Green synthesis of silver-coated zinc oxide nanocomposite using the walnut green husk extract and assessment of its cytotoxicity and antimicrobial activity against Streptococcus mutans.

OBJECTIVE: This study aimed to synthesize and characterize silver-coated zinc-oxide (ZnO/Ag) nanocomposite using green husk extract of Persian Walnut (Juglans regia L.) and evaluate its cytotoxicity and antimicrobial activity against Streptococcus mutans. MATERIALS AND METHODS: In this in vitro experimental study, five groups were tested: hydroethanolic green husk extract (group A), green-synthesized ZnO/Ag nanocomposite using the extract (group B), chemically synthesized ZnO/Ag nanocomposite (group C), 0.2% chlorhexidine (positive control), and distilled water (negative control). Antibacterial activity against S. mutans was assessed via agar-well diffusion (inhibition zone diameters) and broth microdilution (MIC and MBC). Cytotoxicity was evaluated using the MTT assay. Tests were performed in triplicate across serial dilutions, with data analyzed by ANOVA. RESULTS: Group A exhibited no inhibitory or bactericidal effect on S. mutans. Group B inhibited growth at 125 ppm, and group C at 31.25 ppm. Both groups B and C reduced S. mutans colony counts by approximately 3 logs (significant vs. group A) and showed minimal cytotoxicity. CONCLUSION: Both green-synthesized and chemically synthesized ZnO/Ag nanocomposites displayed lower antibacterial activity than chlorhexidine (CHX) against S. mutans, along with lower cytotoxicity, yet achieved a significant reduction in bacterial colony count.