A microfluidic column of water index-matched packed microspheres for label-free observation of water pollutants

A microfluidic, label-free optical sensor for water pollutants, which is based on a packed micro-column of microspheres with refractive index similar to that of water, is presented. The perfluoropolyether microspheres are synthetized by membrane emulsification followed by UV irradiation. The microfluidic channel hosting the packed column is transparent when filled with pure water as a consequence of refractive index matching, whereas it scatters light in presence of compounds with lipophilic moieties that spontaneously adsorb on the fluorinated microspheres. The device is characterized by investigating the response to cationic and anionic surfactants. Both the signal growth rate and the recovery rate measured during washing with water depend on the type and concentration of the compounds. The cationic surfactants tested display a larger signal increase, linearly scaling with concentration. A limit of detection of 1 μM is obtained in the current configuration. The water index-matched microspheres enable to access an additional analytical parameter, that is the propagation velocity of the scattering signal along the column. This parameter is also found to scale linearly with concentration, hence providing a complementary analytical tool sensitive to the adhesion kinetics.