Reaction-Based Fluorescent Probe for Selective Discrimination of

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Reaction-Based Fluorescent Probe for Selective Discrimination of Thiophenols over Aliphaticthiols and Its Application in Water Samples Zheng Wang,‡ De-Man Han,*,† Wen-Ping Jia,† Qi-Zhong Zhou,† and Wei-Ping Deng*,‡ †

Department of Chemistry, Taizhou University, 605 Dongfang Road, Linhai 317000, China School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

‡

S Supporting Information *

ABSTRACT: The development of highly sensitive and selective detection techniques for the discrimination of relevant toxic benzenethiols and biologically active aliphatic thiols is of considerable importance in the fields of chemical, biological, and environmental sciences. In this article, we describe a new design of reaction-based fluorescent probe for discrimination of thiophenols over aliphaticthiols through intramolecular charge transfer pathways using N-butyl-4amino-1,8-naphthalimide as a fluorophore, the strongly electron-withdrawing 2,4dinitrobenzenesulfonamide group as a recognition unit, and 2,3-dihydroimidazo[1,2-a] pyridine moiety as a linker. This rational design not only affords finely tunable spectroscopic properties by adding 2,3-dihydroimidazo-[1,2-a] pyridine moiety but also provides the chance to regulate the selectivity and sensitivity of the probe due to the formation of a new type of potentially reversible sulfonamide bond through 4-dimethylaminopyridine-like resonance. The developed probe displayed high off/on signal ratios, good selectivity, and sensitivity with a detection limit of 20 nM and a relative standard deviation of 1.7% for 11 replicate detections of 0.33 μM thiophenol and was successfully applied to the determination of thiophenols in water samples with quantitative recovery (from 94% to 97%) demonstrating its application prospect for thiophenols sensing in environmental and biological sciences.

T

charge transfer (ICT) mechanism in spite of the drawbacks of its relatively weak fluorescence intensity and low sensitivity.8 Recently, several fluorescent probes with high sensitivity and selectivity toward thiophenols have been reported.9−12 However, to the best of our knowledge, only one fluorescent probe has been used for detection of thiophenols in real environmental and biological samples.10 A brighter and more selective probe for thiophenols is still highly desirable from a practical application point of view. Herein, we report a rational fabrication of a novel type of highly sensitive and selective fluorescent probe toward thiophenols through ICT pathways by selecting N-butyl-4amino-1,8-naphthalimide as a fluorophore, 2,4-dinitrobenzenesulfonamide group as a recognition unit, and 2,3-dihydroimidazo-[1,2-a] pyridine (DHIP) moiety as a linker to extend the fluorophore and reactive sulfonamide bond. The practicality of this probe was further demonstrated through the detection of thiophenols in water samples.

hiols are an important class of molecules in biological systems and chemical science. Aliphaticthiols are found in several biologically important molecules, including cysteine, homocysteine, and glutathione,1−3 which are associated with a wide range of biological functions, whereas thiophenols, as widely used chemical intermediate for pesticides, pharmaceuticals, and amber dyes, are a class of highly toxic and pollutant compounds with a median lethal dose (LC 50) of 0.01−0.4 mM for fish.4,5 Long-term exposure to thiophenols can cause severe damage to the central nervous and other nervous systems including shortness of breath, muscular weakness, paralysis of the hind limbs, coma, and even death. Therefore, a number of analytical methods such as high-performance liquid chromatography- (HPLC) based methods have been reported for determination of thiols.6a Nevertheless, a simple method that can selectively differentiate toxic thiophenols over biologically important aliphaticthiols is still of considerable significance. Recently, great efforts have been directed toward the development of fluorescent probes for thiols.6b However, these previous fluorescent probes are designed mainly for discrimination of aliphaticthiols such as cysteine, glutathione, and homocysteine from other amino acids, and in general they exhibit poor selectivity for aliphatic thiols over thiophenols.7 A probe capable of distinguishing aliphatic thiols and thiophenols had not been realized until Wang et al. developed the first fluorescent probe for thiophenols based on an intramolecular © 2012 American Chemical Society

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EXPERIMENTAL SECTION Materials and Chemicals. All reagents and solvents were of the highest available purity and at least of analytical grade. 2-

Received: February 21, 2012 Accepted: April 26, 2012 Published: April 26, 2012 4915

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Scheme 1. Synthesis Route of Probe 1

compound 3 in MeOH was hydrogenated over 10% Pd/C at a 1 bar H2 pressure for 3 h to afford compound 4. Under nitrogen, the reaction of compound 4 with 6-bromo-2-butyl1H-benzo[de]-isoquinoline-1,3(2H)-dione afforded compound 5. Next, 1 M solution of tetrabutylammonium fluoride (TBAF) in tetrahydrofuran (THF) was dropped to a solution of compound 5 in THF, the reaction mixture was extracted and then treated with Et3N followed with methanesulfonyl chloride (MsCl). The crude product was purified by preparative thin layer chromatography (silica gel) to give the compound 2 as a dark red solid. A solution of compound 2 in anhydrous CHCl3 was added to 2,4-dinitrobenzene-1-sulfonyl chloride under N2. After being stirred at room temperature overnight, the reaction mixture was treated with Et2O. The precipitate was collected to afford probe 1 as a yellow solid. 1H NMR (400 MHz, CDCl3) for major isomer: δ 12.79 (s, 1H), 9.16 (s, 1H), 8.64−7.89 (m, 5H), 7.62 (brs, 2H), 7.50−7.02 (m, 6H), 6.92 (brs, 2H), 5.88 (brs, 1H), 4.88 (brs, 1H), 4.51 (brs, 1H), 4.06 (brs, 2H), 1.63 (brs, 2H), 1.39 (brs, 2H), 0.94 (brs, 3H); 13C NMR (100 MHz, CDCl3): δ 163.4, 163.1, 156.1, 156.0, 150.4, 147.7, 147.2, 146.3, 143.4, 140.2, 139.0, 138.7, 133.9, 133.3, 131.2, 128.9, 128.6, 127.3, 126.7, 126.1, 125.6, 125.4, 122.5, 122.2, 121.1, 120.7, 119.4, 119.0, 118.3, 105.3, 89.1, 64.1, 59.1, 57.3, 52.8, 40.2, 30.0, 20.4, 13.8; HRMS (ESI, m/z): Calcd for C35H26N6O8S+: 693.1762, found: 693.1763; IR ν 2958, 1701, 1655, 1586, 1540, 1389, 1353, 1237, 788, 754 cm−1. Measurement Procedures. To a 6 mL calibrated test tube, phosphate buffer solution (0.01 M, 1 mL, pH 8.0), aqueous solution of probe 1 (0.2 mM, 0.1 mL), and certain amounts of thiophenol standard solution (0.2 mM) or other various testing species (2.0 equiv) were sequentially added at room temperature. After 20 min, the reaction solution was diluted to volume with ultrapure water, mixed thoroughly, and sampled for fluorescence measurements at an excitation wavelength of 481 nm and an emission wavelength of 590 nm. Real Samples. Water samples were collected from Linjiang River and Chemical Factory wastewater near Linhai city, respectively. The water samples were filtered through 0.2 μm cellulose acetate membranes (Whatman, UK) prior to use. The pretreated samples (3 mL) were then subjected to the determination of the concentrations of thiophenol by using probe 1.

chloro-4-nitropyridine was purchased from TCI Development Co. Ltd. (Shanghai, China). Palladium 10% on carbon, 4methylthiophenol, thiohydracrylic acid, propylmercaptan, cysteine, acetylcysteine, homocysteine, glutathione and histidine were from Aladdin Reagent Corporation (Shanghai, China). Pd(OAc)2, 2,2′-bis(diphenylphosphino)-1,1′-binaph-thalene, tetrabutylammonium fluoride were from Alfa Aesar. Triethylamine was from Sigma-Aldrich. Pd2(dba)3, 2,4-dinitrobenzene1-sulfonyl chloride were from J&K Scientific Ltd. (Shanghai, China). Thiophenol was from Shoufu Chemical Co. (Zhejiang, China). Milli-Q ultrapure water was used in all experiments. Instrumentation. 1H nuclear magnetic resonance (NMR) spectra were recorded on the Bruker AVANCE DPX-400 NMR Spectrometer (400 MHz) using an internal deuterium lock for the residual protons in CDCl3 (δH = 7.26) at ambient temperatures. Data were presented as follows: Chemical shift (in ppm on the scale relative to δTMS = 0), integration, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, and m = multiplet), coupling constant (J/Hz), and interpretation. 13 C NMR spectra were recorded on the Bruker AVANCE DPX-400 NMR Spectrometer (100.6 MHz) by broadband spin decoupling using an internal deuterium lock for CDCl3 (δ = 77.2) at ambient temperatures. Chemical shift values were given in ppm on the scale (δTMS = 0). FTIR spectra (4000−400 cm−1) were collected on a Varian Excalibur 3100 FTIR spectrometer. HRMS (EI) spectra were obtained on a Finigann MAT8401 instrument. All pH measurements were made with a Sartorius basic pH meter PB-10. Fluorescence spectra were determined on a Varian Cary Eclipse Fluorescence Spectrophotometer with a 1 cm quartz cell. Comparative analysis was performed on a FULI 9790 gas chromatograph equipped with a flame photometric detector (GC-FPD). A KB-5 (30 m × 0.53 mm ID × 0.50 μm) capillary column was used for the analysis under the following conditions: Initial temperature 100 °C for 1 min, then programmed from 100 to 250 °C at 30 °C/min; injection and detector temperature were set at 250 and 220 °C, respectively; nitrogen was used as carrier gas; injections were carried out in the splitless mode, and the injection volume was 1 μL. Synthesis of Probe 1. Probe 1 was prepared in five steps according to the route shown in Scheme 1. Reaction of 2chloro-4-nitropyridine with (S)-2-(tert-butyl-dimethylsilyloxy)1-phenylethanamine afforded compound 3. A suspension of 4916

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RESULTS AND DISCUSSION Design of the Fluorescent Probe. Conjugation of a wellestablished and efficient recognition site with a suitable signaling moiety is the most popular and rewarding approach, which has been widely used for the design of selective and sensitive probes for discrimination between chemically closely related analysts.11 As a signaling unit, fluorophores generally have many desirable characteristics in both high sensitivity and ease of signal transduction. Among many of the fluorophores, naphthalimide fluorescent dyes have been recognized as the promising ones for the construction of molecular probes because of their excellent characteristics, such as intense fluorescence quantum yields, high photo- and chemo-stability, and ease of modification.13 However, to our knowledge, no reports have been published concerning naphthalimide as a signaling unit for discrimination of thiophenols and aliphaticthiols so far. Recently, DHIP has emerged as a core structure of acylation catalysts for asymmetric kinetic resolution.14 With the inspiration from Wang’s design8 and high nucleophilic property of DHIP core (Scheme 2), we envisaged that the incorporation

We hypothesized that the reaction of the nucleophilic nitrogen of DHIP derivatives with an electron-withdrawing 2,4dinitrobenzenesulfonyl chloride would give rise to a nonfluorescent molecule. In principle, the resulting sulfonamide can be readily cleaved by nucleophile such as thiophenol under basic conditions (Scheme 3), and the removal of 2,4dinitrobenzenesulfonyl group should recover the highly fluorescent compound. By taking advantage of the unique reactivity profile, a new highly selective fluorescent probe 1 for thiophenols based on an ICT mechanism with improved features of water solubility and high sensitivity was designed and synthesized. Fluorescence Property and Optimal Measurement Conditions. The fluorescence property of probe 1 was examined in the absence and presence of a thiol and the optimal measurement conditions was established. Its water solubility enables us to perform the investigation in an aqueous phosphate buffer solution containing probe 1 at room temperature in the absence and presence of a thiophenol with different concentrations. After 20 min, the reaction solution was diluted with ultrapure water (the total concentration of probe 1 was 3.3 × 10−6 M) for emission measurement (λex = 481 nm). As expected, probe 1 exhibited almost no fluorescence in the absence of a thiophenol at λem = 590 nm (Figure 1). When thiophenol (6.6 × 10−6 M, 2.0 equiv) was added, a dramatic increase in fluorescence intensity (>60 times) was observed. The reaction product was monitored and confirmed by a comparison study with a standard pure compound 2 through 1H NMR analysis. The quantum yield of fluorescence for the product 2 was determined to be 0.36. Kinetic studies indicate that probe 1 reacted rapidly with thiophenol in aqueous solution (pH 8.0) at room temperature (Figure S1 of the Supporting Information). A pronounced intensity increase was obtained even after 5 min. The reaction completed after around 15 min. A limited change of fluorescence intensity was observed when longer reaction time was examined. Therefore, a reaction time of 20 min was selected to explore the selectivity of probe 1 toward thiols. Next, we evaluated the effect of reaction pH on probe 1. As designed, no fluorescence intensity enhancement of probe 1

Scheme 2. Design of a Fluorescent Probe for Thiophenols

of DHIP moiety into fluorophore 1,8-naphthalimide would generate a new probe not only affording finely tunable spectroscopic properties by adding DHIP moiety but also providing the chance to regulate the selectivity and sensitivity of the probe due to the formation of a new type of potentially reversible sulfonamide bond through DMAP-like resonance.15

Scheme 3. Reaction Mechanism of the Fluorescent Probe for Thiophenols

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Figure 1. Fluorescence emission spectra of the probe 1 in the absence and presence of a thiophenol with different concentrations. Probe 1 was studied in a phosphate buffer solution at room temperature in the absence and presence of a thiophenol with different concentrations. After 20 min, the reaction solution was diluted with ultrapure water (the total concentration of probe 1 was 3.3 × 10−6 M) for emission measurement (λex = 481 nm).

was observed for either thiophenol or aliphatic thiols at pH 11. Probe 1 was treated with a variety of thiols including thiophenol, 4-methylthiophenol, thiohydracrylic acid, cysteine, propylmercaptan, acetylcysteine, homocysteine, glutathione, and other nucleophile such as histidine to evaluate the selectivity. As shown in Figure 2, the fluorescence intensity of probe 1 was highly enhanced by adding thiolphenols, whereas it did not change noticeably by adding representative aliphatic thiols, such as propylmercaptan, cysteine, glutathione. This phenomenon can be explained as follows: In a pH 8.0 reaction medium, the high degree of dissociation of thiophenols resulted in the predominant generation of the corresponding thiolate, which could effectively react with 2,4-dinitrobenzenesulfonamide. However, under the same reaction conditions, the aliphatic thiols remained as a less reactive neutral form and thus the cleavage of the sulfonamide was very slow. Furthermore, the nucleophilic capacity of thiophenols is stronger than that of aliphaticthiols. More significantly, adding the same amount of any other nucleophiles (such as cysteine, glutathione) to the thiophenol solution resulted in a similar fluorescence intensity increase to that of a pure thiophenol, which indicates that probe 1 is particularly selective toward thiophenol without any obvious interference. An interference test was carried out to further demonstrate the selectivity of probe 1. The results show that no significant change in the fluorescence response of probe 1 toward thiophenol was observed in the presence of other substances (2.0 equiv) such as amino acids, sugar, vitamins, aromatic alcohols and amines, or aliphatic thiols (part B of Figure 2) indicating no interference from these substances. The sensitivity of probe 1 was then examined by the fluorescence response of the probe toward different concentrations of thiophenol under the same reaction conditions described above (Figure 3). The increase in fluorescence intensity was displayed in a concentration dependent manner. However, when more than 2 equiv of thiophenol were used, the enhancement of fluorescence intensity reached a maximum

Figure 2. (A) Fluorescence responses of probe 1 toward thiols and other substances (2.0 equiv) in a phosphate buffer solution at room temperature. After 20 min, the reaction solution was diluted and sampled for fluorescence measurement at λex = 481 nm (the total concentration of probe 1 was 3.3 × 10−6 M). The fluorescence intensity at λem = 590 nm was plotted versus substances: (1) 4-methyl thiophenol, (2) thiophenol, (3) glutathione, (4) cysteine, (5) homocysteine, (6) acetylcysteine, (7) thiohydracrylic acid, (8) propylmercaptan, (9) histidine, (10) glucose, (11) vitamin C, (12) phenol, (13) aniline, (14) alanine, (15) blank. (B) Fluorescence response of probe 1 toward thiophenol in the presence of other substances (2.0 equiv): (1) thiophenol, (2) thiophenol + glutathione, (3) thiophenol + cysteine, (4) thiophenol + homocysteine, (5) thiophenol + acetylcysteine, (6) thiophenol + thiohydracrylic acid, (7) thiophenol + glucose, (8) thiophenol + vitamin C, (9) thiophenol + phenol, (10) thiophenol + aniline, (11) thiophenol + alanine.

Figure 3. Enhanced fluorescence intensity of probe 1 versus the concentration of added thiophenol at λem = 590 nm. The inner panel displays the fluorescence enhancement of probe 1 toward thiophenol from 3.3 × 10−8 M to 3.3 × 10−6 M.

without further alteration. Notably, a pronounced change in the fluorescence signal was observed when the thiophenol concentration was 3.3 × 10−7 M (0.1 equiv). The detection 4918

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Table 1. Comparison of Fluorescent Probes for Thiophenols

Table 2. Analytical Results for the Determination of Thiophenols in Real Water Samples sample

the proposed method thiophenol spiked (M)

Linjiang river

wastewater

0 3.96 3.96 3.96 0 3.96 3.96 3.96

× 10−8 × 10−7 × 10−6 × 10−8 × 10−7 × 10−6

GC-FPD method

thiophenol determined (mean ± s, n = 3) (M) not detected (3.75 ± 0.08) (3.85 ± 0.06) (3.83 ± 0.05) (5.77 ± 0.12) (9.47 ± 0.12) (4.35 ± 0.07) (3.85 ± 0.06)

× × × × × × ×

10−8 10−7 10−6 10−8 10−8 10−7 10−6

limit was estimated to be 2.0 × 10−8 M based on a signal-tonoise ratio of 3, which was much lower than that of the ICT probe reported by Wang et al.,8 and a good precision (relative standard deviation) of 1.7% was obtained for 11 replicate detections of 3.3 × 10−7 M thiophenol. A summary of the comparison of recently developed fluorescent probes for thiophenols is given in Table 1. The probe 1 showed its excellent analytical performance. Detection of Thiophenols in Water Samples. To validate the practicality of this new method, probe 1 was employed to determine thiophenols concentrations in water samples from Linjiang River and Chemical Factory wastewater. The water samples were directly determined first, and then spiked with thiophenol at different levels of 3.96 × 10−8, 3.96 × 10−7, and 3.96 × 10−6 M. The recoveries of thiophenol ranged from 94% to 97%. To validate the proposed method, a GCFPD Method was used as a reference method. The analytical results obtained by the proposed method were in good agreement with those obtained by the GC-FPD method. The above results show that the thiophenol in the water samples could be accurately measured with good recovery, indicating that probe 1 is effective for quantitative detection of benzenethiols in water samples.

thiophenol spiked (M) 0 3.96 3.96 3.96 0 3.96 3.96 3.96

thiophenol determined (mean ± s, n = 3) (M) not detected (3.64 ± 0.11) (3.65 ± 0.08) (3.73 ± 0.06) (5.42 ± 0.18) (9.04 ± 0.18) (4.20 ± 0.08) (3.77 ± 0.07)

× 10−8 × 10−7 × 10−6 × 10−8 × 10−7 × 10−6

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CONCLUSIONS

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ASSOCIATED CONTENT

× × × × × × ×

10−8 10−7 10−6 10−8 10−8 10−7 10−6

We have successfully developed a novel, sensitive, and highly selective fluorescence probe 1 for thiophenols based on the analyte reactivity profile and reaction conditions through ICT process. Dramatic fluorescence intensity enhancement in the presence of thiophenol was observed as a result of effective nucleophilic cleavage of the electron withdrawing 2,4dinitrobenzenesulfonyl moiety from nonfluorescent probe 1 to generate highly fluorescent reagent 2 in an aqueous weakly basic buffer solution in very short reaction time. Moreover, its excitation and emission wavelength were 481 and 590 nm respectively, which would have less background interferences. These good features of the probe warranted its practical application for the determination of highly toxic thiophenols in environmental samples with satisfactory results.

S Supporting Information *

Detailed description of the synthesis and additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. 4919

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AUTHOR INFORMATION

Corresponding Author

*Fax: (86)576-85137029, e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Natural Science Foundation of China (21172068, 21172166), and Natural Science Foundation of Zhejiang Province (Y4090166).

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