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Nanoparticles of various manganese dioxide crystalline modifications (amorphous, b-phase, and g-phase) were compared with respect to their mediator properties in electrochemical detection of thiols. The highest catalytic activity towards thiocholine, cysteine, and glutathione was demonstrated by screen-printed graphite electrodes modified with g-MnO 2 at the optimized working potential of 450 mV versus the Ag/AgCl reference electrode. High sensitivity of g-MnO 2 modified electrodes towards thiocholine (345 mA cm 2 /M) and therefore low detection limit of butyrylcholinesterase (1 pM) enabled their use for subnanomolar detection of an organophosphate pesticide diazinon, an irreversible inhibitor of butyrylcholinesterase. The detection limit for diazinon was estimated as 0.6 nM with assay duration of less than 15 min.

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