Modeling and analysis of a Na$\sp+$ selective membrane
Ion-selective electrodes (ISE's) are routinely used to measure electrolyte concentrations in biological fluids such as blood and urine. The process involves the transduction of ion activity to a potential within a sensing membrane. This membrane typically consists of a plastic phase (e.g., plasticized PVC) impregnated with a highly specific ion-binding molecule. It is desirable to determine the relationship between formulation and function. Such knowledge would enable developers to achieve required performance specifications for sensor slope, longevity, and transient response. This research focused on the analysis of a planar format Na+ sensor, fabricated with the use of thick-film screen printing technology. A model was developed to interpret electrode responses in terms of fundamental physical processes. These processes included interfacial transport (kinetics, solubility), electrodiffusion (Nernst-Planck), and ion-ionophore complexation. Potentials were calculated from the Poisson equation using the resulting time-dependent concentration distributions. Model mechanisms were converted to mathematical expressions and discretized for computer simulation. Experimental design techniques were used to efficiently determine the significance of model input parameters. Results from this exercise specified membrane formulations to be fabricated, tested, and compared to the model. Both model and laboratory sensors revealed a diffusion controlled "warm-up" conditioning phase upon sample exposure. When membrane formulations and thicknesses were varied, electrode responses were altered in the direction predicted by the model. It was concluded that the model could predict the direction of "warm-up" and transient responses with regard to voltage and time as a result of changing inputs. Such correlation between model and laboratory advocates that sensor performance is determined by the physical and chemical parameters of the membrane; hence, formulation and thickness. In addition, the research showed that membrane function is not limited to the electrode/electrolyte interface; rather, it also depends on processes within the membrane bulk. However, because of the disparity between model and laboratory electrodes for voltage spans and response times, it was concluded that the model was neglecting essential mechanisms such as ion-exchange and water interactions. ^
Jay Kevin Bass,
"Modeling and analysis of a Na$\sp+$ selective membrane"
(January 1, 1993).
Dissertations available from ProQuest.