Innovation in pH sensor technology

The Fraunhofer Institute for Photonic Microsystems IPMS has announced what it says is a significant advance in chemical and biochemical analysis in the form of a pioneering sensing layer for chip-based pH measurement, successfully integrating it into an ion-sensitive field-effect transistor (ISFET).

The pH of a solution is the most important parameter in chemical and biochemical analysis. An ISFET with a novel sensor layer developed by Fraunhofer IPMS, in combination with a conventional reference electrode, enables highly precise pH measurement in the range from pH 1 to pH 13. The compact, unbreakable sensor chip with dimensions of 5 mm² is characterised by a minimal drift of less than 20 µV/h as well as low hysteresis, while being easy to integrate. A significant improvement has also been achieved in the reduction of light sensitivity.

The dry storage capability of the sensor is one of its primary advantages, and the electrical operating point of the sensor can be flexibly adjusted via the design and the operating parameters, whereby an operating voltage (VDS) of below 1 V is possible.

“With these properties, the new ISFET is particularly suitable for on-site environmental analysis,” said Dr Olaf Hild, Head of the Chemical Sensor Technology Business Unit. He said that the next important research goal is to develop a sensor layer that enables a purely chip-based pH measurement that eliminates the need for conventional reference electrodes. This breakthrough technology would then have the potential to continuously collect environmental data over long time periods without the need for intervention by operating personnel.

Simplified illustration of an ISFET with an Ag/AgCl reference electrode. © Fraunhofer IPMS. For a larger image click here.

The novel ISFET from Fraunhofer IPMS is based on metal-oxide-semiconductor (MOS) field-effect transistor technology, whereby the sensor area in contact with the medium consists of an amphoteric metal oxide layer. Hydronium or hydroxide ions from the medium are reversibly adsorbed on this sensing layer, allowing for the pH of the medium to be measured. The gate-to-source voltage (VGS) response versus a reference electrode (Ag/AgCl in 3 M KCl) is then used as the measurement signal.

Top image: ISFET pH sensor chip. © Fraunhofer IPMS