Researchers at Sweden's Karolinska Institute have developed a tiny sensor for detecting pesticides on fruit. The technique, which was described as a proof-of-concept in the journal Advanced Science, uses flame-sprayed silver nanoparticles to boost the chemical signals. While the research is still in its early stages, the researchers hope that these nano-sensors will be able to detect pesticides in food before they are consumed.
"Reports show that up to half of all fruits sold in the EU contain pesticide residues that have been linked to human health problems in larger quantities," says Georgios Sotiriou, principal researcher at Karolinska Institutet's Department of Microbiology, Tumor and Cell Biology and the study's corresponding author.
"Current methods for detecting pesticides on single products before consumption, on the other hand, are limited in practice due to the high cost and time-consuming manufacturing of sensors. To address this, we created low-cost, repeatable nano-sensors that could be used to monitor traces of pesticides in fruits at a store, for example."
Surface-enhanced Raman scattering, or SERS, a powerful sensing technique that can increase the diagnostic signals of biomolecules on metal surfaces by more than 1 million times, is used in the new nano-sensors. Chemical and environmental analysis, as well as the detection of biomarkers for various diseases, have all benefited from the technology. High production costs and limited batch-to-batch reproducibility have, however, limited their use in food safety diagnostics so far.
The researchers used flame spray to deposit small droplets of silver nanoparticles onto a glass surface to create a SERS nano-sensor in this study. The flame spray is a well-established and cost-effective technique for depositing metallic coatings.
"The flame spray can be used to quickly produce uniform SERS films across large areas, removing one of the key barriers to scalability," says Haipeng Li, the study's first author and a postdoctoral researcher in Sotiriou's lab.
To improve the sensitivity of the silver nanoparticles, the researchers fine-tuned the distance between them. They used a spectrometer to uncover their molecular fingerprints after applying a thin layer of tracer dye on top of the sensors to test their substance-detecting ability. According to the researchers, the sensors reliably and uniformly detected molecular signals, and their performance remained unchanged after 2.5 months of testing, indicating that they have a long shelf life and are feasible for large-scale production.
The researchers calibrated the sensors to detect low concentrations of parathion-ethyl, a toxic agricultural insecticide that is banned or restricted in most countries, to test their practical application. On a portion of an apple, a small amount of parathion-ethyl was applied.
Later, the residues were collected using a cotton swab soaked in a solution to dissolve the pesticide molecules. The pesticides were detected after the solution was sprayed on the sensor.
"Without destroying the fruit, our sensors can detect pesticide residues on apple surfaces in five minutes," says Haipeng Li.
"While larger studies are needed to validate them, we provide a proof-of-concept practical application for food safety testing at scale prior to consumption." The researchers now want to see if the nano-sensors can be used in other areas, such as finding biomarkers for specific diseases at the point of care in resource-constrained environments.
