Abstract: A solution of aldose 1-dehydrogenase in 10 mM sodium acetate of pH 5 containing 0.1% of Triton X-100 was applied to the polished tip (diameter 1.7 mm) of a carbon paste electrode; results were less satisfactory when the enzyme was first mixed with the carbon paste. Incorporation of 2% (relative to graphite) of dimethylferrocene in the paste led to high catalytic response. The prepared electrode was coated with Eastman AQ-29D from aqueous 0.5% solution; this improved stability and minimized non-specific responses. Amperometric measurements with the electrode were made at +200 mV vs. Ag/AgCl (Pt counter electrode) in a flow injection system with 50 mM sodium phosphate buffer of pH 6.5 as carrier at 0.7 ml/min. Solutions of glucose and xylose standards and aldose samples were prepared in the same buffer. Xylose from a yeast fermenter was separated by LC on an Aminex HPX-87P column (30 cm x 7.8 mm i.d.) in Pb(II) form. The enzyme was active mainly towards the β-anomer of glucose. Response was linear up to 100 mM xylose and 10 mM glucose.

Abstract: Four different polymer-bound ferrocene derivatives were evaluated as electron transfer mediators in aldose biosensors based on aldose dehydrogenase with pyrroloquinoline quinone (PQQ) as co-enzyme. The polymeric ferrocene derivatives (syntheses described) were mixed with C paste and packed into a plastic syringe holder that had been pre-packed with unmodified C paste. A solution of aldose dehydrogenase in 10 mM sodium acetate of pH 5 containing 0.1% Triton X-100 was applied to the electrode surface and allowed to dry. The electrodes were evaluated in a flow injection system equipped with a wall-jet amperometric cell using 50 mM sodium phosphate buffer of pH 6.5 as carrier (0.7 ml/min). Maximum response was obtained at +300 mV vs. Ag/AgCl but measurements were normally made at +200 mV to minimize interference from non-specific oxidation processes. The electrodes were more stable than those modified with monomeric dimethylferrocene. The best response was obtained with polymethyl(11-ferrocenyl-4,7,10-trioxa-undecanyl)methyl (12-amino-4,7,10-tioxa-dodecyl)siloxane (1:1) and electrodes prepared from this derivative was applied to the determination of xylose in fermentation mixtures; the results agreed with those obtained by LC.