Characterizing the uptake, accumulation and toxicity of silver sulfide nanoparticles in plants

Silver nanoparticles (Ag-NPs) are used in a wide range of everyday products, leading to increasing concerns regarding their accumulation in soils and subsequent impact on plants. Using single particle inductively coupled plasma mass spectrometry (spICP-MS) and synchrotron-based techniques including X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (XFM), we characterized the uptake, speciation, and translocation of insoluble Ag2S-NPs (an environmentally-relevant form of Ag-NPs in soils) within two plant species, a monocot and a dicot. Exposure to 10 mg Ag L-1 as Ag2S-NPs for one week resulted in a substantial increase in leaf Ag concentrations (3.8 to 5.8 mu g Ag g(-1) dry mass). Examination using XAS revealed that most of the Ag was present as Ag2S (> 91%). Furthermore, analyses using spICP-MS confirmed that these Ag2S particles within the leaves had a markedly similar size distribution to those supplied within the hydroponic solution. These observations, for the first time, provide direct evidence that plants take up Ag2S-NPs without a marked selectivity in regard to particle size and without substantial transformation (dissolution or aggregation) during translocation from roots to shoots. Furthermore, after uptake, these Ag(2)SNPs reduced growth, partially due to the solubilisation of Ag+ in planta, which resulted in an upregulation of genes involved in the ethylene signalling pathway. Additionally, the upregulation of the plant defense system as a result of Ag2S-NPs exposure may have contributed to the decrease in plant growth. These results highlight the risks associated with Ag-NP accumulation in plants and subsequent trophic transfer via the food chain