Gold Nanoparticle Triggered Chemiluminescence between Luminol

Jul 3, 2008 - When Au colloid was injected into the mixture of luminol and AgNO 3, AgNO 3 reacted rapidly with luminol under the catalysis of gold nan...
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J. Phys. Chem. C 2008, 112, 11319–11323

11319

Gold Nanoparticle Triggered Chemiluminescence between Luminol and AgNO3 Hua Cui,/ Ji-Zhao Guo, Na Li, and Li-Jia Liu Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui 230026, China ReceiVed: January 25, 2008; ReVised Manuscript ReceiVed: May 3, 2008

It was found that luminol could react with AgNO3 in the presence of gold colloid to generate chemiluminescence (CL) at 425 nm. UV-visible spectra and X-ray photoelectron spectra showed that AgNO3 was reduced by luminol to Ag in the CL reaction, which covered on the surface of gold nanoparticles to form Au/Ag core/ shell nanoparticles. The luminophor was identified by the CL spectrum as 3-aminophthalate. The effects of Au/Ag core/shell nanoparticles with various compositions on the luminol-AgNO3 system and the CL kinetics were also studied. On this basis, a CL reaction mechanism involving catalysis is proposed. When Au colloid was injected into the mixture of luminol and AgNO3, AgNO3 reacted rapidly with luminol under the catalysis of gold nanoparticles to produce Ag and luminol radicals, which reacted with the dissolved oxygen, accompanying a light emission; then, with instant deposition of Ag atoms on the surface of Au particles, the catalytic activity declined, leading to a sharp decrease in CL intensity and a slow growth of core/shell nanoparticles. The new CL system has the advantages of low background and good stability, and gold nanoparticles have excellent biocompatible property. It may find future applications in immunoassay and DNA detection. Introduction In recent years, nanoparticles have been widely studied for their excellent properties1,2 and their potential applications in microelectronics, optics, electronics, magnetic devices, and catalysis.3–5 Light emission accompanied by a chemical reaction, 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.6–13 In these systems, nanoparticles can participate in CL reactions as reductants, catalysts, and luminophors. Among them, metal nanoparticles as catalysts in CL reactions have received much attention. Au, Ag, and Pt nanoparticles could catalyze the decomposition of hydrogen peroxide to produce reactive oxygen species and enhance the CL from the reaction between luminol (3-aminophthalhydrazide) and hydrogen peroxide.8–11 Ag nanoparticles could strongly enhance the CL reaction among citrate, tris(2,2′-bipyridyl)ruthenium(II), and cerium(IV).12 Silver island films prepared by chemical deposition could enhance phenyl oxalate ester CL because the electronic excited states of CL species coupled to the surface plasmon of silver films.14–16 The gold thin films elaborated by a pulse-laser deposition technique could catalyze the luminolH2O2-peroxidase CL.17 Luminol is an extremely versatile and widely used CL reagent, which readily produces CL when reacting with strong oxidants such as hydrogen peroxide,18 hypochlorite,19 and potassium permanganate.20 Silver nitrate is a relatively weak oxidant, and no CL between luminol and silver nitrate can be detected in general conditions. In the present work, we found that gold nanoparticles could trigger a strong and flash CL between luminol and silver nitrate. The effect of reaction conditions on the CL intensity was examined. The CL spectra, the UV-visible spectra, and X-ray photoelectron spectrometry (XPS) of the reacted solutions and the CL kinetics were studied. / To whom correspondence should be addressed. Phone: +86-5513606645. Fax: +86-551-3600730. E-mail: [email protected].

The effect of Au/Ag core/shell nanoparticles with various compositions on the luminol-AgNO3 system was explored. A CL reaction mechanism involving the catalysis of gold nanoparticles is proposed. Experimental Section Chemicals. A 1.0 × 10-2 mol/L stock solution of luminol was prepared by dissolving luminol (Sigma, America) in 0.10 mol/L sodium hydroxide solution. AgNO3, HAuCl4 · 4H2O (48% w/w), sodium citrate, NaBH4, polyvinyl pyrrolidone (PVP, Mw ) 36000) and polyethylene glycol (PEG, Mw ) 20000) were obtained from Shanghai Reagent (Shanghai, China). All reagents were of analytical grade and used as received. Triple-distilled water was used throughout. Colloid Preparation. Gold colloids with a diameter of 8, 16, 25, 38, and 68 nm were synthesized by the citrate reduction method.21 The Au concentration in the prepared colloids was 3.0 × 10-4 mol/L. Au/Ag core/shell nanoparticles of various Au/Ag ratios were prepared by using the prepared 8-nm Au nanoparticles as seeds, according to a reference.22 In brief, 1.0 mL AgNO3 of various concentration was added to 49.0 mL of 8-nm Au colloid. Then the mixture was boiled for 30 min and cooled down to room temperature. The remaining citrate in the Au colloid acted as a reductant to reduce AgNO3. The size and shape of the synthesized 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 8.0 ( 2.5, 16.0 ( 1.0, 25.0 ( 2.0, 38.0 ( 3.6, and 68.0 ( 5.1 nm. Au/Ag core/shell colloids stabilized by PVP or PEG was prepared as follows: 2.5 mL of 6 mmol/L HAuCl4 was added dropwise to a 20 mL of 4 mmol/L NaBH4 solution with vigorous stirring simultaneously. Then, 2.5 mL of 6 mmol/L AgNO3 was added dropwise to grow Ag on the formed Au nanoparticles. Ten minutes later, 0.2 g PVP or PEG in 25 mL water was added to stabilize the colloids. CL Measurements. The CL detection was conducted on a flow injection CL system (Ruimai Electronic Science Co.,

10.1021/jp800749y CCC: $40.75  2008 American Chemical Society Published on Web 07/03/2008

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Figure 1. Schematic diagram of flow-injection CL detection system.

Figure 2. CL profiles when injecting Au colloids with various sizes into the mixture of luminol and AgNO3. from top to bottom: Au colloids with a diameter of 8, 16, 25, 38, 68 nm, and blank (1 × 10-3 mol/L sodium citrate) Conditions: luminol, 1 × 10-4 mol/L in pH 12 borate buffer; AgNO3, 1 × 10-5 mol/L. The inset is the CL spectra.

China), including a model IFFM-D flow injection system, a model IFFS-A luminometer, and a computer. Luminol solution was first mixed with AgNO3 and then with metal colloids, as described in Figure 1. When investigating the effects of nitrogen and oxygen, nitrogen and oxygen were bubbling into AgNO3 solution, luminol solution, and Au colloids for 15 min, respectively, and then the solutions driven by peristaltic pumps were mixed to obtain a continuous CL. In static experiments, Au colloid was injected by a 100-µL syringe into the mixture of AgNO3 and luminol in a glass vessel in front of the IFFS-A luminometer. The high potential of the photomultiplier tube was set as -400 V, except for special statements. Optical Measurements. The CL spectra were measured on a model FL 5401 spectrofluorometer (Shimadzu, Japan). UV-visible spectra were measured on a model UV-2401 PC spectrophotometer (Shimadzu, Japan). XPS was measured on a model ESCALAB MK II electron spectrograph (VG, England). The XPS sample was prepared as follows: 1 mL of 0.01 mol/L luminol and 1 mL of 0.01 mol/L AgNO3 was added into 100 mL of Au colloid; after reaction, KNO3 was added into the mixture to precipitate the particles; then, the mixture was centrifuged and the precipitates obtained was thoroughly washed by water. Results and Discussion CL Accompanied by Gold Nanoparticle-Luminol AgNO3 Reaction. When Au colloids with different diameters were injected into the mixture of luminol (1 × 10-4 mol/L) and AgNO3 (1 × 10-5 mol/L), a strong CL was detected as shown in Figure 2. The CL intensity was even comparable with that of well-known luminol-H2O2-peroxidase system. In the absence of Au nanoparticles, no obvious CL emission was observed when luminol was mixed with AgNO3. All the prepared Au colloids in diameter of 8, 16, 25, 38, and 68 nm could initiate the CL, but the CL intensity decreased with

Figure 3. Effects of reaction conditions on the CL intensity. (A) Effect of Au nanoparticle concentration, reaction conditions: pH 12, 1 × 10-5 mol/L of AgNO3, 1 × 10-4 mol/L of luminol. (B) Effect of pH, reaction conditions: 1 × 10-5 mol/L of AgNO3, 1 × 10-4 mol/L of luminol, 3 × 10-4 mol/L of 8-nm Au. (C) Effect of AgNO3 concentration, reaction conditions: pH 12, 1 × 10-4 mol/L of luminol, 3 × 10-4 mol/L of 8-nm Au. (D) Effect of luminol concentration, reaction conditions: pH 12, 1 × 10-5 mol/L of AgNO3, 3 × 10-4 mol/L of 8-nm Au.

Figure 4. Absorption spectra at 5 min (A) and 72 h (B) after mixing Au nanoparticles with 1 × 10-4 mol/L of luminol and AgNO3 of various concentrations: (a) 8 × 10-5 mol/L, (b) 4 × 10-5 mol/L, (c) 2 × 10-5 mol/L, (d) 0 mol/L.

increasing the diameter of gold nanoparticles. The CL spectra showed that the maximum emission wavelength was 425 nm, indicating that the luminophor was 3-aminophthalate, an oxidation product of luminol.23 The effects of reaction conditions on the CL intensity were investigated as shown in Figure 3. In the tested range, the CL intensity increased with pH from 9.5 to 13.0, with luminol concentration over the range 5 × 10-6-2 × 10-4 mol/L, and with Au colloid concentration over the range 0-3 × 10-4 mol/ L. For AgNO3 concentration, the CL intensity increased in the range from 5 × 10-7 to 1 × 10-5 mol/L, but it decreased when AgNO3 concentration was higher than 1 × 10-5 mol/L. It may be due to that AgNO3 at higher concentrations would form precipitates that covered the surface of Au nanoparticles. CL Mechanism. The absorption spectra of the system before and after the CL reaction were studied as shown in Figure 4 in order to check what happened after the CL reaction. The prepared Au colloid had a surface plasmon resonance (SPR) absorption band at 523 nm, and the addition of luminol and AgNO3 led to a little increase in SPR absorbance. After 72 h, an evident blue shift of the absorption band was observed and

Gold Nanoparticle Triggered CL

Figure 5. XPS of the Au nanoparticles after the CL reaction.

the SPR absorbance increased. At the same time, the color of solution turned orange from wine red. The solution with higher concentrations of AgNO3 exhibited a greater shift in absorption band and a larger increase in SPR absorbance. The use of preformed metallic seeds as nucleation centers in nanoparticle synthesis has a long history. The seeds as nucleation centers can catalyze the reduction of metal ions at their surfaces.24–27 When Au nanoparticles were used as seeds to grow Au/Ag core/ shell nanoparticles, the SPR absorption of the colloid shifted to blue and had an increase in absorbance when increasing Ag concentration.22,28 Accordingly, it was deduced that AgNO3 was reduced by luminol to Ag atoms by virtue of the catalysis of Au colloid, which formed an Ag shell on the surface of Au nanoparticles. According to the hypothesis, an Au/Ag core/shell structure should be detected after the CL reaction. However, an Au/Ag core/shell structure could not be observed under TEM, which may be due to the fact that the Ag shell was very thin and Au and Ag had a similar diffraction contrast in TEM images. Furthermore, XPS was used to characterize the reacted nanoparticles as shown in Figure 5.The peaks of Au4f7/2 at 83.8 eV and Ag3d5/2 at 367.9 eV corresponding to metal Au and Ag were found, and an Au/Ag atom ratio of 40/60 was obtained in the nanoparticles after 0.5 h reaction, indicating that the Au nanoparticles were not oxidized in the CL reaction and that AgNO3 was reduced to Ag under the catalysis of gold nanoparticles, which deposited on the surface of gold nanoparticles. According to the absorption spectra, it seems that a long time is required to form Au/Ag core/shell nanoparticles in the luminol-AgNO3-Au colloid CL reaction. To examine the reaction rate, the CL kinetic curve was studied by static injection experiments. It was expected that the reaction was slow and thus tens of hours was required for the formation of Ag shell on the Au particles before the shift of SPR absorption band was evident. From the kinetic curve in the inset of Figure 6, it was surprising that the CL reaction was almost completed in 2 s after injecting 8-nm Au colloid (0.1 mL) into the mixture of luminol (0.1 mL, 0.01 mol/L) and AgNO3 (4.0 mL, 2 × 10-4 mol/L). Therefore, the reaction among luminol, AgNO3, and Au colloid was fast at the beginning and slowed down later on for some reasons. Furthermore, as seen in Figure 6, when the CL induced by the injection of Au colloid was over, similar CL peaks could be regenerated by injecting repeatedly Au colloid (0.1 mL, 3 × 10-4 mol/L) into the reacted mixture. Therefore, the sharp decrease in the CL induced by the injection of Au colloid was due to the “consumption” of Au nanoparticles rather than luminol or AgNO3. By consideration that Au colloid acted as a catalyst in the CL reaction between luminol and AgNO3 and Au/Ag core/shell

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Figure 6. CL on static system when injecting repeatedly 0.1 mL of 8-nm Au colloid into a mixture of 0.1 mL of 0.01 mol/L of luminol and 4 mL of 2 × 10-4 mol/L of AgNO3. The inset is CL kinetic curve in an injection.

Figure 7. Catalysis model of Au nanoparticles in the CL reaction between luminol and AgNO3.

nanoparticle was produced in the catalyzed CL reaction, a model of the catalysis process is proposed in Figure 7. Once Au colloid was injected into the mixture of luminol and AgNO3, AgNO3 was rapidly reduced by luminol to Ag under the catalysis of Au colloid, which covered instantly on the surface of Au particles. It is presumed that the Au/Ag core/shell nanoparticles could also catalyze the luminol-AgNO3 CL reaction, but their catalytic ability was much weaker than that of Au nanoparticles. Accordingly, the CL intensity decreased with the deposition of Ag atoms on the surface of Au particles. This model is also consistent with the change in SPR absorption in the CL reaction. Though the formation of the first layer of Ag atoms was fast, the further deposition of Ag atoms was slow because of the weak catalytic ability of Au/Ag core/shell nanoparticles. Therefore, a long time was required for the reaction before obvious change in SPR absorption was observed. To validate this model, Au/Ag core/shell nanoparticles with various compositions were prepared and their catalytic ability for the luminol-AgNO3 CL system was investigated, as shown in Figure 8. The Au/Ag core/shell nanoparticles could indeed catalyze the CL reaction and the CL intensity decreased dramatically when core/shell nanoparticles of high Ag/Au ratio were used. Actually, when the high voltage of the PMT was set as -800 V, it was observed that Ag nanoparticles could also enhance slightly the CL from the luminol-AgNO3 system. These results supported the model and revealed that the short CL life of the luminol-AgNO3 system in the static experiments was due to the fast formation of Ag shell on the Au particles. Why could gold nanoparticles catalyze the reaction between luminol and AgNO3? The direct reduction of AgNO3 to metal Ag should be very slow in the absence of nuclei because the redox potential of single metal ions is very negative.29,30 Gold nanoparticles offered the advantage of eliminating nucleation and promoting the reduction of AgNO3 on the surface of the nanoparticles. Furthermore, Au and Ag have almost the same crystal lattice constant (0.408 vs 0.409 nm). The good crystal lattice match could facilitate the growth of Ag on Au nanopar-

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LH- + Ag+ + OH- f L•- + Ag + H2O

(1)

L•- + O2 f L + O2•-

(2)

L•- + O2•- f AP2-* + N2

(3)

AP2-*fAP2- + hV

(4)

Conclusion

Figure 8. CL profiles when injecting Au/Ag core/shell colloids with various compositions into the mixture of luminol and AgNO3. Conditions: luminol, 1 × 10-4 mol/L in pH 12 borate buffer; AgNO3, 1 × 10-5 mol/L. The inset is the enlarged segment.

ticles.31 Accordingly, gold nanoparticles showed strong catalytic ability for the CL reaction between luminol and AgNO3. At first, it was guessed that the reason the catalytic ability of the formed Au/Ag core/shell nanoparticles was weaker than that of gold nanoparticles may be due to the fact that the strong interaction between the Ag shell and citrate ions32 might retard the deposition of Ag. Therefore, Au and Au/Ag nanoparticles stabilized by PVP and PEG were synthesized to investigate the CL behavior in the absence of citrate ions. The CL results showed that, whatever stabilizer was used, Au nanoparticles always led a stronger CL than Au/Ag core/shell nanoparticles under the similar experimental condition. The results indicated that Au nanoparticles was inherently better catalyst than Au/ Ag core/shell nanoparticles for this CL reaction, being independent of the stabilizer of nanoparticles. Finally, the dependence of the catalysis on the size of particles should be ascribed to the colloid with small size had the large surface and the high particle concentration because the concentration of Au atoms was same in all colloids. Finally, how did the CL reaction proceed? Luminol (LH-) can be oxidized by various oxidants to luminol radical (L•-), which may react with the dissolved oxygen to produce diazaquinone (L) and superoxide anion (O2•-).33 The reaction between L•- and O2•- produces the excited state 3-aminophthalate (AP2-*), giving rise to light emission.34 This indicates that the dissolved oxygen plays an important role in the luminol CL reactions. Accordingly, the effect of oxygen on the CL reaction was also studied. When nitrogen or oxygen was bubbled into the reaction solution, respectively, the CL intensity declined by 60% under a nitrogen atmosphere and increased by 40% under an oxygen atmosphere over under an air atmosphere. The results revealed that oxygen was involved in the CL process. It should be noted that the bubbling of nitrogen could reduce the oxygen concentration in the reaction solution, but a completely oxygenfree solution could not be achieved. Colarieti et al. validated that water solution could not be entirely oxygen-free, and the residual oxygen was 0.03 ( 0.01 mmol/L even after 30 min of nitrogen sparging.35 We also confirmed by voltammetry that oxygen could not be removed completely under our experimental conditions. As a result, the CL was decreased but not eliminated by the bubbling of nitrogen. Overall, in this case, it is believed that luminol (LH-) was oxidized by AgNO3 to produce luminol radical (L•-) and Ag. The luminol radical (L•-) reacted with the dissolved oxygen, leading to the CL. The CL reaction of the Au colloid-luminol-AgNO3 system may proceed as follows

It was found that luminol could react with AgNO3 to generate the CL in the presence of Au nanoparticles. Au nanoparticles as nucleation centers catalyzed the reduction of AgNO3 to Ag atoms by luminol to yield Au/Ag core/shell nanoparticles. Meanwhile, luminol was oxidized to luminol radical, which further reacted with the dissolved oxygen, giving rise to light emission. The first layer of Ag shell was fast and the formed Ag shell decreased sharply the catalytic ability of the particles, which resulted in a strong and flash CL. The new CL system has the advantages of low background and good stability, which may be of great potential for the CL-based immunoassay and DNA analysis due to excellent biocompatible property of gold nanoparticles. Further work is under the way. Acknowledgment. The financial support of the research by the National Natural Science Foundation of the People’s Republic of China (Grant Nos. 20573101 and 20625517) and the Overseas Outstanding Young Scientist Program of China Academy of Sciences are gratefully acknowledged. References and Notes (1) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409– 453. (2) Chen, S. W.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540–547. (3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Berlin, 1995. (4) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441–3452. (5) Adams, R. D.; Captain, B.; Zhu, L. J. Am. Chem. Soc. 2004, 126, 3042–3043. (6) Poznyak, S. K.; Talapin, D. V.; Shevchenko, E. V.; Weller, H. Nano Lett. 2004, 4, 693–698. (7) Cui, H.; Zhang, Z. F.; Shi, M. J.; Xu. Y.; Wu, Y. L. Anal. Chem. 2005, 77, 6402–6406. (8) Zhang, Z. F.; Cui, H.; Lai, C. Z.; Shi, M. J. Anal. Chem. 2005, 77, 3324–3329. (9) Guo, J. Z.; Cui, H.; Zhou, W.; Wang, W. J. Photochem. Photobiol. A: Chem. 2008, 193, 89–96. (10) Chen, H.; Gao, F.; He, R.; Cui, D. X. J. Colloid Interface Sci. 2007, 315, 158–163. (11) Xu, S. L.; Cui, H. Luminescence 2007, 22, 77–87. (12) Gorman, B. A.; Francis, P. S.; Dunstan, D. E.; Barnett, N. W. Chem. Commun. 2007, 395–397. (13) Zhang, Z. F.; Cui, H.; Lai, C. Z.; Shi, M. J. Phys. Chem. Chem. Phys. 2006, 8, 1017–1021. (14) Chowdhury, M. H.; Aslan, K.; Malyn, S. N.; Lakowicz, J. R.; Geddes, C. D. J. Fluoresc. 2006, 16, 295–299. (15) Aslan, K.; Malyn, S. N.; Geddes, C. D. J. Am. Chem. Soc. 2006, 128, 13372–13373. (16) Chowdhury, M. H.; Aslan, K.; Malyn, S. N.; Lakowicz, J. R.; Geddes, C. D. Appl. Phys. Lett. 2006, 88, 173104. (17) 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. (18) Marquette, C. A.; Blum, L. J. Anal. Bioanal. Chem. 2006, 385, 546–554. (19) Francis, P. S.; Barnett, N. W.; Lewis, S. W.; Lim, K. F. Luminescence 2004, 19, 94–115. (20) Wang, Z. P.; Zhang, Z. J.; Fu, Z. F.; Zhang, X. Anal. Bioanal. Chem. 2004, 378, 834–840. (21) Kim, C. K.; Kalluru, R. R.; Sing, J. P.; Fortner, A.; Griffin, J.; Darbha, G. K.; Ray, P. C. Nanotechnology 2006, 17, 3085–3093. (22) Ji, X.-H.; Wang, L.Y.; Yuan, H.; Ma, L.; Bai, Y.-B.; Li, T.-J. Acta Chim. Sin. 2003, 61, 1556–1560.

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