Nanoparticles Synthesis
The stabilized polyvinylpyrrolidone-coated silver nanoparticles (AgNPs-PVP) were synthesized by a chemical reduction reaction as described in the literature46. In short, silver nitrate (AgNO3, 99.9% trace metals basis, Sigma, USA) and polyvinylpyrrolidone (PVP, MW 40000, Sigma, USA) were dissolved in deionized (DI) water separately and stirred on a magnetic stir plate until the solutions became homogeneous. Then AgNO3 and PVP solutions were added together and stirred for 30 mins at 0°C, meantime sodium borohydride (NaBH4, ≥98.0%, Sigma, USA) was dissolved in DI water at 0°C in a separate flask. Next, the reducing agent solution was added dropwise into the AgNO3-PVP mixture solution to reduce the Ag+-PVP to AgNPs-PVP. The color of the mixture solution became transparent bright yellow indicating formation of AgNPs-PVP. The final molarity concentration ratios of each component were AgNO3:NaBH4:PVP=2:1.4:1.
The AgNPs (AgNPs) was synthesized by chemical reduction reaction as a control to compare with the stabiliity of AgNPs-PVP, using a protocol reported previously47. In short, 0.25 mM AgNPs was made by adding 10 mL of 1.0 mM AgNO3 dropwise to 30 mL of chilled 2.0 mM NaBH4. The reaction mixture was stirred vigorously on a magnetic stir plate for 10 mins. Color of the mixture became clear yellow indicating the formation of AgNPs.