Rayleigh Mapping for Rapid and Precise Nanoplastic Distribution Analysis on Flat Surfaces

The widespread presence of nanoplastics in the environment has created an urgent need for analytical techniques that can efficiently and accurately detect and analyse their distribution. This study introduces an innovative approach that combines Rayleigh mapping with targeted single-point Raman spectroscopy, significantly reducing analysis time for nanoplastics on flat surfaces. By rapidly identifying regions that potentially contain nanoplastics, Rayleigh mapping provides an efficient initial overview, followed by a more detailed and selective Raman spectroscopic analysis at specific points. This approach minimises the need for extensive point-by-point or line scanning, thus offering an effective and efficient solution that yields precise spatial distribution information. Raman spectroscopy, a non-destructive analytical technique, relies on the inelastic scattering of monochromatic light to obtain molecular information. However, when samples are irradiated with monochromatic laser light, most of the incident light is scattered elastically with the same frequency. This Rayleigh scattering occurs with a probability 10 8 times higher than inelastic scattering [ 1]. Common methods of microplastic identification using Raman spectroscopy typically involve mapping large areas, mainly using the Stokes-Raman signal, to generate an overview of particle distribution on filters or substrates. Accurate polymer identification then requires acquiring a full Raman spectrum at a designated map co-ordinate, which is compared against a database of reference spectra [ 2]. However, for nanoplastics, these mapping techniques often demand several hours of analysis time, as the collection time of the map is dependent on parameters such as particle size, region of interest (ROI) and step size. Our approach accelerates the analysis by first generating a particle distribution map using Rayleigh-scattered photons, which retain the energy of the incident laser photons. <!-- named anchor --> Fig. 1 500 nm polystyrene (PS) particles (concentration: 10 -4 mg/mL) drop-cast on a pre-cleaned glass slide: a) Microscopic image captured with a 100× objective; b) Raman point map (53 µm × 32 µm) with a measurement time of 14 min 35s (Map of PS generated using the component analysis function, conducted using Renishaw’s WiRE software, highlights PS at the red pixels); c) Rayleigh map with a measurement time of 39s with red pixels indicating detected particles.