**3. Ru(II)-polypyridyl linked DPA chelate based sensors**

Because of the strong coordinating affinity of N donor atoms of DPA (bis- (pyridin-2-ylmethyl)amine) unit with Zn2+ and Cu2+ ion, DPA tethered luminophores are gaining increasing interest in this research area. In year 2011, Zhang et al. constructed [10] a luminescent probe **3** having a free terminal dipicolylamine or DPA unit to detect Cu2+ ions (**Figure 2**). The emission signal of probe **3** at 612 nm was drastically quenched upon introducing 1.0 equiv. Cu2+ into an aqueous solution of probe **3** (10 mM HEPES buffer solution; pH 7.2). Job's plot data analyses displayed the formation of complex **3**-Cu2+ with 1:1 stoichiometric ratio (**Figure 2**). Furthermore, non-luminescent complex **3**-Cu2+ became emissive in the presence of sulfide ions. In the presence of S2− ions, Cu(II) ion is effectively removed from **3**-Cu2+ to form a stable CuS species which ultimately led to a turn-on fluorescence response.

Xianghong et al. reported a Ru(II)-based probe **4** containing two DPA units as receptors for Cu2+ ions (**Figure 2**) [11]. The absorption and emission spectral changes observed after Cu2+ addition with **4** clearly indicated the coordination of Cu2+ with DPA moieties of complex **4**. The luminescence intensity of probe **4** at 630 nm was quenched upto a significant extent when Cu2+ was successively added in ethanol solution of probe **4**. Job's plot analyses revealed the formation of **4**-Cu2+ with 1: 2 ratio which has also been corroborated with mass spectral data. Probe **4** exhibited a selective detection of Cu(II) over other cations with a binding constant

**Figure 2.** *Chemical drawing of probes* **3–4** *with proposed binding of* **3***-Cu2+.*

**Figure 3.** *Chemical drawing of probes 5–6 with their proposed binding to Cu2+ ions.*

value of 5.89 × 104 M−1. The selective recognition of Cu(II) has been attributed to the high thermodynamic affinity of this metal ion towards N and O coordinating sites.

Liu et al. designed [12] a DPA tethered Ru(II) luminophore (**5**) which serves as an excellent luminescent probe for Cu2+ ion detection in pure water (**Figure 3**). The luminescence emission of probe **5** has been selectively quenched in the presence of Cu2+ among various other cations. An appreciable water solubility and usage in wide pH range make probe **5** a potential candidate for practical applications. The LOD value of **5** for Cu2+ has been calculated as 1.55 × 10−7 M. The DPA chelate of probe **5** coordinated to the copper centre through N3 atoms and form a nonluminescent **5**-Cu2+ complex.

Recently, an imidazo-phenanthroline linked Ru(II) complex **6** with DPA as terminal binding site has been reported by Arora et al. (**Figure 3**) [13]. Probe **6** serves as selective and phosphorescent sensor for recognition of Cu2+ metal ion in aqueous medium. The addition of Cu2+ to probe **6** leads to coordination, as evidenced from the adequate quenching in emission signal of probe **6** at 615 nm. Probe **6** also acted as a colorimetric sensor towards Cu**2+** ions in aqueous solution as the red-orange color of **6** was turned to light yellow (visible to naked eyes) upon adding Cu2+ ions to it. The Job's plot data, LOD (1.89 M) and association constant (1.14 × 105 M−1) values exhibited a 1: 1 complex formation of Cu2+ with probe **6**. Copper(II) selectivity of **6** is barely affected in the presence of other metal ions and biological targets such as amino acids and glucose. The emission of probe **6** was recovered when a sodium salt of EDTA was added to the non-luminescent complex **6**-Cu2+.

### **4. Ru(II)-polypyridyl linked macrocyclic chelate based sensors**

Macrocycles are particulary attractive classes of compound in different research areas because of their relative ease of functionalization and the availability of a central cavity with different conformations and sizes. Depending on the size of their macrocyclic crown, these compounds exhibit strong binding to various alkali and/or transition metal ions. A number of macrocyclic compounds have found applications and uses in sensing and other fields.

Paul et al. developed [14] a luminescent probe **7** containing a macrocyclic receptor for detection of Cu2+ ion in acetonitrile solution (**Figure 4**). Probe **7** displayed a typical UV–visible spectrum with absorption maxima at 453 nm (attributed to

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

**Figure 4.** *Chemical drawing of probes 7–10 with their proposed binding to Fe3+ ion.*

MLCT). Upon excitation at 460 nm, probe **7** exhibited an emission response at 603 nm. Successive addition of Cu2+ to CH3CN solution of **7** resulted in a significant quenching of emission intensity. A strong affinity of coordinating sites (N and O donor atoms) available in receptor unit towards Cu2+ ion is favorable for appreciable binding.

Two novel Ru(II)-based fluorescent probes **8** and **9** having terminal NS2O3 macrocyclic rings as metal ion receptor were reported by Boricha et al. in 2012 (**Figure 4**) [15]. Probes **8** and **9** exhibited the characteristic absorption bands near 454 nm due to a MLCT transition and an emission signal in the range of 602–632 nm in acetonitrile solution. Addition of Cu2+ to probe **8** leads to the binding as evidenced by 87% luminescence quenching in emission intensity. On the other hand, addition of Fe3+ yielded a quenching in emission signal of probe **8** upto 96%. Probes **8** and **9** also displayed strong interactions with soft metal ions such as Pb2+ and Hg2+ ions. The presence of S atoms in the macrocyclic rings facilitated the affinity of these sensors towards soft acids. Probes **8** and **9** showed highest selectivity with Fe3+ ion and form hexa-coordinated complexes **8**-Fe3+ and **9**-Fe3+.

For comparison, another structurally similar probe **10** containing a macrocyclic ring with NO5 donors has also been developed (**Figure 4**) [15]. Interestingly, probe **10** served as a highly selective sensor for the detection of only Cu2+ ions over other metal ions, and a binding constant of 9.51 x 102 M−1 has been reported in this case. Replacement of soft donor S atoms with hard donor N atoms in the macrocycilc ring resulted with the selectivity enhancement of probes.

Due to their strong binding affinity towards metal ions and appreciable water solubility, cyclen (1, 4, 7, 10-tetraazacyclododecane) based derivatives have gained huge attraction in the research areas of chemistry and biology. The metal ion binding with cyclen unit induces a perturbation in electronic structure which results in the change of photophysical properties of luminophores.

A cyclen tethered luminescent probe **11** has been designed and synthesized by Li and group [16] for Cu2+ ion detection in pure water (**Figure 5**). Probe **11** exhibited classical UV–visible and emission spectra with absorption maxima at 450 nm and emission maxima at 604 nm. Upon addition of 1.0 equiv. of Cu2+ ions, the emission intensity was quenched to significant amount. Interaction between probe **11** and Cu2+ were believed to entail 1: 1 complex formation which is consistent with the availability of only one receptor per luminescent probe. Probe **11** was found suitable for Cu2+ detection in pH range of 5–11, and the binding constant value was

**Figure 5.** *Chemical drawing of probes 11–12 and their proposed binding to Cu2+ ion.*

calculated as 2.36 × 104 M−1. The strong Cu binding of **11** has been attributed to high thermodynamic stability and huge formation constant value of ensemble **11**-Cu2<sup>+</sup> . Moreover, probe **11** displayed a off–on–off emissive response with an alternative addition of Cu2+ and S2− ions in water.

Ye et al. developed another cyclen unit based fluorescent probe **12** to prepare a complex **12**-Cu2+ (**Figure 5**) [17]. Non-luminescent complex **12**-Cu2+ was used to selectively detect sulfide ions under physiological conditions. Upon excited with 450 nm light, probe **12** showed a luminescence response at 605 nm. The redorange luminescence of probe **12** was significantly quenched with the addition of 10 μM Cu2+ ions in HEPES buffer solution. The interaction between **12** and Cu2+ are believed to involve 1:1 complex formation as evidenced by Job's plot and mass spectral analyses. The luminescence intensity of **12** has been almost completely recovered by treating H2S with the non-emissive complex **12**-Cu2+.
