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 (IBD) 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.