pH electrodes evolve into the silicon age

Thursday, 08 March, 2001

Silicon chip pH sensors overcome breakage, maintenance and storage problems.

Since the early 1930s when the first pH meter was invented for testing citrus juice, the core technology for quantitative pH measurement has, until recently, remained fundamentally unchanged. From inventor Arnold Beckman's first primitive pH meter housed in a handmade wooden box to today's advanced digital pH meters, most pH systems still consist of a delicate glass bulb which is sensitive to hydrogen ions, a reference electrode, a high input impedance volt meter, and often a temperature sensor. Although glass pH electrodes are, in general, simple to use and available at reasonable cost, they are limited by the potential for glass breakage and difficulties in maintenance and storage.

In the mid-1980s silicon chip pH sensors, usually referred to as ISFETs (Ion Sensitive Field Effect Transistors), began appearing in the laboratory pH market. Well over $100 million had been spent, primarily by medical device companies, in the development of ISFET sensors. The small size, accuracy, fast response time and batch fabrication capability made the sensors ideal for medical applications. The ISFET pH sensors now available for laboratory and industrial pH applications come in a variety of configurations including 316 alloy stainless steel probes.

The ISFET is a derivative of a common electronic component called a metal oxide silicon field effect transistor (MOSFET), which consists of a silicon semiconductor substrate with two electrical contacts (source and drain) a small distance part. Overlaying the substrate between the source and drain is a silicon electrical insulator which itself is overlaid with a metal electrode called a gate. When a potential is applied to the gate of the MOSFET, the induced electrical field changes the freedom with which the current flows between the source and drain. In the case of an ISFET, however, there is no gate electrode and the insulator is in direct contact with the solution to be measured. With the selection of an appropriate insulator material, such as silicon nitride or aluminium oxide, hydrogen ions will reside at the surface of the insulator in proportion to the pH. Their positive charges produce an electric field that modulates the current between the source and drain. In order to quantify this effect, the control voltage is measured that must be applied (via a reference electrode) to maintain the drain-source current at a constant value. The pH can then be derived to a very high level of accuracy.

ISFET sensors facilitate pH measurements that were formerly difficult of impractical. The use of glass electrodes in many food, beverage, cosmetic and pharmaceutical applications is prohibited. QC personnel are required to take a sample from the line and carry it to the laboratory for pH measurement. Now using ISFET pH systems, measurements can be taken on the plant floor without the risk of glass breakage in production areas. One common use of the stainless steel pH probe is in meat applications, particularly pork, where the pH of a carcass 45 minutes post mortem has been shown to have a high correlation with the ultimate meat quality. The ISFET pH probes are inserted directly into meat carcasses without the risk of glass breakage.

Unlike glass electrodes that have a shelf life and must be stored in solution to prevent dehydration, ISFET pH probes can be stored dry. The shelf life is indefinite, and probes that have been stored for seven years have been put into use after only four minutes of rehydration. In normal laboratory use, ISFET probes typically last about 18 months and many have been in regular use for three and four years.

Over the last half-century of glass pH electrode evolution, many refinements have been made and glass formulas developed for almost every conceivable situation. Since ISFET sensors are relatively new, data is still being gathered on their use in many applications. In most cases, measurements correlate closely with glass. The main constraint in the use of ISFET pH systems appears to be physical damage to the sensor from impact or abrasion. Also, prolonged exposure to strong organic solvents can delaminate the silicon chip from the chemical resistant epoxy used to isolate the sensor's electrical connections from the solution.

Proteins, fats and oils can be difficult to remove from glass electrodes and the performance typically slowly degrades over time. Silicon chip pH sensors are easy to clean with a toothbrush and mild detergent. If food particles, grease, fat or other materials cover the sensor, a wooden toothpick and a drop of isopropyl alcohol can even be used to gently clean the sensor itself. There is no polarisation effect from wiping or scrubbing such as can occur with glass pH electrodes.

The sensing surface of the ISFET can be silicon nitride - an ultra-pure version of the same material used as a hard coating for machine tools. Alternative hydrogen sensitive coatings include aluminium oxide, which is often used as the abrasive in sandpaper. The use of such dense, hard materials as sensor surfaces makes ISFETs especially durable.

Data indicates that ISFET sensors respond more quickly than glass pH electrodes. Stable readings at 0.01 pH resolution are usually achieved within 14 seconds. The fast response time is attributed to the ISFET sensor's surface effect (ie, hydrogen ions reside only on the surface of the sensor's silicon nitride layer) as opposed to the membrane diffusion effect of glass electrodes.

Just as silicon semiconductors have replaced glass vacuum tubes, silicon chip ISFET sensors are becoming more common as an alternative to glass bulb pH electrodes.

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