Development of a suitable detection method for silver nanoparticles in fish tissue using single particle ICP-MS†
To determine bioaccumulation potential, suitable methods to extract and quantify engineered nanomaterials (ENMs) from fish tissues are required. The aim was to develop single particle inductively coupled plasma mass spectrometry (spICP-MS) protocols for liver tissue from rainbow trout and to assess the suitability of enzymatic (proteinase K) and alkali (tetramethylammonium hydroxide; TMAH) methods of extraction. A total of four different extractants were used: proteinase K or TMAH both with/without CaCl2. Spike recovery tests using equal mass concentrations (50 ng L−1 of Ag) as silver nanoparticles (Ag NPs) or AgNO3 were conducted. Extractants alone spiked with Ag NPs showed recovery similar to ultrapure deionised water (95–105%). However, the TMAH alone caused AgNO3 to precipitate and was not a suitable extractant. A second series of experiments looked at spike recovery on liver tissue samples. Proteinase K, with or without CaCl2, failed to completely digest the tissues. Only TMAH + CaCl2 demonstrated the ability to solubilise the liver. Ag NPs spiked onto liver tissues and analysed 24 h later, also showed no significant change in particle size distribution or particle mass concentration compared to those freshly spiked without liver present. The particle number concentration fell significantly to around 80% of the freshly spiked Ag NPs. Samples from an in vivo dietary study where fish were fed nominally 100 mg kg−1 Ag as either AgNO3 or Ag NPs were analysed to demonstrate the utility of the method. There was no significant difference between the particle number concentration, mean particle size or particle mass concentration between the in vivo AgNO3 and Ag NP treatment liver tissues. For example, the particle number concentrations were 68.3 ± 33.1 and 76.9 ± 51.6 × 109 particle per g dw liver in the AgNO3 and Ag NPs, respectively. In conclusion, a TMAH + CaCl2 extraction method was developed with good recovery and utility for detecting Ag NPs from in vivo exposures of trout liver.