Spectroelectrochemical planar wave guide design for transport studies in clay films

A new Planar Wave Guide System (PWGS) was designed and constructed with a prototype optimal flow cell for the purpose of developing a spectroelectrochemical sensor involving an electroactive planar waveguide. The spectroelectrochemical flow cell was made from white Delrin plastic with an internal volume of ∼8.0 mL and optical path of 4.5 cm where sample solution is in contact with an ITO slide. The Attenuated Total Reflection (ATR) signal was monitored by using OceanOptics USP 2000 miniature fiber optic spectrometer. The oxidation/reduction current of cationic species was measured by Obbligato Objective Faraday potentiostat. The two SF-6 coupling prisms and optical setups, employed in the PWGS, process light in the entire visible range (400--700 nm) with sufficient sensitivity. The system was validated by using a thin spin-coated Nafion film on an ITO to show that the data obtained by the EA-PWG resemble the results obtained by Heineman's group. The study demonstrated that the chemical sensor recognized a molecular species within clay, sol-gel and/or PVC films. The clay study showed that the EA-PWG is capable of optically recognizing Ru(bpy) 32+ at 460 nm at the electrode surface while an electrochemical reaction is conducted. The modulated absorbance measurements were shown to be accurate in determining the amount of adsorbate in a monolayer within a modified clay-PWG electrode compared to the theoretical calculated values. The current ratio (R) was calculated to be approximately 600 times greater response for an electrochemical experiment of Na+-exchanged clay film at CEC to a monolayer. This means that under the experimental conditions of this study, the adsorbed monolayer on the ITO is electrochemically "invisible for Na+-exchanged film", but it is observed by the ATR method. For K+-exchanged clay film, the monolayer was clearly observed in both modes of detection. In conclusion, the goal of constructing the spectroelectrochemical planar waveguide system to study the mechanism of freely diffusing ions and an adsorbed monolayer in clay films was achieved.