Development of a flow injection analysis mediated biosensor for sulfite

Among several mediators tested including N,N,N′,N′-tetramethyl-1,4-phenylenediamine, 2,3,5,6-tetramethyl-1,4-phenylenediamine, tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate and 2,6-dichlorophenolindophenol, an organic conducting salt of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) was selected to form a mediated biosensor for sulfite in view of its superior mediating capability to sulfite oxidase. The conducting TCNQ/TTF organic salt and sulfite oxidase were sequentially deposited onto the sensing area of a vitreous carbon electrode which was then covered by a dialysis membrane (MW cut-off 6000-8000). The mediated TCNQ/TTF working electrode was used in connection with flow injection analysis for the determination of sulfite. The linear range of the system was 0 to 5 mM with a detection limit of 5 μM. The electrode achieved 90% of maximum response in 20 s, but required 10 min to return to baseline. The system was tested for 55 repeated analyses during 26 h of continuous operation with no loss of activity. Electrodes bearing both the conducting salt and sulfite oxidase stored in 3 M ammonium sulfate and at 4°C retained 100% of their original activity after 2 months. The biosensor system was applicable for the determination of sulfite in wine, beer and dried fruit samples and the data obtained agreed very well with the pararosaniline reference method.