Ratiometric Detection of Viscosity Using a Two‐Photon Fluorescent Sensor

In the thick of it: A carbazole-based cyanine derivative has been used as a ratiometric fluorescent sensor to detect mitochondrial viscosities over the range 1–950 cP within live cells and living tissues by using two-photon microscopy. Its success is based on the fast- and low-barrier rotation around its methine bond (see figure) and the huge increase of the fluorescence quantum yield (QY) from the loss of mobility in viscous media.