Core-shell nanocomposites (R-Au) bearing well-defined gold nanoparticles as surface atoms of variable sizes (8-55 nm) have been synthesized exploiting polystyrene-based commercial anion exchangers. Immobilization of gold nanoparticles, prepared by the Frens method, onto the resin beads in the chloride form is possible by the ready exchange of the citrate-capped negatively charged gold particles. The difficulty of nanoparticle loading, avoiding aggregation, has been solved by stepwise operation. Analysis of the gold particles after immobilization and successive elution confirm the unaltered particle morphology while compared to those of the citrate-capped gold particles in colloidal dispersion. It was observed that the rate of the reaction increases with the increase in catalyst loading, which suggests the catalytic behavior of the gold nanoparticles for the reduction of the aromatic nitrocompounds. The rate constant, k, was found to be proportional to the total surface area of the nanoparticles in the system. Kinetic study for the reduction of a series of aromatic nitrocompounds reveals that the aromatic nitrocompound exclusively adsorbs to atop sites of gold particles and that the rate of the reduction reaction increases as the particle size decreases. Similar reaction kinetics was observed involving gold sol of variable size (homogeneous catalysis) as catalyst. The induction time and the activation energy of the reaction decreases with decrease in particle size indicating the decrease in activation energy for the smaller particles, which also speaks for the increase of surface roughness with decrease in particle size. The observed rate dependence, in relation to particle size, is attributed to a higher reactivity of the coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in larger particles.

Beta-cyclodextrin (β-CD) in alkaline solution has been observed to produce mono-and bimetallic nanoparticles of silver and gold and to provide in-house stability to both types of particles. Thus, the weak reducing capability of the β-CD molecule (oxidation occurs at +1.33 V vs Ag/AgCl) and its unique kinetic control over the evolution of both normal and inverted core-shell bimetallic architectures have been established. The structure and composition of the bimetallic particles were characterized by UV-visible spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, electron dispersive spectroscopy, and X-ray photoelectron spectroscopy. Bimetallic core-shell particles containing silver shells have been shown to provide an elegant SERS-active substrate compared to the corresponding monometallic nanoparticles, and therefore, they highlight the importance of electronic ligand effects on the enhancement of the Raman signals of molecular probes on nanostructured metallic surfaces.

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Adsorption of dopamine (DA) on a Au core -Ag shell bimetallic nanocolloidal surface has been investigated using surface-enhanced Raman spectroscopy (SERS). The normal Raman spectra (NRS) of DA molecules in bulk and in aqueous solution have been investigated in depth. The vibrational signatures, as observed from the Raman and FTIR spectra of the molecule, have been assigned from the potential energy distributions. The pH-dependent NRS of the DA molecule in aqueous solution has been recorded to elucidate the protonation effect and preferential existence of different forms of the molecule. The pH-dependent SERS spectra of the molecule adsorbed on the bimetallic Au core -Ag shell nanocolloidal surface are also reported. The enhancement of bands in the pH-dependent SERS spectra suggests that the molecules are adsorbed onto the bimetallic Au core -Ag shell surface with the molecular plane tilted with respect to the silver surface of Au core -Ag shell bimetallic nanoparticles. The model study authenticates the spectral disposition and orientation of the molecule. Thus, experiment and theory keep abreast of the variety of DA structures envisaged from SERS studies on a new substrate.

Silver nanoparticle aggregates were synthesized in large scale using resorcinol under alkaline condition to obtain an assembly of silver clusters. Stable dispersion of the cluster in aqueous medium has been examined out of resorcinol-capped silver nanoparticle assemblies. The UV-vis spectroscopy during the particle evolution has been studied in detail. From the high-resolution TEM (HRTEM) image and XRD pattern it was confirmed that the particles are made of pure silver only. The capping action of resorcinol has been authenticated from the FTIR spectra. UV-vis spectroscopy and TEM images reveal that the temperature, effect of vibrational energy, heat shock, and time-dependent particle evolution have unique bearing on the stability and surface properties of the clusters. The concentrations of silver nitrate, resorcinol, and NaOH have important influence on the particle evolution and its size. TEM images incite us to examine the aggregates to capitulate surface-enhanced Raman scattering (SERS) to the single molecular level using crystal violet (CV) and cresyl fast violet (CFV) as molecular probes. The SERS intensity of CV increases with increasing the size of the silver aggregate.

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