a School of Physics and Material Studies, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia

b NANO-SciTech Lab (NST), Centre for Functional Materials and Nanotechnology (CFMN), Institute of Sciences (IOS), Universiti Teknologi MARA, 40450 Shah

Alam, Malaysia

c Institute for Biodiversity and Sustainable Development (IBSD), Universiti

Teknologi MARA, 40450 Shah Alam, Malaysia

^dAtta-ur-Rahman Institute for Natural Products Discovery (AuRIns), Universiti Teknologi MARA (UiTM) Puncak Alam Campus, 42300 Puncak Alam, Selangor, Malaysia

e Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400

UPMSerdang, Selangor, Malaysia

f Industrial Technology Division, Malaysian Nuclear Agency, 43000 Kajang,

Selangor, Malaysia

This study investigates the eco-friendly synthesis of zinc oxide nanoparticles (ZnONPs) using Citrus hystrix extract as a reducing agent. HRTEM and SAED analyses confirmed spherical nanoparticles (~20 nm) with a hexagonal wurtzite structure. Photoluminescence (PL) results showed that lower zinc acetate concentrations (0.1 M) produced higher luminescence intensity, indicating an inverse relationship between concentration and optical properties. Antibacterial tests against five bacterial strains revealed greater efficacy at lower concentrations (0.1 M and 0.2 M), likely due to reduced aggregation. Raman spectroscopy confirmed the hexagonal wurtzite phase. Optimizing nanoparticle concentration could enhance applications in optoelectronics and catalysis. Lower concentrations (0.1 M and 0.2 M) often exhibited stronger antibacterial action against bacterial strains such S. aureus, E.

coli, S. typhimurium, K. pneumoniae, and B. subtilis. The antibacterial performance of the ZnONPs was concentration-dependent. Lower doses produced the largest zones of inhibition; 0.2 M was the most effective against S. aureus, while 0.1 M was the most effective against S. typhimurium and B. subtilis.