Bacteria-mediated biosynthesis of silver nanoparticles and their characterization

In this thesis study, silver nanoparticles (AgNPs) were synthesized using a biosynthesis (biological) method that is environmentally friendly, economical, reliable, and allows for controlled production. For this biosynthesis study, bacterially mediated AgNPs were obtained using Pseudomonas fluorescens. The selection of this bacterium was influenced by its ability to remain viable for up to three days even at high concentrations in preliminary trials conducted in buffer solution. Among the 18 different bacterial isolates obtained from stock cultures, Pseudomonas fluorescens was identified as the active isolate that remained viable and demonstrated high biosynthesis capacity, and the study was continued with this isolate. Thus, both an ideal environment for the optimal activity of the bacterium and suitable conditions for nanoparticle formation were established. To determine the optical properties of the synthesized AgNPs, ultraviolet-visible spectroscopy (UV-Vis) was used; for structural properties, dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX) analyses were conducted; and for morphological and size characterization, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) imaging techniques were employed. The expected surface plasmon resonance peak was not achieved in the UV-Vis analysis, which is believed to be due to the absorption properties being affected by organic residues present in the bacterial supernatant. Nevertheless, other characterization techniques confirmed the successful synthesis of AgNPs and verified the presence of organic residues. The AgNPs were found to have particle sizes in the range of 5–50 nm, with spherical appearance, polydisperse morphology, and a biological halo structure surrounding the particles. Antimicrobial activity evaluation was performed using the well diffusion and disk diffusion methods, and the results were compared with commercial antibiotic discs. The synthesized AgNPs exhibited inhibitory effects particularly against pathogenic bacteria such as Acinetobacter baumannii, Enterococcus faecalis, Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli. It was observed that AgNPs produced significant inhibition zones even in cases where some commercial antibiotics were ineffective. The anticancer activity evaluation was conducted on the HCT116 colorectal cancer cell line, and application of AgNPs at high concentrations resulted in approximately 90% death of these tumor cells. A potentiometric sensor application was also carried out for the detection of lead ions; however, the expected selectivity was not achieved, indicating the need for further advanced studies in this area. In conclusion, this study demonstrated that AgNPs can be successfully produced using a bacteria-mediated biosynthesis method that is environmentally conscious, cost-effective, and efficient. Additionally, this study shows that organic residues in biological synthesis prevent AgNP agglomeration, ensuring stable and discrete distribution. Keywords: Silver nanoparticles, Biosynthesis, Bacterial synthesis, Characterization techniques, Antimicrobial activity, Antibacterial activity, Anticancer activity, Potentiometric sensor