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Highly selective fluorescent chemosensor based on benzothiazole for detection of Zn2+
Accepted Manuscript Title: Highly selective fluorescent chemosensor based on benzothiazole for detection of Zn2+ Author: Yingzhi Jin Shuai Wang Yajun Zhang Bo Song PII: DOI: Reference:
S0925-4005(15)30621-3 http://dx.doi.org/doi:10.1016/j.snb.2015.11.039 SNB 19297
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
16-9-2015 2-11-2015 6-11-2015
Please cite this article as: Y. Jin, S. Wang, Y. Zhang, B. [email protected], Highly selective fluorescent chemosensor based on benzothiazole for detection of Zn2+, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.11.039 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.
Highly selective fluorescent chemosensor based on benzothiazole for detection of Zn2+ Yingzhi Jin, Shuai Wang, Yajun Zhang, Bo Song*
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Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu
Key Laboratory of Advanced Functional Polymer Design and Application, College of
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Chemistry, Chemical Engineering and Materials Science, Soochow University,
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Corresponding author: E-mail: [email protected]
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Suzhou 215123, China.
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A benzothiazole dye bearing a long alkyl chain (BTA-12) was synthesized and used as chemosensor to detect Zn2+. In presence of Zn2+, due to the formation of coordination complex and further aggregation, the fluorescence of BTA-12 was significantly enhanced. This chemosensor shows a very good selectivity to Zn2+ and a detection limit as low as 7.5 × 10-7 mol L-1.
Abstract
Chemical detection of Zn2+ ion plays an important role in both environmental and biological systems. In this paper, a new chemosensor based on benzothiazole was designed and synthesized. The fluorescent intensity of the chemosensor enhanced significantly with existence of Zn2+ ion. The control experiments show that the
analogous ions (Pb2+, Co2+, Ni2+, Mn2+, Cu2+, Mg2+, Cd2+, Na+ and K+) do not have fluorescence enhancement effect; and some of them even quench the fluorescence dramatically. The intensity increase upon addition of Zn2+ is due to the coordination between Zn2+ and benzothiazole. This chemosensor shows a high sensitivity for Zn2+ with a detection limit as low as 7.5 × 10-7 mol L-1 and high selectivity through a ‘turn-
on’ fluorescence response over the other tested metal ions.
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Graphical Abstract (for review)
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Keywords: benzothiazole derivative; chemosensor; zinc ion; fluorescent enhancement
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1. Introduction In the past decades, chemosensors for relevant metal ions have become a
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highly attractive research area.[1-7] The chemosensors based on fluorescence signal show many advantages, such as simplicity, low cost, high sensitivity,
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reproducibility and instantaneous response.[8-13] Developing highly selective
sensors with low detection limit is demanded in both biological and
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environmental applications. Zn2+ is the second most abundant transition metal ion in the human body.[14] And it plays an important role in a wide range of
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biochemical processes, such as brain function and pathology, apoptosis, DNA synthesis, gene transcription, enzyme regulation, immune system function and mammalian reproduction.[15-17] However, both deficiency and overload from
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the permissible limit can cause serious diseases such as Parkinson’s disease, Alzheimer’s disease, epilepsy, hypoxia-ischemia and amyotrophic lateral
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sclerosis.[18, 19] In addition, excessive Zn2+ is harmful to the environment, and
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makes water smelly and muddy. Therefore, it is important to develop an efficient and sensitive method to detect Zn2+. For this purpose of application, a
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number of fluorescent chemosensors based on different chromophores have been exploited.[20-27] Yet, the selectivity and / or sensitivity of the available sensors are insufficient for some practical applications. Benzothiazole is known for excited state intramolecular proton transfer
(ESIPT) through tautomerization.[28-33] This chromophore emerges as a promising luminescent material in many practical applications, such as optical memories and switches,[34] UV absorbers,[35] laser dyes,[36] radiation detection scintillators,[37] OLEDs[38-40] and fluorescent probes.[41-43] Besides ESIPT, benzothiazole also possesses aggregation-induced emission (AIE) property.[44-47] That is to say, the fluorescence of the molecules will be enhanced upon aggregation, thus leading to a high quantum yield.[48] Besides these two unique properties, benzothiazole, bearing heterocyclic ring, is able to
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coordinate with different metal ions.[49, 50] The coordination can likely influence the fluorescence of the molecule.[51-54] We speculate that the variation of the fluorescence should be a perfect signal in detection of the metal ions.
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Herein, a derivative of benzothiazole (denoted by BTA-12) has been designed and synthesized, and employed as a novel chemosensor to detect Zn2+. The emission
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properties of BTA-12 were studied in presence of different metal ions. We found that Zn2+ caused significant enhancement of the fluorescence of BTA-12, whilst the rest
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ions have no such effect, showing a very good selectivity. The detection limit (DL) based on IUPAC definition[55, 56] is as low as 7.5 × 10-7 mol L-1.
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2. Experimental
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2.1. Materials
4-Hydroxyacetanilide, 1-bromododecane, 4-hydroxyphenylboronic acid, salicylic
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acid, phosphorus trichloride (PCl3), tertbutyldimethylsilyl chloride (TBDMSCl),
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imidazole, Lawesson reagent, and potassium ferricyanide were bought from Tokyo Kasei Kogyo Co., Ltd. DMF, dichloromethane, hydrochloric acid (HCl), petroleum
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ether, chlorobenzene, toluene and ethanol were purchased from Chinasun Specialty Products Co., Ltd. Anhydrous MgSO4, anhydrous Na2CO3, NaOH, anhydrous K2CO3, KI,
CH3COOK
(KAc),
NaAc,
Mg(Ac)2⋅4H2O,
Co(Ac)2⋅4H2O, Cd(Ac)2⋅2H2O, Cu(Ac)2⋅H2O,
Pb(Ac)2⋅3H2O,
Mn(Ac)2⋅4H2O,
Zn(Ac)2,
Ni(Ac)2⋅4H2O,
ZnCl2, ZnBr2, Zn(NO3)2⋅6H2O were acquired from Sinopharm Chemical Reagent Co., Ltd. Milli Q water was produced by Direct-Q5uv manufactured by Merck Millipore. 2.2. Instruments 1
H NMR spectra were recorded on Avance III 400 MHz produced by Bruker Co..
The mass spectra (MS) were taken either on gas chromatograph with electron ionization (EI) resource (Trace-ISQ, Thermo Fisher Scientific Inc.) or on matrixassisted laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrometer (Ultraflextreme, Bruker Co.). The fluorescence spectra were measured on Fluromax 4
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manufactured by HORIBA, Ltd. The UV-vis spectra were recorded on Cary 60 produced by Agilent Technologies. Dynamic light scattering (DLS) measurement was performed under ambient conditions using a Zetasizer Nano-ZS (Malvern Instruments, UK) equipped with a 633 nm He-Ne laser and a back-scattering detector.
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The atomic force microscope (AFM) images were captured on a Multimode 8 microscope (Bruker Co.). Peak force quantitative nanomechanical mapping (QNM) in
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air scan mode with ScanAsyst-Air probes (nominal spring constant 0.35 N/m,
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frequency 70 kHz, from Bruker Co.) were used during the measurement. 2.3. Synthesis
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2.3.1 Synthesis of 4-dodecyloxyacetanilide (compound A in Scheme 1).
4-Hydroxyacetanilide (2.0 g, 13.2 mmol), 1-bromododecane (4.8 mL, 19.8
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mmol), K2CO3 (3.7 g, 26.5 mmol) and KI (415 mg, 2.5 mmol) were dissolved in 40 mL DMF. The mixture was heated to and kept at 85 °C. After 12 h, DMF was
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removed, and the residue was extracted with brine / dichloromethane for three times.
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The organic phase was dried with anhydrous MgSO4. Then the solvent was removed by a rotavapor. The crude product was purified by column chromatography (silica gel,
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dichloromethane) to give a white solid. Yield: (3.8 g, 90%). δH (400 MHz; CDCl3; TMS): 7.36 (2H, d, J = 8.9 Hz, Ar-H), 7.05 (1H, br, Ar-NH-), 6.85 (2H, d, J = 9.0 Hz, Ar-H), 3.92 (2H, t, J = 6.6 Hz, -CH2O-), 2.15 (3H, s, -COCH3), 1.76 (2H, m, CH2CH2O-),1.25-1.49 (20H, m, -CH2-), 0.88 (3H, t, J = 6.8 Hz, CH3). MS (EI):
Calculated for C20H33NO2, m/z: 319.25; found, 319.21. 2.3.2 Synthesis of 4-dodecyloxyaniline (compound B in Scheme 1). Under refluxing, 12.5 mL of concentrated HCl was slowly added into a flask with compound A (2.5 g, 7.8 mmol) and 25.0 mL of H2O. The reactant was continuously
refluxed for 24 h, and then alkalized by adding NaOH solution (1 mol L-1). The suspension was filtered, and the solid was thoroughly washed with water and then dried in air. Yield: (1.85 g, 86%). δH (400 MHz; CDCl3; TMS): 6.86-6.70 (4H, m, ArH), 3.88 (2H, t, J = 6.6 Hz, -CH2O-), 1.78-1.66 (2H, m, -CH2CH2O-), 1.46-1.19 (18H,
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m, -CH2-), 0.88 (3H, t, J = 6.8Hz, CH3). MS (EI): Calculated for C18H31NO, m/z: 277.24; found, 277.21. 2.3.3 Synthesis of N-(4-(dodecyloxy)phenyl)-2-hydroxybenzamide (compound C in
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Scheme 1.). Compound B (2.5 g, 9.0 mmol) and salicylic acid (1.4 g, 10.0 mmol) were
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dissolved in 100 mL of chlorobenzene. Then PCl3 (0.5 mL, 4.5 mmol) was added.
The reactant was refluxed for 1.5 h. After being cooled down to room temperature,
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the mixture was poured into a beaker filled with ice water. The precipitate was collected by filtration and dried in a vacuum oven. The product was purified by
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recrystallization in mixed solvent of chloroform and ethanol (1:1). Yield: (2.5 g, 70%). δH (400 MHz; CDCl3; TMS): 12.08 (2H, s, Ar-NH-), 7.83 (1H, s, Ar-OH), 7.50
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(1H, d, J = 8.0 Hz, Ar-H), 7.47 - 7.40 (3H, m, Ar-H), 7.03 (1H, d, J = 8.4 Hz, Ar-H), 6.95-6.89 (3H, m, Ar-H), 3.96 (2H, t, J = 6.6 Hz, -CH2O-), 1.85-1.73 (2H, m, CH2CH2O-), 1.40-1.24 (18H, m, -CH2-), 0.88 (3H, t, J = 6.8 Hz, CH3). MS (EI):
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Calculated for C25H35NO3, m/z: 397.26; found, 397.24.
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2.3.4 Synthesis of compound D in Scheme 1.
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Compound C (2.0 g, 5.0 mmol), TBDMSCl (1.5 g, 10.1 mmol) and imidazole (690 mg, 10.1 mmol) were dissolved in 100 mL of dry dichloromethane. The mixture was stirred at room temperature for 12 h under the protection of nitrogen. The mixture was extracted with brine / dichloromethane for three times. The organic phase was dried with anhydrous MgSO4. After removing the solvent, the product (i.e. silyl ether,
compound E in Scheme 1) was used directly in the next reaction. To the above product, Lawesson reagent (3.1 g, 7.6 mmol) and 100 mL of toluene were added. The solution was refluxed for 12 h under an atmosphere of nitrogen. After removing the solvent, the residue was isolated through column chromatography (silica gel, dichloromethane). A yellowish solid (i.e. compound D) was obtained. Yield: (1.0 g, 38%). δH (400 MHz; CDCl3; TMS): 10.15 (1H, s, Ar-NH-), 8.34-8,28 (1H, d, J = 8.0 Hz, Ar-H), 7.09 (2H, m, Ar-H), 7.37-7.27 (1H, m, Ar-H), 7.08 (1H, m, Ar-H), 6.93-
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6.84 (3H, m, Ar-H), 3.97 (2H, t, J = 6.6 Hz, -CH2O-), 1.80-1.75 (2H, m, -CH2CH2O-), 1.44-1.30 (18H, m, -CH2-), 0.94 (9H, s, Si-C-CH3), 0.87 (3H, t, J = 6.8 Hz, CH3), 0.25 (6H, s, Si-CH3). MS (EI): Calculated for C31H49NO2SSi, m/z: 527.33; found,
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527.30. 2.3.5 Synthesis of BTA-12.
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Compound D (1.0 g, 1.9 mmol) was wetted with a few drops of ethanol, then 30% NaOH (1.6 g, 40 mmol) was added. The mixture was added to aqueous solution
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of potassium ferricyanide (2.6 g, 8 mmol) under stirring at 100 °C. After being stirred for 12 h, the reactant was cooled down to room temperature, and neutralized with
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dilute HCl, and extracted with dichloromethane. The crude product was purified by column chromatograph (silica gel, petroleum ether /dichloromethane = 4:1). The
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white solid obtained is the target molecule, BTA-12. Yield: (100 mg, 12%). δH (400 MHz; CDCl3; TMS): 12.42 (1H, s, Ar-OH), 7.86 (1H, d, J = 8.9 Hz, Ar-H), 7.64 (1H, d, J = 7.9 Hz, Ar-H), 7.38-7.32 (2H, m, Ar-H), 7.09 (2H, m, Ar-H), 6.95 (1H, m, Ar-
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H), 4.04 (2H, t, J = 6.6 Hz, -CH2O- ), 1.87-1.78 (2H, m, -CH2CH2O-), 1.27 (18H, m, -
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CH2-), 0.88 (3H, t, J = 6.8 Hz, CH3), MALDI-TOF: Calculated for C25H33NO2S, m/z:
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411.22; found, [M + H] 412.23. 3. Results and discussion
Bearing a heterocyclic ring and a hydroxyl group in the vicinity, benzothiazole
are very likely coordinate with transition metal ions. If the coordination happens, the response of the fluorescence signal can very possibly be utilized in detection of metal ions. Herein, a benzothiazole covalently attached with a dodecoxyl group (i.e. BTA12) was designed. The aromatic benzothiazole and the alkyl chain make an anisotropic structure, which endows BTA-12 with the ability of self-assembly. The synthetic route is shown in Scheme 1, and the details are described in the experimental section.
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O HO
N H
+ Br
O
K2CO3 / KI
9
9 O
A HO 9 O
NH2 + HOOC
9 O
TBSO H O N C
PCl3
HCl
N H
9 O
9 O
NH2 B
HO H O N C
TBDMSCl imidazole
C 9 O
D
TBSO H S N C
NaOH / K 3Fe(CN)6
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Lawesson reagent
E HO
N S BTA-12
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Scheme 1. Synthetic route of BTA-12.
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9 O
To investigate the effect of different ions on the fluorescence of BTA-12, a series
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of divalent metal ions including Pb2+, Co2+, Ni2+, Mn2+, Cu2+, Mg2+, Cd2+ and Zn2+ were selected. These metal ions are often appeared in either biological or environmental systems. Additionally, to exclude the possible influence of ionic
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strength, K+ and Na+ were also tested. To avoid the effect of anions, here acetate was chosen as the anion for all the cations. Before the spectral measurement, BTA-12 and
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the metal ions were pre-mixed in ethanol solutions. For a parallel comparison, the
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final concentrations of BTA-12 and metal ions were controlled to 5.0 × 10-5 and 2.5 × 10-4 mol L-1, respectively. Fig. 1a shows the pictures of the solutions under
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illumination of 365 nm light. Clearly, the one with Zn2+ is much brighter than the rest. The fluorescent spectra were recorded under excitation wavelength of 378 nm. As shown in Fig. 1b, in spite of the type of metal ions added in BTA-12 solution, the shape of fluorescence spectra changed from three peaks to one peak located at around 450 nm. In presence of Zn2+, the fluorescence intensity was remarkably enhanced.
However, the rest metal ions did not show such enhancement effect to the fluorescence intensity. The ions (Na+, K+, Mg2+ and Pb2+) have little effect on the fluorescence intensity, whereas Co2+, Ni2+, Mn2+ and Cu2+ quenched the fluorescence in different extents. Only Cd2+ caused a little increase of the fluorescence intensity. These results indicate that BTA-12 can be regarded as a good fluorescent sensor for Zn2+. It is worth to note that although Zn2+ and Cd2+ are not easy to differentiate, herein they cause significant differences to the emission of BTA-12.
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The fluorescent intensity of BTA-12 versus the concentration of the metal ions was systematically investigated. Herein， the concentration of BTA-12 was 5.0 × 10-5 mol L-1. In order to make a good comparison, the intensities were converted into relative intensity (RI) by dividing over the fluorescent intensity of BTA-12. As shown
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in Fig. 1c, when the concentration of Zn2+ is less than 1.0 × 10-4 mol L-1, the RI at 450
nm increases almost linearly with the concentration of Zn2+. When the concentration
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is larger than 1.0 × 10-4 mol L-1, the RI does not increase so rapidly, and tend to reach
to a constant as further increase the concentration of Zn2+. The similar trend also
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happened to the solutions upon addition of Cd2+. However, the RI is far smaller with existence of equal amount of Zn2+. For the rest ions, the RIs of the solutions either
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kept constant or decreased with increasing the concentration of the metal ions. But all the RIs were invariably small. It can be concluded that Zn2+ can be differentiated from
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these ions by the enhancement of fluorescent intensity. That is to say, BTA-12 can be applied as a chemosensor to detect Zn2+ with a high selectivity. (a) BTA-12 K+ Na+
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Pb2+ Co2+ Ni2+ Mn2+ Cu2+ Mg2+ Cd2+ Zn2+
400
450
500
550
BTA-12 K+ Na+ Pb2+ Co2+ Ni2+ Mn2+ Cu2+ Mg2+ Cd2+ Zn2+
(c)
+
K + Na 2+ Pb 2+ Co 2+ Ni 2+ Mn 2+ Cu 2+ Mg 2+ Cd 2+ Zn
15 10
RI
Fluorescent intensity
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(b)
5 0
600
Wavelength (nm)
650
0.0
0.2
0.4
0.6
0.8
-3
1.0
-1
1.2
Concentration (x 10 mol L )
Fig. 1. (a) Ethanol solutions of BTA-12 in presence of different metal ions under illumination of 365 nm light; (b) Fluorescence spectra of BTA-12 in presence of different metal ions; (c) RI of BTA-12 versus the concentrations of different metal ions. We speculate that the response of fluorescence upon addition of Zn2+ should be attributed to the formation of complexes or assemblies in the solution. UV-vis
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absorption spectra were employed to verify this speculation. The absorption spectra of BTA-12 and Zn2+ (with fixed concentration of BTA-12 and varying the concentration of Zn2+) show a new peak located at 392 nm upon addition of Zn2+ (see Fig. 2). The intensity of this peak increases with the amounts of Zn2+. Meanwhile, the intensity of
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the peak at around 340 nm decreases. These spectral changes imply the formation of
new species in the solution, either coordinate complexes of BTA-12 with Zn2+ or
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aggregates.
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Concentration of Zn 2+ BTA-12 5.0 × 10-6 mol L-1 1.5 × 10-5 mol L -1 2.5 × 10-5 mol L -1 3.5 × 10-5 mol L -1 5.0 × 10-5 mol L -1
0.2
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Absorbance
0.4
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0.0
300
400
500
d
Wavelength (nm)
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Fig. 2. The UV-vis spectra of BTA-12 (5.0 × 10-5 mol L-1) in presence of different concentration of Zn2+.
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MALDI-TOF MS give direct evidence for the formation of coordinate complex. The MS of BTA-12 and the mixture of BTA-12 and Zn2+ were shown in Fig. 3. The corresponding magnifications of both experimental and theoretical MS were presented as insets. The experimental isotopic distributions of BTA-12 and the mixture of BTA-12 and Zn2+ show excellent agreement with the corresponding
theoretical predictions. Fig. 3a indicates the mono-molecular dispersion of BTA-12 in the solution, whereas Fig. 3b suggests the existence of a coordinate complex formed by two molecules of BTA-12 with one Zn2+ (denoted by Zn(BTA-12)2). The coordination of BTA-12 and Zn2+ should happen between the hydroxyl group and
nitrogen, as show in the inset of Fig. 3b.[57] This strong interaction interlocks the possible spinning of the benzene ring, and makes one reason for the enhanced fluorescence of BTA-12.
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(a)
(b)
HO N 9
S
S
O
9
O N Zn N O
O
O
9
S 884.36
Theoretical
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Theoretical
884.42
412.23
Experimental
Experimental
400
414
500
m/z
885
416
600
700
890
800
895
850
m/z
900
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412
cr
412.22
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Fig. 3. The MALDI-TOF MS of BTA-12 (a) and the mixture of BTA-12 and Zn2+ (b). Insets: magnified theoretical and experimental isotopic distributions, molecular structure and possible structure of coordinate complex.
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Since benzothiazole is also known for the AIE effect, we wish to determine if there are any aggregates formed before and after addition of Zn2+ into the solution of BTA-12. This study can be helpful for further understanding the enhanced emission.
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Herein, DLS was employed to investigate the scattering of the ethanol solutions of
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BTA-12 with / without Zn2+. To avoid the interference of fluorescence with the light source, a 633 nm laser was selected for the detection. As for the BTA-12 solution, an
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average hydrodynamic diameter (Dh) of 2.4 nm was reflected by a tiny scattering peak, which is not visible in the intensity scale of Fig. 4. This size is no bigger than a single molecule of BTA-12, indicating that BTA-12 should be dissolved in ethanol as individual molecules instead of aggregates. As for the solution of BTA-12 with Zn2+,
an average Dh of 342 nm was observed. This result suggests the formation of aggregates in the solution. Since BTA-12 itself will not self-assemble, here the aggregates must be assemblies of Zn(BTA-12)2. Combining this result with the UVvis spectra (Fig. 2), it can be concluded that the new peak appears at 392 nm should correspond to the aggregates of Zn(BTA-12)2. We assume that the fluorescence enhancement effect of Zn2+ to BTA-12 should be attributed to the formation of the coordinate complex, as well as the aggregates.
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342 nm
BTA-12
10
100
1000
cr
1
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Intensity
BTA-12 + Zn
2+
Dh (nm)
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Fig. 4. DLS profiles of BTA-12 (1.0 × 10-3 mol L-1 in ethanol solution) with and without Zn2+ (5 equivalent). The aggregation of Zn(BTA-12)2 can also be confirmed by AFM image. As
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shown by AFM image in Fig. 5, particles with size of approximately 200 nm were observed. The sizes are slightly smaller than the average Dh determined by DLS.
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What’s more, the particles seem depressed as shown in AFM image. We assume that the assemblies formed in the solution should be vesicle-like structures. This
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assumption is reasonable to explain the discrepancy of the sizes indicated by DLS and
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process.
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microscope images. Very possibly, the vesicle-like structures shrank during the drying
Fig. 5. AFM image of the aggregates formed in ethanol solution of BTA-12 with Zn2+. The concentration here was 1.0 × 10-3 mol L-1. For a chemosensor, selectivity and DL are two crucially important parameters. These two characteristics are separately investigated in the following. The selectivity of BTA-12 to Zn2+ over other metal ions was evaluated by the fluorescent intensities. In doing so, firstly BTA-12 and 5 equivalent of Zn2+ were
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dissolved in ethanol, and then 5 equivalent of the second ion was added. The fluorescent intensities at 450 nm of the solutions before (FZn) and after (FM) addition of the second ion were recorded. The ratios of FM / FZn were plotted versus the type of ions, as shown in Fig. 6a. K+, Na+, Pb2+, Mg2+ and Cd2+ have little effect on the
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fluorescence of Zn(BTA-12)2, implying the strong interactions between BTA-12 and Zn2+. However, upon addition of Mn2+, Co2+ and Ni2+ the fluorescent intensities
cr
become relatively smaller. Nevertheless, at least approximately 50% of the intensities
were retained. These results suggest that although Mn2+, Co2+ and Ni2+ in some extent
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can quench the fluorescence, the total intensity is still strong enough for detection of Zn2+. When the second ion is Cu2+, as shown in Fig. 6a, the fluorescence decreased
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dramatically. For one thing, the coordinate ability of Cu2+ is competitive with Zn2+; for the other thing, the paramagnetic nature of Cu2+ shows strong quenching effect to
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the fluorescence.[19, 27] Based on this fact, it should be noted here that the possible existence of Cu2+ in the testing sample should be excluded in the practical
0.6 0.4 0.2 0.0
Zn2+ K+ Na+ Pb2+ Mg2+ Cd2+ Mn2+ Co2+ Ni2+ Cu2+
(b) BTA-12 BTA-12 + ZnBr 2 BTA-12 + Zn (CH3COO )2 BTA-12 + ZnCl 2 BTA-12 + Zn (NO3)2
Fluorescent intensity
0.8
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FZn / FM
1.0
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(a)
d
applications.
400
450
500
550
600
650
Wavelength (nm)
Fig. 6. (a) Disturbance of second ion to Zn2+. Concentration of BTA-12 was 5.0 × 10-5 mol L-1, and both Zn2+ and the second ion were 5 equivalent of BTA-12. FZn and FM denote the fluorescence intensity of BTA-12 at 450 nm in the presence of Zn2+ only and in the presence of Zn2+ as well as the competing ions, respectively. (b) The effect of anions (Br-, CH3COO-, Cl- and NO3-) on the fluorescence of complex of BTA-12 and Zn2+. The excitation wavelength was 378 nm. To generalize the potential application of BTA-12 as chemosensor, the effect of different counter-anions were evaluated. In addition to Ac-, three common used counter-anions (Br-, Cl- and NO3-) were investigated. The concentration of BTA-12 was 5.0 × 10-5 mol L-1, and 5 equivalent of zinc salts were added. As shown in Fig.
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6b, the fluorescence spectra have little differences as varying the type of the anions. These results reveal that the counter-anions have little interference to the detection of Zn2+. Therefore, we assume that BTA-12 should be practically useful for the detection
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Y = 45228x + 437235 R = 0.97
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Fluorescent intensity
of Zn2+.
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0 20 40 60 -6 -1 Concentration ( × 10 mol L )
Fig. 7. The calibration curve of fluorescence intensities at 450 nm.
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To determine the DL of BTA-12 for Zn2+, the fluorescence of the solution was measured by varying the content of Zn2+. The concentration of BTA-12 was 5.0 × 10-5
d
mol L-1. The emission intensities at 450 nm were plotted as a function of Zn2+
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concentration (Fig. 7). The DL was extrapolated from the calibration curve based on the definition by IUPAC, i.e. DL = 3 σ/k. Here, σ is the standard deviation of 10
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blank measurements, and k is the slope of the curve. The DL of BTA-12 for detection of Zn2+ is as low as 7.5 × 10-7 mol L-1. This value is hundred fold lower than the WHO guidline (7.6 × 10-5 mol L-1) for Zn2+ ions in drinking water.[58]
4. Conclusions
In this work, a benzothiazole covalently attached with long alkyl chain
BTA-12 was synthesized, and investigated the possibility as a chemosensor for detection of Zn2+. The fluorescence intensity of BTA-12 shows a remarkable
enhancement in presence of Zn2+. Such strong enhancement effect was not observed in terms of other divalent metal ions (Pb2+, Co2+, Ni2+, Mn2+, Cu2+, Mg2+ and Cd2+). Results from MS, DLS and microscope images indicate that BTA-12 and Zn2+ form coordinate complex, and the complex further self-
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assemble into aggregates. The formation of complex and aggregates explain the fluorescence enhancement of BTA-12 by Zn2+. As a sensor, BTA-12 shows very good selectivity and low DL, might be potentially applicable for detection
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of Zn2+ in biological or environmental systems. Acknowledgements
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The authors are great indebt of Professor Jian Fan and Mr. Bin Wang for discussion in molecule design and synthesis. This work was supported by the
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National Natural Science Foundation of China (21204054), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education
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Institutions.
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References
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Biographies
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Yingzhi Jin graduated from Hangzhou Normal University in 2013, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in
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College of Chemistry, Chemical Engineering and Materials Science, Soochow
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University. Her research interest is supramolecular fluorescent materials. Shuai Wang graduated from Qingdao University of Science & technology in
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2012, and in the same year joined Prof. Bo Song’s group in Soochow University. He obtained master degree in 2015. His research interest is supramolecular self-assembly of amphiphilic molecules.
Yajun Zhang graduated from Qingdao University of Science & technology in
2014, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in College of Chemistry, Chemical Engineering and Materials Science, Soochow University. Her research interest is supramolecular selfassembly of amphiphilic molecules. Bo Song received his Ph.D. degree from Tsinghua University, Chemistry department under supervision of Prof. Xi Zhang in 2009. During his Ph.D. training, he worked in KU Leuven for joint-project in 2008-2009. After graduation, he acquired the Humboldt fellowship and worked as a postdoc in
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Siegen University cooperating with Prof. Holger Schoenherr till spring of 2012. The same year, he joined Soochow University and became a professor in College of Chemistry, Chemical Engineering and Materials Science. His research interests include fluorescent materials, supramolecular chemistry and
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organic solar cells.
Biographies
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Yingzhi Jin graduated from Hangzhou Normal University in 2013, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in
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College of Chemistry, Chemical Engineering and Materials Science, Soochow University. Her research interest is supramolecular fluorescent materials.
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Shuai Wang graduated from Qingdao University of Science & technology in 2012, and in the same year joined Prof. Bo Song’s group in Soochow
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University. He obtained master degree in 2015. His research interest is supramolecular self-assembly of amphiphilic molecules.
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Yajun Zhang graduated from Qingdao University of Science & technology in
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2014, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in College of Chemistry, Chemical Engineering and Materials
Science, Soochow University. Her research interest is supramolecular selfassembly of amphiphilic molecules.
Bo Song received his Ph.D. degree from Tsinghua University, Chemistry department under supervision of Prof. Xi Zhang in 2009. During his Ph.D. training, he worked in KU Leuven for joint-project in 2008-2009. After graduation, he acquired the Humboldt fellowship and worked as a postdoc in Siegen University cooperating with Prof. Holger Schoenherr till spring of 2012. The same year, he joined Soochow University and became a professor in College of Chemistry, Chemical Engineering and Materials Science. His research interests include fluorescent materials, supramolecular chemistry and organic solar cells.
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S0925-4005(15)30621-3 http://dx.doi.org/doi:10.1016/j.snb.2015.11.039 SNB 19297
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
16-9-2015 2-11-2015 6-11-2015
Please cite this article as: Y. Jin, S. Wang, Y. Zhang, B. [email protected], Highly selective fluorescent chemosensor based on benzothiazole for detection of Zn2+, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.11.039 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.
Highly selective fluorescent chemosensor based on benzothiazole for detection of Zn2+ Yingzhi Jin, Shuai Wang, Yajun Zhang, Bo Song*
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Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu
Key Laboratory of Advanced Functional Polymer Design and Application, College of
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Chemistry, Chemical Engineering and Materials Science, Soochow University,
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Corresponding author: E-mail: [email protected]
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Suzhou 215123, China.
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A benzothiazole dye bearing a long alkyl chain (BTA-12) was synthesized and used as chemosensor to detect Zn2+. In presence of Zn2+, due to the formation of coordination complex and further aggregation, the fluorescence of BTA-12 was significantly enhanced. This chemosensor shows a very good selectivity to Zn2+ and a detection limit as low as 7.5 × 10-7 mol L-1.
Abstract
Chemical detection of Zn2+ ion plays an important role in both environmental and biological systems. In this paper, a new chemosensor based on benzothiazole was designed and synthesized. The fluorescent intensity of the chemosensor enhanced significantly with existence of Zn2+ ion. The control experiments show that the
analogous ions (Pb2+, Co2+, Ni2+, Mn2+, Cu2+, Mg2+, Cd2+, Na+ and K+) do not have fluorescence enhancement effect; and some of them even quench the fluorescence dramatically. The intensity increase upon addition of Zn2+ is due to the coordination between Zn2+ and benzothiazole. This chemosensor shows a high sensitivity for Zn2+ with a detection limit as low as 7.5 × 10-7 mol L-1 and high selectivity through a ‘turn-
on’ fluorescence response over the other tested metal ions.
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Graphical Abstract (for review)
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Keywords: benzothiazole derivative; chemosensor; zinc ion; fluorescent enhancement
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1. Introduction In the past decades, chemosensors for relevant metal ions have become a
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highly attractive research area.[1-7] The chemosensors based on fluorescence signal show many advantages, such as simplicity, low cost, high sensitivity,
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reproducibility and instantaneous response.[8-13] Developing highly selective
sensors with low detection limit is demanded in both biological and
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environmental applications. Zn2+ is the second most abundant transition metal ion in the human body.[14] And it plays an important role in a wide range of
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biochemical processes, such as brain function and pathology, apoptosis, DNA synthesis, gene transcription, enzyme regulation, immune system function and mammalian reproduction.[15-17] However, both deficiency and overload from
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the permissible limit can cause serious diseases such as Parkinson’s disease, Alzheimer’s disease, epilepsy, hypoxia-ischemia and amyotrophic lateral
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sclerosis.[18, 19] In addition, excessive Zn2+ is harmful to the environment, and
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makes water smelly and muddy. Therefore, it is important to develop an efficient and sensitive method to detect Zn2+. For this purpose of application, a
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number of fluorescent chemosensors based on different chromophores have been exploited.[20-27] Yet, the selectivity and / or sensitivity of the available sensors are insufficient for some practical applications. Benzothiazole is known for excited state intramolecular proton transfer
(ESIPT) through tautomerization.[28-33] This chromophore emerges as a promising luminescent material in many practical applications, such as optical memories and switches,[34] UV absorbers,[35] laser dyes,[36] radiation detection scintillators,[37] OLEDs[38-40] and fluorescent probes.[41-43] Besides ESIPT, benzothiazole also possesses aggregation-induced emission (AIE) property.[44-47] That is to say, the fluorescence of the molecules will be enhanced upon aggregation, thus leading to a high quantum yield.[48] Besides these two unique properties, benzothiazole, bearing heterocyclic ring, is able to
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coordinate with different metal ions.[49, 50] The coordination can likely influence the fluorescence of the molecule.[51-54] We speculate that the variation of the fluorescence should be a perfect signal in detection of the metal ions.
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Herein, a derivative of benzothiazole (denoted by BTA-12) has been designed and synthesized, and employed as a novel chemosensor to detect Zn2+. The emission
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properties of BTA-12 were studied in presence of different metal ions. We found that Zn2+ caused significant enhancement of the fluorescence of BTA-12, whilst the rest
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ions have no such effect, showing a very good selectivity. The detection limit (DL) based on IUPAC definition[55, 56] is as low as 7.5 × 10-7 mol L-1.
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2. Experimental
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2.1. Materials
4-Hydroxyacetanilide, 1-bromododecane, 4-hydroxyphenylboronic acid, salicylic
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acid, phosphorus trichloride (PCl3), tertbutyldimethylsilyl chloride (TBDMSCl),
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imidazole, Lawesson reagent, and potassium ferricyanide were bought from Tokyo Kasei Kogyo Co., Ltd. DMF, dichloromethane, hydrochloric acid (HCl), petroleum
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ether, chlorobenzene, toluene and ethanol were purchased from Chinasun Specialty Products Co., Ltd. Anhydrous MgSO4, anhydrous Na2CO3, NaOH, anhydrous K2CO3, KI,
CH3COOK
(KAc),
NaAc,
Mg(Ac)2⋅4H2O,
Co(Ac)2⋅4H2O, Cd(Ac)2⋅2H2O, Cu(Ac)2⋅H2O,
Pb(Ac)2⋅3H2O,
Mn(Ac)2⋅4H2O,
Zn(Ac)2,
Ni(Ac)2⋅4H2O,
ZnCl2, ZnBr2, Zn(NO3)2⋅6H2O were acquired from Sinopharm Chemical Reagent Co., Ltd. Milli Q water was produced by Direct-Q5uv manufactured by Merck Millipore. 2.2. Instruments 1
H NMR spectra were recorded on Avance III 400 MHz produced by Bruker Co..
The mass spectra (MS) were taken either on gas chromatograph with electron ionization (EI) resource (Trace-ISQ, Thermo Fisher Scientific Inc.) or on matrixassisted laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrometer (Ultraflextreme, Bruker Co.). The fluorescence spectra were measured on Fluromax 4
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manufactured by HORIBA, Ltd. The UV-vis spectra were recorded on Cary 60 produced by Agilent Technologies. Dynamic light scattering (DLS) measurement was performed under ambient conditions using a Zetasizer Nano-ZS (Malvern Instruments, UK) equipped with a 633 nm He-Ne laser and a back-scattering detector.
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The atomic force microscope (AFM) images were captured on a Multimode 8 microscope (Bruker Co.). Peak force quantitative nanomechanical mapping (QNM) in
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air scan mode with ScanAsyst-Air probes (nominal spring constant 0.35 N/m,
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frequency 70 kHz, from Bruker Co.) were used during the measurement. 2.3. Synthesis
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2.3.1 Synthesis of 4-dodecyloxyacetanilide (compound A in Scheme 1).
4-Hydroxyacetanilide (2.0 g, 13.2 mmol), 1-bromododecane (4.8 mL, 19.8
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mmol), K2CO3 (3.7 g, 26.5 mmol) and KI (415 mg, 2.5 mmol) were dissolved in 40 mL DMF. The mixture was heated to and kept at 85 °C. After 12 h, DMF was
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removed, and the residue was extracted with brine / dichloromethane for three times.
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The organic phase was dried with anhydrous MgSO4. Then the solvent was removed by a rotavapor. The crude product was purified by column chromatography (silica gel,
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dichloromethane) to give a white solid. Yield: (3.8 g, 90%). δH (400 MHz; CDCl3; TMS): 7.36 (2H, d, J = 8.9 Hz, Ar-H), 7.05 (1H, br, Ar-NH-), 6.85 (2H, d, J = 9.0 Hz, Ar-H), 3.92 (2H, t, J = 6.6 Hz, -CH2O-), 2.15 (3H, s, -COCH3), 1.76 (2H, m, CH2CH2O-),1.25-1.49 (20H, m, -CH2-), 0.88 (3H, t, J = 6.8 Hz, CH3). MS (EI):
Calculated for C20H33NO2, m/z: 319.25; found, 319.21. 2.3.2 Synthesis of 4-dodecyloxyaniline (compound B in Scheme 1). Under refluxing, 12.5 mL of concentrated HCl was slowly added into a flask with compound A (2.5 g, 7.8 mmol) and 25.0 mL of H2O. The reactant was continuously
refluxed for 24 h, and then alkalized by adding NaOH solution (1 mol L-1). The suspension was filtered, and the solid was thoroughly washed with water and then dried in air. Yield: (1.85 g, 86%). δH (400 MHz; CDCl3; TMS): 6.86-6.70 (4H, m, ArH), 3.88 (2H, t, J = 6.6 Hz, -CH2O-), 1.78-1.66 (2H, m, -CH2CH2O-), 1.46-1.19 (18H,
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m, -CH2-), 0.88 (3H, t, J = 6.8Hz, CH3). MS (EI): Calculated for C18H31NO, m/z: 277.24; found, 277.21. 2.3.3 Synthesis of N-(4-(dodecyloxy)phenyl)-2-hydroxybenzamide (compound C in
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Scheme 1.). Compound B (2.5 g, 9.0 mmol) and salicylic acid (1.4 g, 10.0 mmol) were
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dissolved in 100 mL of chlorobenzene. Then PCl3 (0.5 mL, 4.5 mmol) was added.
The reactant was refluxed for 1.5 h. After being cooled down to room temperature,
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the mixture was poured into a beaker filled with ice water. The precipitate was collected by filtration and dried in a vacuum oven. The product was purified by
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recrystallization in mixed solvent of chloroform and ethanol (1:1). Yield: (2.5 g, 70%). δH (400 MHz; CDCl3; TMS): 12.08 (2H, s, Ar-NH-), 7.83 (1H, s, Ar-OH), 7.50
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(1H, d, J = 8.0 Hz, Ar-H), 7.47 - 7.40 (3H, m, Ar-H), 7.03 (1H, d, J = 8.4 Hz, Ar-H), 6.95-6.89 (3H, m, Ar-H), 3.96 (2H, t, J = 6.6 Hz, -CH2O-), 1.85-1.73 (2H, m, CH2CH2O-), 1.40-1.24 (18H, m, -CH2-), 0.88 (3H, t, J = 6.8 Hz, CH3). MS (EI):
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Calculated for C25H35NO3, m/z: 397.26; found, 397.24.
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2.3.4 Synthesis of compound D in Scheme 1.
Ac ce p
Compound C (2.0 g, 5.0 mmol), TBDMSCl (1.5 g, 10.1 mmol) and imidazole (690 mg, 10.1 mmol) were dissolved in 100 mL of dry dichloromethane. The mixture was stirred at room temperature for 12 h under the protection of nitrogen. The mixture was extracted with brine / dichloromethane for three times. The organic phase was dried with anhydrous MgSO4. After removing the solvent, the product (i.e. silyl ether,
compound E in Scheme 1) was used directly in the next reaction. To the above product, Lawesson reagent (3.1 g, 7.6 mmol) and 100 mL of toluene were added. The solution was refluxed for 12 h under an atmosphere of nitrogen. After removing the solvent, the residue was isolated through column chromatography (silica gel, dichloromethane). A yellowish solid (i.e. compound D) was obtained. Yield: (1.0 g, 38%). δH (400 MHz; CDCl3; TMS): 10.15 (1H, s, Ar-NH-), 8.34-8,28 (1H, d, J = 8.0 Hz, Ar-H), 7.09 (2H, m, Ar-H), 7.37-7.27 (1H, m, Ar-H), 7.08 (1H, m, Ar-H), 6.93-
Page 7 of 22
6.84 (3H, m, Ar-H), 3.97 (2H, t, J = 6.6 Hz, -CH2O-), 1.80-1.75 (2H, m, -CH2CH2O-), 1.44-1.30 (18H, m, -CH2-), 0.94 (9H, s, Si-C-CH3), 0.87 (3H, t, J = 6.8 Hz, CH3), 0.25 (6H, s, Si-CH3). MS (EI): Calculated for C31H49NO2SSi, m/z: 527.33; found,
ip t
527.30. 2.3.5 Synthesis of BTA-12.
cr
Compound D (1.0 g, 1.9 mmol) was wetted with a few drops of ethanol, then 30% NaOH (1.6 g, 40 mmol) was added. The mixture was added to aqueous solution
us
of potassium ferricyanide (2.6 g, 8 mmol) under stirring at 100 °C. After being stirred for 12 h, the reactant was cooled down to room temperature, and neutralized with
an
dilute HCl, and extracted with dichloromethane. The crude product was purified by column chromatograph (silica gel, petroleum ether /dichloromethane = 4:1). The
M
white solid obtained is the target molecule, BTA-12. Yield: (100 mg, 12%). δH (400 MHz; CDCl3; TMS): 12.42 (1H, s, Ar-OH), 7.86 (1H, d, J = 8.9 Hz, Ar-H), 7.64 (1H, d, J = 7.9 Hz, Ar-H), 7.38-7.32 (2H, m, Ar-H), 7.09 (2H, m, Ar-H), 6.95 (1H, m, Ar-
d
H), 4.04 (2H, t, J = 6.6 Hz, -CH2O- ), 1.87-1.78 (2H, m, -CH2CH2O-), 1.27 (18H, m, -
te
CH2-), 0.88 (3H, t, J = 6.8 Hz, CH3), MALDI-TOF: Calculated for C25H33NO2S, m/z:
Ac ce p
411.22; found, [M + H] 412.23. 3. Results and discussion
Bearing a heterocyclic ring and a hydroxyl group in the vicinity, benzothiazole
are very likely coordinate with transition metal ions. If the coordination happens, the response of the fluorescence signal can very possibly be utilized in detection of metal ions. Herein, a benzothiazole covalently attached with a dodecoxyl group (i.e. BTA12) was designed. The aromatic benzothiazole and the alkyl chain make an anisotropic structure, which endows BTA-12 with the ability of self-assembly. The synthetic route is shown in Scheme 1, and the details are described in the experimental section.
Page 8 of 22
O HO
N H
+ Br
O
K2CO3 / KI
9
9 O
A HO 9 O
NH2 + HOOC
9 O
TBSO H O N C
PCl3
HCl
N H
9 O
9 O
NH2 B
HO H O N C
TBDMSCl imidazole
C 9 O
D
TBSO H S N C
NaOH / K 3Fe(CN)6
ip t
Lawesson reagent
E HO
N S BTA-12
us
Scheme 1. Synthetic route of BTA-12.
cr
9 O
To investigate the effect of different ions on the fluorescence of BTA-12, a series
an
of divalent metal ions including Pb2+, Co2+, Ni2+, Mn2+, Cu2+, Mg2+, Cd2+ and Zn2+ were selected. These metal ions are often appeared in either biological or environmental systems. Additionally, to exclude the possible influence of ionic
M
strength, K+ and Na+ were also tested. To avoid the effect of anions, here acetate was chosen as the anion for all the cations. Before the spectral measurement, BTA-12 and
d
the metal ions were pre-mixed in ethanol solutions. For a parallel comparison, the
te
final concentrations of BTA-12 and metal ions were controlled to 5.0 × 10-5 and 2.5 × 10-4 mol L-1, respectively. Fig. 1a shows the pictures of the solutions under
Ac ce p
illumination of 365 nm light. Clearly, the one with Zn2+ is much brighter than the rest. The fluorescent spectra were recorded under excitation wavelength of 378 nm. As shown in Fig. 1b, in spite of the type of metal ions added in BTA-12 solution, the shape of fluorescence spectra changed from three peaks to one peak located at around 450 nm. In presence of Zn2+, the fluorescence intensity was remarkably enhanced.
However, the rest metal ions did not show such enhancement effect to the fluorescence intensity. The ions (Na+, K+, Mg2+ and Pb2+) have little effect on the fluorescence intensity, whereas Co2+, Ni2+, Mn2+ and Cu2+ quenched the fluorescence in different extents. Only Cd2+ caused a little increase of the fluorescence intensity. These results indicate that BTA-12 can be regarded as a good fluorescent sensor for Zn2+. It is worth to note that although Zn2+ and Cd2+ are not easy to differentiate, herein they cause significant differences to the emission of BTA-12.
Page 9 of 22
The fluorescent intensity of BTA-12 versus the concentration of the metal ions was systematically investigated. Herein， the concentration of BTA-12 was 5.0 × 10-5 mol L-1. In order to make a good comparison, the intensities were converted into relative intensity (RI) by dividing over the fluorescent intensity of BTA-12. As shown
ip t
in Fig. 1c, when the concentration of Zn2+ is less than 1.0 × 10-4 mol L-1, the RI at 450
nm increases almost linearly with the concentration of Zn2+. When the concentration
cr
is larger than 1.0 × 10-4 mol L-1, the RI does not increase so rapidly, and tend to reach
to a constant as further increase the concentration of Zn2+. The similar trend also
us
happened to the solutions upon addition of Cd2+. However, the RI is far smaller with existence of equal amount of Zn2+. For the rest ions, the RIs of the solutions either
an
kept constant or decreased with increasing the concentration of the metal ions. But all the RIs were invariably small. It can be concluded that Zn2+ can be differentiated from
M
these ions by the enhancement of fluorescent intensity. That is to say, BTA-12 can be applied as a chemosensor to detect Zn2+ with a high selectivity. (a) BTA-12 K+ Na+
te
d
Pb2+ Co2+ Ni2+ Mn2+ Cu2+ Mg2+ Cd2+ Zn2+
400
450
500
550
BTA-12 K+ Na+ Pb2+ Co2+ Ni2+ Mn2+ Cu2+ Mg2+ Cd2+ Zn2+
(c)
+
K + Na 2+ Pb 2+ Co 2+ Ni 2+ Mn 2+ Cu 2+ Mg 2+ Cd 2+ Zn
15 10
RI
Fluorescent intensity
Ac ce p
(b)
5 0
600
Wavelength (nm)
650
0.0
0.2
0.4
0.6
0.8
-3
1.0
-1
1.2
Concentration (x 10 mol L )
Fig. 1. (a) Ethanol solutions of BTA-12 in presence of different metal ions under illumination of 365 nm light; (b) Fluorescence spectra of BTA-12 in presence of different metal ions; (c) RI of BTA-12 versus the concentrations of different metal ions. We speculate that the response of fluorescence upon addition of Zn2+ should be attributed to the formation of complexes or assemblies in the solution. UV-vis
Page 10 of 22
absorption spectra were employed to verify this speculation. The absorption spectra of BTA-12 and Zn2+ (with fixed concentration of BTA-12 and varying the concentration of Zn2+) show a new peak located at 392 nm upon addition of Zn2+ (see Fig. 2). The intensity of this peak increases with the amounts of Zn2+. Meanwhile, the intensity of
ip t
the peak at around 340 nm decreases. These spectral changes imply the formation of
new species in the solution, either coordinate complexes of BTA-12 with Zn2+ or
cr
aggregates.
us
Concentration of Zn 2+ BTA-12 5.0 × 10-6 mol L-1 1.5 × 10-5 mol L -1 2.5 × 10-5 mol L -1 3.5 × 10-5 mol L -1 5.0 × 10-5 mol L -1
0.2
an
Absorbance
0.4
M
0.0
300
400
500
d
Wavelength (nm)
te
Fig. 2. The UV-vis spectra of BTA-12 (5.0 × 10-5 mol L-1) in presence of different concentration of Zn2+.
Ac ce p
MALDI-TOF MS give direct evidence for the formation of coordinate complex. The MS of BTA-12 and the mixture of BTA-12 and Zn2+ were shown in Fig. 3. The corresponding magnifications of both experimental and theoretical MS were presented as insets. The experimental isotopic distributions of BTA-12 and the mixture of BTA-12 and Zn2+ show excellent agreement with the corresponding
theoretical predictions. Fig. 3a indicates the mono-molecular dispersion of BTA-12 in the solution, whereas Fig. 3b suggests the existence of a coordinate complex formed by two molecules of BTA-12 with one Zn2+ (denoted by Zn(BTA-12)2). The coordination of BTA-12 and Zn2+ should happen between the hydroxyl group and
nitrogen, as show in the inset of Fig. 3b.[57] This strong interaction interlocks the possible spinning of the benzene ring, and makes one reason for the enhanced fluorescence of BTA-12.
Page 11 of 22
(a)
(b)
HO N 9
S
S
O
9
O N Zn N O
O
O
9
S 884.36
Theoretical
ip t
Theoretical
884.42
412.23
Experimental
Experimental
400
414
500
m/z
885
416
600
700
890
800
895
850
m/z
900
us
412
cr
412.22
an
Fig. 3. The MALDI-TOF MS of BTA-12 (a) and the mixture of BTA-12 and Zn2+ (b). Insets: magnified theoretical and experimental isotopic distributions, molecular structure and possible structure of coordinate complex.
M
Since benzothiazole is also known for the AIE effect, we wish to determine if there are any aggregates formed before and after addition of Zn2+ into the solution of BTA-12. This study can be helpful for further understanding the enhanced emission.
d
Herein, DLS was employed to investigate the scattering of the ethanol solutions of
te
BTA-12 with / without Zn2+. To avoid the interference of fluorescence with the light source, a 633 nm laser was selected for the detection. As for the BTA-12 solution, an
Ac ce p
average hydrodynamic diameter (Dh) of 2.4 nm was reflected by a tiny scattering peak, which is not visible in the intensity scale of Fig. 4. This size is no bigger than a single molecule of BTA-12, indicating that BTA-12 should be dissolved in ethanol as individual molecules instead of aggregates. As for the solution of BTA-12 with Zn2+,
an average Dh of 342 nm was observed. This result suggests the formation of aggregates in the solution. Since BTA-12 itself will not self-assemble, here the aggregates must be assemblies of Zn(BTA-12)2. Combining this result with the UVvis spectra (Fig. 2), it can be concluded that the new peak appears at 392 nm should correspond to the aggregates of Zn(BTA-12)2. We assume that the fluorescence enhancement effect of Zn2+ to BTA-12 should be attributed to the formation of the coordinate complex, as well as the aggregates.
Page 12 of 22
342 nm
BTA-12
10
100
1000
cr
1
ip t
Intensity
BTA-12 + Zn
2+
Dh (nm)
us
Fig. 4. DLS profiles of BTA-12 (1.0 × 10-3 mol L-1 in ethanol solution) with and without Zn2+ (5 equivalent). The aggregation of Zn(BTA-12)2 can also be confirmed by AFM image. As
an
shown by AFM image in Fig. 5, particles with size of approximately 200 nm were observed. The sizes are slightly smaller than the average Dh determined by DLS.
M
What’s more, the particles seem depressed as shown in AFM image. We assume that the assemblies formed in the solution should be vesicle-like structures. This
d
assumption is reasonable to explain the discrepancy of the sizes indicated by DLS and
Ac ce p
process.
te
microscope images. Very possibly, the vesicle-like structures shrank during the drying
Fig. 5. AFM image of the aggregates formed in ethanol solution of BTA-12 with Zn2+. The concentration here was 1.0 × 10-3 mol L-1. For a chemosensor, selectivity and DL are two crucially important parameters. These two characteristics are separately investigated in the following. The selectivity of BTA-12 to Zn2+ over other metal ions was evaluated by the fluorescent intensities. In doing so, firstly BTA-12 and 5 equivalent of Zn2+ were
Page 13 of 22
dissolved in ethanol, and then 5 equivalent of the second ion was added. The fluorescent intensities at 450 nm of the solutions before (FZn) and after (FM) addition of the second ion were recorded. The ratios of FM / FZn were plotted versus the type of ions, as shown in Fig. 6a. K+, Na+, Pb2+, Mg2+ and Cd2+ have little effect on the
ip t
fluorescence of Zn(BTA-12)2, implying the strong interactions between BTA-12 and Zn2+. However, upon addition of Mn2+, Co2+ and Ni2+ the fluorescent intensities
cr
become relatively smaller. Nevertheless, at least approximately 50% of the intensities
were retained. These results suggest that although Mn2+, Co2+ and Ni2+ in some extent
us
can quench the fluorescence, the total intensity is still strong enough for detection of Zn2+. When the second ion is Cu2+, as shown in Fig. 6a, the fluorescence decreased
an
dramatically. For one thing, the coordinate ability of Cu2+ is competitive with Zn2+; for the other thing, the paramagnetic nature of Cu2+ shows strong quenching effect to
M
the fluorescence.[19, 27] Based on this fact, it should be noted here that the possible existence of Cu2+ in the testing sample should be excluded in the practical
0.6 0.4 0.2 0.0
Zn2+ K+ Na+ Pb2+ Mg2+ Cd2+ Mn2+ Co2+ Ni2+ Cu2+
(b) BTA-12 BTA-12 + ZnBr 2 BTA-12 + Zn (CH3COO )2 BTA-12 + ZnCl 2 BTA-12 + Zn (NO3)2
Fluorescent intensity
0.8
Ac ce p
FZn / FM
1.0
te
(a)
d
applications.
400
450
500
550
600
650
Wavelength (nm)
Fig. 6. (a) Disturbance of second ion to Zn2+. Concentration of BTA-12 was 5.0 × 10-5 mol L-1, and both Zn2+ and the second ion were 5 equivalent of BTA-12. FZn and FM denote the fluorescence intensity of BTA-12 at 450 nm in the presence of Zn2+ only and in the presence of Zn2+ as well as the competing ions, respectively. (b) The effect of anions (Br-, CH3COO-, Cl- and NO3-) on the fluorescence of complex of BTA-12 and Zn2+. The excitation wavelength was 378 nm. To generalize the potential application of BTA-12 as chemosensor, the effect of different counter-anions were evaluated. In addition to Ac-, three common used counter-anions (Br-, Cl- and NO3-) were investigated. The concentration of BTA-12 was 5.0 × 10-5 mol L-1, and 5 equivalent of zinc salts were added. As shown in Fig.
Page 14 of 22
6b, the fluorescence spectra have little differences as varying the type of the anions. These results reveal that the counter-anions have little interference to the detection of Zn2+. Therefore, we assume that BTA-12 should be practically useful for the detection
ip t cr
Y = 45228x + 437235 R = 0.97
us
Fluorescent intensity
of Zn2+.
an
0 20 40 60 -6 -1 Concentration ( × 10 mol L )
Fig. 7. The calibration curve of fluorescence intensities at 450 nm.
M
To determine the DL of BTA-12 for Zn2+, the fluorescence of the solution was measured by varying the content of Zn2+. The concentration of BTA-12 was 5.0 × 10-5
d
mol L-1. The emission intensities at 450 nm were plotted as a function of Zn2+
te
concentration (Fig. 7). The DL was extrapolated from the calibration curve based on the definition by IUPAC, i.e. DL = 3 σ/k. Here, σ is the standard deviation of 10
Ac ce p
blank measurements, and k is the slope of the curve. The DL of BTA-12 for detection of Zn2+ is as low as 7.5 × 10-7 mol L-1. This value is hundred fold lower than the WHO guidline (7.6 × 10-5 mol L-1) for Zn2+ ions in drinking water.[58]
4. Conclusions
In this work, a benzothiazole covalently attached with long alkyl chain
BTA-12 was synthesized, and investigated the possibility as a chemosensor for detection of Zn2+. The fluorescence intensity of BTA-12 shows a remarkable
enhancement in presence of Zn2+. Such strong enhancement effect was not observed in terms of other divalent metal ions (Pb2+, Co2+, Ni2+, Mn2+, Cu2+, Mg2+ and Cd2+). Results from MS, DLS and microscope images indicate that BTA-12 and Zn2+ form coordinate complex, and the complex further self-
Page 15 of 22
assemble into aggregates. The formation of complex and aggregates explain the fluorescence enhancement of BTA-12 by Zn2+. As a sensor, BTA-12 shows very good selectivity and low DL, might be potentially applicable for detection
ip t
of Zn2+ in biological or environmental systems. Acknowledgements
cr
The authors are great indebt of Professor Jian Fan and Mr. Bin Wang for discussion in molecule design and synthesis. This work was supported by the
us
National Natural Science Foundation of China (21204054), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education
an
Institutions.
M
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d
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Ac ce p
te
d
M
an
us
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Biographies
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Yingzhi Jin graduated from Hangzhou Normal University in 2013, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in
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College of Chemistry, Chemical Engineering and Materials Science, Soochow
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University. Her research interest is supramolecular fluorescent materials. Shuai Wang graduated from Qingdao University of Science & technology in
Ac ce p
2012, and in the same year joined Prof. Bo Song’s group in Soochow University. He obtained master degree in 2015. His research interest is supramolecular self-assembly of amphiphilic molecules.
Yajun Zhang graduated from Qingdao University of Science & technology in
2014, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in College of Chemistry, Chemical Engineering and Materials Science, Soochow University. Her research interest is supramolecular selfassembly of amphiphilic molecules. Bo Song received his Ph.D. degree from Tsinghua University, Chemistry department under supervision of Prof. Xi Zhang in 2009. During his Ph.D. training, he worked in KU Leuven for joint-project in 2008-2009. After graduation, he acquired the Humboldt fellowship and worked as a postdoc in
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Siegen University cooperating with Prof. Holger Schoenherr till spring of 2012. The same year, he joined Soochow University and became a professor in College of Chemistry, Chemical Engineering and Materials Science. His research interests include fluorescent materials, supramolecular chemistry and
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organic solar cells.
Biographies
us
Yingzhi Jin graduated from Hangzhou Normal University in 2013, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in
an
College of Chemistry, Chemical Engineering and Materials Science, Soochow University. Her research interest is supramolecular fluorescent materials.
M
Shuai Wang graduated from Qingdao University of Science & technology in 2012, and in the same year joined Prof. Bo Song’s group in Soochow
d
University. He obtained master degree in 2015. His research interest is supramolecular self-assembly of amphiphilic molecules.
te
Yajun Zhang graduated from Qingdao University of Science & technology in
Ac ce p
2014, and in the same year joined Prof. Bo Song’s group. She is currently a graduate student in College of Chemistry, Chemical Engineering and Materials
Science, Soochow University. Her research interest is supramolecular selfassembly of amphiphilic molecules.
Bo Song received his Ph.D. degree from Tsinghua University, Chemistry department under supervision of Prof. Xi Zhang in 2009. During his Ph.D. training, he worked in KU Leuven for joint-project in 2008-2009. After graduation, he acquired the Humboldt fellowship and worked as a postdoc in Siegen University cooperating with Prof. Holger Schoenherr till spring of 2012. The same year, he joined Soochow University and became a professor in College of Chemistry, Chemical Engineering and Materials Science. His research interests include fluorescent materials, supramolecular chemistry and organic solar cells.
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