Bactericidal efficiency of silver nanocomposites obtained using Brewer’s spent grains

Abstract

Silver nanocomposites were prepared using extracts resulting from different extraction methods of brewer’s spent grains (BSG) herein called BW3. Extracts were prepared using solvent (acetone and methanol aqueous solutions, and water) extraction methods, ultrasonic and microwave-assisted methods, and alkaline-acid hydrolysis.Extraction conditions influenced the composition of extract solutions differing in the amount of Na, K, P, Mg, Ca, Zn, and SO4−, Cl−, and NO3– anions, wherein predominantly varying contents of P and anions influenced the properties of the nanocomposites. Nanocomposites exhibited three major crystalline phases, namely, Ag3PO4 (9–74 wt%), Agmet (25–84 wt%), and AgCl (∼1–5 wt%) with different Ag crystallite sizes. Nanoparticles of different Ag phases were capped by organic structures consisting of chemical groups such as C–OH, C = O, C–OOH, –NH, and –PH. The thermal stability of the nanocomposites increased with increasing Agmet phase content.The minimum inhibitory concentration (MIC) of nanocomposites for Escherichia coli was as low as 4 μg mL−1. The antibacterial efficiency depended on the amount of (i) Ag+ ions from different Ag phases and their solubility, (ii) positively charged P and N hydrogen forms, (iii) negatively charged C groups and vacancies, and (iv) Ag phases crystallite size.