(175av) Biosynthetic Conversion of Ag+ to Ag0 Nanoparticles By Chlamydomonas Reinhardtii: Effects of Extracellular Polymeric Substance and Cell Components on Synthesis & Stability

In the current study, the freshwater microalga Chlamydomonas reinhardtii bioreduced Ag⁺ to silver nanoparticles (AgNPs), demonstrating a more sustainable alternative to conventionally produced AgNPs. The bioreduction takes place in cell cultures of C. reinhardtii at ambient temperature, atmospheric pressure and in an aqueous media, which eliminates the need for harmful reducing agents, specialized equipment or the generation of toxic byproducts. The AgNPs were synthesized in either the presence of whole cell cultures, an exopolysaccharide (EPS)-containing cell culture supernatant or living cells that had been separated from the EPS-containing supernatant and washed before being suspended again in fresh media. While AgNPs were produced by all three methods, the washed cultures had no supernatant-derived EPS and produced only unstable AgNPs, so the supernatant-EPS was necessary to cap and stabilize the biogenic AgNPs. In addition to the visual changes in the cell culture, the production of AgNPs was confirmed by the characteristic surface plasmon resonance (SPR) band in the range of 415­-425 nm using UV-Vis spectrophotometry and further evolution of the SPR peaks were studied by comparing the peak intensities at maximum absorbance. X-ray diffraction (XRD) determined that the NPs were Ag⁰,while the selected area electron diffraction (SAED) pattern of these AgNPs determined the material to be polycrystalline. Transmission electron microscope (TEM) images showed stable AgNPs were mostly spherical and had a bimodal size distribution with size ranges of 3.0 ± 1.3 nm and 19.2 ± 5.0 nm for whole cultures and 3.5 ± 0.6 nm and 17.4 ± 2.6 nm for EPS only. Moreover, Ag⁺ to AgNP conversion 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 identified that polysaccharides, polyphenols and proteins were responsible for the observed differences in the AgNP stability. Additionally, Raman spectroscopy indicated carboxylate and amine groups were bound to the AgNP surface.