Direct wiring of redox cofactors onto gold electrodes via inorganic iron-sulfur clusters enhances charge transport

Abstract

Inorganic iron-sulfur clusters (Fe–S) have emerged as potential electrode anchors and electron shuttles for enzymatic biosensor systems involving nicotinamide adenine dinucleotide (NAD) cofactor. In this work, we report how other cofactors that include nicotinamide adenine dinucleotide phosphate (NADP), flavin adenine dinucleotide (FAD), pyrroloquinoline quinone (PQQ), and coenzyme-Q10 (CoQ10), electrochemically respond to iron-sulfur mediated gold electrodes. Here, select redox cofactors were tethered onto gold electrode supports via inorganic Fe–S (FeS, FeS2, Fe2S3, Fe3S4) and organic Fe–S (Ferredoxin), using the layer-by-layer self-assembly. A combination of docking simulations, secondary ion mass spectrometry, and Fourier-transform infrared spectroscopy revealed the formation of iron carbonyl bonds between FeS and NAD, leading to electrochemical interactions between Fe–S and the redox cofactors. Electrochemical studies confirmed both inorganic and organic iron-sulfur molecules can anchor cofactors onto the gold electrode supports. Up to a 300% increase in anodic peak currents was observed when the inorganic Fe–S replaced the organic counterpart, Ferredoxin, to wire the cofactors onto the gold electrodes. Electrode kinetic measurements revealed the reversible and quasi-reversible behavior of the inorganic and organic Fe–S electrodes, respectively. Reductive desorption studies indicated Fe3S4 closely followed by FeS, forming robust wiring associations with gold by displaying the highest surface coverage, desorption power, and the lowest resistance. Consequently, the inorganic iron-sulfur clusters facilitate an effective method for fabricating enzymatic biosensors for many analytes based on the enzyme-cofactor combination.