Luminescent nafion membranes dyed with ruthenium(II) complexes as sensing materials for dissolved oxygen

The absorption spectroscopy, photophysics, and dioxygen quenching of [RuL3](2+) luminescent probes, where L stands for 2,2'-bipyridine, 1,10-phenanthroline, 5-octadecanamide-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline (dip), electrostatically loaded onto Nafion ionomer membrane have been investigated in air and in organic solvents and water, with the aim of developing rugged materials for optical sensing of molecular oxygen. The significant differences in size and hydrophobicity of the Ru(II) dyes have been used to probe their location within the perfluorinated ionomer pore network, as well as to gain insight into the oxygen accesibility to its microcrystalline and interfacial domains. While the absorption maximums of the probes (444-458 nm) remain relative unchanged, their emission wavelengths (578-622 nm) are extremely sensitive to the degree of Nafion swelling by the solvent. This feature has been characterized by measuring the density (1.19-2.04 g cm(-3)) of the solvent-saturated ionomer and the mass and volume fractions of solvents (0.0-0.7) uptaken by the original acidic Nafion and Li+-, Na+-, or K+-exchanged films. The excited-state lifetimes of the [RuL3](2+) complexes (0.03-4.9 mu s) reflect important variations of the microenvironment around the luminescent probes, which are rationalized in terms of their location and oxygen accessibility when loaded onto the polysulfonated material. Emission quenching rate constants of 1.7 +/- 0.3 M-1 s(-1) have been measured for the [Ru(dip)(3)](2+)-dyed films dipped in methanol; their oxygen sensitivity turns out to be independent of the Ru(II) loading and counterion of Nafion. Highly oxygen-sensitive luminescent membranes, suitable for continuous monitoring in organic solvents, water, or gas phase, have been prepared by immobilization of [Ru(dip)(3)](2+) indicator in 178-mu m thick Nafion, with response times below 1.5 min. Nevertheless, a slow evolution of the oxygen response over 1 week has been detected for very solvent-swollen membranes; some ways of minimizing such an effect are also given.