**2. Ru(II)-polypyridyl linked terpyridine chelate based sensors**

Terpyridine (terpy) and its derivatives are the most frequently employed N-heterocyclic chelating agents which exhibit an exceptional binding ability for various metal ions. The typical cation binding area of terpy unit contains three nearly coplanar N atoms and its complexes have widely been used as signaling units at molecular and supramolecular levels.

In 2013, Wang and group developed [8] a Ru(II)-polypyridyl based luminescent sensor **1** containing a terminal terpyridine (terpy) moiety for Fe2+ ions recognition (**Figure 1**). The fluorescence emission studies of probe **1** were investigated in acetonitrile: HEPES buffer solution (1/71, v/v) of pH 7.2. Luminescence based titration of Fe2+ (0.5 equiv) with sensor **1** in acetonitrile solution clearly displayed a visible color change (light yellow to red-purple) with concomitant changes in emission and absorption spectra of probe **1**. The emission of probe **1** was quenched at 608 nm upon successive addition of Fe2+ ions in aqueous CH3CN solution. Probe **1** has exhibited excellent selectivity towards Fe2+ ions with a detection limit of 4.58 x 10−8 M and also served as a good colorimetric sensor for Fe2+ ion among other metal ions. A 2: 1 binding stoichiometry of Fe2+ with complex **1** has been found in accordance with the coordination of terminal terpy units with Fe2+ ion and formation of **1**-Fe2+ was confirmed by spectroscopic methods (**Figure 1**).

**Figure 1.** *Chemical drawing of probes* **1–2** *and proposed binding of* **1***-Fe2+.*

*Detection of Bio-Relevant Metal Ions by Luminescent Ru(II)-Polypyridyl Based Sensors DOI: http://dx.doi.org/10.5772/intechopen.96453*

The same group reported [9] another Ru(II)-polypyridyl based probe **2** bearing a dipyrazinylpyridine moiety in 2015 which exhibited a sensitive a selective detection for Cu2+ ion in presence of other metal ions (**Figure 1**). The UV–visible and emission spectral changes clearly revealed the coordination of Cu2+ ion with the neutral N donors of dipyrazinyl-pyridine moiety of sensor **2**. A significant quenching (upto 97%) in the luminescence intensity of probe **2** at 607 nm has been observed when 2.0 equiv. of Cu(II) ions were added to a CH3CN/HEPES buffer solution of probe **2**. The detection limit and association constant have been calculated as 2.73 × 10−6 M and 1.88 × 104 M−1 respectively, with a 1:1 binding stoichiometric ratio for complex **2**-Cu2+. The luminescence of probe **2** was almost regenerated when a solution of complex **2**-Cu2+ was treated with excess EDTA. Probe **2** could be used for Cu2+ detection by probe in a wide range of pH upto 3.0–7.0 as the luminescence of **2** was independent of pH in this range.
