Enzymes Confinement in CNT Doped Carbon Microstructures Electrodes (CNT-CME) for Miniaturized Enzyme Biofuel Cell Application
Authors: Lee Yooseok , Lee Hyeryeong , Lee Soo-Kyung , Jang Jae-Hyung , Chang In Seop 
Published: February 2020

Fast development of biomedical technology has led us to the invention and use of implantable medical devices to be utilized for direct monitoring of human health. In general, batteries is used as the main micropower supply of these implanted devices, however it has some drawbacks as it has to be replenished in due time. One of the promising continuous energy supplies to replace the use of batteries is enzyme biofuel cell (EBFC) which harvests electrical energy from the oxidation-reduction reactions as it generates electricity by glucose oxidation and oxygen reduction by the anode and cathode enzymes respectively; in which two of its main substrate are abundant and essential components in human physiological fluid.

EBFC is expected to be minimal in size but still concerning to have maximized performance during operation including; high electrode potential range, current and power densities which dependent to the stability of the enzyme used and enzyme environment construction. In order to achieve long operation of EBFC, it is necessary to be assured that enzyme will not be denatured due to external potent factor such as shear stress, especially if EBFC is employed as micro-power supply of implanted devices in human in which the blood velocity will depend on the diameter of blood vessel as implantation site.

One of the modest methods to protect immobilized enzyme from the shear stress is by using polymer such as Nafion to form physical entrapment layer. However, there is high possibility that Nafion as polymer will be degraded under shear stress leading to leaching of enzyme during long operation. Previous studies showed that encapsulated protein in porous materials, or immobilized inside layer by layer material showed more stability due to less exposure of the external stress. The use of mesoporous material such as sol-gels and entrapment in mesoporous silica have proven to increase the enzyme stability, however this materials are lack of conductivity which is not suitable for bioelectronics application. Due to the necessity of electrical communication, another study was employing conducting material such as Si wafer with micro culverts as enzyme protection platform which enhance the electron transfer between enzyme-electrodes material .

The success of enzyme immobilization also depends on its carrier used for particular application. The platform for enzyme immobilization should have high capacity as enzyme anchoring sites, chemically stable, and having the structural ability to prolong enzyme stability. The combination of nanostructures with enzyme immobilization will be a promising direction since it is possible to engineer enzyme environment to limit the effect of external forces as called enzyme-in-cage. Carbon as the most popular and widely studied as electrode materials due to its physical and electrochemical stability properties as inert material, low cost and easy to be functionalized and decorated. Carbon can be used for miniaturized electrode fabricated through nanolithography by pyrolizing photoresist, decompose co-composite, leaving only carbon in microstructured pattern. The use of photoresist to form carbon after pyrolysis step has the advantage due to its ability to be patterned by photolithography techniques which is reproducible. The properties of pyrolized electrodes are comparable than glassy carbon electrode (GCE) which has similar electrochemical properties such as low background current as the effect of low capacitance

Further advancement for this system is the integration of CNT which has high surface area, excellent conductivity due to its sp2carbon hybridization along its wall. In order to prevent any proximity effect of CNT-pyrolyzed carbon electrode, CNT was grown directly in carbon post in which the catalyst of CNT growth was deposited through electrostatic spray deposition (ESD). Grown CNT on pyrolyzed carbon is expected to keep high structural stability.

In this study we elaborated the fabrication and application of CNT doped carbon microstructures (CNT-CME) as platform for enzyme confinement and immobilization. Thus eventually this method then applied to immobilize the FAD-dependent glucose dehydrogenase (FADGDH) α-subunit with menadione as mediator (FADGDH/menadione) system which has proven to have better performance as anode for biofuel cell application as its native character of low Km value. The parameter such as K
mof free and immobilized enzyme, current and power densities of the EBFC were also tested.

Figure 1