Avidin-Biotin System-Based Enzyme Multilayer Membranes for Biosensor Applications: Optimization of Loading of Choline Esterase and Choline Oxidase in the Bienzyme Membrane for Acetylcholine Biosensors

The loading of choline esterase (ChE) and choline oxidase (ChOx) in the enzyme membrane of an acetylcholine biosensor was optimized based on a layer-by-layer construction of the bienzyme layers on the surface of a platinum (Pt) and a Pt-black electrode. To this goal, ChE and ChOx were tagged with biotin residues, so that the enzymes could be built into the multilayer assemblies composed of monomolecular layers of the enzymes and avidin. The ChE/ChOx bienzyme multilayer-modified electrodes showed amperometric response to acetylcholine in solution, showing that ChE and ChOx catalyzed hydrolysis reaction of acetylcholine and an oxidation reaction of resulting choline, respectively, successively in the bienzyme layer. It was found that the acetylcholine sensor which is modified with 10-layer ChOx exhibited its maximum response to acetylcholine when additional 2 layers of ChE were added to the surface of the ChOx layer. This optimum ratio of ChE to ChOx in the bienzyme layer was reasonably explained in terms of the relative values of catalytic activity of the enzymes. The usefulness of the layer-by-layer deposition technique in optimizing the enzyme loading in bienzyme system is discussed in detail.