Chemically reduced electrospun polyacrilonitrile–carbon nanotube nanofibers hydrogels as electrode material for bioelectrochemical applications

Abstract

Nanosized fibers containing carbon nanotube (CNT) were produced by electrospinning from a DMF solution containing CNT and polyacrilonitrile (PAN). The resulting fibers form a felt like fiber tissue that was used as an electrode. The nitrile groups of these fibers were chemically reduced to amines groups that were protonated at pH 5. The resulting positively charged nanofibers swell in aqueous solutions increasing the exposed surface of CNT and facilitating the diffusion of small molecules and ions to the conducting CNTs. The electrochemical behavior and the morphology of as prepared and reduced nanofibers were characterized by cyclic voltammetry and scanning electron microscopy (SEM) respectively. In addition, the influence of the reduction process determining the amount of amino groups, was investigated. The biofunctionalization of the nanofiber tissue electrodes was carried out by activation of the amino groups via incubation in glutaraldehyde vapor. Then, an enzyme model, polyphenol oxidase (PPO), was chemically grafted onto the nanofiber surface. The efficient covalent binding of PPO onto PAN–NH₂–CNT electrodes (1 × 1 cm size), was exemplified through the electro-enzymatic detection of catechol. The resulting sensitivity and maximum current values at saturated catechol concentration are 118 mA mol⁻¹ L and 10.66 μA respectively, with a detection limit of 0.9 μmol L⁻¹.

Introduction

Electrospinning is a powerful tool to form in high quantities well defined fibers with submicron diameters from many different types of organic or inorganic polymers [1]. Within these polymers, polyacrylonitrile (PAN) is one of the most used due to its high tensile strength, thermal and mechanical stability, and resistance toward solvent and abrasion [2]. Such nanosized fibers have great potentials for various types of applications like tissue or membrane engineering, or as high surface area support in biosensors [3], [4] or fuel cell devices [5]. In order to use these insulating nanofibers as electrode materials, the necessary conductivity can be obtained via carbonization after annealing processes [6], [7], [8], metallic coatings [9] or by choosing appropriate conductive fillers like metal particles or nanostructured carbon [10]. However, high loadings of such conductive materials can prevent fiber formation of homogeneously distributed diameters or can even short circuit the electrospinning setup. These limitations generally lead to poorly conductive fibers. Beside the possibility to form stable nanosized fibers, PAN has also the capacity to form hydrogels in water and became an important compound in biomedical electrophoresis [11]. Furthermore, the chemical transformation of the nitrile groups into NH₂ functions can improve water absorption [12].

Here, we investigated the effect of hydrogel formation reduced PAN fibers on improved diffusion of small molecules like electroactive species. Carbon nanotube doped PAN (PAN–CNT) fibers were chemically reduced and the resulting amine groups served for both, anchor points for biomolecule immobilization and improved hydrogel formation. The potential of these fibers for the development of electroactive biomaterials in biosensors or biofuel cells was exemplified by a standard biosensor setup. Polyphenol oxidase (PPO) was used as bioreceptor unit model for the detection of catechol to evaluate the electroactivity of the CNT–Hydrogel fibers and the diffusion of the enzymatically formed o-quinone to the conductive CNT network.