Lanthanide complexes as redox and ROS/RNS probes: A new paradigm that makes use of redox-reactive and redox non-innocent ligands

Abstract

Lanthanides complexes are indispensable in fields related to medicine and biology. Their success lies in the great number of unpaired electrons (4f7 configuration of the Gd3+ ion) that makes them ideal contrasts agents for magnetic resonance imaging. On the other hand their narrow emission bands and long lifetime of their excited state provide them unique luminescence properties. The aqueous solution chemistry of the lanthanides is dominated by the (+III) oxidation state under oxygenated atmosphere. This severely limits their use as redox probes, and not surprisingly very few redox-switches employing lanthanide ions were reported until 2010. A very promising approach based on redox-reactive and redox non-innocent ligands has recently emerged in the literature, which overcomes this limitation and expands the application of lanthanides to redox monitoring. Herein the ligand acts as a redox-sensor by either changing its oxidation state or reacting with reactive oxygen (or nitrogen) species (ROS/RNS). Its response induces changes in the environment of the lanthanide ion, which acts as a reporter of the redox status. The detection is based on a modification of the magnetic or optical properties of the complexes, with detection by conventional spectroscopic techniques. We summarize in this review article the recent advances in this burgeoning field, with special emphasis on the detection of biological relevant ROS, RNS and redox status.