A Novel, Smart Microsphere with K+-Induced Shrinking and Aggregating Properties Based on a Responsive Host-Guest System

A novel type of smart microspheres with K+-induced shrinking and aggregating properties is designed and developed on the basis of a K+-recognition host-guest system. The microspheres are composed of cross-linked poly(N-isopropylacrylamide-co-acryloylamidobenzo-15-crown-5) (P-(NIPAM-co-AAB15C5)) networks. Due to the formation of stable 2:1 "sandwich-type" host-guest complexes between 15-crown-5 units and K+ ions, the P(NIPAM-co-AAB15C5) microspheres significantly exhibit isothermally and synchronously K+-induced shrinking and aggregating properties at a low K+. concentration, while other cations (e.g., Na+, H+, NH4+, Mg2+, or Ca2+) cannot trigger such response behaviors. Effects of chemical compositions of microspheres on the K+-induced shrinking and aggregating behaviors are investigated systematically. The K+-induced aggregating sensitivity of the P(NIPAM-co-AAB15C5) microspheres can be enhanced by increasing the content of crown ether units in the polymeric networks; however, it is nearly not influenced by varying the monomer an d cross-linker concentrations in the microsphere preparation. State diagrams of the dispersed-to-aggregated transformation of P(NIPAM-co-AAB15CS) microspheres in aqueous solutions as a function of temperature and K+ concentration are constructed, which provide valuable information for tuning the dispersed/aggregated states of microspheres by varying environmental K+ concentration and temperature. The microspheres with synchronously K+-induced shrinking and aggregating properties proposed in this study provide a brand-new model for designing novel targeted drug delivery systems.