Antibacterial activity of PDDA-stabilized GO-AgNP nanocomposites

Abstract Graphene oxide (GO)-silver nanoparticle (AgNP) nanocomposites are widely studied for their antimicrobial synergy. In this study, the antibacterial activity of GO–AgNP nanocomposites prepared with poly(diallyldimethylammonium chloride) (PDDA) as a stabilizing linker was investigated. The composites were characterized by UV–Vis spectroscopy, FTIR, zeta potential analysis, XRD, TGA, and MP-AES. GO–PDDA–AgNPs exhibited a strong surface plasmon resonance band at ~ 420 nm, new C–H stretching bands at 2865–3012 cm⁻¹ from PDDA, and a high positive zeta potential (+ 57.5 mV) compared to bare GO (–40 mV), confirming successful functionalization and improved colloidal stability. Antibacterial activity was evaluated against E. coli and S. aureus using disk diffusion assays, growth curves, and MIC determination. GO and AgNPs alone showed no inhibition at concentrations up to 20 µg/mL, whereas PDDA alone produced inhibition zones of 6–11.5 mm against S. aureus (2.5–100 µg/mL) and induced selective bacterial aggregation. The minimum inhibitory concentration (MIC) of PDDA was between 6.25 and 12.5 µg/mL for S. aureus and between 25 and 50 µg/mL for E. coli , indicating greater potency toward Gram-positive bacteria. The GO–PDDA–AgNP composite inhibited S. aureus with zones of 8.5–10 mm at 10–20 µg/mL, while no significant inhibition was observed for E. coli at the tested concentrations. Molecular docking simulations examining interactions between PDDA and quorum-sensing regulatory proteins AgrA in S. aureus and LsrR in E. coli predicted higher binding affinity to AgrA (–5.11 kcal/mol) than LsrR (–3.76 kcal/mol). While in vitro assays using a Chromobacterium violaceum CV026 biosensor showed no inhibition of AHL-mediated signalling, it should be noted that this model differs mechanistically from the Gram-positive agr system, leaving the predicted AgrA interaction as a potential target for future investigation. Ultimately, this study demonstrated that PDDA is the primary antibacterial component of the GO–PDDA–AgNP composite, exhibiting potent activity against Gram-positive bacteria through a mechanism involving selective bacterial aggregation.