Semiconductor technology used to measure pH and ion concentrations

M H B Grote Gansey reports on the use of semiconductor technology for the measurement of calcium, sodium, potassium and nitrate ion concentrations.

Direct, glass free measurement of pH in various liquids is a well-established application of the ion sensitive field effect transistor (ISFET). Applying the same semiconductor technology for the measurement of ion concentrations is now possible for frequently analysed ions such as calcium, sodium, potassium and nitrate.

Sentron Europe made the step from ISFET to the new CHEmically Modified Field Effect Transistor (CHEMFET) in co-operation with the MESA+ institute of the University of Twente. Sentron translated the scientific knowledge in practically applicable sensors that give reliable results under demanding circumstances.

In order to understand the working principles of the CHEMFET, it is best to consider the basic ISFET. This device has an affinity for hydrogen ions, which is the basis for the determination of the pH. The surface of the sensitive area of the sensor contains hydroxyl groups that are bound to an oxide layer.

At low pH values hydrogen ions in the sample will bind to these hydroxyl groups resulting in a positively charged surface.

In alkaline environments hydrogen ions are abstracted from the hydroxyl groups, leading to a negatively charged surface.

Thus, each pH change has a certain influence on the surface charge. On its turn, this attracts or repulses the electrons flowing between two electrodes in the semiconductor device.

The electronics compensates the voltage in order to keep the current between the two electrodes at its set point. In this way this potential change is related to the pH.

Attachment of a polymer membrane on the ISFET introduces the possibility to go beyond the measurement of pH toward other ions. In this plastic layer certain chemicals (ionophores), which can recognise and bind the desired ion, are put in.

Now, complex formation of the ionophore and the ion introduces a charge. The potential change is a measure for the ion concentration. Typically, these sensors can be used in a concentration range between approximately 10­5 up to 1 mol/l.

A sufficient selectivity is necessary for reliable measurement; ie the influence of potentially interfering ions has to be minimised. This is achieved via supramolecular chemistry: the ionophore is synthesised in such a way that unique, energetically favourable interactions of only the desired ion are obtained and therewith create selectivity over other ions.

Advantages of the applied materials are that the ionophores may be covalently bound to the membrane and therefore cannot leak out. Furthermore, the membrane does not need plasticisers to be incorporated, so again no leakage, associated with other ion selective electrodes, occurs. These features improve the lifetime of these chip-based sensors.

Diffusion of carbon dioxide through the membrane to the chip surface and reaction with water giving hydrogen ions resulted in disturbed measurements: the basic device is after all still a pH measuring one. This problem was circumvented by attachment of a polyHEMA hydrogel layer in between the chip and the ion selective membrane. Inside this hydrogel a pH buffer was dissolved, taking away the disturbing pH influence resulting in true ion selectivity.

These sensors can be applied where fast and easy analyses in aqueous environments are requested. The electrodes are not much bigger than a pencil. Both portable and bench top meters are available. To give an impression of the demanding areas where these sensors can be used: Successful tests have been performed in agriculture and environmental laboratories, as well as inside dishwashing machines and food industry.

Enter 56 or at www.scientistlive.com/elab

Dr Ir M H B Grote Gansey is Engineering Manager with Sentron Europe BV, Roden. The Netherlands. www.sentron.nl

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