Esterase-Activated Precipitating Strategy to Achieve Highly Specific Detection and Long-Term Imaging of Calcium Ions by Aggregation-Induced Phosphorescence Probe

Spatial and temporal monitoring of bioactive targets such as calcium ions is vitally significant for their essential roles in physiological and biochemical functions. Herein, we proposed an esterase-activated precipitating strategy to achieve highly specific identification and long-term bioimaging of calcium ions via lighting up the calcium ions by precipitation using a water-soluble aggregation-induced phosphorescence (AIP) probe. The designed probe CaP2 has an AIP behavior and can be efficiently aggregated by calcium ions through the coupling coordination of carboxylic acid and cyanide groups, which enables it to light up Ca2+ by precipitating-triggered phosphorescence. Four hydrophilic groups of tetraethylene glycol were introduced to endow the resulting probe CaP3 with extraordinary water solubility as well as excellent cellular penetration. Only when the probe CaP3 penetrates inside the live cells the existing esterase in cells can activate the probe to be transformed active CaP2 probe selectively binding with calcium ion in the surroundings. The probe was used to further evaluate the imaging of intracellular calcium ions in model organisms. The excellent imaging performance of CaP3 in Arabidopsis thaliana seedling roots demonstrates that CaP3 has the excellent capability of monitoring calcium ions in live-cell imaging, and furthermore CaP3 exhibits much better photostability and thereby greater potential in long-term imaging. This work established a general esterase-activated precipitating strategy to achieve specific detection and bioimaging in situ triggered by esterase in live cells, and established a water-soluble aggregation-induced phosphorescence probe with high selectivity to achieve specific sensing and long-term imaging of calcium ions in live cells.