A novel turn-on fluorescent probe for cyanide detection in aqueous media based on a BODIPY-hemicyanine conjugate

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Research paper

A novel turn-on fluorescent probe for cyanide detection in aqueous media based on a BODIPY-hemicyanine conjugate Yanhua Yu a,∗ , Tingting Shu a , Bingjie Yu a , Yun Deng a , Cheng Fu a , Yangguan Gao a , Changzhi Dong a,c , Yibin Ruan b,∗ a

Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, China Technology Center of China Tobacco Guizhou Industrial Co. Ltd., Guiyang 550003, China c University Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France b

a r t i c l e

i n f o

Article history: Received 3 July 2017 Received in revised form 19 September 2017 Accepted 20 September 2017 Available online xxx Keywords: Cyanide BODIPY-hemicyanine conjugate Fluorescence enhancement Michael addition

a b s t r a c t A novel BODIPY-hemicyanine based fluorescent probe 1 for cyanide (CN− ) detection was designed and synthesized. Among the tested anions, the probe showed high selectivity towards CN− . Only addition of CN− to probe 1 in aqueous solution (VH2O /VEtOH = 1/1) could result in a remarkable blue shift of the absorption band from 385 nm to 265 nm and induce 20-fold fluorescence enhancement at 515 nm. The sensing mechanism was based on nucleophilic addition between CN− and indolium group, which was confirmed by 1 H NMR and mass spectrum analysis. Plot of fluorescence intensity as a function of CN− concentrations exhibited a good linear relationship in the range of 0–10 ␮M, with a detection limit to be 1.53 ppb. Moreover, the proposed sensing approach could work in a wide pH range from 5.0–9.0 and reach an equilibrium instantaneously. Finally, probe 1 was successfully applied in detection of CN− in natural water samples and on test paper strips with satisfactory results. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Cyanide compounds are one of the most toxic species to human health and environment [1], due to their powerful interaction with ferric iron atoms in metalloenzymes, which can disorder some important biological processes like cellular respiration and oxidative metabolism [2]. Still they are widely utilized in chemical and industrial processes, such as plastics production, gold mining, metallurgy, synthetic fibers and the resins industry [3]. Any accidental release of cyanide into the environment could cause serious health problems. It was reported that in 2015 a large amount of cyanide was leaked out in Tianjin Port by accident, which was considered to be a serious environmental disaster. Moreover, according to the World Health Organization (WHO), the maximum permissive level of cyanide in the drinking water is 1.9 ␮M [4]. Therefore, it is great of importance to develop novel approaches for cyanide anion detection. The strategies for detecting cyanide include mass spectrometry [5], electrochemistry [6], ion chromatography [7], headspace sorptive solid phase microextraction with a spectrophotometry [8], flow

∗ Corresponding authors. E-mail addresses: [email protected] (Y. Yu), [email protected] (Y. Ruan).

injection [9], colorimetric and fluorescent sensors [10–31]. Among these methods, fluorescent sensors have drawn much attention due to their various advantages over other techniques, such as simple operation, versatile adaptation, low-cost, portability, excellent sensitivity and rapid response. The reported CN− fluorescent sensors were mainly based on metal complex ensemble displacement [32–39], nucleophilic addition [40], nanoparticles and polymeric frameworks [41,42]. Among them, nucleophilic-reactions based on dicyano-vinyl [43–47], pyridinium ring [48], oxazines [49], acridinium [50], aldehyde [51,52], boronic acid [53], indolium groups [54–61], generally exhibit satisfactory selectivity due to the exceptional nucleophilicity of CN− . Indolium has recently emerged as one of most promising reactive group for CN− because of its positive charge character which results in a strong attractive force between indolium group and CN− . The detailed computational calculations further demonstrated that solvation of anions by H2 O finally give the unique selectivity for CN− [62]. However, many of them still existed several drawbacks in aqueous environment such as low sensitivity, narrow pH working range, which cannot fulfill the requirement for the real sample analysis [63–65]. Borondipyrromethene (BODIPY) derivatives have been widely used in CN− detection due to its distinct advantages such as large molar excitation coefficient, high fluorescence quantum yield, narrow emission bandwidth, high photostability, small stokes shifts

https://doi.org/10.1016/j.snb.2017.09.142 0925-4005/© 2017 Elsevier B.V. All rights reserved.

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Scheme 1. Design and synthesis of BODIPY-hemicyanine dye 1.

and tunable fluorescence characteristic [66–74]. In this work, we herein report a novel fluorescent BODIPY–hemicyanine probe 1 (as shown in Scheme 1) as a highly selective and sensitive turn-on fluorescent probe for CN− detection in aqueous solution. Hemicyanine unit containing indolium group which was incorporated to BODIPY not only acted as the binding unit for CN− , but also promoted the intramolecular charge transfer (ICT) process and quenched fluorescence of BODIPY. When hemicyanine was attacked by CN− through nucleophilic addition, ICT process was blocked which then led to turn-on fluorescence. As expected, this BODIPY-hemicyanine based probe showed excellent selectivity to CN− over other tested anions. Furthermore, It’s worthy of notice that the BODIPY group was not a good electron donor, which could lead to a more positive charge distribution on indolium group and give a fast kinetics and low detection limit for CN− detection. In our protocol the response time was within 3 s in aqueous media and the detection limit was calculated to be 59 nM, far lower than the permissive level of WHO in drinking water. Finally, this probe was successfully applied to detect CN− in real water samples and on the test paper strips. 2. Materials and methods 2.1. Materials 4-Hydroxymethylbenzaldehyde, 2,4-dimethylpyrrole, 2,3dicyano-5,6-dichlorobenzoquinone (DDQ), trifluoroacetic acid (TFA), boron trifluoride ether complex (BF3 ·OEt2 ) were purchased from Sigma-Aldrich without further purification. Triethylamine, manganese dioxide, piperidine and solvents were purchased from Aladdin (Shanghai, China). In anhydrous reaction, ethanol was dried by distillation over sodium according to the standard procedures. Solvents used for extraction treatment and column chromatography such as petroleum ether, ethyl acetate, dichloromethane and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) in commercial grade and distilled before use. All column chromatography was carried out using silica gel (200–300 mesh). Thin layer chromatography (TLC) was performed on silica gel coated on aluminum plates. Milli-Q water was used in all aqueous analytical experiments. All anions were prepared from their tetrabutylammonium (TBA+ ) salts. 2.2. Apparatus Ultraviolet-visible (UV-vis) spectra were measured on a Perkin Elmer Lambda 25 spectrometer. Fluorescence spectra measurements were performed on a Perkin Elmer LS 55 spectrometer. Perkin Elmer quartz cells with an inner path length of 10 mm were used for absorption and fluorescence spectroscopy mea-

surement. 1 H NMR, 13 C NMR spectra were collected on a Bruker Advance 400 MHz spectrometer in CDCl3 , tetramethylsilane (TMS) as internal standard, chemical shifts are given in ppm related to the protonated solvent as internal reference (1 H: CHCl3 in CDCl3 , 7.26 ppm, 13 CDCl3 in CDCl3 , 77.16 ppm), coupling constants (J) are given in Hz, the following abbreviations were used to explain the multiplicities: s = singlet; d = doublet; m = multiplet. MS spectra were recorded on a Bruker amaZon SL instrument using standard conditions (ESI). All the measurement experiments were performed at room temperature.

2.3. Synthesis and characterization 2.3.1. Synthesis of 1,3,5,6-tetramethyl8-(4-(hydroxymethyl)phenyl)BODIPY 2 1,3,5,6-tetramethyl- 8-(4-(hydroxymethyl)phenyl)BODIPY 2 was synthesized according to the literature reported procedures [52]. To a solution of 2,4-dimethylpyrrole (2.2 mL, 20.0 mmol) in deoxygenated CH2 Cl2 (150 mL) was added 4hydroxymethylbenzaldehyde (1.4 g, 10.0 mmol) and one drop of TFA at room temperature. The mixture was stirred over 12 h under argon, then treated with DDQ (2.3 g, 10.0 mmol), the mixture was continued stirring for 1 h followed by addition of Et3 N (30 mL). After stirring for 15 min, the solution was cooled to 0 ◦ C, then BF3 ·OEt2 (30 mL) was added to the solution, the mixture was stirred at room temperature for further 3 h. After the completion of reaction, washed with saturated NaHCO3 solution, the organic phase was separated, dried over MgSO4, filtered, and concentrated. The residue was purified by silica gel column chromatography (CH2 Cl2 /petroleum ether: 1/2) to give compound 2 (0.9 g, 2.5 mmol) as a red solid in 50% yield. 1 H NMR (400 MHz, CDCl3) ı 7.49 (d, J = 7.8 Hz, 2H), 7.27 (d, J = 7.8 Hz, 2H), 5.98 (s, 2H), 4.80 (s, 2H), 2.55 (s, 6H), 1.38 (s, 6H). 13 C NMR (101 MHz, CDCl3 )ı155.49, 143.10, 141.89, 141.56, 134.20, 131.48, 128.18, 127.39, 121.22, 64.66, 14.48. HRMS: calculated for [M + H]+ : 355.1793, measured: 355.1712.

2.3.2. Synthesis of 1,3,5,6-tetramethyl8-(4-(formylphenyl))BODIPY 3 To a solution of compound 2 (0.9 g, 2.5 mmol) in CH2 Cl2 (150 mL) was added MnO2 (5.4 g, 62.5 mmol), the mixture was refluxed over 12 h. After the completion of reaction, the mixture was filtered through celite, the filtrate was evaporated, then the solid residue was further purified by silica gel column chromatography (CH2 Cl2 /petroleum ether: 1/2) to give compound 3 (0.7 g, 2.0 mmol) as a red solid in 80% yield. 1 H NMR (400 MHz, CDCl3 ) ı 10.14 (s, 1H), 8.03 (d, J = 7.8 Hz, 2H), 7.51 (d, J = 7.8 Hz, 2H), 6.00 (s, 2H), 2.56 (s, 6H), 1.36 (s, 6H). 13 C NMR (101 MHz, CDCl3 ) ı 191.46, 156.26, 142.76, 141.41, 139.69, 136.68, 130.83, 130.34, 129.16, 121.64,

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14.63, 14.52. HRMS: calculated for [M + Na]+ : 375.1456, measured: 375.1475. 2.3.3. Synthesis of BODIPY-hemicyanine conjugate 1 To a solution of compound 3 (0.5 g 1.4 mmol) in anhydrous ethanol was added compound 4 (0.4 g, 1.4 mmol) and piperidine (3 drops). The mixture was refluxed over 12 h, After cooling the solid was collected, washed with anhydrous ethanol, then dried, giving a solid. The solid was purified by silica gel column chromatography (CH2 Cl2 /MeOH: 100/1) to give compound 1 (0.5 g, 0.78 mmol) as a dark purple solid in 56% yield. 1 H NMR (400 MHz, CDCl3 ) ı 8.36-8.23 (m, 3H), 7.96 (d, J = 16.3 Hz, 1H), 7.62-7.59 (m, 4H), 7.497.47 (m, 2H), 5.99 (s, 2H), 4.54 (s, 3H), 2.56 (s, 6H), 1.91 (s, 6H), 1.42 (s, 6H).13 C NMR (101 MHz, CDCl3 ) ı 182.66, 174.25, 156.14, 152.78, 143.08, 142.89, 141.53, 140.51, 139.82, 134.37, 131.70, 130.84, 130.43, 129.86, 129.48, 128.94, 125.39, 122.65, 122.01, 121.57, 115.35, 114.66, 110.90, 105.02, 52.92, 48.86, 37.66, 32.77, 28.11, 26.70, 14.88, 14.66. HRMS: calculated for [M]+ : 508.2730, measured: 508.2751. 2.4. Photophysical property study The stock solution of probe 1 (1 mM) was prepared in ethanol. The UV–vis absorption spectra of the solutions were measured in a 1 cm quartz cell from 250 nm to 600 nm at room temperature. The fluorescence spectra were measured from 475 nm to 625 nm at room temperature using an excitation wavelength at 385 nm. The spectra were acquired from 10 ␮M solutions, diluted from the stock solution with their respective solvents. 2.5. UV–vis absorption and fluorescence spectra study All the UV–vis absorption and fluorescence spectra were acquired from ethanol and water (V/V = 1/1). The stock solutions of the tested anions (F− , Cl− , Br− , I− , ClO4 − , AcO− , H2 PO4 − , NO3 − , HSO4 − , CN− ) (5 mM) were prepared by dissolving their tetrabutylammonium (TBA+ ) salts in Milli-Q water. For selectivity study, the volumes of the mixtures were adjusted to 2.5 mL by ethanol and water (V/V = 1/1) to afford the final concentration of 10 mM for the fluorophores and 80 mM for the anions. The mixture was shaken for 3 s before recording absorption and fluorescence spectra. For CN− titration study, to a solution of probe 1 (10 ␮M) was added CN− (0.1 equiv.) and the mixture was shaken for 3 s before recording absorption and fluorescence spectra. This step was repeated until reaching the desired amount of CN− . The fluorescence quantum yield (ФF ) was calculated by the standard method using Rhodamine 6G in ethanol as a reference. 2.6. Job’s plot measurements A series of solutions containing probe 1 and CN− in the mixture of ethanol and water (V/V = 1/1) were prepared, which the sum of the concentrations remained constant (10 ␮M). The molar fraction of probe 1 was varied from 0 to 1. After measuring fluorescent intensity of each solution,F(F = F(probe 1) − F(probe 1 + CN− )) was plotted against the molar fractions of probe 1. 2.7. Paper-based sensor The solution of probe 1 (0.1 mM, 20 ␮L) in aqueous solution (VH2O /VEtOH = 1/1) was pipetted onto the filter paper and then dried by air. Various anions and CN− at different concentrations (20 ␮L) were pipetted onto the surface of the spots of probe 1. After air drying, the images of the filter paper under 365 nm UV lamp were photographically recorded.

Fig. 1. Absorption spectra of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) with increasing CN− concentrations; inset: the absorption intensity at A385nm versus concentration of CN− from 0 to 60 ␮M.

3. Results and discussions 3.1. Design and synthesis of probe 1 Synthesis of probe 1 was illustrated in Scheme 1. BODIPY 2 was synthesized through a condensation of 4hydroxymethylbenzaldehyde and 2,4-dimethylpyrrole in the presence of trifluoroacetic acid (TFA) as catalyst in CH2 Cl2 solution, followed by oxidation with 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ). The boron difluoride bridge was formed by treatment with boron trifluoride diethyl etherate (BF3 ·Et2 O) and triethylamine. Hydroxyl group was oxidized by MnO2 to afford BODIPY 3 in 80% yields. BODIPY-hemicyanine conjugate 1 was prepared in 56% yields from a Knoevenagel condensation between BODIPY 3 with 1-methyl-2,3,3-trimethyl-3H-indolium, which was synthesized according to the reported procedures [53]. Compounds 2, 3 and 1 were well characterized by 1 H NMR, 13 C NMR and HRMS. 3.2. Optical properties of probe 1 With probe 1 in hand, we firstly examined its sensing ability towards CN− in aqueous solution (VH2O /VEtOH = 1/1) by UV–vis and fluorescence spectroscopy. As shown in Fig. 1, probe 1 exhibits three main absorption bands peaked at 385 and 502, 545 nm, which are respectively attributed to intramolecular charge transfer from BODIPY to hemicyanine and BODIPY’s typical absorption. With the incremental additions of CN− , the absorption band of probe 1 at 385 nm decreases progressively with appearance of a new band at 256 nm; and the absorption bands at 502 nm and 545 nm decreased slightly. An isosbestic point at 295 nm was observed, which indicated formation of a new species. Plot of change in the absorption band at 385 nm of probe 1 as a function of the added CN− concentrations ranging from 0 to 11 mM gave a good linear regression (R2 = 0.991) and reached a plateau in the presence of 2.0 equiv. of CN− (Fig. 2). Fluorescence response of probe 1 towards CN− was also investigated. As shown in Fig. 3, probe 1 exhibits an emission band centered at 515 nm upon excitation at 385 nm, which could be attributed to emission of BODIPY moiety. With increasing addition of CN− , fluorescence intensity at 515 nm increased gradually and reached a plateau in the presence of 20 ␮M CN− . Meanwhile, the fluorescence quantum yield (ФF ) increased from 0.015 to 0.33 (SI). The initial weak fluorescence emission of probe 1 was proba-

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Table 1 Comparison of CN− detection limit of probe 1 with other reported sensors. Sensors

Solvent

Detection limit

Ref.

Coumarin-based Naphthaldehyde-based Benzofurazan-based Indanedione-based BODIPY-salicylaldehyde-based Diketopyrrolopyrrole-based Gold nanodots-based Dicyanovinyl-based Carbazole-based BODIPY-hemicyanine conjugate

MeCN/buffer (V/V = 1/1) EtOH/H2 O (V/V = 95/5) CH3 CN/H2 O (V/V = 95/5) THF/H2 O (V/V = 1/9) DMSO/buffer (V/V = 9/1) THF Water DMSO/H2 O (V/V = 2/8) CH3 CN/H2 O (V/V = 95/5) EtOH/H2 O (V/V = 1/1)

9.37 ␮M 1.6 ␮M 1.5 ␮M 0.94 ␮M 0.88 ␮M 0.36 ␮M 0.15 ␮M 0.14 ␮M 0.13 ␮M 0.059 ␮M

[58] [75] [76] [16] [51] [77] [18] [78] [79] present work

Fig. 2. Plot of change in absorption band of probe 1 at 358 nm as a function of concentrations of CN− from 0 to 11 ␮M in aqueous solution (VH2O /VEtOH = 1/1).

Fig. 4. Linear relationship between fluorescence intensity ratio (I/I0 ) of probe 1 and the low concentrations of CN− from 0 to 10 ␮M in aqueous solution (VH2O /VEtOH = 1/1).

(1.53 ppb) according to the equation 3s/k, where s is the standard deviation of the blank measurements and k is the slope of the intensity ratio versus the sample concentration plot. The detection limit in present work is much lower than other reported CN− -selective sensors (Table 1) and three times lower than maximum permissible level of CN− in drinking water proposed by the World Health Organization (WHO). 3.3. Selectivity of probe 1 for CN− detection

Fig. 3. Fluorescence spectra of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) as a function of CN− concentration; inset: fluorescence intensity ratio (I/I0 ) versus concentration of CN− from 0 to 60 ␮M.

bly due to the ICT from BODIPY unit to the strong electron acceptor indolium group through its ␲-conjugated bridge. After nucleophilic attack of CN− to indolium group, the electron accepting ability of indolium group was strongly reduced and the ICT process was blocked, which then led to turn-on fluorescent signal of BODIPY. Plot of fluorescence intensity (I/I0 ) as a function of the concentrations of CN− also exhibited a good linear relationship (R2 = 0.993) in the range of 0–10 ␮M (Fig. 4). The limit of detection for CN− based on fluorescence spectral changes was calculated to be 59 nM

Selectivity of probe 1 to CN− was evaluated by screening a series of anions such as F− , Cl− , Br− , I− , ClO4 − , AcO− , H2 PO4 − , NO3 − , HSO4 − , CN− in aqueous solution (VH2O /VEtOH = 1/1). As shown in Figs. 5 and 6, only addition of CN− can lead to a considerable absorption spectral change and turn-on fluorescence emission, while other tested anions show no influence. This demonstrated that probe 1 showed excellent selectivity for CN− over the other tested anions. Emission color changes from blank to green were easily observed by the naked-eye under the illumination with a 365 nm UV-lamp (Fig. 7). While color changes from orange to pink under daylight could be only be observed with increasing the solution concentration to 0.1 mM (Fig. S1) 3.4. Competition studies of probe 1 for CN− detection To further confirm practical applicability of probe 1 for CN− detection, a competition experiment was carried out. As shown in Fig. 8, introducing the interfering anions including F− , Cl− , Br− , I− , ClO4 − , AcO− , H2 PO4 − , NO3 − , HSO4 − , CN− to the solution of probe 1 did not cause any fluorescence emission changes. A significant fluorescence enhancement was observed after the addition of CN−

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Fig. 5. Absorption spectra of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) in the presence of 8 equiv. different anions.

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Fig. 8. Fluorescence intensity ratio (I/I0 ) of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) in the presence of different anions (80 ␮M) without/with CN− (40 ␮M).

Fig. 6. Fluorescence spectra of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) in the presence of 8 equiv. different anions. (lex = 385 nm, slit: 5 nm/5 nm).

in all the interfering samples. This phenomenon demonstrated that probe 1 had excellent selectivity for CN− detection over the other anions even in the complicated environment. 3.5. pH influence It’s well known that the existing forms of CN− are highly pHdependent, which would then strongly affect the nucleophilic addition reaction. Therefore, pH influence on spectral response of probe 1 to CN− was investigated at different pH values (10 mM NaH2 PO4 -citric acid buffer for pH 2.2–8.0 and 10 mM glycine-

Fig. 9. Fluorescence intensity at 515 nm of probe 1 (10 ␮M) in the absence and presence of CN− (80 ␮M) as a function of pHs.

hydroxide buffer for pH 9.0–10.6). As depicted in Fig. 9, no significant fluorescence changes could be observed at pH values lower than 9 in the absence of CN− , while a 4-fold fluorescence enhancement was detected when pH was higher than 10. This might be attributed to the nucleophilic attack of OH− to probe 1. In presence of CN−, almost no spectral change could be observed at pH below 4, which indicated that no reaction occurred between probe 1 and HCN. While in the pH range from 5 to 9, the fluorescence intensity at 515 nm of probe 1 increased remarkably in presence

Fig. 7. A visual fluorescence changes of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) in the presence of 8 equiv. different anions under illumination with a 365 nm UV lamp.

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Scheme 2. The proposed mechanism of probe 1 for CN− detection.

Table 2 Recovery data for CN− detection in spiked water samples.

Fig. 10. Time-dependent fluorescence intensity at 515 nm of probe 1 (10 ␮M) in aqueous solution (VH2O /VEtOH = 1/1) in the presence CN− (20 ␮M).

of CN− . The results indicated that the probe 1 was stable in the pH range from 2 to 9 and could successfully detect CN− in a range of pH values from 5 to 9.This is more applicable in real sample when compared with other reported results. 3.6. Effect of reaction time Using chemodosimeters as a sensing approach usually takes minutes or hours to complete the reactions, which restricts their applications in the real-time detection. As shown in Fig. 10, we use fluorescence spectroscopy to monitor the reaction between probe 1 and CN− in aqueous solution (VH2O /VEtOH = 1/1). Quite surprisingly, this reaction was very fast and reached a plateau instantaneously, which might be attributed to the poor electron-donating ability of BODIPY unit in probe 1. Due to this fast kinetics, it showed promising applications in real-time detection for CN− . 3.7. Mechanism study Based on the data collected from the absorption and fluorescence spectra, it’s deduced that nucleophilic addition between CN− and hemicyanine moiety in probe 1 was responsible for the blue

Samples

Spiked (␮M)

Found (␮M)

Recovery (%)

RSD (%)

Lake water

5.0 8.0

5.23 ± 0.11 7.87 ± 0.12

104.05 ± 2.03 98.39 ± 1.51

1.81 1.35

Minerals water

5.0 8.0

5.13 ± 0.18 8.17 ± 0.20

102.56 ± 3.73 102.16 ± 2.28

3.64 2.79

Drinking water

5.0 8.0

5.27 ± 0.19 7.83 ± 0.14

105.46 ± 3.67 97.93 ± 1.81

1.67 1.68

shift of the absorption spectrum and fluorescence enhancement. To confirm this plausible mechanism, 1 H NMR titration with CN− was carried out in CDCl3 . As illustrated in Scheme 2 and Fig. 11, upon addition of TBACN to probe 1, the vinyl protons (Ha and Hb ) displayed upfield shifts from 8.25 ppm, 8.00 ppm to 6.62 ppm, 6.33 ppm and the N-methyl protons (He ) displayed an upfield shift from 4.55 ppm to 3.36 ppm. Moreover, the singlet peak corresponding to C-methyl protons (Hc and Hd ) in probe 1 shifted from 1.90 ppm (s, 6H) to 1.58 ppm (s, 3H) and 1.33 ppm (s, 3H), which indicated C-methyl was adjacent to a chiral center. All these information supported the above assumption that nucleophilic attack of CN− to hemicyanine moiety of probe 1 led to formation of 1–CN adduct, in which the positive charged indolium group changed into a neutral indole group with poor electron-withdrawing property. Job’s plot analysis also revealed that the probe 1 formed a 1: 1 adduct with CN− (Fig. S2). Furthermore, 1–CN adduct was further confirmed by mass spectrometry analysis, where a peak at m/z 573.73 was observed, corresponding to [1-CN-K]+ (Fig. S3). All these results strongly support the proposed mechanism that CN− attacks the hemicyanine moiety of probe 1.

3.8. CN− detection in spiked water samples To investigate the practical application of probe 1 for CN− detection in complicated environment, probe 1 was applied to determine CN− in the real water samples, such as lake water, mineral water and drinking water. The water samples were spiked with set amount of CN− (5 and 8 ␮M). As shown in Table 2, CN− was tested with a satisfactory analytical accuracy and precision (RSD < 4%).

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Fig. 11.

1

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H NMR spectra of probe 1 in CDCl3 in the absence and presence of CN− (1.0 equiv.).

Fig. 12. The photos of the filter paper containing probe 1 after exposure to different anions under UV light (365 nm).

Fig. 13. The photos of the filter paper containing probe 1 after exposure to CN− solution at different concentrations under UV light (365 nm).

3.9. CN− detection on filter paper To verify if this approach could be utilized as a portable sensing device, the paper test strips were constructed. As shown in Fig. 12, after dropping the solution of probe 1 (100 ␮M) on the neutral filter paper and drying them by air, no fluorescence emission was observed under UV light (365 nm). The filter paper containing probe 1 were carefully exposed to different anions solutions (100 ␮M), such as F− , Cl− , Br− , I− , ClO4 − , AcO− , H2 PO4 − , NO3 − ,

HSO4 − , CN− . Only CN− could switch on the emission color from dark to green. Moreover, CN− at different concentrations were tested by the paper-based sensor. As depicted in Fig. 13, dropping the solution of CN− at different concentrations from 5 ␮M to 100 ␮M on the filter paper containing probe 1, the green emission can be clearly observed, the more concentrated of the CN− solutions, the brighter colors are observed on the filter paper. These results demonstrated that probe 1 could selectively detect CN− based on a portable device by naked eye.

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4. Conclusions In summary, a BODIPY-hemicyanine based turn-on fluorescent probe 1 has been designed and easily synthesized for sensitive and selective CN− detection. The detection limit was calculated to be 59 nM in aqueous media. Nucleophilic addition of CN− to indolium interrupted the ␲-conjugation system and restore fluorescence of BODIPY unit. Combined its fast kinetics and wide pH range, probe 1 could be a good approach for the real-time, on-spot, selective and sensitive detection of CN− in the complicated systems. Moreover, probe 1 was successfully applied to detect CN− in real water samples and on paper test strips with satisfactory results.

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Acknowledgement

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This research work was financially supported by National Natural Science Foundation of China (No. 21708015).

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2017.09.142.

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Biographies Dr. Yanhua Yu has earned her MS degree under the supervision of Prof. Jie Tang at East China Normal University in 2010 and PhD degree under the supervision of Prof. Joanne Xie at École normale supérieure de Cachan (France) in 2013. She then moved to Jianghan University as Assistant Researcher. Her research field refers to analytical chemistry and organic chemistry. Her research subject of PhD focused on the rational design and synthesis of fluorescent molecules based on benzothiadiazole, coumarin, BODIPY and DCM with click reaction and investigation on their applications in biology and analytical chemistry. In Jianghan University, her research work is focused on fluorescent probes and fluorescent peptide synthesis. Dr. Yibin Ruan, currently a staff of Technology Center of China Tobacco Guizhou Industrial Co. Ltd., worked with Prof. Yun-Bao Jiang at Xiamen University for his MS degree. He received his PhD degree under the supervision of Dr. Isabelle Leray and Prof. Joanne Xie at École normale supérieure de Cachan (France). And then he spent 6 months as a post-doc to work in Laboratoire de Spectrochimie Infrarouge et Raman with Dr. Stéphane Aloïse. His research interests are the development of molecular sensors for anionic and cationic species and photophysics of metal-complex in the excited state. Prof. Changzhi Dong has earned his BS and MS degrees from Beijing University in 1983 and 1986 respectively. He moved then to Yantai University as Assistant Professor. In 1990, he was awarded of an one-year scholarship from the French Atomic Energy Commissariat (CEA) to work as a visiting scholar in CEA Cadarache Research Centre. He pursued then in France his education and defended successfully his PhD thesis in 1996 in the School of Pharmacy, Paris V University with the major in medicinal chemistry under the supervision of Dr. Bernard P. Roques. He spent then one year as a post-doc to work in Dr. Marc Julia’s lab in the Chemistry Department of “Ecole Normale Supérieure de Paris (ENS Paris)”. He was appointed as assistant professor in 1997 and full professor in 2011 in Paris VII University. From 2001–2003, he has worked as a research associate in Ziwei Huang’s lab in UIUC, USA.S ince 2011, he has been the PI of the Peptide and Protein Chemistry Center of the Institute for Interdisciplinary Research, Jianghan University. The research interests of Prof. Dong mainly focus on two domains: drug development and peptide/protein chemistry. In the first one, he has worked on design, synthesis and SAR study of small organic compounds for the treatment of different diseases, such as AIDS, AD, cancer and inflammation.In the second one, he has performed total chemical synthesis of certain small proteins with full biological activities and their derivatives and these works provided powerful tools for the structure-function relationship study of these proteins. He is recently interested also in the development of chemical sensors for detecting ions, small molecules or peptides.

Please cite this article in press as: Y. Yu, et al., A novel turn-on fluorescent probe for cyanide detection in aqueous media based on a BODIPY-hemicyanine conjugate, Sens. Actuators B: Chem. (2017), https://doi.org/10.1016/j.snb.2017.09.142