A novel, rapid, seedless, in situ synthesis method of shape and size controllable gold nanoparticles using phosphates

We hereby report a novel synthesis method of size and shape controllable gold nanoparticles that is rapid, in situ and seedless. Unlike most currently employed size and shape controllable synthesis methods, it takes place in a single step under room temperature within ~15 minutes. While mixtures of gold nanospheres around 70 nm and gold nanoplates with width ranging from 100 nm to 1000 nm can be synthesized in about 15 minutes by standard synthesis method using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) to reduce Au(III), gold nanoflowers or mixtures of smaller gold nanospheres and nanoplates can be synthesized with the addition of disodium phosphate (Na₂HPO₄) or monosodium phosphate (NaH₂PO₄), respectively. Increasing the concentration of phosphate added significantly reduces the formation time of gold nanoparticles to seconds. By increasing the molar ratio of Na₂HPO₄: HEPES and NaH₂PO₄: HEPES, the size of gold nanoflowers and gold nanoparticle mixtures can be tuned from ~60 nm down to 1 nm and from ~70 nm to ~2.5 nm, respectively. The systematic structural changes are accompanied by similarly systematic colour changes associated with shifting of the surface plasmon resonance. The proposed mechanism of the synthesis process is also presented.