Polymer-enzyme hydrogel films

Figure 1. Schematic depiction of the Polymer/Enzyme hydrogel films.

The fourth and probably most popular enzyme immobilization technique is the entrapment technique. In this case, monomer or low molecular weight water soluble polymers are crosslinked in the presence of the enzyme to entrapment the enzyme into the polymer matrix. This has been done with a variety of redox polymer (osmium and ferrocene-based), as well as sol-gels and other hydrogels. This technique effectively covalently links the enzyme to the electrode surface and minimizes leaching and most of these polymers are hydrogels with facile transport of substrate/product in and out of the film. However, frequently this crosslinking affects specific activity of the enzyme. 

Just one example will be given here.195 Evaporation of water from aqueous solutions of MEEP and the enzyme urease yields films that can be cross-linked by exposure to gamma rays. The cross-linked films absorb water to form hydrogels in which the enzyme molecules are trapped within the interstices of the gel network. Some of the enzyme molecules may also be covalently grafted to the polymer side groups. The immobilized enzyme retained approximately 80% of its activity for the conversion of urea to ammonia. This system can, in principle, be used for the immobilization of a wide variety of enzymes, and for their use in biochemical flow reactors, or in sensors. 

In contrast to the area of redox protein electrochemistry, redox enzyme electrochemistry has received much greater attention, driven in many cases by the desire to construct practical, self-contained enzyme electrodes for commercial applications. Redox enzyme electrochemistry is also easier to study in many ways because the substrate or product is often detected electrochemically rather than the enzyme itself. Various types of electroactive polymers have been used with redox enzymes, including redox polymers, redox-active hydrogels, and electropolymer-ized films of conducting and nonconducting, polymers. We discuss each type of polymer in turn, starting with electropolymerized films. 

The non-aqueous diagnostic polymer must impart particular desirable properties to the resulting coating—film. Most importantly, hydrophilidty and hydrogel character to the film must be imparted, thus allowing intimate contact with aqueous whole blood sample. Furthermore, non-aqueous polymer may also provide some enzyme stabilizing effects. 

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