Lucigenin Chemiluminescence Induced by Noble Metal Nanoparticles

Aug 2, 2007 - It was found that noble metal nanoparticles including Ag, Au, and Pt ... X-ray diffraction patterns and fluorescence spectra indicated t...
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12254

J. Phys. Chem. C 2007, 111, 12254-12259

Lucigenin Chemiluminescence Induced by Noble Metal Nanoparticles in the Presence of Adsorbates Ji-Zhao Guo and Hua Cui* Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui 230026, China ReceiVed: May 17, 2007; In Final Form: June 27, 2007

It was found that noble metal nanoparticles including Ag, Au, and Pt nanoparticles in the presence of adsorbates such as iodide ion, cysteine, mercaptoacetic acid, mercaptopropionic acid, and thiourea could reduce lucigenin (bis-N-methylacridinium) to produce chemiluminescence (CL). Lucigenin-Ag-KI system was chosen as a model to study the CL process. Absorption spectra and X-ray photoelectron spectra showed that when the Ag colloid was mixed with KI, Ag nanoparticles were covered by adsorbed iodide ions. X-ray diffraction patterns and fluorescence spectra indicated that Ag nanoparticles were oxidized to AgI and lucigenin was converted to N-methylacridone in the CL reaction. The addition of superoxide dismutase could inhibit the CL. According to Nernst’s equation, the presence of iodide ions decreased the oxidation potential of Ag nanoparticles. As a result, lucigenin was rapidly reduced by Ag nanoparticle to a monocation radical, which reacted with oxygen to generate a superoxide anion; then the superoxide anion reacted with the monocation radical to produce CL. Other adsorbates such as cysteine, mercaptoacetic acid, mercaptopropionic acid, and thiourea that could decrease the oxidation potential of Ag nanoparticles could also induce the CL reaction.

Introduction The applications of metal nanoparticles have drawn much attention in electrochemistry,1-3 catalysis,4-6 and chemical sensing7-9 because of their special size-dependent physical and chemical properties. Light emission induced by chemical reactions, known as chemiluminescence (CL), has been intensively investigated for many years. Recently, the CL study has been extended to nanoparticle systems from traditional molecular systems. In these systems, metal nanoparticles can participate in CL reactions as a catalyst, reductant, luminophor, and energy acceptor. The related CL systems include a luminol-hydrogen peroxide system,10-12 phenyl oxalate ester-hydrogen peroxide system,13-16 potassium permanganate system,17 and Ru(bpy)32+Ce(IV) system.18 Among them, some CL systems have been used for the determination of some analytes such as -SH and -NH2 compounds,10 citrate,18 DNA, and IgG.19 Lucigenin (bis-N-methylacridinium) as an important CL reagent is widely used as a unique assay for superoxide formation in biological systems.20 It is readily reduced by some compounds to generate CL under alkaline conditions.21,22 The electrogenerated chemiluminescence (ECL) of lucigenin was enhanced on a Au nanoparticles modified Au electrode compared with that on a bulk electrode.23 A new ECL peak of lucigenin was observed during the hydrogen evolution process when Pt nanoparticles were dispersed in a basic lucigenin solution, in which hydrogen atoms adsorbed by Pt nanoparticles as a reductant induced the new ECL peak.24 However, nanoparticle-involved lucigenin CL in liquid phase has not yet been reported. In the present work, it was found that in the presence of KI, Ag nanoparticles as a reductant could induce the liquid-phase CL of lucigenin. The CL mechanism was investigated by analyzing the interaction among the Ag colloid, KI, lucigenin, and the reacted products by virtue of * Corresponding author. Phone: +86-551-3606645. Fax: +86-5513600730. E-mail: [email protected].

absorption spectra, transmission electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectra, X-ray diffraction patterns, and fluorescence spectra. Furthermore, the CL behavior of lucigenin-Ag colloid in the presence of other adsorbates such as thiourea, sodium thiosulfate, cysteine, mercaptoacetic acid, and mercaptopropionic acid was explored. Instead of Ag nanoparticles, Au and Pt nanoparticles were also examined for inducing lucigenin CL in the presence of various adsorbates. This work provides a new way to design CL systems. The new CL system obtained may find applications in the future. Experimental Section Chemicals. Lucigenin (Luc2+•2NO3-) was obtained from Fluka Chemie AG (Switzerland). AgNO3, HAuCl4, H2PtCl6, NaBH4, and sodium citrate were obtained from Shanghai Chemical Reagent Company (Shanghai, China). All reagents were of analytical grade and used as received. Triple-distilled water was used throughout. Colloid Preparation. The Ag colloid was prepared by a common method (NaBH4 reduction method) according to the literature.25 A 25 mL 1 × 10-3 mol/L AgNO3 aqueous solution was added dropwise to 75 mL 2 × 10-3 mol/L NaBH4 aqueous solution with vigorous stirring simultaneously. 10 min later, a 5 mL 1% (w/w) sodium citrate aqueous solution was added to stabilize the colloid. The colloid was stirred for another 20 min and aged for 2 days at room temperature before being used. To keep a similar concentration of metal atoms in Ag, Au, and Pt colloids, the preparation of Au and Pt colloids was carried out according to references.26,27 A 5.0 mL of 1% sodium citrate solution was added to a 100 mL of a boiling 3 × 10-4 mol/L solution of HAuCl4 or H2PtCl6. After boiling for 30 min for gold and 1 h for platinum, the colloid was cooled to room temperature. The concentration of metal atoms was 2.5 × 10-4 mol/L in Ag colloids and 2.86 × 10-4 mol/L in Au and Pt

10.1021/jp073816w CCC: $37.00 © 2007 American Chemical Society Published on Web 08/02/2007

Lucigenin Chemiluminescence

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Figure 1. Schematic diagram of flow-injection chemiluminescence detection system. PMT: photomultiplier tube.

colloids. Statistical analysis of TEM data revealed that the average diameter of the nanoparticles was 20 ( 5, 8.0 ( 2.5, 2.5 ( 0.7 nm for Ag, Au, and Pt colloids, respectively. CL Measurements. The CL detection was conducted on a flow injection CL system (Ruimai Electronic Science Co., China), including a model IFFM-D flow injection system, a model IFFS-A luminometer, and a computer. Metal colloid was first mixed with adsorbates such as KI, and then the mixture was injected into the lucigenin alkaline solution, as described in Figure 1. Static injection experiments were done to investigate the effect of superoxide dimutase (SOD). The Ag colloid was injected by a 100 µL syringe into the mixture of lucigenin and KI in a glass vessel in front of the IFFS-A luminometer to produce the CL. Then, a 100 µL SOD solution was injected into the CL system. Optical Measurements. The CL spectrum of this system was measured on a model F-7000 spectrofluorometer (Hitachi, Japan). UV-visible spectra were measured on a model UV2401 PC spectrophotometer (Shimadzu, Japan), and the cell for the absorption spectra was a 1 cm standard cell. The adsorption of iodide ion on the Ag nanoparticles was characterized by X-ray photoelectron spectrum (XPS) on a model ESCALAB MK II electron spectrograph (VG, England). The XPS sample was prepared as follows: KNO3 was added to a 100 mL Ag colloid containing 1 × 10-4 mol/L KI to precipitate the particles; then, the mixture was centrifuged, and the precipitates obtained were thoroughly washed by water. The Ag colloid after the reaction was characterized by transmission electron microscopy (TEM; H-800, Hitachi) and energy dispersive spectroscopy (EDS) on a field emission scanning electron microscope (JEOL 6500, Japan). The reaction product of the lucigenin-KI-Ag colloid system was filtrated by a mixed cellulose membrane with pore size of 0.22 µm (XingYa, Shanghai). The obtained precipitates were thoroughly washed by water and ethanol and then dried at 60 °C for a day, which was used for the studies of X-ray powder diffraction (XRD). XRD pattern was obtained with a model D/max-rA diffractometer (Rigaku, Japan). The fluorescence spectrum of the filtered solution was measured on a model F-7000 spectrofluorometer (Hatachi, Japan). Results and Discussion When the mixture of the Ag colloid in diameter of 20 ( 5 nm and 5 × 10-3 mol/L KI was injected into a 5 × 10-5 mol/L lucigenin solution containing 0.01 mol/L NaOH, a reproducible CL was observed as shown in Figure 2. When the Ag colloid and the KI solution were injected individually into the lucigenin alkaline solution, no CL was detected. Moreover, no CL was observed when other halide ions including 0.01 mol/L Cl- and Br- were used instead of I- (not shown in the figure). The CL spectrum showed that the maximum emission wavelength was at 475 nm, indicating that the luminophor was N-methylacridone (NMA), the product of lucigenin.28,29

Figure 2. CL profiles of lucigenin when mixed with KI-Ag colloid (solid line), Ag colloid (dashed line), and KI solution (dotted line), respectively. Conditions: lucigenin, 5 × 10-5 mol/L; NaOH, 0.01 mol/ L; KI, 5 × 10-3 mol/L. The inset is the CL spectrum.

Figure 3. (A) Absorption spectra of the Ag colloid after mixing with various concentrations of KI. (B) XPS result of Ag nanoparticles after adding 1 × 10-4 mol/L KI into the Ag colloid.

The effects of reagent concentrations, including the Ag colloid, KI, lucigenin, and NaOH, on the CL intensity of lucigenin-KI-Ag system were studied (Supporting Information, S1). The CL intensity increased with the Ag colloid over the range of 0-(2.5 × 10-4) mol/L, with KI over the range of (1 × 10-5)-0.02 mol/L, and with lucigenin over the range of 0-(2 × 10-4) mol/L. For NaOH concentration, the CL intensity increased in the range of (1 × 10-3)-0.02 mol/L; but it reached a platform in the range of 0.02-0.03 mol/L and decreased when NaOH concentration was higher than 0.03 mol/L. The effect of the size of nanoparticles should be taken into account. But in the present work, Ag nanoparticles with various sizes were hardly synthesized by one method so that we could not study the size effect. The absorption spectra of the Ag colloid before and after addition of KI of various concentrations were studied. The prepared Ag colloid had a strong surface plasmon resonance (SPR) absorption band around 395 nm. The addition of KI decreased the SPR absorbance, and the absorption band shifted slightly toward longer wavelength, as shown in Figure 3(A). The change of SPR absorption was due to that the chemisorption of the iodide ion on the Ag nanoparticles demetalized the surface atoms. When increasing the KI concentration, the SPR absorbance first decreased, then reached a minimum at the KI concentration of 5 × 10-5 mol/L, and did not change anymore with further increase of the KI concentration. This could be due to that fact that the monolayer coverage of iodide ion on the

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Figure 4. (A) Absorption spectra of 8.3 × 10-5 mol/L Ag colloid (solid line), 3.3 × 10-5 mol/L lucigenin (dashed line), and the sum of absorption spectra of the Ag colloid and lucigenin (dotted line). (B) Absorption spectra of the solution containing 8.3 × 10-5 mol/L Ag colloid + 3.3 × 10-5 mol/L lucigenin + 3.3 × 10-3 mol/L KI (solid line); 8.3 × 10-5 mol/L Ag colloid + 3.3 × 10-5 mol/L lucigenin (dashed line); 3.3 × 10-5 mol/L lucigenin + 3.3 × 10-3 mol/L KI (dotted line).

Ag nanoparticles was complete.30,31 The adsorption of iodide ion was validated by X-ray photoelectron spectrum, as shown in Figure 3B. In the figure, besides the peaks of Ag 3d (368.27 and 374.31 eV), two other remarkable peaks at 619.74 and 631.25 eV were found, which exactly corresponded to the peak of I 3d5/2 and I 3d3/2, respectively. The molar ratio of Ag/I in the sample given by the XPS results was 69:31. The interaction among Ag nanoparticles, KI, and lucigenin was also studied by the absorption spectra as shown in Figure 4. The absorption spectrum of lucigenin was the same as the spectrum of the mixture of lucigenin and KI. Accordingly, no obvious reaction between lucigenin and KI occurred. The absorption of the mixture of Ag colloid and lucigenin was exactly equal to the sum of the individual absorbance of the Ag colloid and lucigenin, suggesting that the Ag colloid could not reduce lucigenin directly. These results were consistent with the CL results that neither KI nor Ag colloid could react with lucigenin to generate CL. However, when lucigenin was added to the mixture of the Ag colloid and KI, the SPR absorption peak of the Ag colloid at 395 nm disappeared immediately, as seen in Figure 4B. After a while, some precipitates were formed from the solution. Ag nanoparticles before and after the reaction were characterized by TEM and EDS (Supporting Information, S2). From the TEM micrographs, it was observed that the prepared Ag nanoparticles were sphere-like, and the average diameter was about 20 nm; after the reaction, sphere-like Ag nanoparticles disappeared, and a great deal of short rod-like particles were observed. The size of these rod-like particles was about 1 µm. EDS results showed that the particles were composed of elemental silver and iodine. Furthermore, to ascertain the reaction product, the solution was filtered, and the precipitates were characterized by X-ray diffraction analysis, as shown in Figure 5. The main peaks at 22.5, 23.8, 25.5, 32.9, 39.3, 42.8, and 46.4° exactly corresponded to the peaks of (100), (002), (101), (102), (110), (103), and (112) of β-AgI (wurtzite), respectively.32,33 Considering the reactants were Ag, lucigenin, and KI, it was believed that the precipitates were AgI. The synchronous fluorescence spectra of the filtrated solution, NMA, and lucigenin are shown in Figure 6. The maximal emission wavelengths of NMA and lucigenin were 425 and 526 nm, respectively, when the difference between emission wavelength and exciting wavelength (λem - λex) was 160 nm.

Guo and Cui

Figure 5. XRD pattern of the precipitates generated from the lucigenin-Ag-KI system. Initial reagent concentration: 2.5 × 10-4 mol/L lucigenin, 2.5 × 10-4 mol/L Ag, 2 × 10-3 mol/L KI.

Figure 6. Synchronous fluorescence spectra of the reacted solution (solid line), 1.0 × 10-6 mol/L lucigenin (dashed line), and 1.0 × 10-4 mol/L NMA (dotted line). Fluorescence conditions: λem - λex ) 160 nm; EX and EM slit, 10 nm; high potential of PMT, -400 V.

Obviously, the two emission bands of the filtrated solution corresponded to the emission of NMA and lucigenin, respectively. These results indicated that Ag nanoparticles were oxidized to AgI, and lucigenin was converted to NMA in the CL reaction. Therefore, the disappearance of the SPR absorption peak should be ascribed to the oxidation of Ag particles to AgI by lucigenin in the presence of KI. Henglein’s group has done pioneering work for the property of metal colloids in the presence of adsorbates.30,34-37 The surface atoms of metal nanoparticles are unsaturated coordinatively; a nucleophile can donate electron density to the unoccupied orbits of the particle surface. Thus, excess electron density is donated into the metal particle, and the Fermi potential of the particle shifts to a more negative value by the chemisorption of a nucleophile.35 For example, the Fermi potential of colloidal palladium could be pushed to a sufficiently negative potential to reduce water in the presence of CN-.38 In this case, the adsorption of the iodide ion on the Ag particle shifts the Fermi potential of the particle negatively; that is, the reducing ability of Ag nanoparticles is enhanced by the chemisorption of the iodide ion. Actually, the oxidation potential of Ag to Ag+ can be expressed by the Nernst’s equation:

Ag - e- f Ag+

E1 ) E0 +(RT/F)*ln[Ag+]

(1)

where, E0 is the standard potential of Ag+/Ag. In the presence of iodide ion, the oxidation potential of Ag to Ag+ can be expressed as follows:

Ag+ + I- f AgI

Ksp) [Ag+][I-] ) 8.3 × 10-17

(2)

Lucigenin Chemiluminescence

Ag + I- - e- f AgI

E2 ) E0 + (RT/F)*ln(Ksp/[I-])

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(3)

where, Ksp is the solubility product of AgI. One can see if the iodide ion concentration increases, the oxidation potential of Ag decreases according to eq 3. Therefore, the tendency of the oxidation of Ag particles to AgI by lucigenin increases with the KI concentration. Generally, the mechanism of lucigenin CL in the absence of hydrogen peroxide involves three reaction steps: (i) one electron transfer to Luc2+ producing radical Luc•+, (ii) Luc•+ then reacts with superoxide anion (O2•-) to yield an extremely unstable dioxetane-type intermediate (Luc(OO)), and (iii) the decomposition of this intermediate provides an exited state of NMA, which gives the light emission.39-41 In the lucigenin-KI-Ag colloid system, the luminophor was identified as NMA and Ag nanoparticles were oxidized to AgI in the reaction. When O2 was bubbled into the reaction system, the CL was enhanced; whereas the CL was decreased by bubbling N2 (Figure 7). Apparently, O2 participated in the CL reactions. Earlier work about lucigenin CL42,43 also showed that the dissolved oxygen was an essential reactant for the CL reaction of lucigenin in the absence of hydrogen peroxide. In a static injection experiment, SOD was used to determine whether a superoxide anion was involved in the CL reaction, as seen in Figure 8. When the Ag colloid was injected into a 3 mL lucigenin-KI-NaOH solution, a continuous CL was generated. Then, the CL was sharply decreased by injecting a 0.1 mL SOD solution. This result indicated that O2 participated in the CL reaction through its mono-electron reducing product, the superoxide anion. Lucigenin radical was reported to be able to reduce O2 to O2•-.44 On the basis of these results, it was suggested that Luc2+ was first reduced by Ag nanoparticle-KI to the monocation radical Luc•+, which reacted with the dissolved oxygen to generate the superoxide anion; then the superoxide anion reacted with the monocation radical to produce CL. The CL process may proceed as follows:

Ag + I- + Luc2+ f AgI + Luc•+

(4)

Luc•+ + O2 f Luc2+ + O2•-

(5)

Luc•+ + O2•- f Luc(OO) f NMA* f hν

(6)

Since Ag nanoparticles acted as reductants in the CL reaction, a high concentration of KI is required to decrease the oxidation potential of Ag particles. Furthermore, a high concentration of KI is favored in accelerating the oxidation of Ag particles because the formed AgI shell can be partially dissolved at a high concentration of iodide ion,45 and the inner Ag could be continuously oxidized. Thus, it is comprehensible that the CL intensity increased with KI concentration all along, though 5 × 10-5 mol/L KI was enough to complete the monolayer adsorption on the particles. No CL was observed even when KCl and KBr at a concentration of 0.01 mol/L were used. It is known that the solubility products of AgBr and AgCl are much larger than that of AgI. Accordingly, the reducing ability of Ag nanoparticles might not be enough to reduce lucigenin in the presence of 0.01 mol/L KBr or KCl. The oxidation of metal as facilitated by nucleophilic reagents is well-known, and the extraction of gold from ores by cyanide in the presence of air is a typical example. According to the Nernst’s equation, the compounds which readily form complexes with Ag+ ion can also decrease the oxidation potential of Ag.

Figure 7. CL profiles when bubbling oxygen and nitrogen into the lucigenin-KI-Ag colloid system. Conditions: lucigenin, 5 × 10-5 mol/L; 2.5 × 10-4 mol/L Ag; KI 10 mmol/L; NaOH, 0.01 mol/L.

Figure 8. Chemiluminescence when injecting 0.1 mL Ag colloid and then 0.1 mL 300 U/mL superoxide dismutase (SOD) into a 3 mL lucigenin-KI-NaOH solution. Condition: lucigenin, 1 × 10-4 mol/ L; KI, 3.3 × 10-3 mol/L; NaOH, 0.01 mol/L.

Figure 9. Curves of CL intensity versus the concentration of compounds when the Ag colloid was mixed with lucigenin and various compounds. Conditions: lucigenin, 5 × 10-5 mol/L; NaOH, 0.01 mol/ L; 2.5 × 10-4 mol/L Ag.

It is reasonable to deduce that in the presence of some ligands, lucigenin might react with the Ag colloid to generate CL. As expected, it was found that, in the presence of thiourea, the Ag colloid could also induce the CL of lucigenin (Figure 9). Thiourea as an efficient ligand of Ag+ accelerated the oxidation of Ag nanoparticles by lucigenin, leading to the CL. In fact, thiosulfate ion and ammonia which can complex with Ag+ like thiourea could also induce the CL at a high concentration (Supporting Information, S3). These results supported the idea that Ag nanoparticles acted as a reductant in the CL reaction. Compounds containing -SH can form a S-Ag bond with a Ag electrode, and an oxidative adsorption occurred at a very negative potential.46 The adsorption of compounds containing -SH on Ag nanoparticles can form the Ag-S bonds and produce free hydrogen atoms,45 which should be reductive.

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Guo and Cui decrease the oxidation potential of Ag nanoparticles and induce the CL of lucigenin. Au and Pt nanoparticles show a similar chemical reactivity with lucigenin. However, the CL induced by Au and Pt nanoparticles was weaker than that induced by Ag nanoparticles because Au and Pt were less reactive than Ag. Acknowledgment. The financial support of the research by the National Natural Science Foundation of China (Grant Nos. 20573101 and 20625517) are gratefully acknowledged.

Figure 10. CL profiles when 2.86 × 10-4 mol/L Au (A) or Pt (B) colloid was mixed with lucigenin and (a) 5 × 10-4 mol/L cysteine, (b) 5 × 10-4 mol/L mercaptoacetic acid, (c) 5 × 10-4 mol/L mercaptopropionic acid, (d) 3 × 10-3 mol/L KI, (e) 5 × 10-4 mol/L thiourea, (f) 0.01 mol/L sodium thiosulfate, and (g) 4.5 mol/L ammonia.

Therefore, lucigenin might be reduced during the adsorption of these compounds on the Ag nanoparticles. As shown in Figure 9, the lucigenin CL was indeed observed when iodide ions were replaced by 5 × 10-4 mol/L cysteine, mercaptoacetic acid, or mercaptopropionic acid. The CL intensity in the presence of cysteine was stronger than that in the presence of mercaptoacetic acid and mercaptopropionic acid at the same concentration. It may be due to the fact that cysteine was not only a -SH provider but also an efficient ligand of silver ion, while mercaptoacetic acid and mercaptopropionic acid were only a -SH provider. As a result, Ag nanoparticles were more easily oxidized, leading to stronger CL. Though the -S group in methionine can also conduct chemisorption on Ag nanoparticles,47 no reducing species was produced during the adsorption like free hydrogen atoms. As a result, no CL was detected even when 0.01 mol/L methionine was used. In the presence of these compounds, Au and Pt nanoparticles could also reduce lucigenin to generate the CL. The experimental results are shown in Figure 10. In the presence of 5 × 10-4 mol/L compounds containing -SH such as cysteine, mercaptoacetic acid, and mercaptopropionic acid, both Au and Pt colloids could induce an evident CL. For each compound, the CL intensity induced by various nanoparticles decreased in the following order Ag > Au > Pt, which was consistent with their chemical reactivity. For the Au colloid, 10 mmol/L sodium thiosulfate, 5 × 10-4 mol/L thiourea, 4.5 mol/L ammonia, and 3 × 10-3 mol/L KI generated a very weak CL. For the Pt colloid, 5 × 10-4 mol/L thiourea and 4.5 mol/L ammonia could produce a weak CL. The results reveal that noble metal nanoparticles are reductive in the presence of these adsorbates. Conclusion In the presence of adsorbates such as iodide ion, cysteine, mercaptoacetic acid, mercaptopropionic acid, and thiourea, Ag nanoparticles could react with lucigenin, accompanied by chemiluminescence. Lucigenin-Ag-KI system was chosen as a model system to study the CL process. The CL mechanism is likely due to the fact that lucigenin was reduced by Ag nanoparticle-KI to monocation radical, which reacted with oxygen to generate a superoxide anion; then, the superoxide anion reacted with the monocation radical to produce CL. The adsorbates that have strong interaction with Ag nanoparticles such as thiourea, sodium thiosulfate, ammonia, cysteine, mercaptoacetic acid, and mercaptopropionic acid were able to

Supporting Information Available: The effects of reagent concentrations, including Ag colloid, KI, lucigenin, and NaOH, on the CL intensity of lucigenin-KI-Ag system; TEM and EDS characterization of the Ag colloid before and after the reaction; CL profiles when lucigenin-Ag colloid was mixed with sodium thiosulfate and ammonia, respectively. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322-13328. (2) Chaki, N. K.; Kakade, B.; Sharma, J.; Mahima, S.; Vijayamohanan, K. P.; Haram, K. J. S. J. Appl. Phys. 2004, 96, 5032-5036. (3) Georganopoulou, D. G.; Mirkin, M. V.; Murray, R. W. Nano Lett. 2004, 4, 1763-1767. (4) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552-560. (5) Jakob, M.; Levanon, H.; Kamat, P. V. Nano Lett. 2003, 3, 353358. (6) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B 2001, 105, 92669272. (7) Dong, T. Y.; Shih, H. W. Inorg. Chem. Commun. 2004, 7, 646649. (8) Li, D.; Li, J. H. Surf. Sci. 2003, 522, 105-111. (9) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. AdV. Mater. 1999, 11, 737-740. (10) Zhang, Z. F.; Cui, H.; Lai, C. Z.; Shi, M. J. Anal. Chem. 2005, 77, 3324-3329. (11) Xu, S. L.; Cui, H. Luminescence 2007, 22, 77-87. (12) Lu, G. W.; Cheng, B. L.; Shen, H.; Chen, Z. H.; Yang, G. Z.; Marquette, C. A.; Blum, L. J.; Tillement, O.; Roux, S.; Ledoux, G.; Descamps, A.; Perriat, P. Appl. Phys. Lett. 2006, 88, 023903. (13) Cui, H.; Zhang, Z. F.; Shi, M. J.; Xu. Y.; Wu, Y. L. Anal. Chem. 2005, 77, 6402-6406. (14) Aslan, K.; Malyn, S. N.; Geddes, C. D. J. Am. Chem. Soc. 2006, 128, 13372-13373. (15) Chowdhury, M. H.; Aslan, K.; Malyn, S. N.; Lakowicz, J. R.; Geddes, C. D. J. Fluoresc. 2006, 16, 295-299. (16) Chowdhury, M. H.; Aslan, K.; Malyn, S. N.; Lakowicz, J. R.; Geddes, C. D. Appl. Phys. Lett. 2006, 88, 173104. (17) Zhang, Z. F.; Cui, H.; Shi, M. J. Phys. Chem. Chem. Phys. 2006, 8, 1017-1021. (18) Gorman, B. A.; Francis, P. S.; Dunstan, D. E.; Barnett, N. W. Chem. Commun. 2007, 395-397. (19) Wang, Z. P.; Hu, J. Q.; Jin, Y.; Yao, X.; Li, J. H. Clin. Chem. 2006, 52, 1958-1961. (20) Gyllenhammar, H. J. Immunol. Methods 1987, 97, 209-213. (21) Veazey, R. L.; Nieman, T. A. Anal. Chem. 1979, 51, 2092. (22) Perez-Ruiz, T.; Martinez-Lozano, C.; Tomas, V.; Fenoll, J. Microchim. Acta 2003, 141, 73-78. (23) Dong, Y. P.; Cui, H. J. Electroanal. Chem. 2006, 595, 37-46. (24) Guo, J. Z.; Cui, H.; Xu, S. L.; Dong, Y. P. J. Phys. Chem. C 2007, 111, 606-611. (25) Creighton, J. A.; Blathchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 1979, 75, 790-798. (26) Frens, G. Nat. Phys. Sci. 1973, 241, 20-22. (27) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129-14136. (28) Okajima, T.; Ohsaka, T. Luminescence 2003, 18, 49-57. (29) Maskiewiez, R.; Sogah, D.; Bruice, T. C. J. Am. Chem. Soc. 1979, 101, 5347-5354. (30) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679-682. (31) Mulvaney, P.; Langmuir 1996, 12, 788-800. (32) Ida, T.; Kimura, K. Solid State Ionics 1998, 107, 313-318.

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