The syntheses and photophysical behaviour of nine, strongly luminescent nonadentate Eu(III) complexes are reported. Each complex is based on N-functionalised 1,4,7triazacyclononane, and linkage to other groups or targeting vectors can occur either via amide bond formation to a coordinated pyridine p-aminopropyl group or via a nucleophilic substitution reaction involving thiol attack on a metal coordinated p-nitropyridyl moiety. Evidence is presented in favour of the latter conjugation strategy, as parallel work with maleimide conjugates was complicated or compromised by the propensity to undergo postconjugation thiol exchange or succinimide ring hydrolysis reactions. Confocal microscopy and spectral imaging studies revealed that the peptide conjugate of AcCFFKDEL was found to localise selectively in the endoplasmic reticulum of mouse fibroblast cells, whereas the related maleimide conjugate was only observed in cellular lysosomes.
A set of four luminescent EuIII 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 pKa 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.
We report the design and evaluation of pH responsive luminescent europium(III) probes that allow conjugation to targeting vectors to monitor receptor internalisation in cells. The approach adopted here can be...
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