Small-Molecule NIR-II Pushes In Vivo Imaging Deeper

While living tissue significantly attenuates and scatters visible light, attenuation, and scattering are less for two spectral regions at slightly longer wavelengths. The first and second near-infrared (NIR) windows refer to these frequencies. Although labels emitting in the NIR-II band are challenging, this band is especially attractive. A group of scientists has created an organic small-molecule NIR-II dye that is safe and has a higher quantum yield.

While the NIR-II window is open from roughly 1000 to 1800 nm, the NIR-I window is open from roughly 700 to 900 nm. Autofluorescence, or fluorescence signals from endogenous molecules, is significantly less in NIR-II because scattering typically decreases with longer wavelengths. Because of this, NIR-II can have a high signal-to-noise ratio even though deep imaging is made possible by its low attenuation.

While IR light alters the motion of an atom that is accessible to the outside world, visible light alters the motion of an electron that is “internal” to a molecule. Temperature is correlated with spinning or vibrational energy. Therefore, matter’s temperature rises as a result of infrared absorption.

The issue occurs in the NIR-II window directly between these two extremes. The absorption of a NIR particle increases the energy of an electronic state.

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