A biocompatible phosphorescent Ir(iii) oxygen sensor functionalized with oligo(ethylene glycol) groups: synthesis, photophysics and application in PLIM experiments

Abstract

The synthesis of phosphorescent transition metal complexes with emission in the near infrared (NIR) region, appreciable quantum yields, compatibility with physiological media and suitability for application as effective oxygen sensors in biological systems is still a challenge in the field of synthetic organometallic chemistry. In addition to the sufficiently high requirements in photophysics, they also have to meet strict conditions for biocompatibility and selectivity of sensory responses in a multicomponent biological medium. Building on our previous research, we have synthesized three novel iridium [Ir(N^C)₂(N^N)]⁺ complexes (N^C – metalated ligands based on benzothienyl-phenanthridine core; N^N – pyridine-triazole diimine chelate), with their periphery decorated with 1, 2 or 3 relatively small “double-tail” oligo(ethylene glycol) (OEG) {–C(O)NHCH(CH₂OC₂H₄OC₂H₄OCH₃)₂} functions. All these complexes display desirable photophysical characteristics: excitation at the high energy limit of the “window of transparency”, NIR emission at ca. 710–720 nm, rather high quantum yields (13–15% in degassed methanol solution) and strong lifetime responses to variations in oxygen concentration. However, the study of their behavior in model biological systems (human serum albumin (HSA) and fetal bovine serum (FBS) solutions) by gel permeation chromatography and absorption/luminescence spectroscopy showed that only the complex bearing OEG functions at all three chelate ligands is compatible with the requirements of imaging experiments, i.e. it is water soluble and bleaching stable, has low toxicity and its sensory response is insensitive to the presence of the components of biological media, temperature, and pH. The sensing properties of this molecular probe were studied in detail and test imaging experiments in phosphorescence lifetime imaging (PLIM) mode were carried out on cancer cell cultures. We demonstrate that this probe is accumulated in the cell cytoplasm and exhibits low cytotoxicity at concentrations up to 125 μM. Phosphorescence lifetime imaging microscopy (PLIM) of live mouse colorectal cancer cells (CT26) and human oral squamous carcinoma (SCC-4) stained with the probe has shown lifetimes in the 1.35–2.51 μs interval with pronounced responses to oxygen under hypoxic conditions, thus providing the basis for qualitative detection of hypoxia and semi-quantitative oxygen concentration measurements.