Formation of silver nanoparticles in aquatic environments facilitated by algal extracellular polymeric substances: Importance of chloride ions and light

Abstract

Natural conversion of metal species is an important source for nanoscale metal particles in the aquatic environment, and it could affect their fate and toxicity. Extracellular polymeric substances (EPSs) are ubiquitous and abundant in the aquatic environment, thus likely can reduce metal ions to nanoscale particles. However, the effect of natural inorganic ligand and light on this process has not been well investigated. In this work, Ag⁺ was readily reduced to silver nanoparticles (AgNPs, around 15 nm in size) by the EPS collected from Chlorella pyrenoidosa. AgNPs could be generated in the dark environment but at a slow rate. Visible light accelerated the photoreduction. The reaction mechanism probed by Fourier transform infrared spectroscopy and three-dimensional excitation-emission matrix spectrometry demonstrated that the reduction in Ag⁺ was attributed to the protein and polysaccharides in the EPS. The presence of chloride ions (Cl^-) largely shortened the duration of photoreduction. Scanning electron microscopy results indicated that with the aid of EPS, the AgCl nanocrystal was converted to core-shell structure, with dot-like nano Ag acting as the shell and the AgCl nanocrystal acting as the core. Size and morphological changes were observed on transmission electron microscopy. This study adds new knowledge of the joint effect of light exposure, Cl^-, and EPS on the formation of AgNPs from Ag⁺ and advances the understanding of the natural formation mechanism of AgNPs.

Keywords: AgCl nanocrystal; Chloride ions; Extracellular polymeric substance; Photoreduction; Silver nanoparticles.