Sensor Based on the Room-Temperature Phosphorescence of Nano

Apr 20, 2007 - SiO2 can be recovered when it is dipped in a hydroxy- .... peroxo moieties formatted in TiO2/SiO2 composite oxide after adding H2O2.24...
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Anal. Chem. 2007, 79, 3695-3702

H2O2 Sensor Based on the Room-Temperature Phosphorescence of Nano TiO2/SiO2 Composite Xiaohong Shu,† Ying Chen,† Hongyan Yuan,‡ Shangfeng Gao,‡ and Dan Xiao*,†,‡

College of Chemistry and College of Chemical Engineering, Sichuan University, Chengdu, 610065 P.R. China

A TiO2/SiO2 composite prepared by the sol-gel route can produce highly emissive broadband room-temperature phosphorescence at an excitation wavelength of 403 nm. The white phosphorescence of TiO2/SiO2 could be quenched by H2O2. The phosphorescence quenching effect demonstrated excellent sensitivity and high selectivity to H2O2. Furthermore, the phosphorescence of TiO2/ SiO2 can be recovered when it is dipped in a hydroxylamine hydrochloride solution. Therefore, the TiO2/SiO2 was used to develop a reproducible phosphorescence sensor for H2O2. It has been successfully applied to the determination of H2O2 in the enzymatic catalytic reaction and real samples. Phosphors are important materials of considerable interest in many fields including fluorescent lighting, displays, and X-ray scintillation. Green et al.1 reported highly emissive broad and phosphors that can be synthesized from a tetraalkoxysilane solgel precursor and a variety of organic carboxylic acids. Here, we report a TiO2/SiO2 composite oxide white phosphor via a solgel route. It produces highly emissive broadband phosphorescence from 450 to 650 nm at an excitation wavelength of 403 nm. The material could be used for the determination of trace hydrogen peroxide. Hydrogen peroxide can be harmful to biological systems and appears to be involved in the neuropathology of central nervous system diseases.2 Hydrogen peroxide is an important trace gas that plays a significant role in the troposphere. In addition, H2O2 has an indirect impact on the acid rain procedure that is harmful to the atmosphere.3 Because the determination of H2O2 is also very important in enzymatic reactions,4 the trace determination of H2O2 is of considerable importance in clinical and environmental applications. For the determination of H2O2, methods such as titrimetric,5 spectrophotometric,6-8 fluorescence,9-11 electrochemical,12 and * To whom correspondence should be addressed. E-mail: [email protected]. † College of Chemistry. ‡ College of Chemical Engineering. (1) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science 1997, 276, 1826-1828. (2) Mazzio, E. A.; Soliman, K. F. A. J. Appl. Toxicol. 2004, 24, 99-106. (3) Komazaki, Y.; Inoue, T.; Tanaka, S. Analyst 2001, 126, 587-593. (4) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Nano Lett. 2005, 5, 21-25. (5) Klassen, N. V.; Marchington, D.; McGovan, H. C. E. Anal. Chem. 1994, 66, 2921-2925. (6) Zhu, M.; Huang, X.; Liu, L.; Shen, H. Talanta 1997, 44, 1407-1412. (7) Matsubara, C.; Kudo, K.; Kawashita, T.; Takamura, K. Anal. Chem. 1985, 57, 1107-1109. 10.1021/ac0624142 CCC: $37.00 Published on Web 04/20/2007

© 2007 American Chemical Society

chromatographic13-16 are usually used. The titrimetric detection method uses simple apparatus but it is less sensitive. The spectrophotometric methods have to react with a chromogenic hydrogen donor in the presence of peroxides such as Eriochrome Black T,6 a mixture of titanium(IV) and 2-((5-bromopyridyl)azo)5-(N-propyl-N-sulfopropylamino)phenol,7 oxoperoxopyridine-2,6dicarboxylic acid, and vanadate(V).8 The fluorescence methods involve the fluorescence reaction between fluorescein hydrazide and hydrogen peroxide,9 the oxidation of acetaminophen with hydrogen peroxides in acidic medium,10 or Eu3+-tetracycline complex binding H2O2 to form fluorescent complex in buffer solution.11 This method has low detection limit, but it needs several reagents, such as a fluorogenic precursor. Electrochemical methods have been reported for the determination of hydrogen peroxide using a platinum electrode in a supporting electrolyte12 or a copper electrode by direct and catalytic reduction.13 Although the electrochemical method is very sensitive, it makes use of enzymes as reagents that is unstable and expensive. Chromatographic methods offer an alternative way of developing a sensor for H2O2. For example, high-performance liquid chromatography (HPLC) with fluorescence detection is used for the determination of organic peroxides and hydrogen peroxide,14 HPLC with electrochemical detection,15 with UV detection.16 The chromatography can sensitively determine H2O2 in the presence of organic peroxides; however, the equipment is also expensive and depends on other detection. To the best of our knowledge, a H2O2 sensor based on room-temperature phosphorescence is seldom reported. In this study, the phosphorescence material of TiO2/SiO2 composite oxides was synthesized and its phosphorescence was remarkably selectively quenched by H2O2 in the presence of common ions, acids, and bases. The phosphorescence of TiO2/ SiO2 composite oxide can be recovered in hydroxylamine hydrochloride solution and exhibit good linear response to H2O2 concentration ranging from 7.0 × 10-6 to 7.0 × 10-2 M. This method was successfully applied to H2O2 determination in com(8) Tanner, P. A.; Wong, A. Y. S. Anal. Chim. Acta 1998, 370, 279-287. (9) Mori, I.; Takasaki, K.; Fujita, Y.; Matsuo, T. Talanta 1998, 47, 631-637. (10) Jie, N.; Yang, J.; Huang, X.; Zhang, R.; Song, Z. Talanta 1995, 42, 15751579. (11) Wolfbeis, O. S.; Durkop, A.; Wu, M.; Lin, Z. Angew. Chem., Int. Ed. 2002, 41, 4495-4498. (12) Harrar, J. E. Anal. Chem. 1963, 35, 893-896. (13) Somasundrum, M.; Kirtikara. K.; Tanticharoen, M. Anal. Chim. Acta 1996, 319, 59-70. (14) Wang, K.; Glaze, W. H. J. Chromatogr., A 1998, 822, 207-213. (15) Osborne, P. G.; Yamamoto, K. J. Chromatogr., B 1998, 707, 3-8. (16) Pinkernell, U.; Effkemann, S.; Karst, U. Anal. Chem. 1997, 69, 3623-3627.

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mercial samples. The primary interest of this work is to develop the TiO2/SiO2 composite oxide to the analysis of H2O2 for the first time. EXPERIMENTAL SECTION Chemicals. Tetraethoxysilane, isopropyl alcohol, and hydroxylamine hydrochloride were obtained from Chengdu Chemicals (Sichuan, China). Tetrabutyl titanium was purchased from Shanghai Chemicals (Shanghai, China). A 30% H2O2 aqueous solution was obtained from Chongqing Chemicals (Chongqing, China). All chemicals were of analytical reagent grade and were used without further purification. A stock solution of 1 × 10-2 M H2O2 was prepared by 30% H2O2 solution and diluted with redistilled water. The H2O2 standard solution was standardized by KMnO4, which was previously standardized by sodium oxalate. Redistilled water was used throughout all experiments. Instrumentation. Phosphorescence measurements were performed at room temperature using a fluorescence spectrophotometer F-4500 (Hitachi). Infrared (IR) spectra were performed on a FI-IR670 infrared spectrophotometer (Nicolet Corp.). The electron spin resonance (ESR) spectra were recorded on a Bruker ER200D-SRC. Microwave frequency was 9.44 GHz. X-ray absorption spectra (Ti K edge at 4.996 keV) were measured at the National Synchrotron Radiation Laboratory (NSRL). Scanning electron microscopy (SEM) image was captured by a JSM-5900LV scanning electron microscope (JEOL). XPS analysis was performed on a V4105 X-ray photoelectron spectroscopy instrument (Noran). Preparation of the TiO2/SiO2 Composite Oxide. The nanometer particles of TiO2/SiO2 composite oxides were prepared by the sol-gel method according to the literature.17-20 Tetrabutyltitanium and tetraethoxysilane were selected as the source of Ti and Si, respectively. Tetraethoxysilane was dissolved in isopropyl alcohol, and then the solution was stirred continuously for 20 min. Tetrabutyltitanium was dripped into the solution. Distilled water was injected drop by drop until a transparent sol formed and gelled at room temperature. The molar ratio of Si and Ti was 1:4.6. Finally, calcinations were performed at 550 °C for 1 h, and nanometer-sized TiO2/SiO2 particles were obtained. (Warning: The isopropyl alcohol is flammable; keep fume hood closed and ensure the system is away from flames and sparks. Isopropyl alcohol can be volatilized during the calcinations of the gels. Therefore, calcinations should be done in a well-ventilated fume hood.) The composite oxide was then kept in the desiccators and triturated before use. Measurement Procedure. Phosphorescence measurements of the sensor were carried out on a fluorescence-phosphorescence spectrophotometer in the absence and presence a series of H2O2 solutions (7 × 10-7-7.0 × 10-2 M). The powder of TiO2/SiO2 composite oxides was pressed into disks of 12 mm in diameter and ∼0.5 mm in thickness. The response of the sensors to H2O2 solutions of different concentrations was detected in a flow cell (Supporting Information Figure S1). A 10.0-mL sample of H2O2 (17) Yu, J.; Zhao, X.; Yu, J. C.; Zhong, G.; Han, J.; Zhao, Q. J. Mater. Sci. Lett. 2001, 20, 1745-1748. (18) Lee, J. H.; Choi, S. Y.; Kim, C. E.; Kim,G. D. J. Mater. Sci. 1997, 32, 35773585. (19) Zhai, J. W.; Zhang, L. Y.; Yao, X.; Shi, W. S. J. Mater. Sci. Lett. 1999, 18, 1107-1109. (20) Samantaray, S. K.; Parida, K.; Kinet, R. Catal. Lett. 2003, 78, 381-387.

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was manually injected into the flow cell before it was determined. The time scan phosphorescence spectra were obtained at the maximum excitation wavelength of 403 nm and the emission wavelength of 535 nm. The wavelength scan phosphorescence spectra were also excited at 403 nm, and the emissions ranged from 450 to 650 nm. The excitation and emission bandwidth slits were set at 10 and 20 nm, respectively. RESULTS AND DISCUSSION The nanometer particles of TiO2/SiO2 composite oxides were prepared by a sol-gel method according to the literature.17-20 The molar ratio of Si and Ti was 1:4.6. Calcinations were finally performed at 550 °C for 1 h to obtain nanometer-sized TiO2/SiO2 particles. The phosphorescence intensity and experimental condition can be seen in Table S1 (Supporting Information). When the Ti/Si molar ratio was equal to 1:4.6, the phosphorescence intensity of TiO2/SiO2 was stronger than that of Ti:Si ) 1:1.5 and Ti:Si ) 1:7.5. The phosphorescence intensity of TiO2/SiO2, which was calcined at 550 °C for 1 h, is the strongest of all. If the calcination temperature exceeded 600 °C, the TiO2/SiO2 composite oxides have no phosphorescence. A morphological study of the material was carried out by SEM. Figure S2 (Supporting Information) shows the SEM image of the material (Ti/Si molar ratio is 1:4.6) obtained at 550 °C for 1 h. The particle diameter was from 20 to 50 nm. Fourier transform infrared of composite oxides is shown in Figure 3. The IR band observed at 910-960 cm-1 is widely accepted as the characteristic vibration due to the formation of Ti-O-Si bonds.21,22 This indicates that Ti substitutes for Si, which is assumed to form the trap levels. This results in the broadband phosphorescence (Scheme 1). When the gel is calcined at much higher temperature (>600 °C), the “Ti-O” bond is very stable. It is difficult to produce traps and cannot form the defect, so there is no phosphorescence observed. XPS analysis was performed on the TiO2/SiO2 composite oxide sample. Figures 1 and 2 show XPS core level spectra for the TiO2/ SiO2 composite oxide sample before and after adding H2O2. According to the literature,23 the binding energies of Ti 2P3/2 for the different oxidation states of titanium are positioned on the graph as follows: Ti0 at 454.1 eV; Ti2+ at 455.3 eV; Ti3+ at 457.2 eV; and Ti4+ at 459.2 eV. Hence, it can be inferred from Figure 1 (the top curve) that the titanium of TiO2/SiO2 composite oxide is mostly confined to its highest oxidation state (IV). In addition, according to the result of the IR spectra of TiO2/SiO2 composite oxide (Supporting Information Figure S3), the majority of framework Ti centers are tetrahedral surrounded by four -OSi linkages. In Figures 2 and 3, the Ti (2p) spectrum is slightly shifted to the higher binding energy (BE) after adding H2O2; however, the Si (2p) spectrum shifts to lower BE. This shift in binding energy is attributed to the change in the chemical environment of the TiO2/ SiO2 composite oxide. It was reported that the Ti-O-O-Si peroxo moieties formatted in TiO2/SiO2 composite oxide after adding H2O2.24 (21) Jung, M. J. Sol-Gel Sci. Technol. 2000, 19, 563-568. (22) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 4125-4132. (23) Mayer, J. T.; Diebold, U.; Madey, T. E.; Garfunkel, E. J. Electron Spectrosc. 1995, 73, 1-11.

Figure 1. XPS of the Ti (2p) of TiO2/SiO2 composite oxide sample (a) before and (b) after adding H2O2.

Figure 2. XPS of the Si (2p) of TiO2/SiO2 composite oxide sample (a) after and (b) before adding H2O2.

Scheme 1. Proposed Phosphorescence Excitation/Emission Mechanism of TiO2/SiO2 Composite Oxide

Figure 3. ESR spectra of Ti-Si material before (a) and after (b) treatment with H2O2 solution.

Electron spin resonance (ESR) spectroscopy was used to detect and identify radicals formed after TiO2/SiO2 composite oxide interaction with H2O2 solution. As shown in Figure 3, no ESR signal was detected for the material. However, after reaction with H2O2 solution, one signal appeared at gz ) 2.0200, gy ) 2.0094, (24) Munakata, H.; Ozumi, Y.; Miyamoto, A. J. Phys. Chem. B 2001, 105, 34933501.

and gx ) 2.0034, which is assigned to superoxide O2-.25 This indicated that H2O2 was a prerequisite for the formation of this radical addict. X-ray absorption fine structure (XAFS) spectra were analyzed by the Athena and Artemis program in the IFEFFIT computer package.26 To obtain more detailed information about the Ti local environment in the TiO2/SiO2 composite sample, an extended X-ray absorption fine structure (EXAFS) spectrum was fitted to (25) Bonoldi, L.; Busetto, C.; Congiu, A.; Marra, G.; Ranghino, G.; Salvalaggio, M.; Spano, G.; Giamello, E. Spectrochim. Acta A 2002, 58, 1143-1154. (26) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J. Am. Chem. Soc. 1991, 113, 5135-5140.

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Figure 4. Fourier transform magnitude resulting in a radial distance structure for the Ti-O-Si compound. Experimental data, solid line; and the best fit, dash line.

Figure 5. Fourier transform Ti K edge EXAFS spectra of Ti-Si compound (a) and after interaction with H2O2 (b).

anatase TiO2. The result from fitting the EXAFS spectra is shown in Figure 4. Only one prominent peak corresponding to the first shell was observed for the nano TiO2/SiO2 composite. This indicated that local structure around Ti atom in the Ti-O-Si sample was amorphous. The first peak in the FT was the result of the nearest-neighbor Ti-O shell. In fact, fitting the spectrum yields a coordination number close to four for the first Ti-O shell. Figure 5 shows the EXAFS spectrum of the sample is compared with sample interaction with H2O2. As seen in Figure 5, after adding H2O2, the Ti-O bond distance of the sample increases. This was due to the change of local environment around the Ti atom. It changes from substituted within the silica network to phase separated into domains of TiO2.27 So, EXAFS results confirmed that the local structure around Ti atoms in the nano TiO2/SiO2 composite was changed after adding H2O2. In the presence of H2O2, the phosphorescence intensity of TiO2/SiO2 composite oxide was measured using a fluorometer. (27) Pickup, D. M.; Sowrey, F. E.; Newport, R. J. J. Phys. Chem. B 2004, 108, 10872-10880.

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We found that the intensity was reduced gradually (shown in Figure 6) as H2O2 concentration increased. In general, the quantitative relations between phosphorescence intensity and concentration of analyte exhibit the following relationship: lg P versus lg C, lg P versus C, P versus lg C, and P versus C. In this paper, however, the above quantitative relations do not exhibit a linear relationship. Instead, we found a logarithmic quantitative equation, lg [(P0 - P)/P] ) a + blg C, which is similar to lg P versus lg C fitting the experimental data. The curve exhibits an excellent linear relationship (as shown in Figure 6 and Figure 8). From Figure 6, when H2O2 concentration changed from 7.0 × 10-7 to 7.0 × 10-2 M, its linear equation could be represented as y ) 0.5123x - 1.8974, where y is -lg[(P0 - P)/P] and x is -lgC. Here, P0 is the phosphorescence intensity of TiO2/SiO2 in redistilled water, P is the phosphorescence intensity of TiO2/SiO2 in H2O2 of different concentrations, and C is the concentration of hydrogen peroxide (M). The linear relative coefficient is R ) 0.9925 and the detection limit (signal/noise ) 3) is 1.6 × 10-7 M. To the best of our knowledge, TiO2/SiO2 nanometer material is

Figure 6. Phosphorescence spectra of TiO2/SiO2 composite oxide at H2O2 concentration of (a) 0 , (b) 7.0 × 10-7, (c) 7.0 × 10-6, (d) 7.0 × 10-5, (e) 7.0 × 10-4, (f) 7.0 × 10-3, and (g) 7.0 × 10-2 M.

the first sensor with a large dynamic response range for H2O2. After adding the H2O2, TiO2/SiO2 composite had been oxidized into peroxide (Scheme 2)·25 The product after washing with redistilled water repeatedly was put into KI solution. Starch test paper turned blue immediately when it was put into the solution. This demonstrated that the material had the ability to oxidize Ito I2 and the reaction occurred:

(Si - O)3Ti(O2-) + I- f (Si - O)3 Ti - O - Si + I2

The existence of peroxide bond causes the reducing of the phosphorescence intensity. After reaction with KI, phosphorescence of the TiO2/SiO2 composite reappeared. Scheme 2 illustrates the process of H2O2 quenching and KI recovering the phosphorescence of the TiO2/SiO2 composite, respectively. The species in the scheme were confirmed with IR, XPS, ESR, and EXAFS. IR spectra (Supporting Information Figure S3) suggest that the Ti-O-Si bond be present in TiO2/SiO2 composite oxide. According to the literature,28 the phosphorescence of TiO2/SiO2 is related to the Ti-O-Si stretching mode. However, after interaction with H2O2, anionic triangular Ti (O2-) was formed in the TiO2/SiO2 composite oxide (in Scheme 2), as supported by the XPS (Figures 1 and 2), ESR (Figure 3), and EXAFS (Figures 4 and 5). The oxidation state of Ti was confirmed by XPS. Figure 2 showed increase of in binding energy of the Ti 2P3/2 electrons after adding H2O2 compared with that in the pure TiO2/SiO2, which indicates the formation of Ti-O-O-Si peroxo moieties in the TiO2/SiO2. To complement the XPS, the TiO2/SiO2 sample was further probed by ESR. The data reveal that, with the addition of H2O2 in the TiO2/SiO2 composite oxide, the Ti sites are highly reactive to H2O2 and form superoxide ion (O2-) with a characteristic average g value. These are also combined with EXAFS to supplement ESR result. Analysis of EXAFS gives valuable information regarding the environment of Ti center in TiO2/SiO2 oxides. Figure 5 reports that the addition of H2O2 to TiO2/SiO2 composite oxide can affect the Ti environment by causing the cleavage of the silica network. (28) Soult, A. S.; Poore, D. D.; Mayo, E. I.; Stiegman, A. E. J. Phys. Chem. B 2001, 105, 2687-2693.

Therefore, the formation of this triangular Ti(O2-) blocks the TiO-Si vibration, which results in the quenching of phosphorescence of TiO2/SiO2. Phosphorescence lifetime was measured with employing a Hitachi FL-4500 instrument. The exaction wavelength was at 403 nm. The decay curves of TiO2/SiO2 composite oxide before and after added H2O2 are shown in Figure 7. As an analytical method, time-resolved, room-temperature phosphorescence can minimize the light source noise or quench the background signal, and this is desirable for routine analytical chemistry. From the report,29 an extended equation lnIt ) lnI0 - t/τ was gotten from the formula It ) I0e-t/τ, where lnIt ) lnI0 - t/τ was the intensity of the phosphorescence as a function of time. I0 was the maximum intensity at time t ) 0, It was the phosphorescence intensity at decay time t after excitation, and τ was the apparent lifetime of decay. According to the equation lnIt ) lnI0 - t/τ, a plot of lnIt versus t gives a straight line, with a slope equal to 1/τ, which is used for calculations. In Figure 7, it is apparent that the lifetime decreases considerably with addition of 1.0 × 10-2 M H2O2. The phosphorescence lifetime of TiO2/SiO2 composite oxide in the absence of H2O2 is 3.3 and 0.4 s for interaction with 1.0 × 10-2 M H2O2. This difference was dependent on the concentration of H2O2 and tended to decrease at concentration ∼10-6 M. The effect of H2O2 on the phosphorescence lifetimes provides further evidence for quenching by H2O2. The large difference in phosphorescence lifetime in the two different conditions also supports the change of the structure of the TiO2/SiO2 composite oxide. The phosphorescence quenching mechanism between H2O2 and TiO2/SiO2 sample has been discussed in the introduction. In the presence of H2O2, anionic triangular Ti(O2-) formed in the TiO2/SiO2 oxide (Scheme 2). In view of the oxidation of peroxide product, its phosphorescence may be recovered by some reducing agents. In this work, we found potassium iodide and hydroxylamine hydrochloride can recover the phosphorescence of TiO2/SiO2 composite oxides. Because iodide ion cannot completely recover the phosphorescence of TiO2/SiO2 and will interfere in some tests and measurements, hydroxylamine hydrochloride was selected as the reductant. The information of the phosphorescence recovery in two solutions which was mentioned above is shown in Figure S4 (Supporting Information). Using this method, the materials of TiO2/SiO2 can be used repeatedly and the curve exhibits a linear relationship in the H2O2 concentration range from 7.0 × 10-6 to 7.0 × 10-2 M as shown in Figure 8. From this graph, the results of two successive determinations demonstrated a highly reproducible response to H2O2, and the linear relative coefficient R was better than 0.9950. In this way, the detection limit (signal/noise ) 3) of this reversible method is 4.0 × 10-6 M. It satisfies the desire of routine hydrogen peroxide determination. In addition, we found that an immediate color change occurs in Ti-O-Si compound from white to orange when different concentrations of H2O2 solution were added. In Figure 9, redistilled water (1 drop) was added to the TiO2/SiO2 compound and it kept its original color; a light yellow developed immediately when the concentration of H2O2 solution was 1.0 mM, a deeper yellow color with 10.0 mM, and orange to 1.0 M. Therefore, we can determine (29) Wei, Y.; Dong, C.; Shuang, S.; Liu, D. Spectrochim. Acta A 2005, 61, 25842589.

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Figure 7. Decay of the phosphorescence (a) TiO2/SiO2 composite oxide added H2O and (b) TiO2/SiO2 composite oxide added 1.0 × 10-2 M H2O2.

Figure 8. Time scan phosphorescence spectra of two times successive determinations and its calibration curves. The H2O2 concentration: (a) 0, (b) 7.0 × 10-6, (c) 7.0 × 10-5, (d) 7.0 × 10-4, (e) 7.0 × 10-3, and (f) 7.0 × 10-2 M.

Scheme 2. Proposed Oxidation and Reduction of H2O2 and the Reductant between TiO2/SiO2 Composite Oxide

the concentration of H2O2 solution with the naked eye. The color change can be used as a colorimetric sensor for determining the presence of H2O2 in solution. It is obvious that the determination of H2O2 is of great importance in enzyme reaction. To demonstrate the present H2O2 sensor available for the determination of H2O2 production during the process of the enzymatic catalytic reaction, the enzymatic reaction of glucose oxidase catalyzing glucose oxidation and H2O2 3700

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production was applied. In the presence of glucose oxidase, the color and the phosphorescence intensity change of TiO2/SiO2 composite oxides also took place when different concentrations of glucose solution were added. In Figure 10A, we can see that the color of TiO2/SiO2 composite oxides deepens gradually with the increase of the glucose concentration. The color changes from white (0 mM glucose) to a distinct yellow (60 mM glucose) in a few minutes. Therefore, the concentration of glucose solution could be observed with the naked eye. In addition, the phosphorescence intensity of TiO2/SiO2 composite oxides was decreased with the increase of glucose concentration in the presence of glucose oxidase. Figure 10B shows the room-temperature phosphorescence photograph captured with a commercial digital camera, exposed right after turning off the UV lamp, of the solutions containing glucose in concentrations from 0 to 60 mM in the presence of glucose oxidase. From the photograph, we can see the phosphorescence emission of the TiO2/SiO2 composite oxides. The phosphorescence of the TiO2/SiO2 composite oxides was nearly completely quenched by 60 mM glucose solution, only weak phosphorescence emission could be observed when 6 mM glucose solution was added, bright phosphorescence emission from the composite oxides could be seen when 0.6 mM glucose solution was added, and strong phosphorescence emitted from the composite oxides could be seen when buffer solution without glucose was added. It indicates that the sensor can also be used to monitor the enzymatic catalytic reaction. In order to illustrate the reliability of this sensor, the H2O2 concentration in two commercial samples (∼3 wt %), which were bought from a local market, was measured. The commercial samples were diluted 100-fold with redistilled water before the H2O2 was determined. The TiO2/SiO2 composite oxides were dipped in the sample solution, and then the phosphorescence intensity of the material was determined. At the same time, the concentration of H2O2 in the samples was also standardized with potassium permanganate, and the results of the two methods are summarized in Table 1. Satisfactory agreement between the

Figure 9. Photograph of TiO2/SiO2 composite oxide after adding different concentrations of H2O2: (A) 0, (B) 1.0 × 10-3, (C) 1.0 × 10-2, and (D) 1.0 M.

Figure 10. (a) Photograph of TiO2/SiO2 composite oxide at glucose concentration (contain glucose oxidase) of (A) 0, (B) 6.0 × 10-4, (C) 6.0 × 10-3, and (D) 6.0 × 10-2 M. (b) Room-temperature phosphorescence photograph of TiO2/SiO2 composite oxide at different glucose concentrations (contain glucose oxidase): (A) 0, (B) 6.0 × 10-4, (C) 6.0 × 10-3, and (D) 6.0 × 10-2 M. Phosphorescence photograph was captured with a digital camera exposed immediately right after turning off the UV lamp with the exposure time of 3 s.

Table 1. Determination of H2O2 in Commercial Samples Using the Proposed Sensor and Titrimetric Method proposed method

titrimetric method

sample

ca (wt %)

RSD (%)

ca (wt %)

relative error (%)

antiseptic solution contact-lenses solution

2.49 2.48

6.08 3.10

2.46 2.47

+1.50 +0.40

a

Average value from eight determinations.

proposed method and titrimetric method was obtained, and the relative standard deviation (RSD) was less than 7.0% for each sample. The results indicate that the sensor is suitable for the determination of H2O2 real samples. In this study, influences of foreign species on the determination of 7.0 × 10-4 M hydrogen peroxide were also examined. We found that the measurements were not significantly affected in the presence of a maximum tolerance molar ratio of ions, such as Na+, K+, Zn2+, NH4+, Ca2+, Al3+, Ba2+, HPO42-, NO3-, Ac-, Cl-, SO42-, SO32-, S2O32-, CO32-, Br-, NO2-, tartrate, or urea, to H2O2 was 1000; a maximum tolerance molar ratio of ions Pb2+, Mn2+, or citrate to H2O2 was 100. Because the colors of Cu2+, Co2+, and Ni2+ can absorb the phosphorescence at the wavelength of measurement, when the maximum tolerance molar ratio of these ions to H2O2 was 20, there was no interference. Organic molecules, such as ethanol, methanol, formaldehyde, benzene,

toluene, petroleum ether, acetone, phenol, nitrophenol, dichloromethane, and chloroform, did not affect the phosphorescence intensity. CONCLUSIONS We prepared nano TiO2/SiO2 composite by a sol-gel scheme and it can produce highly emissive broadband phosphorescence from 450 to 650 nm at an excitation wavelength of 403 nm. The phosphorescence of TiO2/SiO2 can be quenched by H2O2. And this was successfully used to establish a new sensor of high sensitivity, high reproducibility, fast response, and high selectivity to H2O2. The phosphorescence of TiO2/SiO2 can be recovered by simply dipping in hydroxylamine hydrochloride solution; thus a reproducible sensor is obtained. This H2O2 sensor was successfully applied to the determination commercial samples containing H2O2. The experimental results also demonstrated the possibility of its further development into a phosphorescence lifetime resolved-based sensor. ACKNOWLEDGMENT We thank Prof. Shiqiang Wei in the National Synchrotron Radiation Laboratory for the measurement of EXAFS spectrum and Prof. Chunpei Zhao in Analytical & Testing Center of Sichuan University for the measurement of XPS. Financial support from National Natural Science foundation of China (No. 20570542) and Education Ministry of China (No. 105141) are gratefully acknowledged. Analytical Chemistry, Vol. 79, No. 10, May 15, 2007

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SUPPORTING INFORMATION AVAILABLE Tables of the effect of volume ratio, calcinated temperature, and time on phosphorescence intensity. The diagram of flow cell, SEM photos, FT-IR spectra, and the phosphorescence spectra of TiO2/SiO2 composite oxide in original and hydroxylamine hydro-

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chloride solution. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review December 21, 2006. Accepted March 19, 2007. AC0624142