Isotopic labelling for sensitive detection of nanoparticle uptake and translocation in plants from hydroponic medium and soil

Unintended releases of nanoparticles (NPs) into agricultural soil have recently raised concerns regarding NP accumulation in plants. In this study, specially synthesised isotopically labelled ¹⁰⁷Ag-NPs, ⁶⁵Cu-NPs and ⁷⁰ZnO-NPs were exposed to three representative plants (Arabidopsis thaliana, Solanum lycopersicum (tomato) and Phragmites australis (common reed)) in hydroponic cultivation and, separately, to tomato plants cultivated in soil at concentrations of 2 mg L^-1. Metal concentrations in all samples were analysed by inductively coupled plasma mass spectrometry following acid digestion. The use of isotopically labelled NPs confirmed that elevated levels of metals were from the NP source used for the experiments. Although the highest concentrations of NPs or metals were detected in roots in both hydroponic and soil cultivations, varied levels of translocation to shoots were observed in different plants under hydroponic cultivation. In soil cultivation, where tomato plants were grown to full maturity, low levels of ¹⁰⁷Ag (0.38 mg kg^-1) with respect to controls were recorded in tomato fruits; ⁷⁰Zn showed the highest level of translocation to tomato stems (2.72 mg kg^-1) and leaves (13.93 mg kg^-1). Furthermore, the amounts of NPs retained in the soil (at different depths) after harvesting tomato plants were also determined; the highest concentrations of respective isotopes (1.25 mg kg^-1 of ¹⁰⁷Ag, 0.79 mg kg^-1 of ⁶⁵Cu, 4.06 mg kg^-1 of ⁷⁰Zn) were found in the top soil layer (∼3 cm). Analysis of NPs exposed to plants in hydroponic medium indicated that the presence of plants increases the dissolution of NPs. Scanning electron microscopy analysis enabled determination of the location of ¹⁰⁷Ag-NPs in the roots of tomato plants grown in soil; these NPs were found to accumulate mainly in the cortical cells.