Opto-microfluidic sensor for the detection of microplastics in aqueous solutions

Microplastics (MPs) pollution represents an increasingly important environmental issue, threatening almost every kind of ecosystem. The widely documented contamination of both aquatic and terrestrial environments constitutes a serious issue, demanding the exploration of new methods to sense, identify and quantify MPs. Over the last years, various techniques have been proposed for MPs sensing, such as optical microscopy, SEM and TEM imaging, µRaman and µFTIR spectroscopy, as well as more compact methods like hyperspectral imaging, digital holography and polarization imaging. Several of the proposed approaches, such as µRaman, µFTIR and electron microscopy, allow the detection of particles down to 1 µm and below but tend to be time consuming and expensive, and typically requires to be combined with other methods to detect MPs when dispersed in water. As matter of fact, a technique able to provide alone an effective and fast detection of MPs with size ranging from 100 nm to 10 µm has not been proposed yet. Due to the capability of dealing with aqueous samples, optofluidics potentially constitutes a promising alternative to achieve effective MPs detection even at the micrometre scale, as optofluidic systems inherently exploit the advantages of microfluidics as well as of integrated optics. In this work, we propose a droplet-based optofluidic device realized on a monolithic substrate of Lithium Niobate (LN) integrating two different stages. A microfluidic stage, namely a cross-junction engraved on the LN substrate, is employed in a T-junction configuration, and an optical stage, i.e., an array of optical waveguides in a Mach-Zehnder interferometer (MZI) configuration is realized on the same substrate, perpendicularly crossing the main microchannel. Laser light in the visible range can be coupled to a single MZI waveguide, enabling the sensing of flowing objects in the microchannel. Polystyrene (PS) microparticles with diameters ranging from 153 nm up to 6 µm are dispersed in water droplets, which are generated in an immiscible phase. When a droplet flows in front of a coupled waveguide, light interacts with the latter giving rise to scattering and interference phenomena due to the geometry of the system. Proper analysis of the transmitted light allows to obtain information on each droplet content, as well as providing accurate estimation of droplets size and velocity. The proposed device can detect PS particles dispersed in water down to 153 nm (concentration 0.03 mg/mL), distinguishing between droplets containing the particles and pure water droplets. Monodisperse aqueous suspensions with dispersed PS particles are systematically investigated as a function of particles concentration and size, and the device specificity is tested by employing poly(methyl methacrylate) particles of diameters 2.5 µm and comparing the obtained results with the case of PS particles of the same size. As a perspective tool to manipulate droplets, the integration of an additional iron-doped LN cover is presented. As the doping of LN with iron enhances the photovoltaic properties of the latter, the cover is exploited to generate a photo-induced space-charge electric field in such a way that the field lines enter the main microchannel. The interaction between the photo-induced field and pure water droplets is comprehensively investigated. Waveguides in straight configuration are also tested in a different device, to explore potential alternatives in terms of waveguide geometry. As a complementary tool to extend the detection range of the optofluidic device, a portable multispectral imaging system is adopted and tested with different types of common plastics of dimensions ranging from 0.5 mm up to few tens of mm. By exploiting principal component analysis and proper data processing, different types of plastics are distinguished from the background and identified. Plastic fragments down to 500 µm in a thin film of water are detected with this method.