Recent development of small-molecule organic fluorophores for multifunctional bioimaging in the second near-infrared window

The optical wavelength between 1000 and 1700 nm, commonly referred to as the second near-infrared (NIR-II) window, has become a highly popular optical region for multifunctional bioimaging. Compared to both of visible (400–700 nm) and traditional NIR-I (700–900 nm) light, NIR-II bioimaging has numerous advantages, including higher spatial resolution, deeper penetration, and minimized tissue autofluorescence. Thus, the use of fluorophores in the NIR-II window has attracted increasing attention for multifunctional bioimaging, especially in vivo imaging. In this review, we attempt to summarize the rational design of various small-molecule NIR-II fluorophores and their recent developments. Furthermore, we will also discuss their challenges and prospects in the following clinical application.