A biodegradable metal-organic framework (MOF) that does double duty as a pesticide delivery system and a plant fertilizer could help improve the effectiveness of pest-killing compounds (ACS Agric. Sci. Technol. 2023, DOI: 10.1021/acsagscitech.2c00295).
MOFs are crystalline lattices containing metal nodes held together by organic linker molecules, and their pores can store guest molecules. In this latest example, researchers added avermectins, a family of compounds produced by the bacterium Streptomyces avermitilis, to a MOF. Inserting these broad-spectrum pesticides within a structure can protect them from degrading easily when exposed to light and make them more potent against pests.
Previous attempts to protect and deliver avermectins have included encapsulating the pesticides in polymers—which may add microplastics to the environment—or loading them into MOFs. But most MOFs use toxic metals or expensive organic linkers, making them unsuitable for agriculture.
Lidong Cao of the Chinese Academy of Agricultural Sciences and colleagues instead used MOFs made from potassium ions, an essential plant nutrient, and cone-shaped sugar molecules called cyclodextrins (CDs). Such CD-MOFs are nontoxic and biodegradable and have been tested in applications such as drug delivery. But this is “the first example of a CD-MOF being used for pesticide delivery,” Cao says.
The CD-MOF can carry up to 20% of its weight in avermectins, which when nestled in the CD-MOF’s pores are twice as stable under bright light as naked avermectins. The CD-MOF grains, roughly 3 µm wide, could release up to 70% of their cargo over 8 days, depending on the pH of the surroundings.
The researchers found that their avermectin-loaded CD-MOF was up to 20 times as effective as powdered avermectins alone at killing the citrus red mite Panonychus citri, which commonly infests citrus plants. Cao suggests that this result might be due to a combination of effects, including the CD-MOF’s gradual release of avermectins and the formulation’s higher surface area compared with powdered avermectins.
Applying the loaded CD-MOFs to cotton plants, another crop that is commonly protected by avermectins, yielded a modest increase in root and stem growth after 2 weeks. In contrast, applying powdered avermectins inhibited plant growth.
“I think the main selling point for this MOF is its biocompatibility—its nontoxicity. That’s where it stands out,” says Sanjit Nayak, a materials chemist at the University of Bradford. Nayak has developed MOFs for pesticide and herbicide delivery but was not involved in this work. He adds that Cao’s team should assess the long-term stability of the avermectin-loaded CD-MOFs to see whether they are suitable for storage. “You want them to degrade [to release the pesticide], but you don’t want them to degrade too fast,” Nayak says.
For now, Cao says the work serves as a proof of principle. His team is testing the CD-MOF with other pesticides and hopes to carry out field trials of the material.
This story was updated on Feb. 23, 2023, to correctly state the size of the cyclodextrin metal-organic framework’s grains. It is 3 µm. The original included a paragraph separator symbol instead of the micro symbol.