pH-Responsive lignin-modified copper-based metal-organic frameworks for precision thiamethoxam delivery in the targeted control of red imported fire ants ( Solenopsis invicta)

The red imported fire ant (RIFA), Solenopsis invicta, is a globally invasive pest, posing a significant challenge in developing effective control strategies. HKUST-1, a classical metal-organic framework (MOF), holds potential in pest control; however, its water instability limits its practical application. In this study, we address this limitation by synthesizing lignin-modified metal-organic frameworks (L/HKUST-1), using alkaline lignin as a biopolymer carrier in a one-pot synthesis process. The effects of various temperatures, stirring speeds, and lignin concentrations on the morphology and particle size of L/HKUST-1 were systematically investigated. The resulting L/ HKUST-1 exhibited an octahedral morphology with an average particle size of 530 nm, along with improved water stability and a thiamethoxam (TMX) loading rate of 26.5 %. A pH-responsive pesticide delivery system (TMX@L/HKUST-1) was developed using L/HKUST-1 as the carrier and TMX, a neonicotinoid insecticide, as the active ingredient. This system effectively controls RIFA by precisely delivering TMX in response to the acidic pH environment of the RIFA gut. In vitro release kinetics demonstrated that TMX@L/HKUST-1 rapidly releases TMX under pH 4.5 conditions. Bioassay results revealed that TMX@L/HKUST-1 significantly enhances toxicity against RIFA by increasing feeding activity and causing damage to intestinal cells. The LC50 of TMX@L/HKUST-1 was 1.225 mg/L (24 h) and 0.3164 mg/L (48 h), respectively. The system also inhibited key enzymes in detoxification and oxidative stress in RIFA. Furthermore, 16S rDNA analysis showed that TMX@L/HKUST-1 reduced the relative abundance of Serratia, while increasing the abundance of Pseudomonas and Bacillus, thereby weakening RIFA's resistance to pesticides and pathogens. These findings suggest that TMX@L/HKUST-1 is a promising nanopesticide delivery system, offering a novel approach for the efficient control of invasive species such as RIFA.