Composite Biosensor for Sulfite Assay: Use of Water-Insoluble Hexacyanoferrate(III) Salts as Electron-Transfer Mediators

Water-insoluble salts of hexacyanoferrate(III) and cationic surface active agents were synthetized and used as electron-mediators for sulfite oxidase. The biosensor was prepared from a composite consisting of modified graphite (50 % w/w) and n-eicosane (50 % w/w). Graphite was modified with mediators or with both mediator and sulfite oxidase for surface- and bulk-modified electrode, respectively. The main advantage of biosensors with insoluble mediators is the possibility to operate at a potential of 0 mV (vs. SCE), thus less interferences are expected, in comparison to soluble hexacyanoferrate(III) where a potential of +300 mV must be used. The maximum sensitivity 7.8 × 10−4 μA/μM was obtained for bulk-modified biosensor, prepared from graphite modified with 5 % w/w of hexadecyltrimethylammonium hexacyanoferrate(III) and 1.25 units/mg (of graphite) of sulfite oxidase. The sensitivity of the biosensor decreased to 24 % of the initial sensitivity after one month storage in dry state at ambient temperature. The use of trehalose as an enzyme stabilization agent has led to the improved stability: 40 % of the initial stability was retained after one month.