Determination of thiamethoxam with a Nafion/l-ornithine coated graphite paper electrode and its application to celery residue analysis

Abstract

Thiamethoxam (TMX), a widely used neonicotinoid insecticide, is electrochemically detected based on the reducibility of the nitro group. However, complex electrodes and matrix interferences limit the application. Due to the response attenuation of the graphite paper electrode (GPE) toward TMX detection, Nafion solution (NF) and L-ornithine hydrochloride (LO) were adopted to improve the measurement stability of TMX and enhance the resistance to matrix interferences. After drip-coating modification, a new electron-conducting layer was formed, accompanied by a scaly film with stacked interlayer gaps of approximately 100 nm. LO acted as a bridge, which bonded with NF via a sulfonylation reaction and linked to TMX through C–O–NO₂ during the electro-sorption and enrichment process. This binding to TMX corresponded to a reversible electrochemical process involving one-electron transfer. Accordingly, the analytical stability was greatly improved. The modified NF–LO/GPE also exhibited excellent anti-interference ability against extractable organic acids from celery, such as chlorogenic acid. This was mainly attributed to the negative charge repulsion of NF and acid-base neutralization under moderate alkaline activation. This work demonstrates good modifiability of NF–LO based on the sheet-like graphite electrodes and conducts a valuable exploration of matrix interference in real samples. Enhancing the adsorption capacity of the electrode surface by constructing unsaturated C–O sites may be the primary issue for the detection of organic pesticides.