The last three decades have seen major advancements in the field to enhance the open circuit potentials, current and power densities, fuel efficiency and biocatalyst stability. These advances have resulted in open circuit potentials increasing from 175 mV to almost 1 V and current densities increasing from μA/cm2 to mA/cm2. These increases come from enhanced electrochemically assessable surface areas with nanomaterials, increasing the oxidation reaction activity of anodes with enzyme cascades, metabolons, and optimal cell design to minimize internal resistance.
However, further researches into improving power density over 1~5 mW are still needed to supply the power for stable operation of target devices. In this situation, the stable power output is hampered by a low level of electron transfer rate since electron transfer is known as crucial factor that determines performance of EBFC systems.
The low level of electron transfer rate is resulted from several reasons that are related to protein structure and enzyme-layer formation. In detail, the redox center where electron is generated is often buried inside the protein matrix and moreover, enzymes are randomly oriented or forms multi-layers of enzymes during loaded on electrode, all of which attribute to long distance of electron transfer and degrade the performance of enzymatic electrode as well.
To alleviate these problems, many groups have studied to develop the methods to enhance the electron transfer in these circumstances. Especially, construction of nanostructured high surface electrodes has been spotlighted because it could facilitate the electron transfer and thus performance of enzymatic electrode.
In this research, we attempted to fabricate the nanostructured bioanode, optimizing the electron transfer from enzymes to electrode by control of enzyme layer thickness on the electrode surface. Genetically expressed FAD-dependent glucose dehydrogenase is immobilized on Au electrode modified by metal assisted chemical etching (MaCE).
Recently, the fabrication of silicon (Si) subwavelength structure (SWS) by means of metal-assisted chemical etching (MaCE) has received increasing attention for its several merits during the modification of electrode surface that low-cost, fast, and scalable. Through this methodology, we fabricated nanospikes in pyramidal structure on the electrode surface, in which the nanospikes exist periodically with a sub-wavelength pitch all over the electrode surface.
The effect of nanofabrication in this study is that immobilized enzyme on electrode surface are facilitated to be evenly distributed on the electrode surface. The periodically inclined structure grants equal opportunity of enzyme loading for unit area of electrode surface. By achieving the uniform distribution of enzymes, the distance between enzyme redox center and electrode surface can be reduced by excluding enzyme multilayers.
Here, we experimentally studied the effect of pyramidal structure on enzymatic behaviors. During the experiment, the silicon (Si) wafer was selected with high conductive type for eliminating the possible overpotential that could be occured from the support material. Ag nanoparticles were used to assist the etching process of Si substrate, providing catalytic sites. The fabrication utilizing various diameter of Ag nanoparticles and etching times were conducted for having electrode candidates with different periodicities and pitch sizes.
The optimized structure was confirmed by testing with electrochemical method, cyclic voltammetry (CV), and the optical slope which includes periodicity and pitch size was determined.
Based on this research, it was concluded that electrode morphology is parameter that determines the performance of EBFC. Therefore, electrode structure for EBFC should be constructed in consideration of bio-electrode compatibility, not just of high surface area.