Refractive Index of Benchmark Polystyrene Nanoplastics by Optical Modeling of UV-Vis Spectra

Optical recognition and identification of nanoplastics such as polystyrene nanobeads (PSbs), a widely used polymer and an actual source of environmental pollution, is a challenging task relying on knowledge of the PSbs' refractive index (RI) and its relation to the PSbs' morphology. This is, however, lacking for PSbs' sizes lower than 1 μm. Here, we bridge this gap by measuring UV-vis spectra of PSbs deposited on a sapphire substrate via spin coating and by connecting the experimental data to the RI, PSbs' morphology, and optical transitions through a new optical model based on the Mie theory. Specifically, the new model allows us to fit the total and diffuse light components considering the particles' size distribution, the superficial density, and the substrate effects, thus correlating the PSbs' wavelength-dependent RI features to the experimentally validated specific sample morphology. Two critical model assumptions, i.e., Mie theory and substrate effects, are also discussed employing discrete dipole approximation simulations. Moreover, we identify the optically allowed molecular electronic transitions in nanometric PSbs as potential fingerprints for nanoparticle characterization in more complex matrixes.