Simple method to prepare size-controllable gold nanoparticles in solutions and their applications on surface-enhanced Raman scattering

We report here, for the first time, a simple method to prepare size-controllable Au nanoparticles (NPs) in aqueous solutions from bulk Au substrates. First, chitosan (Ch)-capped Au-containing complexes were prepared by electrochemical oxidation-reduction cycles in 0.1 N NaCl and 1 g/l Ch solutions. Then the solutions were heated from room temperature to boiling at different heating rates to synthesize size-controllable Au NPs. The particle sizes of the prepared Au(111) NPs could be controlled from 5 to 30 nm with an increase of the heating rate during preparation. Experimental results indicate that the prepared Au(111) NPs with diameters ranging from 10 to 30 nm can serve as surface-enhanced Raman scattering active probes for molecules of rhodamine 6G. These SERS spectra exhibit two competitive effects. One is the fluorescence from R6G dissolved in the aqueous solution. The other is SERS from R6G adsorbed on Au NPs, where SERS becomes more dominant for R6G adsorbed on larger NPs.