Controlled synthesis of colloidal monodisperse gold nanoparticles in a wide range of sizes; investigating the effect of reducing agent

The diverse applications of gold nanoparticles (Au-NPs) with sizes less than 50nm require a comprehensive information about how to produce Au-NPs with different sizes and narrow size distributions for a desired application. In this work, a wide range of sizes of Au-NPs from 2.5 to 35nm has been synthesized by using two reducing agents of sodium citrate and sodium borohydride. We have studied the dependency of the size, polydispersity, surface potential, plasmonic spectrum, and pH of the resultant Au-NPs on the reducing agents' concentrations. The findings show that uniform and monodispersed Au-NPs can only be produced in specific ranges of concentrations of the reducing agents. In addition, the absorption spectra of Au-NPs cannot uniquely determine the dominant size of the nanoparticles and polydispersity of the sizes is an important parameter. Measuring Zeta potential of the Au-NPs shows that by increment of pH of the solutions, the surface potential in both methods monotonically increases which results in more stability of the nanoparticles.