**6. Ru(II)-polypyridyl linked triazole chelate based sensors**

In recent years, 1,2,3-triazole based synthetic receptors have been considered to be an excellent motif for recognition of different analytes [25]. As N-atom containing Lewis bases, the triazole-based derivatives display strong metal ion binding properties and have been employed in different areas of research.

Triazole Ramachandran et al. constructed [26] a luminescent probe **23** containing a benzothiazole unit connected to RuII(bpy)3 luminophore through a triazole linker (**Figure 9**). The probe was found highly selective towards Cu2+ ion detection and the red-orange emission of **23** at 630 nm was quenched upto 80% with addition of Cu2+ in HEPES buffer solution of pH 7.4. A 1:1 ratio of Cu binding with **23** has been confirmed with the help of Job's plot and ESI-mass spectral analyses. As evidenced by selectivity studies, other cations hardly affect the sensing ability of probe **23** towards Cu2+ ion. The binding constant and limit of detection values were in order of 5.11 x 104 M−1 and 7.00 x 10−7 M respectively.

The same group reported a dinuclear RuII(bpy)3 based luminescent probe **24** which contain a *p*-tert-butylcalix[4]arene fragment as receptor for Cu2+ ions (**Figure 9**) [27]. As indicated by phosphorescence based titration experiments, probe **24** was found effective and selective Cu2+ ion sensor with a *turn-off* emission signal at 637 nm. Binding of Cu2+ was evidenced by the large decrease in luminescence intensity of probe **24**. The strong interaction of calixarene based receptor with Cu2+ is believed to involve 1:1 formation of **24**-Cu2+ with a binding constant value of order 2.31 x 104 M−1. The red-orange luminescence of **24** was revived when S2− ions were added to complex **24**-Cu2+. Due to a very low cytotoxicity of probes **23** and **24**, cell imaging experiments have also been performed using these probes in human lung cancer A549 and MCF-7 cell lines.

**Figure 9.** *Chemical drawing of probes* **23–24** *and their proposed binding to Cu2+ ions.*
