Environment-related parameters, including viscosity, polarity, temperature, hypoxia, and pH, play pivotal roles in controlling the physical or chemical behaviors of local molecules. In particular, in a biological environment, such factors predominantly determine the biological properties of the local environment or reflect corresponding status alterations. Abnormal changes in these factors would cause cellular malfunction or become a hallmark of the occurrence of severe diseases. Therefore, in recent years, they have increasingly attracted research interest from the fields of chemistry and biological chemistry. With the emergence of fluorescence sensing and imaging technology, several fluorescent chemosensors have been designed to respond to such parameters and to further map their distributions and variations in vitro/in vivo. In this work, we have reviewed a number of various environment-responsive chemosensors related to fluorescent recognition of viscosity, polarity, temperature, hypoxia, and pH that have been reported thus far.

Traditional photothermal therapy requires high‐intensity laser excitation for cancer treatments due to the low photothermal conversion efficiency (PCE) of photothermal agents (PTAs). PTAs with ultra‐high PCEs can decrease the required excited light intensity, which allows safe and efficient therapy in deep tissues. In this work, a PTA is synthesized with high PCE of 88.3% based on a BODIPY scaffold, by introducing a CF3 “barrier‐free” rotor on the meso‐position (tfm‐BDP). In both the ground and excited state, the CF3 moiety in tfm‐BDP has no energy barrier to rotation, allowing it to efficiently dissipate absorbed (NIR) photons as heat. Importantly, the barrier‐free rotation of CF3 can be maintained after encapsulating tfm‐BDP into polymeric nanoparticles (NPs). Thus, laser irradiation with safe intensity (0.3 W cm−2, 808 nm) can lead to complete tumor ablation in tumor‐bearing mice after intravenous injection of tfm‐BDP NPs. This strategy of “barrier‐free rotation” provides a new platform for future design of PTT agents for clinical cancer treatment.

A self-calibrating bipartite viscosity sensor 1 for cellular mitochondria, composed of coumarin and boron-dipyrromethene (BODIPY) with a rigid phenyl spacer and a mitochondria-targeting unit, was synthesized. The sensor showed a direct linear relationship between the fluorescence intensity ratio of BODIPY to coumarin or the fluorescence lifetime ratio and the media viscosity, which allowed us to determine the average mitochondrial viscosity in living HeLa cells as ca. 62 cP (cp). Upon treatment with an ionophore, monensin, or nystatin, the mitochondrial viscosity was observed to increase to ca. 110 cP.