Fluorine-18 is the most frequently used radioisotope in positron emission tomography (PET) radiopharmaceuticals in both clinical and preclinical research. Its physical and nuclear characteristics (97% β+ decay, 109.7 min half-life, 635 keV positron energy), along with high specific activity and ease of large scale production, make it an attractive nuclide for radiochemical labeling and molecular imaging. Versatile chemistry including nucleophilic and electrophilic substitutions allows direct or indirect introduction of 18F into molecules of interest. The significant increase in 18F radiotracers for PET imaging accentuates the need for simple and efficient 18F-labeling procedures. In this review, we will describe the current radiosynthesis routes and strategies for 18F labeling of small molecules and biomolecules.

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 S 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.

NIR-II fluorescence imaging greatly reduces scattering coefficients for nearly all tissue types at long wavelengths, benefiting deep tissue imaging. However, most of the NIR-II fluorophores suffer from low quantum yields and/or short circulation time that limit the quality of NIR-II imaging. Here, we engineered a supramolecular assembly of protein complex with lodged cyanine dyes to produce a brilliant NIR-II fluorophore, providing a NIR-II quantum yield of 21.2% with prolonged circulation time. Computational modeling revealed the mechanism for fluorescence enhancement and identified key parameters governing albumin complex for NIR-II fluorophores. Our complex afforded high-resolution microvessel imaging, with a 3-hour imaging window compared to 2 min for free dye alone. Furthermore, the complexation strategy was applied to an antibody-derived assembly, offering high-contrast tumor imaging without affecting the targeting ability of the antibody. This study provides a facile strategy for producing high-performance NIR-II fluorophores by chaperoning cyanine dyes with functional proteins.