NIR-II Fluorophores: From Synthesis to Biological Applications

Fluorescence imaging is a useful tool in the field of biomedical applications. However, its imaging capacity is limited by the depth of tissue that can be penetrated when using visible light (400-700 nm) or the first near-infrared window (NIR-I, 700-900 nm). To overcome the problem, fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) has been developed to reduce photon scattering, auto-absorption and tissue autofluorescence to achieve high spatiotemporal resolution and deep imaging penetration. The key to NIR-II imaging is obtaining and analyzing highly selective information from functional fluorophores that emit in the 1000-1700 nm range. With the rapid development of multidisciplinary research, various types of NIR-II fluorophores have been produced and used in non-invasive, real-time NIR-II biomedical applications. This review summarizes some of the most prevalent NIR-II fluorophores and their synthesis, such as organic fluorophores (OFs), single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), and rare-earth nanoparticles (RENPs). On this basis, we describe the applications of these fluorophores in biomedical fields, including bioimaging, biosensing, phototherapy and surgical navigation. Additionally, major challenges and prospects of NIR-II biomedical application will be further explored.