New ratiometric two-photon fluorescent probes are developed from 6-substituted quinolines for biological Zn²⁺ detection. They show large red shifts and good ratiometric responses upon Zn²⁺ binding. They also exhibit high ion selectivities and large two-photon absorption cross sections at nearly 720 nm. Because the new probes are cell-permeable, they can be used to detect intracellular zinc flux under two-photon excitation.

The first two-photon probe, Lyso-OC, was proposed for use in monitoring cell autophagy by detection of the change in the lysosomal polarity during the membrane fusion process of autophagy. The Lyso-OC probe exhibited desirable optical properties and a detection signal that was selective to the polarity change. More importantly, Lyso-OC displayed real-time monitoring of autophagy in living cells.

The sulfide anion is significant in industrial processes, biology and environmental science. Hence, robust fluorescent sensors for sulfide which are water soluble and biocompatible are highly desirable. Here, we developed a novel red-emitting fluorescent probe (SP1) for the sulfide anion based on cleaving the dinitrobenzenesulfonate ester to restore the fluorescence, accompanied by a long wavelength emission at 644 nm. The detection process took place in 100% PBS buffer, which suggested it had excellent water solubility. Most importantly, the ability of SP1 to detect sulfide anions in living cells (MCF-7 cells) and zebrafish has also been proven.

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 (R² = 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 (λ_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 (∼130 μM) under two-photon excitation. Moreover, MCN also showed the capacity for in vivo imaging viscosity in zebrafish with obvious fluorescence emission.