FD-838 is primarily used as a fluorescent dye in biological imaging and diagnostic applications. It exhibits strong fluorescence in the near-infrared range, making it suitable for in vivo imaging where deep tissue penetration and low background interference are critical. The compound is often employed in labeling antibodies, peptides, or nanoparticles to track molecular targets in live cells and animal models. Its high photostability and quantum yield enhance signal detection in fluorescence microscopy, flow cytometry, and optical imaging systems. FD-838 is also utilized in research settings for monitoring drug delivery systems and visualizing tumor margins during preclinical studies. Due to its favorable toxicity profile and solubility in aqueous buffers, it is compatible with a variety of biological assays requiring real-time visualization.