New chip technology for reliable pH measurements

Researchers at Fraunhofer IPMS have developed a new chip technology for pH measurements. Instead of sensitive reference electrodes, the system uses a reference ISFET. The solution could make measuring devices more robust, easier to use, and better suited for mobile applications in medicine, biology, and environmental analysis.

Determining the pH value is a central task in chemical sensor technology, for example in medical diagnostics, biology, agriculture, and environmental analysis. Instead of the reference electrodes commonly used to date, a resistant sensor chip is used. Initial test kits are already available to test the technology in various applications.

Conventional pH meters work with silver/silver chloride/potassium chloride reference electrodes. Although these provide a stable reference potential, they are considered prone to errors. Changes in electrolyte concentration, clogged membranes, or drying electrodes can affect the measured values.

Ion-sensitive field-effect transistors (ISFETs) have been used as replacements for conventional glass electrodes for some time now. Fraunhofer IPMS has now succeeded in replacing the sensitive reference electrode with a reference ISFET (REFET).

Dr. Olaf Hild, head of the Chemical Sensors and Systems department at Fraunhofer IPMS, explains: "Our ISFETs are reliable, stable, and durable. They overcome common problems of conventional reference electrodes, such as clogged or charged diaphragms, drift due to concentration changes, and contamination of the measurement solution."

These properties make the chips particularly suitable for mobile measuring systems or compact analyzers.

The new technology combines a classic pH ISFET with a reference ISFET. The reference sensor has a significantly lower pH slope—for example, around 20 mV per pH compared to the usual 59 mV per pH at 25 °C according to the Nernst equation.

Both sensor chips are coated with thin layers of niobium pentoxide (Nb₂O₅) or tantalum pentoxide (Ta₂O₅). These materials contribute to the stability and good shelf life of the sensors.

The two ISFETs are operated together via an auxiliary electrode. The pH value can then be calculated from the measured electrical signals.

Dr. Olaf Hild points out the historical basis of the approach: "Unfortunately, we did not develop the ISFET-REFET concept. It was created in the 1980s by Professor Piet Bergveld, the inventor of the ISFET, and his colleagues," explains Hild. "However, the concept has so far not been adopted commercially."

Current technology allows pH values to be measured in the range from approximately pH 4 to pH 8. This range covers many typical applications, for example in:

• Biological and medical analyses
• Environmental monitoring
• Agricultural applications
• Integrated sensor systems

Currently, two chips, each measuring 5 × 5 mm², are still used to ensure sufficient wetting. In the future, both sensors are to be combined on a single chip with integrated temperature measurement.

Electronics developer Hans-Georg Dallmann explains an important prerequisite for stable measurements: "Long-term measurements after a 2- or 3-point calibration require precise control of sensor drift. The drift can be compensated for with the help of the control electronics."

The researchers are now planning further optimizations, including expanding the pH measurement range, reducing sensor drift, and fully integrating the sensors on a single chip.

COMPAMED-tradefair.com; Source: Fraunhofer Institute for Photonic Microsystems