engleski

Influence of structure, interstitial cations, and structural defects on electrocatalytic properties and stability of Prussian blue-based catalysts for sensor applications

Prussian blue (iron(III) hexacyanoferrate(II), PB) is one of the most commonly used electrochemical catalysts in the field of chemical sensors in the last 25 years. Popularity of PB is based primarily on the fact that in the reduced form it permits sensitive determination of H2O2 at low cathodic potentials, at which the electrochemical interferences are minimal. This property makes PB an ideal catalyst for the direct sensing of H2O2 or the use as a transducer in first- generation amperometric biosensors based on oxidase enzymes. Crystal structure of PB corresponds to a three-dimensional cubic covalent network consisting of alternating Fe2+ and Fe3+ ions bridged by cyanide ligands. Two forms of PB are usually distinguished: the so- called "soluble PB" (KFe[Fe(CN)6]·nH2O) and "insoluble PB" (Fe4[Fe(CN)6]3·mH2O). Due to the low solubility of PB and its rapid precipitation from aqueous solutions, PB formed under ordinary conditions usually exhibits defect crystal structure and stoichiometry differing from the ideal soluble and insoluble forms. In this work the influence of structure, interstitial cations, and structural defects on electrochemical properties of thin (25–100 nm) PB films was investigated. PB films were electrochemically deposited on glassy carbon, Au, or ITO electrodes under galvanostatic or potentiostatic conditions from solutions of different composition (Fe3+/ferricyanide ratio, presence of complexing agents, pH). Insertion of cations other than preferential K+ into the interstitial cavities of PB was performed by cyclical reduction and oxidation of PB in the solution of target mono- and divalent cations. Composition of prepared PB films was determined by laser-induced breakdown spectroscopy (LIBS). Electrochemical and electrocatalytic properties of PB films were investigated by the combination of voltammetric, spectroelectrochemical, and electrochemical quartz crystal microbalance measurements. Performed investigations revealed that the electrocatalytic activity of PB towards H2O2 reduction can be significantly enhanced through the manipulation of PB structure at molecular level (presence of structural defects / ferrocyanide vacancies, type of interstitial cation). It was found that the structural defects in PB can act as highly active electrocatalytic sites capable of reducing H2O2 at potentials higher than the N-coordinated Fe2+/Fe3+ redox couple of PB. It was also found that the nature of interstitial cations strongly affects both the electrocatalytic properties of PB and the stability of thin PB films in alkaline medium or in the presence of millimolar concentrations of H2O2. Examples of PB-based electrocatalytic films at which the reduction of H2O2 commences at potentials as high as 0.55–0.60 V (vs. Hg/Hg2Cl2/3.5 M KCl), at which the diffusion limited sensing of H2O2 can be performed at potentials as high as 0.20 V, and which are fully stable in the presence of millimolar concentrations of H2O2 at pH 10 will be presented and their applicability in construction of amperometric biosensors based on oxidase enzymes having optimal pH in the slightly alkaline medium will be demonstrated.

prussian blue; electrocatalysis; structural defects

nije evidentirano