Researchers at IIT Jodhpur have fabricated a portable sensor for detecting lead in water. It can detect lead even when present at extremely low concentration of 0.018 ppb, much lower than the WHO limit of 5 ppb for drinking water.
A highly sensitive, portable device that can detect the presence of lead in water even when present in extremely small amount of 0.018 parts per billion (ppb) has been fabricated by a team of researchers from the Indian Institute of Technology (IIT) Jodhpur. The World Health Organisation limit for lead in drinking water is 5 ppb.
Besides being highly sensitive, the sensor is also highly specific to lead even in the presence of other metals such as mercury, copper, zinc, cadmium and chromium. It takes just about four seconds to detect lead.
“The sensor can be reused multiple times. We tested it for 50 cycles but as the materials used in the sensor are stable, it can be used several times for real-time monitoring,” says Prof. Mahesh Kumar from the Department of Electrical Engineering at IIT Jodhpur who led the team. The results were published in the journal IEEE Electron Device Letters.
The research was done in collaboration with the Singapore-based Agency for Science, Technology and Research.
Fabricating the sensor
The sensor is fabricated using silicon wafer as the base material. However, the crystal structure of silica and gallium nitride/aluminium gallium nitride is different and so they experience stress when grown on silicon. “Also, the thermal expansion of silicon and gallium nitride is different. As the layers are grown at 1000 degree C, it causes stress on the grown material,” says Prof. Kumar.
To produce a layer of gallium nitride that is stress-free, the researchers produced a stack of five layers of gallium nitride and aluminium gallium nitride on the silicon wafer base (with the percentage of aluminium in gallium nitride reducing from 100% to 0%). This helped in producing an almost defect-free gallium nitride layer at the top. A layer of AlGaN is then grown on the defect-free gallium nitride.
The researchers then fabricated the source and drain terminals that allow the electrons to flow and then used a gate (akin to a valve) to regulate the flow of electrons. “At the junction of the top layers of GaN and AlGaN, a two-dimensional electron gas is formed naturally, which is the conducting layer that moves the electrons from the source to the drain,” he explains.
How it works
Finally, the gate was functionalised to facilitate lead ion adsorption. When lead ions get adsorbed on the gate, the rate of flow of electrons increases leading to increased current flow. The amount of increase in current depends on the amount of lead ions that get adsorbed.
“There is a slight increase in current even when lead concentration in water is as low as 0.01 ppb. The increase is more at higher lead concentration,” says Adarsh Nigam from IIT Jodhpur and the first author of the paper. “We used 0.5 volt to ensure a constant flow of current from the drain to the source.”
“We tested the sensor using 15 samples each of tap and lake water. The sensor shows good agreement with the results from standard techniques used for lead detection,” says Nigam.
The sensor can be reused by washing with water. The sensor response time — the time taken to detect lead — is hardly affected when reused, Nigam says.
Fabricating an array of sensors
“We are fabricating an array of devices on a single chip to detect different heavy metal ions. We have already developed a cadmium ion sensor,” says Nigam. “The cadmium sensor has higher sensitivity (0.255 ppb) than other electrochemical approaches and can detect cadmium in about 3 seconds.” The results of cadmium sensor were published in April 2019 in the journal IEEE Electron Device Letters.
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