Biomimetic synthesis of nanocrystalline silver sol using cysteine: stability aspects and antibacterial activities

Abstract

The study reports the development of a simple, environmentally benign green chemical route to produce stable silver nanoparticle (Ag-np) sols with excellent antibacterial properties under ambient conditions. The method involves the room temperature reduction of AgNO₃ by cysteine (aq) and requires no additional capping/stabilizing agent. It essentially mimics the redox reaction that takes place during incubation of the cell-free extract from Trichoderma asperellum in the presence of AgNO₃ (aq) (P. Mukherjee, M. Roy, B. P. Mandal, G. K. Dey, P. K. Mukherjee, J. Ghatak, A. K. Tyagi and S. P. Kale, Nanotechnology, 2008, 19, 075103), wherein cysteine, a biomolecule present in the fungal extract, acts as a potential reducing agent. Additionally, cysteine acts as a capping molecule in the present case. Formation of Ag-nps was evidenced from UV-Vis, TEM, XRD and EDS studies. The stability of Ag sols has been shown to depend strongly on the concentration of cysteine relative to that of AgNO₃. Sols obtained by reacting 0.1 mM of cysteine with 1 mM of AgNO₃ remained stable for more than one month at 24 °C. The role of cysteine as capping molecule and the possible modes of its linkages with Ag-nps was studied by FT-IR, XPS and Raman spectroscopy. Bonding of Ag with either or all the three, thiolate, amino and carboxylate groups of the cysteine molecule via stable P_H configuration is believed to have resulted in the stabilization of the Ag-nps. Antibacterial activity of the cysteine capped Ag sol was studied along with that of the Ag sol obtained by fungal route. Both the sols exhibited excellent and comparable efficacies as bactericidal agents against gram negative bacteria E. coli BW (25113), with one of the lowest minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values published so far.