Surface-enhanced Raman spots low levels of pesticide contamination

12 Aug 2024

Anhui Agricultural University designs metal-coated membrane for testing of fruit and produce.

A project at China's Anhui Agricultural University (AHAU) has developed a Raman-based imaging technique to detect low levels of pesticide contamination in fruit.

Published in ACS Nano Letters, the method further develops the use of Raman methods for the analysis of organic produce and points towards improved future food safety standards.

Spectroscopic methods of assessing food quality have been studied for some time, with the goal of detecting the molecular fingerprints of undesirable contaminants or decaying material. Several of these have aimed to observe the volatile organic compounds (VOCs) that are given off as food rots.

Examples have included a 2022 UK project developing an optics-based sensor to monitor the health of strawberry crops, alongside bespoke machine learning hardware to process the spectroscopic data.

Elsewhere a Canadian team designed a cheap hand-held device to assess fruit ripeness using LEDs emitting at selected wavelengths, with the level of reflected light from the target used as a direct indication of changes to the texture of the fruit and its condition.

Raman-based methods, in particular surface-enhanced (SERS) variants where metal nanoparticles or nanosheets are used to amplify the inherently weak Raman signals, have been a particular focus of research, thanks to their potential applicability in current agriculture workflows to detect low levels of specific biomarkers, such as the presence of pesticides left over from fruit cultivation.

AHAU aimed to develop a SERS method primarily for testing of fruit but which could be versatile enough for use in other testing scenarios, based on the use of a metal-coated membrane that could be placed directly onto farm-grown produce or other targets and provide the signal amplification required for SERS.

New guidance indicated for food preparation

The researchers started with a cellulose hydrogel film, first stretched to form aligned nanoscale wrinkles along its surface and then immersed in a solution of silver nitrate to coat the grooves with signal-enhancing silver nanoparticles. This left the membrane highly flexible and practically transparent in visible light, essential features for SERS signal detection.

Using apples as test subjects, AHAU sprayed the commercial pesticide and fungicide chemicals thiram and carbendazim onto the fruit, air-dried and washed them to mimic everyday practices, and then applied the SERS method to analyze for remaining contamination.

SERS detected pesticides on the apples even when the chemicals were present at low concentrations, commented AHAU. The team was able to clearly resolve scattered-light signatures for each pesticide, as well as detect pesticide contamination through the fruit's peel and into the outermost layer of pulp.

"These three-dimensional nanowrinkled structures considerably improve the spatial-electromagnetic-coupling effect of metal nanoparticles on the membrane surface, providing a 2.3 × 10⁸ enhancement factor for the SERS effect for trace detection of pesticides in foods," noted the team in its paper.

Further tests on cucumbers, shrimp, chili powder and rice confirmed that the AHAU membrane assisted a SERS-based detection of contaminants in those products too, and suggested that washing alone could be insufficient to prevent pesticide ingestion in domestic environments. Peeling would be required to remove potential contamination in the skin and outer pulp, the researchers said.

"This study provides a new idea for designing nanowrinkled structures and broadening cellulose utilization in food safety," commented AHAU.