Method development for speciation analysis of silver nanoparticles and silver ions in green algae and surface waters at environmentally relevant concentrations using single particle ICP-MS

Abstract

Interactions of silver(I) and PVP-stabilized silver nanoparticles (AgNPs) of different nominal diameters (40, 60 and 75 nm) with green microalgae Desmodesmus subspicatus during a 24 h incubation period in matrices that mimic surface water and natural river water were studied by single particle inductively coupled plasma mass spectrometry (spICP-MS). It was found that the uptake of silver species by algae was dependent on the type of matrix and was the lowest in ultrapure water and the highest in the presence of high concentrations of inorganic cations. AgNP uptake was size dependent and decreased with increasing particle size. Higher stability of AgNPs was observed in the presence of organic compounds, for both surface water and green algae origin. Three procedures of AgNP extraction from algal cells were tested: direct extraction, extraction by enzymatic digestion with pectinase (Macerozyme R-10) and by alkaline digestion with tetramethylammonium hydroxide (TMAH). In order to identify the source of overestimation of the results obtained in samples rich in organic matrix, interference studies were carried out. Two matrix-matched variants of calibration (using digestion reagents with or without cell lysates) for both AgNPs and Ag⁺ were tested for different extraction procedures. The silver recovery and AgNP size determined after enzymatic digestion of algal cells were highly dependent on the type of calibration used showing the occurrence of non-spectral interferences. Extraction of AgNPs using the alkaline digestion procedure (10% TMAH, 24 h, room temperature) and calibration using 0.1% TMAH + 0.1% Triton X-100 was found to give accurate results in terms of both the recovery and size of AgNPs. The developed procedure was applied to determine the fate of silver species in surface water – green algae environments.