Nanotechnology offers a solution to bacterial antibiotic resistance, which poses a serious threat to global health. Green synthesis of metallic nanoparticles is gaining increasing attention due to its environmental benefits. This study aimed to biosynthesize silver nanoparticles (AgNPs) by microwave irradiation through silver nitrate reduction using starch and microalgae biomass; characterize them using UV–visible spectroscopy, scanning electron microscopy-energy-dispersive X-ray microanalysis, and X-ray diffraction; and evaluate their antibacterial activity against Escherichia coli, Bacillus clausii, and Staphylococcus aureus using disk diffusion and broth dilution methods. Synthesized AgNPs showed a single peak related to surface plasmon resonance at 430 nm. Size range of spherical AgNPs was 40–150 or 90–400 nm for starch- or biomass-mediated NPs, respectively. Biomass-mediated AgNPs exhibited antibacterial activity with the inhibition zones of 8, 12, and 10.5 mm against E. coli, B. clausii, and S. aureus, respectively; those starch-mediated showed inhibition of 7, 13, and 12 mm, respectively. AgNPs’ minimum inhibitory concentrations were 6.25 μg·mL−1 toward both E. coli and S. aureus and 12.5 μg·mL−1 against B. clausii when using starch in biosynthesis, whereas they were 19.6 μg·mL−1 against both E. coli and S. aureus and 9.81 μg·mL−1 toward B. clausii when using biomass. Synthesized AgNPs have promising antibacterial potential.

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Green synthesis is one of the fastest and best ways for ecofriendly nanoparticle synthesis. This study aims to investigate the use of the green microalgae Parachlorella kesseleri and Cyclotella spp. for the biological synthesis of silver nanoparticles (AgNPs). This work focuses on optimizing various parameters necessary for the production and stability of AgNPs. The nanoparticle formation was confirmed by UV-Visible analysis, which revealed the surface plasmon resonance band at 420 nm. The characterization of the AgNPs was performed using UV-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive X-ray microanalysis (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The antimicrobial properties of these bioactive AgNPs were also tested, showing excellent antibacterial activity against six bacterial strains, Escherichia coli, multidrug-resistant Escherichia coli, Bacillus clausii, Pseudomonas aeruginosa, Staphylococcus aureus, and Salmonella typhi. The biosynthesis of AgNPs from living cultures of microalgae has remarkable antibacterial properties. Other studies are underway in our laboratory to clarify the mechanism of the biosynthesis of these nanoparticles, and their action on bacteria.