Azo-Based Fluorogenic Probes for Biosensing and Bioimaging: Recent Advances and Upcoming Challenges

The use of nonfluorescent azo dyes as dark quenchers in activatable optical bioprobes based on the Förster resonance energy transfer (FRET) mechanism and designed to target a wide range of enzymes has been established for over two decades. The key value of the azo moiety (-N=N-) to act as an efficient "ON-OFF" switch of fluorescence once introduced within the core structure of conventional organic-based fluorophores (mainly fluorescent aniline derivatives) has recently been exploited in the development of alternative reaction-based small-molecule probes based on the "profluorescence" concept. These unprecedented "azobenzene-caged" fluorophores are valuable tools for the detection of a wide range of reactive (bio)analytes. This review highlights the most recent and relevant advances made in the design and biosensing/bioimaging applications of azo-based fluorogenic probes. Emphasis is also placed on relevant achievements in the synthesis of bioconjugatable/biocompatible azo dyes used as starting building blocks in the rational and rapid construction of these fluorescent chemodosimeters. Finally, a brief glimpse of possible future biomedical applications (theranostics) of these "smart" azobenzene-based molecular systems is presented.