Near-Infrared and Shortwave Infrared Building Blocks for Activity-Based Sensors in Animals

Activatable fluorescent probes operating in the near-infrared (NIR, 700-1000 nm) and shortwave-infrared (SWIR, also known as NIR-II, 1000-1700 nm) windows have become essential tools for bioimaging due to their deep tissue penetration, low background interference, and high signal-to-noise ratios. With growing numbers of such probes in the red-shifted windows developed for detection in animal models, this review summarizes how and what NIR and SWIR fluorophores can be coupled with selective triggers to create activity-based sensing NIR/NIR-II probes rather than listing probes based on their bioanalytes. To date, a variety of probes have been constructed this way, involving the modular combination of a fluorophore scaffold that defines photophysical properties, defined modification sites where caging groups are installed, and connection chemistry that links responsive moieties to the fluorophore core. Through representative examples, we illustrate how the same fluorophore can be adapted for diverse applications by varying the caging group, and how the same responsive chemistry can be implemented across different scaffolds by preserving the linkage type. This modular framework provides researchers with a practical design toolkit for constructing activatable probes for new biological targets.