While the general practice of chemical pest control dates back several thousand years — the Sumerians were known to use sulfur compounds to repel insects and mites from their crops  — the industrial production of modern pesticides is thought to stem from 1950s India . The development of these technical grade pesticides has since enabled farmers to reduce greatly crop losses and increase food production.
Despite these advantages, it is important to recognize that pesticides are toxic by design, and so while we make use of these chemical pesticides, it is also important that we have sufficient technology through which to monitor their presence in our produce and on our farmland.
Presently, chromatography-based methods, such as gas chromatography with multiple detectors, or high-performance liquid chromatography with fluorescence, are the most commonly used pesticide screening methods for pesticide residue . These methods are normally highly selective and sensitive, and so are able to detect pesticide residue even at low concentrations. However, these techniques are often time consuming, and can require multistep sample pretreatment and the service of highly knowledgeable staff who can operate the machinery. To address these disadvantages, some researchers have begun to investigate the application of alternate techniques for pesticides screening.
Detecting harmful organophosphate residue
One of the most widely used class of pesticides are the organophosphates. These pesticides work by irreversibly disrupting the function of an enzyme central to the insect nervous system, proving fatal to common insect pests . Further study of these pesticides has shown that organophosphate residue in agricultural products or in water can cause adverse health events in wildlife  and in humans . Medical management of organophosphorus poisoning can be very difficult  and so it is important that highly sensitive detection methods are developed for the detection of organophosphate pesticide residue in environmental samples.
Richard Bruce is a man who was seriously affected by organophosphate poisoning which working as a farm manager many years ago. He now spends his time raising awareness about the dangers that can be presented by organophosphate pesticide exposure, and how difficult it can be to seek medical treatment after the fact.
“I was poisoned in 1992 by an illegal mix of organophosphates, but neither I nor my GPs knew anything about poisoning,” he recounts. “Had it not been for the advice of other pesticide awareness groups, I have no doubt that I would not be here today.”
“I was treated with intravenous drips for weeks, and this was followed by carefully chosen dietary and enzyme supplements to slow the deterioration of my health.”
Bruce believes that due to confusing packaging, many farmers may actually have no idea that they are working with organophosphate pesticides, and so could be unintentionally putting themselves at risk with their farming practices. Developments in pesticide residue screening, especially in the detection of the presence of organophosphate pesticide residues, could help to protect the health of farmers, and downstream manufacturers, and consumers as they come into contact with farm equipment and produce.
A recent publication from a research team at the Chinese Academy of Agricultural Sciences in Beijing, China, reports a significant advancement in the development of a simple dual modal sensor for the detection of organophosphate residue . The sensor works by exploiting the unique optical properties of gold nanoparticles (AuNPs) during dispersion and aggregation, and how these properties may change in reaction to changes in their immediate environment.
AuNP-based colorimetric and fluorescent assays of this type are already used commonly in chemical and biological sensing  as these methods are normally very fast and easy to perform while still being accurate and highly sensitive. Despite this, AuNP-based methods are typically considered unsuitable for the study of organophosphates. As the authors explain in their report, this is because many of the most common organophosphate pesticides also contain sulfur; the strong Au-S bond can electrostatically interfere with normal AuNP aggregation, resulting in a loss of specificity when using these detection methods.
In this new work, the researchers describe a variation of this method which would allow for better study of the organophosphates using rhodamine B-modified gold nanoparticles (RB-AuNPs) that were capped with adenosine triphosphate (ATP) on the particle surface. The effectiveness of these modified nanoparticles for use as a colorimetric and fluorescent sensor was tested using tap water samples that were mixed to contain varying levels of the common organophosphate pesticide, ethoprophos.
Colorimetric and fluorescence imaging showed that the modified nanoparticles were successfully able to detect the sulfur-containing ethoprophos down to a concentration of 37.0 nM (detection limit = 3σ/k). The simple assay also demonstrated strong reliability, sensitivity, and selectivity, and given its performance with these spiked samples, it is thought that it would also be able detect pesticides residue in real water samples. It is also believed that the simplicity and ease of the nanoparticle preparation and the detection procedure could enable future speedy on-site detection of organophosphate residue.
Taking analysis into the field
The desire for in-field or on-site pesticide screening is a common theme among new advancement efforts. Another recent publication  in the field from a team at the Università degli Studi di Firenze, Italy, supported by researchers at the European Laboratory for Non-Linear Spectroscopy, Italy, and St. Petersburg Electrotechnical University, Russia, has reported positive results for the use of portable Raman spectroscopy as an in-field detection method for pesticide residues on olive leaves.
The pesticide dimethoate (DMT) is an organophosphate insecticide that is commonly applied to fruit trees, including olive trees . Dimethoate is highly water-soluble  and so it is usually considered safe for use as the pesticide residue should then be removed during the normal course of the oil extraction process. Still, the detection and determination of DMT levels (as well as levels of omethoate, which DMT degrades to over time) is important, as the produce being treated with DMT still needs to be pesticide free and suitable for human consumption. A simple, sensitive, portable screening method would be an ideal answer to the needs presented by the olive and olive oil production industry.
“Surface-enhanced Raman spectroscopy, or SERS, is a novel experimental method for the study of matter based on lasers and nanotechnologies which has high sensitivity and specificity,” explains Professor Giulietta Smulevich, one of the authors of the new study. “Recently, advances in SERS-based spectroscopy have led to its further application in analytical settings and for the detection of pesticides on vegetables.”
A previous study has already shown that SERS can effectively detect the presence of DMT in aqueous solution and on a solid substrate . In this new work , the research team reported a number of new conclusions relevant to the use of SERS specifically for the detection of DMT as it relates to the olive farming industry.
The group performed SERS analysis on a DMT-treated olive leaf substrate using a portable microRaman setup. The concentration of DMT used varied from 10−2 to 10−5 M in order to mimic the theoretical testing of an olive leaf sometime after treatment with a standard 10−2–10−3 M DMT pesticide solution. An untreated segment of the leaf was used as a control.
Sensitivity and good signal-to-noise ratio were retained throughout the 10−4–10−2 M DMT concentration range, indicating that this sort of portable SERS apparatus could detect DMT effectively up to 1-2 months after the initial DMT treatment. However, the study authors do note that this timeframe would be impacted by the local climate and weather, as well as any particularly strong variation in the amount of pest activity in the area. Even so, it was concluded that this sort of method could be beneficial at least as a pre-screening test on leaf samples.
“We have shown that it is indeed possible to detect, by SERS, the presence of DMT — a pesticide commonly used for the control of the olive flies — on olive leaves,” Smulevich continued. “The method allows in-field analysis, enabling detection of dimethoate at levels 10-100 times below those commonly used for the treatment of olive plants.”
“Moreover, it does not require long and tedious sample preparation. This approach could be extremely rewarding for the protection of the final consumer, especially in cases of organic products.”
These two studies together represent only a small segment of the work that is currently being done to improve the pesticide screening technology that is available to the agricultural industry. And yet both already show promise as a way to improve upon the current accessibility of pesticide screening.
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