Conducting Polymers Covalently Linked to Enzymes and Mediators and Electropolymerized on Microelectrode Arrays

Enzymes have proven to be effective electrode surface catalysts for the oxidation and reduction of fuels and oxidants, respectively, in biofuel cell applications.¹  Due to low power densities enzymatic biofuel cells (EBFCs) are best suited for low power miniature systems.²  Microfabricated arrays of individually addressable electrodes can serve as an ideal platform for this application.  As the size of the cell and electrode arrays decrease it is necessary to simplify the overall cell design using as few components as possible. 

One way to minimize the number of components is to remove the separator that is usually required to keep the anodic and cathodic compartments separate.  This can be accomplished by immobilizing the enzymes and electron mediators to the electrode surface.³  There are three main ways of accomplishing immobilization: adsorption, entrapment, and covalent linkage.  Adsorption of the mediator and enzyme can include weak interacting forces that may be disturbed upon shear flow and suffers from poor spatial control. Entrapment is an immobilization strategy in which the species of interest is held inside a 3D polymer matrix on the electrode surface. This can be accomplished in several different ways but the two most common methods are drop casting and electropolymerization.  Drop casting is suitable for large area electrodes but is impractical for interdigitated microelectrodes.  Electropolymerization can affect a much higher spatial resolution and can allow for faster processing.  However, entrapment methods may result in the species diffusing away and an overall inefficient cell.  Covalent linkage of mediators and enzymes provides strong bonds to the polymer backbone while maintaining high spatial resolution, electroactivity, and catalytic activity.  Therefore it is of interest to use electropolymerizable monomers with functional groups for covalent coupling with enzymes and mediators. 

The electropolymerizable monomers studied in this present work are 3,4-ethylenedioxythiophene (EDOT) and functionalized derivatives such as hydroxymethyl 3,4-ethylenedioxythiophene (HM-EDOT), ferroceneacetic acid derivatized  3,4-ethylenedioxythiophene (FcAA-EDOT)⁴, and carboxylic acid functionalized 3,4-ethylenedioxythiophene (COOH-EDOT)⁵.  The resulting electropolymerized conducting polymers (PEDOT, HM-PEDOT, FcAA-PEDOT, COOH-PEDOT) were then used to either immobilize enzymes of interest such as horseradish peroxidase (HRP) and glucose oxidase (GLOX) or serve as electron mediators to shuttle electrons from the enzymes’ catalytic center to the electrode surface.  Pre and post polymerization covalent linkage was studied as well as combinations of the different monomers polymerized to a single electrode.  Electropolymerizations were carried under aqueous conditions. Electrochemical and spectroscopic characterization of the modified surfaces will be described, with an emphasis on the facilitation of electron transfer between the electrodes and enzymes and cross communication with neighboring electrodes.

Acknowledgments

Research was partially supported through the National Science Foundation (CHE-0719097 and CBET-1336853) and the Arkansas Biosciences Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000.

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