Growth Mechanism of Flowerlike Gold Nanostructures: Surface Plasmon Resonance (SPR) and Resonance Rayleigh Scattering (RRS) Approaches to Growth Monitoring

A facile method is proposed for the room-temperature synthesis of flowerlike gold nanostructures (AuNFs) with a size of 50-115 nm by reducing HAuCl4 with ascorbic acid (AA) in the presence of chitosan. It was found that the concentration of chitosan controlled the size, while that of AA influenced the morphology of as-prepared AuNFs. With higher concentration of AA flowerlike nanostructures were produced, whereas with lower concentration of AA quasi-spherical nanoparticles were formed. Time-dependent surface plasmon resonance (SPR) absorption spectroscopy and the resonance Rayleigh scattering (RRS) technique were used to monitor the growth processes. According to the temporal evolutions of SPR maximum absorption wavelength and RRS intensity, a second growth mechanism is proposed to explain the effect of AA concentration on the morphology and the effect of chitosan concentration on the size of obtained gold nanostructures. In order to determine whether or not the present method is suitable for synthesis of flowerlike gold nanostructures by use of other reductants in the presence of chitosan, four other conventional reductants, including gallic acid (GA), oxalate acid (OA), tartaric acid (TA), and sodium citrate (Cit), instead of AA were examined. The intrinsic reason-for the different performances of these reductants was further investigated, and the results also supported the proposed second growth mechanism.