The release of cations and anionic pollutants due to industrial and farming practices has increased threats to human health and the environment. To have sensitive, specific ion sensors is a major goal. Various methods have been developed for ion detection, including flame atomic absorption spectroscopy, inductively coupled plasma emission spectroscopy, etc. However, the cost of these methods is too high yet the sensitivity is not high enough. Hence, different sensors have been explored for the determination of ions to overcome the previously mentioned limitations of these methods. Coordination complexes have been explored successfully for the determination of diverse ions. For example, Schiff bases are generally known as imines or azomethines and act as ligands in various metal complexes. To date, Schiff base complexes have been primarily explored as optical metal ion sensors. The signals of these sensors exhibited strong dependency upon the interactions between ions and Schiff bases.
Recently, a large class of ion sensors based on coordination complexes has been developed. These sensors present a good selectivity for ions, which is of great importance for both biological and environmental fields.
The [Ru(bpy)3]2+ unit is known to display unique photophysical and redox properties. In 1993, Beer and co-workers  prepared the first ruthenium (II) complexes for anion sensors utilizing the [Ru(bpy)3]2+ unit as a chromophore and bipyridyl ligands bearing amide substituents as the anionic recognition site. Later, a series of [Ru(bpy)3]2+-based complexes (Figure 1) containing triarylboron groups has been synthesized for anion detection . The addition of fluoride or cyanide ions caused a 10-40 nm blue-shift in the emission of the complexes to 585-600 nm, consistent with the change of the boryl group from electron-withdrawing to electron-donating upon anion binding. The shift in phosphorescence wavelength upon analyte binding could be potentially exploited for the ratiometric detection of anions.
Figure 1. Boryl-functionalized Ru (II) complexes as anion detection
Similar to anion sensors, cation sensors based on coordination complexes have been developed rapidly. For example, Guerchais and co-workers  reported the use of the [Ir(ppy)2(bpy)] + complex appended with two π-conjugated DPA moieties for the sensitive detection of Zn2+ ions. The iridium complex is highly luminescent in the absence of analyte due to intense emission from the mainly ligand-centered 3IL state localized on the styryl-substituted bpy ligand. Both the absorption and emission of iridium complex were significantly altered upon the addition of Zn2+ ions, but not with other divalent cations. The presence of Zn2+ ions specifically perturb the excited state, giving rise to a blue-shifted absorption and emission, and a shorter luminescence lifetime.
Figure 2. The iridium (III) complex and its ORTEP plot
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- Wang, S.; et al. Tuning and switching MLCT phosphorescence of [Ru(bpy)3]2+ complexes with triarylboranes and anions. Inorg. Chem. 2011, 50: 3373-3378.
- Guerchais. V.; et al. Modulating the luminescence of an iridium (III) complex incorporating a di(2-picolyl) anilino-appended bipyridine ligand with Zn2+ cations. New J. Chem. 2010, 34: 21-24.