In recent years, the rapid development of bioimaging technology has provided powerful tools for life science research. Among them, fluorescence imaging, as an important imaging technique, enables real-time and non-invasive visualization of physiological activities in biological systems. Since biological tissues have lower absorption and scattering of photons in the near-infrared region Ⅱ (NIR-Ⅱ), combined with the weaker autofluorescence of tissues in this region, the signal-to-background ratio is greatly improved. Therefore, NIR-Ⅱ fluorescence imaging can achieve deeper and higher-resolution biological imaging, and is expected to be widely used as an ideal precision imaging technique in basic research and clinical practice in the future.

NIR-Ⅱ fluorescence probes can be mainly categorized into inorganic and organic probes. Organic probes have advantages such as strong near-infrared absorbance, good biocompatibility, and easy metabolism, making them the preferred choice for in vivo imaging. Currently, there are two major classes of organic probes used in NIR?Ⅱ fluorescence imaging. One class is dyes with a donor-acceptor-donor structure, and the other class is cyanine dyes connected by conjugated polymethyl chains with a certain length of carbon atoms. Compared with donor-acceptor-donor dyes, the synthesis process of NIR-Ⅱ cyanine dyes is relatively simple, and they have higher brightness, thus possessing significant advantages in NIR-Ⅱ imaging. Cyanine dyes, as a highly valuable class of molecular probes, exhibit excellent fluorescence characteristics in the NIR-Ⅱ region, so that they have attracted extensive research interests and continue to develop in the field of disease diagnosis and treatment. Due to the high tissue penetration depth and low interference from biological background signals, NIR‐Ⅱ cyanine dyes can overcome the drawbacks of traditional fluorescence probes and be applied in the diagnosis of diseases.

Cancer is one of the leading causes of death of the global population, characterized by high mortality and recurrence rates. NIR-Ⅱ cyanine dyes can be used for tumor detection and visualization in the NIR-Ⅱ region. Their high sensitivity and high-resolution imaging make them important tools for early tumor diagnosis. Additionally, cyanine dyes also have significant advantages in real-time dynamic display of tumor boundaries, providing critical information for tumor resection surgery. Inflammation is a protective response to stimuli. However, if inflammation persists without timely diagnosis and effective control, the detrimental effects will outweigh its biological benefits. NIR-Ⅱ cyanine dyes also have important value in the application of inflammatory diseases. On the one hand, they can precisely locate the inflammatory area, and on the other hand, by monitoring the distribution and concentration changes of the dye in the body, the activity of inflammation can be evaluated, providing guidance for treatment. NIR-Ⅱ cyanine dyes can also effectively differentiate and locate the sites of injury, visualize injuries, and help assess the extent of tissue injuries. Furthermore, the combination of NIR-Ⅱ cyanine dyes with drugs enables targeted drug delivery to tumors, inflammatory areas, or injured sites. By monitoring the distribution of the drug-dye complexes in the body, the therapeutic effect can be assessed in real time. These advanced applications demonstrate the tremendous potential of NIR-Ⅱ cyanine dyes in the field of modern medicine and their broad application prospects in diseases.

The latest advancements in the applications of NIR‐Ⅱ cyanine dyes in various diseases are summarized. First, the structural characteristics, classifications, and applications of cyanine dyes are introduced. Then, the utilization of NIR-Ⅱ cyanine dyes in various tumors such as brain tumors, breast cancer, pancreatic cancer, liver cancer, bladder cancer, colorectal cancer, gastric cancer and other tumors is comprehensively reviewed, referencing prior researches. Tian’s research group from University of Chinese Academy of Sciences and Gambhir et al. from Stanford University have developed an integrated visible and NIR-Ⅰ/Ⅱ multispectral imaging instrument to perform the first human liver tumor surgery. They have taken relatively pioneering studies on treatment of tumors. Additionally, the research group from Fudan University, led by Zhang, has developed a tumor-microenvironment-responsive lanthanide-cyanine fluorescence resonance energy transfer (FRET) sensor for NIR-Ⅱ luminescence-lifetime in situ imaging of hepatocellular carcinoma [Fig. 5(a)]. The applications of NIR-Ⅱ cyanine dyes in various inflammatory diseases like acute vascular inflammation, rheumatoid arthritis, gastritis, and in injuries related to liver, kidney, and biliary tract are further discussed. Liu et al. collaborated on cyanine-doped lanthanide metal-organic frameworks for NIR-Ⅱ bioimaging [Fig. 6(a)]. It is noted that the application research of NIR-Ⅱ cyanine dyes is still limited and not comprehensive. Finally, the challenges and research trends in this field are discussed.

NIR?Ⅱ cyanine dyes have tremendous potential in the imaging and treatment of tumors, inflammatory diseases, and injuries. In summary, further in-depth exploration is needed for the development of NIR-Ⅱ cyanine dyes to promote their wider clinical applications, and bring new breakthroughs and developments to the fields of medical imaging and clinical diagnosis.