AgNPs were synthesized using a modified procedure by Leopold and Lendl . All solutions were freshly prepared prior to synthesis. In brief, 300 µL of 1.0 M sodium hydroxide solution was added to 80 mL of 1.87 mM solution of hydroxylamine hydrochloride under vigorous magnetic stirring. After about a minute, a NE-1000 syringe pump (New Era Pump Systems Inc., USA) was used to dispense 10 mL of 8.57 mM AgNO3 solution to the stirred hydroxylamine solution at a constant rate over 30 minutes. The formed gray-brown colloid was kept stirred for 10 additional minutes after which it was moved to a refrigerator (6 °C) until use.