The spectral and microscopical study of phytosynthesized plasmonic gold nanoparticles

Here, we present a facile and environmentally friendly method for the synthesis of gold nanoparticles (Au NPs) with an infrared response. The structure of the obtained Au NPs was investigated by transmission electron microscopy. Small and large Au NPs with different morphologies, including spheres, triangles, and hexagons, were imaged and studied, and the reasons for the morphological diversity were discussed. From the selected area diffraction data, the Au NPs showed sufficient crystallinity. The optical properties of the Au nanocolloids, investigated by UV-visible absorption spectroscopy, confirmed the presence of localized surface plasmon resonance (LSPR) peaks at 500…540 nm for Au NPs smaller than 30 nm. An increase in absorption intensity in the 600…1050 nm region indicates the formation of larger non-spherical Au NPs. The optical absorption spectra show the redshift of the second LSPR peak to the near-infrared region with a longer wavelength with increasing HAuCl4 concentration in the synthesis solution. In addition, we recorded the maxima of photoluminescence (PL) bands at 370 and 458 nm for the water-diluted Au colloids under 320 nm excitation and considered the possible reasons for PL. Attempts were made to elucidate the optical and PL behavior of the nanocolloids within the known models