A fluorescein-based highly specific colorimetric and fluorescent probe for hypochlorites in aqueous solution and its application in tap water

Sensors and Actuators B 166–167 (2012) 44–49

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A fluorescein-based highly specific colorimetric and fluorescent probe for hypochlorites in aqueous solution and its application in tap water Fang-Jun Huo a,1 , Jing-Jing Zhang b , Yu-Tao Yang a,b , Jian-Bin Chao a , Cai-Xia Yin b,∗,1 , Yong-Bin Zhang a , Ting-Gui Chen b a b

Research Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, China Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China

a r t i c l e

i n f o

Article history: Received 5 September 2011 Received in revised form 23 November 2011 Accepted 29 November 2011 Available online 8 December 2011 Keywords: Hypochlorite Cuprous Fluorescein Fluorescence UV–vis spectroscopy

a b s t r a c t A fluorescein derivative, 2-pyridylaldehyde fluorescein hydrazone (FHP), was developed for detection of hypochlorites. This probe functioned by oxidizing Cu+ to Cu2+ to form a Cu2+ complex, which produced colorimetric and fluorescent signals that were monitored by ultraviolet–visible and fluorescence spectrophotometry, respectively. Other common anions, including F− , Cl− , ClO3 − , NO2 − , CN− , S2− , SCN− , P2 O7 4− , AcO− , CO3 2− , SO4 2− , ClO4 − , did not interfere with the hypochlorite detection, and hypochlorites could be detected at low micromolar levels in aqueous solutions. The color change on reaction with hypochlorites was rapid and visible to the naked eye. The sensor could be applied to the detection of hypochlorites tap water. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Sodium hypochlorite (NaOCl) is used extensively as a bleaching agent and disinfectant at concentrations ranging from 10−5 to 10−2 mol/L [1,2]. In its protonated form (hypochlorous acid, HClO), it is a biologically important reactive oxygen species (ROS) [3] and functions as an antimicrobial agent in organisms and for water treatment [4–8]. However, excess HClO can cause tissue damage and diseases such as hepatic ischemia-reperfusion injury [9], atherosclerosis [10], lung injury [11], rheumatoid [12,13] and cardiovascular diseases [14], neuron degeneration [15], arthritis [16], and cancer [17,18]. Therefore, sensitive and selective probes are required for the detection of hypochlorite (ClO− ). A number of colorimetric, luminescent, electrochemical and chromatographic methods [3,19–29] have been reported for hypochlorite detection. Recently, some recent probes have been developed for HClO based on its strong oxidation properties [25,30–34]. For example, oxidation reactions of p-methoxyphenol to benzoquinone, dibenzoylhydrazine to dibenzoyl diimide, thiol to sulfonate, and p-alkoxyaniline have been used in the design of ClO− -selective probes [35,30,36,37]. In addition, rhodamine and fluorescein-based

fluorescent probes have been reported for HClO [14,30]. Chen et al. developed two fluorescent probes for HClO that functioned in an organic co-solvent system [38]. However, most of these probes have relatively complicated structures and are not easy to synthesize. Consequently, new probes are required for the detection of HClO/ClO− . Chen et al. reported phosphorescent sensor for hypochlorite [39]. Fluorescein derivatives have been used extensively in the design chemosensors because they are water-soluble, have long excitation and emission wavelengths, high fluorescence quantum efficiencies, and can be produced via short synthetic routes [40–43]. Previously, a rhodamine-based colorimetric HClO/ClO− probe was developed that functioned by oxidizing Cu+ to Cu2+ to form a Cu2+ complex [37]. In the present study, 2-pyridylaldehyde fluorescein hydrazone (FHP) (Scheme 1), was synthesized as a probe for the detection of ClO− . This probe remained unchanged in the presence of Cu+ ions, but produced fluorescent and colorimetric signals when Cu+ was oxidized to Cu2+ in the presence of trace ClO− . 2. Materials and methods 2.1. Materials

∗ Corresponding author. Tel.: +86 351 7011022; fax: +86 351 7011022. E-mail address: [email protected] (C.-X. Yin). 1 Both authors contributed equally to this work. 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.081

4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) was purchased from Sigma–Aldrich (St. Louis, MO). FHP was synthesized using a modification of a literature method [44].

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Scheme 1. Structure (left) and thermal ellipsoids of the FHP probe drawn at the 50% probability level.

Sodium hydroxide solution (0.1 mol/L) was added to aqueous HEPES (10 mmol/L) to adjust the pH to 7.0. Cationic salts were purchased from Shanghai Experiment Reagent Co., Ltd (Shanhai, China). All other chemicals used were of analytical grade. 2.2. Instruments A (50 pH meter (Beckman Coulter, Brea, CA) was used to determine the pH. Ultraviolet–visible (UV–vis) spectra were recorded on a HP8453 spectrophotometer. A PO-120 quartz cuvette (10 mm) was purchased from Huamei Experiment Instrument Plants (Shanhai, China). 1 H NMR and 13 C NMR spectra were recorded on a Bruker DRX-300 MHz NMR spectrometer (Billerica, MA). Fluorescence spectra were measured using a Cary Eclipse fluorescence spectrophotometer (Agilent, Santa Clara, CA). A light yellow single crystal of FHP was mounted on a glass fiber for data collection. Cell constants and an orientation matrix for data collection were obtained by least-squares refinement of diffraction data from reflections between 1.45–26.0◦ using a Bruker SMART APEX CCD automatic diffractometer. Data were collected at 296 K using Mo ˚ and the ␻-scan technique, and corK˛ radiation ( = 0.710713 A) rected for the Lorentz and polarization effects (SADABS) [45]. The structures were solved by direct methods (SHELX97) [46], and subsequent difference Fourier maps were inspected and then refined in F2 using a full-matrix least-squares procedure and anisotropic displacement parameters. 2.3. Preparation of FHP FHP was synthesized in one step by reaction of fluorescein hydrazine with picolinaldehyde in methanol containing acetic acid. 2-pyridine (0.107 g) was added to 0.35 g of fluorescein hydrazine dissolved in 20 mL of methanol, and then the reaction solution was refluxed in an oil bath for 2 h. A white solid appeared, which was isolated by filtration. The crude product was recrystallized with CH3 OH and petroleum ether (v/v, 1/1), to provide FHP as a yellow powder in 60% yield. The H2 O/CH3 CH2 OH solution of FHP was allowed to evaporate slowly at room temperature for several days to produce yellow crystals suitable for X-ray crystallography. 1 H NMR (300 MHz, 25 ◦ C, DMSO-d6 ): ı 9.95 (bs, 2H), 8.57 (s, N C H, 1H), 8.49 (d, Ar H, 1H), 7.96 (t, pyridine-H, 1H), 7.81 (t, pyridine-H, 1H), 7.65 (m, Ar H, 3H), 7.35 (t, pyridine-H, 1H), 7.12 (d, pyridine-H, 1H), 6.69(d, xanthene-H, 2H, J = 2.4 Hz), 6.54 (d, xanthene-H, 2H, J = 8.8 Hz), 6.48 (dd, xanthene-H, 2H, J = 8.8 Hz, J = 2.4 Hz); 13 C NMR (75 MHz, DMSO-d6 ): ı 164.22, 158.84, 153.17, 151.90, 151.21, 149.54, 146.49, 137.00, 134.56, 129. 25, 127.93, 127.80, 124.59, 123.52, 119.21, 112.65, 109.50, 102.73, 64.95; ESI-MS m/z 436[FHP + H]+ , 458 [FHP + Na]+ ; Elemental analysis: calcd. for C26 H17 N3 O4 : C, 71.72; H, 3.94; N, 9.65%; found: C, 71.70; H, 4.01; N, 9.68%. Crystal data for C26 H17 N3 O4 : crystal size: 0.20 × 0.05 × 0.05, monoclinic, space group P21/c (No. ˚ b = 26.813(2) A, ˚ c = 16.8152(14) A, ˚ ˇ = 100.685◦ , 14). a = 9.7947(8) A, V = 4339.5(6) Å3 , Z = 4, T = 296 K,  max = 26.0◦ , 24,570 reflections

Fig. 1. Changes in the absorption spectra of FHP (40 ␮mol/L)-Cu+ in HEPES 10 mmol/L buffer (pH 7.0) on gradual addition of ClO− (0–100 ␮mol/L). Each spectrum was recorded 2 min after ClO− addition.

measured, 8501 unique (Rint = 0.0633). Final residual for 614 parameters and 8501 reflections with I > 2␴(I): R1 = 0.0686, wR2 = 0.1738 and GOF = 0.999 (Figs. 1–3). 2.4. General UV–vis and fluorescence spectra measurements A Cu+ solution was prepared by dissolving cuprous chloride in acetonitrile. FHP stock solutions were prepared in ethanol. UV–vis

Fig. 2. Optical density three-dimensional graph of the intensity of the FHP–Cu+ 502 nm peak on addition of other anions. Inset: a photograph of the color change of the probe solution in the presence of ClO− and other anions.

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Fig. 3. Reaction-time profile of probe (4.0 × 10−5 M), CuCl (8.0 × 10−5 M) and sodium ascorbate (1.0 × 10−6 M) in the presence of different concentrations of hypochlorite anions.

and fluorescence spectra were obtained in HEPES aqueous buffer (10 mmol/L, pH 7.0) solutions. Aqueous anion solutions were also prepared with deionized water. Fluorescence measurements were carried out with a slit width of 5 nm. 2.5. Detection range The main band in the UV–vis spectrum was centered at about 502 nm. The detection threshold for ClO− was 10−5 –10−6 mol/L, and at this level the color change was very obvious. Fluorescence spectra were measured from 450–650 nm with excitation at 325 nm, and the sensitivity for ClO− was 10−7 –10−5 mol/L. 3. Results and discussion 3.1. UV–vis spectra The HEPES buffer (10 mmol/L, pH 7.0) solution of FHP (40 ␮mol/L) was colorless, and did not change color in the presence of Cu+ and sodium ascorbate as a stabilizer (Fig. 4). This system was stable in air for at least 30 min. Fig. 1 shows the change in the UV–vis spectra when ClO− was added to the HEPES buffer (10 mmol/L, pH 7.0) solution containing the FHP probe (40 ␮mol/L), Cu+ , and a moderate concentration of sodium ascorbate. As the ClO− concentration

Fig. 4. Changes in the fluorescence spectra of FHP (4 ␮mol/L) with Cu+ on addition of ClO− (0–11 ␮mol/L) (ex = 325 nm, em = 518 nm, slit width: 5 nm/5 nm) in 10 mmol/L HEPES (pH 7.0) buffer. Each spectrum was recorded 48 h after ClO− addition. Inset: shows the color (left) and visual fluorescence (right) of the FHP probe complex on addition of ClO− in HEPES (pH 7.0) buffer under illumination with a UV lamp (365 nm).

Fig. 5. Changes in the spectra of FHP (40 ␮mol/L)–Cu+ in HEPES 10 mmol/L (pH 7.0) buffer on gradual addition of H2 O2 (0–100 mol/L). Each spectrum was recorded 2 min after H2 O2 addition.

increased, an absorption peak appeared at about 502 nm and gradually increased in intensity. This was accompanied by a change in the color of the solution from colorless to magenta. Isosbestic point was observed at 301 and 335 nm (Fig. 5). 3.2. pH dependence The optimum pH for the system was investigated using solution with pH values between 2.0 and 13.0. Fig. 6a showed the UV–vis absorption spectra obtained for the free probe in different pH values. The intensity of the peak at 502 nm increased in alkaline solutions compared to that at other pH values. The free probe had no absorption peak at 502 nm when the solution pH was between 2.0 and 7.0. The increase in strongly alkaline solutions would interfere with the detection of ClO− . HClO is a stronger oxidant in strongly acidic solutions than in weakly acidic solutions. However, in strongly acidic solutions, HClO will decompose rapidly. By comparison, alkaline solutions prevent generation of HCIO. Consequently, the optimum pH for this probe system was 7.0. Fig. 6b showed the UV/vis absorption spectra for FHP Cu+ ClO− . No absorption was observed between 400 and 600 nm in solutions with pH values of 2.0 and 3.0. An absorption peaks was observed at 524 nm when the solution pH was between 4 and 6. When the solution pH was between 11 and 13, the absorption peaks were weak. Between pH 8 and 10 the adsorption peaks were similar in

Fig. 6. Standard curve for the determination of ClO− in HEPES buffer (10 mmol/L, pH 7.0) plotted using the absorbance (max = 502 nm) for various concentrations of ClO− .

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Scheme 2. Proposed mechanism for the detection of ClO– using the FHP–Cu+ system in an aqueous solution.

intensity to that of the free probe. This is because alkaline solutions prevent generation of HClO.

3.3. Selectivity over other anions and metal ions The probe system (10 mmol/L HEPES, 40 ␮mol/L FHP, 80 ␮mol/L Cu+ ) showed high selectivity for ClO– over other anions, such as F− , Cl− , ClO3 − , NO2 − , CN− , S2− , SCN− , P2 O7 4− , AcO− , CO3 2− , SO4 2− , and ClO4 − (Fig. 2). Among the ions tested, only ClO− generated a large change in the UV–vis spectrum at 502 nm, and a solution color change from colorless to magenta. The presence of these other anions (up to 30 equiv.) did not interfere with the detection of ClO− (Fig. S7). Fig. S8 shows that various general metal ions, including Fe3+ did not interfere with the detection of ClO− . These results show that probe has high selectivity for ClO− , and can be used for visual determination without the need of any sophisticated instruments. This method can be used for simple qualitative and quantitative determination of ClO− , and is a large improvement over previously reported methods.

3.4. Time-dependence in the detection process of hypochlorite ions The influence of the reaction time of the FHP probe with the ClO− solution on the detection result is shown in Fig. 3. The difference between the absorption intensities at 502 nm at 30 min (A30 min ) and 2 min (A2 min ) was calculated using (A30 min − A2 min )/A2 min . The results were 5.08, 4.30, 0.415, 0.009, 0.012, 0.01, 0.009, 0.023, 0.003 for solutions with hypochlorite concentrations between 0.5 × 10−5 and 10.0 × 10−5 mol/L. When the ClO− concentration was >4.0 × 10−5 mol/L, there were no apparent differences between the absorption intensities at 2 and 30 min. When the ClO− concentration was between 0.5 × 10−5 and 2.0 × 10−5 mol/L, the absorption intensity at 30 min was five times larger than that at 2 min. These results indicate that at high ClO− concentrations, the reaction of the probe with ClO− is almost complete within 2 min, while at low ClO− concentrations, the reaction time is longer. These differences can be attributed to the different oxidation capacities of solutions with different amounts of ClO− .

3.6. Response to H2 O2 and ClO2 − The probe also produced a signal in response to H2 O2 (in 10 mmol/L, pH 7.0 HEPES buffer), but this signal was not as strong as that for ClO− (Fig. 5). When the concentration of H2 O2 was 1.0 × 10−4 mol/L, the peak intensity was about 0.17 and lower than that for ClO− (A = 0.58, 1.0 × 10−4 mol/L). The color change with H2 O2 was also not as distinct as that for ClO− (Fig. S9). The weaker signal is probably because H2 O2 is a relatively weak oxidant compared to HClO. Similarly, for other oxidants containing oxygen and/or chlorine, the signal will be related to the strength of the oxidant. The strengths of some of these oxidants relative to hypochlorous acid are in the order sodium hypochlorite > chlorous acid > chloric acid > perchloric acid. This relationship between oxidant strength and adsorption peak intensity was evident in a solution containing 1.0 × 10−4 mol/L ClO2 − , which gave a peak intensity of about 0.20 that was far lower than that for the corresponding ClO− solution (Fig. S10). 3.7. Proposed mechanism The results showed that FHP did not change in the presence of Cu+ , but produced detectable colorimetric and fluorescent signals when Cu+ ions were oxidized to Cu2+ ions in the presence of trace concentrations of ClO− . The fluorescence produced was related to ring opening/closing in FHP that occurred during its interaction with Cu2+ with FHP. In the presence of Cu+ , the ring was close and no fluorescence was produced. In the presence of HClO, Cu+ was oxidized to Cu2+ , which formed a complex with FHP and the 502 nm peak appeared in the UV–vis spectrum. However, there was no fluorescence at this point, and it only appeared after storing the FHP Cu2+ solution for a few hours. After storage, the system displayed strong fluorescence because of FHP ring opening (Scheme 2). ESI-MS of this solution showed two peaks at m/z 536 and 558 for the FHP Cu2+ complexes [FHP Cu2+ 2H2 O + H]+ and [FHP Cu2+ 2H2 O + Na]+ respectively. (Fig. S11). The color change in the presence of HClO was very distinct. The color of FHP Cu2+ complex with the FHP ring closed was magenta, and this changed to green after FHP ring opened. This compound showed green fluorescence under illumination with a UV lamp at 365 nm (Fig. S9). 3.8. Real sample analysis

3.5. Fluorescence spectra A series of solutions of FHP Cu+ ClO− in HEPES buffer with different amount-of-substance ratios for the probe complex were prepared and stored at room temperature. The fluorescence spectra of all these solutions showed an emission peak at 518 nm (ex = 325 nm) and the fluorescence intensity increased rapidly with the concentration of ClO− (Fig. 4). These results show the response of the FHP probe increases linearly with the ClO− concentration from 7.5 × 10−7 to 1.1 × 10−5 ␮mol/L. Therefore, the probe could be used to detect ClO− at low micromolar levels.

The probe was applied to detection of ClO− in tap water. No signal was produced with deionized water, even when the sample volume was 300 mL. These results show that there is no interference from deionized water. The addition of tap water (30 mL) increased the absorption intensity at 502 nm, and magnitude of the changes in the absorption intensity increased with the volume of tap water (Fig. S12). These results show that the probe is sensitive toward ClO− . The standard addition method was used to determine the concentration of ClO− in the tap water samples. A linear correlation was obtained between the absorbance and the concentration of ClO− from 0 to 1.0 × 10−4 mol/L (R2 = 0.99) (Fig. 6).

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Table 1 Results for the determination of oxalate in tap water. Sample

Determined ClO− (10−5 mol/L)a

Added ClO− (10−5 mol/L)

Found (10(5 mol/L)a

1

1.03 ± 0.06

2

1.37 ± 0.08

3

2.07 ± 0.05

1.00 2.00 3.00 1.00 2.00 3.00 1.00 2.00 3.00

2.08 3.05 4.00 2.35 3.40 4.46 3.02 4.00 5.15

a

± ± ± ± ± ± ± ± ±

0.07 0.05 0.08 0.04 0.06 0.02 0.05 0.03 0.08

% Recovery 104.9 100.7 99.3 99.1 100.9 102.1 98.4 98.3 101.6

Mean ± standard deviation (n = 5).

According to the linear regression equation, the ClO− concentrations in the three tap water samples were 1.03 × 10−5 , 1.37 × 10−5 and 2.07 × 10−5 mol/L. These results are higher than the recommended concentrations for tap water (8.4 × 10−6 mol/L) [47]. The recoveries for added ClO− were good, which shows that this method is accurate. These results indicate that this sensing system has potential for quantitative analysis of ClO− in environmental samples (Table 1). Iodometric titration was used to confirm the accuracy of these results. The hypochlorous acid concentrations calculated for the three tap water samples using iodometric titration were 1.06 × 10−5 , 1.33 × 10−5 , and 2.0 × 10−5 (Fig. S13). 4. Conclusions A fluorescein derivative, FHP, with a simple structure was developed as a colorimetric and fluorescent chemosensor for ClO− in water. In the presence of ClO− , Cu+ was oxidized to Cu2+ , which formed a complex with FHP. This produced changes in the color and fluorescence of the probe that could be monitored by UV–vis and fluorescence spectrometry, respectively. The probe could be used for qualitative and quantitative detection of ClO− in water, and the sensor was applied to analysis of tap water. This is the first report of a fluorescein derivative for rapid colorimetric and fluorescent detection of HOCl/ClO− in water. Acknowledgments The work was supported by the National Natural Science Foundation of China (Nos. 20801032, 21072119, 21102086), the Shanxi Province Science Foundation for Youths (No. 2009021006-2), the Shanxi Province Foundation for Returnee (No. 200815), Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (TYAL), the Shanxi Province Foundation for Selected Returnees (No. 2010) and Shanxi Province Graduate Innovation and Creativity Funds (20103019). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2011.11.081. References [1] T. Aoki, M. Munemori, Continuous flow fluorometric determination of ammonia in water, Anal. Chem. 55 (1983) 209–212. [2] D.Q. Zhang, Highly selective and sensitive colorimetric probes for hypochlorite anion based on azo derivatives, Spectrochim. Acta A. 77 (2010) 397–401. [3] S.M. Chen, J.X. Lu, C.D. Sun, H.M. Ma, A highly selective naked-eye probe for hypochlorite with the p-methoxyphenol-substituted aniline compound, Analyst. 135 (2010) 577–582. [4] J. Shepherd, S.A. Hilderbrand, P. Waterman, J.W. Heinecke, R. Weissleder, P. Libby, A fluorescent probe for the detection of myeloperoxidase activity in atherosclerosis-associated macrophages, Chem. Biol. 14 (2007) 1221–1231.

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Biographies Fang-Jun Huo is an associate professor in Research Institute of Applied Chemistry at Shanxi University, major in organic chemistry. His current research interests are sensors, supramolecular chemistry. Jing-Jing Zhang is studying for master in Institute of Molecular Science at Shanxi University. She received her B.Sc. in chemistry at Yuncheng University in 2010. Yu-Tao Yang is studying for master in Institute of Molecular Science at Shanxi University. She received her B.Sc. in chemistry at Shanxi University in 2008. Jian-Bin Chao is an associate professor in Research Institute of Applied Chemistry at Shanxi University major in organic chemistry. His current research interests are supramolecular chemistry. Cai-Xia Yin is a professor in Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science at Shanxi University, major in inorganic chemistry. Her current research interests are molecular recognition and sensors chemistry. Yong-Bin Zhang is an associate professor in Research Institute of Applied Chemistry at Shanxi University major in organic chemistry. His current research interests are synthesis chemistry. Ting-Gui Chen is an associate professor in Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science at Shanxi University, major in inorganic chemistry. His current research interests are biology and chemistry.