**7. Ru(II)-polypyridyl linked carboxamide chelate based sensors**

The coordination behavior of carboxamide group with different transition metal ions has extensively been investigated and well documented in the literature. The rapid growth in the development of carboxamide based organic and inorganic synthetic receptors is due to the realization of their important roles in chemistry, catalysis, medicine and the biology. In the present section, we have highlighted few luminescent sensors containing carboxamide group in the framework of metal ion receptors.

A series of luminescent Ru(II)-polypyridyl based sensors **25–27** have been designed for efficient sensing of Cu2+ ions. Sensors **25–27** show a MLCT absorption band at 450 nm and emission band at 620 nm after excitation this MLCT band. (**Figure 10**) [28]. Two equivalents of Cu2+ ion was enough to quench the emission of **25–27** completely. To selectivity of **25** towards Cu2+ ion in presence of other metal ions was studied in different pH and found the best selectivity and quenching at pH 5 only.

Another carboxamide linked Ru(II)-polypyridyl based sensor **28**, was developed [29] by Gopidas and co-workers showed unique chemical oxidation properties and Turn-ON emission with Cu2+ ion in CH3CN (**Figure 10**). The emission intensity at 620 nm of **28** quenched by unique and fast electron transfer from the phenothiazine moiety to the Ru2+ core. Interestingly, the luminescence intensity of **28** enhanced by Cu2+ ion due to the oxidation of phenothiazine moiety Cu2+ ion. In presence of Cu2+ ion, phenothiazine is unable for emission quenching of Ru2+ centre.

**Figure 10.** *Chemical drawing of probes* **25–28***.*

### **8. Ru(II)-polypyridyl linked imine chelate based sensors**

Imine base ligands (Schiff bases) play important role in coordination chemistry due to their easy synthesis, high stability and insensitive properties towards air and moisture. Moreover, the electronic and steric features of these imine based ligands could easily be tuned by varying appropriate condensing partners.

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

These ligands bind through imine-N atom and display adequate structural flexibility and strong binding ability for various cations. Over the past decades, imine based derivatives are gaining increasing interest in the research area of electrochemical and optical sensing. In the present section, we have discussed luminescent Ru(II)-polypyridyl sensors containing an imine group in the metal ion binding site.

Kumar et al. described [30] a luminescent probe **29** containing a terminal thiophene unit linked with Ru(II)-polypyridyl based luminophore *via* imine bond (**Figure 11**). The water soluble probe having imine-N and thiphene-S coordinating sites detected Fe3+ ions through turn-off luminescence response at 615 nm, over other metal ions. However, a minor change in intensity has also been observed in the presence Cu2+ and Ag+ ions. A 1: 1 stoichiometry of complex **29**-Fe3+ was validated by Job's plot data and mass spectroscopic studies. The detection limit (LoD) and the binding constant for Fe3+ ion were computed as 0.11 ppm and 1.57 × 103 M−1 respectively. Moreover, the red-orange luminescence of probe 29 was restored with the addition of EDTA to a buffer solution of complex **29**-Fe3+. For practical applications on real samples, probe **29** has been investigated for paper strips, polystyrene films and live cell imaging experiments.

Zhang et al. reported [31] the formation of luminescent probe **30** which displays a switch-off emission response in presence of Cu2+ ions in aqueous medium (**Figure 11**). Probe **30** detects Cu2+ selectively over various other metal ions, except a marginal quenching observed in case of Zn2+ ions. Binding of Cu2+ with **30** was evidenced by a red shift observed in absorption wavelength and a significant decrease in emission intensity. The luminescence of probe **30** was revived by treatment of non-luminescent complex **30-**Cu2+ with L-histidine. A limit of detection of order 3.50 x 10−10 M and an association constant value of 4.44 × 103 M−1 were measured. The effect of pH on sensing ability of probe 30 has been established in this report. The resultant data from pH studies revealed that the most significant quenching occurs in mild basic conditions. The deprotonation of phenolic-OH in basic medium increases its coordinating ability to form non-luminescent complex **30-**Cu2+. Applications of probe **30** were investigated for imaging Cu2+ ions in live cells and real water samples.

**Figure 11.** *Chemical drawing of probes* **29***–***30** *and proposed binding of complexes* **29***-Fe3+/***30***-Cu2+.*
