A two-photon fluorescent probe for viscosity imaging in vivo

A new two-photon fluorescent probe (MCN) for viscosity imaging was developed based on a 6-substituted quinoline framework. MCN showed an excellent off-on'' fluorescence response (ca. 90-fold enhancement) with viscosity increasing in the glycerol-water viscosity system. MCN showed great sensitivity to viscosity (R-2 = 0.98, x = 0.65), which gave rise to cell imaging for micro-viscosity or real-time cell imaging during apoptosis with low cytotoxicity under two-photon excitation (lambda(ex) = 800 nm). Fluorescence lifetime imaging (FLIM) of living HeLa cells stained with MCN revealed that the intracellular average viscosity value was 73.45 +/- 21.55 cP in cytosol. Imaging in living tissue slices indicated that MCN can work in deep tissue (similar to 130 mu M) under two-photon excitation. Moreover, MCN also showed the capacity for in vivo imaging viscosity in zebrafish with obvious fluorescence emission.