Spectroelectrochemical Sensing Based on Multimode Selectivity Simultaneously Achievable in a Single Device. 20. Detection of Metal Ions in Different Oxidation States

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Abstract
Spectroelectrochemical sensing a metal in two different oxidation states, both of which are weakly absorbing in the visible wavelength range, was demonstrated with ferrous and ferric ion. The sensor consisted of an indium tin oxide optically transparent electrode (ITO OTE) coated with a thin film of Nafion preloaded with the ligand 2,2‘-bipyridine (bipy). Fe2+ in the sample partitioned into the film where it reacted with bipy to form Fe(bipy)32+, which absorbs strongly at 520 nm. The change in absorbance (ΔA) at 520 nm associated with the accumulation of Fe(bipy)32+ complex in the film was measured by attenuated total reflectance spectroscopy and was proportional to the concentration of Fe2+ in the sample. Iron in the Fe3+ form can also be determined, but it has a more complex coordination chemistry involving formation of [Fe2(bipy)4O(H2O)2]4+ as well as Fe(bipy)33+ in the film. Fe3+ was detected indirectly by reducing the nonabsorbing Fe3+−bipy complexes that accumulated in the film to absorbing Fe(bipy)32+ and monitoring ΔA at 520 nm. The effects of film thickness and ligand concentration in the film on sensor sensitivity and response time for Fe2+ were evaluated. Detection limits of 0.6 × 10-6 M for Fe2+ and 2 × 10-6 M for Fe3+ were obtained with 300 nm thick films after 30 min of exposure to a quiescent sample. Careful manipulation of the potential applied with simultaneous optical detection enables Fe2+ to be distinguished from Fe3+, which is the first step in developing a sensor for speciating the two oxidation states in a mixture.