1 mL of HAuCl4.3H2O (39.38 mg, 0.1 mol) was added to 89 mL of ultrapure water and was brought to boil under constant stirring. Aqueous sodium citrate solution (38.8 mM, 10 mL) was rapidly added to the boiling solution causing a colour change from yellow to burgundy. The solution was boiled for 10 minutes before allowing it to cool down at room temperature under stirring. The solution was then filtered using a 0.22 µm nylon filter to remove any large aggregates, leaving monodispersed cit-AuNPs.