Recent advances in organic near-infrared ratiometric small-molecule fluorescent probes

The burgeoning organic near-infrared (NIR) ratiometric small-molecule fluorescent probes with excellent merits have attracted great attention over the past few years. The traditional intensity-based probes are unable to output reliable and accurate detection signals as well as to realize quantitative determination of analytes due to the lack of an “internal standard method”. The smart dual emission probes with built-in correction ability can overcome the “Achilles' heel” of the single channel fluorescent probes, such as concentration-dependent and microenvironment sensitive, to enable reliable quantitative image analysis. Furthermore, fluorescent chemosensors in the NIR window (650–1700 nm) have great advantages, including high signal-to-noise (S/N) ratio, low tissue damage, and deep tissue penetration. Therefore, the NIR ratiometric fluorescent probes combine the advantage of the ratiometric fluorescent probes and the single-intensity-based NIR probes. Nowadays, these robust multichannel probes have been widely applied for biomedical research and clinical practice. In this review, we systematically summarized the recent advances (2020–2022) in these kinds of probes. The design strategies, sensing mechanisms, and bioapplications of these intelligent fluorescent probes were discussed. Moreover, the challenges and prospects in this promising field were addressed, and we anticipated that our work would inspire the development of organic NIR ratiometric fluorescent probes.