Stable Flexible Electrodes With Enzyme Cluster Decorated Carbon Nanotubes for Glucose-Driven Power Source in Biosensing Applications

Over the years, implantable sensor technology has found many applications in healthcare. Research projects have aimed at improving power supply lifetime for longevity of an implanted sensor system. Miniature power sources, inspired from the biofuel cell principle, can utilize enzymes (proteins) as catalysts to produce energy from fuel(s) that are perennial in the human body. Bio-nanocatalytic hierarchical structures, clusters made of enzyme molecules, can be covalently linked to the electrode’s surface to provide better enzyme loading and sustained activity. Carbon nanotube base electrodes, with high surface area for direct electron transfer, and enzyme clusters can achieve efficient enzymatic redox reaction. A redox pair of such bioelectrodes can make up a power source with improved performance. In this study, we have investigated high throughput processes for coupling enzyme catalysts with power harvesting mechanisms via a screen printing process and solution processing. The process incorporates enzyme (glucosse oxidase and catalase) micro-/nanocluster immobilization on the surface of carboxylated (functionalized) carbon nanotubes with screen printed electrodes. The 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and $N$-hydroxysulfosuccinimide amide linkage chemistries were used to bind the enzyme molecules to nanotube surface, and bis[sulfosuccinimidyl] suberate (BS3) was used as the cross-linker between enzymes. Optimized enzyme cross-linking was obtained after 25 min at room temperature with 0.07 mmol BS3/nmol of enzymes, with which 44% of enzymes were immobilized onto the surface of the bioelectrode with only 24% enzyme activity lost. A cell, redox pair of bioelectrodes, was tested under continuous operation. It was able to maintain most of the enzyme activity for 7 days before complete deactivation at 16 days. Thus, the power harvesting mechanism was able to produce power continuously for 7 days. The results were also analyzed to identify impeding factors such as competitive inhibition by reaction byproduct and cathode design, and methods to rectify them have been discussed. Coupling this new and improved nanobiopower cell with a product removal mechanism and enzyme mutagenesis should provide enzyme protection and longevity. This would bring the research one step closer to development of compatible implantable battery technology for medical applications.