Polymer redox-relay modified

Figure 1.6 Schematic representation of reagentless biosensors (a) ET via conductive polymer chains (b) ET via redox-relay modified polymer chains.

To summarize, the advent of redox-relay modified polymers, such as redox hydrogels, conducting polymers, or EDPs, enabled the development of biosensors that even made it to commercial applications such as implantable glucose sensors. In addition, this approach is now increasingly used for the development of biofuel cells. 

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

A direct electron transfer from entrapped quinohemoprotein alcohol dehydrogenase (QH-ADH) to a Pt electrode, via chains of the polypyrrole, acting as immobilization matrix, was demonstrated [152]. QH-ADH is able to translocate in a fast inner-enzymatic reaction, the electrons primarily accepted by PQQ to heme units located close to the outer protein shell, from where they can be transferred on the conducting-polymer chains (Fig. 13). A similarity between the electron-transfer pathway in multicofactor proteins and that of mediator-modified electroenzymes is apparent, if one considers that a multicofactor enzyme can be regarded as a combination of a primary redox site and protein-integrated electron-transfer relays. 

In contrast, Boguslavsky and co-workers used a flexible polymer chain to put electron relays [15]. Their polymers provide communication between redox centres in glucose oxidase (GOD) and the electrode. No mediation occurred when ferrocene was attached to a non silicone backbone. Their ferrocene-modified siloxane polymers are stable and non diffusing. Therefore, biosensors based on these redox polymers give good response and stability. 

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