A pH-responsive self-fluorescent polymeric micelle as a potential optical imaging probe

A series of methoxypolyethylene glycol-terminated self-fluorescent polyurethane multi-block copolymers with excellent pH-responsivity, self-fluorescence, and biocompatibility are designed and synthesized. In our design, 1, 4-bis (hydroxyethyl) piperazine is chosen as a pH-responsive segment which can donate or accept protons in response to the change of environmental pH, and fluorescein isothiocyanate is used as a fluorescent dye conjugated into the micelles to offer self-fluorescence. The chemical structure of the polyurethane multi-block copolymers is characterized by nuclear magnetic resonance and Fourier transform infrared spectroscopy. The results of the acid-based titration, the fluorescence spectrometry, and the ultraviolet visible spectroscopy indicate that the polyurethane multi-block copolymers own an excellent pH-buffering capacity responded to the change of pH values and the favorable self-fluorescence property in an aqueous solution. And the ultraviolet absorption peaks of samples are strengthened with increasing of pH values, indicating that methoxypolyethylene glycol-terminated self-fluorescent polyurethane multi-block copolymer can be a pH-dependent fluorescent probe in a broad pH range. In addition, the in vitro cytotoxicity test showed that the polyurethane multi-block copolymer has low cytotoxicity and good biocompatibility, which make it a promising nanoplatform for molecular imaging, diagnosis, and treatment of disease.