Fraunhofer Institute for Photonic Microsystems (IPMS) has developed a breakthrough chip for pH measurement. Instead of the traditional, frequently prone to error reference electrodes, the new chip is more durable, it is pressure resistant, and easily integrated into various devices. Initial test kits are already available for rapid use in medical, biological, agricultural and environmental applications.
Precise pH measurement is a major challenge in chemical sensor technology. To date, conventional silver/silver chloride/potassium chloride electrodes have been used. Although these electrodes provide a stable reference potential, they are highly prone to errors. Changes in electrolyte concentration or a clogged or dried-out membrane can make the readings unreliable.
While the pH-sensitive glass electrode has already been successfully replaced by robust chips called ion-sensitive field-effect transistors (ISFETs), replacing the sensitive reference electrode has only recently been achieved at Fraunhofer IPMS with a reference ISFET (REFET). “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,” explains Dr. Olaf Hild, division director of the Chemical Sensors and Systems department at Fraunhofer IPMS. This makes them particularly suitable for mobile or integrated measurement systems.
In this technology a second ISFET is used with a standard pH ISFET instead of a sensitive reference electrode. This reference ISFET has a significantly lower pH slope of 20mV/pH at 25°C, compared to the usual 59mV/pH according to the Nernst equation.
The ISFETs are coated with thin layers of niobium pentoxide (Nbâ‚‚Oâ‚…) or tantalum pentoxide (Taâ‚‚Oâ‚…). These materials provide the sensors with high stability, ease of handling, and good storage properties. The two ISFETs operate together via an auxiliary electrode to reliably calculate the pH value from the measured electrical signals.
Currently, two 5 mm × 5 mm chips are required for proper wetting, but their size can be reduced. In the future, the both chips are planned to be combined into a single chip of the same size, with integrated temperature measurement.
“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,” said electronics developer, Hans-Georg Dallmann, describing how the control system works. This establishes the roadmap for further development at Fraunhofer IPMS: expanding the pH range of the REFET through improved sensor layers, reducing REFET sensor drift, and integrating both functions on a single chip with temperature measurement.
