Pesticides and herbicides are critical to global food safety, but can pose a safety risk to people who inadvertently ingest them. Protecting human health therefore requires sensitive analytical methods to identify even trace amounts of potentially harmful substances. Now researchers report in the ACS study on Nano letters have developed a state-of-the-art imaging technique to detect pesticide contamination at low levels. Applying this method to fruit shows that current food safety practices may be inadequate.

The analytical technique called surface-enhanced Raman spectroscopy (SERS) is gaining popularity as a nondestructive method for detecting chemicals from modern agriculture on agricultural produce. SERS uses metal nanoparticles or nanosheets to amplify the signals produced by molecules when exposed to a Raman laser beam. The patterns created by the metal-enhanced scattered light serve as molecular signatures and can be used to identify small amounts of specific compounds. To improve SERS sensitivity for pesticide detection, Dongdong Ye, Ke Zheng, Shaobo Han and colleagues developed a metal-coated membrane that they could place on agricultural produce. They also wanted to develop the material to be versatile enough to be suitable for a range of other applications.

The researchers started with a cellulose hydrogel film, which they stretched to form aligned nano-wrinkles on its surface. They then dipped the film in a silver nitrate solution to coat the grooves with SERS-enhancing silver nanoparticles. The resulting membrane was highly flexible and virtually transparent in visible light—essential properties for SERS signal detection.

In testing the silver-embedded membrane for food safety applications, the researchers sprayed the pesticides thiram and carbendazim individually or together on apples, allowed the fruit to air dry, and then washed them to mimic everyday practices. When they placed their membrane over the apples, SERS detected pesticides on the apples, even though the chemicals were present at low concentrations. The team was also able to clearly resolve scattered light signatures for each pesticide on apples sprayed with both thiram and carbendazim, and detect pesticide contamination through the fruit’s skin and in the outermost layer of pulp.

These results suggest that washing alone is not enough to prevent pesticide absorption and that peeling is necessary to remove possible contaminants in the peel and outer flesh of the fruit, the researchers say. In addition to apples, they also used the SERS membrane system to detect pesticides on cucumbers, shrimp, chili powder and rice.

Reference: Lin Z, Fu X, Zheng K, Han S, Chen C, Ye D. Cellulose surface nanoengineering for food safety visualization. Nano-Lett. 2024. doi: 10.1021/acs.nanolett.4c01513

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