CO2-Responsive Water-Soluble Conjugated Polymers as a Multifunctional Fluorescent Probe for Bioimaging Applications

We describe important progress in the synthesis and development of gas-responsive water-soluble conjugated polymers (WSCPs) with potential as multifunctional fluorescent materials for biomedical imaging and probes. A water-soluble WSCP (I-PT) composed of a hydrophobic fluorescent polythiophene backbone and a hydrophilic imidazole side chain was successfully prepared through a facile and efficient two-step synthetic route. Owing to the repulsive force between the hydrophilic and hydrophobic segments and the highly sensitive carbon dioxide (CO2)- and nitrogen (N2)-responsive imidazole groups in its structure, I-PT can spontaneously self-assemble into spherical-like nanoparticles in an aqueous environment, and thus exhibits unique light absorption and fluorescence properties as well as rapid responsiveness to CO2 and N2. In addition, its structure, optical absorption/fluorescence behavior, and surface potential can be quickly turned on and off through alternating cycles of CO2 and N2 bubbling and exhibit controllable cyclic switching stability, thereby allowing effective manipulation of its hierarchical structure and chemical–physical characteristics. More importantly, a series of in vitro cell experiments confirmed that, compared to the significant cytotoxicity of pristine and N2-treated I-PT nanoparticles, CO2-treated I-PT nanoparticles exhibit extremely low cytotoxicity in normal and cancer cells and undergo greatly accelerated cellular uptake, resulting in a significant increase in the intensity and stability of their fluorescence signal in the intracellular environment. Overall, this newly discovered CO2/N2-responsive system provides new insights to effectively enhance the biocompatibility, cellular internalization, and intracellular fluorescence characteristics of WSCPs and holds great potential for biomedical imaging/sensing applications.