Carbon-nanotubes doped polypyrrole glucose biosensor

Abstract

We report on the one-step preparation route of amperometric enzyme electrodes based on incorporating a carbon-nanotube (CNT) dopant and the biocatalyst within an electropolymerized polypyrrole film. Cyclic voltammetric growth profiles indicate that the anionic CNT is incorporated within the growing film for maintaining its electrical neutrality. The entrapment of the CNT has little effect upon the electropolymerization rate and redox properties of the resulting film. The CNT dopant retains its electrocatalytic activity to impart high sensitivity and selectivity. Linearity prevails up to ca. 50 mM glucose, with a slight curvature thereafter. Relevant parameters of the film preparation were examined and optimized. Such an electropolymerization avenue represents a simple, one-step route for preparing enzyme electrodes and should further facilitate the widespread production of CNT-based electrochemical biosensors.

Introduction

Electropolymerization represents an attractive well-controlled one-step avenue for preparing amperometric enzyme electrodes [1], [2]. The main advantages of this immobilization avenue are the simple one-step preparation, exclusion of electroactive and surface-active interferences, control of the film thickness, and localization of biocatalysts onto tiny electrode surfaces. Numerous enzymes have thus been confined into electropolymerized conducting and non-conducting films. Most commonly, the preparation of these amperometric biosensors involves the physical entrapment of the enzyme within the electropolymerized film in the presence of a charge-balancing small inorganic anion (dopant) [1], [2], [3], [4]. Polypyrrole (PPy) has been particularly useful for this task [5], [6], since it can be electropolymerized at neutral pH (which allows the entrapment of a wide range of biocatalysts).

Here we describe the preparation of amperometric enzyme electrodes based on the co-immobilization of carbon-nanotube (CNT) dopants and glucose oxidase (GOx) within an electropolymerized polypyrrole film. Composite materials based on the coupling of conducting polymers and CNT have recently shown to possess properties of the individual components [7], [8]. The greatly enhanced redox activity of hydrogen peroxide at CNT-based electrochemical transducers makes CNT extremely attractive for numerous oxidase-based amperometric biosensors [9], [10], [11]. Glucose biosensors, coupling electrocatalytic CNT modifiers and conducting-polymer immobilization matrices, have also been described [12], [13]. However, unlike previous work on glucose biosensors based on CNT/PPy electrodes where the CNT was physically entrapped within the growing film, our PPy/GOx films were formed by using ‘oxidized’-CNT as the sole charge-balancing anionic dopant. This represents the first example of anionic CNT acting as dopants in the preparation of conducting-polymer enzyme electrodes. Voltammetric growth profiles indicate that the CNT is incorporated within the growing film for maintaining its electrical neutrality in a manner analogous to the doping of PPy by small inorganic anions. Such entrapment of the CNT dopant does not compromise its electrocatalytic activity, facilitates a highly sensitive biosensing of glucose, and represents a simple and effective route for preparing amperometric enzyme electrodes. While the concept has been presented within the context of glucose sensing, it could be readily extended to other biocatalytic electrodes based on the judicious selection of the enzyme. Details of the preparation, performance and advantages of the new PPy/CNT-GOx biosensor are given in the following sections.