Activatable small molecular luminescence probes for autofluorescence-free bioimaging

Autofluorescence-free luminescence imaging, which captures optical output without using real-time light excitation, enables non-invasive visualization of biological events with extremely low backgrounds. It is endowed with high specificity and sensitivity at the molecular level by adjuvant stimulus-activatable probes, which turn their luminescent signals "on" exclusively at the stimulus-rich region of interest. Particularly, those activatable autofluorescence-free imaging probes with small molecular weights (i.e., activatable small molecular probes) also possess high molecular programmability, well-defined chemical structure, tunable optical properties, and relatively high biosafety. In this review, we provide an overview of the activatable small molecular probes with autofluorescence-free techniques (including chemiluminescence, bioluminescence, time-gated luminescence, and afterglow luminescence) for imaging of endogenous bioanalytes (i.e., enzymes and bioactive species), with the emphasis on the very recent breakthroughs in the past three years (from 2020 to early 2023). Finally, challenges and trends in this field are discussed.