The development of highly selective materials is a critical task in separation fields because of their increasing importance in environmental treatment, biological medicine, and clinical diagnosis. Polymer microparticles are leading separation materials that have been frequently used for the removal of contaminants from water, extraction of drugs from matrixes, and detection of biomarkers from biofluids. Among existing methods for the preparation of polymer microparticles, emulsion polymerization takes the predominant place due to the good controllability on particle diameter and pore size. Polymer microparticles prepared by emulsion polymerization almost exhibit homogeneous composition. They are competent for the separation of high-concentration species from simple samples but often encounter bottlenecks in the separation of trace species from complex samples. Although surface modification with specific monolayer molecules can improve separation selectivity, unspecific adsorption still exists due to inevitable modification defects. Therefore, the development of novel synthesis methods and separation microparticles is warranted to enable highly selective separation.In recent years, our group has been dedicated to addressing the challenge of the separation of trace species from complex samples by developing next-generation separation microparticles through the innovation of synthesis technologies. We developed the synthesis technology of emulsion interfacial polymerization, prepared a series of new types of polymer microparticles, heterostructured microparticles, and achieved efficient and rapid separation of complex fluidic samples. In this Account, we formally define the concept of "emulsion interfacial polymerization", systematically summarize the recent progress in the physical and chemical parameter regulation for heterostructured microparticles, and comprehensively review related separation applications. In a typical emulsion interfacial polymerization system, an oil-in-water emulsion is established, in which hydrophobic monomers are included in the oil phase, while hydrophilic monomers are included in the aqueous phase. Hydrophobic monomers and hydrophilic monomers copolymerize at the oil-water interface upon initiation, obtaining microparticles with hydrophilic-hydrophobic heterostructures. We show that emulsion interfacial polymerization is a general strategy for the synthesis of heterostructured microparticles with controllable physical and chemical parameters. A series of heterostructured microparticles, including Janus microparticles with tunable topology, heterostructured porous microparticles with tunable pore size, and nanofractal microparticles with tunable surface structures, have been synthesized by rationally regulating the polymerization conditions. Subsequently, we demonstrate that these heterostructured microparticles can be used for various separation systems, including separation of trace microsized oil droplets and organic dyes from water, separation of similarly sized proteins, and separation of trace glycopeptides, glycoproteins, nucleic acids, and circulating tumor cells (CTCs) from complex biofluids. Finally, we prospect the future developments of emulsion interfacial polymerization for the synthesis of heterostructured microparticles toward the separation of species from molecule scale to subcellular scale and cellular scale, and envision that these microparticles can be widely used in environmental and healthcare applications.
