Pesticides that contaminate food products like fruit can have negative health consequences for consumers, but methods to detect these chemicals are typically complex, time-consuming and expensive. Technologies like surface-enhanced Raman scattering (SERS), which can increase the diagnostic signals of molecules on metal surfaces by more than a million times, pose the challenge of high production costs and limited batch-to-batch reproducibility, which hinders their widespread application in food safety diagnostics. Researchers at Karolinska Institutet have now developed a faster, more reproducible and less costly fabrication method to produce SERS nanosensors that can detect pesticides in minutes.
The researchers created the sensors using a flame spray technique to deliver droplets of silver nanoparticles onto a glass surface. The flame spray method is well-established, inexpensive and highly scalable, and also provides a high level of batch-to-batch reproducibility, the authors wrote. Another benefit of flame spray is its ability to produce uniform SERS films across large areas very quickly, said first author Haipeng Li. The researchers further enhanced the sensitivity of the sensors by fine-tuning the distance between individual silver nanoparticles using a dielectric spacer.
To first test the sensors’ ability to detect substances, the team applied a thin layer of R6G and obtained the SERS spectra using both a confocal Raman microscope and a portable Raman spectrometer, confirming the sensors’ ability to detect the dye with high sensitivity. To assess the sensors’ practical application, the researchers calibrated 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 the surface of an apple, and the residues were later collected using a cotton swab that was immersed and sonicated in an ethanol solution to dissolve the pesticide. The solution was then dropped on the sensor, which confirmed the presence of the pesticide. This proof-of-concept study was published in Advanced Science.
“Our sensors can detect pesticide residues on apple surfaces in a short time of five minutes without destroying the fruit,” said Li. “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 new sensors could potentially be used for cost-effective, easier, more flexible nondestructive food testing such as testing at the store where the food is sold. The researchers hope to next explore whether their SERS nanosensors could be applied in other areas such as point-of-care disease biomarker detection.
Photo: Flame nanoparticle deposition was used to produce robust nanosensors that can detect pesticide residues on apple surfaces within minutes. Credit: Artwork by Haipeng Li and Georgios A. Sotiriou