Correlation of refractive index to morphology for polystyrene nanospheres by optical modelling of UV-VIS spectra

The complex refractive index (RI) is a necessary quantity to characterize nanoplastics in complex environments, aiming at monitoring their potential harm and pollution. However, the nanoplastics’ RI determination remains challenging due to their small size, heterogeneity, and morphological complexity. Although polystyrene nanobeads (PSbs) are a perfect benchmark system to tackle the issue due to their widespread use and reduced sample variability, there are two major gaps in the RI -morphology correlation, for sizes lower than 1μm and for wavelength less than 400 nm. Here we aim at bridging this gap by a) measuring total and diffusive UV-Vis spectra of 55nm sized PSbs deposited via spin coating on a sapphire substrate; b) connecting the data to the RI, PSbs morphology and optical transitions through a new Mie-based optical model. The assumptions regarding particle–substrate interactions, described using Mie theory, are justified through discrete dipole approximation simulations, at the same time highlighting the gap on PSbs RI knowledge. The diffused and absorbed light components are fitted through the model accounting for the PSbs’ size distribution, superficial density and substrate effects, yielding the PSbs complex RI linked to the specific sample morphology. Finally, the study of the optical transition expected from the model provides further evidence of the reliability and potentiality of our approach for the determination of the RI dispersion of nanoscale PSbs.