**5. Ru(II)-polypyridyl linked imidazole chelate based sensors**

Luminescent Ru(II)-polypyridyl complexes linked with 2-hydroxyphenylimidazo unit are gaining increasing interest in the monitoring and detection of copper ions owing to the strong binding affinity and straight coordination of 2-hydroxyphenylimidazo unit. Zhang et al. constructed [18] a luminescent probe **13** containing 2-hydroxyphenylimidazo moiety which acts as highly selective sensor for Cu2+ ion recognition in aqueous media (**Figure 6**). Upon exciting at 467 nm light under physiological conditions, probe **13** displayed an emission spectrum with emission maxima at 585 nm. Probe **13** provides two donor (N, O) sites to link with Cu2+ ion to form complex **13**-Cu2+ in 1:1 binding stoichiometry.

Later, Zheng's group introduced [19] another 2-hydroxyphenylimidazo based luminescent probe **14** for highly selective and effective detection of Cu2+ ions in CH3CN-HEPES buffer solution of pH 7.2 (**Figure 6**). To confirm the Cu2+ binding with terminal 2-hydroxyphenylimidazo of **14**, the absorption and emission spectral changes have been observed. Introduction of Cu2+ leads to coordinate with receptor, as evidenced by the quenching in the emission intensity of probe **14**. A strong interaction of 2-hydroxyphenylimidazo moiety with Cu(II) (**14**-Cu2+) is validated with a binding constant value of 1.09 × 105 M−1.

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

**Figure 6.**

*Chemical drawing of probes 13–15 and their proposed binding to Cu2+ ion.*

Recently, a novel quinoline-tethered Ru(II)-based luminescent probe **15** has been developed by Kumar et al. (**Figure 6**) [20]. Probe **15** displayed an absorption maxima at 470 nm and emission intensity at 604 nm. Addition of Cu2+ in the solution of probe **15** leads to the binding as evidenced by the measurement of UV–visible and emission spectral changes. A bathochromic shift in absorption wavelength at 470 nm and appearance of new band between 620 nm to 720 nm (Cu based *d-d* transition) indicated the coordination of Cu2+ with probe **15**. A large decrease in emission intensity at 604 nm has also been only in the presence of Cu(II), over other cations. The LoD and binding constant values are calculated as 5.07 x 10−8 M and 5.00 x 105 M−1. Interaction of probe **15** with Cu2+ is believed to entail 1:1 formation of complex **15**-Cu2+.

In a very recent report, Song and group reported [21] a luminescent probe **16** containing a terminal pyrozole fragment connected with Ru(II) luminophore *via* imidazole linker (**Figure 7**). Probe **16** proved to be fast and highly selective fluorescent chemosensor for Cu2+ ions in aqueous buffer solution (pH 7.4). The emission intensity of probe **16** at 621 nm (*λ*ex 460 nm) was adequately quenched upon introducing paramagnetic Cu2+ ion with a detection limit of 8.33 x 10−8 M. Depending on the pH of probe's solution, a protonation-deprotonation process of N atoms of imidazole fragment has also been experienced. Job's plot analyses demonstrated the formation of complex **16**-Cu2+ with a 1:1 binding ratio. The formation of **16**-Cu2+ was also confirmed by broadening in resonances and disappearance of NH signals in 1 H NMR spectrum of **16** on adding Cu2+ ions. The red luminescence of probe **16** was regenerated with addition of an excess of EDTA to complex **16**-Cu2+.

**Figure 7.** *Chemical drawing of probes* **16***–***17** *and oxidative cyclization of* **17** *to* **18** *by Cu2+ ion.*

In 2015, Zhang et al. described [22] the synthesis of *o*-(phenylazo)aniline based non-luminescent probe **17** for sensing of Cu2+ ions with a emissive switch-on response (**Figure 7**). Addition of Cu2+ to probe **17** caused the oxidative cyclization of probe **17** to produce a highly luminescent complex **18** containing a benzotriazole fragment. Probe **17** was found completely soluble in water and exhibited an appreciable photostability in presence of light. The cyclization of *o*-(phenylazo) aniline moiety could easily be performed by introducing only 1 equiv. of Cu(II). Addition of 1 equiv. of Cu resulted with the large increase in luminescence intensity of **18**. The Cu sensing by probe **17** is unique as Cu2+ is only participating in cyclization reaction but does not coordinate with the receptor. Probe **17** was found highly selective for Cu2+ in the presence of various cations, with a detection limit of 4.42 × 10−9 M. Probe **17** has also been employed to detect Cu2+ in live-pea aphids with a switch-on emissive signal.

In 2013, Chao and co-workers reported [23] a dinuclear Ru(II)-based luminescent probe **19** for sensitive and selective detection of Cu2+ ions (**Figure 8**). Upon addition of Cu2+ ions (2.0 equiv.) into probe solution, the emission intensity at 600 nm was significantly quenched (96%). A naked eye color change could also be observed under UV light exposure. Binding of Cu with **19** is reported to involve 1:1 complex formation as evidenced by ESI-MS, NMR and EPR measurements, and the detection limit is computed to be 3.33 × 10−8 M. The luminescence of **19**-Cu2+ was recovered with an excess addition of EDTA to the mixture of Cu2+ and probe **19**. The selectivity studies clearly demonstrated no interference of other cations in sensing of probe **19** towards Cu2+ ions.

Cheng et al. developed [24] two dinuclear ruthenium complexes **20** and **21** for the luminescence based recognition of Cu2+ ions (**Figure 8**). Investigation of luminescence properties of these probes indicated higher emission response for probe **20** at 609 nm compared to probe **21**. Probe **20** detected Cu2+ selectively over other cations, and the luminescence of this probe was almost completely quenched in presence of Cu(II) ion. Nitrogen atoms from imidazole fragments and oxygen atom of furan participated in coordination to form complex **20**-Cu2+ with 1:1 ratio of binding. It is noteworthy that probe **21** displayed an increase in luminescence in various metal ion with no metal ion selectivity. The enhancement in emission intensity of probe **21** is attributed to the disturbance of photoinduced electron transfer process as the lone pair on S donor site becomes unavailable after metal ion coordination.

**Figure 8.** *Chemical drawing of probes* **19–22** *and proposed binding of* **19** *to Cu2+ ion.*

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

Zheng et al. developed [19] a 2-hydroxyphenylimidazo based luminescent probe **22** for Cu(II) ion recognition in aqueous buffer solution (pH 7.2; HEPES) containing 1% acetonitrile (**Figure 8**). To validate the binding of Cu2+ with probe **22**, the absorption and emission spectral changes were investigated. Binding of Cu2+ with 2-hydroxyphenylimidazo fragment leads to a significant quenching in emission intensity of probe **22**. This probe showed a ON–OFF–ON emissive response with an alternative addition of Cu2+ and CN− ions. The detection limit and the association constant for Cu ion sensing by **22** were calculated as 3.77 × 10−7 M and 4.31 × 104 M−1 respectively.
