Organic ligand-mediated synthesis of shape-tunable gold nanoparticles: An application of their thin film as refractive index sensors

A simple wet-chemical in situ reduction method has been developed for the synthesis of shape-tunable gold nanoparticles (GNPs) using 1,4,8,11-tetraazacyclotetradecane (cyclam) as a reducing-cum-stabilizing agent. The growth of different shaped GNPs occurred at ambient temperature in the absence of any added stabilizers/templates. Transmission electron microscopic analysis of the as-prepared GNPs shows that an increase in the metal precursor concentration in the reaction medium resulted in the formation of triangular or hexagonal nanoplates along with spherical GNPs. The addition of foreign metal ions like Ni2+ or Cu2+ in the reaction medium also affects the shape of the formed GNPs. Mechanistic and kinetic studies of GNPs formation have been studied by UV-vis spectroscopy. We have also demonstrated that the GNPs could easily be deposited on unfunctionalized glass and quartz substrates by a simple dip-coating technique to fabricate thin films that were also characterized by UV-vis spectroscopic and high-resolution microscopic techniques. The formed GNPs present in aqueous suspension or in the deposited thin films showed shape-dependent optical properties. A refractive index sensor for organic solvents was successfully fabricated by using this GNP's thin-film-coated glass or quartz substrate on the basis of the surface plasmon resonance (SPR) property. The sensor measured the changes of the SPR property of GNPs in the presence of various solvents.