Fluorescent Probes for HOCl Detection in Living Cells

Hypochlorous acid (HOCl) plays an important role in the immune system not only protecting the organism from pathogens, but also, due to its high reactivity, provoking the development and complication of many diseases. Therefore, the development of highly sensitive and selective HOCl biosensors, including fluorescent probes, to better understand the roles of HOCl in living systems, is of great importance. Nevertheless, there are many difficulties associated with HOCl registration in biological probes (chemo- and photostability, cytotoxicity, fluorescence and absorption characteristics, selectivity, sensitivity, etc.). Thus, the development of new chemosensors has been relevant for many years. In this review, we describe classification of small-molecule probes for HOCl detection in biological systems, as well as summarize the results of the development of chemosensors, their photophysical properties, and biological applications. Particular attention is paid to the achievements in this area over the years 2016–2021. A number of problems related to the design and application of small-molecule probes are formulated. Due to the absence of a "gold standard" among the HOCl chemosensors, which is associated with the commercial unavailability or complex methods of synthesis of new sensors as well, some recommendations are given regarding the field of the chemosensor application. Trends in the development of fluorescent chemosensors for HOCl visualization in living cells are outlined.