MEASURING DONNAN-RELATED PHENOMENA USING A SOLID-STATE ION SENSOR ANDA CONCENTRATION-STEP METHOD

Measurements are performed with a device consisting of an ISFET pH-sen sor in the middle of a Ag/AgCl electrode, on top of which a microporou s composite membrane is deposited. A sudden change of the salt concent ration in the bathing electrolyte causes a transient change in the ele ctrical potential of these sensors when measured vs. a reference elect rode in the bathing electrolyte. The potential transient is modulated by adsorption of protein to the membrane. To explain the measured tran sients, a model is presented for the measuring device describing the i on transport by the Nernst-Planck and Poisson equations, incorporating the different proton-dissociation reactions occurring in the system, and the sensor responses to their potential determining ions (the prot on or the Cl- ion). A finite-difference solution method is presented t o solve the resulting differential equations. Measurements are perform ed before and after the adsorption of the model protein lysozyme to th e membrane. Analysis of the measurement results indicates that the mea sured potential transient is caused by a change of the Donnan potentia l of the membrane, followed by a compensating change in the concentrat ion of the potential determining ion. It is proven that no diffusion p otential is generated, In addition, it is shown that an interlayer of electrolyte between membrane and measuring electrode will not influenc e the measured response. The potential transients measured by the ISFE T have a larger amplitude and a longer duration than the Ag/AgCl-measu red transients. An analysis shows that this is caused by the buffering action of the proton-dissociating membrane groups. The longer duratio n results from the release of a large amount of protons from binding t o fixed groups, while chloride ions are not bound. The larger amplitud e can be explained by refining the Donnan model to account for the inh omogeneous charge distribution in the membrane. The proton-dissociatin g groups reside at the surface of the polystyrene beads, at which plac e the potential change on an ion step is larger than the average in th e membrane pore solution. This surface-potential change can be measure d by the pH-sensitive ISFET because a proton release occurs from the s urface-bound groups into the membrane pores, changing the pore pH.