Enhancing the antimicrobial and antibiofilm effectiveness of silver nanoparticles prepared by green synthesis

The use of polyphenol-rich plant extracts is well established for the green synthesis of silver nanoparticles (AgNPs). However, the size of the AgNPs varies substantially depending on the extract used and many researchers report sizes above 20 nm, which are not optimal for antimicrobial activity. Herein, using catechin as a model polyphenol, we have explored two techniques to improve its stabilising capacity and therefore decrease the subsequent AgNP size: cross-linking catechin with sodium tetraborate (borax); and preparation of a water soluble oligomer from catechin (polycat). The prepared AgNPs from the three stabilising systems, cat@AgNPs, cat-borax@AgNPs and polycat@AgNPs, were characterised by UV-Vis spectroscopy, dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry (ICP-MS). Cat-borax produced smaller AgNPs (18.4 nm) than catechin (42.3 nm) but the smallest particles were prepared with polycat (8.5 nm). Antimicrobial efficacy was assessed against Gram positive and Gram negative bacteria and was compared with 10 nm sodium citrate capped AgNPs (citrate@AgNPs). Polycat@AgNPs showed superior antimicrobial activity to cat@AgNPs and cat-borax@AgNPs as well as citrate@AgNPs, exhibiting MICs of only 1.25 μg mL^-1 (Ag) for Pseudomonas aeruginosa and Acinetobacter baumannii. Polycat@AgNPs also demonstrated substantially enhanced antibiofilm activity. An Ag concentration of only 5 μg mL^-1, was sufficient for a 99.9% reduction in biofilm cell viability and a 99.1% reduction in biofilm biomass with polycat@AgNPs. Uptake of polycat@AgNPs by bacteria was determined to be significantly higher than for citrate@AgNPs and tomographic and SEM images showed evidence of destruction of bacteria cells by polycat@AgNPs.