Synthesis, stabilisation and surface modification of gold and silver nanoparticles by rosmarinic acid and its analogues

The aim of this study was to show that an antioxidant may be able to synthesise and stabilise metal nanoparticles while the same molecule might be adsorbed on the surfaces of the particles for their targeted delivery. Biochemical assay and pulse radiolysis technique with kinetic spectrophotometric detection have been employed to show the antioxidant activity of rosmarinic acid. It has been observed that the molecule can protect thiol from free radical induced damage. The reaction of a model peroxyl radical with rosmarinic acid is very fast in the first step possibly through electron transfer. An intramolecular rearrangement takes place in the second step with the formation of phenoxyl radical. Rosmarinic acid and its analogues caffeic acid and dihydrocaffeic acids were not only able to reduce gold and silver ions but they could stabilise nanoparticles of these noble metals. The particles were characterised by transmission electron microscopy. The UV Visible absorption spectra of the silver sol before and after the addition of rosmarinic acid show the transverse plasrnon peak, and the longitudinal plasmon peak for silver nano-rods. TEM image of the nanoparticles after surface modification shows the formation of nano-rods. These were formed by assembly of spherical nanoparticles. Along with the nano-rod formation other nanostructures could also be detected. Rosmarinic acid here is acting as a stabiliser and a surface-passivating agent.