Peroxidase mimicking: Fe(Salen)Cl modified electrodes, fundamental properties and applications for biosensing

Iron(lII) N,N′-bis(salicylidene)ethylenediamine (denoted as Fe(Salen)⁺) was prepared and characterized for its application in chemical analysis. From the stability constant of Fe(III)(Salen)⁺ (7.1×10²⁵ M^-1) and the formal potentials of Fe^3+/2+ and Fe(Salen)^+/0, the stability constant of Fe(II)(Salen) was calculated to be 3×10¹⁷ M^-1. This relatively weaker stability constant, compared with that of Fe(III)(Salen)⁺, led to the occurrence of the electron transfer reactions between Fe(Salen) and electron acceptors, like oxygen and H₂O₂. Experimental results supported this hypothesis, showing that the pseudo-first order rate constants for the reactions of Fe(II)(Salen) with O₂ (DMSO-H₂O, v/v 4:1) and H₂O₂ (pH 7) are 330 and 4400 M^-1 s^-1, respectively. Because of this catalytic effect, a sensing electrode for glucose or uric acid was constructed on the basis of Fe(Salen)⁺ and glucose oxidase (GOx) or uricase (UOx). According to the flow injection analysis (FIA). the detection limits were 1 μm for glucose at pH 7 and 0.1 μM for uric acid at pH 8.5, respectively. The linear response to each substrate covered a region of 1 μM-10 mM for glucose and 5-40 μM for uric acid. Fe(Salen)⁺ might form a 1:1 adduct with β-cyclodextrin (β-CDx); the equilibrium constant was determined to be about 6 M^-1. Although this chemical equilibrium, in terms of the numerical value, was not significant, the formation of {Fe(Salen)}₂O was effectively limited as β-CDx was incorporated.