Drying-Induced Flash Nanoprecipitation in a Sessile Drop: A Route to Synthesize Polymeric Nanoparticles.

Flash nanoprecipitation is a simple and scalable method to produce nanoparticles by rapid mixing of a polymer solution with an antisolvent. High-speed mixing devices for the continuous synthesis of polymeric nanoparticles and drug-encapsulated nanoparticles have been designed. In this work, we demonstrate a different approach to induce flash nanoprecipitation using the differential evaporation of solvents in a sessile drop. To show proof of concept, we use polymethyl-methacrylate (PMMA) dissolved in a tetrahydrofuran (THF)-water mixture as a model system. A sessile drop of the polymer solution is allowed to dry under controlled conditions. The sessile drops of the PMMA-THF-water ternary mixture are observed to dry in the constant radius mode. As THF in the drop evaporates faster than water, PMMA supersaturates and precipitates as nanoparticles. Although coffee-ring formation is well-studied in the drying of colloidal suspensions, this work demonstrates the formation of nanoparticles in situ due to a change of solvent quality and subsequent deposition of particles at the pinned contact line. Using the theory of drying of binary solutions, we calculate the temporal variation of composition. The drying paths passing through the low-concentration branch of the binodal give rise to nanoparticles, whereas those passing through the high-concentration branch yield porous films. Spherical polymeric nanoparticles in the size range of 250-700 nm were synthesized using this technique starting from drops with different initial polymer concentration. The method is a cost-effective (no high-speed mixing is required) and scalable alternative to conventional flash nanoprecipitation for synthesizing polymeric nanoparticles for potential applications in drug delivery, diagnostics, and polymer recycling.