Bart H. van der Schoot

ISFET based enzyme sensors

This paper reviews the results that have been reported on ISFET based enzyme sensors. The most important improvement that results from the application of ISFETs instead of glass membrane electrodes is in the method of fabrication. Problems with regard to the pH dependence of the response and the dynamic range as well as the influence of the sample buffer capacity have not been solved. As a possible solution we introduce a coulometric system that compensates for the analyte buffer capacity. If the pH in the immobilized enzyme layer is thus controlled, the resulting pH-static enzyme sensor has an output that is independent of the sample pH and buffer capacity and has an expanded linear range.

Comparison of the hysteresis of Ta2O5 and Si3N4 pH-sensing insulators

Abstract It has been shown that the accuracy of pH sensor devices based on insulator/electrolyte interface is limited by a slow response, which manifests itself as hysteresis and drift. In this paper, we use hysteresis measurement to compare the performance of Ta2O5 and Si3N4 as pH-sensing surfaces. The results depend on the time taken to perform a complete pH loop. At short loop times (960-9600 s) Ta2O5 is superior to Si3N4 is reduced. These data can be combined with earlier measurements of the hysteresis of Al2O3 surfaces to establish the following order to hysteresis magnitude at a loop time of 1920 s: si3N4 > Al2O3 > Ta2O5.

A silicon integrated miniature chemical analysis system

Abstract Piezoelectrically driven micromachined silicon pumps are shown to have excellent characteristics for application in miniaturized chemical analysis systems. A system is demonstrated using two micro pumps and a separate glass flow-through cell with a potassium-sensitive ISFET. The measurement protocol is such that the sample solution enters the detector but does not pass the sensitive pump valves, thus improving the practical applicability of the system. During its operation, the sensor is continuously calibrated with a very low consumption of calibrating solution. With a measurement rate of four samples per minute, the use of calibrant is less than 3 ml/h.

Thermal optimization of micro-hotplates that have a silicon island

We have performed thermal measurements and electrothermal simulations (finite element modelling) with the aim of optimizing the power consumption and the temperature distribution of micro-hotplates for gas-sensing applications. A silicon island was added underneath the membrane of the micro-hotplate to improve the temperature distribution of drop-coated metal-oxide gas sensors and to thermally isolate MOSFET gas sensors. The temperature distribution over the sensing area and the power consumption depend on the silicon island thickness, which was optimized for both applications using the software MEMCAD from Microcosm Technologies. In the optimization process, we considered the thermal conductivity of silicon and dielectric membrane, the operating temperature, the geometry and the area of the heater, and the processing of the silicon island. The thickness of the silicon island was optimized to ensure a good temperature distribution over the gas-sensing area for metal-oxide and MOSFET gas sensors with specific geometry.

Modular setup for a miniaturized chemical analysis system

Abstract A miniaturized system for chemical analysis is presented, based on two piezo-electrically driven silicon micropumps and an ISFET-based flow through cell for the measurement of ion concentrations. The active elements of the system are stacked together and measure only 22 × 22 × 11 mm in total. The modular approach allows for great flexibility in the setup of the system.

Coulometric sensors, the application of a sensor-actuator system for long-term stability in chemical sensing

We present a new type of chemical transducer, the coulometric sensor. This sensor is in fact an integrated sensor-actuator system that is able to measure the concentration of acids and bases by means of coulometric titration. An ISFET is used as the sensor to monitor the pH changes induced by the actuator, which is a gold electrode that fits closely around the ISFETs gate area. Coulometry is an absolute method and therefore the output of the new sensor is only determined by its dimensions and is not subject to changes in offset and sensitivity of the indicator electrode. It is thus expected that the operation of the sensor will be stable for a long time, so that only a one-time calibration is needed. As a first example of this new class of chemical transducers, a carbon dioxide sensor is presented. It is shown that the stability is some orders of magnitude better than that for a ‘classical” potentiometric sensor.

The pH-static enzyme sensor : An ISFET-based enzyme sensor, insensitive to the buffer capacity of the sample

An ISFET-based urea sensor is combined with a noble-metal electrode which provides continuous coulometric titration of the products of the enzymatic reaction. The sensor thus becomes independent of the buffer capacity of the sample; and because the enzyme is operating at a constant pH, the linear response range is expanded.

On-wafer fabricated free-chlorine sensor with ppb detection limit for drinking-water monitoring

Note: 85 Reference SAMLAB-ARTICLE-1993-002 Record created on 2009-05-12, modified on 2016-08-08

A microsensor array for biochemical sensing

A microsensor array to measure chemical properties of biological liquids is presented. A hybrid integration technique is used to mount four sensor chips on a micro flow channel: a pressure, temperature, pH, combined pO2 and pCO2 sensor chip. This results in a microsensor array which is developed to meet the technical requirements for space applications. The integration method allows to integrate other types of sensor chips. This multi-purpose and multi-user approach makes the microsensor array suitable for various biochemical applications.

A modular miniaturized chemical analysis system

Abstract A modular miniaturized system for flow-injection analysis (FIA) is presented, based on two piezoelectric silicon micropumps and an ISFET-based flow-through cell for the measurement of ion concentrations. The active elements of the system are stacked together and measure only 22 mm × 22 mm × 12 mm in total. The small dimensions ensure a very low consumption of calibration and carrier solutions and are very attractive for the development of hand-held analysers.