New colorimetric detection of glucose by means of electron-accepting indicators: Ligand substitution of [Fe(acac)_3-n(phen)_n]ⁿ⁺ complexes triggered by electron transfer from glucose oxidase

A new colorimetric detection technique for glucose, based on electron transfer from glucose oxidase (GOD_red) to iron(III) acetylacetonate(acac)/phenanthroline(phen) mixed complexes, is developed. When GOD is added to an aqueous mixture that contains tris(acetylacetonato)iron(III) complex (Fe^III-(acac)₃), 1,10-phenanthroline (phen), and glucose, the color immediately changes from pale yellow to red. The red color originates from formation of tris(1,10-phenanthroline)iron(II) complex ([Fe^II(phen)₃]²⁺). Differential pulse voltammetry indicates that cationic, mixed-ligand complexes of [Fe(acac)_3-n-(phen)_n]ⁿ⁺ are formed upon mixing the labile Fe^III(acac)₃ complex and phenanthroline. The cationic mixed-ligand complexes electrostatically bind to GOD (pI 4.2), and are easily reduced by electron transfer from GOD_red. This electron transfer is not affected by the presence of oxygen. The reduced complex [Fe^II(acac)_3-n(phen)_n]^(n-1)+ then undergoes rapid ligand exchange to Fe^II (phen)₃. Formation of the colored Fe^II complex is repressed when the salt concentration in the mixture is increased, or when anionic bathophenanthroline disulfonate (BPS) is employed in place of phenanthroline. The use of labile metal complexes as electron acceptors would be widely applicable to the design of new biochromic detection systems.