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

Abstract

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.

In Situ Rolling Circle Amplification Förster Resonance Energy Transfer (RCA-FRET) for Washing-Free Real-Time Single-Protein Imaging

“…The improved signal-tobackground ratios of TG and TG-FRET imaging (compared to CW imaging) underline the significant benefits that lanthanide complexes can bring to advanced high-contrast PL imaging. 9,34,37,[41][42][43][44][45][46] Notably, lanthanide-based TG PL imaging of fixed cells or tissues is not common and considering that the well-established solution-phase TG-FRET assays have only recently been implemented into advanced live cell and in vivo imaging, we can anticipate that the same performance can be expected for carefully optimized TG imaging on fixed cells. Similar to immunohistochemistry, the performance of in situ PLA depends on the fixation and pretreatment conditions, the applied antibody-DNA probes, and the careful adaption of assay components and their concentrations, which are important optimization steps that are beyond our in situ RCA-FRET proof-of-concept study.…”

The design of responsive luminescent lanthanide probes and sensors

“…2). 23,24,38 The set of three Eu complexes, [EuL [7][8][9] ], containing increasing numbers of electron rich chromophores and can be compared with the set of complexes, [EuL [10][11][12] ] where the chromophore also possesses a conjugated nitrogen lone pair in the upper aryl ring, 38 (Table 4). With [EuL [7][8][9] ], there is a small decrease in the lifetime of emission as the number of chromophores increases, suggesting that electron transfer from the electron rich sensitising moiety quenches the Eu excited state, to some extent.…”

“…Europium(III) complexes have been described that on protonation show a change in lifetime of a factor of 4 with an emission intensity pH enhancement of two orders of magnitude between pH 8 and 4. 38 The pK a value of complexes such as [EuL 23 ] 2À , can be tuned to the pH range 5 to 6, (Fig. 12).…”

“…Europium excited state lifetimes for [EuL[7][8][9][10][11][12] ]23,24,38 pH 4, 0.1 M NaCl, H 2 O; pK a values are 6.75, 6.30 and 6.21, respectively. Scheme 2), the rates of energy transfer from the ligand triplet to Eu 5 D 1,0 or Tb 5 D 4 are around 10 5 s À1 , while k ET is 10 7 s À1 to the Nd 4 F 3/2 state.…”

Targeted pH switched europium complexes monitoring receptor internalisation in living cells

“…[2][3][4][5] Recently, we have identified europium(III) complexes that show a pH dependent change in lifetime of a factor of 3 and an emission intensity pH switching ratio of around two orders of magnitude, using time-gated acquisition methods. 6 Each of these Eu(III) complexes is C 3 symmetric and contains strongly absorbing alkynyl-aryl chromophores, e.g. EuL 1a with a brightness of 10 200 M À1 cm À1 (332 nm) in acidic conditions.…”