Cationic Two-Photon Lanthanide Bioprobes Able to Accumulate in Live Cells

Abstract

An original cationic water-soluble cyclen-based Eu(III) complex [EuL(1)](+) featuring a chromophore-functionalized antenna to increase the two-photon (2P) absorption properties was synthesized. The photophysical properties were thoroughly studied in various solvents and rationalized with the help of theoretical calculations. The complex exhibits an optimized 2P absorption cross section. Finally, 2P microscopy imaging experiments on living T24 human cancer cells highlighted the spontaneous internalization and the biological stability of this 2P bioprobe in vitro. Macrocyclic-based antennas open new perspectives for future optimization of the photophysical properties and allows envisaging the design of Eu, Tb, Yb, and Sm bioprobes. This result also opens the way for the design of functional two-photon Ln complexes able to monitor intracellular physicochemical parameters.

Cationic Biphotonic Lanthanide Luminescent Bioprobes Based on Functionalized Cross‐Bridged Cyclam Macrocycles

“…These two parameters are qualitative indicators of the symmetry of the coordination polyhedron. Comparison with similar parameters in other macrocyclic complexes featuring similar antennas confirmed that they are lower than that of the C 3 symmetric tacn based complex (r=10.9, R 2 =79 %) but compare well with those of C 2 diMe‐cyclen [Eu L 1b ] + (r=2.5, R 2 =56 %) and distorted C 3 pyclen derivatives (r=3.0, R 2 =65 %) . Contrarily to what we observed at 77 K, at room temperature the J=0 transition of [Eu L 2b ] + presents two components at 581 (main peak B, Figure ) and 580.1 nm (minor peak A, Figure ) signature of the presence of at least two different emitting species.…”

“…A value of 0.2 was obtained for [Eu L 2b ] + similar to that of the parent complex (q=0.25–0.35 for [Eu L 2a ] + ) and 0.1 for [Yb L 2c ] + . Interestingly, these values are lower than that of their diMe‐cyclen analogues (0.5 and 0.2 for [Eu L 1b ] + and [Yb L 1c ] + , respectively) suggesting that the more sterically constrained cross‐bridged cyclam scaffold offers a stronger protection against water coordination ( vide infra ). This conclusion is further corroborated by longer lifetimes observed in water for the cb‐cyclam based complexes compared to their diMe‐cyclen analogues and the six‐fold increase of the quantum yield measured for [Eu L 2b ] + (0.06 vs 0.01 for [Eu L 1b ] +[7g] ).…”

“…A valuable strategy to optimize the brightness consists in taking advantage of charge transfer (CT) antennas to increase simultaneously 1P‐ and 2P‐absorption coefficients and displace the excitation wavelengths to the visible due to a direct sensitization mechanism . In this context, following our initial report in 2008 on functionalized CT‐antennas tris‐dipicolinate europium complexes, we systematically explored the effect of such functionalization using existing polydendate or macrocyclic complexes such as cyclen bis‐bipyridine, tacn, pyclen and diMe‐cyclen ([Ln L 1a – c ] + , Figure ). It appears that anionic polydentate and neutral tacn or pyclen complexes featuring CT antennas are among the brightest complexes reported in the literature for Eu 3+ and/or Tb 3+ , but are not able to stain living cells through passive internalization processes .…”

“…It appears that anionic polydentate and neutral tacn or pyclen complexes featuring CT antennas are among the brightest complexes reported in the literature for Eu 3+ and/or Tb 3+ , but are not able to stain living cells through passive internalization processes . On the other hand, cationic bis‐pyridine or diMe‐cyclen ([Ln L 1b,c ] + , Figure ) complexes exhibit lower quantum yield efficiency due to partial coordination of a water molecule and/or increased fluxionality in protic solvents but showed spontaneous internalization in living cells . A similar charge effect was already described by Parker and Bünzli, who proposed a passive non‐diffusive internalization process based on macropinocytosis with a localisation of the complexes in endosomal vesicles inside the cytosol…”

Responsive, Water‐Soluble Europium(III) Luminescent Probes

“…[60] Fig. 12 Schematic mechanism of single and two--photon excitation Various examples of europium complexes for two--photon applications have been reported so far, [61], [62], [63] including bright alkynyl--pyridine systems, functionalised with hydrophilic groups (PEG, sulfo--betaine, carboxylic acids) to ensure water solubility and cellular permeabilisation. Unfortunately, the relatively low two--photon cross--sections (<100 GM) of these systems require the use of rather high energy excitation sources that may result in lower signal intensity compared to organic dyes, such as the series of squaraine--rotaxane derivatives (up to 10,000 GM).…”