Semiconducting materials are those which, if you ask them “do you conduct electricity?” will reply “yes and no”. Most of the semiconducting materials — such as Silicon — are inorganic solids. However, in recent times, a new class of semiconductors are emerging: organic semiconductors. Organic materials are those that have carbon and hydrogen atoms. Imagine a plastic sheet for conducting electricity. That is roughly organic semiconductors or OSCs.

The OSCs are now getting noted for their attractive properties. They are light, cost less to make, flexible and abundantly available. Fabrication of solar cells, transistors, photodetectors, and lasers with OSCs can be highly feasible and highly efficient. Further, they can be processed using simple solution processing techniques (e.g. ink-jet printing, reel-to-reel fabrication), making the fabrication of electronic devices much easier and cheaper, according to a 2018 paper by Fang-Chung Chen of the National Yang Ming Chiao Tung University.

Flexible pH meters

Now, scientists at the Indian Institute of Science Education and Research (IISER-TVM) Thiruvananthapuram have developed an organic semiconductor-based device that can be used to design disposable, flexible pH meters to test the full-scale acidity and alkalinity of substances.

The research team led by Dr Bikas C. Das, Assistant Professor in the institute’s School of Physics has developed a device for measuring pH. It requires only a drop of the fluid whose pH needs to be measured accurately in a few milliseconds without the need of calibration.

The pH meter is an extensively used instrument in laboratories and industries to measure the acidity and alkalinity of various substances. There are various kinds of pH sensing materials that are used to make these pH meters – even the litmus paper that changes colour from blue to red or vice versa, is a simple pH sensor.

However, advanced applications require sensitive pH meters without frequent calibration that can detect minute variations in acidity values and require small sample volume for analysis. Some of its applications include detecting the pH of bodily fluids such as blood, which has limited the amount of fluid available for testing.

In such cases, a variety of pH sensors are made using semiconductors – ion-sensitive field-effect transistors (ISFET), explains Das. But those are mostly inorganic compounds or inert polymer protected semiconductors. In recent years, there has been an interest in the development of organic semiconductors, but the development of organic ISFET has remained challenging.

IISER’s solution

The IISER TVM team has developed a highly sensitive pH sensor using an organic thin film as the sensing layer. Das explains thus: The organic layer is a semiconducting polymer called P3HT (or poly(3-hexylthiophene-2,5-diyl)) that is extensively used in the areas of organic photovoltaics, photodetectors, OLEDs and OFETs. It works with low-voltage and requires only a small volume of the fluid whose pH needs to be measured. The analyte drop is placed on the P3HT film channel between the source and drain, and used as the ‘gate dielectric’ to measure the pH value in dual mode from the change of threshold voltage and drain current modulation.

Das notes that the existing methods of pH sensing using organic channel-based ISFETs “are soft and flexible but requires a thin passivation layer, which increases the operating voltage and detection speed.” Furthermore, organic ISFETs have shown sensitivity only for limited pH value regions.

The team tested their organic ISFET device and found that it effectively senses pH values ranging from 3 to 12 within few milliseconds. The device needs no calibration, has a stable performance for 5 minutes and can be used as a single-use, rapid pH meter.

The organic ISFET pH sensor can be used to sense the acidity of bodily fluids such as blood, sweat, saliva, etc. and may be used to fabricate wearable acidity sensors. The absence of calibrating the pH sensor means that it can be used to make hand-held, disposable pH meters that farmers can use to check the acidity of their soil for optimum harvest.

“The sensitivity and accuracy of our technique can be improved further in future by using customised functional organic molecules as expert chemists are regularly synthesising different functional molecules in their laboratory,” says Das.