Repurposing cyanine NIR-I dyes accelerates clinical translation of near-infrared-II (NIR-II) bioimaging

The significantly reduced tissue autofluorescence and scattering in the NIR-II region (1000–1700 nm) opens many exciting avenues for detailed investigation of biological processes in vivo. However, the existing NIR-II fluorescent agents, including many molecular dyes and inorganic nanomaterials, are primarily focused on complicated synthesis routes and unknown immunogenic responses with limited potential for clinical translation. Herein, the >1000 nm tail emission of conventional biocompatible NIR cyanine dyes with emission peaks at 700–900 nm is systematically investigated, and a type of bright dye for NIR-II imaging with high potential for accelerating clinical translation is identified. The asymmetry of the π domain in the S₁ state of NIR cyanine dyes is proven to result in a twisted intramolecular charge-transfer process and NIR-II emission, establishing a general rule to guide future NIR-I/II fluorophore synthesis. The screened NIR dyes are identified to possess a bright emission tail in the NIR-II region along with high quantum yield, high molar-extinction coefficient, rapid fecal excretion, and functional groups amenable for bioconjugation. As a result, NIR cyanine dyes can be used for NIR-II imaging to afford superior contrast and real-time imaging of several biological models, facilitating the translation of NIR-II bioimaging to clinical theranostic applications.