J.C. van Kerkhof

The ISFET based heparin sensor with a monolayer of protamine as affinity ligand

The ion-step measuring method was used to determine absolute heparin concentrations in PBS and blood plasma with a Ta2O5 ISFET on to which a monolayer of protamine had been immobilized. Heparin is a highly negatively charged polysaccharide, which is used clinically to delay the clotting of blood. Protamine acts as an affinity ligand for heparin. The response of the ISFET system on a step-wise increase in the electrolyte concentration (a so-called ion-step) is a transient change of the output voltage, which is related to the surface charge density of the ISFET gate oxide. After 2 mins of incubation in a plasma sample containing heparin, the amplitude of the transient ISFET response to an ion-step showed a linear relation to the heparin concentration. In blood plasma, heparin concentrations between 0.3 and 2.0 Units/ml could be determined with an accuracy of +/- 0.08 Units/ml. Heparin concentrations in different plasma samples of heparinized patients were determined and compared with the APTT. No direct relation was found between the APTT and the heparin concentration, but this result was not surprising.

ISFET responses on a stepwise change in electrolyte concentration at constant pH

Responses on a stepwise increase of the electrolyte concentration of bare ISFETs can interfere with responses of an ISFET with an affinity membrane deposited on the gate. In this paper the responses of bare ISFETs are studied. Results of experiments and simulations are presented and the mechanism is explained.

Development of an ISFET based heparin sensor using the ion-step measuring method

The ion-step measuring method was used to determine heparin concentrations in PBS and blood plasma. Heparin is a sulphated polysaccharide which is clinically used as a drug to prevent the clotting of blood. The measuring method is based on detection of changes in charge density in a porous membrane which is deposited on the gate of an ISFET. Protamine was used as affinity ligand in the membrane. In PBS a linear relation was found between the heparin concentration and the ISFET response. The incubation time was reduced from 18 h to 15 min by increasing the porosity of the membrane. The results of the measurements in blood plasma show a significant nonspecific binding of plasma components in the membrane. Suggestions are given to prevent this nonspecific adsorption. The results described in this paper show a detection limit for the ion-step measuring method of at least 5 ± 10-11 Mol/l which is promising for future practical applications.

Preparation of iridium oxide and its application in sensor-actuator systems

Acid or base concentrations can be determined by coulometric titration. A new all-iridium oxide sensor-actuator device is presented, of which the titrant is generated by a reversible redox reaction occurring in the electroactive iridium oxide actuator electrode, and the equivalence point in the titration curve is detected by the pH-sensitive iridium oxide sensor. The advantage with respect to the formerly used noble metal actuator electrode/ISFET sensor device is twofold: on the one hand, interference of Cl− ions, which oxidize at the noble metal actuator electrode during titration, is avoided and on the other hand, the preparation of the all-iridium oxide device is easier than of the ISFET-based devices. In addition to the conventional potential sweep method of oxide growth, a new current-pulse method is proposed, of which the parameters for optimal growth depend less on the pH of the solution.

Tungsten trioxide (WO3) as an actuator electrode material for ISFET-based coulometric sensor-actuator systems

Acid or base concentrations can be determined by performing an acid-base titration with OH− or H+ ions, coulometrically generated by the electrolysis of water at a noble metal actuator electrode. This can be done very rapidly if the actuator electrode is in close proximity to an ISFET which is used as the indicator electrode to detect the equivalence point in the titration curve. In order to restrict the effect of interfering redox reactions at the actuator electrode during coulometric generation, electroactive actuator materials have been studied which can exchange H+ ions at a lower electrode potential than the potential of anodic water electrolysis. In this paper, electrochemically grown tungsten trioxide (WO3) is proposed as an actuator electrode material. At a WO3 electrode, H+ ions can be generated by a redox reaction at approximately 0.1 V versus SCE in a mildly alkaline solution (0.5–7 mM KOH) (anodic water electrolysis at a Pt electrode occurs at 1.5 V versus SCE). The observed thermodynamic and kinetic behaviour of the redox reaction is in good agreement with the theoretical predictions. Disadvantages of WO3 are its slow dissolution in aqueous solutions and the restriction that a titration at a WO3 electrode can only be performed in alkaline solutions.

New Results of the ISFET Based Heparin Sensor Using a Dynamical Measuring Method

This chapter describes the results of the ISFET-based heparin sensor using a dynamical measuring method. The measuring method used is the ion-step measuring method. The affinity ligand is immobilized in a membrane of polystyrene beads in an agarose gel. As a response on a stepwise change of the electrolyte concentration, the ISFET shows a transient response which is related to the net charge density in the membrane. It was shown that heparin concentrations could be determined in buffer solutions by using protamine as an affinity ligand. The measurements in blood plasma showed that nonspecific binding of plasma proteins in the sensor membrane interfered with the specific binding of heparin. The biological activity of heparin is also measured by detecting the formation of thrombin–antithrombin III complexes. The use of other techniques to immobilize the affinity ligand in the sensor membrane, or even directly to the ISFET surface, appears to improve the selectivity, and the sensitivity of the sensor.