Speciation of gold nanoparticles and total gold in natural waters: A novel approach based on naked magnetite nanoparticles in combination with ascorbic acid.

A novel method for AuNPs/total Au speciation based on the combination of magnetic solid phase extraction and graphite furnace atomic absorption spectrometry (GFAAS) is described. Ascorbic acid enabled the quantitative extraction of both AuNPs and Au(III) by naked FeONPs, whereas a selective extraction of AuNPs was achieved in the presence of sodium thiosulfate. Experimental parameters influencing the extraction of both AuNPs and total Au, namely FeONPs mass, L-ascorbic acid concentration, pH, extraction time, sample volume, NaSO concentration and re-dispersion volume of magnetic solid phase prior to introduction in the graphite tube, were evaluated. Under optimal conditions, the proposed method yielded detection limits of 19.5 and 19.7 ng L for AuNPs and Au(III), respectively. Intraday repeatability and inter-day reproducibility were lower than 5.3% (N = 6) and 7.6% (N = 4) for both species, respectively. Enrichment factors over 196, corresponding to extraction efficiencies higher than 98%, were obtained for both species. Remarkably, non-significant differences in the extraction of AuNPs over a wide range of AuNPs sizes and morphologies with different capping agents were observed. The reported method was applied to the analysis of superficial waters, groundwater, seawater and artificial wastewater with good recoveries. The method represents a suitable alternative to other reported methodologies for AuNPs quantification in environmental waters at (ultra)trace levels.