A colorimetric and fluorescent probe containing diketopyrrolopyrrole and 1,3-indanedione for cyanide detection based on exciplex signaling mechanism

A colorimetric and fluorescent probe containing diketopyrrolopyrrole and 1,3-indanedione for cyanide detection based on exciplex signaling mechanism

Sensors and Actuators B 198 (2014) 455–461 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevie...

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Sensors and Actuators B 198 (2014) 455–461

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

A colorimetric and fluorescent probe containing diketopyrrolopyrrole and 1,3-indanedione for cyanide detection based on exciplex signaling mechanism Lingyun Wang ∗ , Jiqing Du, Derong Cao School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 23 January 2014 Received in revised form 13 March 2014 Accepted 13 March 2014 Available online 26 March 2014 Keywords: Chemosensors Cyanide anion Colorimetric Diketopyrrolopyrrole Exciplex

a b s t r a c t A new fluorescent probe 1 comprising diketopyrrolopyrrole and indanedione-based Michael receptor, which recognized cyanide anion with high selectivity was designed and synthesized. Addition of CN− aqueous solution to 1 in THF resulted in a rapid color change from purple to yellow together with a large blue shift from 553 to 480 nm, while other anions did not induce any significant color change. Furthermore, the Michael addition of cyanide to 1 elicited 3.11 and 6.13-fold PL enhancements at 484 and 572 nm, respectively, which constituted the fluorescence signature for cyanide detection. The mechanism of exciplex formation was proposed for this interesting observation. The detection limits were 0.36 ␮M using the fluorescence spectra changes, which was far lower than the WHO guideline of 1.9 ␮M. The results indicated 1 can act as dual-channel, high selective, immediacy and sensitive optical probe for CN− . © 2014 Elsevier B.V. All rights reserved.

1. Introduction Cyanide is well known as one of the most toxic species and is extremely harmful to mammals. Any accidental release of cyanide to the environment causes serious problems. Nevertheless, cyanide salts are still widely used as industrial materials in gold mining, electroplating, plastics production and other fields. Therefore, it is highly desirable to develop sensitive, selective and quick detection methods for toxic cyanide anions. Traditionally, hydrogen bonding or supramolecular interactions have been used in the detection of low concentrations of cyanide in solution [1–3]. These approaches, however, usually result in poor selectivity over other common anions [4]. The burgeoning field of reaction-based indicators has made progress in this area because of the unique reactivity of CN− toward a variety of organic functional groups including C O, C N, C C, and so on [5]. Unfortunately, none of these is ideal. More importantly, there are practical limitations to their use. For instance, the cationic borane receptor was very promising in that reaction with cyanide took place in water, but failed to produce a color change, a shortcoming that restricted its utility [5k]. Likewise, the oxazine-based indicators required specific

∗ Corresponding author. Tel.: +86 20 87110245; fax: +86 20 87110245. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.snb.2014.03.046 0925-4005/© 2014 Elsevier B.V. All rights reserved.

biphasic conditions [5q,5r], while the acridinium salts required an elevated reaction temperature [5s]. There thus remains a need for yet-improved reaction-based cyanide indicator systems. Reaction-based cyanide indicators form the irreversible formation of chemical bonds that can provide chemodosimetric information and develop ratiometric fluorescent probes. However, up to now, only a limited number of Michael addition type probes for the ratiometric fluorescent detection of cyanide have been reported in literatures [5d,5f,5q]. There is an ever-present need to develop new reaction-based CN− sensors, as these can help overcome lingering obstacles in its detection, such as selectivity, sensitivity, response times, sensor stability, reaction conditions, etc. The signaling mechanisms of fluorescent chemosensor are generally based on photoinduced electron transfer, metal-ligand charge transfer, excimer formation, and intramolecular charge transfer, etc. However, exciplex formation, to the best of our knowledge, is not usually used as a signaling mechanism for anion recognition, though it has been documented previously [6]. We wished to explore this mechanism of achieving detectable optical response upon interaction of CN− with probe. One design element was a CN− –induced conformational change of probe, which subsequently triggered a fluorescence altertion. Zhou et al. have previously shown the viability of such this approach [7]. Considering our objectives, we designed probe 1 (Scheme 1) keeping in mind the following. (1) The probe should be

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CHO

O CH2OH CH2OH

O

CH

O

O

O

O CH O

CH

t-C5H11ONa t-C5H11OH CN 2

t-C4H9OK

H N

O

NH2SO3H

O

O O

O

CN

C8H17Br

O

N H

O

O

3 C8H17

C8H17 O

N

O

CH

N

CHO

O HCl/H2O

CH

N

O

THF

OHC

C8H17

4

C8H17 C8H17

O

O

N

O

O

N

5 O

O O

O

N O

C8H17 1 Scheme 1. Synthesis of probe 1.

conformationally rigid before the interaction with CN− . This would inhibit intramolecular interactions. (2) Interaction of such a probe with a CN− should disrupt rigid structure and therefore, make the probe’s frame work more flexible, which is easy to fold yielding ␲-stacking exciplex. (3) The signal reporting of probe derives from more reactive indandione-derived vinyl moiety which could selectively react and enhance the sensitivity of the nucleophilic addition reaction between the vinyl group and CN− . (4) Electron-deficient diketopyrrolopyrrole (DPP) is selected as chromophore considering its many advantageous features including its enhanced vinyl moiety reactivity with CN− , strong fluorescence as well as its distinct rigid ring structure. Herein, we report a DPP–indanedione conjugate (1) as doubly activated Michael addition type probe for the colorimetric fluorescent detection of cyanide. The probe displays high selectivity and sensitivity for CN− over other anions with a fast response.

concentrations were mixed, they were measured by UV–vis and fluorescence spectroscopy.

2.2. Synthesis of compound 1 A mixture of 5 (56.8 mg, 0.10 mmol), 1,3-indanedione (86 mg, 0.60 mmol) and Al2 O3 in dry CH2 Cl2 (20 mL) was mixed at room temperature. After 24 h, the reaction mixture was filtered and the filtrate was evaporated under reduced pressure. The purification by silica gel column chromatography (petroleum ether/dichloromethane/THF = 100/100/1, v/v/v) yielded the desired compound 1 (29.8 mg, 35%) as a dark red solid. 1 H NMR (CDCl , 400 MHz, ı): 8.60 (d, 2H), 8.07–7.98 (m, 8H), 3 7.89–7.85 (m, 8H), 3.81(t, 4H), 1.61–0.86 (m, 30H).

2. Experimental

3. Results and discussions

2.1. Chemicals and instruments

3.1. Synthesis and structural characterization

Nuclear magnetic resonance spectra were recorded on Bruker Avance III 400 MHz and chemical shifts are expressed in ppm using TMS as an internal standard. The UV–vis absorption spectra were recorded using a Helios Alpha UV-Vis scanning spectrophotometer. Fluorescence spectra were obtained with a Hitachi F-4500 FL spectrophotometer with quartz cuvette (path length = 1 cm). N-Methyl-2-pyrrolidone (NMP) was dried with CaH2 and distilled under nitrogen atmosphere. Other solvents were obtained from commercially available resources without further purification. 2,5-Dioctyl-3,6-bis(4 -formylphenyl)pyrrolo[3,4-c] pyrrole1,4-dione (compound 5) was synthesized according to our published literature [8]. The recognition between 1 and different anions was investigated by UV–vis and fluorescence spectroscopy in THF solution at room temperature. The stock solution of 1 and anions was at a concentration of 10.0 mM. After the 1 and anions with desired

Compared with the general Michael acceptors, the doubly activated acceptors, in which two electron-withdrawing groups are attached to the C C group [9,10], are more reactive [11–14], and allow the Michael addition reaction to occur under mild condition. Clearly, to improve the sensitivity, it is necessary to enhance the reactivity of the probe to cyanide. We reasoned that this could be accomplished by increasing the electrophilicity of the ␤-carbon. With this in mind, we decided to introduce electron withdrawing 1,3-indanedione and electron-deficient DPP moiety to afford probe 1. Thus, it is anticipated that probe 1 should be highly reactivity to CN− . 2,5-Dioctyl-3,6-bis(4 -formylphenyl)pyrrolo[3,4-c] pyrrole1,4-dione (compound 5) was prepared from p-cyanobenzaldehyde as a starting material, in four steps following Scheme 1. Probe 1 was obtained by a straightforward aldol-type condensation reaction of 1,3-indanedione with compound 5 in 35% yield as a purple solid.

L. Wang et al. / Sensors and Actuators B 198 (2014) 455–461

457

1.4

1.2

553 nm 480 nm 359 nm

Absorbance

1.0

0.8

0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

t/s Fig. 1. Time-dependent absorption intensity of probe 1 in THF at 551( in black), 480 (䊉 in red) and 359 nm ( in blue) in the presence of CN− (3 equiv.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The 1 H NMR spectrum of probe 1 showed the distinctive signals corresponding to the resonance of vinyl proton at 8.60 ppm. 3.2. Photophysical properties

Fig. 2. (a) UV–vis spectral changes of 1 in THF (20 ␮M) with the increasing concentrations of cyanide anion aqueous solution. (b) Photographs of 1 (40 ␮M) in THF with cyanide anion aqueous solution ranging from 0, 20, 40, 80, and 400 ␮M from left to right.

For the purpose of evaluating selectivity of 1 to cyanide, the absorption spectral change of 1 upon addition of other anions was also investigated. Dramatic change of the absorption spectrum induced by CN− was observed, while almost no changes could be found in presence of other anions, including F− , Cl− , Br− , I− , H2 PO4 − , SO4 2− , CO3 2− , PO4 3− and OAc− (Fig. 3). More importantly, the color change from purple to yellow can be clearly observed by the naked eye in presence of CN− , while other anions did not induce any

1.4 1.2

Blank and other anions

1.0

Absorbtance

The photophysical properties of 1 were studied in THF. 1 showed two major absorption peaks at 359 and 553 nm. To exploit its sensing behavior, we first examined the time dependent changes in the absorption spectra of 1 (20 ␮M) upon reaction with CN− (3 equiv.). Generally, reaction-based chemosensors suffer from a long response time. In our case, the response of 1 to CN− was found to be very fast. The addition of CN− aqueous solution elicited not only a significant absorption increase around 480 nm but also dramatic absorption decreases around 359 and 553 nm (Fig. 1). After 10 s, the UV–vis spectra of 1 were unchangeable, indicating nucleophilic addition reaction between the vinyl group and CN− was completed. It is well known that the nucleophilicity of anions is greatly decreased in water due to hydrogen bond formation between the anions and water molecules. For instance, Guo’s group reported that the reaction of anthracene–indanedione Michael receptor and cyanide could be completed within 2 min at an elevated temperature of 50 ◦ C in 9:1 CH3 CN–water, and 10 equiv. of cyanide was required to reach the spectral saturation [7]. Herein, in the presence of 3 equiv. of cyanide, the nucleophilic CN− addition to 1 occurred very rapidly (within 10 seconds), indicating high reactivity was the unique feature of 1, which was impressive as many reported cyanide probes require high equivalents of cyanide and long reaction time to reach a maximal spectral signal [15–26]. To get insight into the binding of CN− with 1, the absorption spectra of 1 in THF upon titration with CN− aqueous solution were recorded. As shown in Fig. 2(a), the absorption peaks of 1 at 359 and 553 nm gradually decreased following the formation of two new bands centered at 314 and 480 nm. Meanwhile, two isosbestic points at 408 and 507 nm were observed, which implied new species with less conjugation were formed. Fig. 2(b) showed the color photographs of 1 in the presence of different CN− concentrations. With increased CN− concentrations, color of 1 solution gradually changed from purple to yellow, an agreement with the appearance of a new absorption band at about 480 nm in the corresponding UV/Vis spectra. At [CN− ] = 4.0 × 10−4 M, the color of 1 solution was yellow, which offered the feasibility of naked-eye detection.

CN

0.8

-

0.6 0.4 0.2 0.0 400

600

Wavelength (nm) Fig. 3. UV–vis absorption changes of 1 in THF (20 ␮M) upon addition of 10 equiv. of each anion aqueous solution.

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Fig. 4. (a) Color and (b) fluorescence changes of 1 in THF (20 ␮M) in the absence and presence of various anions aqueous solution (10 equiv.) under UV light (365 nm). From left to right: control (1 only), CN− , F− , Cl− , Br− , I− , H2 PO4 − , SO4 2− , CO3 2− , PO4 3− and OAc− .

significant color change, which suggested that naked-eye selective detection of CN− became possible (Fig. 4(a)). The CN− 1 sensing property was further examined through fluorescent titration studies in detail. As shown in Fig. 5, 1 showed a major emission peak at 484 nm and two very weak emission peaks at 572 and 654 nm upon excitation at 479 nm. Upon addition of CN− 1 aqueous solution to 1, the PL intensity at 484 nm was enhanced gradually. Interestingly, the emission at 572 nm did not decrease, but significantly increased, while the band at 654 nm disappeared. Finally, 3.11 and 6.13-fold PL enhancements at 484 and 572 nm were reached after adding 50 ␮M CN− , respectively. Because Michael addition of CN− with 1 really interrupted the ␲conjugation of the molecule, the fluorescence enhancement at long wavelength of 1 upon addition of CN− was abnormal. In fact, the case can be rationalized by exciplex formation [6]. On the basis of the spectral results and possible structural change of 1 upon addition of CN− , the luminescence at 572 nm can be reasonably attributed to the exciplex formation by a

5400 4800

3 eq.

FL intensity (a.u.)

4200 3600 3000 2400 1800

0

1200 600 0 500

600

700

Wavelength (nm) Fig. 5. Fluorescence spectral changes of 1 (10 ␮M) in THF with the increasing concentrations of CN− aqueous solution. Excitation wavelength of 1 was 479 nm.

photoinduced electron-transfer process between the 1,3indanedione moiety and DPP moiety of [1+CN]− adduct. As noted above, in the absence of CN− , probe 1 contained two C C bonds, rigid DPP and indanedione rings, which was difficult to fold. However, when CN− was added, the original C C bond was broken due to nucleophilic addition reaction of CN− with C C bond, resulting in freely rotating carbon carbon bond. As a result, the whole molecular structure became flexible, which made it easy for the three parts of [1+CN]− adduct to fold in a sandwich-like mode. Consequently, a photoinduced electron-transfer process may induce the interaction of the 1,3-indanedione with DPP, leading to exciplex formation, as shown in Scheme 2. Similar results that used exciplex formation as signaling mechanism for CN− recognition were reported by Guo’s group [7]. To confirm the mechanism proposed above, the experiments have been carried out as follows. When n-Bu4 NCN was dissolved in THF, the nucleophilic CN− addition to 1 occurred in THF in absence of water. Different optical changes were present compared with that in THF–H2 O (v/v = 95/5) system, as shown in Fig. 6. Both the emission bands at 484 and 572 nm increased with gradual addition of CN− . Finally, 1.50 and 1.91-fold PL enhancements at 484 and 572 nm were reached after adding 50 ␮M CN− , respectively, which was much lower than that in THF–H2 O (v/v = 95/5) system. The possible reason maybe that three parts of [1+CN]− adduct is not easy to fold in a sandwich-like mode exciplexes or excimers in pure THF, since THF is a good solvent for [1+CN]− adduct. The results suggested that the exciplex emission favorably occurred only in THF–H2 O (v/v = 95/5) but not in pure THF, in contrast with other exciplexes or excimers reported in literature which were only stable in non-polar solvents [27]. To evaluate the selectivity of 1 for detection of cyanide, fluorescence changes caused by the addition of other anions including F− , Cl− , Br− , I− , H2 PO4 − , SO4 2− , CO3 2− , PO4 3− and OAc− were evaluated. Most of these anions did not cause significant changes in the fluorescence intensity at 572 nm of 1, as shown in Fig. 7. Meanwhile, only CN− generated a yellow fluorescence, while 1 with and without other anions showed red fluorescence (Fig. 4(b)). Another important feature of 1 is its high selectivity toward the CN− in presence of other competitive anions. Changes of

L. Wang et al. / Sensors and Actuators B 198 (2014) 455–461

C8H17

H

C8H17

a H O

N

O

CN-

a O

N

CN

O

N

H a' O

C8H17

O

H a'

O

O

CN

N

O

Step 1

O

459

O

O

C8H17

1

1-CN O

H

Exciplex formation Step 2

O

O

NC

N C8H17

C8H17 N

H

O

O CN O

Scheme 2. Schematic illustration of nucleophilic addition reaction of CN− with C C bond. The molecular structure changes from rigid to flexible.

2500

3 eq.

FL intensity (a.u.)

2000

1500

1000

0 500

0 500

600

700

Wavelength (nm) Fig. 6. Fluorescence spectral changes of 1 (10 ␮M) in THF with the increasing concentrations of CN− in THF. Excitation wavelength of 1 was 479 nm.

3500

fluorescence spectra of 1 (10 ␮M) caused by CN− (5 equiv.) and miscellaneous competing species (5 equiv.) were recorded in Fig. 7. As can be seen, these competitive species, did not lead to any significant interference. In the presence of these ions, the CN− still produced similar optical spectral changes. These results showed that the selectivity of sensor 1 toward CN− was not affected by the presence of other anions. The detection limit of 1 for CN− was calculated based on the fluorescence titration data according to a reported method [28]. Under optimal conditions, calibration graphs for the determination of CN− were constructed. The enhanced fluorescence intensity of the system showed a good linear relationship with the concentration of CN− in the range of 0–10 ␮M (R2 = 0.9964), as shown in Fig. 8. The detection limit for CN− was determined as 0.36 ␮M based on S/N = 3, which was far lower than the WHO guideline of 1.9 ␮M cyanide. To gain further insight into the nature of 1-cyanide interactions, the changes of 1 H NMR spectra and high resolution mass spectrum produced via the addition of cyanide anion to 1 were monitored. As shown in Fig. 9, it was obvious that the resonance signal corresponding to the vinyl proton (Ha ) at 8.60 ppm decreased, whereas a new signal grew at 5.24 ppm corresponding to the ␣-proton (Ha ) upon addition of CN− 1 to a CDCl3 solution (1 mM) of 1. Meanwhile,

3000

2200

2500

-

[I] =1.93E8[CN ]+262.86 2 R =0.9964

2000 2000

FL intensity at 572 nm

FL intensity at 572 nm

2400

1500

1000

500

1800 1600 1400 1200 1000 800

0 1

2

3

4

5

6

7

8

9

10

Anions Fig. 7. Selectivity of 1. The black bars represent fluorescence intensity at 572 nm of 1 in THF in the presence of other anions aqueous solution (30 ␮M). The red bars represent the fluorescence intensity that occurs upon the subsequent addition of 30 ␮M of CN− to the above solution. From 1 to 10: control, F− , Cl− , Br− , I− , H2 PO4 − , SO4 2− , CO3 2− , PO4 3− and OAc− . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

600 400 0.0

-6

2.0x10

-6

4.0x10

-6

6.0x10

-6

8.0x10

-5

1.0x10

-

[CN ] Fig. 8. The linear relation for concentration of CN− in the range of 0–10 ␮M. ex = 479 nm.

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4. Conclusions In summary, we developed a new type of DPP-based turn-on fluorescence chemodosimeter, which displayed high selectivity and sensitivity for the detection of cyanide. Addition of CN− aqueous solution to 1 in THF resulted in a rapid color change from purple to yellow, while other anions did not induce any significant color change, suggesting naked-eye selective detection of CN− using 1 became possible. In the presence of CN− , [1+CN]− exhibited a considerable excimer emission at 572 nm in THF–H2 O. Exciplex formation was responsible for the recognition process. 1 H NMR spectra and high resolution mass spectrum confirmed the cyanide anion was added to the vinyl group of 1. Acknowledgements

Fig. 9. Partial 1 H NMR changes upon the addition (a) 0 equiv.; (b) 0.4 equiv.; (c) 0.8 equiv.; (d) 1.2 equiv.; (e) 1.6 equiv.; (f) 2.0 equiv.; (g) 5.0 equiv.; (h) 10.0 equiv. of Bu4 + CN− to 1 (1.0 mM) in CDCl3 .

the indanedione protons and benzyl protons displayed upfield shift compared to those of 1 due to the breaking of the conjugation. With the addition of 2.0 equiv. of CN− , the peak at 8.60 ppm disappeared completely, which suggested that 1:2 reaction between 1 and CN− ions occurred. In Fig. 10, a high resolution mass spectrometry of negative ions mode revealed its cyanide adduct (calculated for [1+CN]− , 878.4049; found, 878.4012). All these observations clearly indicated that the cyanide anion was added to the vinyl group, and the anionic species was formed.

Fig. 10. Found (top) and calculated (bottom) high resolution mass spectrometry of negative ions mode for [1+CN]− .

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Biographies Lingyun Wang received her PhD degree in Chemistry from Sun Yat-sen University, China, in 2006. After post-doctoral training at the South China University of Technology (SCUT), she joined SCUT where she is currently an assistant professor in the School of Chemistry and Chemical Engineering. Her current research focuses on the design and synthesis of functional water-soluble conjugated polymers and exploration of their applications in chemosensors, biosensors and optoelectronic. She has over 30 peer-reviewed publications. Jiqing Du received the BSc in Chemistry in Shenzhen University, China, in 2012. Currently, he is a graduate student in South China University of Technology, China. Derong Cao is a professor in the School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, China. His research interest focuses on organic synthesis and materials science.