Characterization of interacting ferrocene–cyclodextrin systems and their role in mediated biosensors

Inclusion complexes of 1,1′‐dimethylferrocene (DMFe) with α‐, 2‐hydroxypropyl‐β‐ or γ‐ cyclodextrins were formed in aqueous systems. For the first time, the interacting DMFe‐cyclodextrin systems have been characterized using cyclic voltammetry which enabled the estimation of the complexation order, the formation constant and the diffusivity. Cyclic voltammetry was performed at a stationary electrode, yielding complexation ration of 1:2 for DMFe with α‐cyclodextrin and 1:1 with the other two cyclodextrins, as expected from the known cavity dimensions for the cyclodextrins. Formation constants of 4.4 × 10⁴ M⁻², 1.2 × 10³ M⁻¹ and 2.9 × 10² M⁻² were than determined for the complexes between DMFe and the α‐, β‐, and γ‐cyclodextrins, respectively, in relative agreement with the literature. The maximum complexation efficiency, diffusivity and solubility were observed for the DMFe:2‐hydroxypropyl‐β‐cyclodextrin inclusion complex, which, combined with cost factors, resulted in the selection of this form for bioelectrocatalysis studies. The efficiency of the complex as a mediator for the glucose: glucose oxidase system was determined by measuring the rate constant (k_S) for reaction of the oxidized DMFe with the reduced enzyme. The k_S value decreased from 3.4 × 10⁴ M⁻¹ s⁻¹ to 1.9 × 10⁴ M⁻¹ s⁻¹ with an increase in 2‐hydroxypropyl‐β‐cyclodextrin concentration from 4mM. In this range, the k_s value is similar to that of free ferrocene, implying a high efficiency of the DMFe:2‐hydroxypropyl‐β‐cyclodextrin inclusion complex.