Researchers at the Indian Institute of Technology (IIT) Kharagpur have successfully developed a small device to measure the pH of any solution, with preliminary studies showing its usefulness to measure the pH of blood. Unlike the conventional approach of measuring the pH of blood potentiometrically, the device works by measuring the impedance. The device was found to have satisfactory pH sensitivity in the physiological pH range.

In contrast to conventional arterial blood-sampling devices that are bulky as they use three electrodes and with time-consuming processes, the miniature device relies on venous blood sampling for pH measurement. The results were published in the journal RCS Advances.

Three electrodes

The sensor has an aluminium electrode that is deposited on a glass substrate and made into two electrodes with a minimum gap by manually removing a small portion of aluminium around the midpoint of a long strip of aluminium film. “The electrodes are then coated with molybdenum disulphide nanoparticles and Nafion in a small region around the gap between the two electrodes. So only a small amount of analyte is required for pH measurement,” says Prof. Soumen Das from the School of Medical Science and Technology, IIT Kharagpur, who is the corresponding author of the paper.

The pH of a solution is a measure of the number of hydrogen ions produced or available in a solution; when the solution is acidic the pH will be less than 7 and the number of hydrogen ions will be more.

The Nafion layer that is coated on aluminium electrodes selectively allows the hydrogen ions to diffuse into the layer. This leads to an increase in the conductivity of the Nafion layer and a reduction in its impedance. “Because of the Nafion coating, the hydrogen ions become preferential mobile carrier ions. That’s why the conductivity increases when the pH reduces,” says Ranjan Mukherjee also from the School of Medical Science and Technology, IIT Kharagpur and a coauthor of the paper.

The sensitivity of the device improves when the electrodes are coated with molybdenum disulphide nanoparticles and then coated with Nafion. While the sensitivity of pH measurement for the electrodes when coated with only Nafion was 347 ohms per pH, it was as much as 6,193 ohms per pH when electrodes were coated with molybdenum disulphide nanoparticles and a Nafion layer.

Preliminary study using capillary blood showed that the estimated pH was close to that of the venous blood pH. For example, the pH of venous blood was 7.39 when measured using blood gas analyser and 7.38 when measured using the device developed by the researchers. So the device can be developed for use as a point of care device for measuring the pH of blood.

The device was able to detect the pH of the blood in 10 seconds which is far better than commercial blood pH sensors where the response time is 30-40 seconds. The time taken to measure the blood pH is lesser than the time taken for the blood to clot.

Hence, there is no need to use heparin (an anti-coagulant drug) when the tiny device is used for measuring the pH of blood, thus avoiding any errors associated with the use of heparin.

“As it is a new approach we need to carry out more tests to validate the device. This is a proof-of-concept study,” says Prof. Das.