Green synthesis of poly-L-lysine-coated sericin nanoparticles and their molecular size-dependent antibacterial activity.

Investigating the role of molecular size and interfacial potential dependent antimicrobial propensity of nanoparticles (NPs) against bacteria is the important goal for secure usage of NPs to any living systems. In this study, crude silk sericin protein of Antheraea mylitta cocoon was fractionated into three different molecular size-ranges fractions such as fraction-1 (50-300 kDa), fraction-2 (30-50 kDa) and fraction-3 (10-30 kDa), and used to prepare crude sericin nanoparticles (CRSNPs), as well as fraction specific negative surface potential nanoparticles : n-SNP1, n-SNP2 and n-SNP3, respectively. SNPs were coated with poly-l-lysine to make the surface potential positive (p-SNPs) and confirmed through UV-vis spectroscopy, FTIR, zeta sizer and zeta potential measurement. The shape and sizes of all SNPs were determined by electron microscopy and found spherical in shape having diameter ranging from 110-165 nm (CRSNPs), 66-85 nm (SNP1), 33-49 nm (SNP2) and 14-24 nm (SNP3) for n-SNPs and p-SNPs, respectively. Evaluation of antibacterial activity using different concentrations (50, 100, 200 µg/mL) of all these SNPs showed significantly more activity of p-SNPs than n-SNPs against Staphylococcus aureus and Escherichia coli. Among these, SNP2 showed the strongest antibacterial activity followed by SNP3, SNP1 and CRSNPs. Relatively higher amounts of reactive oxygen species (ROS) generation were observed after treatment of bacteria with p-SNP2 (50 µg/mL) which is non-toxic to human cells. FE-SEM analysis showed more disruption of bacterial cell membrane after treatment with p-SNPs than n-SNPs. All these data suggested that molecular size and interfacial potential of SNPs enhance ROS generation to exert their antibacterial activity.