An aggregation-induced emission star polymer with pH and metal ion responsive fluorescence

A one-pot strategy was applied to synthesize an aggregation-induced emission (AIE) star polymer with pH and metal ion responsive fluorescence. The star polymer 1 was prepared via ring-opening metathesis polymerization (ROMP) of norbornene-based monomers M1, M2, M3, and crosslinker bi-Nor. In this approach, M1 with a poly(ethylene glycol) side chain was polymerized to form poly(M1) as the hydrophilic arm of the star polymer. The mixture of M2 (with phosphonate groups), M3 (with AIE fluorogen), and bi-Nor was then added in situ to extend poly(M1) chains and form the star polymer core. Subsequently, the functional star polymer 2 with phosphonic acid groups was derived by hydrolyzing the phosphonate groups in the core of star polymer 1, which demonstrated strong fluorescence when pH < 4 and weak fluorescence when pH > 5 in water. In addition, by virtue of the coordination of phosphonic acid groups with various metal ions, the aqueous solution of star polymer 2 (pH = 7 and 11) emitted significantly enhanced fluorescence in the presence of Mg2+, Ca2+, Cd2+, Ni2+, In3+, Zn2+, etc. The fluorescence intensity of the aqueous solution linearly increased with increasing concentration of metal ions in a wide range.