J. Phys. Chem. C 2007, 111, 4561-4566
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Size-Dependent Inhibition and Enhancement by Gold Nanoparticles of Luminol-Ferricyanide Chemiluminescence Chunfeng Duan, Hua Cui,* Zhifeng Zhang, Bo Liu, Jizhao Guo, and Wei Wang Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui, 230026, People’s Republic of China ReceiVed: December 21, 2006; In Final Form: January 30, 2007
It was found that gold nanoparticles of small size (10 nm) could enhance this CL, and the most intensive CL signals were obtained with 25-nm-diameter gold nanoparticles. The luminophor was identified as the excited-state 3-aminophthalate anion. The studies of UV-visible spectra, CL spectra, X-ray photoelectron spectra, effects of concentrations of luminol and ferricyanide solution, and fluorescence quenching efficiency of gold colloids were carried out to explore the CL inhibition and enhancement mechanism. The CL inhibition by gold nanoparticles of small size was supposed to originate from the competitive consumption of ferricyanide by gold nanoparticles and the relatively high quenching efficiency of the luminophor by gold nanoparticles. In contrast, the CL enhancement by gold nanoparticles of large size was ascribed to the catalysis of gold nanoparticles in the electron-transfer process during the luminol CL reaction and the relatively low quenching efficiency of the luminophor by gold nanoparticles. This work demonstrates that gold nanoparticles have the size-dependent inhibition and enhancement in the CL reaction, proposing a perspective for the investigation of new and efficient nanosized inhibitors and enhancers in CL reactions for analytical purposes.
Introduction With the explosive growth of the field of nanoparticle research, the size- and/or shape-dependent properties of nanoparticles have attracted great attention because of their potential application in many areas such as catalysis,1,2 chemical and biochemical sensing,3-6 and biological imaging.6-8 For example, the size- and shape-dependent surface plasmon resonance (SPR) of metal nanoparticles (in particular, gold and silver nanoparticles) has been widely studied and used in sensing.9,10 The catalytic activity of metal nanoparticles can be changed as well when varying their size and/or shape.11 Furthermore, metal nanoparticles of smaller size are more unstable because of the higher surface energy, leading to higher redox activity.12 Smaller-sized gold nanoparticles exhibit stronger fluorescence quenching effects on the molecules near the surface.13 Recently, in our group, it was found that gold nanoparticles larger than 6 nm could enhance the luminol-H2O2 chemiluminescence (CL) in different degrees.14 The size-dependent enhancement of gold nanoparticles was ascribed to their catalytic activities. However, further investigation of the effect of gold nanoparticles in a size regime from ∼2.7 to 71 nm on the luminol-ferricyanide CL system showed that gold nanoparticles could not only enhance the CL (for sizes larger than 10 nm) but also inhibit the CL (for sizes smaller than ∼5 nm). Herein, we reported the findings. The size-dependent CL inhibition and enhancement mechanisms of gold nanoparticles were investigated carefully by virtue of UV-visible spectra, CL spectra, X-ray photoelectron spectra (XPS), the effects of concentrations of luminol and ferricyanide solution, and the fluorescence quenching efficiency of gold colloids. Finally, the rule that * Corresponding author. Tel: +86-551-3606645; fax: +86-551-3600730; e-mail:
[email protected].
controls the behavior of nanomaterials as CL inhibitors or enhancers is briefly concluded. Experimental Section Chemicals and Solutions. A 1.0 × 10-2 mol/L stock solution of luminol (3-aminophthalhydrazide) was prepared by dissolving luminol (Acros Organics) in 0.1 mol/L sodium hydroxide solution without further purification. Working solutions of luminol were prepared by diluting this stock solution in 0.02 mol/L Na2CO3-NaHCO3 buffers. A 1.0 × 10-4 mol/L stock solution of 3-aminophthalic acid (APA) was prepared by dissolving 3-aminophthalic acid hydrochloride (Acros Organics) in redistilled water without further purification. Working solutions of APA were prepared by diluting the stock solution in 0.01 mol/L sodium hydroxide solution. Potassium ferricyanide and potassium ferrocyanide were purchased from Wenzhou Chemicals (Wenzhou, China) and Xuzhou Chemicals (Jiangsu, China). Sodium borohydride (NaBH4) and tetrachloroauric acid (HAuCl4‚4H2O, 48% w/w) were 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 was prepared by dissolving trisodium citrate (Sanpu Chemical Co. Ltd., Shanghai, China) solids in redistilled water. All of the reagents were of analytical grade, and redistilled water was used throughout. Synthesis of Gold Nanoparticles. The colloidal solutions of gold nanoparticles smaller than 5 nm in diameter were synthesized by the borohydride reduction method,15 while colloidal solutions of gold nanoparticles larger than 10 nm in diameter were synthesized by the citrate reduction method reported by Frens.16 The sizes and shapes of the synthesized gold nanoparticles were characterized by a model JEM-2010
10.1021/jp068801x CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
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Duan et al.
Figure 1. Schematic diagram of the flow injection CL detection system.
high-resolution transmission electron microscope (HRTEM) (JEOL, Japan) for gold nanoparticles smaller than 5 nm and by a model H-800 transmission electron microscope (TEM) (Hitachi, Japan) for gold nanoparticles larger than 10 nm, respectively. Statistical analysis of HRTEM and TEM data revealed that the average diameters of the gold colloids were about 2.7 ( 0.6, 4.9 ( 1.6, 11 ( 1, 16 ( 3, 25 ( 4, 39 ( 5, and 71 ( 7 nm, respectively. The gold colloids were used as synthesized in the CL reaction. For the CL measurement of a blank solution of 4.9 nm gold nanoparticles, the supernatant after a salting out of the colloidal solution with NaCl was used. CL Measurements. The CL detection was conducted on a flow injection CL system (Remax, China) consisting of a model IFFM-D peristaltic pump, a model IFFS-A CL detector, and a photomultiplier tube (PMT), as shown in Figure 1. Redistilled water was used as a carrier for the colloidal solution of gold nanoparticles, which first mixed with luminol working solution (in Na2CO3-NaHCO3 buffer) and then with K3Fe(CN)6 solution. The CL signals were monitored by the PMT adjacent to the flow CL cell. Optical Measurements. The CL spectra of this system were measured on a model RF 5301PC spectrofluorometer (Shimadzu, Japan) when the Xe lamp was turned off. Fluorescence spectra, UV-visible spectra, and XPS were measured on a model RF 5301PC spectrofluorometer (Shimadzu, Japan), a model UV-2401PC spectrophotometer (Shimadzu, Japan), and a model Escalab MK II electron spectrograph (VG), respectively. For XPS measurements, solid precipitates could be obtained by a salting out from the gold colloids with NaCl, collected after centrifugation, and dried in vacuum at room temperature. Results and Discussion Size-Dependent Inhibition and Enhancement of Luminol CL. The effects of gold colloids of different size on the luminol-K3Fe(CN)6 chemiluminescent system were investigated. As shown in Figure 2, the CL signal was inhibited by the gold colloids smaller than 5 nm in diameter, while it was enhanced by the 11-71-nm-diameter gold colloids, and the most intensive CL signal was obtained for gold colloids of 25 nm in diameter. The blank experiments, including Na3C6O7, NaBH4-Na3C6O7, and HAuCl4 solutions with the concentrations used as the synthesis conditions, were also carried out to explore the origins of the inhibition and enhancement. As shown in the inset of Figure 2A, the NaBH4-Na3C6O7 blank solutions (used for synthesis of
