Enzyme electrodes their characteristics

The response characteristics of enzyme electrodes depend on many variables, and an understanding of the theoretical basis of their function would help to improve their performance. Enzymatic reactions involving a single substrate can be formulated in a general way as... 

Table 7.3 Typical enzyme electrodes and their characteristics. 

The electrochemical characteristics of a solid electrode can be chemically, electrochem-ically, as well as physically modified in order to increase its stability, sensitivity, or most commonly selectivity. An important example of selectivity modification is the formation of a mercury amalgam on gold electrodes to specifically detect thiols. Other examples are the adsorption of metals, polymers, self-assembled monolayers (SAMs), or enzymes but their applications to HPLC are yet scarce. 

More recently, a new series of water dispersed anionic polymers, the AQ 29D, 38D and 55D polymers were released by Eastman Kodak. Since that time, these polymers were used as electrode modifier (12, 13), as covering membrane (14) and as support for enzyme immobilization (15, 16). AQ polymers are high molecular weights (14,000 to 16,000 Da) sulfonated polyester type polymers (17, 18). Their possible structures have been recently presented (18). The AQ polymer serie shows many interesting characteristics useful for the fabrication of biosensors. They are water dispersed polymers and thus compatible with enzymatic activity. They have sulfonated pendant groups similar to Nafion and they can act as a membrane barrier for anionic interferring substances and they offer the possibility to immobilize redox mediators by ion exchange. 

Several strategies for immobilizing proteins on CNTs modified electrodes have been proposed. The step of immobilization is critical, since the enzyme has to remain as much active as possible in order to perform an efficient biorecognition of the substrate. The other aspect to consider is that the transducer where the protein will be immobilized has to allow a fast charge transfer to ensure a rapid and sensitive response. Therefore, it is important to take into account that the noncovalent functionalization of the sidewalls of SWCNTs is the best way to preserve the sp nanotube structure and their electronic characteristics. 

On the basis of all the available data, we can assert that enzymes immobilized on an electrode which catalyze electrochemical transformations preserve their molecular integrity and principal bioelectrocatalytic characteristics. The proof of this is found in the following three groups of experimental facts. 

In the past decade, much work has been done on the exploration and development of redox polymers that can rapidly and efficiently shuttle electrons. Several research groups have wired the enzyme to the electrode with a long chain polymer having a dense array of electron relays. The polymer penetrates and binds the enzyme to the electrode. Gregg and Heller have done extensive work on osmium-containing polymers. They have made a large number of such polymers and evaluated their electrochemical characteristics [14]. Their most stable reproducible redox polymer was poly(4-vinyl pyridine) to which Os(bpy)3Cl2... 

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