A Mechanistic View of the Light-Induced Synthesis of Silver Nanoparticles Using Extracellular Polymeric Substances of <i>Chlamydomonas reinhardtii</i>

• Main Authors: Ashiqur Rahman, Shishir Kumar, Adarsh Bafana, Julia Lin, Si Amar Dahoumane, Clayton Jeffryes
• Format: Article
• Language: English
• Published: MDPI AG 2019-09-01
• Series: Molecules
• Subjects: eps; nanobiomaterials; bottom-up; agnps; algal synthesis; factorial design; photon
• Online Access: https://www.mdpi.com/1420-3049/24/19/3506

In the current study, extracellular polymeric substances (EPS) of <i>Chlamydomonas reinhardtii</i> and photon energy biosynthetically converted Ag⁺ to silver nanoparticles (AgNPs). The reaction mechanism began with the non-photon-dependent adsorption of Ag⁺ to EPS biomolecules. An electron from the EPS biomolecules was then donated to reduce Ag⁺ to Ag⁰, while a simultaneous release of H⁺ acidified the reaction mixture. The acidification of the media and production rate of AgNPs increased with increasing light intensity, indicating the light-dependent nature of the AgNP synthesis process. In addition, the extent of Ag⁺ disappearance from the aqueous phase and the AgNP production rate were both dependent on the quantity of EPS in the reaction mixture, indicating Ag⁺ adsorption to EPS as an important step in AgNP production. Following the reaction, stabilization of the NPs took place as a function of EPS concentration. The shifts in the intensities and positions of the functional groups, detected by Fourier-transform infrared spectroscopy (FTIR), indicated the potential functional groups in the EPS that reduced Ag⁺, capped Ag⁰, and produced stable AgNPs. Based on these findings, a hypothetic three-step, EPS-mediated biosynthesis mechanism, which includes a light-independent adsorption of Ag⁺, a light-dependent reduction of Ag⁺ to Ag⁰, and an EPS concentration-dependent stabilization of Ag⁰ to AgNPs, has been proposed.