Introduction: Fluorescent Probes in Biology

Seeing is believing!Chemistry makes the invisible visible through the use of imaging agents that can monitor the elements and molecules of life at a variety of length scales with both spatial and temporal resolution.In this context, fluorescence and luminescence provide visual readouts that can be widely adopted by specialists and non-specialists alike, from laboratory researchers to medical clinicians and field technicians to children playing with glowsticks and fireflies during the long nights of summer.This thematic issue, Fluorescent Probes in Biology, delves into the latest research achievements in this storied and highly active field in the design and development of chemical reagents to decipher new fundamental biology and to translate this knowledge to advanced diagnostic and/or therapeutic platforms.Each paper in this issue focuses on molecular principles of probe design, applied to a particular biological question for analyte detection, chemical platform consisting of a small molecule, macromolecule, nanomaterial, or hybrid scaffold, or disease biomarker.State-of-the art research and future prospects in unmet needs pervade all these informative reviews.A key emerging theme in this collection is the broad use of activity-based sensing, a termed coined by our laboratory and advanced by our team and many others across the world, which is defined as using molecular reactivity rather than molecular recognition for analyte detection. 1We organize this discussion across fluorescent probes for specific bioanalytes, fluorescent probes derived from a specific type of scaffold (e.g., small molecule, protein, nanomaterial, or hybrid), or fluorescent probes for biomedical applications.A foundational use of fluorescent probes in biology is their application in detecting the chemistry of elements and molecules of life to decipher their physiological and/or pathological contributions to living systems.New and our laboratory have collaborated to write a review on small-molecule sensors for transition metal ions in biological specimens (https://doi.org/10.1021/acs.chemrev.3c00819).A focus is on the use of bindingbased sensing and activity-based sensing approaches, where the former strategy exploits traditional lock-and-key metal-ligand coordination bonding for selective metal chelation and detection, whereas the latter concept leverages the diverse reactivity of metal ions for their sensing.These approaches are applied to image bioavailable metal pools, termed the labile metal pool, over a variety of biological length scales and in a variety of cell and animal models with metal and oxidation state selectivity, revealing new biological concepts such as transition metal signaling and metalloallostery in health and disease.Activity-based sensing offers a powerful approach to use reaction chemistry as a strategy for analyte detection.Lippert and Domaille and their teams review activity-based sensing