Luminescence cell imaging

Zhang L., L. Tian, Z. Ye, B. Song, J. L. Yuan. Preparation of visible-light-excited europium biolabels for time-resolved luminescence cell imaging application, Talanta, 108,143-149 (2013). 

The three Ndm, Er111, and Ybm chelates display sizeable metal-centered NIR luminescence in HBS-buffered (pH 7.4) aqueous solutions. Their photophysical characteristics are summarized in table 18. The hydration numbers calculated from eqs. (10a) and (9a) are very small, 0.31 and 0.16 for Ndm and Ybm, respectively, and compare well with the results obtained for the tetrapodal ligand H890a. The overall luminescence quantum yields in aqueous solution are comparable to those obtained for H890a, but smaller than those determined for chelates withH890b (compare tables 17 and 18). Upon deuteration of the solvent, from 3- to 10-fold increases are observed in the luminescence quantum yields. Moreover cytotoxicity studies on several cell lines have shown the Ybm chelate to be non-toxic, opening the way for applications in cell imaging (Comby et al., 2007). 

Modification of the C2-symmetrical ligands to allow water solubility for the assembled helicate is another landmark (Elhabiri et al., 1999 Vandev)Tver et al., 2007) since it opens the way for the binuclear complexes to be used as lanthanide luminescence probes with a variety of applications ranging from cell imaging (Biinzli et al., 2008) to nucleic acid detection (Song et al., 2008b). The chemical robustness of the helicates, both thermod)mamically and kinetically speaking, their inherent... 

Iridium metal compounds have shown some remarkable effects recently for luminescent properties such as very high photoluminescent quantum yields [27, 29, 44] and high stability [46]. Two reviews [2, 4] have recently reported on phosphorescent heavy-metal complexes (Re, Ru, Os, Ir, and Rh) as bioimaging probes, including their photophysical properties, cytotoxicity, and cellular uptake mechanisms [2], and on transition metals (Ir, Re, Ru) in fluorescent cell imaging applications such as uptake and toxicity [4], respectively. 

Luminescent organometallic complexes used for cell imaging can be divided in two main families, namely (i) carbonyl complexes, most commonly octahedral rhenium c-tricarbonyl derivatives incorporating a polypyridine ligand and (ii) cyclometalated complexes such as iridium and platinum derivatives containing the ubiquitous 2-phenylpyridine unit (complexes 1-3 in Scheme 11.2). 

Scheme 11.2 Structure of the three main types of luminescent organometallic complexes used for cell imaging. 1 fac-[Re(CO)3(NAN)L] type, with N N = any...

In addition to the specific and tunable photophysical properties of organometallic complexes, many other physicochemical features make them very appealing luminescence probes for cell imaging. First, it should be pointed out that excited states of organometallic complexes are particularly stable toward degradation to nonemissive species, thus low photobleaching is alvrays observed with these molecules. 

The history of organometallic complexes suitable for cell imaging goes back to 2004 with the synthesis by Valliant and coworkers of the c-rheniirm tricarbonyl bispyridyl complex 4 (Scheme 11.3) [48,49]. Like most, if not all, of the rhenium mononuclear complexes used as luminescent agents for cell imaging, this complex contains a c-Re(CO)s unit and possesses the general formula of l /flc-[Re(CO)3(N N)L]. 

Scheme 11.7 Luminescent Ir(lll) and Rh(lll) complexes used in cell imaging (PFe counterion omitted for clarity).

Only two rhodium complexes, 38 and 39, have been employed for cell imaging and their behavior resembles that of their iridium counterparts. In particular, the monocationic rhodiimi(III) cyclometalated, [Rh(pba)2(bpy-Et)] complex 38 (Scheme 11.7) showed weak emission in fluid solutions at 298 K but more intense luminescence in glass matrix at 77 K [110]. The emission is attributed to a triplet intraligand ( IL)(3t -> 7t )(pba) excited state. This complex was internalized in HeLa cells after incubation at 37 °C for 24 h and found localized in the cytoplasm, without significant uptake in the nucleus. 

Scheme 11.8 Molecular structure of luminescent Pt(ll) complexes for cell imaging.

Shen JM, Xu L, Lu Y et al (2012) Chitosan-based luminescent/magnetic hybrid nanogels for insulin delivery, cell imaging, and antidiabetic research of dietary supplements. Int J Pharm 427 400-409... 

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