Optofluidic light-droplet interaction for rapidly assessing the presence of plastic microspheres within aqueous suspensions

Microplastics (MPs) pollution has recently become an increasingly important environmental issue threatening both marine and terrestrial ecosystems as a result of the uncontrolled release of plastic waste as well as a health concern especially when the interactions with pharmaceutical products are considered. This serious contamination, widely documented over the last years, demands the development of new methodologies to sense MPs, particularly when the latter are dispersed in water. Due to the inherent capability of handling and dealing with aqueous samples, ensuring at the same time high-throughput analysis, optofluidics constitutes a promising way to achieve effective and fast MPs detection. In this work we propose a droplet based optofluidic device, realized on a lithium niobate substrate and exploiting a cross-junction configuration in combination with Ti-indiffused optical waveguides in the visible range. Plastic microspheres with dimensions between 340 nm and 2500 nm are dispersed within water droplets, and their detection is achieved by analyzing the transmitted light intensity, distinguishing between pure water and suspensions with particles down to a concentration of 0.13 mg/g (0.015 - 6.3 ·10⁹ particles/mL). Particular focus is placed on the light-droplet interaction, assessing the impact of the different factors in play influencing the sensing as well, i.e., particles size, concentration, refractive index, and coupled light wavelength.