Precise determination of the pH value is a major challenge in chemical sensor technology. Until now, classic silver/silver chloride/potassium chloride electrodes have been used for this purpose. Although they provide a stable reference potential, they are also very prone to errors. If the electrolyte concentration changes, the membrane becomes clogged or dries out, the measured values become unreliable.
While the pH-sensitive glass electrode has already been successfully replaced by robust chips known as ion-sensitive field-effect transistors (ISFETs), the sensitive reference electrode has only now been successfully replaced by a reference ISFET (REFET) at Fraunhofer IPMS. “Our ISFETs are reliable, stable and durable. They avoid typical problems of classic reference electrodes, such as clogged or charged diaphragms, drift due to concentration changes or contamination of the measuring solution,” explains Dr. Olaf Hild, Head of the Department of Chemical Sensors and Systems at Fraunhofer IPMS. This makes them particularly suitable for mobile or integrated measuring systems.
Physical principles of the new technology for pH measurement
In the new technology for pH measurement from Fraunhofer IPMS, a second ISFET is used together with a classic pH ISFET instead of the sensitive reference electrode. This reference ISFET shows a significantly smaller pH slope, for example 20 mV/pH, compared to the usual 59 mV/pH at 25°C, according to Nernst’s equation.
The ISFETs are coated with thin layers of niobium pentoxide (Nb₂O₅) or tantalum pentoxide (Ta₂O₅). These materials give the sensors high stability, easy handling and good shelf life. The two ISFETs are operated together via an auxiliary electrode so that the pH value can be reliably calculated from the measured electrical signals.
“Unfortunately, this concept of the ISFET-REFET does not originate from us, but was developed back in the 1980s by Professor Piet Bergveld, the inventor of the ISFET, and his colleagues,” explains Hild. “However, the concept has not yet found its way into commercial use.” This is set to change with the new components: pH measurement is initially possible in the range of around pH 4 to pH 8, which corresponds to the typical measurement requirements for many applications in biology, medicine, agriculture and the environment.
Currently, the solution still requires two 5 × 5 mm² chips to ensure good wetting. However, these can be reduced in size at the customer’s request. In the future, the plan is to reduce both chips to a single chip of the same size with integrated temperature measurement.
“The prerequisite for long-term measurements after a 2- or 3-point calibration is the control of sensor drifts, which can be compensated for with the control electronics if necessary,” explains electronics developer Hans-Georg Dallmann, explaining how the control works. This sets the roadmap for further development at Fraunhofer IPMS: increasing the pH range of the REFET by improving the sensor layers, reducing the REFET sensor drift and developing integration on a chip with temperature measurement. “Although there is still a lot of work ahead of us, we are pleased to already be able to offer test kits and present them to trade visitors at Analytica 2026 (Hall 3, Stand 312),” concludes Hild. Individual appointments at the trade fair can be arranged in advance via the Fraunhofer IPMS website.
About Fraunhofer IPMS
Fraunhofer IPMS is a leading international research and development service provider for electronic and photonic microsystems in the application fields of intelligent industrial solutions, medical technology and health, mobility as well as green and sustainable microelectronics. Research focuses on customer-specific miniaturized sensors and actuators, MEMS systems, microdisplays and integrated circuits as well as wireless and wired data communication. The range of services extends from consulting and conception to process development and pilot series production.
– – – – –
Further links
👉 www.ipms.fraunhofer.de
Photo: Fraunhofer IPMS
