Application of NIR (near-infrared) emitting transition metal complexes in biomedicine is a rapidly developing area of research. Emission of this class of compounds in the “optical transparency windows” of biological...

NIR emitting Ir(iii) complexes decorated with oligo(ethylene glycol) were used to assess the degree of hypoxia in biosamples.

Synthesis of biocompatible near infrared phosphorescent complexes and their application in bioimaging as triplet oxygen sensors in live systems are still challenging areas of organometallic chemistry. We have designed and synthetized four novel iridium [Ir(N^C)2(N^N)]+ complexes (N^C–benzothienyl-phenanthridine based cyclometalated ligand; N^N–pyridin-phenanthroimidazol diimine chelate), decorated with oligo(ethylene glycol) groups to impart these emitters’ solubility in aqueous media, biocompatibility, and to shield them from interaction with bio-environment. These substances were fully characterized using NMR spectroscopy and ESI mass-spectrometry. The complexes exhibited excitation close to the biological “window of transparency”, NIR emission at 730 nm, and quantum yields up to 12% in water. The compounds with higher degree of the chromophore shielding possess low toxicity, bleaching stability, absence of sensitivity to variations of pH, serum, and complex concentrations. The properties of these probes as oxygen sensors for biological systems have been studied by using phosphorescence lifetime imaging experiments in different cell cultures. The results showed essential lifetime response onto variations in oxygen concentration (2.0–2.3 μs under normoxia and 2.8–3.0 μs under hypoxia conditions) in complete agreement with the calibration curves obtained “in cuvette”. The data obtained indicate that these emitters can be used as semi-quantitative oxygen sensors in biological systems.

Near-infrared (NIR) molecular emitters based on transition-metal complexes have attracted growing attention due to their potential application for in vivo and in vitro bioimaging experiments. Their photophysical characteristics (large Stokes shift and lifetime in the microsecond domain) offer some important advantages in comparison to organic fluorophores and may provide better imaging resolution and higher sensitivity: for example, in mapping the oxygen concentration in biological objects. We have synthesized a series of [Ir(N ∧ C) 2 (N ∧ N)] + complexes with emission in the NIR region (N ∧ C = (2-benzothienyl)phenanthridine and 6-(2-benzothienyl)phenanthridine-2-carboxylic acid; N ∧ N = functionalized pyridine-triazole chelates), which also display a considerable red shift of their excitation spectra to the edge of the window of transparency. The flexible protocol for the synthesis of the N ∧ N ligands makes possible wide variations in the peripheral ligand environment: e.g., insertion of hydrophilic carboxyl group and further attachment of the other biologically relevant functions. The compounds obtained were completely characterized using spectroscopic methods, and their ground-state structures and photophysical properties were studied by DFT and TD DFT methods. To analyze the behavior of these emitters in biological systems, we investigated their interaction with human serum albumin (HSA), as the most abundant serum protein. It was found that these complexes readily form noncovalent {HSA−complex} adducts by embedding into hydrophobic cavities of this protein that also induced its partial aggregation. The complexes demonstrated preferential redistribution toward aggregated forms of HSA; the complex:HSA molar ratio did not exceed 1:3 for nonaggregated species. It was also shown that interaction of the hydrophobic complexes with albumin and the resulting aggregation dramatically change their important photophysical parameters such as emitter lifetime and its sensor response onto molecular oxygen.

Two NIR-emitting platinum [Pt­(N^N^C)­(phosphine)] and iridium [Ir­(N^C)­2­(N^N)]+ complexes containing reactive succinimide groups were synthesized and characterized with spectroscopic methods (N^N^C, 1-phenyl-3-(pyridin-2-yl)­benzo­[4,5]­imidazo­[1,2-a]­pyrazine, N^C, 6-(2-benzothienyl)­phenanthridine, phosphine-3-(diphenylphosphaneyl)­propanoic acid N-hydroxysuccinimide ether, and N^N, 4-oxo-4-((1-(pyridin-2-yl)-1H-1,2,3-triazol-4-yl)­methoxy)­butanoic acid N-hydroxysuccinimide ether). Their photophysics were carefully studied and analyzed using time-dependent density functional theory calculations. These complexes were used to prepare luminescent micro- and nanoparticles with the “core–shell” morphology, where the core consisted of biodegradable polymers of different hydrophobicity, namely, poly­(d,l-lactic acid), poly­(ε-caprolactone), and poly­(ω-pentadecalactone), whereas the shell was formed by covalent conjugation with poly­(l-lysine) covalently labeled with the platinum and iridium emitters. The surface of the species was further modified with heparin to reverse their charge from positive to negative values. The microparticles’ size determined with dynamic laser scanning varies considerably from 720 to 1480 nm, but the nanoparticles’ diameter falls in a rather narrow range, 210–230 nm. The species with a poly­(l-lysine) shell display a high positive (>30 mV) zeta-potential that makes them essentially stable in aqueous media. Inversion of the surface charge to a negative value with the heparin cover did not deteriorate the species’ stability. The iridium- and platinum-containing particles displayed emissions the spectral patterns of which were essentially similar to those of unconjugated complexes, which indicate retention of the chromophore nature upon binding to the polymer and further immobilization onto polyester micro- and nanoparticles for drug delivery. The obtained particles were tested to determine their ability to penetrate into different cells types: cancer cells, stem cells, and fibroblasts. It was found that all types of particles could effectively penetrate into all cells types under investigation. Nanoparticles were shown to penetrate into the cells more effectively than microparticles. However, positively charged nanoparticles covered with poly­(l-lysine) seem to interact with negatively charged proteins in the medium and enter the inner part of the cells less effectively than nanoparticles covered with poly­(l-lysine)/heparin. In the case of microparticles, the species with positive zeta-potentials were more readily up-taken by the cells than those with negative values.