Evaluation of enhanced darkfield microscopy and hyperspectral analysis to analyse the fate of silver nanoparticles in wastewaters

Nanomaterials are a class of emerging contaminants that are increasingly found in industry and the environment. In environmental media, their detection is often complicated by: their low concentration, the presence of high concentrations of natural colloids and the complexity of the matrix. Enhanced darkfield microscopy (EDM) coupled to hyperspectral analysis (HSI) is a recently developed tool that can theoretically be used to facilitate the localization and identification of nanoparticles and follow their fate in situ. The objective of this work was to evaluate the capacity of EDM-HSI to evaluate the fate of silver nanoparticles in wastewaters and biosolids. By coupling EDM with a pixel by pixel spectral analysis, transformation kinetics (attributed to the adsorption of organic matter) of several different silver nanoparticles (40, 80 and 100 nm) with different coatings (citrate and polyvinylpyrrolidone) could be analysed on a particle by particle basis in wastewaters. Results demonstrated that when spiked into complex systems containing natural organic matter, many of the nanoparticles were rapidly transformed (over a period of 2 hours), as shown by a wavelength shift attributed to their plasmonic resonance (450 to 570 nm). Based upon complementary results obtained by dynamic light scattering (DLS) and single particle inductively coupled plasma spectrometry (SP-ICP-MS), it was possible to attribute the changes not to particle aggregation or dissolution but rather to the adsorption of natural organic matter on the surface of the nanoparticle. The paper will demonstrate that the spectral properties of the nanoparticles in complex systems are very different from those in simple systems.