Modeling of enzyme‐potentiometric sensors involving acid‐ or base‐forming reactions

Abstract

Enzyme-potentiometric sensors in which the chemical conversion of the analyte leads to the formation of an acid and/ or a base can often display complex response characteristics. In these sensors, the electrochemically monitored species is usually the H + ion (pH-based sensors). However, in some cases the conjugate-ion of the H' (or OH-) ion of the acid (or base) produced can also be monitored-a specific example being the urease-NH4+ sensor. The response of both types of sensors is strongly affected by: (1) the degree of dissociation of the products and their transport properties in the enzymic film, (2) the amounts of pH-buffers present in the test solution, (3) the test solution's pH, and (4) the diffusion coefficients of the various species. In this article, a previously developed theoretical model for pH-based sensors-in which the differences in diffusivities of t h e various species were ignored-is generalized to accommodate for such differences, and extended to the latter of the above two types of sensors. It is shown that when the sensor operates under analyte diffusioncontrolled conditions, the response of either type of sensor can be predicted by a simple algebraic equation which is independent of the actual kinetics of the enzymic reaction.

Comparison of label-free ACh-imaging sensors based on CCD and LAPS

“…Due to the fact that the measurement spots of the LAPS are defined by external light sources, an alignment to an additional micro-fluidic set-up might be possible. Both sensor set-ups show the same pH changes to different ACh concentrations up to a concentration of 5 mM ACh, and these pH changes also fit well to the theoretical model for enzyme-based potentiometric sensors, discussed in [30]. The long-term stability and adhesion of the enzymatic membrane is better on the LAPS surface, which is probably due to the smooth and unstructured surface.…”

“…(1) has been considered. For this, we used partition coefficients of ˛S = ˛H = ˛E = 0.5, These values have been also suggested in [30]. The hydrogen-ion concentration of the bulk solution was C b H + = 10 −7 M, and the total concentration of the phosphate buffer was C b TE = 20 mM according to the experimental conditions.…”

“…Ogundiran et al [30] provide a model that describes the complex response of enzymepotentiometric sensors utilising a pH-sensitive membrane. This model shows that, when the enzyme sensor operates under analyte diffusion-controlled conditions, the response can be predicted by a simplified algebraic equation, which is independent of the actual kinetics of the enzymatic reaction.…”

A mathematical model for the generation and control of a pH gradient in an immobilized enzyme system involving acid generation

“…A buffer may be required to maintain a pH of 5.0 in the bulk solution. A buffer affects the pH gradient because it provides a parallel path for protons to diffuse into the solid enzyme matrix (Byers et al, 1992;Carleysmith et al, 1980;Engasser and Horvath, 1974;Ogundiran et al, 1991;Ruck- Rajaro, 1985;Ruckenstein and Varanasi, 1984;Varanasi et al, 1987). As mentioned earlier, the dissociation of WH and the neutralization of its conjugate base W − by H + are usually very fast reactions, thus a local equilibrium exists between these three species at every point inside the pellet and in the bulk solution.…”

“…When in an immobilized form, it is assumed that the matrix does not significantly alter the internal pH environment. The assumption is justified by the fact that there is, in general, a buffer present in the biological system, which facilitates the transport of hydrogen ions and thus decreases the pH difference between the matrix and the bulk solution (Byers et al, 1992;Carleysmith et al, 1980;Engasser and Horvath, 1974;Ogundiran et al, 1991;Ruckenstein and Rajora, 1985;Ruckenstein and Varanasi, 1984;Varanasi et al, 1987). With the bulk solution being acidic to favor enzyme E 1 , enzyme E 2 , which has a basic optimal pH, must be immobilized.…”