Bimetallic Co2+ and Mn2+ Hexacyanoferrate for Hydrogen Peroxide Electrooxidation and Its Application in a Highly Sensitive Cholesterol Biosensor

In this work, we report the synthesis and characterization of a bimetallic Prussian blue analogue containing Co and Mn as outer‐sphere metals (CoMnHCF). The material was a solid solution and its characterization revealed a chemical composition of Co2.05Mn0.95[Fe(CN)6]2 ⋅ 12H2O. The electrochemistry of this novel material showed the existence of two redox waves displaying quasi‐reversible kinetics, as expressed by the larger peak‐to‐peak separation upon increasing the potential sweep rate. Interestingly, the CoMnHCF solid exhibited high electrocatalytic activity for the oxidation of hydrogen peroxide, thus representing an appealing scaffold for the construction of biosensors. As a proof of concept, cholesterol oxidase was immobilized at the electrode surface by using a sol‐gel method, and the cyclic voltammograms were recorded at increasing concentrations of cholesterol. The biosensor showed a detection limit of 30 μM and two linear ranges with excellent sensitivity of 385 mA cm−2 M−1 between 50 and 150 μM, and an adequate sensitivity of 80 mA cm−2 M−1 between 150 and 1 mM. To the best of our knowledge, this is the first biosensor application of a pre‐synthesized bimetallic hexacyanometallate, thus exploiting its potential as an H2O2 electrooxidation catalyst. Cholesterol sensor: A bimetallic analogue of Prussian blue containing Co2+ and Mn2+ is fabricated through chemical co‐precipitation, yielding the following composition: Co2.05Mn0.95[Fe(CN)6]2 ⋅ 12H2O. This material exhibits excellent electrocatalytic properties towards hydrogen peroxide oxidation and is implemented as an electrochemical transducer in a highly sensitive biosensor for the determination of cholesterol.