Molecular engineering of a D-A-D-type fluorescent probe for detecting viscosity in environmental and living systems

Organellar viscosity is an important biomechanical parameter and is closely associated with multiple physiological and pathological processes. Push-pull molecular rotors can realize viscosity visualization via changes in fluorescence intensity, providing a promising noninvasive strategy for organellar viscosity detection. However, most such molecular rotors are susceptible to interference from environmental polarity, human serum albumin and other substances, and tend to induce the aggregation-caused quenching (ACQ) effect, which severely limits the accuracy and sensitivity of viscosity detection.

Based on the π-bridge molecular engineering strategy, a donor-acceptor-donor (D-A-D) molecular skeleton was constructed using carbazole as the donor and pyridinium as the acceptor. By incorporating a thiophene group into the molecular π-bridge to tune the degree of molecular twisting and conjugation length, a novel AIE probe, SPS, was successfully designed and synthesized. Experimental results demonstrate that the fluorescence intensity of the SPS probe can be enhanced by up to approximately 21-fold in high-viscosity systems. Moreover, the probe exhibits excellent viscosity selectivity, with its response barely affected by potential interfering substances, including environmental polarity and human serum albumin (HSA). Furthermore, SPS was encapsulated within DSPE-PEG₂₀₀₀ to fabricate the nanoprobe SPS NPs, which enables precise endoplasmic reticulum (ER) targeting and real-time monitoring of dynamic variations in ER viscosity. In addition, using this probe, a gradual increase in ER viscosity was visually observed during erastin-induced cellular ferroptosis.

Collectively, the newly developed probe provides a new insight into the design of high-performance viscosity-responsive probes and serves as a reliable and efficient detection tool for mechanistic research and rapid diagnosis of related diseases.