(HAuCl4) (99.9% purity-trace metal basis) was acquired
from Sigma-Aldrich Co., Ltd, USA. Trisodium citrate dihydrate (HOC(COONa)(CH2COONa)2·2H2O) was acquired from
Sigma-Aldrich Co., Ltd, USA. Milli-Q water was obtained using a Thermo
Scientific Barnstead Nanopure system, having 18.2 MΩ ionic purity.