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 (DAg) 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 (Dcl), 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 Dcl ∼ DAg whereas those lying outside the bacteria to have Dcl ∼ 3DAg, consistent with a strong size selectivity of antimicrobial efficacy.