Sniffing Out New Methods for In-Field Pesticide Detection

Between a global pandemic, the tricky management of the Earth’s dwindling natural resources, and the climate crisis — it’s no exaggeration to say that the agricultural sector has a fair few major challenges on its plate that it will need to learn to adapt to.

With pesticides generally being an effective and economical way for the modern farmer to improve their crop yields and quality, global pesticide usage has increased dramatically over recent years in response to the threats these challenges pose. According to figures from the mid-2010s, approximately 2 million tonnes of pesticides were being used annually worldwide at this time; but estimates for 2020 now put this figure at close to 3.5 million tonnes.

Such a spike in pesticide use comes with its own set of challenges that farmers must deal with. In order to ensure that pesticide usage on agricultural products stays within the allowable safety limits, there is now a strong demand from within the sector for advanced pesticide screening methods that can be used by farmers in the field as a part of their existing crop monitoring regimens.

Smart farming and pesticide vapors

So-called ”smart farming” approaches give farmers a way to completely optimize their operations through constant real-time monitoring of their farm. With a good smart farming system, farmers can collect data on all aspects of their practices and discover where they might cut back or improve on certain elements to save money, improve crop yield or quality, or to lessen their environmental impact.

To make such a technological revolution possible, compatible smart-sensors must be developed that are capable of monitoring every possible point of interest for farmers, including the levels of pesticides that their crops are being exposed to.

“The incorporation of remote, standalone sensors is key for the realization of potent smart-farming solutions,” explains Dr Evangelos Skotadis, a post-doctoral researcher at the National Technical University of Athens. Skotadis is also the first author of a newly published research piece, detailing the development of a nanomaterial-based sensing array that can detect levels of pesticide in the air.

“Our approach utilizes nanomaterial-based sensors that are sensitive towards pesticide vapors, and that are able to discern between pesticides and other environmental co-factors. In addition, the sensors tick all necessary boxes by being low-power, low-cost, reusable, simple to operate, and with a small footprint.”

The sensor developed by Skotadis’ group had been proven to be capable of detecting the active substances present in pesticide vapors in earlier tests, but never before had it been tested with commercially available pesticide solutions. In their latest research, the group found the sensing array was easily able to differentiate between ordinary relative humidity levels and an environment where a locally available chlorpyrifos-based insecticide, Chloract 48 EC, had been used.

When the group analyzed the unique response patterns using principal component analysis (PCA), a statistical analysis tool, they found that the gas-sensing array was also able to quantify the concentration of chlorpyrifos present in each gas sample being analyzed successfully. With further study, the group believes that this PCA technique in conjunction with the sensing array could also be used more generally to detect and quantify many different individual gases within a single sample, raising the possibility that the sensor could be used as a multi-purpose “e-nose” in the future.

The combination of such a pesticide sensor with a smart farming system could be invaluable to farmers. Gone would be the days of waiting on busy laboratories to process samples and send back pesticide testing results; farmers would have access to real-time data that could help them to ensure compliance with pesticide regulations throughout a crop’s lifetime. For Skotadis and the team at the National Technical University of Athens, the next major steps will be to evaluate the sensor’s use further in analyzing multiple gases, and to implement this sensor into a real smart farming system and test its performance in real-world conditions.

“There are very few reports on the detection of pesticide vapors, rendering our study a valuable contribution in the field of environmental monitoring as well as gas-sensing technology,” Skotadis said. “The evaluation of our sensing array in real-life, in-the-field conditions will be critical for our next steps.”

Detecting harmful levels of pesticide air pollution

Beyond the development of monitoring systems for smart farming, being able to detect pesticide vapor on-site is also important from the perspective of air pollution and human and animal safety.

While pesticides and insecticides are incredibly effective at repelling pests from the crops that they are applied to, they themselves cannot be so easily removed. Organophosphorus-based pesticides, such as chlorpyrifos, are largely resistant to physicochemical conditions and can remain embedded in the farm environment for a long time. This resilience makes the pesticides particularly difficult to deal with if they begin to leach out of the farming environment and contaminate nearby groundwater, or are distributed through the air streams as aerosol mist. Breathing in this aerosol mist can cause disruption to the body’s immune system through the inhibition of the acetylcholinesterase enzyme (AChE), leading to lung problems, cardiovascular and nervous system dysfunction, and in some cases even death.

“Pesticides are designed to kill some type of living organism — weed, insect, disease — and as such need to be handled carefully and according to the label,” cautions Jeffery S. Graybill, an Extension Educator in Agronomy at Penn State University.

“Each specific [pesticide] product has a United States Environmental Protection Agency (EPA)-determined re-entry interval (REI) which specifies how long workers and others must stay out of the applied area. However, this REI can vary with temperature, air movement, and humidity.  [Pesticide vapor detection] devices would be a great tool to verify that the chemical and its activity are no longer present at levels which are of health concern.”

In new research published in Scientific Reports, a collaboration of researchers from Iran and Vietnam report the development of a simple colorimetric ”e-nose” sensor that uses nanoparticles arranged on filter paper to sense the presence of aerosolized pesticides, and change color when they are detected.

The researchers printed groups of gold or silver nanoparticles (AuNPs and AgNPs, respectively), each capped with different functional groups to interact with each pesticide of interest, onto different zones marked out on standard laboratory filter paper. They then sprayed several aerosolized pesticides — including malathion, parathion, chlorpyrifos, and diazinon — in the direction of the paper e-nose. The nanoparticle zones were evaluated with the naked eye and also with image analysis software to record and measure any changes in color in response to the vapors accurately.

They found distinct patterns appearing on the paper sensing array, that reliably indicated the presence of individual pesticides. For example, when exposed to malathion every printed nanoparticle zone changed color; and when exposed to chlorpyrifos only three zones would change.

In the paper, the scientists explain that these color changes are caused by the pesticides’ interaction with the printed functional nanoparticles. In the presence of the correct analyte, the electrostatic repulsion between the nanoparticles will lessen, causing them to aggregate together. As these nanoparticles begin to cluster they will scatter visible light differently, prompting a change in color that is noticeable to the human eye.

“The color of sensing elements changed after exposure to the analyte due to the NPs aggregation,” the scientists explain in their paper. “The presence of a pesticide reduced the electron repulsion or created a bridge between two NPs, leading to a decrease in the distance [between] NPs. Based on this event, the yellow color of AgNPs turned to orange or brown, and the color of AuNPs changed from red to pale or intense purple.”

Further testing confirmed that these results were unaffected by the introduction of other chemicals, such as other non-organophosphate pesticides or similar organic compounds.

Similar color-based e-noses have been developed previously, but the fabrication of these systems had often required expensive or complicated processes. In addition, these sensors would usually be built on a polymer substrate that was brittle and prone to leaching. In using regular laboratory filter paper as a basis for this pesticide detecting e-nose, suddenly these e-nose sensors become a lot easier to work with and to manufacture.

The researchers behind the paper-based pesticide sensor also say that this ease of manufacture and flexibility means that the sensor could be easily embedded in clothes, accessories, or masks belonging to farm workers, effectively allowing for an on-the-go pesticide pollution detection kit.

Both pesticide sensors profiled in these two studies could “have especially good application within greenhouse applications” Graybill explains, as here “pesticide vapors and residues may not degrade and dissipate as quickly as they do in an outdoor setting.”

“Many pesticides are also expensive and so farmers generally apply them when needed. Sometimes, however, they are applied as a preventative and any technology which can aid farmers in the appropriate use of these chemicals should be well received by both farmers and their customers.”

Effective methods for in-field pesticide detection are a crucial line of defense in protecting both the wellbeing of farm workers and the health of the land on which they work. With access to smart or easy-to-use pesticide screening systems, farmers benefit from an increased control over their operations that can assist them in making more informed decisions about their pesticide usage. Such systems could also help to ensure that these operations stay in line with relevant statutory safety limits at all times.