Spectroelectrochemical Sensing Based on Multimode Selectivity Simultaneously Achievable in a Single Device. 1. Demonstration of Concept with Ferricyanide

A new type of spectroelectrochemical sensor that demonstrates three modes of selectivity (electrochemistry, spectroscopy, and selective partitioning) is demonstrated. The sensor consists of an optically transparent electrode (OTE) coated with a selective film. Sensing is based on the change in the attenuation of light passing through the OTE that accompanies an electrochemical reaction of the analyte at the electrode surface. Thus, for an analyte to be detected, it must partition into the selective coating and be electrolyzed at the potential applied to the electrode, and either the analyte or its electrolysis product must absorb light at the wavelength chosen. Selectivity for the analyte relative to other solution components is obtained by choice of coating material, electrolysis potential, and wavelength for optical monitoring. The sensor concept is demonstrated with an OTE consisting of an indium−tin oxide coating on glass that has been over-coated with a sol−gel-derived charge-selective thin film. Attenuated total reflection (ATR) is used as the optical detection mode. The selective coating was an anionically charge-selective sol−gel-derived PDMDAAC−SiO2 composite film, where PDMDAAC = poly(dimethyldiallylammonium chloride). Fe(CN)6 4- was used as a model analyte to demonstrate that the change in the transmittance of the ATR beam resulting from oxidation of Fe(CN)6 4- to Fe(CN)6 3- can be used to quantify an analyte. The unoptimized sensor exhibited the following characteristics: linear range, 8.0 × 10-6−5.0 × 10-5 M; sensitivity, 8.0 × 103ΔA/M; and detection limit, 8.0 × 10-6 M.