Spectral and Electrochemical Study of the Response of Mechanism of Ionphore-Based Polymeric Membranes

Abstract

Ion-Selective electrodes based on solvent polymeric membranes, typically plasticized poly-(vinyl chloride), are routinely used and fundamental studies on ion transfer mechanisms that dictate their practicality continue. This dissertation is aimed at elucidating some of the features of these ionophore-based sensors in hope of reaping the most benefit from their accurate response. The initial goal of this research was to develop a theory based on normal pulse voltammetry (NPV) applied to a conducting polymer membrane electrode doped with highly selective, lipophilic ion-complexing molecules. Upon discreet potential pulses (uptake of target ion) in a three-electrode setup, ions are selectively transferred from a conducting aqueous electrolyte phase into the polymeric membrane phase. After a short uptake pulse (1 s), the membrane is allowed to relax at a potential 0.0 V (30 s). This allows the membrane to expel previously transferred ions back into the aqueous phase, renewing the original membrane composition prior to subsequent pulses. This method was used to examine 18 neutral ionophores in an attempt to calculate ion-ionophore complexation constants. The NPV scans were conducted at 20 mV intervals. These scans were fit with a theory that depends on three separate regions of the NPV plot. The most striking feature in the NPV scans is the diffusion limited plateau in the current/potential plots. These plateau potentials vary in range for membranes containing different ionophores with different complex formation constants and different ion-ionophore stoichiometry. The same principles mentioned above were also applied to polymeric films containing the proton-selective chromoionophore ETH 5294 without ionic sites. In addition, a calibration curve of potential versus pH was made and compared to similar potentiometric ion-selective membranes showing a Nernstian response and good lower and upper detection limits. The same NPV membranes (or ion amperometric membranes) used in the pH calibration were also fitted in a special transparent electrochemical cell coupled to a microscope and potentiostsat. The chromoionophore, ETH 5294, changes its absorbance maximum upon protonation. The cell was positioned under the microscope, a potential was then applied which transferred protons from an aqueous buffer into the polymer membrane protonating the chromoionophore and changing its absorbance characteristics. The absorbance profiles were used to estimate the diffusion coefficient of the chromoionophore in the membrane. In view of realizing accurate diffusion coefficients within solvent polymeric ion-selective membranes, an additional optical method was developed. ETH 5294 can be photobleached using ultra-violet light. This is not good for sensor performance, however, it was utilized to gain information on the effects of plasticizer and polymer content on the diffusion coefficient of ETH 5294. The method uses light from a high-pressure Hg arc lamp to photobleach a portion of a single polymer membrane containing ETH 5294. The microscope is then repositioned and absorbance measurements are taken over fixed time intervals, allowing for the estimation of the apparent diffusion coefficient of the chromoionophore in 20 different membranes. This method was also used to investigate other chromophores including an Indium(III) porphyrin and three grafted ionophores with an MMA-DMA copolymer backbone.