One of the most measured chemical parameters is pH. Currently, the glass electrode is commonly used. However, especially in the food industry, risk of breakage limits the usability of glass electrodes. Based on the ISFET principle, microchip technology now enables the design of robust, glass-free pH electrodes, which enables safe, reliable pH measurement. IJcke Schaepman reports.
Commonly used to express the acidity of products, pH is an essential parameter, especially in the food and beverage industry. The taste and customer's perception are good examples. Also parameters as freshness and shelf life (limiting micro-biological activity) are pH dependant.
This is why pH determination has become a frequently performed analysis, not only at quality control of raw materials, but also during production.
Currently, the aglass electrode' is most frequently used. The chance of breakage of the glass is large, which limits usability for applications in food-industry. This is why this sector is looking for alternative methods: sensors that deliver at least equal accuracy and less risk for the environment in which they are used.
The ISFET principle
An alternative method for measuring pH is available now. This method uses semiconductor technology. The heart of the measurement principle is an ISFET (Ion Selective Field Effect Transistor). This ion-sensitive sensor is derived from the MOSFET (Metal Oxide Screen Field Effect Transistor). The working principle is based on controlling the current that flows between two semiconductor electrodes. These aDrain' and aSource' electrodes are placed on one element, with the third electrode, the aGate', between them.
In an ISFET, the metallic aGate' electrode is replaced by a special chemical layer, which is sensitive to free Hydrogen ions, eg Aluminum Oxide (Al2O3). In other words, this forms a pH measuring system with a very short response time and low impedance.
Unlike glass-electrodes, where the pH sensitive bulb has to be filled with a buffer solution, the semiconductor technology results in a true Solid State ISFET pH sensor.
The whole microchip is embedded in plastic, only the agate' surface is left open to connect to the sample.
By replacing the fragile glass bulb with this embedded microchip, robust and glass-free pH probes can be designed. Together with a reference electrode and a temperature sensor, the ISFET can be combined in one plastic housing.
With glass electrodes, the pH sensitive membrane's impedance is quite high, a relatively large area of the bulb's surface has to be in contact with the sample in order to get an accurate, stable and reliable measurement.
Other physical parameters of the material limit the shapes in which a glass electrode can be made.
The ISFET's small dimensions however enable a wide variety of probe designs without the need to modify the ISFET pH sensor. This allows for the design of efficient, application specific pH probes.
As with the glass electrodes, several reference systems with non-flowing gelled reference solutions or flowing reference solutions can be applied, while still only using plastic.
The resulting system enables fast and reliable pH determination. For many applications in the food industry, an optimal probe choice is possible.
Because these probes are completely glass-free, the use in production areas is allowed. It is no longer needed to transfer samples to a laboratory: direct measuring into the product at the line is possible.
While this saves time, the faster response speed allows for measuring more samples than previously.
Possible further developments
Glasselectrodes for pH measuring are available for a long time already. The last years have shown little new developments. Seen the vulnerability of this type of electrodes and the limitations associated with it, alternatives are wanted for. The ISFET probe is a robust alternative, or could grow further into this role.
Safer methods for use in the production process are asked for more and more. It can be conceived that in future the ISFET probe can play an active role here as well. Also, more interest is generated for sensors that can directly measure other parameters, or ion-concentrations. Various institutes have initiated research to develop such sensors.
As the use of semiconductor technology seems to be a suitable method for these sensors, it can be anticipated that ISFET based ISE's may become available.
Enquiry No 75
IJcke Schaepman is with Sentron Europe BV, AC Roden, The Netherlands. www.sentron.nl
