Using autofluorescence for microplastic detection - Heat treatment increases the autofluorescence of microplastics1.

Introduction: More and more researchers are studying the effects of microplastics on the environment and the organisms living in it. Existing detection methods still require a heavy workload, complex sample preparation and high costs. In this study, autofluorescence of plastic was used as a new method for microplastic detection.
Material and Methods: Particles of common plastics were incubated at various temperatures (21-230 °C) for different time periods to investigate the influence of these conditions on their autofluorescence using methods like fluorescence microscopy, and measurement of absorption and emission. To give an example of an autofluorescence application, ImageJ was used to determine the contamination of microplastic in sea salt samples.
Results: After treatment at 140 °C for 12 h the plastics ABS, PVC and PA showed a distinct increase in their fluorescence intensity. For PET higher temperatures were necessary to achieve higher fluorescence intensities. Using ImageJ, the particle contamination in sea salt samples was determined as 4903±2522 (aluminium membrane) / 5053±2167 (silicone membrane) particles in 10 g salt, which is a much higher number than counted in other publications.
Discussion: Probably the increase in fluorescence intensity is due to the movement of atomic bonds caused by the thermic energy during the heat treatment. The high number of counted particles by using ImageJ is most likely based on the smaller pore size of the used filter membranes and other contaminations like dust and fibers, which could be avoided by alternative sample treatment.
Conclusion: Considering the outcomes of this study, heat treatment is a useful tool to make microplastic particles more visible in microscopic applications without readable destruction of their composition. The heat treatment of plastics for defined incubation times and temperatures can lead to a distinct increase in autofluorescence intensity of the plastics and therefore serve as an easy and cost-effective applicable method for microplastic detection.