Fluorescent Dyes for Probing Intracellular and Subcellular Polarity

Herein, the use of solvatochromic fluorescent probes for detecting polarity in cellular microenvironments, a complex physicochemical parameter that influences various biological processes, is reviewed. The nature of polarity itself is related to the physicochemical properties of the environment that affect the spectroscopic properties of luminophores. The mechanisms of solute–solvent interactions and photophysical processes, such as intramolecular charge transfer and photoinduced electron transfer, which are fundamental to understanding the response of the probes to polarity changes are discussed. In the intracellular context, solvatochromic probes have been used to determine polarity in different organelles and subcellular structures. The review puts emphasis on the various targeting strategies and the most commonly utilized photophysical parameters for estimating polarity are focused here. A special attention is paid to those approaches capable of providing absolute quantification of intracellular polarity, for instance, through ratiometric measurements or the use of fluorescence lifetime imaging to improve the accuracy of measurements. Reliable intracellular polarity measurements may provide invaluable information to understand physiological and pathological processes, including neurodegenerative diseases or cancer.