Electrogenerated Chemiluminescence of Au Nanoclusters for the

Jan 12, 2011 - discussed according to the presented results and ascribed to the effective electron transfer from the conduction-band of excited indium...
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Electrogenerated Chemiluminescence of Au Nanoclusters for the Detection of Dopamine Lingling Li, Hongying Liu, Yuanyuan Shen, Jianrong Zhang, and Jun-Jie Zhu* Key Laboratory of Analytical Chemistry for Life Science (Ministry of Education of China), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China

bS Supporting Information ABSTRACT: Electrogenerated chemiluminescence (ECL) emission was observed from the water-soluble, bovine serum albumin (BSA)-stabilized Au nanoclusters for the first time. The possible ECL mechanism was discussed according to the presented results and ascribed to the effective electron transfer from the conduction-band of excited indium tin oxide (ITO) to Au nanoclusters (NCs). A simple label-free method for the detection of dopamine has been developed based on the Au NCs ECL in aqueous media. The Au NCs could be an effective candidate for new types of ECL biosensors in the future due to their fascinating features, such as good water solubility, low toxicity, ease of labeling, and excellent stability.

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uantum-sized Au nanoparticles, also called Au nanoclusters (Au NCs), composed of very few atoms with a ultrasmall size ranging from subnanometer to approximately 2 nm (core diameter),1 have drawn wide attention in recent years. Because of the strong quantum confinement of electrons in the ultrasmall size regime, the Au NCs exhibit discrete size-dependent electronic energy level and lose the bulklike electronic properties, the disappearance of surface plasmon resonance, for instance.1-3 Hence, Au NCs show “molecule-like” properties, such as enhanced catalytic activity, enhanced photoluminescence, intrinsic magnetism, and unique charging properties,1,4,5 which render the material promising in various areas ranging from fundamental studies6-10 to potential applications from catalysis,11 sensing,2,12 to bioimaging.4,13 Furthermore, Au NCs exhibit other fascinating features, including ease of synthesis, good water solubility, low toxicity,2 surface functionalities, biocompatibility, and excellent stability, which also make them hold great promise in bioanalysis. Electrogenerated chemiluminescence (ECL) is a light emission that arises from the high-energy electron transfer reaction between electrogenerated species,14 which has special advantages over photoluminescence techniques, such as low cost, wide range of analytes, and high sensitivity.15 Because of their unique quantum size dependent optical and electrochemical properties, the ECL of quantum dots (QDs) has become more and more fascinating14-19 since Ding and Bard reported the ECL of Si QDs in 2002.20 However the inherent toxicity in most of the QDs used in ECL studies, such as CdTe,14,15 CdS,17,19 and CdSe,18 would limit their applications in a bioassay. Thus it is necessary for developing novel low-toxicity or nontoxic ECL species. Clearly, it is of considerable interest for the Au NCs ECL on the basis of their low toxicity.2 To the best of our knowledge, no research has been reported involving the Au NCs ECL study. r 2011 American Chemical Society

Herein, we explored the ECL properties of Au NCs. Red fluorescent BSA-stabilized Au NCs, consisting of 25 gold atoms (Au25), were prepared with a simple, environmental friendly synthetic route as reported previously (see the Supporting Information).21 High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, and UV-vis spectra analyses were performed and presented the consistent results with previous reports (see the Supporting Information).2,21 To investigate the ECL behavior, the Au NCs were first immobilized onto hydroxylated indium tin oxide (ITO) electrodes through 3-aminopropyl-triethoxysilane (APTES) and glutaric dialdehyde (Figure S1 in the Supporting Information). The BSA layer surrounding the Au NCs could afford abundant functionalities such as carboxyl and amine groups (Figure S2 in the Supporting Information), facilitating the assembly of Au NCs. The thickness of the Au NCs film was characterized by AFM (See Figure S7 in the Supporting Information), which revealed a uniform Au NCs monolayer with a thickness of about 1.5 nm. The ECL of the Au NCs at the ITO electrode was observed in the electrolyte containing K2S2O8, a coreactant commonly used in ECL systems, when the potential was cycled between þ2.0 and -1.5 V. Figure 1a shows the cyclic voltammograms (CV) of clean ITO in 0.1 M PBS (pH 7.4) containing 0.1 M KCl with or without K2S2O8. The obvious cathodic peak at about -1.3 V (C1) represents the reduction of ITO. Moreover, with the presence of K2S2O8, the cathodic current began to flow at -0.6 V, earlier than that Received: October 5, 2010 Accepted: January 4, 2011 Published: January 12, 2011 661

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Figure 1. (a) CVs of the bare ITO electrode in the absence (1) or presence (2) of K2S2O8. Inset: DPV of the bare ITO in the absence of K2S2O8, the arrows indicate the scan direction. (b) ECL-potential curves of the bare ITO electrode (1) and Au NCs modified ITO electrode (2) with the corresponding CV and DPV (inset). (c) ECL-potential curves of the bare ITO electrode (1) and Au NPs modified ITO electrode (2). (d) ECL-potential curves of the Au NCs modified ITO electrode (1) before and (2) after the assembly of Au NPs. Inset: Electrochemical impedance spectroscopy of (1) Au NCs and (2) Au NPs modified electrodes. (e) ECL-potential curves of the Au NCs modified ITO electrode with the potential cycled between (1) þ2.0 and -1.5 V and (2) 0 and -1.5 V. (f) ECL-potential curves of the bare (dark) and Au NCs modified (red) Au electrodes (1, 2) and glassy carbon (3, 4) electrodes. Scan rate: 100 mV s-1.

in the absence of K2S2O8, due to the reduction of peroxydisulfate on ITO.22 As for Au NC modified ITO electrodes, the cathodic ECL signal is about 5 times higher than that of bare ITO, whereas the change of the anodic signal is negligible (Figure 1b). Note also that in the voltammograms, the Au NCs film blocks the electrochemical process on the modified ITO surface as indicated by the decreased current compared to CV of bare ITO in Figure 1a, which could be ascribed to the nonconductive property of the BSA layer surrounding the Au NCs. The differential pulse voltammograms (DPV) inset in Figure 1b revealed two cathodic peaks at about -1.3 V (C1) and -1.0 V (C2), corresponding to the reduction of the reduction of ITO and peroxydisulfate, respectively.22 It was reported that Au nanoparticles (Au NPs) exhibited a significant ECL amplification effect, which acted as a conductive wire to facilitate the electron transfer in the ECL reaction.19 To confirm that the obtained strong signal at the Au NCs

modified ITO electrode indeed originated from the Au NCs, trisodium citrate reduced Au NPs were used for the comparison. As shown in Figure 1c, no obvious enhanced signal could be observed in the presence of Au NPs, suggesting the material is not ECL active. Furthermore, the Au NCs showed much higher electron-transfer resistance (Ret) value than that of Au NPs, due to the nonconductive property of the BSA layer in the Au NCs (inset in Figure 1d). On the other hand, when Au NPs were assembled onto the Au NCs modified ITO electrode, significantly enhanced signal could be obtained due to a signal amplification effect (Figure 1d). Furthermore, we measured the ECL spectra by employing a series of optical filters, and a distinguished ECL peak at about 620 nm was observed, which was approximate to the PL peak (see the Supporting Information). All the obtained results showed strong evidence that the observed emission was indeed the ECL of Au NCs. 662

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In ECL system, there are two main mechanisms: one is ion annihilation ECL, the other is coreactant ECL.16 It was found that the ECL signal of the Au NCs modified ITO electrode in the absence of S2O82- was so weak to be distinguished from that of bare ITO electrode, indicating that S2O82- played an important role in the ECL process as a coreactant. In addition, the cathodic ECL of the Au NCs modified ITO electrode was also studied as the potential was cycled between 0 and -1.5 V. Compared with the ECL between þ2.0 and -1.5 V, the cathodic ECL just decreased slightly (Figure 1e), indicating that the coreactant ECL contributed to the whole ECL dominantly. Meanwhile, the small decrease suggested that the annihilation ECL via electron

transfer between redox species of Au NCs might also contribute to the overall ECL. According to our previous reports concerning the cathodic ECL of QDs,18,19 it could be confirmed that the cathodic ECL herein was originated from the formation of excited-state Au NCs (Au25*) via electron-transfer (ET) annihilation of negatively charged Au25•- and the strongly oxidizing SO4•- radicals produced by electroreduction of S2O82-. Furthermore, the ECL behaviors of Au NCs modified glassy carbon and Au electrodes have also been investigated as shown in Figure 1f. Obviously, the ECL signal of those electrodes was even lower than that of the bare glassy carbon and Au electrodes, revealing that ITO played an important role in ECL process. The ITO participated ECL behaviors have also been reported previously, where the ITO acted as a medium for the electron transfer between QDs and dissolved oxygen.14,15 It has been demonstrated that when metal nanoparticles, such as Ag or Au, come in contact with a charged semiconductor, the Fermi levels of the two materials equilibrate and the metals accept charges, such as the confirmed electron transfer from the conduction-band of excited TiO2 to Au NCs.23 Au25 NCs exhibits an electronic molecular-type HOMO-LUMO bandgap of 1.3 eV with the orbital energies of -10 and -8.7 eV, respectively.9,11,24 Meanwhile, similar to that of TiO2 (with a band gap of 3.4 eV15), ITO has a band gap of 3.5 eV with the Fermi level of -3.7 eV,14 which is much higher than the energy level of Au NCs. Furthermore, the conduction and valence band potentials for ITO were similar to those for In2O3, that were at-0.4 and 2.1 V (vs SCE) at pH 7, respectively.25 Thus in the cathodic ECL process, electrons could be injected into the conduction-band of ITO and then transfer to the LUMO of Au NCs, resulting the generation of Au25•- (Figure 2a). In a word, ITO acted as an effective reductant for Au25. The possible ECL mechanisms are described in Figure 2b

Figure 2. Schematic illustration of (a) electron transfer between ITO and Au NCs and (b) the ECL mechanisms of Au NCs.

Figure 3. (a) CV and DPV (inset) of Au NCs modified ITO electrode with 20 μM DA in the electrolyte. (b) Cathodic ECL profiles of the Au NCs modified ITO electrode in the presence of different dopamine concentrations. The inset shows the correlating smoothed curves for ease of observation. (c) Linear calibration plot for DA detection. (d) Consecutive ECL intensity in the presence (dark) and absence (red) of interfering agents. 663

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’ ASSOCIATED CONTENT

with the following equations: ITO

bS

Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Au25 sf Au25 • -

ð1Þ

S2 O8 2 - þ e - f S2 O8 •2 -

ð2Þ

S2 O8 •2 - f SO4 2 - þ SO4 • -

ð3Þ

Au25 • - þ SO4 • - f Au25 / þ SO4 2 -

ð4Þ

*E-mail: [email protected]. Phone and fax: þ86-25-83594976.

Au25 / f Au25 / þ hν

ð5Þ

’ ACKNOWLEDGMENT L.L. and H.L. contributed equally to this work. This research was financially supported by the National Natural Science Foundation of China for the Key Program (Grants 20635020, 50972058, and 20821063) and the National Basic Research Program of China (Grant 2011CB933502). We also thank the support from Discipline Crossing Foundation of Nanjing University, Analysis & Test Fund of Nanjing University and the Foundation from State Key Laboratory of Bioelectronics, Southeast University.

Actually, it was reported that silver nanoclusters (Ag NCs) exhibited cathodic hot electron-induced ECL with S2O82as a coreactant, and the ECL mechanisms of Ag NCs were ascribed to the ox-red excitation pathway,26 where the luminophore was oxidized by the same cathodically produced oxidizing radical, SO4•-, as described above. However, the reported Ag NCs ECL was obtained under a pulse voltage of -30 V that was unfavorable to its application owing to some unforeseen reaction under such draconian conditions. Herein, we could observe cathodic ECL of Au NCs, integrating with ITO and S2O82-, under attainable operating conditions, which could provide a fascinating method for both experimental and theoretical studies. As is well-known, dopamine (DA) is an important neurotransmitter and plays a significant role in the function of the central nervous, renal, and hormonal systems.The detection of DA has been reported based on its quenching effect on the anodic ECL of CdSe QDs resulting from an energy-transfer process.15 In this work, when DA was injected into the electrolyte, the cathodic ECL of the Au NCs modified ITO electrode showed an obvious increase. Also, the ECL increased linearly with the increasing concentration of DA ranging from 2.5 to 47.5 μM as shown in Figure 3b,c. It has been reported that the adsorption of DA on the TiO2 surface can form a charge transfer complex, and electrons can transfer from the dopamine ligands directly to the conduction band of TiO2.27 Thus the ECL enhancement mechanism of DA here might be due to the formation of a similar charge transfer complex between DA and ITO, thus accelerating the electron injection into the conduction-band of ITO. Obviously, compared to the curves in Figure 1b, well-defined CV and DPV could be obtained in the presence of DA (Figure 3a), indicating that DA could indeed facilitate the electrochemical behavior in this system. The stability of the Au NCs modified electrodes was also tested, which showed stable ECL signal under consecutive potential scans from 0 to -1.5 V for 4 cycles with 40 μM DA in the electrolyte. An interference investigation was performed by using the solution containing 30 μM of DA and 100 μM of uric acid and ascorbic acid respectively, which are two common interfering agents for DA detection. Also no obvious change could be observed (Figure 3d), indicating an acceptable selectivity for DA detection. In conclusion, ECL was first observed from the water-soluble and low-toxicity BSA-stabilized Au NCs. The ECL behavior and mechanism of Au NCs have been studied in detail and could be ascribed to cathodic coreactant ECL based on electron transfer between Au NCs and ITO. In addition, the ECL system showed acceptable selectivity for DA detection. The Au NCs could become a promising material for ECL investigations.

’ AUTHOR INFORMATION Corresponding Author

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