A highly sensitive sensor that can detect real-time the presence of zinc over a wide concentration range — 0.1-500 ppm — has been fabricated by researchers at the Indian Institute of Technology (IIT) Bombay. The electrochemical sensor can be used for detecting zinc in the soil even in the presence of other elements and also at minute levels as seen in human sweat.
Accurate determination of zinc in soil samples will help in soil-nutrient assessment and prevent overuse of fertilisers while measuring zinc in sweat samples can help signal early onset of muscular fatigue. It can be used as a noninvasive point-of-care sensor. Further, the sensor requires only few microliters of analyte, thus making it possible to detect zinc in sweat samples.
The sensor is mechanically sturdy and so the signal response remains unaffected even when the sensor is bent 180 degrees. The sensor can also be employed over 4-7 pH range, indicating its usefulness for both sweat-based physiological sensing and soil-nutrient assessment.
The two-member team led by Prof. Chandramouli Subramaniam from the Department of Chemistry at IIT Bombay have already tested the sensor for the presence of zinc on sweat samples and in three different soil samples — deep black soil, read loamy soil and red clayey soil — collected from Maharashtra, Rajasthan and Tamil Nadu.
The sensor has one working electrode and a reference electrode. These electrodes are made of cellulose fibre coated with carbon nanotubes. The working electrode is coated with a polymeric ion-receptor (tetrakis aminophenyl porphyrin) that binds specifically to zinc.
A fixed voltage is applied to the electrodes and there is an increase in current when zinc binds to the porphyrin receptor on carbon nanotubes. The amount of increase in current depends on the concentration of zinc that binds to the electrode.
“We are able to detect extremely low concentration (0.1 ppm) of zinc because of the very high surface area and electrical conductivity of carbon nanotubes,” says Sudeshna Mondal from IIT Bombay and first author of a paper published in the journal ACS Sustainable Chemistry & Engineering .
The porphyrin receptor in mixed with polyvinyl chloride (PVC) and then coated on carbon nanotubes. “Using the PVC matrix ensures uniformity in terms of porphyrin coverage on carbon nanotubes. If we directly coat the carbon nanotubes with porphyrin receptor, we cannot control the uniformity and number density of porphyrin,” says Prof. Subramaniam.
Both the electrodes were then laminated. “We sealed the electrodes by laminating it and provided a well-defined opening in the middle of the lamination to allow direct interaction between porphyrin and zinc,” says Mondal.
Explaining the rationale behind lamination, Prof. Subramaniam says: “A fixed-size opening controls the area of interaction as well as achieves uniformity of interaction between zinc and porphyrin. The signal from the sensor depends on the area of contact with zinc.”
The porphyrin receptor has a cage-like structure with a void in the centre. “The size and charge of the void matches perfectly with zinc. The matching size makes it possible for zinc to bind to porphyrin and the charge allows the interaction between the two,” he says. “It is akin to only iron binding to porphyrin in haemoglobin.” The researchers are working on developing a read-out device to use the sensor in collaboration with Electrical Engineering Department at IIT Bombay. Meanwhile, field trials are under way to further test the device. “In six months we will be able to make a full-fledged product,” Prof. Subramaniam says. “This project is funded by the Nanomission program of DST India and is intended to complement the soil-health card program of the Indian government.”
The researchers are already working on developing similar sensors for other plant nutrients such as potassium, nitride, phosphate and sodium.
“We already know which ion-receptor to use on carbon nanotubes to achieve perfect binding with each analyte,” he says.