Concentration of Capping Agent Controls Size Selection, Agglomeration and Antimicrobial Action of Silver Nanoparticles

Polyvinyl Alcohol (PVA) capped Ag nanoparticles (AgNPs) formed through reduction of Silver Nitrate (AgNO₃) by Sodium Borohydride (NaBH₄) were studied for different PVA concentrations (C, %w/w of solution) in the bulk. Diameter (D_Ag) of Ag metallic core, obtained from plasmon resonance in Ultraviolet-Visible spectra, had a peak (≈12.8 nm) at C~0.24, which almost exactly matched the gyration sphere diameter of PVA molecule (∼13 nm). Cluster diameter (D_cl), measured after a month through Dynamic Light Scattering (DLS) study, gave the number of NPs per cluster showing a dip at C∼0.24. Minimum inhibitory concentration (MIC) of AgNPs on Escherichia coli (E. coli), assayed from standard broth dilution method (CLSI M07-A8), showed MIC of 0.66 μg/ml at the PVA concentration of 0.24%. Scanning Electron Microscopy (SEM) showed NP clusters accumulating over E. coli to have D_cl ∼ D_Ag whereas those lying outside the bacteria to have D_cl ∼ 3D_Ag, consistent with a strong size selectivity of antimicrobial efficacy.