ICT from the naphtholate to the imide moiety. Sensor 19 can be used for imaging
O
N
O
O
N
O
CH 3CN O
Si
us
O
cr
F
ip t
fluoride in A549 cells with high selectivity and sensitivity.
20
NA
an
TBS-NA
Scheme 18. Cleavage of the SiO bond of 20 by F [145].
M
Ren developed a colorimetric fluoride chemosensor, 20, based on a fluorescent naphthalimide [145]. In MeCN solution, the addition of TBAF triggers a decrease in the intensity of the main absorption band at 362 nm along with
d
an increase in the intensity of a new band at 474 nm resulting in an obvious and relatively rapid color change from
Ac ce p
te
colorless to yellow in ca. 3 min. The lower detection limit for F was 0.59 µM.
Figure 5 The HOMO and LUMO energy levels and the angular nodal patterns for 20
and NA at an isosurface value of 0.02 a.u. (Adapted
with permission from ref. 146. Copyright © 2011 Elsevier.)
Chu assessed the sensor mechanism of 20 by using DFT and TDDFT calculations to analyze the frontier molecular orbitals (MOs), and the predicted electronic transition energies and oscillator strengths [146]. Structures were 22 Page 22 of 79
optimized for the groundstates and first-excited-states of 20 and the desilylation product (NA) (Figure 5). Although the groundstates of 20 and NA have similar structures, there is a large red-shift in the main absorption band of NA due to the strong ICT character of the S0–S1 transition. The calculated fluorescence emission spectrum exhibited a large red-shift of ca. 100 nm and this was also ascribed to the relatively strong ICT character of the S1 state. The
N
O
O F CH 3CN
O
O
an
OTBDPS
21
N
us
O
cr
ip t
large red-shift of both the absorption and emission bands is what makes 20 suitable for use as a F chemosensor.
M
Scheme 19. Cleavage of the SiO bond of 21 by F [147]. Hu designed a highly selective colorimetric and ratiometric fluorescent chemosensor, 21, by using a 4-silylhydroxy-
d
1,8-naphthalimide to detect F [147]. When F was added gradually, the maximum absorption peak at 361 nm
te
decreased in intensity and a new absorption band was observed at 471 nm, resulting in a color change from
Ac ce p
colorless to yellow due to the cleavage of the Si–O bond to form the fluorescent 4-hydroxy-N-butyl-1,8naphthalimide. 21 can therefore serve as a naked-eye chemodosimeter for F. The emission peak undergoes a red shift of 123 nm from 430 to 553 nm and a 510-fold enhancement (F553/F430) in intensity. O N
O Si
N
O
O F
O O
N O Si
O
O
22
Scheme 20. Cleavage of the SiO bonds of 20 and 22 by F [148]. Cao reported a fluoride chemosensor, 22, comprised of an N-aryl-1,8-naphthalimide bearing a trimethylsilyl ether that is cleaved in the presence of F to give a ratiometric fluorescence response for an excimer-monomer conversion 23 Page 23 of 79
[148]. Upon addition of F in DCM, the main emission band shifted to the blue from 524 to 378 nm, because the excimer is disrupted by the presence of F, since desilylation results in the formation of a phenoxide ion. The excimer emission at 524 nm is completely quenched within 2 min resulting in a 2.7-fold enhancement in the
F
OMe
O O
N
OMe
an
23
O
cr
N
us
O
OTMS O
ip t
monomer emission intensity at 378 nm. The lower detection limit was 7 µM.
Ac ce p
te
d
M
Scheme 21. Cleavage of the SiO bond of 23 by F [149].
Figure. 6 The postulated mechanism for PET between the naphthalene and aryl moieties, which results in the fluorescence quenching observed in the presence of the fluoride ion. (Reprinted with permission from ref. 149. Copyright © 2014 Royal Society of Chemistry.)
Cao also developed a chemosensor, 23, based on the 1,8-naphthalimide fluorophore by using a trimethylsilyl ether as the recognition unit for the detection of F in its inorganic forms (e.g. KF and NaF). The response time of ca. 5 min in MeCN/H2O media is relatively rapid [149]. The main absorption band lies at 366 nm in MeCN/H2O (v/v = 7:3), while the emission band lies at 442 nm. Upon addition of increasing concentrations of F, the emission band is quenched drastically due to what was described as a strong PET effect related to the formation of the phenolate ion that is formed by the fluoride-triggered desilylation reaction. The overall rate constants for the pseudo-first-order 24 Page 24 of 79
reactions of 10 M solutions of TBAF and KF and a 30 mM solution of NaF were calculated to be 7.8 × 103, 7.1 × 103, and 4.2 × 103 s1, respectively. The authors used calculation results (Figure 6) to postulate that the electron of the HOMO of the aryl moiety can transfer to that of the naphthalene moiety after the formation of a phenolate due to the desilylation of 23 and that this results in significant fluorescence quenching. The lower detection limit was
O
O
an
F
Si
N O
Bu N
cr
O
O
us
Bu N
ip t
2.1 µM.
O
HO
M
24
Scheme 22. Cleavage of the SiO bond of 24 by F [150].
d
Liu reported a completely selective and highly sensitive chemodosimeter for F, 24, by exploiting a dual reaction-
te
based site for F-triggered Si–O bond cleavage and the deprotonation/autoxidation of the secondary nitrile group
Ac ce p
[150]. Upon addition of 0.0–2.0 equiv of F, two new bands were observed at 370 and 625 nm in the UV–visible absorption spectrum, due to formation of [C–H…F] complexes, resulting in a blue solution. On further addition of F, the intensity of the band at 625 nm decreased and there was a slight red-shifted to 660 nm. This was ascribed to the formation of a phenolate anion with a strong negatively charged donor oxygen atom, accompanied by a strong increase in the intensity of the band centered at 367 nm. Upon the addition of F, a significant decrease in the fluorescence intensity of 24 was observed at 500 nm, which was ascribed to enhanced PET. The lower detection limit during the fluorescence studies was 0.105 μM. O
O N
N O Si
O
N
O
N
F O
O
O
25
25 Page 25 of 79
Scheme 23. Cleavage of the SiO bond of 25 by F [151]. Cao developed another chemosensor for F, 25, using fluoride-triggered desilylation to provide a quantitative measurement of the F concentration [151]. Upon addition of F, new absorbance peaks were observed at 423 and 552 nm resulting in a color change from colorless to red in MeCN. In addition, the intensity of the emission
cr
ip t
observed at 486 nm decreased. The detection limit to F was 2.6 μM.
2.1.6 Cyanine dye -Based Sensors
us
The structure of cyanine dyes are comprised of two aromatic or heterocyclic rings, which are linked by a
an
polymethine chain [152-153]. Cyanines are highly fluorescent and have found use as fluorescent chemosensors,
d
M
photographic sensitizers and biomolecular labels [154-157].
N
O
Ac ce p
26
N
THF/H 2O 7:3 (v:v) TBDMS
te
O
F
Scheme 24. Cleavage of the SiO bond of 26 by F [158].
Xu reported a novel highly sensitive colorimetric method based on selective attack by the fluoride ion on the SiO bond of cyanine dye 26 [158]. On addition of F, in THF/water (70:30, v/v) 26 undergoes a dramatic color change from yellow to green accompanied with a blue shift of the main absorption band from 600 to 440 nm, due to the increased ICT character of the S1 state. The lower detection limit was 0.1 μM in THF/water (70:30, v/v) with a long reaction time of 1.5 h. The analytical approach has been successfully applied to the determination of F concentrations in rainwater samples.
26 Page 26 of 79
I N
I N
I N F
excess F
CH 3CN
CH 3CN
O Si O
ip t
OH 28
cr
27
us
Scheme 25. Cleavage of the SiO bond of 27 by F [159].
Zhang designed a dual-channel “off-on-off” chemosensor, 27, comprised of a phenylpyridylvinylene fluorophore
an
and a Si–O bond receptor, by making use of the ICT quenching mechanism [159]. Upon addition of F, the main absorption band undergoes a drastic red-shift from 362 to 545 nm, accompanied by a striking color change from
M
colorless to purple. The Si–O bond is cleaved and a highly fluorescent molecule, 28, is released, which has enhanced fluorescent intensity at 476 nm. 28 is subsequently converted into a phenolate ion and the emission
te
d
intensity gradually decreases. The lower detection limit was 0.1 μM.
Si
Ac ce p
O
OH
O PBS pH 7.4
NaF
S
N
S
N
S
N I
I
I
29
Scheme 26. Cleavage of the SiO bond of 29 by F [160]. Ma developed a new chemosensor, 29, with an ICT quenching mechanism, which provides colorimetric and ratiometric detection of F in buffered aqueous solutions and living matrices [160]. A conjugated benzothiazolium 27 Page 27 of 79
hemicyanine dye provides the latent fluorophore. Upon addition of F, a red shift of the absorption band from 407 to 517 nm is observed in an ethanol/water mixture (3:7, v/v, phosphate buffered saline (PBS), pH 7.4) and this provides a color change from light yellow to orange, making 29 suitable for use as a naked-eye indicator. Similarly, the reaction of 29 with F triggers the cleavage of the SiO bond resulting in a ratiometric red shift in the emission
ip t
maximum from 500 to 558 nm. The lower detection limit was 0.08 mM. Sensor 29 can be used as a fluorescent
cr
ratiometric biosensor for F in live RAW 264.7 macrophage cells.
OH
O
us
Si O
F
F
H 3C
H 3C
30
I
H 3C
N
I
M
I
N
H 3C
d
N
H 3C
PBS pH 7.4
an
Ethanol/H 2O 3/7 PBS pH 7.4 H 3C
te
Scheme 27. Cleavage of the SiO bond of 30 by F [161].
Ac ce p
Feng have reported a fluorine chemosensor, 30, consisting of a 1,3,3-trimethyl-3H-indolium hemicyanine dye as the chromophore and a TBDPS group as the reaction site. Promising colorimetric and fluorescent turn-on properties were observed in aqueous solution with reaction times of only a few minutes [161]. Upon addition of F, there is a large bathochromic shift of the main absorption band from 420 to 535 nm, resulting in a color change from light yellow to red, which is easily discernable by the naked-eye. The fluorescence emission intensity at 558 nm increases by about 25-fold and there is a lower detection limit of 6.6 µM. The optimal pH range for the desilylation process lies between 7–9. Test papers can be readily prepared and were used to demonstrate the potential applicability of 30 for F determination in aqueous samples. 2.1.7 Thiazoles-Based Sensors
28 Page 28 of 79
Thiazoles are an important class of fluorescent heterocyclic compounds, which provide many potent biologically active molecules for medical applications [162]. They contain an electron-deficient core and hence have a strong electron-accepting ability. Thiazoles have found applications as drugs for the treatment of HIV and bacterial
HO N
NaF
N
S S
O
cr
Si O
O
HN
us
HN
31
ip t
infections, inflammation, as inhibitors of bacterial DNA gyrase B, and as fluorescent sensors for ions [163-165].
31-BTTPB
an
BTTPB
Scheme 28. Cleavage of the SiO bond of 31 by F [166].
M
Yang have described a thiazole fluoride chemosensor, 31, with a rapid reaction time in aqueous cetyltrimethylammounium bromide (CTABr) micellar media, which has two independent modes of signal
d
transduction based on fluoride-induced changes in the fluorescence color and intensity [166]. In the absence of F,
te
31 has a blue-violet emission band at 418 nm. Upon addition of F, a new band appears at 560 nm due to the
Ac ce p
formation of a benzothiazole derivative and there is a gradual increase in the intensity of the bright yellow emission. The lower detection limit was 5.3 µM. Test papers were prepared by immersing filter paper into a THF solution of 31 and were successfully used to detect the presence of F in aqueous solutions.
29 Page 29 of 79
Figure. 7 The angular nodal patterns calculated for the HOMOs and LUMOs of 31 and 31-BTHPB at the DFT/B3LYP/TZVP level of theory and an isosurface value of 0.02 a.u. (Reprinted with permission from ref. 167 Copyright © 2014 Royal Society of Chemistry.)
Chu investigated the F sensing mechanism of 31, with DFT and TDDFT calculations to examine the electronic structures of the ground and excited states of the molecules that are involved in the sensor process [167]. Figure 7
ip t
contains the frontier MOs of the optimized structures of 31 and its desilylated product (31-BTHPB). In each case, it is obvious that that there are larger MO coefficients on the thiazole moiety in the LUMO than in the HOMO. The
cr
excited states of both BTTPB and 31-BTHPB, therefore, have significant ICT character. In the 31-BTHPB structure, the proton of the hydroxyl hroup can transfer to the nitrogen atom of the thiazole moiety, and this excited-
us
state intramolecular proton transfer (ESIPT) process results a significant red shift of the main fluorescence emission
an
band and hence also a very large Stokes shift value.
Si O
S
O
N S
O HN
d
HN
te
32
M
F
N
HO
Ac ce p
Scheme 29. Cleavage of the SiO bond of 32 by F [168]. Yang used hydrogel as the support for a F-specific reaction between a water insoluble sensor molecule, 32, and F in aqueous environments [168]. 32/polyvinylpyrrolidone (PVP) composites were prepared by adding a methanol solution of 32 (2.0 μL, 2.0 mM) into a xerogel film (1.0 mg) followed by drying to remove the solvent. When NaF solution was added, the maximum absorption band shifted to the red from 345 to 370 nm, and the main fluorescence emission band maximum shifted from 406 to 494 nm with a color change from blue−violet to green. The 32/PVP system is sufficiently sensitive to provide naked-eye detection for F in pure water media with a detection time of only 15 s at concentrations relevant to the USEPA’s drinking water standard.
30 Page 30 of 79
S
N F
S
O Si
N OH
ip t
33
Scheme 30. Cleavage of the SiO bond of 33 by F [169].
cr
Goswami reported a ratiometric and colorimetric chemosensor, 33, based on an ESIPT process [169]. Upon
us
treatment with F, the main absorption band decreases in intensity and a weak band at 330 nm is formed along with a large increase in the intensity of a new absorption band at 440 nm, resulting in a color change from colorless to
an
green in CH3CN-H2O (8/2, v/v). When F is added, the fluorescence emission is red-shifted from 407 to 477 nm and intense green fluorescence is observed. Test strips were formed by immersion in 33 solution and provided
M
convenient and efficient detection of F. The lower detection limit was 6.5 μM. Sensor 33 can be used as a fluoride
N
O
Si
te
N
d
sensor in bioimaging applications.
S
F
N
N
O
S
Ac ce p
34
Scheme 31. Cleavage of the SiO bond of 34 by F [170].
Beckert reported another novel O-silyloxy-1,3-thiazole sensor with a fast-response ratiometric fluorescent response, 34, based on a desilylation-triggered chromogenic reaction in polar aprotic solvents and in aqueous CTABr micellar media [170]. After the addition of TBAF to a DMSO solution, the O-silyloxy-1,3-thiazole is converted into an enolate and there is a red shift of the absorption band from 344 to 481 nm. The fluoride-triggered deprotection of these silyl ethers leads to a bathochromic-shift of the main fluorescence emission band from 418 to 602 nm in DMSO from the blue end of the visible region to the red. The lower detection limit was 0.1 µM.
31 Page 31 of 79
OH O
HO HO S
TBSO
OH
NH
N
OH
F
O
HO HO S
O
NH
OH
PBS, pH 7.4
O
N
O
ip t
35
Scheme 32. Cleavage of the SiO bond of 35 by F [171].
cr
Wang successfully designed a stable sugar-functionalized fluorescent fluoride chemosensor, 35, based on a desilylation reaction in aqueous media [171]. The sugar moiety was incorporated to improve the solubility and has
us
no measurable effect on the fluoride sensing. In the presence of other ions, such as Cl, Br, I, F, H2PO4, N3,
an
NO2, NO3, AcO, and SO42, only F triggers significant fluorescence intensity changes. After the addition of 0.1 M NaF, the fluorescence emission intensity at 508 nm increases 30 fold in 10 min in PBS (10 mM, DMSO 0.5%,
M
pH = 7.4), due to the high reactivity of TBS and F. Sensor 35 is non-cytotoxic and can be used to monitor the
d
presence of F in KB human carcinoma cell lines by fluorescence imaging.
S
O
N
NH
te
O
Ac ce p
Si
O
S
N
37
F
S
O
N
NH
N
36
Si
O
N Cl
Cl
O
S
O NH
N
Cl
F
O S
S
N
N
NH
N
N Cl
Scheme 33. Cleavage of the SiO bonds of 36 and 37 by F [172]. Tang reported the rapid detection of fluoride ion (NaF) in aqueous solution and living cells by sensitive fluorescence thiazole chemosensors with quaternary ammonium moieties, 36 and 37, [172]. The quaternary 32 Page 32 of 79
ammonium not only improves the solubility of the probes, but also enhances their sensitivity and selectivity. Upon addition of F, the TBDPS group of 36 is desilylated and the fluorescence intensity at 520 nm increases more than 121- and 143-fold, respectively, for NaF and TBAF solutions, with detection limits as low as 30 μM.. In a similar manner, only F
induces a significant increase in fluorescent intensity of 37 at 550 nm along with a decrease in
ip t
intensity at 450 nm. 37 can be used to detect fluoride ion in cells by both green channel and ratiometric imaging.
cr
2.1.8 Quinoline-Based Sensors
us
Quinoline derivatives are pharmacologically active synthetic compounds that are widely used in applications such as their use in ion detection and as antitumor DNA intercalating agents. [173-177].
an
Br
Br
N
Br O
N
Br OH
d
38
Si
M
F
te
Scheme 34. Cleavage of the SiO bond of 38 by F [178].
Ac ce p
Bai designed a ratiometric and colorimetric fluorescent fluoride chemosensor, 38, based on the combination of a desilylation reaction and an excited-state proton transfer (ESPT) from the product [178]. After the addition of TBAF in THF, the absorption peak at 313 nm gradually decreases in intensity and two new peaks are observed at 363 and 410 nm. The color of the solution changes from colorless to yellowish-green in a manner that can be readily observed by the naked-eye within 10 s. Blue fluorescence is observed in THF, which on addition of F changes color to strong yellowish-green, since the band maximum shifts from 403 to 520 nm due to the ESPT process and the formation of an OH···F hydrogen bond. The lower detection limit was below 1.0 µM. When increasing amounts of water are added to the solvent mixtures, the intensity of the emission peak decreases significantly.
33 Page 33 of 79
TBSO
CHO
O
CHO
F
N
N 39
ip t
Scheme 35. Cleavage of the SiO bond of 39 by F [179]. Bai designed another new strategy for the highly selective and sensitive detection of both F and Zn2+ based on an
cr
F-induced desilylation reaction and a Zn2+ chelation-enhanced fluorescence effect [179]. Upon addition of F, the
us
bright blue fluorescence that is associated with an emission band at 458 nm is quenched due to efficient radiationless relaxation to the ground state via intra- and intermolecular ESPT. In THF, however, 39 exhibits a
an
different set of photophysical changes. The emission intensity at 418 nm gradually decreases, and a new peak is observed at 520 nm with the color of the solution and fluorescence changing from blue-violet to yellowish-green.
M
This can be attributed to the combination of a desilylation reaction and an excited state proton transfer from the product to the fluoride ion. The lower detection limit was 1 µM.
d
2.1.9 Azo-Based Sensors
te
Azo dyes contain a N=Nbond. When an N=N bond is attached to two aryl groups there are photochromic
Ac ce p
properties, since the relative orientations of the chromophores have a significant effect on the energies of the main π-π* absorption bands [180-181]. The light- or heat-induced isomerization reactions result in the formation of trans and cis isomers. Many fluorescent sensors have been reported based on the orientation of anchored azo dye systems [182-183].
Si
O O F N N
N
N
N N 40
34 Page 34 of 79
Si
O O F N
N
N
ip t
N
N
MeO MeO
Si Si
OMe
OMe
us
MeO
MeO
cr
N
41
an
Scheme 36. Cleavage of the SiO bonds of 40 and 41 by F [184].
M
Martínez-Máñez reported a fluoride chemosensor, 40, in which a pyridine azo dye is functionalized with a protecting TBDMS moiety. A selective phenolate fluoride response is observed in acetonitrile/water mixtures [184].
d
Upon the addition of F‾ to 9:1 (v/v) solutions, the main absorption band is red-shifted from 350 to 470 nm and there
te
is a visible color change from colorless to orange-red due to the cleavage of the SiO bond and the formation of a phenolate anion. The lower detection limits are 0.04, 0.09, 0.14 ppm, respectively, for 15, 40, 60 min reaction times
Ac ce p
in 90:10, 75:25, 50:50 (v/v) acetonitrile/water mixtures.
41 can be anchored onto a silica support with a
trialkoxysilane moiety. Treatment with TLC silica foils functionalized with 41 by immersion at concentrations of 104, 103, 102 M, results in red, green, and blue coloration, respectively in a manner that can also be used to detect F‾ in solution.
35 Page 35 of 79
O O F N
N
N
N
N
N
ip t
Si
OH
NO2
NO2
us
cr
NO2 42
Scheme 37. Mechanism of the reaction of 42 with F. [185].
an
Li reported a novel ratiometric and colorimetric fluoride chemosensor, 42, by taking advantage of both the azobenzene structure and the selective reaction site provided by the SiO bond [185]. Upon addition of F‾, a
M
significant color change from colorless to deep blue is observed, with a dramatic red-shift of the absorption wavelength maximum from 365 to 595 nm. The lower detection limit was 15 µM. Test strips were successfully
d
used to detect F. There is an obvious color change from light yellow to blue based on the fluoride-initiated
te
cleavage of the SiO bond and the resulting formation of an azobenzene compound. It is noteworthy that 42 can be
Ac ce p
used as colorimetric indicator for convenient analyses even in the absence of a spectrometer. 2.2.10 Pyrene-Based Sensors
Pyrene molecules are widely-used as fluorophores, since they possess excellent properties, such as large fluorescence quantum yields, long lifetimes, and ratiometric signals for the monomer and excimer emission [186187]. Pyrenes have been successfully used in the selective detection of ions, DNA and small molecules [188-190].
O
Si
Si
O
O F
43
Scheme 38. Cleavage of the SiO bond of 43 by F [184]. 36 Page 36 of 79
ip t cr
Figure 8. The use ratiometric pyrene fluorescence probes as chemodosimeters. (Reprinted with permission from ref. 191. Copyright
us
© 2012 Royal Society of Chemistry.)
Shen designed the first example of the use of a pyrene as a ratiometric fluorescent chemosensor, 43. An –O–Si–Si–
an
O– or –O–Si–O– bridging moiety is used to link two pyrene rings. Fluoride-induced Si–O bond cleavage results in a ratiometric excimer/monomer conversion and significant changes in the emission spectrum [191] (Figure 8).
M
Upon incubation with F in THF/H2O (v/v, 50/50), the excimer emission band at 470 nm is significantly quenched, and there is a large increase in the intensity of monomer emission bands at 378 and 396 nm, which can be attributed
d
to the ion-specific cleavage of Si–O bonds by F, leading to the breakage of the stacked conformation of the two
te
pyrene moieties. The lower detection limit was in the micromolar range. Confocal fluorescence microscopy
Ac ce p
experiments have established the utility of water soluble nanoparticles with encapsulated molecules of 43 as a ratiometric biosensor in living cells. 2.1.11 Rhodamine-Based Sensors
Rhodamine has been widely used in many research areas, including use as fluorescent markers in structural microscopic studies, and as laser dyes and photosensitizers, due to their excellent photophysical properties, such as their high photostability, excellent quantum yields, and high extinction coefficients [192-197].
37 Page 37 of 79
(-O)H
R O
O
Si O
O F
N N
O
N H
N H
O
44 , R=H, O Si
cr
45, R=
N H
ip t
N H
N N
us
Scheme 39. Cleavage of the SiO bonds of 44 and 45 by F [198].
an
Bhattacharya designed two dual ‘turn-on’ chemosensors 44 and 45 for the detection of Hg2+ and F‾ ions using the rhodamine fluorophore. At physiological pH in aqueous media, the sensor mechanism is based on the ring-opening The addition of F‾ results in the
M
reaction of the spirolactam and the cleavage of the O-silyl bond [198].
instantaneous appearance of a yellow color, since the absorption band shifts from 300 to 385 nm for 44, and from
d
300 and 350 nm to 395 nm for 45. Bright yellow emission is observed with a maximum at 490 nm, which increases
te
in intensity by 10-fold. The lower detection limit was estimated to be 0.03 µM and 2.3 µM for 44 in MeCN and DMF/water (9/1, v:v), respectively, and 47 nM and 7 µM for 45 in MeCN and DMF/water (8/2, v:v). These sensors
Ac ce p
could be used to analyze the fluoride content in toothpaste.
O
O Si O
F
NH SO2
Et2N
O 46
F Si F
NH SO2
F
SO3
NEt2
SO3
Et2N
O
NEt2
Scheme 40. Cleavage of the SiO bond of 46 by F [199].
38 Page 38 of 79
NH
O O Si O
O
N N
48
ip t
47
N H
cr
O O Si O
NEt2
us
Scheme 41. Structures of 47 and 48 [199].
an
Martínez-Máñez designed a series of sensors for use in aqueous samples, 46-48, based on a fluoride-specific reaction between hydrofluoric acid and MCM-41 mesoporous nanoparticles functionalized with fluorescent or
M
colorimetric signaling units [199]. Upon treatment with F‾, 46-48 is freed from the surface and the colorless solution of 46 in MeCN–H2O (7:3, v:v) (buffered to pH = 2.5 with 0.1 M potassium hydrogenphthalate and HCl)
d
turns pink. Sensor 46 can also be used for the quantitative determination of the fluoride content in commercial
Ac ce p
te
toothpaste.
2.1.12 Other Fluorochrome-Based Sensors
39 Page 39 of 79
MeO
O O MeO
O
O
O
O
O
O
O O
O
CH 3
O
ip t
F
CH 3
O
O
O
O
O
O MeO
O O
MeO
O O
us
49
cr
TBDMSO
O O
Scheme 42. Mechanism of the reaction of 49 with F. [200]
an
Akkaya reported a new chemosensor, 49, which incorporates a self-immolative linker to trigger two simultaneous chemiluminescence processes [200]. When F was added to a DMSO/PBS buffer solution (90/10, v:v, pH 7.2), an
M
emission band was observed at 466 nm with bright blue chemiluminescence due to fluoride-mediated deprotection of the silyl-protecting group and an electronically triggered dioxetane cleavage, followed by self-immolation via a
te
d
1,4-quinone-methide rearrangement. PMMA polymer strips impregnated with 49 can be used to detect F in aqueous solutions at low micromolar concentrations.
Ac ce p
NO2
N
O
N
HN
O
O
NO2
OTBDMS F pH= 7.4
N O N NH2
50
Scheme 43. Cleavage of the SiO bond of 50 by F [201].
Talukdar successfully designed a 7-nitro-2,1,3-benzoxadiazole-based off–on chemosensor, 50, through the use of molecular modelling [201]. In the presence of other ions, only F triggered SiO cleavage, which results in a dramatic color change from colorless to yellow in 9:1 ethanol-HEPES buffer (v:v, 10 mM, pH = 7.4) solution, due to a decrease in the intensity of the main absorption band at 399 nm and the emergence of a new band at 460 nm. 40 Page 40 of 79
Upon addition of 300 equiv of F, a significant “turn-on” response is observed for strongly green fluorescence at 535 nm with a 110-fold enhancement in the intensity and a relatively slow response time of 80 min. Sensor 50 can be used to detect intracellular F ions in live cells. O
O
O
F
O
O
+
Si
N
cr
O
O
ip t
N
O
us
51
an
Scheme 44. Cleavage of the SiO bond of 51 by F [202]. Hong developed a novel chromogenic and fluorescent chemosensor, 51, based on the formation of a highly Upon treatment with fluoride in
M
fluorescent resorufin upon the addition of F at the silicon center [202].
acetonitrile–water (1:1, v/v) and acetonitrile solution, the main absorption band undergoes a large red shift from 445
d
to 586 nm resulting in a change in the color of the solution from yellow to pink. The main fluorescence emission
te
band of the product increases in intensity at 589 nm. In MeCN/H2O (1:1), after the addition of 3000 equiv of F, there is a 200-fold enhancement of the fluorescence intensity. Neither enhanced fluorescence intensity nor a color
Ac ce p
change, was observed in the presence of other ions. NC
CN
O
O
NC
CN
F DMSO/H2O 95:5(v:v)
O
Si
O
52
Scheme 45. Cleavage of the SiO bond of 52 by F [203]. Zhu devised a novel NIR region chemosensor, 52, with a selective “turn-on” fluorescence response for F by combining a tert-butyldiphenylsiloxy group with a dicyanomethylene-4H-chromene moiety [203]. In the presence 41 Page 41 of 79
of other ions, only the F‾ ion triggers the cleavage of the SiO bond resulting in a dramatic color change from pale yellow to blue. In DMSO/H2O (95:5, v/v) solution, there is a decrease in the intensity of the absorption band at 447 nm within only 30s, and a new band is observed at 645 nm. Upon addition of 400 equiv of F‾, a “turn-on” fluorescence emission response is observed at 718 nm with a 1000-fold enhancement in intensity. The lower
Si
F
an
H 2O/CTABr
cr
O
us
O
ip t
detection limit for F was 85 nM.
N
N
M
53
te
d
Scheme 46. Cleavage of the SiO bond of 53 by F [204].
Ac ce p
Martínez‐Máñez designed a novel pyridine-based fluoride chemosensor, 53, with a silyl ether group that enables the
selective recognition of F in pure water at pH 7.4 containing the cationic surfactant cetyltrimethylammonium bromide (CTABr) solutions [204]. The addition of F triggers a change in the color of the solution from colorless to yellow due to a decrease in the intensity of the absorption band at 325 nm, which has been ascribed to the fluorideinduced hydrolysis of the silyl ether moiety and the formation of a phenolate ion. A new absorption band is observed at 445 nm, with an emission band at 540 nm. The lower detection limit was 76 µM.
42 Page 42 of 79
NO2
NO2
F O
OTBDPS
H N CONH(CH2)5 CH3
NHCONH(CH2)5 CH3
ip t
54
Scheme 47. Mechanism of the reaction of 54 with F. [205]
cr
Cho reported a urea-based chemosensor, 54, for fluoride detection in mixed aqueous media. Upon addition of F in
us
HEPES buffered MeCN/water solution (9/1, v/v, 10 mM HEPES buffer, pH = 7.0), produces a readily observed colorimetric response from colorless to yellow due to the formation of a new absorption band at 420 nm. The F‾-
an
induced Si–O bond cleavage leads to the deprotection of a phenolic OH group and the formation of hydrogen bonds with the urea NH protons or carbonyl group, due to its free rotation along the acetylene axis [205]. In DFT
M
calculations, the distance between the oxygen atoms is predicted to be 2.73 Å which is consistent with the presence of a strong hydrogen bond. The lower limit of detection was less than 1 µM when the colorimetric approach is
te
d
taken into consideration. OTBS
Ac ce p
TBSO
N
O 2N
OH
O
F
N
55
HO
O
N NO2
O 2N
OH
N HO
NO2
Scheme 48. Cleavage of the SiO bond of 55 by F [206].
Bhalla designed a novel terphenyl fluorescent chemosensor, 55, for the selective recognition of Cu2+ and F‾ ions in two contrasting modes by making use of an excited state intramolecular proton transfer (ESIPT) process [206]. Upon addition of F‾, there is a decrease in the absorption intensity at 265 and 342 nm and a new red shifted band is formed at 447 nm resulting in a color change from colorless to yellow. The addition of F‾ to a THF solution leads 43 Page 43 of 79
to a significant decrease in the intensity of the emission band at 517 nm due to the inhibition of the ESIPT interaction. Upon further addition of F‾, a new blue-shifted band was observed at 478 nm due to the cleavage of the SiO bond. This results in increased negative charge on the oxygen atoms, which interacts strongly with the -
HO
TBSO N
F
N H
56
cr
N H
N
ip t
conjugation system. The lower detection limit was 10 nM when 55 was used as a fluorescence sensor.
us
Scheme 49. Cleavage of the SiO bond of 56 by F [207].
an
Yang designed a ratiometric fluorescent chemosensor, 56, based on the modulation of the ESIPT process associated with the protection/deprotection reaction of the hydroxyl group [207]. Upon addition of F, the absorption band at
reaction time.
M
293 nm decreases in intensity, while new bands at 315 and 324 nm gradually intensify over the course of a 40-min In aqueous DMF solution, the emission band at 360 nm decreases in intensity with a new
d
fluorescence peak appearing at 454 nm. This was ascribed to the restoration of the ESIPT process. The lower
Ac ce p
toothpaste and tap water samples.
te
detection limit was 0.19 µM. This analytical method has been successfully applied to fluoride determination in
Si
HO
O
N
N
F
N
N
57
Scheme 50. Cleavage of the SiO bond of 57 by F [208]. Lu reported a new highly selective and sensitive probe with a rapid response to the fluoride anion, 57 [208]. At low fluoride concentrations, the main absorption band of 57 was red shifted from 280 to 320 nm. With the further addition of F, the emission band was red-shifted from 384 to 475 nm. The detection limit was 0.43 µM. 44 Page 44 of 79
O
O F
Si O
O
ip t
58
Scheme 51. Cleavage of the SiO bond of 58 by F [209].
cr
Song used the naphthol fluorophore to design a colorimetric and “turn-on” chemosensor, 58, for fluoride detection
us
in acetonitrile [209]. Upon addition of F‾, the main absorption band of 58 gradually decreases in intensity at 304 nm and a new band is formed at 406 nm, resulting in a color change from colorless to yellow. Upon treatment with
an
10 eq. of F‾, the fluorescence emission at 488 nm is enhanced 9663-fold within 30s due to an ICT process. The
O
O
H
N
H
O
te
N
N
O H
Ac ce p
O Si
O H O
d
Si
M
lower detection limit was 40 nM.
O
H
N
N
F
H
Si
O
O
H
N
O H
H O
59
Scheme 52. Mechanism of the reaction of 59 with F. [210]
Lee reported that 59 can be used as a “turn-on” chemosensor for the detection of F based on a conformational switch of a covalently modifiable fluorophore [210]. Upon addition of F‾ in CH2Cl2 solution, there is a 3-fold increase in the emission band intensity at 458 nm, due to F‾-triggered SiO bond cleavage and the formation of a rigid molecular structure under mild conditions (spring-loaded conformational transitions).
45 Page 45 of 79
NC
NC NC
CN
CN
NaF
O
Si O
NC
O
O 60
ip t
Scheme 53. Cleavage of the SiO bond of 60 by F [211].
cr
Zhu developed a colormetric and red-emitting fluorescent dual-channel chemosensor, 60, for imaging F‾ in living cells, by using the Si–O bond as a highly selective recognition receptor [211]. When F‾ is added to a solution of 60,
us
there is a red shift in the maximum of the main absorption band of 158 nm from 438 to 596 nm and the color of the solution changes from yellow to blue. In the fluorescence spectrum, the continuous addition of F‾ results in a
an
gradual increase in emission intensity at 612 nm, due to a strong ICT effect. Sensor 60 can be used to determine F‾ concentrations in the 0–6 mM range with a lower detection limit of 0.07 mM and can also be used for the bio-
M
imaging of F‾ in living HeLa cells.
d
O Si O
O
Ac ce p
61
N
te
N O
F
Scheme 54. Cleavage of the SiO bond of 61 by F [212].
Yang reported a new chemosensor, 61, for the rapid and sensitive colorimetric detection of F‾ in aqueous solution, coupling the specific molecular recognition ability of silylated spiropyran (SPS) dyes with the unique features of graphene oxide to develop a nanocomposite [212]. During the reaction with F‾ in the presence of graphene oxide, a very strong absorption band maximum centered at 490 nm appears within 30 min and there is a rapid color change from colorless to orange-yellow due to the cleavage of the SiO bond that is triggered by the conversion of the closed spiropyran to an open mercyanine. 61 acts as a sensor for the F‾ ion between 710 pH. The SPS/graphene oxide nanocomposite has been successfully used to monitor F‾ levels in serum. 2.1.13 F triggered cyclization reaction 46 Page 46 of 79
COOEt
COOEt F
COOEt O Si(t-Bu)Me 2
S S
O S S
62
COOEt
ip t
COOEt
COOEt OTIPS
O
cr
O
F
S
S
OC 12H 25
C 12H25 O
OC 12H 25
us
C 12H25O
O
n
n
an
63
M
Scheme 55. Cleavage of the SiO bond and cyclization of 62 and 63 by F [213]. Swager reported a new strategy for the detection of F‾ based on the formation of highly fluorescent coumarins, 62
d
and 63, through fluoride-triggered Si−O bond cleavage and cyclization reactions. The response is successfully
te
amplified using exciton migration in a semiconducting organic polymer [213]. Upon the gradual addition of F‾, the absorbance maximum of 62 exhibits a red shift from 384 to 412 nm with a 46-fold enhancement in fluorescence
Ac ce p
emission intensity at 450 nm due to the formation of a coumarin. Upon addition of F‾, the emission band maximum of the electric coupling polymer 63 was red shifted from 482 to 517 nm with a 100-fold intensity enhancement since the structure of the polymer enables dexter energy transfer. COOEt
COOEt
COOEt OTBS OTBS
F
O O
O
64
Scheme 56. Cleavage of the SiO bond and cyclization of 64 by F [214]
47 Page 47 of 79
Wang reported a new sensor, 64, for sensing F‾ that makes use of a rational reaction-based relay recognition strategy [214]. In the presence of other ions, only F‾ generates a significant quenching in the intensity of the fluorescence band at 360 nm and an increase in the intensity of the characteristic monomer peak of a coumarin derivative at 460 nm. The color of the fluorescence changes gradually from blue to yellowish green. The lower
O O
O 113
x O O
y O O
O 113
z O O
O
O
O
O O
O
an
8-9 O
O
O
65 O
M
O
y O O
z O O
us
F
x O O
cr
O O
ip t
detection limit was 1.86 μM.
O
O O
8-9 O O O
O
OTBS
O
COOEt
COOEt
d te
O
COOEt
Ac ce p
Scheme 57. Cleavage of the SiO bond and cyclization of 65 by F [215]
Liu reported a highly selective and sensitive fluorescence “turn-on” chemosensor, 65, based on the preparation of a novel type of responsive double hydrophilic block copolymer in purely aqueous media by exploiting the F‾-induced cyclization reaction of non-fluorescent moieties to induce the formation of fluorescent coumarins [215]. Upon treatment with 100 equiv of TBAF, the emission intensity at 402 nm exhibits a monotonic 88-fold increase in intensity, leading to a fluorometric transition from a non-emissive solution to an intense blue color, due to the generation of coumarin moieties. The lower detection limits were 3.4 and 2.6 µM for diblock monomers and micelle nanoparticles, respectively.
48 Page 48 of 79
S
S N
F
N
CN
Et2 N 66
O TBS
Et2 N
O
NH
ip t
Scheme 58. Cleavage of the SiO bond and cyclization of 66 by F [216] Peng reported that a novel chemosensor, 66, has unprecedented sensing properties for fluoride and copper (II) ions
cr
[216]. Upon addition of F‾, the fluorescence intensity at 523 nm significantly increases by 164-fold in ethanol-
us
water(1:1; v/v), and there is a change in the fluorescence color from almost colorless to bright jade green, since the fluoride ion triggers Si−O bond cleavage and an intramolecular cyclization to form a highly fluorescent
an
iminocoumarin−benzothiazole. In pure ethanol solution there is a dramatic 833-fold enhancement in the emission intensity at 510 nm. The color of the solution changes from yellow to yellowish green. The lower detection limit
M
was calculated to be 19.6 nM.
S NC
te
d
N
O Si
F
N N
O
NH
Ac ce p
N
S
67
Scheme 59. Cleavage of the SiO bond and cyclization of 67 by F [217]
Venkatesan reported a coumarin-based fluorescent F chemosensor, 67,[217]. Upon addition of F, the main absorption band at 465 nm shifts to 483 nm with an obvious color change from yellow to green. A new emission band centered at 521 nm is also observed. 67 can be used as an efficient fluorescent sensor for F both in vitro and in vivo. The lower limit of detection was 1.45 μM.
49 Page 49 of 79
N
O Si N
F N
HEPES pH 7.4 68
NC
O
NH
N
CN
CN
ip t
Scheme 60. Cleavage of the SiO bond and cyclization of 68 by F [218] Songdesigned a novel red emitting fluorescent chemosensor, 68, for the detection of F‾ in aqueous solution (35 mM,
cr
pH 7.4, HEPES buffer with 30% CH3CN) that is based on a F‾ triggered Si–O bond cleavage and a subsequent
us
rigidizing, cyclization reaction [218]. A blue shift from 492 to 473 nm is observed in the main absorption band within 15 min. 68 is essentially non-fluorescent due to fast non-radiative decay of the singlet excited state mediated
an
by internal bond rotations. An intense fluorescence emission band is observed for the product at 616 nm in the red region of the spectrum, with a large Stokes shift of 143 nm. The lower detection limit was calculated to be 5.4 µM.
CN
d
NC
M
68 can penetrate cell membranes and has been successfully used in the imaging of F‾ in living HaCaT cells.
H
O Me Si t-Bu
H 2O
Me 2N
O
NH
Me
Ac ce p
69
te
Me 2N
CN
F
Scheme 61. Cleavage of the SiO bond and cyclization of 69 by F [219]
Ahn reported a novel molecular chemosensor, 69, which exhibits excellent analyte selectivity and sensitivity based on fluoride-mediated desilylation to form an iminocoumarin [219]. A red shift of the absorption band center from 460 to 585 nm is observed upon addition of fluoride ions and there is a significant two-photon absorption crosssection value (GM =180). 69 initially exhibits almost no fluorescence in HEPES buffer solution (10 mM, pH 7.4) containing acetonitrile (20% by volume). Upon the addition of F‾, however, there is a dramatic increase in the fluorescence intensity at 595 nm within 10 min. The lower limit of detection was below 210 µM. Sensor 69 can be used to monitor F levels in living cells and in zebrafish by confocal and two-photon fluorescence microscopy.
50 Page 50 of 79
Si O
O
F
NH
CN CN
CN 70
ip t
Scheme 62. Cleavage of the SiO bond and cyclization of 70 by F [220] Peng reported a highly selective and sensitive organosilicon fluoride ion probe, 70, with intense green fluorescence,
cr
which provided the first example of fluoride ion bioimaging in mitochondria [220]. In the absence of F‾ in CH3CN solution, the major absorption band of 70 lies at 463 nm and no fluorescence is observed. Intriguingly, upon adding
us
F‾ to the solution within 10 min the absorption band is blue shifted to 441 nm and strong fluorescence intensity is
an
observed at 485 nm with detection limits as low as 0.16 μM.. Fluoride paper test strips can easily be prepared with lower detection limits as low as 19 ppb in aqueous solutions.
TBSO
M
I
O
OTBS
O
te
71
I
d
F
Ac ce p
Scheme 63. Cleavage of the SiO bond and cyclization of 71 by F [221]
Kim reported a fluoride-induced aromatic cyclization sensor method which produces ladder-type conjugated molecules [221]. An analysis of the absorption and emission spectroscopy confirmed that their rigid planar structures result in an extension of the -conjugation system. 71 is non-fluorescent. Upon addition of F‾, strong fluorescence is observed at 336 nm due to fluoride-induced cyclization. When only two equiv of fluoride are added in THF, there is complete rapid conversion to the ladder-type product in only 2 min.
51 Page 51 of 79
TBSO
OTBS
HO
OH
F
TBSO
HO
OTBS
72
OH
HO
OH
ip t
TBSO
OTBS
Scheme 64. Cleavage of the SiO bond and cyclization of 72 by F [222]
cr
Bhalla developed a novel terphenyl based chemosensor, 72, based on the fluoride ion induced cyclization of On
us
symmetrically/unsymmetrically-substituted triphenylenes in the absence of any oxidizing reagent [222].
addition of F‾, the main absorption band at 295 nm undergoes a red-shift to 342 nm, due to cleavage of the SiO
an
bond. An extra band is observed at 663 nm resulting in a dramatic color change from colorless to violet. On the further addition of F‾, the absorption band at 663 nm disappears and new bands are observed at 526 and 879 nm
M
which can be ascribed to cyclization of 72 to form a triphenylene. Upon addition of F‾, the fluorescence emission band shifts from 393 to 425 nm with a 32% enhancement in intensity, due to the increased negative charge on the
d
phenolate oxygens and the subsequent cyclization. The lower detection limit for F‾ was 2 μM.
O
Ac ce p
TBSO
OTBS
te
OTBS TBSO
O
OTBS
73
O
OTBS
OTBS OTBS
O
O O
O
F
O
O O
O O
Scheme 65. Cleavage of the SiO bond and cyclization of 73 by F [223]
Bhalla have also reported that 73 provides a novel fluoride sensor, since the oxy-tert-butyldimethylsilyl (OTBS) groups undergo irreversible cyclization to substituted higher quinones in the presence of TBAF in dry THF [223]. Upon the addition of 2 equiv of F‾, the absorption band of 73 shifts from 357 nm to 400 and 299 nm. When a further 3 equiv of F‾ are added, the absorption bands at 400 and 299 nm reach their maximum intensities and the band at 400 nm is further red-shifted to 411 nm, while the band at 299 nm shifts slightly to the blue to 296 nm. This is accompanied with a gradual change in the color of the solution from yellow to brown as fluoride-induced 52 Page 52 of 79
desilylation and the extension of the -conjugation system leads to an enhanced negative charge on the phenolate oxygens and hence a red shift of the spectral bands. Upon addition of 10 equiv of F, the fluorescence band at 551 nm was efficiently quenched, due to charge transfer from the phenolate oxygens to the quinone moiety. OTBS
OH
TBSO OTBS
OTBS
OH HO
OTBS
us
F
OTBS
TBSO
OH
an
OTBS TBSO
OH
HO OH
M
OTBS 74
OH
OH
HO OTBS
OH
cr
TBSO
ip t
HO
Scheme 66. Cleavage of the SiO bond and cyclization of 74 by F [224]
d
Bhalla developed a highly selective triphenylene-based chemosensor, 74, with twelve OTBS groups, which
te
undergoes cyclization to a supertriphenylene [224]. In the absence of F‾, the main absorption band of 74 lies at 306
Ac ce p
nm. Upon treatment with F, this band undergoes a red shift to 325 nm and the color of the solution changes from colorless to green. When 12 µM of F‾ was added to a THF solution of 74, the intense fluorescence emission band at 395 nm was completely quenched. 74 has a lower detection limit of 0.8 μM for F‾. These TLC strips coated with 74 can be used for the instant detection of the fluoride ion in aqueous media.
2.2 F triggered Silicon-Carbon bond cleavage The fluoride ion can also trigger silicon-carbon bond cleavage and this has been used to form fluoride chemosensors. There is a more rapid bond cleavage than is the case with the OSi bond, possibly due to the low bond-dissociation energy of the CSi bond.
In contrast with the silicon-oxygen systems, there is usually no ICT quenching
mechanism available to create a “turn-off” sensor. Colorimetric changes are observed instead based on the changes in the HOMOLUMO gaps of the sensors. 53 Page 53 of 79
2.2.1 BODIPY-Based Sensors TMS
CH3 CH3 F
B
N N
F F TMS
75
B
N
N
F F
B
N
F F 76
TMS
N
B
N
F F
cr
N
ip t
F
Ac ce p
te
d
M
an
us
Scheme 67. Cleavage of the SiC bond of 75 and 76 by F [225].
Figure. 9. The frontier MOs of the protected and deprotected forms of 75 and 76 calculated for the B3LYP optimized geometries by using the CAM-B3LYP functional with 6-31G(d) basis sets at an isosurface of 0.02 a.u. Occupied MOs are highlighted with black diamonds. MOs with angular nodal patterns similar to those of the HOMO and LUMO of the BODIPY chromophore are highlighted with gray lines.
54 Page 54 of 79
Ravikanth developed a novel BODIPY derivative, 75, for use as a fluoride chemosensor by introducing trimethylsilylethynyl groups at the 3,5-positions [225]. On addition of TBAF in CH2Cl2, the main absorption band shifts from 571 to 551 nm and the fluorescence emission band shifts from 584 to 564 nm with a color change from bright orange to green due to a F‾-induced transformation of the electron-rich trimethylsilylethyne group to an
ip t
electron-deficient ethyne group. MO calculations demonstrate that this is due to the change in the HOMOLUMO gap (Figure 8). In contrast, there is no obvious change in the absorption and emission spectra of 76, since the
cr
phenyl group at the 4-position is not conjugated with the main BODIPY -system. Mes
N
Si(C 6H13) 3
N
B
F Acetone
N
77
Si(C6 H13) 3
N
B
F F
an
F F
us
(C 6H13) 3 Si
Mes
M
Scheme 68. Cleavage of the SiC bond of 77 by F [226]. Jiang reported an innovative reaction-based 2,6-trihexylsilylacetylene-conjugated BODIPY dye, 77 [226]. In the
d
presence of other anions in acetone, only the interaction with F‾ results in a decrease in the intensity of the
te
absorption band at 555 nm and the appearance of a new band at 538 nm. At the same time, the fluorescence
Ac ce p
emission band of 77 exhibits a blue shift from 571 to 554 nm and the color is observed to change from orange to green in less than 5 min. The lower detection limit for 77 was 67.4 nM. Mes
THS
N
N
Mes THS
B F F 78
F
N
B
N
F F O2 N O O S O
NO2
O H
Scheme 69. Cleavage of the SiC bond of 78 by F [227].
55 Page 55 of 79
Jiang
designed
a
second
2,6-trihexylsilylacetylene
BODIPY
chemosensor,
78,
containing
2,4-
dinitrobenzenesulfonyl and trihexylsilylacetylene (THS) as a “two-in-one” chemosensor for F‾ and HS‾ [227]. Upon addition of F‾, the fluorescence band shifts from 613 to 553 nm with a marked increase in intensity in ca. 18 min resulting in a color change from purple to yellow. The lower detection limit was 14.5 nM, which is nearly 5
ip t
times lower than that of 77 (67.4 nM). 2.2.2 Pyrene-Based Sensors
TMS
an
F
cr
TMS
us
TMS
TMS
79
Ac ce p
te
d
M
Scheme 70. Cleavage of the SiC bond of 79 by F [228].
Figure. 10 The HOMO and LUMO energy levels and angular nodal patterns of 79 (left) and the product formed after the addition of F‾ (right) at an isosurface value of 0.02. (Reprinted with permission from ref. 228. Copyright © 2011 Royal Society of Chemistry.)
Shen developed a new pyrene-based fluorescent chemosensor, 79 [228]. Upon addition of F‾, the main absorption bands undergo a blue shift from 436 and 410 nm to 417 and 392 nm, resulting in a significant colour change from light green to colorless, which can be attributed to the elimination of the TMS substituents due to the strong interaction between the fluoride ion and the silicon atoms. This results in an increase in the HOMOLUMO gap 56 Page 56 of 79
due to the absence of δ–π interactions between the silicon atoms and the pyrene -system (Figure 10). Upon addition of F‾, the fluorescence emission is blue shifted within a few seconds and the emission changes color from blue to purple.
The detection limit was approximately 53 µM, which approaches standard drinking water
concentrations. Test papers prepared by immersion in THF solutions can be used to detect the presence of F in
ip t
aqueous samples. 2.2.3 Diketopyrrolopyrrole (DPP)-Based Sensors C12H 25
cr
C12H 25
C10H21 S
O TMS
S
O
N
S
O
N
an
TMS
C10 H21
F
us
N
C 12H25
O S
N
C 12H25
C 10H 21
M
80
C 10H 21
Ac ce p
te
d
Scheme 71. Cleavage of the SiC bond of 80 by F [236].
Figure 11. The HOMO and LUMO energy levels and angular nodal patterns of 80 (left) and the desilyl product formed after the addition of F (right) at an isosurface value of 0.02. (Adapted with permission from ref. 236. Copyright © 2014 Elsevier.)
Diketopyrrolopyrrole (DPP) dyes are a synthetically versatile class of visible light-harvesting chromophores with high fluorescence quantum yields, which have been exploited in both small molecule and polymeric formats in areas as diverse as photorefractive materials, fluorescence sensors, thin film transistors, and light-emitting 57 Page 57 of 79
diodes[229]. DPP-based sensitizers have also emerged as promising candidates for use as low cost photovolatics (PVs) based on either organic bulk heterojunctions or dye-sensitized solar cells [230-235]. Choi reported that DPP derivative 80 can act as a highly sensitive, selective and rapid recognition sensor for F‾ in THF/HEPES (8:2, v/v; pH 7.4) by using a chemodosimeter approach [236]. Upon addition of F‾, an absorption
ip t
band at 319 nm increases in intensity and there is a significant decrease in the intensity of bands at 385, 408, 549 and 589 nm. There is also a blue shift of the fluorescence emission maximum from 617 to 604 nm and a slight
cr
change in emission intensity. A red shift is observed for the major bands in both the absorption and emission spectra of 80, relative to those of the desilyl compound, because of a destabilization of the HOMO (Figure 11). The
us
reaction occurs in less than 5 minutes at 25 ˚C at low reactant concentrations. The lower detection limit of 80 was 0.2 µM. 80 can also be used to detect F‾ in the solid state such as in glass films and test strips, opening the door to
an
practical applications such as solid optical sensors.
M
2.2.4 Naphthalene-Diimide-Based Sensors
Naphthalene-diimide derivatives are planar molecules with high electron mobilities, which have been used in
d
artificial photosynthetic reaction centers, in conducting materials, and as chemosensors for ions and small molecules,
te
because of their electron acceptor properties [237-242]. (CH2)7 CH3 N O
Ac ce p
O
TMS
O
N O (CH2)7 CH3
O
TMS
(CH2)7 CH3 N O
F
O
N O (CH2)7 CH3
81
Scheme 72. Cleavage of the SiC bond of 81 by F [243]. Bhosale have reported a naphthalene diimide based colorimetric and fluorescent chemosensor, 81 [243]. Upon addition of TBAF, absorption bands at 289, 382 and 439 nm are blue shifted to 275, 376 and 419 nm and there is a significant visible change in the color of the solution from yellow to dark brown. The emission intensity at 329 and
58 Page 58 of 79
455 nm is enhanced due to the deprotection of both the trimethylsilyl moieties of 81. The analytical process is rapid and can be carried out within tens of seconds or a few minutes. 2.2.5 Other Systems
N
ip t
N
N
N
O
O
F TIPS
O
N
N
us
O
cr
TIPS
N
an
N 82
M
Scheme 73. Cleavage of the SiC bond of 82 by F [244]. Miljanić reported the F‾ detection properties of a silylethynyl-substituted benzobisoxazole cruciform compound, 82,
d
that are caused by the desilylation of silylethynyl and the formation of an ethynyl moiety [244]. Stabilization of the
te
HOMO upon desilylation results in a wider HOMOLUMO gap. When 82 is treated with F, the intensity of the
Ac ce p
emission band at 381 nm increases by more than 50% and the color of the solution changes from cyan to purple. The lower detection limit for F‾ was 50 μM.
2.3 Sensors Based on F-Si Interactions
The addition of a fluoride ion to tetrahedral organosilicon compounds results in a structural change to a trigonal bipyramidal geometry due to pentacoordination. This causes changes in the interligand through-space interactions of the substituent -systems leading to changes in the photophysical properties, which can be used to form a sensor system. F KF Si Ant Ant Ant [2.2.2]cryptand 83
F Ant Ant Si Ant F
K /cryptand
84
59 Page 59 of 79
Scheme 74. Possible structure of the 83−F complex [245]. Yamaguchi reported the use of 83 as a fluorescent chemosensor for F, based on the perturbation in the throughspace interaction between the anthracene moieties of 84 [245]. 83 has red-shifted absorption and emission band maxima relative to anthracene, for this reason. The silicate 84•K+/cryptand was isolated by using KF/[2.2.2]
ip t
cryptand as a source of F. Upon addition of TBAF, there is a blue shift in the absorption band of 83 from 401 nm to 392 nm and the emission band shifts from 416 nm to 396 nm. These photophysical properties can mainly be
cr
ascribed to the electronic perturbation associated with the hypercoordination. A decrease in the degree of the
F
Ant
Si
us
through-space interaction between the anthryl groups would be anticipated based on the crystal structures.
Ant
Ant
Si
Ant F SMe2
an
SMe2
F
F
OTf
M
85
d
Scheme 75. Possible structure of the 85−F complex [246].
te
Gabbaï developed a novel proximal third row onium ion based chemosensor, 85, by making use of the cooperative
effects between the fluoride atom and fluorosilanes [246]. Upon treatment with TBAF, the absorption bands were
Ac ce p
blue shifted due to decreased intramolecular anthryl-anthryl π-stacking interactions that are related to the change in the coordination geometry at the silicon center when 85-F is formed. The trigonal bipyramidal geometry around the Si atom that is formed by the Si–F–S bridge was demonstrated by X-ray crystallography. F3C CF 3 O
Si
O CF3 CF3 86
F F
CF3 CF3 O Si O F3C CF3 87
Scheme 76. Possible structure of the 86−F complex [247].
60 Page 60 of 79
F
F
Si F
Si
89
88
90
ip t
NO2
Na
F
NO2
NO2 92
us
91
cr
Si
an
Scheme 77. Mechanisms of the reactions of 88 and 91 with F [247]. Fensterbank developed three silicon-based sensors, 86, 88, and 91 to detect the presence of fluoride ions in solution
M
[247]. Upon capture of F‾, 86 forms a three-center four-electron hypervalent bond at the apical position to form 87 and this results in a red shift of the emission band from 290 to 311 nm in DCM with a detection limit of 10 μM. 86
d
exhibits high selectivity for F‾ in the presence of other naturally abundant anions. Although 90 is formed from 88
te
in a similar manner, in protic solvents or in the presence of water, the emission signal at 348 nm disappears upon
Ac ce p
addition of 1 equiv of TBAF and is replaced within 17 min by a band at 301 nm which was assigned to the formation of biphenyl, 89. When 91 is treated with TBAF in the presence of traces of water, the emission is blue shifted from 336 to 305 nm with detection limits as low as 5 μM due to the formation of 92.
61 Page 61 of 79
Table 1. Summaries of physiochemical properties of F‾ chemosensors. F¯ type
1 2
TBAF TBAF
498→488 560→682
λemA →λemP (nm) 506→ 575→
3
TBAF
546→644
573→676
4
TBAF
718→780
750→
↓ ↓ 71-fold ↑ ↓
5
TBAF
318→361
394→459
↑
19 nM
6
TBAF
330→434
→500
↑
50 nM 16 μM
7
NaF
－
→461
4-fold ↑
－
8
NaF
－
→468
↑
－
9 10
TBAF TBAF
－ －
→387 →374
－ －
11
TBAF
592→680
606→472
↑ ↑ 17.4fold ↑
12
NaF
－
→532
↑
13
TBAF
－
→520
160-fold ↑
10.5 μM
14
F¯
－
→526
↑
1 μM
15
NaF
－
→520
16
TBAF
－
→523
17
TBAF
→500
→523
18
TBAF
365→421
449→508
220-fold ↑
19
TBAF
365→475
→560
↑
20
TBAF
362→474
－
21
F¯
361→471
430→553
22
TBAF
－
524→410
－ 510-fold ↑ 2.7-fold ↑
23
TBAF/ NaF/KF
366→
442→
24
TBAF
→370, 625
25
TBAF
26
Limit
Media
Probe type
Ref.
－ －
CH3CN CH3CN
turn off turn off
95 95
0.12 μM
DCM
ratiometric
97
2.1 μM
CH3CN
turn off
98
turn on
110
turn on
111
turn on
112
turn on
113
turn on turn on
114 114
us
cr
C3H6O-H2O (7:3, v:v, pH 9.1 ) CH3CN H2O PBS pH 7.4 CH3OH-HEPES buffer (1:1, v:v, pH 7.4 ) CHCl3 CHCl3
0.12 μM
DMSO
ratiometric
121
0.041 μM
DMF-buffer (7:3, v:v, pH 7.0 )
turn on
130
H2O(PBS, pH 7.4 )
turn on
131
turn on
132
turn on
133
M
an
φ ratio
ip t
Dye
λabsA →λabsP (nm)
2.7-fold ↑ 550-fold ↑ ↑
18 μM
HEPES buffer pH 7.4 HEPES-THF (9:1, v:v, pH 7.2 ) DMSO
turn on
134
0.05 μM
CH3CN
turn on
135
－
MeCN
ratiometric
143
18 μM 0.1 μM 0.59 μM
MeCN-H2O (1:1, v:v) MeCN MeCN DMSO-buffer (9:1, v:v, pH 7.5 )
turn on
144
colorimetric
145
ratiometric
147
7 μM
DCM
turn on
148
↓
2.1 μM
MeCN-H2O (7:3, v:v)
turn off
149
500→
↓
0.105 μM
MeCN
turn-off
150
→423, 552
486→
↓
2.6 μM
MeCN
colorimetric
151
NaF
600→440
－
－
0.1 μM
colorimetric
158
27
TBAF
362→545
→476→
↑↓
0.1 μM
off-on-off
159
29
F¯
407→517
500→558
↑
80 μM
ratiometric
160
30
NaF
420→535
→558
colorimetric
161
31
NaF
320→340
418→560
32
NaF
345→370
406→494
33
TBAF
293→440
407→477
Ac ce p
te
d
1.03 μM
25-fold ↑ 6.6-fold ↑ ↑ 55-fold ↑
－
6.6 μM
THF-H2O (7:3, v:v) MeCN Ethanol-H2O (3:7, v:v, pH 7.4,PBS) Ethanol-H2O (3:7, v:v, pH 7.4,PBS)
5.3 μM
H2O
ratiometric
166
－
PVP MeCN-H2O (8:2, v:v, pH
ratiometric
168
turn on
169
6.5 μM
62 Page 62 of 79
0.1 μM
7.4,HEPES) DMSO
ratiometric
170
－
PBS, pH 7.4
turn on
171
30 μM
PBS buffer (10 mM, pH = 7.2)
turn on
172
↑
－
PBS buffer (10 mM, pH = 7.2)
turn on
172
1.0 μM
THF
ratiometric
178
1.0 μM
THF
TBAF
344→481
418→602
35
NaF
－
→508
36
NaF/TBAF
－
→520
37
NaF/TBAF
－
450→550
38
TBAF
313→410
403→520
39
TBAF
－
418→520
40
TBAF
350→470
－
↑
－
41
TBAF
－
－
↑
－
42
TBAF
365→595
－
43
TBAF
－
470→396
44
TBAF
300→385
→490
45
TBAF
350→395
→490
49
TBAF
－
→466
50
TBAF
399→460
→535
51
TBAF
445→586
→589
52
TBAF
447→645
→718
53
F¯
325→445
→540
54
NaF
→420
55 56 57
F¯ NaF TBAF
342→447 293→324 280→320
58
TBAF
304→406
→488
59
TBAF
－
→458
9663fold ↑ 3-fold ↑
60
NaF
438→596
→612
↑
62
TBAF
384→412
489→529
63
TBAF
378→396
482→517
64
TBAF
303→335
360→460
46-fold ↑ 100-fold ↑ ↑
65
NaF
－
→402
↑
179
colorimetric
184
ratiometric
185
THF-H2O (1:1, v:v) MeCN DMF-H2O (9:1, v:v) MeCN DMF-H2O (9:1, v:v) DMSO-PBS (9:1, v:v, pH 7.2 ) Ethanol-HEPES (9:1, v:v, pH 7.4)
ratiometric colorimetric turn on
191
turn on
198
turn on
200
turn on
201
－
MeCN-H2O (1:1, v:v)
turn on
202
85 nM
DMSO-H2O (95:5, v:v)
turn on
203
↑
76 μM
H2O (pH 7.4)
turn on
204
－
↑
1.0 μM
colorimetric
205
517→478 360→454 384→475
↑ ↑ ↑
10 nM 0.19 μM 0.43 μM
MeCN-H2O (9:1, v:v, pH 7.0) THF DMF (pH 7.0) THF
turn on ratiometric turn on
206 207 208
40 nM
MeCN
turn on
209
－
DCM Ethanol-H2O (7:3, v:v, pH 7.4, PBS)
turn on
210
colorimetric
211
290-fold ↑ ↑ 10-fold ↑ 10-fold ↑
1.0 μM 0.03 μM 2.3 μM 47 nM 7 μM
↑
1.0 mM
→510
THF
an
us
15 μM
－
M
110-fold ↑ 200-fold ↑ 1000fold ↑
d
te
－
cr
184
Ac ce p
TBAF
MeCN-H2O (9:1, v:v) MeCN-H2O (9:1, v:v)
turn off
colorimetric
→523 66
2.1-fold ↑ 15-fold ↓
ip t
↑ 30-fold ↑ 121-fold ↑
34
0.07 mM
198
－
DCM
turn on
213
－
DCM
turn on
213
1.86 μM 3.4 μM 2.6 μM
THF Diblock unimers Micelles nanoparticles
ratiometric
214
turn on
215
turn on
216
turn on
217
turn on
218
turn on
219
164-fold ↑ 833-fold ↑
19.6 nM
146-fold ↑
1.45 μM
Ethanol-H2O (1:1, v:v) Ethanol
67
TBAF
465→483
→521
68
NaF
492→473
→616
↑
5.4 μM
69
NaF
460→585
→595
↑
210 μM
MeCN-H2O (1:1, v:v, pH 7.0) HEPES (30% MeCN, pH 7.4) Buffer
63 Page 63 of 79
(20% MeCN, pH 7.4) 463→441
→485
0.16 μM
MeCN
turn on
220
71
TBAF
255→256
→336
72 73 74 75 77
TBAF TBAF TBAF KF TBAF
295→663→879 357→411 306→325 571→551 555→538
393→425 551→ 395→ 584→564 571→554
78
TBAF
－
613→553
79
TBAF
436→417
→423
4600fold ↑ ↑ ↓ ↓ ↑ ↑ 50-fold ↑ ↑
－
THF
turn on
221
2 μM － 0.8 μM 67.4 nM
THF THF THF DCM Acetone
turn on turn off turn off ratiometric colorimetric
222 223 224 225 226
14.5 nM
Acetone
ratiometric
227
53 μM
colorimetric
228
turn on
236
－
THF THF-H2O (8:2, v:v, pH 7.0) DCM
80
TBAF
588,549,408,388→319
617→604
↑
0.2 μM
81
TBAF
289→275
→329,455
82
TBAF
－
→381
83
KF
401→392
416→396
86 88 91
TBAF TBAF TBAF
－ － －
290→311 348→301 336→305
↑ 1.5-fold ↑ 19-fold ↑ ↑ ↑ ↑
turn on
243
50 μM
THF
turn on
244
THF
turn on
245
DCM Ethanol Acetone-H2O(1:1, v:v)
turn on turn on turn on
247 247 247
－
ip t
TBAF
us
70
cr
1049fold ↑
an
10 μM － 5 μM
M
λabsA: the maximum absorption wavelength of the chemodosimeter in the absence of F‾. λabsP: the maximum absorption wavelength of the chemodosimeter in the presence of F‾. λemA: the maximum emission wavelength of the chemodosimeter in the absence of F‾. λemP: the maximum emission wavelength of the chemodosimeter in the presence of F‾. φ ratio: the change
te
detecting F‾.
d
ratio of the emission intensity of the chemodosimeter upon addition of F‾. Limit: the detection limit of the chemodosimeter for
Ac ce p
3. Conclusions and perspectives
A comprehensive account of the research that has been carried out to develop organosilicon compounds for use as chemosensors for the fluoride anion (Table 1) has been provided. Organosilicon compounds, which take advantage of the affinity between fluoride and silicon, have faster response times, better selectivity, higher sensitivity and stability both in organic solvents and in aqueous solution than the other chemosensors that have been used for F‾ detection. There are three main sensor reactions between the F‾ ion and silicon: (i) F triggered silicon-oxygen bond cleavage; (ii) F triggered silicon-carbon bond cleavage; and (iii) direct FSi interactions. Ideally, fluoride chemosensors should meet the requirements for cellular and biological systems, such as easy detection of F in aqueous environments, permeability through cell membranes and nontoxicity to cells. However, most of organosilicon chemosensors that have been reported have been for the recognition of fluoride in organic 64 Page 64 of 79
medium. To date, only a few organosilicon chemosensors have been developed that can be used to detect F in living cells, with high detection sensitivity and short response times, and this limits the wider applicability of fluoride anion fluorescence sensor research. Despite the relative lack of progress in this regard, fluorine sensor research remains a highly active emerging field. In order to promote the further development of fluoride ion
ip t
fluorescent chemosensors, an obvious approach will be to carry out the rational design of sensors using DFT/TDDFT calculations to model trends in the optical properties and electronic structures of organosilicon
cr
compounds. And the development of sensors that are suitable for use in the clinical detection of fluoride ions remains the ultimate goal. Most of the chemosensors that have been reported do not absorb in the NIR region, so
us
the incident light cannot penetrate the cell membrane, limiting their suitability for in vitro and in vivo imaging. Given the progress that has been made in the context of BODIPY dyes, which retain excellent photophysical
an
properties even after structural modification [84], it appears probable that rational strategies for forming reactionbased F‾ chemosensors that are soluble in aqueous solution and absorb and emit strongly in the NIR region will be
M
reported in due course.
d
Acknowledgment
te
Financial support was provided by the National Natural Science Foundation of China (no. 21471042) and Zhejiang
Ac ce p
Provincial Natural Science Foundation of China (Grant No. LY14B010003). Theoretical calculations were carried out at the Centre for High Performance Computing in Cape Town.
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BDE: Bond-dissociation energy
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Glossary
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BODIPY: Boron dipyrromethene difluoride
B3LYP: Becke 3-Parameter (Exchange), Lee, Yang and Parr CTABr: Cetyltrimethylammonium bromide DCM: Dichloromethane
DFT: Density functional theory DMF: Dimethylformamide
DMSO: Dimethylsulfoxide DNA: Deoxyribonucleic acid DPP: Diketopyrrolopyrrole ESPT: Excited-state proton transfer ESIPT: Excited-state intramolecular proton transfer F: fluorescence quantum yield
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HaCaT: Cultured human keratinocyte HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid HIV: Human immunodeficiency virus HOMO: Highest Occupied Molecular Orbital ICT: Intramolecular charge transfer
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LUMO: Lowest Unoccupied Molecular Orbital NI: 1,8-naphthalimides
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NIR: Near-infrared region NMR: Nuclear magnetic resonance
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OTBS: Oxy-tert-butyldimethylsilyl OLED: Organic light-emitting diode
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PBS: Phosphate buffered saline PEG: Polyethylene glycol
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PET: Photoinduced electron transfer PMMA: Poly(methyl methacrylate)
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PVP: Polyvinylpyrrolidone
SPS: Silylated spiropyran
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TBA: Tetrabutylammonium
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RAW: Mouse leukaemic monocyte macrophage cell line
TBAF: Tetrabutylammonium fluoride TBDMS: Tert-butyldimethylsilyl TBDPS: Tert-butyldiphenylsilyl TBS: Tert-butyldimethylsilyl
TDDFT: Time dependent density functional theory calculations THF: Tetrahydrofuran THS: trihexylsilylacetylene TIPS: Triisopropysilyl TMS: Trimethylsilyl TZVP: Triple zeta valence plus polarization USEPA: United States Environmental Protection Agency
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UV: ultraviolet
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