**Abstract**

Biorelevant metal ions such as Cu2+ and Fe2+/Fe3+ participate in various biological events which include electron transfer reactions, delivery and uptake of oxygen, DNA and RNA syntheses, and enzymatic catalysis to maintain fundamental physiological processes in living organisms. So far, several analytical techniques have been investigated for their precise detection; however, luminescence-based sensing is often superior due to its high sensitivity, selectivity, fast and easy operation and convenient cellular imaging. Owing to their immense photophysical and photochemical properties stemming from large Stokes shift, absorption in visible region, good photostability and long excited state lifetimes, Ru(II)-polypyridylbased complexes have gained increasing interest as luminophores. Over past few decades, several Ru(II)-polypyridyl based chemosensors have rapidly been developed for detection of different biorelevant and other metal ions. The main object of this book chapter is to cover a majority of Ru(II)-polypyridyl based chemosensors showing a selective and sensitive detection of bio-relevant Cu2+ and Fe2+/Fe3+ ions. The photophysical properties of Ru(II) complexes, detection of metal ions, sensing mechanism and applications of these sensors are discussed at a length.

**Keywords:** Ru(II)-polypyridyl, phosphorescence, sensing, biorelevant, metal ions

## **1. Introduction**

The aim of this chapter is to familiarize readers about the luminescent sensing applications of Ru(II)-polypyridyl fragment based chemical systems for the detection of bio-relevant metal ions. Biorelevant metal ions such as Cu2+ and Fe2+/Fe3+ participate in various biological events which include electron transfer reactions, delivery and uptake of oxygen, DNA and RNA syntheses and enzymatic catalysis [1, 2]. Ru(II)-polypyridyl complexes have been considered as ideal phosphorescent chemosensors due to their distinguished photochemical and photophysical properties such as absorption in visible region, emission in long wavelength red and near-infrared regions, long lifetimes of excited state, redox- and photo-stability [3]. The UV–visible spectrum of this system displays several interesting features such as ligand centered (π → π\*) transitions at high energy (185–285 nm), two weak signals between 322–344 nm, and most intense peak near λmax 450 nm which is attributed to the MLCT (metal to ligand charge transfer) transition [4, 5]. The Ru(II) polypyridyl centre worked as excited state redox active agent in electron transfer

processes, and showed very good emission properties [4, 5]. Ru(II)-polypyridyl complexes are classical luminophores showing excitation at 450–470 nm and wide emission bands centred at 600–620 nm. In general, three bidentate (bipyridine/ phenanthroline) or two tridentate (terpyridine) ligands have been employed to prepare Ru(II)-polypyridyl chemosensors which exhibit outstanding optical and electrochemical properties. The focus of this chapter is to illustrate the chemical versatility of such chelating systems and their utilization in the detection of different analytes. Over the past few decades, the investigation into the salient properties of ruthenium (II)-polypyridyl complexes has turned out to be a major research area which stems especially from their appealing photochemical and photophysical properties [6, 7]. The next few sections have collected selected examples where an appropriate category of receptors based on Ru(II)-polypyridyl fragment has been selected for showcasing a particular theme.
