Polyethylenimine-Derived Fluorescent Nonconjugated Polymer Dots with Reversible Dual-Signal pH Response and Logic Gate Operation

A kind of nonconjugated polymer dot possessing autofluorescence was synthesized by hyperbranched polyethylenimine and salicylaldehyde via environmentally friendly imine cross-linking and self-assembly in the aqueous phase. Both fluorescence and ultraviolet–visible (UV–vis) absorption of the polymer dots showed a sensitive and reversible response to pH. The mechanism of pH-dependent optical properties was investigated. The existence of two proton-responsive functional groups (phenolic hydroxyl groups and amine groups) on the polymer dot surface is crucial. The pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV–vis absorption is induced by ionization of phenolic hydroxyl groups. Based on the pH-mediated optical properties, a colorimetric and fluorescent dual-signal pH sensor and multiple logic gates were developed. The fluorescent nonconjugated polymer dots with easy preparation and sensitive response to pH are very promising for applications in biological, pharmaceutical, and material science fields.