Molecular Fluorescence and Photoacoustic Imaging in the Second Near-Infrared Optical Window Using Organic Contrast Agents

Noninvasive near-infrared (NIR) light ranging from 650 to 1000 nm (NIR-I) is widely employed in fundamental research and clinical applications; however, a recently discovered second NIR (NIR-II) window from 1000 to 1700 nm exhibits even better deep-tissue imaging capability due to reduced photon scattering, minimized tissue autofluorescence, and increased applicable power at longer wavelengths. This review focuses on recent advances of organic contrast agents developed for in vivo fluorescence and photoacoustic imaging in the NIR-II optical window. The superiority of the NIR-II over the NIR-I window for molecular imaging is first discussed in detail, followed by discussion of fluorescence and photoacoustic imaging of cancer, vasculature, and the brain using organic contrast agents in the NIR-II window. At last, challenges and perspectives of organic contrast agents for NIR-II in vivo imaging are suggested.