Layered core-shell bimetallic silver-gold nanoparticles were prepared by coating Au layers over Ag seeds by a seed-growth method. The composition of Ag100-xAux particles can vary from x=0 to 30. TEM and SEM images clearly show that the bimetallic nanoparticles are of core-shell structure with some pinholes on the surface. Strong surface-enhanced Raman (SER) signals of thiophenol and p-aminothiophenol have been obtained with these colloids. It was found that the SERS activity of aggregated colloids critically depends on the molar ratio of Ag to Au. With the increase of the Au molar fraction, the SERS activity enhances first and then weakens, with the maximal intensity being 10 times stronger than that of Ag colloids. The AgcoreAushell nanoparticles were then labeled with monoclonal antibodies and SERS probes and used for immunoassay analysis. In the proposed system, antibodies immobilized on a solid substrate can interact with the corresponding antigens to form a composite substrate, which can capture reporter-labeled AgcoreAushell nanoparticles modified with the same antibodies. The immunoreaction between the antibodies and antigens was demonstrated by the detection of characteristic Raman bands of the probe molecules. AgcoreAushell bimetallic nanoparticles, as a new SERS active and biocompatible substrate, will be expected to improve the detection sensitivity of immunoassay.

Assemblies consisting of nanosized silver particles, 4-aminothiophenol, and the smooth macroscopic surface of silver metal (Ag/PATP/Ag) were prepared and characterized by surface-enhanced Raman spectroscopy. The Raman spectrum of 4-aminothiophenol in the assemblies is similar to that obtained on the electrochemically roughened electrode, although spectral differences, including slight frequency shifts and changes in relative intensity of the characteristic bands, were observed. The enhancement of the spectrum of 4-aminothiophenol in the assemblies is ascribed to the electromagnetic coupling between the silver particles and the surface of the silver metal. The fact that only the SERS spectrum of 4-aminothiophenol was observed for the assembly, where 4-aminothiophenol is partially replaced with N,N‘-diphenylthiourea, demonstrates that the electromagnetic field is mainly localized in the region between the silver particles and the surface of the silver metal.

Surface properties of an assembled silver nanoparticle/ITO (AAgNP/ITO) electrode and an electrochemically roughened Ag electrode were studied by surface-enhanced Raman spectroscopy and electrochemical techniques. Methyl viologen (MV) and p-aminothiophenol (PATP) were used as the probing molecules. The electrochemical results show that MV can be strongly adsorbed on the surface of the roughened Ag electrode but not on the surface of the AAgNP/ITO electrode. The SERS spectra further indicate that the MV molecules are adsorbed with parallel orientation to the surface of the roughened Ag electrode. The fact that no SERS spectrum was observed on the AAgNP/ITO electrode may imply that there are no required active sites on the surface of the isolated Ag nanoparticle for the interaction between the ring plane of MV and Ag nanoparticle. On the other hand, the SERS spectra of PATP reveal that PATP molecules can be adsorbed on the surfaces of both electrodes, because of the perpendicular orientation of the adsorbed molecules relative to the surface of the Ag nanoparticle on the electrodes. Nevertheless, changes in relative intensities of several a1 and b2 modes also indicate that there are slight differences in the orientation of the adsorbed PATP and the chemical property of the electrode surfaces.

Surface-enhanced Raman spectroscopy has been successfully extended to the study of corrosion inhibition of bare iron and nickel metals. The inhibition effects of benzotriazole (BTAH) for copper, iron, and nickel electrodes in 0.1 M KCl solution were investigated by using both polarization curves and in situ Raman techniques. The protective films formed on copper and iron surfaces, in the presence of BTAH, are characterized as [Cu I BTA]n and [Fe II (BTA)2]n, respectively. The formation of Fe-N coordinated bonds and the deprotonation of the triazole ring may occur while BTAH interacts with the iron surface. On the contrary, BTAH may interact with the nickel surface as neutral molecules in the whole potential range investigated resulting in a poor inhibition effect. The surface complex is characterized as [Ni-BTAH]. The potential dependence of the Raman spectra on copper and iron shows that the BTAion in the surface complex may rebind with H + at more negative potentials and accordingly the inhibition efficiency of benzotriazole decreases.

FT-Raman spectra were obtained for thiophenol UP) and TP on gold nanoparticles. All vibrational fundamentals for the TP molecule are assigned on the basis of the scaled quantum force field procedure. Three model systems are studied and compared for the interactions of TP with the Au atom: (1) TP with a Au atom, C6H5SH-Au; (2) TP anion with a Au atom, C6H5S--Au; and (3) TP with a Au atom and subsequent formation of thiophenylate, C6H5SAu. The equilibrium structures and Raman spectra were calculated for the model systems using density functional theory (DFT) with the B3LYP functionals and the mixed basis set 6-311+G** (for C, S, H) and LANL2DZ (for Au), and theoretical Raman wavenumbers of C6H5SAu and C6H5S--Au were assigned according to potential energy distributions. The third model system is shown to be preferred over the other two. The calculated binding energies are also shown to support the third model system. It is suggested that a simple model, such as the one used in the present study, is reasonable to describe surface-enhanced Raman spectroscopy of thiophenol adsorbed on gold nanoparticles. Copyright (C) 2007 John Wiley & Sons, Ltd

Au@Co and Au@Ni core−shell nanoparticles with controllable shell thicknesses were prepared by reduction of Co2+ and Ni2+ salts with hydrazine hydrate in ethanol over preformed Au seeds. Both cyclic voltammetric and surface-enhanced Raman studies using CO as the probe molecule confirmed the core−shell structure of the synthesized nanoparticles. The Au@Co and Au@Ni nanoparticles dispersed on a glassy carbon electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine. High-quality SERS spectra of CO absorbed on Co and Ni have been obtained through the high enhancement of the core−shell nanoparticles. The originally low surface enhancement of the Co and Ni can be substantially improved giving total enhancement factors up to 103-104. Such a SERS-active substrate can be used as an alternative substrate for investigating adsorption and reactions occurring on the Co and Ni metal surfaces.

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