Green synthesis of silver and zinc oxide nanoparticles for SERS-based detection of resonant fluorescent molecules

Strong fluorescence from resonant organic dyes often limits the applicability of Raman spectroscopy by masking weak vibrational signals. In this study, green-synthesized silver nanowires (Ag NWs) and zinc oxide nanoparticles (ZnO NPs) derived from Azadirachta indica leaf extract were investigated as surface-enhanced Raman scattering (SERS) substrates for the fluorescent dyes resorufin and pyronin-Y. The synthesized nanostructures were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM/TEM), and energy-dispersive spectroscopy (EDS). Ag NWs exhibited a face-centered cubic structure with an interconnected wire-like morphology, whereas ZnO NPs possessed a highly crystalline hexagonal wurtzite structure with a spherical morphology and smaller particle size. The optical properties of resorufin and pyronin-Y were examined through Raman and photoluminescence spectroscopy under 532 nm excitation. Both dyes exhibited intense fluorescence backgrounds that obscured their Raman signatures. The incorporation of Ag NWs and ZnO NPs significantly improved spectral resolution by suppressing fluorescence and enhancing Raman scattering. Distinct vibrational bands associated with aromatic ring deformations and C-C, C-N, C[double bond, length as m-dash]C, C[double bond, length as m-dash]N, and C[double bond, length as m-dash]O vibrations were successfully identified. Among the investigated substrates, ZnO nanoparticles produced markedly stronger Raman enhancement than Ag nanowires, particularly for the highly fluorescent analytes studied. The enhanced SERS activity of ZnO is attributed to the combined effects of efficient light scattering, high surface area, and defect-mediated charge transfer between the semiconductor surface and the adsorbed molecules. The proposed energy-level alignment further supports a chemical enhancement pathway under resonant excitation conditions. This study demonstrates the potential of green-synthesized ZnO nanostructures as sustainable and efficient SERS substrates for fluorescence-suppressed molecular detection and provides new insight into semiconductor-assisted Raman enhancement mechanisms.