Sensing hydrogen peroxide involving intramolecular charge transfer pathway: A boronate-functioned styryl dye as a highly selective and sensitive naked-eye sensor

Analytica Chimica Acta 658 (2010) 175–179

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Sensing hydrogen peroxide involving intramolecular charge transfer pathway: A boronate-functioned styryl dye as a highly selective and sensitive naked-eye sensor Xin-Qi Zhan a,∗ , Bing-Yuan Su b , Hong Zheng c , Jiang-Hua Yan a,∗ a

Cancer Research Center of Medical College, Xiamen University, Xiamen 361005, China Xiamen Center for Disease Control and Prevention, Xiamen 361021, China c Key Laboratory of Analytical Sciences, Ministry of Education, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China b

a r t i c l e

i n f o

Article history: Received 15 June 2009 Received in revised form 3 November 2009 Accepted 10 November 2009 Available online 16 November 2009 Keywords: Colorimetry sensor Hydrogen peroxide Merocyanine

a b s t r a c t The synthesis, properties and applications of a novel boronate-functioned styryl dye, BSD, as a colorimetric sensor for hydrogen peroxide is presented. The dye displayed remarkable color change from colorless (max = 391 nm) to deep red (max = 522 nm) in the presence of H2 O2 and the behavior could be rationalized by the chemoselective H2 O2 -mediated transformation of arylboronate to phenolate, resulting in the release of the merocyanine dye which featured with strong intramolecular charge transfer (ICT) absorption band. The absorption increment of merocyanine at max = 522 nm (ε = 87000 L mol−1 cm−1 ) is linear with the concentration of H2 O2 in the range of 1.0 × 10−7 –2.5 × 10−5 mol L−1 with the detection limit of 6.8 × 10−8 mol L−1 under optimum conditions. There is almost no interference by other species that commonly exist due to the specific deprotection of H2 O2 towards arylboronate group on BSD. The chromogenic sensor has been applied to the detection of trace amounts of hydrogen peroxide in rain water. © 2009 Elsevier B.V. All rights reserved.

1. Introduction As an biochemically and environmentally relevant species, hydrogen peroxide is considered as the most efficient oxidant for the conversion of dissolved sulphur dioxide to sulphuric acid which is one of main contributors to the acidification of rain water. Besides, H2 O2 is a widely used chemical reagent as an essential mediator in many fields such as food, pharmaceutical, clinical and bleaching-related industries. The oxidative damage resulting from cellular imbalance of H2 O2 is connected to aging and severe human diseases. As a consequence, intense research efforts have been directed to develop the analytical methods for the detection of H2 O2 , such as amperometry [1], fluorimetry [2] and chemiluminescence [3]. A number of spetrophotometry methods have been proposed for the determination of H2 O2 [4–5], however, most of them are based on peroxidase-catalysis reaction and their application is limited due to the reliance of instable and high cost of enzymes such as horseradish peroxidase.

∗ Corresponding authors. Tel.: +86 592 2187750/86 592 2180307; fax: +86 592 2186731. E-mail addresses: [email protected] (X.-Q. Zhan), [email protected] (J.-H. Yan). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.11.013

In recent years the development of optical chemosensors has attracted considerable attention. Upon binding of targets, sensor molecules will display completely different absorption and/or fluorescence signals compared with free sensors, promising a nondestructive and convenient way to report targets specially. A number of chemosensors [6–14], involving both fluorescent and colorimetric sensors, have so far been reported for H2 O2 sensing. In this context, colorimetry sensors was particularly attractive due to simple instruments or even can be easily detected immediately by “naked-eye”. So far, however, most established colorimetric sensing systems for H2 O2 generally proceeded with decoloration reactions of chromogenic reagents and thus were unable to afford considerable sensitivity for naked-eye sensing. In general, two following aspects should be considered in favor of sensitivity for naked-eye detection: (1) displaying a large color change, ca. ≥60 nm change in absorption spectra after sensor binds with target; (2) The sensor-analyte (i.e. receptor–guest) complexation is in red color since that is the one which is most sensitive towards eyes. Merocyanine dyes are heterocyclic compounds containing an electron-withdrawing group conjugated to an electron-donating group such as phenols and involving an intramolecular charge transfer (ICT) which can be greatly improved when phenol turned into phenolate [15]. By virtue of their large molar absorptivity and absorption in red-region wavelength range, they have been used

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without further purification. O3 was generated by a O3 generator (Anqimei Co, Xiamen). Cl2 was collected from the reaction of manganese dioxide with hydrochloric acid. Twice-deionized water was further distilled in presence of potassium permanganate to give thrice-deionized water. A stock solution of BSD (1.0 × 10−3 mol L−1 ) was prepared in acetonitrile and stored at 4 ◦ C for future use. Standard solutions of H2 O2 were prepared by diluting an aqueous 30% H2 O2 stock solution that was standardized against standard potassium permanganate. 2.3. Synthesis of boronate of styryl dye (BSD)

Scheme 1. Synthesis of the boronate of styryl dye (BSD). (a) Pinacol, Et2 O, stirring at room temperature; (b) 1,2,3,3-tetramethyl-3H-indolium iodide, AcOH, refluxed for 4 h.

as excellent dye frameworks for designing colorimetric sensors of anionic [16], metallic ions [17,18] as well as solvatochromism [19]. Up to now, no studies have been reported involving H2 O2 colorimetric sensors utilizing merocyanine as a chromogenic unit. On the other hand, boronate and boronic acid groups have been very attractive to be used as receptor sites for the design of chemosensors, such as for monosaccharides [20] and anions [21,22] due to the unique characteristics of electron-deficient Lewis acid. It is also known that arylboronic acid was used as phenol or amine protecting groups and could be removed by hydrogen peroxide specifically [23,24]. Based on this reaction, a few fluorescent sensors for H2 O2 have been repoted by Lo and Chu [10] and Chang and co-workers [9,12] respectively. However, the relatively low molar absorption and short-wavelength spectral characteristics of the chromophores hinder their practical application for colorimetry detection. In this work a boronate-functioned styryl dye, BSD, was developed as a sensor for H2 O2 by facile synthesis from pinacolato4-formylbenzeneboronate under mild conditions (Scheme 1). Our design rationale for H2 O2 colorimetry detection relies on modulating ICT to promote a color change. Specifically, the arylboronate group in styryl dye was selectively cleaved upon reaction with H2 O2 , resulting in the release of the merocyanine and the restore of strong ICT absorption band. It was found that the sensor displayed remarkable color change from colorless (max = 391 nm) to deep red (max = 522 nm) in presence of H2 O2 with large molar absorptivity (ε = 87000 L mol−1 cm−1 ) of the reaction product (merocyanine), making it to be an excellent chromogenic indicator for H2 O2 . In this work a selective and sensitive colorimetric determination procedure for H2 O2 has been developed and applied in rain water with satisfactory results. 2. Experimental 2.1. Apparatus A Varian CARY-300 absorption spectrophotometer and a 1.0 cm quartz cell were used for absorption study. 1 H NMR spectra were measured on a Varian 500 spectrometer. Mass spectra were obtained on an Esquire 3000 mass spectrometer. A model pHs-301 meter (Xiamen, China) was used for pH measurements. 2.2. Chemicals and solutions 1,2,3,3-tetramethyl-3H-indolium iodide, pinacol, 4formylbenzeneboronic acid and 4-hydroxybenzaldehyde were purchased from Aldrich. NOC13 was synthesized according to the literature [25] as a NO carrier. All other reagents were received from Shanghai Chemicals Group Co. as analytical grade and used

The BSD was synthesized as illustrated in Scheme 1. Pinacolato4-formylbenzeneboronate was synthesized according to the literature [26]. 1,2,3,3-tetramethyl-3H-indolium iodide (0.30 g, 0.99 mmol) and pinacolato-4-formylbenzeneboronate (0.30 g, 1.30 mmol) were dissolved in 10 mL acetic acid and the mixture was refluxed for 4 h. After cooling, yellow solid precipitate appeared. The obtained yellow solid was filtered and dried under vacuum, then purified by column chromatography to give BSD (0.18 g, yield: 35%). The structure of BSD was confirmed by 1 H NMR and MS as follows: 1 H NMR (CDCl3 , 400 Hz): ı(ppm) 1.37 (s, 12H); 1.88 (s, 6H); 4.49 (s, 3H); 7.56–7.62 (m, 3H); 7.68–7.71 (m, 1H); 7.78 (d, 16.4 Hz, 1H); 7.95 (d, 8.0 Hz, 2H); 8.02 (d, 8.0 Hz, 2H); 8.19 (d, 16.4 Hz, 1H). ESI mass spectrometry, m/z: 388.2 (M − I + 1)+ . 2.4. Synthesis of MC, the molecular of merocyanine 1,2,3,3-Tetramethyl-3H-indolium iodide (0.50 g, 1.67 mmol) and 4-hydroxybenzaldehyde (0.30 g, 2.45 mmol) were dissolved in dry methanol, the reaction mixture was refluxed for 4 h under N2 atmosphere after adding 100 ␮L dry piperidine [27]. The solvent was removed and the residue was purified by recrystallization with methanol to give MC (0.44 g, yield: 55%). The structure of MC was confirmed by 1 H NMR and MS spectra as follows: 1 H NMR (d6 DMSO, 400 MHz): ı 1.77 (s, 6H), 4.08 (s, 3H), 6.96 (d, J = 8.8 Hz, 2H), 7.46 (d, J = 16.4 Hz, 1H), 7.56–7.63 (m, 2H), 7.83–7.86 (m, 2H), 8.12 (d, J = 8.4 Hz, 2H), 8.36(d, J = 16.0 Hz, 1H), 10.82 (s, 1H). ESI mass spectrometry, m/z: 278.4(M − I)+ . 2.5. General procedure Into a 10.0 mL volumetric flask, transfer 1.0 mL of NaHCO3 –Na2 CO3 buffer solution (0.10 mol L−1 , pH 9.0) and a known volume of 1.0 × 10−3 mol L−1 BSD standard solution. The mixture was diluted to 10.0 mL with distilled water and mixed thoroughly. Then appropriate volume of H2 O2 standard solutions (or samples) were added by a pipette. The mixing solution was kept for 15 min at 37 ◦ C and then the absorbance of the solutions was measured at 522 nm. A calibration graph of the A (A = A − AO ) vs. the concentrations of H2 O2 was plotted, where A and AO are the absorbance of BSD solutions in the presence and absence of H2 O2 respectively. 3. Results and discussions 3.1. Spectra characteristics The absorption spectra of free BSD in different pH conditions were shown in Fig. 1. Within the pH range of 7.0–11.0, BSD itself displayed only one absorption band around UV region, which could be assigned to an intramolecular charge transfer transition [28]. Red shifts of the absorption peaks from 374 nm at pH 7.0 to 403 nm at pH 11.0 were also found and this was probably due to the addition reaction of hydroxyl ions on the electron-deficient boron atom as the pH increase, resulting in the decrease of the energy barrier of the

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Fig. 1. Absorption spectra of BSD in different pH buffer solutions (0.1 mol L−1 NaHCO3 –Na2 CO3 ) ranging from 7.0 to 11.0. [BSD] = 10 ␮M.

ICT. However, it is notable that BSD displayed almost no absorption sign in visible region even when the pH was up to 11.0. After the increasing addition of H2 O2 , the absorption spectra with the peak at 522 nm gradually emerged accompanying by the decline of the absorption band around 391 nm at pH 9.0 as anticipated (Fig. 2), affording a large value of 131 nm in the change of the absorption band. This phenomenon could be explained by the H2 O2 -mediated transformation of the arylboronate group to the phenolate group which is much stronger in its electron-donating ability, leading to the depression of the energy barrier of the ICT and at the same time restoring of the strong ICT absorption band of merocyanine in ionized form. The presence of an isosbestic point at 432 nm also indicates the transformation of these two structures. Accordingly, the color of the sensing solutions also changes remarkably. Fig. S1 (Supporting Information) shows the color changes of the sensing system towards different contents of hydrogen peroxide. It can be seen that the color evolved from colorless into red with the increasing concentrations of hydrogen peroxide. These performances of the sensing system indicated that the sensor indeed can serve as a sensitive indicator for H2 O2 by “naked-eye” detection. 3.2. Reaction mechanism

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Fig. 2. Absorption spectra of BSD in 0.1 mol L−1 NaHCO3 –Na2 CO3 buffer (pH 9. 0) with H2 O2 concentrations ranging from 0.0 to 25.0 ␮M. [BSD] = 10 ␮M.

compared with its acidic counterpart due to the smaller energy barrier of the ICT. Photometric titration was also conducted to determined the pKa of MC in aqueous solution and a value of 6.7 was obtained, indicating the reaction product of BSD with H2 O2 is in its basic form under pH 9.0. It is noteworthy that compared with free BSD whose absorption band is at 391 nm under pH 9.0 (Fig. 2), MC in acidic form locates its absorption at 420 nm (Fig. 3). The shift in spectra could be explained by the fact that the electron-deficient boronate group on BSD is weaker in electron-donating ability than the hydroxyl group on MC, making the larger of the ICT energy barrier of the BSD molecular. Based on the description above, the sensing mechanism is proposed in Scheme 2. 3.3. Effect of pH and reaction time on the detection system The pH-dependent absorbance at 522 nm of the sensor and the assay system were studied. As anticipated, the absorbance of free BSD solution was almost independent of pH at 522 nm, while the absorption value of the assay system with hydrogen peroxide

To further confirm the reaction mechanism of BSD with H2 O2 , the following studies were performed: 1. The compound merocyanine (MC) was independently synthesized (see Section 2.4: Synthesis of merocyanin). Both the absorption spectra of synthesized MC and the sensing system containing BSD and H2 O2 were recorded under the same conditions as shown in Fig. S2. It could be seen that the two spectra displayed the same spectral characteristics, implying the release of MC in the reaction system. Solid evidences come from the comparing the ESI-MS spectra of both the free BSD and the reaction system composed of BSD and H2 O2 . The peak at m/z 278.1 corresponding to MC [M-I]+ (Fig. S3) was clearly observed when 2.0 eq. of H2 O2 was added to BSD solution, whereas free BSD exhibited only a peak at m/z 388.2 corresponding to BSD [M-I + 1] + (Fig. S4). 2. The absorption spectra of the free MC were recorded under different pH conditions (Fig. 3). MC was found to display absorption peak at 420 nm in its acidic species and 522 nm in its basic species respectively. The red shift of the basic form in absorption spectra could be ascribed to the stronger electron-donating ability of the ionized species and thus giving a stronger ICT absorption band

Fig. 3. Absorption spectra of MC in different pH buffer solutions (0.1 mol L−1 NaHCO3 –Na2 CO3 ) ranging from 5.6 to 10.0. [MC] = 10 ␮M. Insert: Absorbance–pH titration curve of MC at 522 nm.

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Scheme 2. Reaction mechanism of the detection of H2 O2 with BSD.

reached its maximum over a pH range from 9.0 to 12.0. As a result a NaHCO3 –Na2 CO3 buffer (pH 9.0) was selected in our study. The time-dependent absorbance at 522 nm of the detection system was assessed and the results showed that the BSD itself gave almost no absorption signal at 522 nm within at least one day. Upon addition of H2 O2 , the absorption signal of the sensing system reached its maximum value within 15 min. Therefore, a 15 min reaction time was selected in our work. For potential biologically use of the determination system, 37 ◦ C was used in our study. 3.4. Influence of foreign substances To assess the usefulness of the proposed method for environmental assays, the effects of certain foreign substances commonly present in environmental water samples were examined. The tolerable concentration ratios with respect to 5.0 × 10−6 mol L−1 of H2 O2 for interference at ±5% level were summarized in Table 1. In the presence of most of transition metal ions and oxygen-containing species such as Co2+ , Ni2+ , Cr(VI), Cu2+ , Cd2+ , Fe3+ , Pb2+ , Mn2+ , Ag+ , NO, NO2 − , NO3 − , O2 , O3 , Cl2 and MnO4 − , the sensing system exhibited high tolerance levels without any masking reagent and this could be ascribe to the specific deprotection reaction of H2 O2 on arylboronate group in BSD. 3.5. Calibration graph The titration curves of BSD with the standard solution of H2 O2 were constructed under the optimal conditions. The results showed that A (A = A − Ao ) of the system was proportional to the H2 O2 concentrations over a wide linear range of 1.0 × 10−7 –2.5 × 10−5 mol L−1 with a detection limit (LOD) of 6.8 × 10−8 mol L−1 . The LOD was given by the equation LOD = kS0 /S, where k is the numerical factor chosen according to the confidence level desired, S0 is the standard deviation of the blank measurements (n = 6) and S is the sensitivity of the calibration graph. Here, a value of 3 was used for k. The linear equation of calibration curve was A = 0.009 + 0.00159CH2 O2 (CH2 O2 : 10−7 mol L−1 ) with a correlation coefficient Table 1 Effect of foreign substances on the determination of 5.0 ␮M H2 O2 . Foreign substances

Molar ratios to H2 O2

K+ , Ca2+ , Mg2+ , CO3 2− , Cl− , NO3 − , SO4 2− , Ac− Zn2+ , Pb2+ , Cd2+ , Ni2+ , Co2+ , Cu2+ , Fe3+ , Ag+ , Hg2+ , Cr3+ , Mn2+ O2 , O3 , SO3 2− , NO2 − NO, Cl2 Mn(VII), Cr(VI)

>2500 200 100 50 10

Table 2 Results of the determination of hydrogen peroxide in rain water samples by the proposed method and referenced fluorimetric method. Samplea

Rain water 1 Rain water 2 Rain water 3 Rain water 4

H2 O2 foundb (␮M) Proposed method

Reference method

3.20 ± 0.04 2.92 ± 0.36 0.82 ± 0.01 0.86 ± 0.01

3.42 ± 0.08 3.41 ± 0.13 0.93 ± 0.02 0.91 ± 0.03

Added (␮M)

Foundb (␮M)

3.0 3.0 1.0 1.0

2.81 ± 0.04 2.70 ± 0.06 0.89 ± 0.02 0.86 ± 0.03

Recoveryc

93% 90% 89% 86%

a Samples were collected on 29th July 2008 (rain water 1), 30th July 2008 (rain water 2) and 12th November 2008 (rain water 3 and 4), respectively, in Xiamen. b Mean ± SD, n = 3. c Mean, n = 3.

of 0.9995 (N = 9). The relative standard deviation (n = 6) was 2.5% for the determinations of 5.0 × 10−6 mol L−1 of H2 O2 . 3.6. Analysis of samples The proposed method was used for the determination of trace amount of hydrogen peroxide in rain water samples collected in summer and autumn days respectively in Xiamen. Each sample was filtered through a 0.45 ␮m cellulose acetate membrane to remove the small amount of insoluble deposition [29] and then determined without delay to avoid any degradation of the analyte in the samples. A fluorimetric assay [30] was also carried out as a reference at the same time and the results are shown in Table 2. It is found that the concentrations of H2 O2 determined by the proposed method are lower than those by the reference method for all rain water samples. T test showed that there exists significant difference between the two method, which may be ascribed to two reasons: First, for the reference method oxygen molecular could also oxidate the fluorescent substrate in addition to H2 O2 in presence of hemin as a catalyst [30]. Second, compared with the reference method under which the procedure was run by incubation of H2 O2 with the fluorescent substrate under room temperature for 8 min before determination, that of the proposed method was run under 37 ◦ C for 15 min which could probably lead to little decrease in H2 O2 concentrations. 4. Conclusion A novel boronate-functioned styryl dye was facilely synthesized as a colorimetry sensor for hydrogen peroxide with selectivity, sensitivity and simplicity. The probe was successfully applied to detect hydrogen peroxide in rain water. The specific H2 O2 -mediated deprotection of arylboronate group to phenolate recovered the

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strong ICT of the merocyanine dye, leading to a remarkable change in color and a red shift up to 131 nm in absorption spectrum. The unique performance of the merocyanine dye make it to be a promising structural scaffold for design chemosensors. Acknowledgment The authors are thankful to the financial support from the Natural Science Foundation of Fujian Province of China (2008J0236) and National Natural Science Foundation of China (No. 20675067 and No. 20835005). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.aca.2009.11.013. References [1] [2] [3] [4] [5] [6]

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