Fiber optic lifetime pH sensing based on ruthenium(II) complexes with dicarboxybipyridine.

The complexes of ruthenium(II) with phenanthroline (Phen), diphenylphenanthroline (Ph(2)Phen) and with 4,4'-dicarboxy-2,2'-bipyridine acid (Dcbpy) ([Ru(Phen)(2)Dcbpy]Cl(2) and [Ru(Ph(2)Phen)(2)Dcbpy]Cl(2)) were synthesized and the variation of the correspondent fluorescence intensity and lifetime with the pH characterized. Luminescence intensity, emission wavelength and excited state lifetime all show a typical sigmoid variation with pH in the range 3-9, demonstrating the suitability of this complex for luminescence sensing applications. In aqueous solutions (28% ethanol) the complexes [Ru(Phen)(2)Dcbpy]Cl(2) and [Ru(Ph(2)Phen)(2)Dcbpy]Cl(2) show, respectively, the following properties: apparent pK(a) of 3.6+/-0.4 and 3.7+/-0.4; lifetimes of the protonated species 0.46+/-0.01micros and 0.38+/-0.02micros and ionised species 0.598+/-0.001micros and 0.61+/-0.08micros. The [Ru(Phen)(2)Dcbpy]Cl(2) complex was immobilised in the tip of optical fibers using a hybrid sol-gel procedure based on tetraethoxysilan and phenyltriethoxysilan enabling pH sensitive fiber probes. The immobilised complex shows the following lifetimes: protonated species 1.05+/-0.04micros and ionised species 1.16+/-0.04micros. These characteristics show that these ruthenium(II) complexes are good indicators for pH sensing, either in aqueous solution or immobilised in sol-gel, and are well suited for intensity and/or frequency domain interrogation.