Green Synthesis of Silver‐Zinc Oxide Nanocomposites Using <i>Spirulina platensis</i> Biomass: Characterization and Biomedical Evaluation

In this study, an aqueous extract of Spirulina platensis was utilized as a reducing and stabilizing agent for the green synthesis of silver-zinc oxide nanocomposites (Ag-ZnO). The S. platensis extract was optimized at a concentration of 0.5%, stored at 4°C and centrifuged at 6000 rpm for 5 min. Ultraviolet-visible spectroscopy of the extract showed a broad absorbance band in the 330- to 380-nm region and a shoulder in the 520- to 620-nm range, corresponding to bioactive metabolites. The formation of silver-zinc oxide nanocomposites was confirmed by characteristic absorbance in the 300- to 400-nm region under optimized conditions. Particle size analysis using a Zetasizer revealed an average particle size of approximately 204 nm with a polydispersity index of 0.28. Functional groups, crystallinity, and surface morphology were characterized using Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. Due to increasing concerns over antibiotic resistance, the antibacterial potential of the synthesized nanocomposites was evaluated by the disk diffusion method showing a maximum zone of inhibition of 35 ± 0.4 nm against Pseudomonas aeruginosa at 13 μg/mL. The Ag-ZnO nanocomposites exhibited a notable antioxidant activity, indicating a concentration-dependent DPPH radical scavenging activity with a maximum inhibition of 67.3% at 517 nm, compared to 60.12 for ascorbic acid. Furthermore, cytotoxic evaluation against the BT-549 breast cancer cell line revealed a half-maximal inhibitory concentration (IC₅₀) of 6.401 μg/mL. These findings indicate that S. platensis-mediated silver-zinc oxide nanocomposites exhibit promising biological activity.