Polymer-Mediated Assembly from Core-Shell Particles to Tunable Structures and Microrotors

We introduce a polymer-mediated approach for assembling binary colloidal particles into core-shell and other tunable structures with their transformation into microrotors via Janus design. By mixing polyvinylpyrrolidone (PVP)-coated polystyrene (PS) microparticles with polymer-free silica nanoparticles, we exploit electrostatic repulsion to maintain dispersion until ionic screening permits a close approach. At this point, PVP acts as a molecular glue, selectively bridging bare silica onto PS surfaces to yield PS@SiO₂ core-shell structures. The number ratio of PS to SiO₂ dictates the assembly outcome. Excess PS leads to shared silica shells that link multiple cores into chains and colloidal gels, while excess silica leads to complete shell coverage and crystallization of microspheres into close-packed hexagonal lattices. Applying this method to Janus PS/Pt particles enables regioselective SiO₂ coating on the PS hemisphere only, producing asymmetric "PS@SiO₂"/Pt Janus microspheres that assemble into dimers and trimers through directional binding on the silica-coated hemispheres only. Remarkably, in 5% H₂O₂, the resulting Janus dimers transform into self-propelled microrotors that exhibit sustained rotation, powered by the catalytic decomposition of H₂O₂ on the exposed platinum hemispheres. These findings present a simple yet powerful strategy for the controlled synthesis of functional colloidal superstructures as well as stimulus-responsive micromachines.