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(Ph2Phen)2Dcbpy]Cl2 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 +/- 0.01 mu s and 0.38 +/- 0.02 mu s and ionised species 0.598 +/- 0.001 mu s and 0.61 +/- 0.08 mu s. 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 +/- 0.04 mu s and ionised species 1.16 +/- 0.04 mu s. 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. (C) 2008 Elsevier B.V. All rights reserved.