A recent study by researchers at Gauhati University, Indian Institute of Technology Bombay and DAIICT in India has demonstrated a soil moisture sensor made from graphene quantum dots, which are nanometer-sized fragments of graphene.

Water sensors are vital for various agriculture applications, like keeping track of the watering schedule for a large number of plants, such as for a field of crops. Soil moisture sensors measure the water content in the soil to avoiid crop destruction by under or over watering the field.

“Our motivations behind this study was to devise a simple, inexpensive and scalable approach for synthesizing graphene quantum dots, and to develop an affordable soil moisture sensor that is suitable for large scale use,” says Prof Hemen Kalita, who is the lead author of this study.

The researchers have proposed a method to produce graphene quantum dots as small as 3–5 nanometers from easily available and low-cost graphene oxide. They coated a thin film of graphene oxide onto a carbon electrode and placed it inside an electrolyte solution. When an electric current is applied to the setup, the carbon bonds in the graphene oxide get cleaved, and molecules of the electrolyte occupy those gaps in the graphene oxide layer. Eventually, they form quantum dots of graphene having oxygen-containing chemical groups.

“At a laboratory scale, we were successful in synthesizing graphene quantum dots through our novel approach, and we have filed a patent for the synthesis method,” says Prof Kalita.

Using the graphene quantum dots, the researchers fabricated a soil moisture sensor which is smaller in size than a lentil seed. The moisture content value displayed by the sensor depends on the resistance measured across it, and with an increasing percentage of water content, there is a fall in resistance. When the sensor is inserted into moist soil, the oxygen atoms present in the graphene quantum dots interact with the hydrogen atoms of the water and form a layer of water molecules on the surface of the sensor. When an external voltage is applied to the sensor via a source meter, the loosely held water molecules in the upper layers get ionized and conduct electrical charge. This leads to a decrease in the resistance of the sensor.

The researchers tested the soil moisture sensors on samples of black and red soil. They found that the moisture content measured by the sensor closely matched the known water content of the soil samples. The sensor gives the final reading within 3 minutes and can be used again after 20 seconds.

Further, the researchers tested the stability of the sensor by continuously using it over five months to measure the water content in soil samples. They found that the sensor gives a consistent reading throughout this time and works well for a range of soil water levels.

“With extensive field testing and improved packaging, our sensors will be suitable for commercialization. A few companies have approached us and initiated discussions with our team to take this project to the industry front,” says Prof Kalita. “We are aiming to develop stable and affordable sensors for the middle-class farmer community,” he concluded.