Chemiluminescence Reactions of a Luminol System Catalyzed by

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Chemiluminescence Reactions of a Luminol System Catalyzed by ZnO Nanoparticles Shi-Feng Li, Xin-Ming Zhang, Wan-Xin Du, Yong-Hong Ni, and Xian-Wen Wei* College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, P. R. China ReceiVed: September 18, 2008; ReVised Manuscript ReceiVed: NoVember 10, 2008

Colloidal ZnO nanoparticles were synthesized by the sol-gel method and were characterized by UV-visible spectroscopy and photoluminescence spectroscopy. The effect of ZnO nanoparticles on the luminol-H2O2 chemiluminescence (CL) system, a popular model CL system, was investigated. It was found that ZnO nanoparticles with a size regime from 6 to 21 nm could enhance the CL of the luminol-H2O2 system, and the strongest CL signal was obtained when the mean diameter of ZnO nanoparticles was 6 nm. To investigate the enhancement mechanism of the CL, we used UV-visible spectroscopy, FT-IR spectroscopy, and CL spectroscopy to study optical properties before and after the CL reaction in the presence/absence of ZnO nanoparticles. The CL enhancement of the luminol-H2O2 system by ZnO nanoparticles should originate from the catalysis of ZnO nanoparticles, which could catalyze the decomposition of H2O2 to produce some reactive intermediates such as hydroxyl radical and superoxide anion. Then, the resulting hydroxyl radical reacted with luminol to form luminol radical, which rapidly reacted with superoxide anion or monodissociated hydrogen peroxide. As a result, the emission was enhanced. Experimental results showed that some organic compounds containing -OH, -NH2, and -SH groups, such as amino acid, ascorbic acid, and dopamine, could inhibit the CL signal of the luminol-H2O2-ZnO system. The above facts indicated that the present system had a wide application for the determination of such compounds. 1. Introduction Semiconductor nanoparticles have attracted extensive attention in the past two decades because of their size-dependent novel electronic and optical properties originating from surface and quantum confinement effects.1-5Luminescent properties of semiconductor nanocrystals (NCs) are usually investigated by photoluminescence (PL) produced using photoexcitation6 or electrochemiluminescence (ECL) generated by electron injection.7,8 In recent years, chemiluminescence (CL) and related analytical techniques have attracted extensive interest and have been developed as important and powerful tools in different fields,9-15 because of their high sensitivity, wide linear range, simple instrumentation, and lack of background scattering light interference. Though CL has been investigated for years, study of CL was limited to some molecular systems. Recently, much attention has been extended to the CL of nanomaterials systems, to improve the sensitivity and the stability. The expanding availability of nanoparticles in CL reactions has attracted widespread attention in the use of catalyst, reductant, luminophor, and energy accepter16-19 due to their high surface areas, good adsorption characteristics, high activity, and high selectivity. Semiconducting and metallic nanoparticles represent increasingly intensively explored materials that bridge bulk material and molecular behavior and offer novel chemical properties. Many investigations have indicated that the use of metal nanoparticles in CL reactions has provided new approaches to enhance the inherent sensitivity and expand new applications of this mode of detection. For example, Cui et al.20,21 have found that noble metal nanoparticles can enhance the CL of the lucigenin-KI and luminol-H2O2 systems. However, the ap* Corresponding author. Tel.: +86 553 3937138. Fax: +86 553 3869303. E-mail: [email protected].

plication of semiconductor nanoparticles as catalysts is rarely used in investigations. To the best of our knowledge, there are only a few reports involving the chemiluminescence of semiconductor nanocrystals. Zhang et al.22,23 have made use of the active catalysis of the nanomaterials to develope gas-sensing modes, such as nanosized TiO2 and ZrO2, and investigated their applications for catalyzing the gas phase chemiluminescence. The Li group24 reported the CL of CdTe NCs directly oxidized by some oxidants, such as H2O2 and KMnO4, and its sizedependent and surfactant-sensitized effects. Talapin et al.25 also described the CL property of CdSe/CdS core/shell nanostructures dealing with the emitting state related to the quantum-confined orbitals. Recently, it has been found that H2O2 can directly oxidize CdS NCs to produce CL emission.26 In the present study, ZnO nanoparticles, the novel classes of semiconductors, were chosen as catalysts for the luminol CL system. The luminol-H2O2 CL reaction, a popular model CL system, has been widely applied for the detection of various substances.27-36 The effect of ZnO nanoparticles on the luminol-H2O2 CL system was investigated, and the enhancement mechanism of ZnO nanoparticles on luminol CL was also researched. Experimental results showed that some organic compounds containing -OH, -NH2, and -SH groups could inhibit the CL signal of the luminol-H2O2-ZnO system. The above facts indicated the present system had a wide application for the determination of such compounds. 2. Experimental Section Reagents and Materials. A 2.5 × 10-2 mol/L stock solution of luminol (3-aminophthalhydrazide) was prepared by dissolving luminol (Fluka) in 0.1 mol/L sodium hydroxide solution without purification. Working solutions of H2O2 were prepared fresh daily by dilution of a 30% reagent solution (Shanghai Taopu Chemical Company, China). Zn(CH3CO2)2 · 2H2O, 1-butanol,

10.1021/jp808312j CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

Chemiluminescence Reactions of a Luminol System absolute ethanol, methanol, NaOH, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), β-cyclodextrin (β-CD), polyvinglpyrrolidone (PVP), Tween-80, Tween20, and Triton-100 were obtained from Shanghai Chemical Reagent Plant (Shanghai, China). All chemicals and reagents used in this study were of analytical grade; the water used for the preparation of solutions was deionized and triple-distilled. Apparatus. The chemiluminescence detection was conducted on a model IFFL-E flow-injection CL analyzer (Xi’an, China). Triple-distilled water was used as a carrier to carry the colloidal solution of ZnO nanoparticles to mix with H2O2 and then with luminol. The chemiluminescence signals were monitored by the PMT adjacent to the flow CL cell. When the CL system was used for investigation of the effects of organic compounds on the CL system, the sample solution and the ZnO colloids were injected simultaneously and mixed with each other before further reaction with luminol-H2O2 solutions, in which both ZnO colloids and organic compounds were injected and mixed before reacting with luminol (7.5 × 10-5 mol/L in 0.0075 mol/L NaOH) and H2O2 (0.15 mol/L) of the optimized concentrations. Preparation of Nanoparticles. Colloidal ZnO nanoparticles 6 nm in diameter were synthesized by the sol-gel method from Zn(CH3CO2)2 · 2H2O in absolute ethanol.37 Synthesis of ZnO nanoparticles involves three major steps: (1) preparation of oraganometallic precursor containing 0.1 mol/L Zn, (2) preparation of nearly stoichiometric 0.1 mol/L ZnO colloids, and (3) solvent removal by rotary evaporation (conditions: 12 torr, 25 °C) to concentration levels resulting in desired products. Colloidal ZnO nanoparticles of 16-21 nm diameter were synthesized from Zn(CH3CO2)2 · 2H2O in 1-butanol.38 In this case 9 mmol of Zn(CH3CO2)2 · 2H2O was dissolved in 180 mL of 1-butanol in a covered flask under vigorous stirring at 50 °C. The solution was then heated to 95 °C. Once at the reaction temperature, 60 mL samples were withdrawn at 6, 11, and 24 h. In these experiments, the Zn(CH3CO2)2 · 2H2O concentration was relatively high, 0.05 mol/L, to produce sufficient ZnO for X-ray powder diffraction measurements. ZnO powders were all obtained by precipitation after 30 min centrifugal separation and vacuum drying for 12 h. Optical Measurements. The CL spectra of this system were recorded with a Hitachi FL-4500 spectrofluorometer (Tokyo, Japan) combined with a flow-injection system, with its excitation source turned off. The UV-visible spectra were recorded with a model UV-4100 PC spectrophotometer (Hitachi). The fluorescence spectra were recorded on a model FL-4500 spectrofluorometer with a quartz cell of 1 cm (Tokyo). X-ray powder diffraction (XRD) patterns of the products were carried out on a XRD-6000 X-ray diffractometer (Shimadzu) with Cu KR radiation (λ ) 0.154 06 nm, U ) 40 kV, I ) 30 mA) at a scanning rate of 0.02°/s in the 2θ range from 20° to 80°, and TEM images were taken on a Hitachi model H-800 transmission electron microscope, with an accelerating voltage of 200 kV. Procedures for CL Detection. A diagram of the flow system is shown in Figure 1. The solutions of luminol, H2O2, and H2O were pumped into the flow cell by the peristaltic pump at the rate of 2.6 mL/min, respectively. The ZnO colloid solution was injected by a valve injector with a 100 µL sample loop. The light output from the CL reaction was detected and amplified by the PMT and luminometer. The signal was imported to the computer for data acquisition. The determination of certain analytes of interest was based on changes in CL intensity, ∆I ) Is - I0, where Is and I0 were the CL intensity of sample and

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Figure 1. Diagram of the flow-injection chemiluminescence detection system.

Figure 2. XRD patterns for the growth of ZnO nanocrystals (a-d): 6, 16, 18, and 21 nm.

blank solutions, respectively, which were used for quantitative analysis of biological molecules. 3. Results and Discussions Crystalline Size and Structure. Figure 2 shows the X-ray powder diffraction (XRD) patterns for ZnO nanoparticles. The diffraction peaks at 31.58°, 34.27°, 36.08°, 47.37°, 56.44°, 62.67°, 66.23°, 67.81°, and 68.34° can be found and be indexed as (100), (002), (101), (102), (110), (103), (200), (112), and (201) of hexagonal wurtzite ZnO crystallites by comparison with the date from JCPDS No. 36-1451. By application of the Sheller-Waren formula, the average crystallite sizes of the samples are 6-21 nm. Figure 3 shows the TEM image of ZnO nanoparticles. It can be seen that the majority of the nanoparticles have an average size in the range of 6-21 nm, which is well consistent with the calculation results by Sheller’s equation. The nanoparticles are approximately spherical and hexagonal in shape and aggregate a little. Enhancement of Luminol CL. In alkaline media, the oxidation of luminol by H2O2 generates CL. Figure 4 shows the kinetic curves of the colloids-enhanced CL system, which indicated that ZnO nanoparticles could highly enhance CL systems. The CL signal was enhanced by the 6-21 nm diameter ZnO colloids, and the most intensed CL signal occurred for ZnO colloids 6 nm in diameter. Blank experiments were also carried out, including solutions with the concentrations used as the synthesis conditions; no significant enhancement effects were found for Zn(CH3CO2)2. In order to explore the CL-enhancing phenomena, the following experiments were performed. First, the CL spectrum was drawn using an FL-4500 model spectrofluorimeter combined with a flow-injection system with the entrance slit shut. The CL spectra for 6 nm ZnO colloid mixed with luminol-H2O2 were acquired as shown in Figure 5, and it was clearly indicated that the maximum emission for all the cases was ∼425 nm,

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Figure 5. CL spectra for the luminol-H2O2-6.0 nm ZnO colloid system: (I) ZnO colloid; (II) luminol-H2O2; (III) luminol-H2O2-ZnO colloid (6.0 nm). Inset: PL spectra for ZnO colloids: a, 16 nm; b, 21 nm; c, 18 nm; d, 6.0 nm. Luminol solution: 2.5 × 10-4 mol/L, pH 11.88. H2O2 solution: 5.0 × 10-2 mol/L. ZnO colloids: 2.82 × 10-4 g/mL.

Figure 3. TEM images of ZnO nanocrystals from Zn(CH3CO2)2 in (a) ethanol and (b-d) 1-butanol: (a) ZnO colloids (6 nm); (b) ZnO colloids (16 nm); (c) ZnO colloids (18 nm); (d) ZnO colloids (21 nm).

Figure 4. Kinetic characteristics of the luminol-H2O2-ZnO colloids CL system. Luminol solution: 2.5 × 10-4 mol/L, pH 11.88. H2O2 solution: 5.0 × 10-2 mol/L. ZnO colloids: 2.82 × 10-4 g/mL. Blank 1: H2O. Zn(CH3CO2)2: 2.82 × 10-4 g/mL.

revealing that the luminophor for the CL system was still the excited-state 3-aminophthalate anions (3-APA/).39,40 Therefore, the addition of ZnO nanoparticles did not lead to the generation of a new luminophor for this CL system. The enhanced CL signals were thus ascribed to the possible catalysis from ZnO nanoparticles or any related species. The inset in Figure 5 is the PL spectra of the ZnO nanoparticles, employing the exciting wavelength of 340 nm. In the PL spectra of the ZnO nanoparticles, an ultraviolet emission peak at 386 nm (3.21 eV), an olivaceous emission peak at 559 nm (2.22 eV) (inset a-d), and a visible emission peak at 428 nm (2.89 eV) (inset a-c) can be easily seen. In alkaline medium, ZnO colloids were found to catalyze the luminol-H2O2 system. Generally, the use of surfactants often

makes the corresponding CL system more sensitive.41 We studied different cationic, anionic, and nonionic surfactants such as CTAB, SDS, Tween-80, Tween-20, Triton-100, β-CD, and PVP. However, we found that they did not show a significant influence on the CL reaction. Optimization of the Experimental Conditions. The experimental conditions were optimized for the luminol-H2O2-6 nm ZnO colloids CL system as shown in Figure 6. The results demonstrated that the CL intensity increased with increasing luminol concentration from 1.0 × 10-5 to 2.5 × 10-4 mol/L. Over 2.5 × 10-4 mol/L, the signal decreased dramatically (Figure 6A). The effect of pH on the CL was tested in the range of pH 11-13 (Figure 6B). The optimized pH condition for the luminol-H2O2-6 nm ZnO colloids CL system was pH 11.88. When the pH of the luminol solution was lower than pH 11.88, the CL intensity increased with increasing pH. When the pH of the luminol solution was higher than pH 11.88, the CL intensity decreased with increasing pH. The effect of H2O2 concentration was tested in the range 1 × 10-3-0.2 mol/L; the result is shown in Figure 6C. The CL intensity increased with increasing H2O2 concentration in the range of 1 × 10-3-0.05 mol/L, and only slight changes in light intensity were observed when the concentration of H2O2 was larger than 0.05 mol/L. The effects of the concentration of ZnO nanoparticles were also investigated, as shown in Figure 6D. The CL intensity increased steadily with increasing concentration of ZnO nanoparticles. Considering the CL intensity and the consumption of the reagents, the optimized conditions for the luminol-H2O2-6 nm ZnO colloids CL system were as follows: 2.5 × 10-4 mol/L luminol in 0.0075 mol/L NaOH, 0.05 mol/L H2O2, and the 6 nm ZnO colloids used as synthesized. Mechanism Discussion. The UV-visible absorption spectra as depicted in Figure 7 showed that in media of NaOH (pH ) 11.88) ZnO colloids had a maximum absorption peak at 364 nm, and the luminol-H2O2 system had two absorption peaks at 294 and 346 nm.42,43 Nevertheless, light absorption of the mixed system was equal to the sum of the light absorption of the two individual systems. UV-visible spectra for ZnO nanoparticles before and after the CL reactions also revealed that the position of the absorption for 6 nm ZnO nanoparticles did not change after the reaction, as shown in the inset in Figure 7. Therefore, we suggest that no complex was formed between the species and ZnO nanoparticles and that it may be involved in the reaction of the luminol-H2O2 system. The FT-IR spectra of ZnO nanoparticles (Figure 8) show that the peaks at 1590 and 1384 cm-1 are ascribed to the vibration

Chemiluminescence Reactions of a Luminol System

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Figure 6. Effects of the reactant conditions on the luminol-H2O2-6 nm ZnO colloids CL system. (A) Effect of luminol concentration: 0.01 mol/L NaOH, 0.1 mol/L H2O2, 2.82 × 10-4 g/mL ZnO colloids. (B) Effect of pH of luminol: 2.5 × 10-4 mol/L luminol, 0.1 mol/L H2O2, 2.82 × 10-4 g/mL ZnO colloids. (C) Effect of H2O2 concentration: 2.5 × 10-4 mol/L luminol, pH 11.88, 2.82 × 10-4 g/mL ZnO colloids. (D) Effect of catalyst concentration: 2.5 × 10-4 mol/L luminol, pH 11.88, 0.05 mol/L H2O2.

Figure 7. UV-visible absorption spectra: (a) luminol-H2O2-ZnO colloid (6 nm); (b) luminol-H2O2; (c) ZnO colloids (18 nm); (d) ZnO colloids (16 nm); (e) ZnO colloids (6 nm); (f) ZnO colloids (21 nm). Inset: (1) 6 nm ZnO nanoparticles before the CL reaction; (2) 6 nm ZnO nanoparticles after the CL reaction. Luminol solution: 2.5 × 10-4 mol/L, pH 11.88. H2O2 solution: 5.0 × 10-2 mol/L. ZnO colloids: 2.82 × 10-4 g/mL.

of crystal water and air water and the strongest peaks at 566 and 403 cm-1 to the vibrations of νZn-O. Curve a is ZnO nanoparticles washed by ethanol, curve b and c are obtained from ZnO nanoparticles before and after the CL reaction washed by triple-distilled water. It can be seen that the ZnO nanoparticles washed by different solvents show no difference for the surface of ZnO nanoparticles, and after the CL reaction ZnO nanoparticles still remain unchanged by comparison with the original ones. Therefore, the experiments above demonstrated that a major fraction of the ZnO nanoparticles did not change, and the enhancement of CL signals may have originated from the catalytic effects of ZnO nanoparticles. The reaction mechanism of oxidation of luminol is believed to involve the superoxide radical O2•- or hydroxyl radical OH• as the important intermediates leading to luminescence.44,45 During the luminol oxidation processes, the presence of oxygenrelated radicals46,47 (for example, OH•, O2•-, and other radical

Figure 8. FT-IR spectra of ZnO nanoparticles: (a) ZnO nanoparticles washed by ethanol; (b) ZnO nanoparticles washed by triple-distilled water; (c) after the CL reaction’s ZnO nanoparticles were washed by triple-distilled water.

derivatives) as oxidants is expected to occur. In case of the luminol-H2O2 system, such oxygen-related radicals were supposed to be generated from H2O2. In the absence of a catalyst, the oxidation of luminol by hydrogen peroxide in alkaline solution is a relatively slow reaction process and the CL intensity is relatively weak. It is assumed that the catalyst ZnO nanoparticles may interact with the reactants or the intermediates of the reaction of luminol with hydrogen peroxide. When ZnO nanoparticles were used as the catalysts, they could catalyze the decomposition of H2O2 to make some reactive intermediates such as hydroxyl radical (OH•) and superoxide anion (O2•-).48-51 Then, the resulting products such as hydroxyl radical reacted with luminol to form luminol radical and diazaquinone, which rapidly reacted with superoxide anion or monodissociated hydrogen peroxide, giving rise to light emission. As a result, the emission was enhanced. We suggested that the O-O bond of H2O2 might be broken up into double HO• radicals by virtue of the catalysis of ZnO

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TABLE 1: Inhibition Effects of Organic Compounds on the Luminol-H2O2-ZnO Colloids CL System organic compoundsa L-leucine L-tryptophan L-arginine L-glycine L-cystine L-proline L-asparagine L-serine L-threonine L-cysteine L-isoleucine L-histidine L-valine L-methionine L-aspartic

acid

∆I

quenching,b %

organic compounds

∆I

quenching,b %

-1511 -1464 -1257 -1181 -2041 -2164 -1213 -1161 -1362 -2118 -1440 -2526 -1370 -1074 -1730

40.3 39.0 33.5 31.5 54.4 57.7 32.3 31.0 36.3 56.5 38.4 67.4 36.5 28.6 46.1

dopamine epinephrine pyrogallic acid tannic acid ascorbic acid phenol hydroquinone pyrocatechol p-aminophenol resorcinol 2,4-dihydroxybenzoic acid

-3626 -3437 -3598 -3506 -3004 -1900 -3558 -3623 -3555 -2870 -1680

96.7 91.6 95.9 93.5 80.1 50.7 94.9 96.6 94.8 76.5 44.8

Concentration of organic compounds:10-5 g/mL. b The percentage of quenching was calculated as (-∆I/I0). The blank CL signal I0 obtained by the luminol-H2O2-ZnO colloids system with organic compounds was 3750. a

TABLE 2: Analytical Performance of the Proposed Luminol-H2O2-ZnO Colloids CL System organic compounds L-cystine L-proline L-cysteine L-histidine

dopamine epinephrine pyrogallic acid tannic acid ascorbic acid hydroquinone pyrocatechol p-aminophenol

linear range (g/mL) -9

-6

2.0 × 10 ∼2.0 × 10 2.0 × 10-9∼5.0 × 10-6 2.0 × 10-9∼2.0 × 10-6 2.0 × 10-9∼1.0 × 10-6 8.0 × 10-10∼1.0 × 10-6 2.0 × 10-9∼3.0 × 10-6 2.0 × 10-9∼1.0 × 10-6 2.0 × 10-9∼2.0 × 10-6 2.0 × 10-9∼5.0 × 10-6 1.0 × 10-9∼1.0 × 10-7 2.0 × 10-9∼1.0 × 10-6 2.0 × 10-9∼2.0 × 10-6

regression equation (C: ng/mL) log(-∆I) ) 1.7299 log(-∆I) ) 2.2534 log(-∆I) ) 2.1188 log(-∆I) ) 2.3393 log(-∆I) ) 2.2407 log(-∆I) ) 2.2225 log(-∆I) ) 2.3726 log(-∆I) ) 2.5769 log(-∆I) ) 2.2178 log(-∆I) ) 2.5544 log(-∆I) ) 2.1066 log(-∆I) ) 1.7502

nanoparticles, and the generated hydroxyl radicals might be stabilized by ZnO nanoparticles via partial electron exchange interactions.50,51 The HO• radicals reacted with luminol anion and HO2- to facilitate the formation of luminol radicals (L•-) and superoxide radical anion (O2•-). It was also possible that oxygen dissolved in the solution reacted with L•- to generate O2•-.45 However, deaeration experiments did not support that the dissolved oxygen was involved in the CL reaction because no significant changes in CL intensity (less than 2% change of average CL intensity) were observed when N2 was bubbled to the reactant solutions for 30 min before the reaction and all the solutions were blanketed with N2 during the experiments. Further electron-transfer processes between O2•- and L•- radicals on the surface of ZnO nanoparticles would take place to produce the key intermediate hydroxyl hydroperoxide52 as indicated in Scheme 1, leading to the enhancement of the CL. 4. Application It has been reported that the reducing groups of OH, NH2, or SH reacted readily with the oxygen-containing intermediate radicals.52,53 In the luminol-H2O2-6 nm ZnO colloids system, some intermediate radicals such as HO• and O2•- were formed during the reaction. The reducing groups of OH, NH2, or SH are likely to compete with luminol for active oxygen intermediates, leading to a decrease in CL intensity. Therefore, these compounds may interact with ZnO nanoparticles to interrupt the formation of luminol radicals (L•-) and superoxide radical anion (O2•-) taking place on the surface of ZnO nanoparticles, resulting in a decrease in CL intensity. Therefore, the effects of such organic compounds on the CL system were investigated. The results are listed in Table 1. As expected, all the tested

+ 0.4250 log C + 0.3529 log C + 0.2634 log C + 0.4262 log C + 0.3751 log C + 0.3640 log C + 0.4696 log C + 0.3822 log C + 0.3941 log C + 0.5257 log C + 0.6611 log C + 0.6183 log C

correlation coefficient

detection limit (g/mL)

0.9961 0.9939 0.9942 0.9955 0.9922 0.9963 0.9958 0.9951 0.9940 0.9975 0.9923 0.9947

8.5 × 10-10 9.1 × 10-10 7.9 × 10-10 5.6 × 10-10 8.0 × 10-10 7.4 × 10-10 4.3 × 10-10 2.7 × 10-10 3.8 × 10-10 6.5 × 10-10 1.1 × 10-9 8.9 × 10-10

compounds with the concentration of 1 × 10-5 g/mL inhibited the CL signal of the luminol-H2O2-6 nm ZnO colloids system. The analytical potential of the inhibition effects of the organic compounds containing OH, NH2, or SH groups on the proposed luminol-H2O2-6 nm ZnO colloids CL system was explored by use of a flow-injection procedure. The linear range and SCHEME 1: Possible Mechanism for the Luminol-H2O2-ZnO Colloids CL System

Chemiluminescence Reactions of a Luminol System detection limits for seven selected compounds are presented in Table 2. The results demonstrate that the luminol-H2O2-6 nm ZnO colloids CL system has a wide application for the determination of such compounds. 5. Conclusions In conclusion, ZnO nanoparticles were found to enhance the luminol-H2O2 CL signals. The luminophors of this CL system were still the excited-state 3-APA/. The CL enhancement of ZnO nanoparticles may be attributed to the catalysis of ZnO nanoparticles on the radical generation and electron-transfer processes during the luminol CL reaction. Some organic compounds containing OH, NH2, or SH groups interacting with ZnO nanoparticles were found to inhibit the CL signals of the luminol-H2O2-6 nm ZnO colloids system under the optimized experimental conditions. Some compounds were detectable at the nanogram level by use of a flow-injection method with inhibited CL detection. This work is important for the investigation of new and efficient catalysts for chemiluminescent reactions. Acknowledgment. We are thankful for the financial support of the Natural Science Foundation of Anhui Province (Nos. 2002JQ128, 2003kj146, and 2006KJ006TD), the Doctoral Fund of Anhui Normal University, and the National Natural Science Foundation of P. R. China (No. 20671002). References and Notes (1) Wehrenberg, B. L.; Guyot, S. P. J. Am. Chem. Soc. 2003, 125, 7806–7807. (2) Shim, M.; Wang, C.; Guyot, S. P. J. Phys. Chem. B 2001, 105, 2369–2373. (3) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226–13239. (4) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025–1102. (5) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016–2018. (6) Qu, L. H.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 2049–2055. (7) Shen, L.; Cui, X.; Qi, H.; Zhang, C. J. Phys. Chem. C 2007, 111, 8172–8175. (8) Zou, G. Z.; Ju, H. X. Anal. Chem. 2004, 76, 6871–6876. (9) Dodeigne, C.; Thunus, L.; Lejeune, R. Talanta 2000, 51, 415–439. (10) Easton, P. M.; Simmonds, A. C.; Rakishev, A.; Egorov, A. M.; Candeias, L. P. J. Am. Chem. Soc. 1996, 118, 6619–6624. (11) Ruengsitagoon, W.; Liawruangrath, S.; Townshend, A. Talanta 2006, 69, 976–983. (12) Yeh, H. C.; Lin, W. Y. Talanta 2003, 59, 1029–1038. (13) Lu, J. Z.; Lau, C. W.; Lee, M. K.; Kai, M. Anal. Chim. Acta 2002, 455, 193–198. (14) Liu, Y. M.; Cheng, J, K. J. Chromatogr., A. 2002, 959, 1–13. (15) Powe, A. M.; Fletcher, K. A.; Luce, N. N. St.; Lowry, M.; Neal, S.; McCarroll, M. E.; Oldham, P. B.; McGown, L. B.; Warner, I. M. Anal. Chem. 2004, 76, 4614–4634. (16) Duan, C. F.; Cui, H.; Zhang, Z. F.; Liu, B.; Guo, J. Z.; Wang, W. J. Phys. Chem. C 2007, 111, 4561–4566. (17) Zhang, Z. F.; Cui, H.; Shi, M. J. Phys. Chem. Chem. Phys. 2006, 8, 1017–1021. (18) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B. 2001, 105, 9266– 9272.

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