Activatable Fluorescence Probes for “Turn-On” and Ratiometric Biosensing and Bioimaging: From NIR-I to NIR-II

The greatest advantage of activatable fluorescence probes (AFPs) is the inherent responsiveness to manipulate spectroscopic properties by chemical/physical interactions with the biological analytes/microenvironmental factors. As alternatives to "always-on" fluorescence probes, AFPs in the first near-infrared (NIR-I) window expanded dramatically over the past decade and served as powerful tools in fluorescence biosensing and bioimaging. Benefiting from the deep tissue penetration, minimal tissue damage, and negligible background signal within longer wavelength, recent progress of fluorescent materials in the second near-infrared (NIR-II) window has been creating vast new opportunities in developing AFPs. Here, we review the current role of AFPs in biosensing and bioimaging, with emphasis on NIR-II AFPs developed for biomedical applications. The challenges and prospects of AFPs are also discussed by considering the clinical translation from bench to bedside.