A highly sensitive and selective nanosensor for near-infrared potassium imaging.

Potassium ion (K+) concentration fluctuates in various biological processes. A number of K+ probes have been developed to monitor such fluctuations through optical imaging. However, the currently available K+ probes are far from being sensitive enough in detecting physiological fluctuations in living animals. Furthermore, the monitoring of deep tissues is not applicable because of short-wavelength excitation prevailingly used so far. Here, we report a highly sensitive and selective nanosensor for near-infrared (NIR) K+ imaging in living cells and animals. The nanosensor is constructed by encapsulating upconversion nanoparticles (UCNPs) and a commercial K+ indicator in the hollow cavity of mesoporous silica nanoparticles, followed by coating a K+-selective filter membrane. The membrane adsorbs K+ from the medium and filters out interfering cations. The UCNPs convert NIR to ultraviolet light, which excites the K+ indicator, thus allowing the detection of the fluctuations of K+ concentration in cultured cells and intact mouse brains.