Nanoporous Gold Electrodes with Tuneable Pore Sizes for Bioelectrochemical Applications

Abstract

Nanoporous gold (NPG) fabricated by sputtering is a material of versatile morphology with pores whose size can be tailored to accommodate enzymes. The process of pore formation and the size of the pores in NPG are influenced by the composition of Au and Ag in the alloy used to prepare the electrodes together with the temperature and time period of the dealloying process. On increasing the time from 1 to 60 min and the temperature from 0.5 °C to 60.5 °C in concentrated HNO3, significant increases in the average pore diameters from 4.4 to 78 nm were observed with simultaneous decreases in the roughness factor (Rf). The pores of NPG were fully addressable regardless of the diameter, with Rf increasing linearly up to an alloy thickness of 500 nm. The influence of the pore size on the bioelectrochemical response of redox proteins was evaluated using cytochrome c as a model system. The highest current densities of ca. 30 µA cm−2 were observed at cytochrome c modified NPG electrodes with an average pore size of ca. 10 nm. The pores in NPG were also tuned for the mediatorless immobilization of Myrothecium verrucaria bilirubin oxidase. High current densities of ca. 65 µA cm−2 were observed at MvBOD modified NPG electrodes prepared by dealloying at 0.5 °C for 5 min with an average pore size of 8 nm, which is too small to accommodate the enzyme into the pores, indicating that the response was from enzyme adsorbed on the electrode surface.

Immobilization of Redox Enzymes on Nanoporous Gold Electrodes: Applications in Biofuel Cells

“…Note also that coupling of BOx through diazonium improved the stability of the electrodes substantially. In a previous report, we demonstrated that physically adsorbed BOx on NPG only showed catalytic activity for 4 h . In addition to the higher electrochemically active surface area, NPG electrodes clearly provide a more stable environment for the immobilized BOx enzyme.…”

“…The structure and morphology of a metal surface produced by chemical etching of a bimetallic alloy can vary substantially depending on the etching conditions . NPG electrodes with a wide range of pore and crack sizes and substrate thicknesses (Table ) were utilized to evaluate the effects on current density and stability.…”

“…BOx was attached covalently (Scheme ) to the NPG electrodes, whereas GDH was immobilized by drop‐casting the enzyme with an Os redox polymer in the presence of a crosslinking polymer, PEGDGE . Because of the nature of the two different coupling methods, the observed catalytic responses of the enzymes may be altered significantly.…”

“…The geometric surface area ( A geo ) was determined by taking a high‐resolution photograph of each NPG electrode on a millimeter grid and measuring the area with ImageJ software . The average pore size and crack length of each electrode was also measured by using ImageJ software on several SEM images of the electrode surface, as reported earlier …”

“…The structure and morphology of am etal surface produced by chemicale tchingo fabimetallic alloy can vary substantially depending on the etchingc onditions. [25] NPG electrodes with aw ide range of pore and crack sizes and substrate thicknesses (Table 1) were utilized to evaluate the effects on current density and stability.F igure 1s hows examples of the electrode surfaces prepared using the same dealloying conditions (20 8Cf or 15 min). Upon varying the dealloying conditions, pores ranging from 9t o6 0nmi nd iameter could be prepared (Table 1), and the cracks on the surface ranged from 220 to 3180 nm in length.T hese electrodes were used as supports for BOx and GDH for use as cathodes and anodes in biofuelc ells.…”