Physicochemical and antimicrobial properties of post-synthesis betanin and chitosan oligosaccharide functionalized silver nanoparticles

Size uniformity and defined surface coating are important parameters for the biological properties of silver nanoparticles (AgNPs). Green synthesis uses biomolecules either in pure form or plant extracts as both reductants and stabilizers yielding a wide size range and capping composed of oxidation products and biomolecules present in the reaction mixture. Herein, we used an established methodology, silver nitrate as the metal precursor, and sodium citrate and tannic acid for reduction and stabilization to produce weakly passivated citrate capped AgNPs (CtAgNPs). Inductively coupled plasma mass spectrometry was used to determine the total concentration of silver in the purified nanoparticles colloidal dispersion and results correlated with absorbance at localized surface plasmon resonance (LSPR) peak for quick estimation of silver concentration. The CtAgNPs were functionalized with chitosan oligosaccharide (mean Mn 5000) and betanin to yield chitosan and betanin capped AgNPs. Their biocidal properties were tested against Staphylococcus aureus American Type Culture Collection (ATCC) 12,600. Citrate-stabilized AgNPs had a LSPR peak at 432 nm, mean core diameter of 37.9 ± 8.6 nm, and a monomodal distribution, and were roughly spherical. A slight red shift in LSPR peak was observed upon capping change. The minimum inhibitory concentration against S. aureus was 12.5 ppm Ag for all the nanoparticles. Our study advances the path of exploiting size reproducible techniques in AgNP synthesis, post-synthesis biomolecule functionalization for defined surface, and a clear procedure for estimating the concentration of Ag in assays. This is envisaged to encourage investigations with distinct sizes and capping for cross-comparison.