Quantitative tracing of uptake and transport of submicrometre plastics in crop plants

Micro/nano-plastics (MNPs) is a new global environmental pollutant, posing a potential threat to ecological environment and human health. Recent studies have revealed the effective pathways and mechanisms for uptake and translocation of MNPs in plants. Nevertheless, no quantitative information on uptake kinetics and internal transport of MNPs in plants is currently available, despite the fact that this information is a key to assess the environmental impact and the potential hazard of MNPs to human health.

Numerous studies have attempted to track fluorescent polystyrene (PS) particles, as a model for MNPs. However, this technique has a number of disadvantages, including high limits of detection and interference from fluorescent background signals, which is a universal challenge in the visualization of MNPs using conventional fluorescence techniques.

Lanthanide chelates are distinctly advantageous, as they have long luminescence lifetimes, large Stokes shifts, sharp emission profiles and visible-light excitation wavelengths. Thus, it is anticipated that such probes could be a useful tool for a broad range of applications in which background-free and time-resolved bioimaging is needed to visualize plastic particles in complex biological samples (Figure 1 and 2). Furthermore, the entrapment of lanthanide chelates within the plastic particles could be a suitable strategy for the indirect determination of the amount of MNPs accumulated in the plants via the quantification of lanthanide metals by ICP-MS, even at low concentrations .

Figure 1 High-angle annular dark-field (HAADF) STEM image (a) and corresponding energy-dispersive X-ray spectroscopy elemental mapping ofC (b), O (c), Eu (d), F (e) and S (f) atoms in the synthesized 0.2 μm PS-Eu particles used in this study. g,h, Line-scanning profiles (h) of C, O, S, F and Eu atoms across the as-prepared PS-Eu particle in the STEM image (g).

The labelling approach described here, on the basis of highly sensitive ICP-MS, may provide a valuable technique by which to investigate the biological fate of MNPs in plants at low environmental concentrations. we showed that even at a concentration of 5 μg l–1, PS-Eu accumulated in the plant’s root and transported to the shoot could subsequently be detected using ICP-MS (Figure 3).

Figure 2 Time-gated imaging of wheat root, stem and leaf after exposure to PS-Eu particles. Bright-field images (a, d, g), time-gated luminescence images (b, e, h) and steady-state luminescence images (e, f, i) of wheat tissues followed by exposure with 50,000 μg l–1 0.2 μm PS-Eu particles in 20% Hoagland solution for six days (B). A control experiment was included (A). Scale bar, 100 μm.

Figure 3 Bioaccumulation of 0.2 μm PS-Eu in roots and shoots of wheat (a) and lettuce (b) grown for six days with different concentrations of PS-Eu particles in solution. c, Bioaccumulation of 0.2 μm PS-Eu in roots and shoots of wheat (left) and lettuce (right) grown for 14 days with 1 mg kg–1 and 10 mg kg–1 PS-Eu in a sandy soil.

The labelling technology used in this study could also be applied in microcosm or mesocosm experiments to enhance the sensitivity of MNPs detection. However, potential lanthanide leaching from the particles should be monitored carefully in the systems due to the complex environmental conditions as well as due to the presence of a wide number of (mico)organisms.

 Using the labelling method, we showed that a transfer of MNPs (and uptake) into the plants occurs, but no significant biomagnification was observed. The method can be extended to other polymer types of MNPs found in the environ ment, which may facilitate future quantitative studies on MNPs–plant interactions and will benefit ongoing quantitative environmental risk assessments of MNPs. The research results will help to comprehensively understand the processes and mechanisms of uptake and transport of microplastic particles in terrestrial food chain, and also provide a methodology and the scientific basis for quantitatively assessing the ecological environment and food chain risks of MNPs.