Cross-linking, chemical, enhanced enzyme

Cross-linked enzyme aggregates (CLEAs) rely on chemical cross-linking to form a self-assembled enzyme matrix that can be fabricated into, or onto, an electrode material. CLEAs find application in EECs, particularly anodic catalysts [65]. GOx, for example, can be cross-linked in the presence of ammonium sulfate onto the surface of various nanomaterials. The aggregation of the enzyme results in a substantial increase in volumetric protein loading and enhanced stability (>200 days). The use of GOx as CLEA was used as the anode of a biological fiiel cell and demonstrated reproducible power density and stability up to 50 "C. 

Figure 18 Schematic of a glucose biosensor assembled from diphenylalanine peptide nanotubes. The enzyme GOX has been cross-linked to these nanotubes, which are further linked to the gold (Au) electrode and immobilized in a polyethyleneimine (PEI) matrix. The nanofibers act in two ways they immobilize the sensing enzyme and enhance the transducer. Reprinted in part with permission from Yemini et al. (2005a) (copyright 2005 American Chemical Society).

By immobilsation, the pH and temperature profiles of the enzymes may be shifted and the stability of the enzymes altered and in most cases the objective is towards enhancement of these properties. Methods used for the immobilization of enzymes fall into the following categories physical adsorption onto an inert carrier, inclusion in the lattices of a polymerized gel, cross-linking of the protein with a bifunctional reagent, and covalent binding to a reactive insoluble support (Figure 6.52). The selection of the carrier depends on the nature of the enzyme itself, as well as the particle size, surface area, molar ratio of hydrophilic to hydrophobic groups and chemical composition. 

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