Determination of silver nanoparticles in complex aqueous matrices by total reflection X-ray fluorescence spectrometry combined with cloud point extraction

The development of methods for the analysis of silver nanoparticles (AgNPs) is of special relevance to achieve detailed insights into the fate, transport and exposure of these emerging pollutants in the environment. In the present contribution, for the first time, the possibilities and drawbacks of cloud point extraction (CPE) in combination with total reflection X-ray fluorescence spectrometry (TXRF) have been evaluated for the determination of AgNPs content in soil and consumer product water extracts. The experimental conditions for the separation and preconcentration of AgNPs with different coatings (PVP, PEG and citrate) and sizes (40-100 nm) were evaluated, paying attention to possible modifications required by the characteristics of the TXRF technique. Single particle inductively coupled plasma mass spectrometry (SP-ICPMS) analysis of the same aqueous extracts was also carried out in order to confirm the presence and percentage of AgNPs. Good agreement with respect to AgNPs concentrations in the analysed soil extracts was obtained between CPE-TXRF and SP-ICPMS, confirming the absence of matrix effects for this type of sample by CPE. On the contrary, large discrepancies were found between both analytical approaches for the studied consumer product water extracts. SP-ICPMS analysis evidenced the presence of ionic silver with a heterogeneous mixture of different sizes including relatively large Ag particles (> 100 nm). This fact was also verified by SEM-EDS analysis. The results from this study highlight the necessity to study in more detail the effectiveness of existing methodologies (including CPE-based methods and SP-ICPMS) for the monitoring of AgNPs in complex aqueous matrices involving mixtures of different NPs sizes and coatings.