Double-enzymes-mediated Fe2+/Fe3+ conversion as magnetic relaxation switch for pesticide residues sensing.

• A double-enzymes-mediated Fe2+/Fe3+ conversion strategy was developed for acetamiprid residue detection (2.66 ng mL−1). • Fe2+ as T 2 signal transformation and amplification can show stable and sensitive signal out-put. • Fe2+, ACh, AChE and CHO are optimized to obtain the best reaction system with one-step mixing assay. • The sensor showed a 335-fold improvement in the sensitivity than that of TEIA method (0.89 μg mL−1). It is a great challenge to develop a newly rapid and accurate detection method for pesticide residues. In this work, based on acetylcholinesterase (AChE) and choline oxidase (CHO), a double-enzymes-mediated Fe2+/Fe3+ conversion as magnetic relaxation switch was explored for the measurement of acetamiprid residue. In the double-enzymes reactions, acetylcholine chloride (ACh) can be catalyzed to produce choline by AChE, which is successively hydrolyzed to betaine and hydrogen peroxide (H 2 O 2) by CHO. According to the enzyme inhibition principle, AChE activity will be inactivated in the presence of acetamiprid, thus leading to the less production of H 2 O 2. Wherein, Fe2+, ACh, AChE and CHO were optimized as the reaction substrates. In the reaction system, acetamiprid can be reflected by the transverse relaxation time (T 2) that related with H 2 O 2 mediated Fe2+ variations, which was further developed as an enzyme cascade amplification method. The detection linear range is 0.01∼1000 μg mL−1 (R2 = 0.99), and the limit of detection (LOD) is 2.66 ng mL−1 (S/N = 3, n = 3), behaving a 335-fold improvement in LOD than that of traditional enzyme inhibition method (0.89 μg mL−1). This method can realize "one-step mixing" detection of acetamiprid, which makes it a promising analytical tool for monitoring pesticide residue in complicated samples. [ABSTRACT FROM AUTHOR]