(60cp) Biosynthetic Conversion of Ag+ to Colloidally Stable Ag0 Nanoparticles By Chlamydomonas Reinhardtii: A Mechanistic View of the Light-Induced, Extracellular Polymeric Substances-Mediated Synthesis Process

In the current study, the freshwater microalga Chlamydomonas reinhardtii bioreduced Ag⁺ to colloidally stable silver nanoparticles (AgNPs), demonstrating a more sustainable alternative process to conventionally produced AgNPs. The process required photons to promote the reaction. The production of AgNPs was confirmed by the characteristic Ag⁰ surface plasmon resonance (SPR) band in the range of 415-430 nm using UV-Vis spectrophotometry. X-ray diffraction (XRD) determined that the NPs were Ag⁰. Transmission electron microscope (TEM) images showed stable AgNPs were mostly spherical with size < 10 nm. Moreover, Ag⁺ to AgNP conversion rate was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The AgNPs were stable over time in the cell culture media, acetone, NaCl and reagent alcohol solutions. This was verified by a negligible change in the features of the SPR band after t > 300 days of storage at 4 °C. Fourier-transform infrared spectroscopy (FTIR) of the as-produced AgNPs confirmed the presence of polysaccharides, polyphenols etc. and the corresponding shifts in the peak intensities and positions indicated the functional groups in the EPS that reduced Ag⁺, capped Ag⁰, and promoted the stability of AgNPs. Based on these findings, EPS-mediated biosynthesis mechanism has been proposed that began with the non-photon-dependent adsorption of Ag⁺ to EPS biomolecules followed by the light-driven reduction of Ag⁺ to Ag⁰ by an electron donated by the biomolecules. Following the reaction, stabilization of the NPs took place as a function of EPS concentration, which was identified as the final step of the mechanism.