Just in! A nano-sensor that detects pesticides on fruit in minutes

The technique could be used to monitor pesticide residues at the store.
Deena Theresa

Last month, the Environmental Working Group (EWG), an American activist group, announced its annual Dirty Dozen List - a list of the most pesticide-contaminated fresh fruits and vegetables based on the latest tests by the Department of Agriculture and Food and Drug Administration. Strawberries, spinach, leafy greens, apples, grapes, and nectarines are the top offenders.

In Europe, Pesticide Action Network (PAN) Europe conducted a nine-year study with government data, analyzing 100,000 popular homegrown fruit samples. The study found that a third of apples and half of all blackberries surveyed had residues of the most toxic categories of pesticides. Some of them have been linked to illnesses such as cancer, heart disease, and birth deformities.

"Reports show that up to half of all fruits sold in the EU contain pesticide residues that in larger quantities have been linked to human health problems," said Georgios Sotiriou, principal researcher at the Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, according to a press release

Sotiriou and his colleagues at Karolinska Institutet in Sweden have developed a tiny sensor for detecting pesticides on fruit in just a few minutes. Described as a proof-of-concept in a paper in the journal Advanced Sciencethe technique uses flame-sprayed nanoparticles made from silver to increase the signal of chemicals.

Based on a 1970s discovery 

The concept is at an early stage, but the researchers hope the nano-sensors can help uncover food pesticides before consumption. 

“Current techniques for detecting pesticides on single products before consumption are restricted in practice by the high cost and cumbersome manufacturing of its sensors. To overcome this, we developed inexpensive and reproducible nano-sensors that could be used to monitor traces of fruit pesticides at, for example, the store,” said Sotiriou, the study's corresponding author. 

The new nano-sensors are based on a 1970s discovery known as SERS (surface-enhanced Raman scattering). It is a powerful sensing technique that can 'increase the diagnostic signals of biomolecules on metal surfaces by more than 1 million times'.

Though the technology has been used in several research fields, high production costs and limited batch-to-batch reproducibility have hindered its widespread application in food safety diagnostics. 

Flame spraying
Flame nanoparticle deposition was used to produce robust nano-sensors that can detect pesticide residues on apple surfaces within minutes. Source: Haipeng Li and Georgios A Sotiriou.

The experiment employed a flame spray technique

The current study saw researchers creating a SERS nano-sensor by using a flame spray, which is a well-established and cost-effective technique for depositing metallic coating. The flame spray delivered small droplets of silver nanoparticles onto a glass surface.  

“The flame spray can be used to quickly produce uniform SERS films across large areas, removing one of the key barriers to scalability,” said Haipeng Li, a postdoctoral researcher in Sotiriou’s lab and the study’s first author.

The distance between the individual silver nanoparticles was calibrated to enhance their sensitivity. To test their substance-detecting ability, the researchers applied a thin layer of tracer dye on top of the sensors and used a spectrometer to uncover their molecular fingerprints. 

The sensors reliably and uniformly detected the molecular signals and their performance remained intact when tested again after 2.5 months, which emphasizes their shelf life potential and feasibility for large-scale production, according to the researchers.

Practical success

Next, the sensors' practical application had to be tested.

The researchers finetuned them to detect low concentrations of parathion-ethyl, a toxic agricultural insecticide that is banned or restricted in most countries. 

A small amount of parathion-ethyl was placed on part of an apple. The residues were later collected with a cotton swab that was immersed in a solution to dissolve the pesticide molecules. The solution was then dropped on the sensor, which confirmed the presence of pesticides.

“Our sensors can detect pesticide residues on apple surfaces in a short time of five minutes without destroying the fruit,” Haipeng Li said. “While they need to be validated in larger studies, we offer a proof-of-concept practical application for food safety testing at scale before consumption.”

The nano-sensors could be a potential application in discovering biomarkers for specific diseases at the point of care in resource-limited settings - the researchers are looking to explore further. 

The research was funded by the European Research Council (ERC), Karolinska Institutet, the Swedish Foundation for Strategic Research (SSF), and the Swedish Research Council.

Abstract: Surface-enhanced Raman scattering (SERS) is a powerful sensing technique. However, the employment of SERS sensors in practical applications is hindered by high fabrication costs from processes with limited scalability, poor batch-to-batch reproducibility, substrate stability, and uniformity. Here, highly scalable and reproducible flame aerosol technology is employed to rapidly self-assemble uniform SERS sensing films. Plasmonic Ag nanoparticles are deposited on substrates as nanoaggregates with fine control of their interparticle distance. The interparticle distance is tuned by adding a dielectric spacer during nanoparticle synthesis that separates the individual Ag nanoparticles within each nanoaggregate. The dielectric spacer thickness dictates the plasmonic coupling extinction of the deposited nanoaggregates and finely tunes the Raman hotspots. By systematically studying the optical and morphological properties of the developed SERS surfaces, structure–performance relationships are established and the optimal hot-spots occur for interparticle distance of 1 to 1.5 nm among the individual Ag nanoparticles, as also validated by computational modeling, are identified for the highest signal enhancement of a molecular Raman reporter. Finally, the superior stability and batch-to-batch reproducibility of the developed SERS sensors are demonstrated and their potential with a proof-of-concept practical application in food-safety diagnostics for pesticide detection on fruit surfaces is explored.

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