Non-toxic, Printable Starch Hydrogel Composite with Surface Functionalized Silver Nanoparticles Having Wide-Spectrum Antimicrobial Property

Potent chemical-free approaches are much needed to control multi-drug resistant pathogens such as biofilm-forming Staphylococcus aureus and rice blast fungus Magnaporthe oryzae. Environmentally safe agricultural practices and wound healing scaffolds based on hydrogels have a limitation arising from formulations containing environmentally toxic cross-linkers or photo-initiators. We have developed non-toxic starch hydrogels with uniformly dispersed silver nanoparticles (AgNP) having wide-spectrum antimicrobial activity for biomedical or agricultural applications. AgNPs were synthesized with Emblica officinalis extract, surface functionalized with the nutraceutical chemical groups in the extract as revealed by Fourier transform infrared spectroscopy. Scanning electron microscopy images show uniform dispersion of AgNP (5 ± 2 nm size) in the hydrogel matrix. AgNP-starch hydrogels are observed to have an excellent antifungal activity to M. oryzae with complete growth inhibition for 10 days compared to the control-starch hydrogel without AgNP and agar gel, even in the presence of the favourable growth medium of yeast extract + glucose. Antibacterial activity was also observed for gram-negative bacteria Escherichia coli and gram-positive bacteria S. aureus using diffusion assay with antibiotic mixture, with four-fold enhancement in zone of inhibition area in case of AgNP-starch hydrogels compared to control starch hydrogels, showing good release efficiency. Sustained release of botanical extracts showed promising bacterial growth inhibition. AgNP hydrogel surface-inhibition of bacterial growth for B. subtilis and E. coli persisted for 8–10 h and S. aureus until 6 h. Starch hydrogels have a high storage modulus G′ = ~ 17 kPa, and AgNP-starch hydrogels have a softer matrix with G′ = 3.6 kPa. Both are stable for a wide range of strain frequencies in the linear viscoelastic regime, revealing the potential for a strong, stable printable scaffold for wound healing. In the dried state, the AgNP-starch films show higher yield point and elongation at break compared to nascent cross-linked starch films under uniaxial tensile forces. Although AgNP-starch hydrogels show a nanoporous morphology enabling controlled drug release, there is negligible AgNP release from the hydrogel matrix.