Access to Diverse Activatable Heptamethine Cyanine Probes with Low Intrinsic Fluorescence via 5‐exo‐trig Cyclization Strategy for High‐Contrast Bioimaging In Vivo

The conversion of conventional “always‐on” cyanine dyes into activatable NIR probes with low inherent fluorescence remains challenging, resulting in poor imaging contrast and nonspecific response in vivo. We herein report a 5‐exo‐trig cyclization strategy to create diverse activatable heptamethine cyanine probes with “zero” intrinsic background fluorescence for high‐contrast bioimaging. This intramolecular ring‐closure approach imparts a built‐in switch to regulate the fluorescence of cyanine dyes, which predominantly exist in the nonfluorescent closed‐loop form at pH over 5.0 and are not affected by environmental polarity and protein interaction, thereby reducing false‐positive fluorescent signals. Upon reaction with various biomarkers, including methylglyoxal (MGO), hydroxyl radical (·OH), and adenosine triphosphate (ATP), they could rapidly transform into fluorescent open‐loop structures, showing significant NIR fluorescence enhancement (up to 184‐fold). Furthermore, we extended this strategy to develop a variety of NIR‐II ATP‐activatable cyanine probes for the first time, as well as a theranostic probe 57 using host‐guest chemistry. Probe 57 not only sensitively monitored ATP levels in inflammatory bowel disease with a high signal‐to‐background ratio of 48/1, but it also precisely detected extracellular ATP in stools. This work opens a new avenue to develop stimuli‐responsive NIR cyanine probes for improving disease diagnosis.