Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 7. Sensing of Fe(CN)(6)(4-)

A spectroelectrochemical sensor that has trimodal selectivity (selective pa rtitioning, electrochemistry, and spectroscopy) is evaluated for the determ ination of ferrocyanide in solution. The sensor is based on attenuated tota l reflection spectroscopy at an indium-tin oxide optically transparent elec trode coated with a cationic PDMDAAC-SiO2, where PDMDAAC means poly(dimethy ldiallylammonium chloride) film into which anionic Fe(CN)(6)(4-) partitions . Fe(CN)(6)(4-) loaded into the film is subjected to spectroelectrochemical modulation, and the absorbance change at 320 nm associated with cycling Fe (CN)(6)(4-)/Fe(CN)(6)(3-) is used to quantitate the analyte. Cyclic voltamm ograms of Fe(CN)(6)(4-), 0.1 M KNO3 at high concentrations showed multiple peaks, which are interpreted as two types of Fe(CN)(6)(4-)/Fe(CN)(6)(3-) in the PDMDAAC-SiO2 film: Fe(CN)(6)(4-) bound to PDMDAAC and free Fe(CN)(6)(4 )- in interstitial regions of the porous film. The sensor response was affe cted by ionic strength and anion type of the supporting electrolyte and fil m thickness. The diffusion coefficient of Fe(CN)(6)(4-) in the PDMDAAC-SiO2 film was determined to be 4.9 x 10(-11) cm(2)/s. An analytical calibration plot of absorbance change at 420 nm versus Fe(CN)(6)(4-) concentration wit h a preconcentration time to equilibration was linear from 5 to 400 muM fer rocyanide with negative deviation from linearity observed at higher concent rations. However, a linear calibration plot at high concentrations (1-8 mM) could be achieved by making sensor measurements in a very short time perio d, only 1 min after exposure to the sample.