Labelling of polystyrene microplastics towards in situ detection in soil

One of the biggest pillars of our modern society relies heavily on the usage of plastics in all kinds of applications. Consequently leaching of plastics in the environment creates pervasive pollution; disturbing wildlife and ecosystems. As Earth’s largest plastics reservoir, soils form the bridge for microplastics (3µm < 5mm) transfer from pollution sources to other ecosystem compartments (to atmosphere by wind erosion – to ground and surface water by leaching and run-off/erosion; to the terrestrial food chain through uptake by soil organisms and by plants).1 Therefore a fundamental understanding of the migration of microplastics (MP) in the soil dynamics is needed for risk analysis and mitigation. For monitoring studies of environmental microplastics, development of identification and analytical methods for quantification of MP is of high importance. Within this field, various methods have been developed, such as visual light microscopy, fluorescence spectroscopy by staining the environmental MP with a fluorescent dye, Raman and Fourier transform infrared spectroscopy, and various chromatographic techniques.2 However since soil is a very complex matrix (consisting of a variety of inorganic and organic matter, (micro-)organisms and pores filled with water and air) it is often a challenge to observe MP directly in soil. Thus the analytical methods stated above are often coupled with extraction methods to remove the mineral and/or organic matrix. In this project, polystyrene (PS) polymers will be synthesized and functionalized with different labels, to act as a model system for commercial PS. These polymers will be processed and weathered to a selected particle size, to generate artificially designed MP. The PS will be functionalized with specific labels such as fluorescent, heavy metals and/or isotopes, in pursuance of studying the MP soil dynamics in situ.3 As the microplastics need to be detected as such, the fluorescent label needs to be designed in order to maximise the quantum yield as a solid particle. Therefore so-called aggregation induced emission (AIE) dyes, such as tetraphenylethene (TPE) and hexaphenylsilole (HPS) derivatives will be explored, as these are known to emit strong fluorescence in the aggregated state.4 The use of heavy metals, such as Ru, Os, and Pd as tracer elements inside the MP will be explored for the identification and quantification of the particles using ICP-MS. Finally incorporation of isotopes, such as 13C and 2H, allows for detection using isotope ratio mass spectrometry (IRMS). Moreover the mineralisation process (in essence conversion from microplastic to CO2 and H2O) of MP labelled with stable isotopes can potentially be monitored by measuring the 13CO2 released during the degradation. During this project the labels stated above will be explored and compared to each other in their ease of traceability of the MP in soil. Moreover the influence of fundamental properties of the PS MP, such as particle size, molecular weight will be investigated on the MP soil dynamics. (1) Wang, Y.; Bai, J.; Liu, Z.; Zhang, L.; Zhang, G.; Chen, G.; Xia, J.; Cui, B.; Rillig, M. C. Consequences of Microplastics on Global Ecosystem Structure and Function. Reviews Env.Contamination (formerly:Residue Reviews) 2023, 261 (1), 22. https://doi.org/10.1007/s44169-023-00047-9. (2) Möller, J. N.; Löder, M. G. J.; Laforsch, C. Finding Microplastics in Soils: A Review of Analytical Methods. Environ. Sci. Technol. 2020, 54 (4), 2078–2090. https://doi.org/10.1021/acs.est.9b04618. (3) Liu, Y.; Li, J.; Parakhonskiy, B. V.; Hoogenboom, R.; Skirtach, A.; De Neve, S. Labelling of Micro- and Nanoplastics for Environmental Studies: State-of-the-Art and Future Challenges. Journal of Hazardous Materials 2024, 462, 132785. https://doi.org/10.1016/j.jhazmat.2023.132785. (4) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, No. 29, 4332–4353. https://doi.org/10.1039/B904665H.