Evanescent wave spectroelectrochemical studies on clay film: Structure, topography, and utilization

Clays are interesting materials finding increase use in sensor technology and nanocomposites. In these uses, transport processes are very important. The transport phenomena are affected by the time dependent arrangement of clay particles, the oxidation state of the crystal lattice, orientation on a template surface, and exposure to various electrolytes.;This study aims to probe transport and reactions in clay films in two different spatial locations simultaneously by the application of electrochemistry and planar waveguide technology, using a home-built spectroelectrochemical instrument.;The work is broken into five categories. The first is spectroelectrochemical system validation and calibration. The validation concept was used to determine the diffusion coefficients of Rubpy2+ 3 and FeCN3- 6 (3.42 x 10-12 and 3.92 x 10-9 cm2/s, respectively), which indicate literature consistent measurements by the system. Calibration curve indicated linearity at Rubpy2+ 3 concentrations below 4.0 x 10-4 M. Conditions affecting sensor performance were also optimized.;Another study demonstrates how evanescent field and electrode potential affect the various electron transfer steps of Rubpy2+ 3 -clay systems. Optical microscopy studies of ex-situ clay films revealed dewetting processes initiated by Rubpy3+ 3 . The hole sizes and dewetting period were dependent on film thickness and evanescent field. Rubpy+ 3 caused repulsion of the negatively charged clay platelets, and thus, thinning of the clay films.;The system was also used for ascorbic acid/ascorbate detection to demonstrate the mediator and charge exclusion concepts. The optical and electrochemical responses of ascorbic acid/ascorbate were monitored using Rubpy2+ 3 as mediator. Ascorbic acid produced larger signals than ascorbate due to charge exclusion of the latter by the clay film.;Additionally, in-situ redox processes of structural iron in clays were investigated using the spectroelectrochemical instrument. In-situ absorption spectra indicated the disappearance of a shoulder around 515 nm (due to tetrahedral Fe3+) and a corresponding growth of a band around 730 nm (due to octahedral Fe2+), after dithionite reduction.;A separate non-spectroelectrochemical study was performed, which is the catalysis of hydroquinone by iron(III) at clay-modified platinum electrode. It was established that the kinetically slow oxidation of hydroquinone can be changed to a faster mechanism at more positive electrode potentials with the use of iron as charge shuttle.