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

Iron(III) 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 Fe3+/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.