An improved enzymatic assay for glucose determination in blood serum using a 1,1'-dimethylferricinium dye

1,1'-dimethylferricinium (DMFe+), a stable and pH-insensitive blue dye, was prepared via enzymatic oxidation of a 1,1'dimethyl-ferrocene (DMFe):2-hydroxypropyl-beta-cyclodextrin (HPCD) water-soluble inclusion complex, using bilirubin oxidase immobilized onto porous aminopropyl glass beads via glutaraldehyde activation. In the presence of glucose, DMFe+ was reduced to DMFe by reacting with the reduced glucose oxidase (FADH2), and the absorbance decrease was followed at 650 nm. In acetate pH 5.2 buffer, the response to glucose in blood serum was nonlinear, especially in the low concentration range, because of a competition for the reduced glucose oxidase between the DMFe+ dye and oxygen. At this pH, endogenous ceruloplasmin was also observed to oxidize residual DMFe (16%) in the dye preparation, causing an increase in absorbance at 650 nm. An assay protocol was then developed using maleate buffer, pH 6.5, to overcome these interferences as well as mutarotation of alpha-D-glucose. The results obtained for glucose in the blood serum samples agreed well with those of the reference hexokinase/glucose-6-phosphate dehydrogenase method.