The pH has key role in many physical and chemical reactions correspond to environmental, oil and gas industry, biochemistry, biotechnology, and biomedical research. Efficient and rapid measurement of pH values has become a hot topic. The measurement of pH by luminescent sensors has recently attracted a lot of attention. This is mainly due to its electrical safety, high accuracy, and the fact that it does not require a reference element. Changes in absorbance or luminescence of certain indicator molecules indirectly indicate changes in pH. The major limitation of most pH sensors is the narrow pH range covered. Recently, there has been considerable interest in extending the use of pH sensors in industrial and hazardous environments and the development of pH sensing dyes with wider working range of pH values is necessary. The simplest method of obtaining direct pH sensing is to attach a pH-sensitive group to a metal complex so that its emissive properties are altered as the pH changes. One class of pH-sensitive metal complexes is Ru(α-diimine)2L, where α-diimine has typically been a 2,2'-bipyridine, 1,10-phenanthroline, or substituted analogue and L is an α-diimine with pyridine, carboxylic acid, amine, or phenol substituents. The substituents can be electronically separated from the ligand by a methylene group or electronically coupled directly to the ligand. Other similar complexes are the Os2+ and Re2+ complexes. Complexes with these metals are normally luminescent, photochemically robust, exhibit either near UV or visible absorption, and can be "tuned" by a variety of synthetic methods.
The complexes applied in pH sensing are usually transition metal complexes. In addition to the signal-to-noise advantages inherent in all luminescent measurements, transition metal complexes have some photophysical properties. These properties include significant stokes shifts for easy separation of excitation and emission, emission color shifts with changes in the local environment, and relatively long lifetimes compared to their purely organic counterparts. These advantages make the application of luminescent transition metal complexes in pH sensors have great potential.
Cyclometalated iridium (III) complexes have received considerable attention due to their high phosphorescent quantum yields, excellent color-tuning capability, larger Stokes’ shifts, and relatively longer lifetimes. For example, Aoki and co-workers  reported the synthesis and pH-dependent photochemical properties of a series of pH-activatable Ir (III) complexes, based on the predicted pKa values of the corresponding aniline derivatives. These Ir complexes show a red emission, and their emission intensity is enhanced to a considerable extent upon the protonation of their basic groups in aqueous solution. Taking the Ir complex shown in Figure 1 as an example, it was found that the emission of this complex is nearly nonexistent at pH > 5, and a red emission is observed at pH < 5.
Figure 1. Photograph showing solutions of Ir complex in degassed DMSO/100 mM buffer (from pH 2 to 8) at 25°C. Excitation at 365 nm
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- Aoki, S.; et al. Photochemical properties of red-emitting tris(cyclometalated) iridium (III) complexes having basic and nitro groups and application to ph sensing and photoinduced cell death. Inorganic Chemistry. 2015, 54: 5342-5357.