Characterization and Bioactivity of Silver Nanoparticles Synthesized from Melissa officinalis Extract: A Green Nanotechnology Approach

Silver nanoparticles (AgNPs) were synthesized via a green chemistry approach using Melissa officinalis aqueous extract as both a reducing and stabilizing agent. The formation of AgNPs was confirmed visually by a color change from light yellow to dark brown and spectroscopically by a distinct surface plasmon resonance band between 350 and 500 nm. The formulation containing 2 mM AgNO₃ and 4% (v/v) extract at pH 8 and 37 °C exhibited optimal nanoparticle formation. Fourier transform infrared (FTIR) analysis revealed the presence of hydroxyl, carbonyl, and amine functional groups, indicating that phenolic and flavonoid compounds acted as reducing and capping agents. Dynamic light scattering (DLS) showed a broad hydrodynamic distribution, while scanning electron microscopy (SEM) confirmed that the primary nanoparticles were spherical with an average diameter of approximately 60 nm. X-ray diffraction (XRD) patterns indicated a face-centered cubic crystalline structure. Phytochemical analysis revealed a significant decrease in total phenolic and flavonoid contents following nanoparticle synthesis, confirming their involvement in the reduction and stabilization processes. Antioxidant assays (FRAP and DPPH) demonstrated that AgNPs retained or slightly enhanced activity compared to the crude extract, with FRAP values of 2.23 vs. 1.93 Eq/mg and DPPH inhibition of 86.72% vs. 78.31% at a concentration of 1 mg/mL. Antibacterial testing revealed selective activity against Staphylococcus aureus, with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of 8 µg/mL and 16 µg/mL, respectively. In contrast, only weak or no activity was observed against Escherichia coli and Proteus mirabilis. Cytotoxicity assays (MTT) on MCF-7 breast cancer cells showed that the M. officinalis extract exhibited greater anticancer activity (IC₅₀ = 32 µg/mL) than the synthesized AgNPs, suggesting reduced bioavailability due to nanoparticle aggregation and lower cellular uptake. Overall, these findings demonstrate that M. officinalis-mediated AgNPs are spherical nanoscale particles (∼60 nm) with confirmed crystallinity, moderate stability, enhanced antioxidant activity, and selective antibacterial effects. Optimization of synthesis parameters could further improve their biomedical potential.