While living tissue strongly absorbs and scatters visible light, there are two spectral regions at slightly longer wavelengths for which attenuation and scattering is lower. These two bands are called the first and second near-infrared (NIR) windows. The NIR-II band is particularly attractive, although labels that emit in this band are difficult to fabricate. A team of researchers at Stanford University (Palo Alto, CA) and Wuhan University (Hubei, China), however, has developed a biocompatible organic small-molecule NIR-II dye with improved quantum yield.1
The NIR-I window covers from about 700 to 900 nm, while the NIR-II window is open from about 1000 to 1800 nm. Because scattering generally diminishes with longer wavelengths, autofluorescence (fluorescence signals from native molecules) is much lower in NIR-II. This gives NIR-II’s signal-to-noise ratio the potential to be high, even while its low attenuation facilitates deep imaging.
Just because there’s a window, though, does not mean that there are tools (in this case, appropriate fluorophores) to open it. It’s not for lack of trying, though, and recent reviews have summarized efforts to develop dyes based on lanthanide nanoparticles, conjugated polymers, small-molecules, carbon nanotubes, and other mechanisms. The challenge is rooted in fundamental physics.