Effect of Synthesis Conditions on the Plasmon Resonance Peak of Silver Nanoparticles

This study explores the influence of synthesis conditions on the surface plasmon resonance (SPR) peak of silver nanoparticles (AgNPs). Key synthesis parameters—including reaction time, extract-to-precursor volume ratio, initial silver nitrate (AgNO₃) concentration, and type of plant extract—were systematically varied. The synthesized nanoparticles were characterized using UV-Visible (UV-Vis) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, and antibacterial assays. UV-Vis spectra were analyzed using Gaussian–Lorentzian curve fitting to determine the resonance wavelength. Additionally, the spectra were interpreted using Mie theory simulations to estimate particle diameter, extinction efficiency, and the number of nanoparticles per unit volume. Size estimates were further validated using empirical relations between resonance wavelength and particle size derived from previous experimental studies. The results reveal that reaction time did not significantly shift the SPR peak for either extract. However, for nanoparticles synthesized with cabbage extract, varying the extract concentration caused a blue shift in the SPR peak from 434 nm to 427 nm—a shift not observed with the tomato extract. The choice of extract played a substantial role in tuning the optical properties of the nanoparticles. Tomato extract yielded smaller, more monodisperse AgNPs with an average diameter of approximately 48 nm and a sharp SPR peak at 419 nm. In contrast, cabbage extract produced larger, more polydisperse particles with an average diameter of 62 nm and a broader SPR peak around 434 nm. FTIR analysis confirmed the involvement of functional groups from the plant extracts in the reduction and stabilization of the nanoparticles. Furthermore, the AgNPs demonstrated notable antibacterial activity against Escherichia coli , Staphylococcus aureus , and Pseudomonas aeruginosa .