Green synthesis of spheroid silver nanoparticles using cyanobacteria with multifunctional biological activities

Cyanobacteria-mediated synthesis of silver nanoparticles (AgNPs) offers a sustainable approach for producing functional AgNPs for biomedical applications. However, controlling their physicochemical properties while maintaining robust biological activities remains a critical challenge. Here, we developed a green, one-pot synthesis method to produce uniform, small AgNPs using the novel Cyanobium gracile PCC 6307 and evaluated their biological activities. PCC 6307 was isolated and identified using microscopic examination and sequencing analysis. The aqueous extract of algal biomass (CB) and cell-free medium (CS) were used to synthesize AgNPs under optimized reaction conditions. Acetyleugenol (38.09%) and eugenol (10.5%) were identified as the predominant metabolites in CB extract by gas chromatography-mass spectrometry. CS@AgNPs and CB@AgNPs showed strong absorption peaks at 412.5 and 430.5 nm and exhibited uniform spherical shapes with core diameters of 4.0 and 12.4 nm, respectively. The synthesized AgNPs exhibited hydrodynamic diameters < 100 nm and negative surface charges. Energy-dispersive X-ray spectroscopy confirmed the elemental composition of AgNPs, while Fourier-transform infrared spectroscopy verified the presence of algal functional groups on their surfaces. AgNPs reduced the colon cancer cell viability to 4.1% at 200 µg/mL exhibiting moderate to low toxicity toward normal human fibroblasts. CS@AgNPs revealed higher biocidal activity (inhibition zone ≤ 20 mm) than CB@AgNPs against Staphylococcus aureus, S. haemolyticus, and S. lentus. The antioxidant activity of AgNPs was assessed using the 2,2-diphenyl-1-picrylhydrazyl assay, where AgNPs showed moderate (46.0%) to low (25.1%) radical scavenging activity. This study provides a sustainable approach for producing uniform, small, and biologically active AgNPs with potential biomedical applications.