Nitric acid

All chemicals were of analytical grade. Gold(iii) chloride trihydrate (HAuCl₄·3H₂O) was purchased from Sigma-Aldrich (USA). Chromium(iii) nitrate nanohydrate was purchased from Alfa-Aesar (USA). Trisodium citrate and disodium hydrogenphosphate dodecahydrate were purchased from Ajax Finechem (Australia). Hydrochloric acid, potassium hydroxide, ethanol and nitric acid were purchased from RCI Labscan (Thailand). Potassium bromide and sodium dihydrogen phosphate dehydrate were purchased from LobaChemi (India). All solutions were prepared using 18 MΩ cm⁻¹ ultrapure water obtained from ELGA Maxima.
Sodium phosphate buffer (pH 6.5 at 0.01 M) was prepared by mixing 34 mL of 0.02 M sodium dihydrogenphosphate and 16 mL of 0.02 M disodium hydrogenphosphate. Subsequently, the buffer was adjusted by either sodium dihydrogenphosphate or disodium hydrogenphosphate to obtain a pH of 6.5 and the volume was made up to 100 mL with ultrapure water. A stock solution of 12.52 mM bromide ion was prepared from potassium bromide. It was necessary that all the glassware was thoroughly cleaned in freshly prepared aqua regia solution (3 : 1 v/v HCl–HNO₃), washed with ultrapure water at least twice and then oven-dried at 150 °C before use to avoid aggregation of gold colloids due to any contaminants.