ISFET Non Glass pH Technology
Up to now, accurate quantitative pH measurements have been made using a system that consists of a thin glass bulb sensitive to hydrogen ions; a reference electrode;a high input impedance voltmeter; and in some cases, a temperature sensor. In fact, although a range of pH electrode glasses have been developed to cater for different applications, and the use of microprocessors have led to increasingly sophisticated meters, the pH meter/electrode combination has changed relatively little since it was first introduced commercially by Arnold Beckman 55 years ago.
Alternatives to the pH electrode have only gained acceptance in certain specialized areas. An optical method, for example, is used to measure the pH of blood; in this case the limited dynamic range of the pH-sensitive dyes used is not a significantly limiting factor.
Despite the lack of a real challenge to its position as the pre-eminent measure of pH, the electrode is not without shortcomings. In addition to a variety of industry-specific problems, the pH electrode is notoriously fragile and has a very high impedance, which means that, to minimise electrical noise, the cable must be shielded and the input stage of the voltmeter (usually a field effect transistor or FET) is as close to the electrode as possible. In some pH electrode, the manufacturers have the FET built into the electrode cap.
The most elegant solution, however, is to combine the ion or pH sensitive membrane and FET into a single microelectronic device, the ion sensitive field effect transistor (ISFET) to produce a very low noise instrument that offers the advantages of small size, high reliability and reproducibility. It also gets the robustness offered by semiconductor fabrication technology.
The ISFET is a derivative of a common electronic component called a metal oxide silicon field effect transistor or MOSFET, which consists of a silicon semiconductor substrate with two electrical contacts (source and drain) a small distance apart. Overlaying the substrate between the source and the drain is an electrical insulator made from silicon oxide or nitride, which itself is overlaid by a metal electrode called the gate.
When a potential is applied to the gate the induced electric field changes the freedom with which current flows between the source and the drain.
In the case of the ISFET, however, there is not gate electrode and the insulator is in direct contact with the solution. If the insulator is silicon nitride, hydrogen ions are absorbed on its surface in proportion to the pH. Their positive charges produce an electric field that modulates the current between the source and the drain. In order to qualify this effect in practice, we can measure the control voltage that must be applied (via a reference electrode) to maintain the drain-source current at a constant value.
Unifet has introduced an ISFET-based pH probe with a miniature integral temperature sensor (which allows automatic temperature compensation up to 100ºC), a built-in reference electrode and a hand-held microprocessor-based meter that provides all the features normally expected of a conventional pH meter.
The probe has been designed to allow users to take advantage of the small sensing area (less than 0.5mm2), which means that it can be used to measure the pH of sample drops down to 30µl and less, as well as by the conventional dip approach.
Unlike the glass electrode, which must be stored in an aqueous environment to prevent dehydration, the Unifet ISFET pH probe can be stored dry, which makes it equally at home in the laboratory and in the field.
The toughness of the probe will appeal to the food industry, where pH often has a significant effect on the flavour, texture, and microbial balance of products. However, on-line pH testing of food has often proved difficult because the fragility of conventional glass bulb-type probe represents an unacceptable safety hazard.
Another advantage of the ISFET arises when the pH of strongly alkaline solutions is being measure. Conventional glass bulb electrodes respond to sodium ions as well as hydrogen ions in alkaline solutions and give an erroneously low reading. The onset of this error depends on the ratio and the composition of the glass. The ISFET pH sensor can reduce this error without the need to resort to a special glass.
To produce ISFETs responsive to ions other than H-, electroactive membranes containing ionophores selective to sodium or potassium can be deposited over the gate insulator. Devices of this sort have already been used in the UK, where clinical trials using whole blood have shown accuracy and precision comparable to that of 'gold standard' clinical analyser.