Luminescence modulation of two individual fluorophores over a wide pH range and intracellular studies

A novel terpyridine derivative conjugated by tetraethylene glycol chains with a coordinative europium (III) center was prepared and the assembled complex with high water solubility exhibited sensitive pH dependence. Modulation of the luminescence could be realized at variable physiological pH values (from 8 to 2) by consecutive changes in the form and intensity of both europium and ligand emissions. The achieved multiple optical signal shifts (blue-red-green) and especially the performance of the well-defined lanthanide complex under strong acidic condition had never been reported. In vitro studies revealed that the europium complex was nontoxic to HeLa and THP-1 cells. Fluorescent microscope images supported the distribution of the emissive species and the measurement of pH fluctuations in pathogenic cells could be achieved.