Screen-printed enzymatic biosensor modified with carbon nanotube for the methimazole determination in pharmaceuticals formulations

This paper describes the development of a screen-printed sensor, modified with carbon nanotubes for the rapid and sensitive quantification of methimazole (MT) in pharmaceuticals formulations. Tyrosinase [EC 1.14.18.1], immobilized on a rotating disk, catalyzed the oxidation of catechols to o-benzoquinone, whose back electrochemical reduction was detected on graphite screen-printed electrodes modified with carbon nanotubes at −150 mV. Thus, when MT was added to the solution, this thiol-containing compound participate in Michael type addition reactions with o-benzoquinone to form the corresponding thioquinone derivatives, decreasing the reduction current obtained proportionally to the increase of its concentration. This method could be used to determine MT concentration in the range of 0.074–63.5 μM (r = 0.998). The determination of MT concentration was possible with a detection limit of 0.056 μM in the processing of as many as 25 samples per hour. The biosensor has a reasonable reproducibility (R.S.D. < 3.50%) and a very stable amperometric response toward this compound (more than 1 month). The application of this analysis to different pharmaceutical samples containing MT supports the utility this biosensor.