Flash nanoprecipitation of monodisperse luminescent nanosensors for simultaneous oxygen and flow imaging

Understanding mass transfer in aquatic biological systems requires simultaneous measurements of velocity fields and transported scalar fields. Such scalar fields, including spatial distributions of dissolved oxygen concentration, can be measured using luminescent sensor particles that also function as flow tracers, but their performance is affected by particle size and dispersity. Therefore, reproducible preparation of monodisperse sensor particles is essential for reliable sensing and velocimetry. Here, we systematically optimized flash nanoprecipitation to prepare oxygen-sensitive nanosensors based on poly(styrene-co-maleic anhydride) (PSMA). Dynamic light scattering identified polymer concentration as the primary determinant of particle size, with additional tunability provided by nonionic surfactants. Below the critical overlap concentration, increasing Reynolds number generally reduced particle size with lower dispersity. By varying these preparation conditions, monodisperse nanosensors with tunable hydrodynamic diameters of approximately 120-410 nm (polydispersity index, PDI < 0.1) were reproducibly obtained. In contrast, polymer concentrations above the critical overlap concentration produced polydisperse particles with mean hydrodynamic diameters exceeding 800 nm. Frame-straddling lifetime imaging showed an approximately linear Stern-Volmer response from anoxia to air saturation, with nonlinear behavior occurring only under supersaturated conditions. The nanosensors were successfully applied within the sensPIV framework, enabling simultaneous mapping of oxygen distributions and velocity fields around a living coral. Estimated hydrodynamic relaxation times indicated negligible inertial lag across the investigated particle-size range, whereas estimated oxygen equilibration times of approximately 0.04-4 ms suggested that finite sensor response may limit the resolution of the smallest scalar structures. These results establish flash nanoprecipitation as a reproducible and highly controllable route for preparing monodisperse polymeric nanosensors with tunable particle sizes for simultaneous scalar and velocity measurements.