Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 13. Detection of aqueous iron by in situ complexation with 2,2'-bipyridine

A spectroelectrochemical sensor with attenuated total reflectance at an indium-doped tin oxide (ITO) optically transparent electrode coated with a thin film of Nafion has been demonstrated for the determination of aqueous iron ion. The novelty of this sensor stems from its ability to take up colorless iron ion (Fe2+) from solution and complex it with an organic ligand, 2,2'-bipyridine (bipy), that has been previously loaded in the optically transparent charge-selective Nafion film coating the electrode. The resulting complex ion, tris(2,2'-bipyridyl)iron(II), Fe(bipy)3(2+), absorbs strongly, making it easily detectable via optical spectroscopy. Fe(bipy)3(2+) loaded into the selective film is oxidized to colorless Fe(bipy)3(3+), which gives rise to an absorbance change for quantifying iron. This paper maps the development of this sensor, from the spectroelectrochemical characterization of the complex ion at an ITO optically transparent electrode to an analysis of the uptake, retention, and optical response of the complex ion in the Nafion film. Finally, an evaluation of the uptake of aqueous Fe2+ by the bipy-loaded Nafion film is reported. These data include preliminary results illustrating the dependence of the sensor response on differing concentrations of Fe2+ in solution.