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Hollow microparticles are important materials, offering a larger surface area and lower density than their solid counterparts. Furthermore, their inner void space can be exploited for the encapsulation and release of guest species in a variety of applications. Herein, we present phosphazene-based silica hollow microparticles prepared via a surfactant-free sol-gel process through self-assembly of the alkoxysilyl-containing polymer in water–ethanol solution. Solely, a silane-derived polyphosphazene was used as the precursor for the microparticle formation, without additional classical silica sources. These novel hollow silica-based microparticles were prepared without surfactant, using a designed amphiphilic polyphosphazene for the particle formation made by two components, a hydrophilic unit consisting of 3-mercaptopropyl(trimethoxysilane), and a hydrophobic unit (dodecanethiol) attached to the double bonds from the poly(allylamine)phosphazene backbone via a thiol-ene photoreaction. Due to these two functionalities, a "vesicle"-like self-assembled structure was formed in the reaction medium, which could be then utilized for the microparticle preparation. The influence of NaOH during the synthesis was shown to affect the size and the wall thickness of the microparticles. This effect may enhance the possibilities to tailor such microparticles for drug delivery purposes or for future controlled release of other substances, such as drugs, fragrances, or anticorrosive pigments. [ABSTRACT FROM AUTHOR]

This work reports a reversible braking system for micromotors that can be controlled by small temperature changes (≈5 °C). To achieve this, gated‐mesoporous organosilica microparticles are internally loaded with metal catalysts (to form the motor) and the exterior (partially) grafted with thermosensitive bottle‐brush polyphosphazenes to form Janus particles. When placed in an aqueous solution of H2O2 (the fuel), rapid forward propulsion of the motors ensues due to decomposition of the fuel. Conformational changes of the polymers at defined temperatures regulate the bubble formation rate and thus act as brakes with considerable deceleration/acceleration observed. As the components can be easily varied, this represents a versatile, modular platform for the exogenous velocity control of micromotors. [ABSTRACT FROM AUTHOR]

Manganese ions are embedded in microporous silica nanoparticles to catalyse the degradation of the H SB 2 sb O SB 2 sb "fuel" to water and gaseous oxygen. Keywords: bottle-brush polymers; mesoporous silica; micromotors; microparticles; polyphosphazenes EN bottle-brush polymers mesoporous silica micromotors microparticles polyphosphazenes 3187 3187 1 02/17/21 20210215 NES 210215 B Self-propelled micromotors b with a reversible braking system are shown. Manganese ions are embedded in microporous silica nanoparticles to catalyse the degradation of the H SB 2 sb O SB 2 sb "fuel" to water and gaseous oxygen. [Extracted from the article]

The incorporation of an extraneous on–off braking system is necessary for the effective motion control of the next generation of micrometer‐sized motors. Here, the design and synthesis of micromotors is reported based on mesoporous silica particles containing bipyridine groups, introduced by cocondensation, for entrapping catalytic cobalt(II) ions within the mesochannels, and functionalized on the surface with silane‐derived temperature responsive bottle‐brush polyphosphazene. Switching the polymers in a narrow temperature window of 25–30 °C between the swollen and collapsed state, allows the access for the fuel H2O2 contained in the dispersion medium to cobalt(II) bipyridinato catalyst sites. The decomposition of hydrogen peroxide is monitored by optical microscopy, and effectively operated by reversibly closing or opening the pores by the grafted gate‐like polyphosphazene, to control on demand the oxygen bubble generation. This design represents one of the few examples using temperature as a trigger for the reversible on–off external switching of mesoporous silica micromotors. [ABSTRACT FROM AUTHOR]