Synthesis of au nanoparticles at "all" pH by H2O2 reduction of HAuCl4.

In this article we report the synthesis of Au nanoparticles (NPs), from HAuCl4, in the pH range of 2.9 to 11.2 using H2O2 as the reducing agent. Ultraviolet-visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction techniques have been used to characterize the Au NPs. UV-Vis spectral observation showed that the Au NPs synthesized in acidic conditions tend to generate particles with absorption maximum around 540 nm. On the other hand the NPs generated at a pH higher than 8.0 generally have broad absorption with maxima occurring beyond 600 nm. Interestingly, TEM analysis showed that the NPs generated at pH lower than 7.0 tend to be smaller and spherical in shape, whereas the particles generated at a pH beyond 7.0 tend to be non-spherical and larger in sizes or agglomeration of small particles. Also, we speculate on the mechanisms of reduction of HAuCl4 by H2O2 under different pH conditions.