Synthesis and Evaluation of Europium Complexes that Switch on Luminescence in Lysosomes of Living Cells

A set of four luminescent Eu^III complexes bearing an extended aryl-alkynylpyridine chromophore has been studied, showing very different pH-dependent behaviour in their absorption and emission spectral response. For two complexes with pK_a values of 6.45 and 6.20 in protein-containing solution, the emission lifetime increases very significantly following protonation. By varying the gate time during signal acquisition, the ‘switch-on’ intensity ratio could be optimised, and enhancement factors of between 250 to 1330 were measured between pH 8 and 4. The best-behaved probe showed no significant emission dependence on the concentration of endogenous cations, reductants, and serum albumin. It was examined in live-cell imaging studies to monitor time-dependent lysosomal acidification, for which the increase in observed image brightness due to acidification was a factor of 50 in NIH-3T3 cells.