Infoscience

Thesis

Luminescent lanthanides bioprobes emitting in the visible and/or near-infrared ranges

This Ph.D. work deals with the development of lanthanide-based bioprobes for biomedical analyses and imaging. For this purpose, two different approaches have been followed, one focusing on visible-emitting complexes, while the second addresses the sensitization of NIR-emitting LnIII ions. The homoditopic ligands H2LC2 and H2LC3 have been tailored to self-assemble with lanthanide ions (LnIII) into neutral triple-stranded bimetallic helicates of overall composition [Ln2(L)3] and also to provide a versatile platform for further derivatization. The grafting of polyoxyethylene groups onto the pyridine (H2LC2) or the benzimidazole (H2LC3) units ensures water solubility, while maintaining large thermodynamic stability and adequate antenna effect for the excitation of LnIII ions. The conditional stability constants are large at physiological pH, with logβ23 reaching values in the range 22-25 for helicates with H2LC2 and H2LC3. The ligands triplet states feature adequate energy to efficiently sensitize the EuIII luminescence (QEu L = 21 and 11 % for L = H2LC2 and H2LC3, respectively) in aerated water at pH 7.4. Conversely, energy back transfer occurring in [Tb2(LC3)3] precludes a good sensitization of the TbIII luminescence, and only the TbIII helicate with H2LC2 displays sizeable quantum yield (QTbL = 11 %). Furthermore, it has been demonstrated that ligand H2LC2 sensitizes the emission of other visible- and NIR-emitting LnIII ions, such as SmIII or YbIII, for which in cellulo luminescence is evidenced, a rare observation. The EuIII emission spectra of both helicates arise from a main species with pseudo-D3 symmetry and without coordinated water. Cytotoxicity experiments reveal no significant effect of the helicates on the viability of several cancerous cell lines and cell imaging properties of the EuIII helicates are demonstrated for the HeLa cell line by luminescence microscopy. Bright EuIII emission is seen after relatively short time (15-30 min) and low helicate incubation concentrations (10 µM). Similar images were obtained for [Sm2(LC2)3] and [Tb2(LC2)3] using slightly higher loading concentration. The helicates stain the cytoplasm and the permeation mechanism is likely to be endocytosis. The described luminescent helical stains are robust chemical species which remain mostly undissociated in the cell medium and which resist to the presence of other complexing agents, as demonstrated in the case of H2LC2. All these properties make these helicates potential bioprobes for interacting with biological material and for cell imaging applications. As near-infrared (NIR) light, in addition to be less energetic than visible light, displays better penetrability into the human body, the development of bioprobes emitting in the near-infrared range is of prime importance for deeper tissue imaging, especially for early diagnosis of tumors. We therefore investigate the ability of new polydentate podands to sensitize the NIR luminescence of NdIII and YbIII ions. These ligands have been designed in a way to take advantage of the chelating effect of bidentate (Tsox, TsoxMe) or tridentate 8-hydroxyquinolinate (8-HQ) subunits (T2soxMe). The interaction between the ligands and LnIII ions in aqueous solution leads to the formation of stable 1:1 chelates (pLn in the range of 12-16), all the chromophoric 8-HQ units being coordinated to the metal center, exploiting the entropic effect generated by the anchor. At physiological pH, one major species is present for all the ligands regardless of the LnIII ion used. The low energy triplet states of the ligands are particularly well-suited for an efficient sensitization of the NdIII- or YbIII-centered luminescence (QNdL = 0.02-0.04 % and QYbL = 0.13-0.37 %) in buffered aqueous solution. ErIII NIR luminescence is also detected in water for the chelates with Tsox and TsoxMe. Lifetime determinations point in all cases to the absence of coordinated water molecules in the inner coordination sphere of the LnIII ion. It has also been demonstrated that methylation of the amide functions of Tsox removes the quenching mechanism induced by the proximate N-H groups and increases both the lifetimes and quantum yields of the TsoxMe chelates, with the YbIII complex being one of the most NIR luminescent lanthanide-based edifices described so far. Ligand TsoxMe thus appears to be the best candidate of the series regarding to the development of NIR probes for bioanalyses. Furthermore, other applications, for instance in telecommunications, are still conceivable since the incorporation of the Yb-TsoxMe chelate into sol-gel silica films has proven to be successful, the chelate withstanding the sol-gel process, while providing a slight enhancement of photophysical properties compared to the same chelate in aqueous solution.

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