Phytogenic silver nanoparticles synthesized from Dendrophthoe falcata and Ocimum tenuiflorum: SERS and ultrafast nonlinear optical studies

Abstract

The surface enhanced Raman scattering (SERS) performance and femtosecond third-order nonlinear optical (NLO) properties of green synthesized silver nanoparticles (AgNPs) were systematically investigated in this work. Three dye molecules (nile blue-NB, crystal violet-CV and methylene blue-MB) and an explosive molecule (picric acid-PA) were utilized as probe analytes for the SERS detection. Among the different plant mediated syntheses, AgNPs synthesized with Dendrophthoe falcata plant extract (AgL) demonstrated a superior SERS enhancement (10⁶) with high signal reproducibility and sensitivity. With AgL as the SERS substrate, the detection sensitivity of NB, PA, MB, and CV were measured to be 10 nM, 10 µM, 100 nM and 50 µM, respectively, with Raman enhancement factors ranging from 10³ to 10⁶. In contrast, AgNPs synthesized with Ocimum Tenuiflorum (AgT) depicted one order lower SERS enhancement, with reduced signal reproducibility. Fourier Transform Infrared (FTIR) spectroscopy was utilized to examine the phytochemical composition of plant extracts used for synthesis. Transmission electron microscopy (TEM) analysis of AgL reveals nearly spherical-nanoparticles with a narrow size distribution (3–10 nm); while AgT displayed large nanoparticles with significant variation in size (10–70 nm) and shape (aspect ratio of 0.9–2). COMSOL Multiphysics simulation based on the finite element method was implemented to investigate the correlation between nanoparticle morphology (AgL and AgT) and the associated electric field enhancements. Furthermore, femtosecond Z-scan experiments were performed on AgL and AgT samples and their optical nonlinearity is found to be dominated by a two-photon process, which can be tailored to meet specific NLO applications. These results highlight the crucial role of green AgNPs mediated via plant-derived agents in tuning nanoparticle morphology and plasmonic behaviour, hence offering an efficient green route to enhance the performance of SERS substrates as well as for NLO applications.