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Detection of microplastics and nanoplastics: Are Raman tweezers and enhanced Raman methods the solution for sub 20 μm particles?
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Raman tweezers — devices that use a laser beam to trap and analyze individual particles — combined with plasmonic enhancement techniques can detect and characterize nanoplastics and microplastics smaller than 20 µm, a size range that defeats most conventional filtration-based detection methods. Improving detection sensitivity for the smallest plastic particles is critical because nanoplastics are thought to be the most biologically active fraction, capable of crossing cell membranes and accumulating in tissues.
Despite the significant progress in the detection of nano and small microplastics, the detection of such particles still faces problems caused by the limitations of current detection methods and instruments. Herein, we present the optical methods for detection of sub 20 μm microplastics. We introduce optical methods for the analysis of individual microplastics and the fabrication of a substrate using plasmonic particles to detect plastic nanoparticles. We summarize recent experimental activities involving the construction of portable Raman tweezers that can be used for optical trapping and analysis of microplastics with size from a few hundred nanometers to lower tens of micrometers. Optical trapping is complemented by another optical manipulation method: nanoimprinting of plasmonic nanoparticles that enables create the “active” aggregates that can be used for Surface Enhanced Raman Spectroscopy (SERS) detection in microfluidic circuits and as plasmon-enhanced thermoplasmonic concentrators for nanoscale particulate matter such as nanoplastics. The principle of nanoimprinting is based on the dominance of the scattering force (compared to the gradient force) for plasmonic particles, this force pushes the particles in the direction of propagation of the light beam. This phenomenon enables the preparation of an aggregate comprising of plasmonic particles that can serve as a substrate for SERS and as a source of the temperature gradient that is able to attract dielectric nanoparticles. In both cases, enhanced sensitivity is demonstrated, allowing the detection of nanoplastics/molecules of size/concentration orders of magnitude lower than what can be achieved by Raman spectroscopy. This study demonstrates that the combination of two optical manipulation techniques with Raman spectroscopy is capable of filling the technological gap in the detection of plastic particles ranging in size from a few tens of nanometers to 20 micrometers. This is an ideal solution for the detection of very small microplastics, which currently lacks a suitable technology.
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