Steady-State Fluorescence and Excitation–Emission Matrix Analysis of Sunlight-Assisted Silver Nanoparticle Formation Using Cucurbita pepo Peel Extract

Sunlight-assisted plant-mediated synthesis of silver nanoparticles (AgNPs) is well established, yet the complementary use of fluorescence spectroscopy to examine the optical response of complex phytochemical-containing media remains of interest. Here, Cucurbita pepo peel extract (CPE) was used for rapid AgNP formation under natural sunlight using single-step (A-Sun) and dropwise (B-Sun) addition strategies. The characteristic localized surface plasmon resonance (LSPR) band developed near 445–450 nm within 5 min, while corresponding dark controls showed negligible nanoparticle formation over the same period. Both dispersions retained their characteristic LSPR response during six months of optical monitoring. Steady-state photoluminescence (PL) measurements at excitation wavelengths of 300–350 nm and excitation–emission matrix (EEM) fluorescence mapping revealed lower recorded fluorescence intensities in the order CPE > A-Sun > B-Sun, with the principal emission region remaining broadly preserved near 435–440 nm. These observations provide complementary fluorescence fingerprints of the investigated preparations; however, inner-filter effects and light scattering may contribute to the measured attenuation, preventing its exclusive assignment to molecular quenching. FTIR revealed changes in oxygen-containing functional-group vibrations, while TEM showed predominantly spherical to quasi-spherical AgNPs, with mean particle diameters of 11.88 ± 3.60 nm for A-Sun and 9.77 ± 3.43 nm for B-Sun. The resulting dispersions also exhibited antibacterial activity against the six investigated bacterial strains. Overall, integrating PL and EEM with conventional characterization provides additional information on fluorescence changes accompanying sunlight-assisted AgNP formation without directly establishing the individual phytochemical reducing species, electron-transfer steps, or molecular surface-capping mechanisms.