Bacillus lipopeptides: powerful capping and dispersing agents of silver nanoparticles

Abstract

This study demonstrates the use of Bacillus lipopeptides as both capping and stabilizing agents in the single-step synthesis of silver nanoparticles (Ag-NPs). With the aim to achieve a stable Ag-NP suspension, two methods of synthesis procedures, methods A and B, were tested and compared. In method A, the excess reactant sodium borohydride (NaBH4) was added to the limiting reactant silver nitrate (AgNO3), while in method B the limiting reactant was added to the excess reactant. It was found that, in both methods, the lipopeptide concentration significantly influenced the morphology and size of Ag-NPs. In case of method A, at low lipopeptide concentrations, Ag-NPs of uniform size distribution were synthesized, whereas at high concentration, incipient small nano-spheres of Ag-NPs were found concentrated at the vesicular surfaces during initial stage of reaction. However, at later stages, the smaller spherical particles radically changed into varied shapes and sizes and eventually detached from the vesicles completely. Ag-NPs of method B showed narrow size distribution. Stability studies revealed that Ag-NP suspension remained stable up to the sodium chloride, NaCl, concentration of 25 g/L. Steric and depletion types of stabilization were found to be the predominant mechanisms that imparted stability to the Ag-NPs. The particles were found to be stable up to test period of 6 months. Antimicrobial studies revealed that the lipopeptide-conjugated Ag-NPs with low lipopeptide were sensitive against all the tested organisms.