Identify and characterise various plastics with fast, high resolution, easy-to-use cathodoluminescence solutions.
Plastics find wide use in our daily lives, with over 380 million tonnes of plastics being produced every year, of which about 40% is single use plastic1,2. Studies have also found that microplastics can act as vectors for heavy metal poisoning or adhere to chemical additives, which are then released and damage nearby tissue. Studying micro and nanoplastics has therefore become important as well as urgent. As an SEM-based high-resolution optical technique, cathodoluminescence (CL) imaging is promising since it can overcome many of the disadvantages in other popular techniques currently used for characterising plastic micro and nanoparticles, such as optical microscopy, Raman imaging, FTIR spectroscopy, gas chromatography, and SEM-EDX. CL is fast, easy to use, as well high resolution, capable of providing quantitative information and imaging particles of all sizes from nanometres into the microns or millimetres.
1. Lawnstarter, Plastic Statistics 2023 2. Plastics Europe, 2022
The general understanding of the mechanism of CL generation from plastics is bond cleavage by the electron beam, and the subsequent formation of luminescent polycyclic structures. The formation of radicals by the cleavage of aliphatic bonds may also lead to crosslinking, resulting in the generation of light under electron exposure. By performing CL spectroscopy, making sure to remain in the regime where the morphology of the particles is intact, we have performed proof of principle measurements to characterise a variety of micro and nanoplastic materials, for instance, polystyrene, polyethylene, polypropylene, as well as a mixture from which polystyrene and Teflon could be identified.
Our recent studies have identified CL imaging as a very promising fast, high-resolution technique for the characterisation of micro and nanoplastics. As shown in the image slider on the right side, the complete spectral information of 8μm polystyrene beads in the spectral range 300-800 nm was acquired for every pixel in the CL map, and overlaid with the SEM image. A peak is visible in this material at around 560 nm, with a shoulder at around 430 nm. Since the beads were large, they were imaged at 20 keV, which would excite the bulk. The high resolution of the technique is evident when imaging nanoparticles: we have found that the spectral map of 600 nm beads of the same material has approximately the same peak positions, but different peak widths and relative intensities. This is often the case for nanoparticles, whose properties can vary with size. Several plastics have been investigated and found to show characteristic CL peaks, demonstrating the promise of CL imaging for adding real value to the characterisation of micro and nanoplastics.
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