Monitoring and managing the usage of pesticides and herbicides is a major challenge in an agriculture-driven economy like India. In recent years, several studies have detailed the health hazards of these chemicals, even as their use continues on a large scale. Among them, a controversial herbicide is paraquat dichloride, marketed as Gramoxone. Paraquat is one of the most extensively used herbicides globally, especially in developing countries, because of its low cost and high effectiveness.
However, it is a highly toxic chemical, known to cause Acute Respiratory Distress Syndrome (ARDS) and Parkinson’s disease, among other complications. More than 70% of farmer deaths in India are linked to accidental paraquat poisoning. Hence, regular monitoring of the chemical’s levels in crops and nearby water sources is of utmost importance. In an attempt to make quality control more accessible to farmers, researchers at the Indian Institute of Science, Bangalore, and National Bureau of Agricultural Insect Resources (NBAIR), Bangalore, led by Prof. Santanu Bhattacharya, from IISc, have developed a simple, inexpensive and robust method to specifically detect paraquat in various samples.
“Many efficient techniques to detect herbicides already exist, but they are time-consuming and involve elaborate and costly experimental set-up and tedious sample-preparation techniques that often limit their applicability in practical use”, says Dr. Deepa Bhagat from NBAIR, and the lead author of the study. In the proposed technique, one can estimate the level of paraquat simply with his or her eyes, making the test it very simple. The nanoparticles that the researchers used were called quantum dots, which emit red fluorescence. When paraquat is added, it forms a complex with the quantum dot thereby diminishing the fluorescence. This reduction in fluorescence is a measure of the amount of paraquat present in the sample.
To ensure that the sensors are cheap and easy to use, the researchers have developed them as low-cost portable strips. “This procedure does not require a trained technician or even a constant electric supply”, elaborates Dr. Bhagat. “It will be useful for poorer farmers who live in remote locations. Even a packet of twenty such paper strips will cost less than one rupee. The same strips can even be used multiple times by just washing with excess salt water”, she adds.
Another merit of this sensor is that it is very selective towards paraquat. Previously developed optical sensors could not distinguish between paraquat and a structurally similar molecule, diquat. Distinction between these is important for diagnostic purposes, as they have very different effects on the human body. The sensor is also suitable for usage on a wide variety of crops. “The response of many sensor molecules is often masked in the presence of other competing analytes. Therefore, one needs to develop a system which can produce an easily detectable signal for paraquat, even in a complex biological environment such as aqueous extract of crops”, explains Mr. Nilanjan Dey from IISc, who is a co-author of the study.
To verify this, the researchers screened a wide variety of crops, including vegetables, fruits, cereals and fodder. They found that the sensor operated as expected in 37 out of 50 test cases. The sensor can also be used for ‘non-invasive’ diagnosis in cases of paraquat poisoning. The researchers showed that the sensors could detect paraquat even in urine samples when poisoning was suspected, without interference from other herbicides or the urine itself. This could be used to identify cases of paraquat poisoning well in time and could save many lives.
The researchers believe the new low-cost, portable nanosensor for detecting paraquat on the field can be successfully transferred to the industry, thus bringing this life-saving tool from laboratories to the farmer’s pocket.