Gold Nanoparticle-Catalyzed Luminol Chemiluminescence and Its

Anal. Chem. 2005, 77, 3324-3329

Gold Nanoparticle-Catalyzed Luminol Chemiluminescence and Its Analytical Applications Zhi-Feng Zhang, Hua Cui,* Chun-Ze Lai, and Li-Juan Liu

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Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China

Gold colloids with nanoparticles of different sizes were found to enhance the chemiluminescence (CL) of the luminol-H2O2 system, and the most intensive CL signals were obtained with 38-nm-diameter gold nanoparticles. UV-visible spectra, X-ray photoelectron spectra, and transmission electron microscopy studies were carried out before and after the CL reaction to investigate the CL enhancement mechanism. The CL enhancement by gold nanoparticles of the luminol-H2O2 system was supposed to originate from the catalysis of gold nanoparticles, which facilitated the radical generation and electron-transfer processes taking place on the surface of the gold nanoparticles. The effects of the reactant concentrations, the size of the gold nanoparticles. and some organic compounds were also investigated. Organic compounds containing OH, NH2, and SH groups were observed to inhibit the CL signal of the luminol-H2O2-gold colloids system, which made it applicable for the determination of such compounds. Since the chemiluminescence (CL) phenomenon of luminol was first reported by Albrecht in 1928,1 investigation of effective catalysts for such CL reactions has been carried out, including metal ions, metal complex, and enzymes.2-10 The catalyzed luminol CL has been successfully applied in bioanalysis and immunoassay11,12 or as sensitive detectors for high-performance liquid chromatography (HPLC)13,14 or capillary electrophoresis (CE).15-17 However, the application of metal nanoparticles as catalysts for * Corresponding author. Phone: +86-551-3606645. Fax: +86-551-3601592. E-mail: [email protected]. (1) Albrecht, H. O. Z. Phys. Chem. 1928, 136, 321-330. (2) Schneider, E. J. Am. Chem. Soc. 1941, 63, 1477-1478. (3) Shevlin, P. B.; Neufeld, H. A. J. Org. Chem. 1970, 35, 2178-2182. (4) Bostick, D. T.; Hercules, D. M. Anal. Chem. 1975, 47, 447-452. (5) White, E. H.; Bursey, M. M. J. Am. Chem. Soc. 1964, 86, 941-942. (6) Mere´nyi, G.; Lind, J. S. J. Am. Chem. Soc. 1980, 102, 5830-5835. (7) Easton, P. M.; Simmonds, A. C.; Rakishev, A.; Egorov, A. M.; Candeias, L. J. Am. Chem. Soc. 1996, 118, 6619-6624. (8) Kricka, L. J.; Voyta, J. C.; Bronstein, I. Methods Enzymol. 2000, 305, 370390. (9) Liu, Y. M.; Cheng, J. K. J. Chromatogr., A 2002, 959, 1-13. (10) Yeh, H. C.; Lin, W. Y. Talanta 2003, 59, 1029-1038. (11) Niazov, T.; Pavlov, V.; Xiao, Y.; Gill, R.; Willner, I. Nano Lett. 2004, 4, 16831687. (12) Pavlov, V.; Xiao, Y.; Gill, R.; Dishon, A.; Kotler, M.; Willner, I. Anal. Chem. 2004, 76, 2152-2156. (13) Dapkevicius, A.; Van Beek, T. A.; Niederlander, H. A. G.; De Groot, A. Anal. Chem. 1999, 71, 736-740. (14) Parejo, I.; Viladomat, F.; Bastida, J.; Schmeda-Hirschmann, G.; Burillo, J.; Codina, C. J. Agric. Food Chem. 2004, 52, 1890-1897.

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the luminol CL system has not yet been reported, to the best of our knowledge. Gold nanoparticles were one of the most widely used nanomaterials in the recent decades. The catalysis of gold nanoparticles for gas-phase18-20 and liquid-phase21 redox reactions is now an expanding area. Recently, our group reported that 16-nm gold nanoparticles could greatly enhance two anodic and one cathodic electrogenerated chemiluminescence (ECL) peaks of luminol in neutral and alkaline media and initiate one new cathodic ECL peak for their unique catalytic property and electrochemical reactivity.22 However, most of these studies were restricted to gold nanoparticles supported on various matrixes. According to Lopez and coworkers,23 the most important factor that rules the catalysis of gold nanoparticles is the availability of many low-coordinated gold atoms on the small particles. Therefore, the interaction between oxide matrix and gold nanoparticles was not essential for the catalytic activity of the gold nanoparticles.24 In other words, unsupported gold nanoparticles, i.e., gold colloids, might be directly used as the catalysts of liquid-phase chemical reactions. For example, Sau et al.25 reported catalyzed reduction of eosin in the presence of 10-46-nm gold colloids. Such unsupported gold colloids may also be employed as catalysts for luminol CL. Luminol-H2O2 CL reaction, a popular CL reaction, has been widely applied for the detection of various substances.11-17 In this work, we chose the luminol-H2O2 CL reaction as a model system and explored the effect of colloidal solutions of gold nanoparticles on the CL for the first time. It was found that gold nanoparticles with a size regime from 6 to 99 nm could enhance the CL from the luminol-H2O2 system. The enhancement mechanism of gold nanoparticles on luminol CL was investigated. The analytical application potential for the luminol-H2O2-gold colloids CL assay was exploited. (15) Wang, J.; Huang, W.; Liu, Y.; Cheng, J.; Yang, J. Anal. Chem. 2004, 76, 5393-5398. (16) Tsukagoshi, K.; Nakahama, K.; Nakajima, R. Anal. Chem. 2004, 76, 44104415. (17) Liu, B.-F.; Ozaki, M.; Utsumi, Y.; Hattori, T.; Terabe, S. Anal. Chem. 2003, 75, 36-41. (18) Haruta, M. Catal. Today 1997, 36, 153-166 and references therein. (19) Wallace, W. T.; Whetten, R. L. J. Am. Chem. Soc. 2002, 124, 7499-7505. (20) O’Hair, R. A. J.; Khairallah, G. N. J. Cluster Sci. 2004, 15, 331-363. (21) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552-560. (22) Cui, H.; Xu, Y.; Zhang, Z. F. Anal. Chem. 2004, 76, 4002-4010. (23) Lopez, N.; Janssens, T. V. W.; Clausen, B. S.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Norskov, J. K. J. Catal. 2004, 223, 232-235. (24) Yang, C. M.; Kalwei, M.; Schuth, F.; Chao, K. J. Appl. Catal. A 2003, 254, 289-296. (25) Sau, T. S.; Pal, A.; Pal, T. J. Phys. Chem. B 2001, 105, 9266-9272. 10.1021/ac050036f CCC: $30.25

© 2005 American Chemical Society Published on Web 04/05/2005

EXPERIMENTAL SECTION Chemicals and Solutions. A 1.0 × 10-2 mol/L stock solution of luminol (3-aminophthalhydrazide) was prepared by dissolving luminol (Sigma) in 0.1 mol/L sodium hydroxide solution without purification. Working solutions of luminol were prepared by diluting the stock solution. Working solutions of H2O2 were prepared fresh daily from 30% (v/v) H2O2 (Xinke Electrochemical Reagent Factory, Bengbu, China). HAuCl4‚4H2O (48% w/w) was obtained from Shanghai Reagent (Shanghai, China). A 1.0 g/L HAuCl4 stock solution was prepared by dissolving 1 g of HAuCl4 in 1 L of redistilled water and stored at 4 °C. A 1% (w/w) trisodium citrate (Na3C6O7) solution were prepared by dissolving trisodium citrate (Sanpu Chemical Co. Ltd., Shanghai, China) solids in double-distilled water. All the reagents were of analytical grade, and double-distilled water was used throughout. Synthesis of Gold Nanoparticles. A colloidal solution of 6-nm-diameter gold nanoparticles was synthesized by the hydroborate reduction method,26 while colloidal solutions of 16-, 25-, 38-, 68-, and 99-nm-diameter gold nanoparticles were synthesized by the citrate reduction method.27 The size and shape of the synthesized gold nanoparticles were characterized by a model H-800 transmission electron microscope (TEM; Hitachi). Statistical analysis of TEM data revealed that the average diameters of the gold colloids were about 6.0 ( 3.5, 16 ( 1.0, 25 ( 2.0, 38 ( 3.6, 68 ( 5.1, and 99 ( 7.0 nm, respectively. The concentrations of gold nanoparticles corresponding to different sizes above were calculated to be 3.26 × 10-8, 1.72 × 10-9, 4.51 × 10-10, 1.29 × 10-10, 2.24 × 10-11, 7.27 × 10-12 mol/L, respectively, by assuming the gold nanoparticles as truncated octahedral-shaped particles.28 Chemiluminescence Measurements. The chemiluminescence detection was conducted on a laboratory-built flow injection chemiluminescence system, consisting of a model IFIS-C flow injection system (Ruimai Electronic Science Co.), a model CR105 photomultiplier tube (PMT) (Bingsong Electronic Co.), a model GD-1 luminometer (Ruimai Electronic Science Co.), and a computer, as shown in the Supporting Information. Doubledistilled water was used as a carrier to carry the colloidal solution of gold nanoparticles to mix with luminol and then with H2O2. 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 gold colloids were injected simultaneously and mixed with each other before further reaction with luminol-H2O2 solutions, as shown in the Supporting Information, in which both gold colloids and organic compounds were injected and mixed before reacting with luminol (0.0002 mol/L in 0.01 mol/L NaOH) and H2O2 (0.15 mol/L) of the optimized concentrations. The value of ∆I ) (I0 - I) showed the effect of organic compounds on the CL intensity of luminol-H2O2-38-nm gold colloids system, where I0 stands for the signal in the absence of organic compounds and I stands for the signal in the presence of organic compounds, which was used for quantitative analysis of organic compounds. Optical Measurements. The CL spectra of this system were measured on a model FL 5401 spectrofluorometer (Shimadzu) (26) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 11541157. (27) Frens, G. Nat. Phys. Sci. 1973, 241, 20-22. (28) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36.

Figure 1. Chemiluminescence profiles of luminol-H2O2 mixed with gold colloids and three blank solutions: HAuCl4, 1 × 10-4 g/mL; blank 1, 2 × 10-4 g/mL Na3 C6O7; blank 2, 5.5 × 10-5 g/mL Na3C6O7, 1.125 × 10-6 NaBH4. Conditions: luminol, 2 × 10-4 mol/L in 0.01 mol/L NaOH; H2O2, 0.01 mol/L.

with the excitation light source being turned off. Fluorescence spectra, UV-visible spectra, and X-ray photoelectron spectra were measured on the model FL 5401 spectrofluorometer (Shimadzu), a model UV-2401 PC spectrophotometer (Shimadzu), and a model Escalab MK II electron spectrograph (VG). RESULTS AND DISCUSSION Enhancement of Luminol CL. The effects of gold colloids on the luminol-H2O2 chemiluminescent system were investigated. As shown in Figure 1, the CL signal was enhanced by the 6-99nm-diameter gold colloids, and the most intensive CL signal was obtained for gold colloids of 38 nm in diameter. Blank experiments were also carried out, including HAuCl4, Na3C6O7, and NaBH4-Na3C6O7 solutions with the concentrations used as the synthesis conditions. As shown in Figure 1, no significant enhancement effects were found for Na3C6O7 and Na3C6O7-NaBH4 blank solutions; whereas 1 × 10-4 g/mL HAuCl4 solution used for synthesizing gold nanoparticles could enhance the luminol CL signal similar to other reports,29 but the enhancement effect of HAuCl4 was much lower than that of the 38-nm gold colloids. Furthermore, the concentration of unreacted HAuCl4 in gold colloids was much lower than 1 × 10-4 g/mL, and thus, its possible contribution to the CL intensity for gold colloids would be negligible. Therefore, the size-related enhancement of gold colloids was ascribed to the gold nanoparticles or related species but not concomitant species such as NaBH4, Na3C6O7, or HAuCl4. The CL spectra for different gold colloids mixed with luminolH2O2 were acquired as shown in Figure 2. It was clearly indicated that the maximum emission for all the cases was ∼425 nm, revealing that the luminophor for the CL system was still the excited-state 3-aminophthalate anions (3-APA). Therefore, the addition of gold 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 gold nanoparticles or any related species.18-21,30 Identification of Gold Nanoparticles after the CL Reaction. Thompson reported that the active species for gold catalysts could be gold species in oxidized state, namely, Au(I) and Au(III) species, or metallic gold nanoparticles, depending on the type of (29) Imdadullah; Fujiwara, T.; Kumamaru, T. Anal. Chem. 1991, 63, 2348-2352. (30) Thompson, D. Gold Bull. 1998, 31, 111-118 and references therein.

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Figure 2. Chemiluminescence spectra for luminol-H2O2-gold colloids system. Conditions are as in Figure 1.

Figure 3. XPS of 38-nm gold nanoparticles before and after the CL reaction.

reactions involved.30 Luminol can react with H2O2 to generate weak CL. The gold nanoparticles may be involved in the luminol-H2O2 CL reaction as catalysts. In this case, the gold nanoparticles still remain after the CL reaction. It is also possible that the gold nanoparticles are oxidized to Au(I) and Au(III) species during the CL reaction. Therefore, it is of vital importance to identify whether the gold nanoparticles or the oxidized gold species were presented in the solution after the CL reaction. For 38-nm gold colloids, XPS studies were carried out to investigate the oxidation state of gold before and after the CL reaction, and the results are shown in Figure 3. No change of the binding energies of gold was found before and after the CL reaction, excluding the possibility of catalysis from Au(I) or Au(III) species. UV-visible spectra for all the reactants before and after the CL reactions in Figure 4 showed that the position of the surface plasma resonance (SPR) absorption for 38-nm gold nanoparticles did not change after the reaction. TEM studies also revealed that the shape and size of 38-nm nanoparticles did not change before and after the CL reaction, as shown in the inset photos in Figure 4. Therefore, the enhancement of CL signals may have originated from the catalytic effects of gold nanoparticles. However, the experiments above only demonstrated that a major fraction of the Au did not change, small concentrations of oxidized gold were difficult to be detected by XPS or UV-visible spectra, and possible powerful effects from small concentrations of oxidized gold could not be totally ruled out. Optimization of the Reaction Conditions. The reaction conditions were optimized for the luminol-H2O2-38-nm gold colloids CL system as shown in Figure 5. The results demonstrated that the CL intensity increased linearly with increasing luminol concentration from 1 × 10-6 to 1 × 10-3 mol/L (Figure 5A). The effect of pH on the CL was tested in the range of pH 3326 Analytical Chemistry, Vol. 77, No. 10, May 15, 2005

Figure 4. UV-visible spectra for 6- and 38-nm gold colloids and blank solutions (A) before the CL reaction and (B) after the CL reaction. The insets were TEM photos of 38-nm gold nanoparticles before and after the CL reaction. Conditions are as in Figure 2.

11-13 (Figure 5B). The optimized pH condition for luminolH2O2-38-nm gold colloids CL system was pH 12. When the pH of luminol solution was lower than pH 12, the CL intensity increased with increasing pH; when the pH of luminol solution was higher than pH 12, the CL intensity would decrease with increasing pH. The effect of H2O2 concentration on the CL was studied in the range of 1 × 10-4 ∼ 0.30 mol/L (Figure 5C), the CL intensity increased with increasing H2O2 concentration in the range of 1 × 10-4 ∼ 0.15 mol/L and only slight changes in light intensity were observed when the concentration of H2O2 was larger than 0.15 mol/L. The effects of the concentration of gold nanoparticles were also investigated, as shown in Figure 5D. The CL intensity increased steadily with increasing concentration of gold nanoparticles. Considering the CL intensity and the consumption of the reagents, the optimized conditions for the luminol-H2O2-38-nm gold colloids CL system were as follows: 2 × 10-4 mol/L luminol in 0.01 mol/L NaOH, 0.15 mol/L H2O2, and the 38-nm gold colloids were used as synthesized. Mechanism Discussion. The major CL-generating mechanism for luminol oxidation in aqueous solution has been summarized by Mere´nyi and co-workers31 to occur in three basic steps, as shown in Scheme 1: (1) oxidation of luminol to the luminol radical; (2) oxidation of the luminol radical to hydroxy hydroperoxide, the key intermediate; (3) decomposition of hydroxy hydroperoxide with or without the emission of CL, among which step 1 was supposed to be the rate-determining step of luminol CL.32 During the luminol oxidation processes, the presence of oxygen-related radicals (for example, OH•, O2•-, and other radical derivatives) as oxidants is expected to occur. As the case for the luminol-H2O2 system, such oxygen-related radicals were supposed to be generated from H2O2. Because the reaction of luminol with hydrogen peroxide in alkaline solution in the absence of a catalyst underwent weak CL, it is assumed that the catalyst gold nanoparticles may interact with the reactants or the intermediates of the reaction of luminol with hydrogen peroxide. When gold nanoparticles were used as the catalysts, the formation of active oxygen-containing reactant intermediates such as OH• and O2•- were frequently reported.33-35 (31) Mere´nyi, G.; Lind, J.; Eriksen, T. E. J. Biolumin. Chemilumin. 1990, 5, 53-56. (32) Burdo, T. G.; Seitz, W. R. Anal. Chem. 1975, 47, 1639-1643. (33) Grisel, R.; Weststrate, K. J.; Gluhoi, A.; Nieuwenhuys, B. E. Gold Bull. 2002, 35, 39-45.

Figure 5. Effects of the reactant conditions on the luminol-H2O2-38-nm gold colloids CL system. (A) Effects of luminol concentration: 0.01 mol/L NaOH, 0.01 mol/L H2O2, gold colloids as synthesized. (B) Effects of pH of luminol: 2 × 10-4 mol/L luminol, 0.01 mol/L H2O2, gold colloids as synthesized. (C) Effects of H2O2 concentration: 2 × 10-4 mol/L luminol in 0.01 mol/L NaOH, gold colloids as synthesized. (D) Effects of catalyst concentration: 2 × 10-4 mol/L luminol in 0.01 mol/L NaOH, 0.01 mol/L H2O2.

Scheme 1. Schematic CL-Generating Mechanism for the Oxidation of Luminol with Three Major Steps

Furthermore, Landon and co-workers36 reported that gold nanoparticles could be used as catalysts for the synthesis of H2O2 from the hydrogenation of O2 at low temperature and would further lead to the decomposition of H2O2. Epoxidation reactions34 were also reported to take place on the surface of gold nanoparticles via the reversible generation of O-O bond. Similarly, we suggested that the O-O bond of H2O2 might be broken up into double HO• radicals by virtue of the catalysis of gold nanoparticles, and the generated hydroxyl radicals might be stabilized by gold nanoparticles via partial electron exchange interactions.37 The HO• radicals reacted with luminol anion and HO2- to facilitate the formation of luminol radicals (L•-) and superoxide radical anion (O2•-).6 It was also possible that oxygen dissolved in the solution reacted with L•- to generate O2•-.31 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% changes of average CL intensity) were observed when N2 or Ar was bubbled to the reactant solutions for 20 min before the reaction and all the solutions were blanketed with N2 or Ar during the experiments. Further electron-transfer processes (34) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566-575. (35) Overbury, S. H.; Ortiz-Soto, L.; Zhu, H. G.; Lee, B.; Amiridis, M. D.; Dai, S. Catal. Lett. 2004, 95, 99-106. (36) Landon, P.; Collier, P. J.; Carley, A. F.; Chadwick, D.; Papworth, A. J.; Burrows, A.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1917-1923. (37) Zhang, Z.; Berg, A.; Levanon, H.; Fessenden, R. W.; Meisel, D. J. Am. Chem. Soc. 2003, 125, 7959-7963.

between L•- and O2•- radicals on the surface of gold nanoparticles would take place to produce the key intermediate hydroxy hydroperoxide8 as indicated in Scheme 2, leading to the enhancement of the CL. The catalysis of gold nanoparticles was found to relate to their sizes (Figure 1). It is well known that the rate of heterogeneous catalysis increases with the available active surface area of the catalyst.38,39 Therefore, a higher number of particles of smaller size would be present in a given mass of catalyst material, leading to a higher rate of catalytic reaction. This has been exactly observed for particles of a size larger than 38 nm in diameter: the active surface areas of the gold nanoparticles decreased with increasing particle size, and the catalytic efficiency of the gold nanoparticles decreased accordingly. However, for gold nanoparticles of an average size smaller than 38 nm in diameter, an opposite effect was observed, indicating that another parameter was playing its role to manifest this effect. It is presumed that the adsorption of H2O2 onto the surface of gold nanoparticles would cause partial electron transfer from the gold nanoparticles to the adsorbed H2O2, which was known as particle-mediated electron transfer.38,39 Therefore, the electron density in the conduction bands of gold nanoparticles would influence the particle-mediated electron-transfer processes and higher electron density would be advantageous for the particle-mediated electron(38) Sharma, R. K.; Sharma, P.; Maitra, A. J. Colloid Interface Sci. 2003, 265, 134-140. (39) Henglein, A. J. Phys. Chem. 1993, 97, 5457-5471.

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Scheme 2. Possible Mechanism for the Luminol-H2O2-Gold Colloids CL System

Table 1. Inhibition Effects of Organic Compounds (10-4 mol/L) on Luminol-H2O2-38-nm Gold Colloids CL System organic compounds

I

quenching,a %

blankb phenol catechol resorcinol phloroglucinol pyrogallol ascorbic acid hydroquinone gallic acid adrenaline noradrenaline dopamine chlorogenic acid p-aminothiophenol sodium thioglycolate

2448 1552 32 348 1054 331 222 370 310 198 47 16 198 572 364

63.4 1.3 14.2 43.1 13.5 9.1 15.1 12.7 8.1 1.9 0.7 8.1 23.4 14.9

organic compounds

I

quenching,a %

L-alanine L-glutamine L-glutamic acid

1902 2106 2110 1698 1707 1611 1579 1533 1460 1378 1093 1614 1117 176

77.7 86.0 86.2 69.4 69.7 65.8 64.5 62.6 59.6 56.3 44.6 65.9 45.6 7.2

glycine L-leucine L-serine L-arginine L-phenylalanine L-threonine L-tryptophan L-histidine L-cystine L-cysteine

glutathione

a The percentage of quenching was calculated as I/I . b The blank CL signal I obtained by luminol-H O -38-nm gold colloids system without 0 0 2 2 organic compounds was 2448.

transfer processes. SPR absorption intensity from UV-visible absorption spectra is considered to be an indicator of the electron density in the conduction bands of nanoparticles.40 UV-visible absorption spectra (Figure 6B) showed that the SPR absorption intensity of the gold nanoparticles decreased with decreasing size below 38 nm in diameter, reached the maximum with 38-nm gold colloids, and then decreased once again with further increase of the particle size. The change trend of the SPR absorption intensity of the gold nanoparticles with their sizes was similar to that of the CL intensity of luminol-H2O2-gold colloids systems with their sizes, and both maximum of SPR absorption intensity and CL intensity were observed for 38-nm gold colloids. Therefore, the catalytic effects would also decrease with decreasing size below 38 nm in diameter due to a decrease in electron density with decreasing size, leading to lower CL intensity. For 6-nm gold (40) Link, S.; El-sayed, M. A. Int. Rev. Phys. Chem. 2002, 19, 409-453.

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nanoparticles, extremely small CL intensity was observed, probably because that quantum size effects began to function with an increase in band gap energy, leading to a higher activation energy that was needed for electron transfer. In conclusion, 38-nmdiameter gold nanoparticles showed the strongest catalysis because of proper electronic structures, proper surface area, and high electron density in the conduction bands. Analytical Application Studies. Organic compounds containing hydroxyl (OH), amino (NH2), or mercapto (SH) groups have been reported to interact readily with gold nanoparticles.41 It is possible that these compounds interact with gold nanoparticles in the CL reaction leading to a change in CL intensity. Herein, the effects of such organic compounds on the luminol-H2O238-nm gold colloids CL system were investigated. The results were (41) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. J. Phys. Chem. B 2004, 108, 13963-13971.

Table 2. Analytical Performance of the Proposed Luminol-H2O2-38-nm Gold Colloids CL System

sample

linear range (mol/L)

equation of linear regression

related coefficient

detection limit (3SD, n ) 5), mol/L

catechol ascorbic acid adrenalin noradrenalin dopamine L-histidine L-cysteine

5 × 10-9-2 × 10-6 5 × 10-9-5 × 10-6 5 × 10-9-5 × 10-6 2 × 10-9-5 × 10-6 1 × 10-9-5 × 10-6 2 × 10-9-2 × 10-6 2 × 10-9-2 × 10-6

lg∆I ) 0.92139 lgC + 8.71296 lg∆I ) 0.69076 lgC + 7.00397 lg∆I ) 0.78724 lgC + 7.78253 lg∆I ) 0.58635 lgC + 6.19813 lg∆I ) 0.48979 lgC + 5.76644 lg∆I ) 0.60106 lgC + 6.27624 lg∆I ) 0.63077 lgC + 6.57657

0.997 0.996 0.995 0.995 0.994 0.991 0.995

1.7 × 10-9 2.0 × 10-9 1.9 × 10-9 8.4 × 10-10 1.9 × 10-10 9.3 × 10-10 6.4 × 10-10

H2O2-38-nm gold colloids CL system is capable of a response to a number of compounds. However, the usefulness of this technique in terms of selectivity may be limited. If it combined with separation techniques, this CL detection will not be problematic. Therefore, it is ideal for the design of a CL detector in HPLC and high-performance capillary electrophoresis by use of this CL system for the simultaneous detection of numerous compounds. Furthermore, the excellent derivatization ability of gold nanoparticles11,44-46 might make this CL system applicable in immunoassay and bioanalysis research.

Figure 6. Size effects on (A) CL intensity of the luminol-H2O2gold colloids system and (B) SPR absorption intensity of gold nanoparticles. Conditions are as in Figure 2.

listed in Table 1. As expected, all the tested compounds with the concentration of 10-4 mol/L inhibited the CL signal of luminolH2O2-38-nm gold colloids system. It has been reported that the reducing groups of OH, NH2, or SH reacted readily with the oxygen-containing intermediate radicals.42,43 In the luminol-H2O2-38-nm gold colloids system, some intermediate radicals such as OH• 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. On the other hands, organic compounds containing OH, NH2, or SH groups have been reported to interact readily with gold nanoparticles.41 Therefore, these compounds may interact with gold nanoparticles to interrupt the formation of luminol radicals (L•-) and superoxide radical anion (O2•-) taking place on the surface of gold nanoparticles, resulting in a decrease in CL intensity. The analytical potential of the inhibition effects of the organic compounds containing OH, NH2, or SH groups on the proposed luminol-H2O2-38-nm gold colloids CL system was explored by use of a flow injection procedure. The linear range and detection limits for seven selected compounds were presented in Table 2. It could be seen that the linear range for all the compounds could reach 3 orders of magnitude, and the detection limits were even as low as 10-10 mol/L. The results demonstrate that the luminol(42) Cui, H.; Shi, M. J.; Meng, R.; Zhou, J.; Lai, C. Z.; Lin, X. Q. Photochem. Photobiol. 2004, 79, 233-241. (43) Lau, C.; Qin, X.; Liang, J.; Lu, J. Anal. Chim. Acta 2004, 514, 45-49. (44) Park, S. J.; Lazarides, A. A.; Storhoff, J. J.; Pesce, L.; Mirkin, C. A. J. Phys. Chem. B 2004, 108, 12375-12380. (45) Zheng, M.; Huang, X. Y. J. Am. Chem. Soc. 2004, 126, 12047-12054. (46) Jaramillo, T. F.; Baeck, S. H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148-7149.

CONCLUSIONS Gold nanoparticles were found to enhance the luminol-H2O2 CL signals in this work. The luminophors of this CL system were still the excited-state 3-APA*. The CL enhancement of gold nanoparticles was suggested to be due to the catalysis of gold nanoparticles on the radical generation and electron-transfer processes during the luminol CL reaction. Some organic compounds containing hydroxyl, amino, or mercapto groups interacting with gold nanoparticles were observed to inhibit the CL signals of the luminol-H2O2-38-nm gold colloids system at the experimental conditions. Some compounds were detectable at the nanomole per liter level by use of a flow injection method with the inhibited CL detection. This work is important for the investigation of new and efficient catalysts for chemiluminescent reactions, and the proposed luminol-H2O2-38-nm gold colloids system is of great analytical potential in developing new immunoassay and CL detectors with a wide response to a range of compounds for HPLC and CE. ACKNOWLEDGMENT The support of this research by the National Natural Science Foundation of P.R. China (Grant 20375037) and the Overseas Outstanding Young Scientist Program of China Academy of Sciences are gratefully acknowledged. We also thank Prof. X. M. Liu, Prof. M. R. Ji, and Prof. H. Z. Zhao for measurements. SUPPORTING INFORMATION AVAILABLE The schematic diagram of the flow injection CL systems, and the UV-visible spectra for gold colloids of various sizes as synthesized. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review January 7, 2005. Accepted March 14, 2005. AC050036F Analytical Chemistry, Vol. 77, No. 10, May 15, 2005

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