Mn(II)-based electron spin resonance (ESR) spectroscopy was used for detecting butyrylcholinesterase (BChE) and organophosphorus pesticides (OPs). MnO₂ nanosheets were synthesized with manganese chloride and hydrogen peroxide. With the catalysis of BChE, S-butyrylthiocholine iodide (BTCh) was hydrolyzed into thiocholine which has a reducing -SH group. In the presence of thiocholine, MnO₂ nanosheets were broken down and Mn(IV) in MnO₂ nanosheets was reduced into Mn(II). Mn²⁺ is a paramagnetic ion and gives a good ESR signal. In contrast, MnO₂ nanosheets have no ESR signal and need not be separated from Mn²⁺. Mn²⁺ can be determined directly by ESR spectroscopy, and no further sensing probe is needed. ESR spectroscopy based on directly detecting Mn²⁺ is much simpler than those using other probes besides MnO₂. The ESR signal of Mn²⁺ is proportional to the catalytic activity of BChE. OPs which inhibit the activity of BChE can also be detected by probing the ESR signal of Mn²⁺. Since there is no ESR signal of MnO₂ nanosheets, the background signal in the absence of BChE was close to zero. The limit of detection (LOD) of BChE was as low as 0.042 U L^-1. The standard curve for determining the OP paraoxon was established by measuring the inhibition of BChE by paraoxon, and the LOD of paraoxon was found to be 0.076 ng mL^-1. The spiked Chinese cabbage extract samples were analyzed, and the experimental results indicated that the recoveries were from 96.5 to 102.8%. The planted Chinese cabbage was sprayed with the paraoxon solution, and the residue amount of paraoxon in the extract was estimated by the method. The result obtained by the present method was consistent with that obtained by HPLC, which proved the practicability of this new method.

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