Detection of Bio-Relevant Metal Ions by Luminescent Ru(II)-Polypyridyl Based Sensors

Abstract

Biorelevant metal ions such as Cu2+ and Fe2+/Fe3+ participate in various biological events which include electron transfer reactions, delivery and uptake of oxygen, DNA and RNA syntheses, and enzymatic catalysis to maintain fundamental physiological processes in living organisms. So far, several analytical techniques have been investigated for their precise detection; however, luminescence-based sensing is often superior due to its high sensitivity, selectivity, fast and easy operation and convenient cellular imaging. Owing to their immense photophysical and photochemical properties stemming from large Stokes shift, absorption in visible region, good photostability and long excited state lifetimes, Ru(II)-polypyridyl-based complexes have gained increasing interest as luminophores. Over past few decades, several Ru(II)-polypyridyl based chemosensors have rapidly been developed for detection of different biorelevant and other metal ions. The main object of this book chapter is to cover a majority of Ru(II)-polypyridyl based chemosensors showing a selective and sensitive detection of bio-relevant Cu2+ and Fe2+/Fe3+ ions. The photophysical properties of Ru(II) complexes, detection of metal ions, sensing mechanism and applications of these sensors are discussed at a length.

1. Introduction

The aim of this chapter is to familiarize readers about the luminescent sensing applications of Ru(II)-polypyridyl fragment based chemical systems for the detection of bio-relevant metal ions. Biorelevant metal ions such as Cu²⁺ and Fe²⁺/Fe³⁺ participate in various biological events which include electron transfer reactions, delivery and uptake of oxygen, DNA and RNA syntheses and enzymatic catalysis [1, 2]. Ru(II)-polypyridyl complexes have been considered as ideal phosphorescent chemosensors due to their distinguished photochemical and photophysical properties such as absorption in visible region, emission in long wavelength red and near-infrared regions, long lifetimes of excited state, redox- and photo-stability [3]. The UV–visible spectrum of this system displays several interesting features such as ligand centered (π → π*) transitions at high energy (185–285 nm), two weak signals between 322–344 nm, and most intense peak near λ_max 450 nm which is attributed to the MLCT (metal to ligand charge transfer) transition [4, 5]. The Ru(II)-polypyridyl centre worked as excited state redox active agent in electron transfer processes, and showed very good emission properties [4, 5]. Ru(II)-polypyridyl complexes are classical luminophores showing excitation at 450–470 nm and wide emission bands centred at 600–620 nm. In general, three bidentate (bipyridine/phenanthroline) or two tridentate (terpyridine) ligands have been employed to prepare Ru(II)-polypyridyl chemosensors which exhibit outstanding optical and electrochemical properties. The focus of this chapter is to illustrate the chemical versatility of such chelating systems and their utilization in the detection of different analytes. Over the past few decades, the investigation into the salient properties of ruthenium (II)-polypyridyl complexes has turned out to be a major research area which stems especially from their appealing photochemical and photophysical properties [6, 7]. The next few sections have collected selected examples where an appropriate category of receptors based on Ru(II)-polypyridyl fragment has been selected for showcasing a particular theme.

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⁺.

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.

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.

Figure 2.

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

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 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 non-luminescent 5-Cu²⁺ complex.

Figure 3.

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

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 Cu²⁺ 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 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.

Figure 4.

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

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 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⁺. Moreover, probe 11 displayed a off–on–off emissive response with an alternative addition of Cu²⁺ and S²⁻ ions in water.

Figure 5.

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

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 red-orange 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⁺.

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.

Figure 6.

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

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.

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 ¹H 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.

Figure 8.

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

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.

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.

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 Ru^II(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.

Figure 9.

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

The same group reported a dinuclear Ru^II(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.

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.

Figure 10.

Chemical drawing of probes 25–28.

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.

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. 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.

Figure 11.

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

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.

9. Conclusions

The present chapter covers a majority of luminescent Ru(II)-polypyridyl based chemosensors for the selective recognition of biorelevant metal ions such as Cu²⁺ and Fe²⁺/Fe³⁺ ions. The sensing behavior of different chemosensors varying from mono- to di-nuclear Ru(II)-polypyridyl complexes has been considered and discussed at a length. The applications encompass many fields including environmental, biological, analytical and medicinal domains. This field of luminescence sensing is quite prosperous and still emerging. Taking advantage of already known ligands topology exploiting their selective binding properties towards a particular metal ion, several chemosensors are developed. The design, detection, mechanisms and applications for different sensors presented in this chapter create huge opportunities for the development of future chemosensors.

Acknowledgments

PK thanks Mahamana Malviya College Khekra to provide infrastructure. SK acknowledges Department of Science and Technology (DST), New Delhi for financial support in the form of Inspire Faculty Award [DST/INSPIRE/04/2015/002971].

Conflict of interest

Authors have no conflict of interest.