Bifunctional Electrocatalytic Films with Gold Nanoparticles and Metalloporphyrin: Preparation and Characterization
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Bifunctional Electrocatalytic Films with Gold Nanoparticles and Metalloporphyrin: Preparation and Characterization Wenjun Yang, Xiaoyuan Li* and Nai-Teng Yu* Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, SAR Hong Kong, P. R. China ABSTRACT Bifunctional electrocatalytic films were prepared employing a layer-by-layer assembly approach. The cation, metalloporphyrin (MP), and the anion, potassium tetrachloroaurate (KAuCl4) were alternatively assembled on glass or indium-doped tin oxide (ITO) supports. The assembled AuCl4- ions were then converted to gold nanoparticles (Aun) under the exposure to ultraviolet light. Film growth at each adsorption step was monitored by UV-Vis absorption spectroscopy and cyclic voltammetry (CV). The formation of gold nanoparticles was characterized by UV-Vis absorption spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Bifunctional electrocatalytic activity of the film was demonstrated in the oxidative detection of nitric oxide (NO) over Aun and the reductive detection of oxygen (O2) over Co(II)P in the same CV scan. INTRODUCTION Electrochemical transducer has been established as one of the primary sensing technologies in the selective and quantitative detection of chemical and biological species. Most of the modified electrodes provide one catalytic function for a specified analyte. In a living system it is often desirable to detect several ananlytes simultaneously at the same location. Layer-by-layer (LbL) electrostatic assembly is a simple while useful method for multilayer construction of oppositely charged species [1-3]. The amount of assembled species can be controlled by the number of repeating dipping/incubation/rinsing cycles. The assembling species ranged from polyelectrolytes, dye molecules, protein, DNA to semiconductive or metallic nanoparticles [2,3]. This technique provides an easy and convenient approach for the fabrication of 2-D arrays and 3-D networks of charged species with versatile functions. We report here the preparation and characterization of bifunctional films assembled on electrodes containing two electrocatalytic species. MP and KAuCl4, precursor of metallic gold, were alternatively assembled on electrodes by LbL deposition. Exposure of the assembled films to ultraviolet irradiation yields gold nanoparticles embedded in
hν MP⊕
AuCl4-
… Figure 1. Schematic illustration of LbL assembly of MP cations and AuCl4- anions on an electrode and the subsequent photochemical conversion of AuCl4- to gold nanoparticles. Y3.26.1
metalloporphyrin matrix. An advantage of the photochemical method for colloidal metal synthesis lies in the fact that a large number of metal atoms are homogeneously and instantaneously produced, which is favorable for the formation of highly dispersed particles [4]. The preparation process is illustrated in Fig. 1. EXPERIMENTAL Materials. Potassium tetrachloroaurate (KAuCl4, 98%) was purchased from Aldrich. Manganese(III)-tetra(N-Methyl-4-pyridyl)-porphyrin chloride
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