Adsorption and electrooxidation of nucleic acids at carbon nanotubes paste electrodes

Abstract

Carbon nanotubes paste electrodes (CNTPE) are shown to be suitable for adsorptive stripping potentiometric measurements of trace levels of nucleic acids. The influence of surface pretreatments, paste composition, nature of the nucleic acid, and accumulation conditions on the adsorption and further electrooxidation of different oligonucleotides and polynucleotides at CNTPE is described. The electroactivity inherent to carbon nanotubes has allowed us to obtain a large enhancement of the guanine oxidation signal compared to that obtained at its analogue carbon (graphite) paste electrode (CPE). Trace (μg/l) levels of the oligonucleotides and polynucleotides can be readily detected following short accumulation periods with detection limits of 2.0 μg/l for a 21 bases oligonucleotide and 170 μg/l for calf thymus dsDNA. The interaction between nucleic acids and CNTPE demonstrated to be mainly hydrophobic. The confined DNA layers demonstrated to be stable in air, in 0.200 M acetate buffer pH 5.00 and in 0.020 M phosphate buffer pH 7.40 + 0.50 M NaCl.

Introduction

Some years after the discovering of fullerenes [1] Iijima [2] reported the synthesis of a new carbon material, the carbon nanotubes. They discovered the multi-walled nanotubes (MWNT) in carbon-soot made by an arc-discharge method [2]. Since then, carbon nanotubes have received enormous attention due to their unique structural, electronic, mechanical, and chemical properties [3]. Carbon nanotubes can be visualized as a sheet of graphite that has been rolled into a tube. Therefore, rolling sheets of graphite into cylinders form carbon nanotubes where each carbon atom has three nearest neighbors [4]. MWNTs consist of concentric single walled cylinders held together by relatively weak Van der Waals forces with an interlayer spacing of 3.4 Å, a typical diameter on the order of 10–20 nm and lengths up to hundreds of microns or even centimeters [5].

The unique electrocatalytic properties of nanotubes either dispersed in bromoform [6], Teflon [7], mineral oil [8], [9]; or dissolved in concentrated acidic solutions [10], [11], [12], [13] or in nafion polymer [14] followed by the immobilization on glassy carbon electrodes, have been reported. Excellent improvements in the electrochemical behavior of dopamine, dopac, ascorbic acid, hydrogen peroxide and proteins like cytochrom b and azurin have been demonstrated [6], [7], [8], [9], [10], [11], [12], [13], [14]. The usefulness of carbon nanotubes for the preparation of enzymatic electrodes has been also reported, allowing the highly sensitive detection of glucose [7], [9], [14] and alcohols [7]. Li et al. [15] have presented the applications of carbon nanotubes for electrochemical determination of small organic and inorganic species as well as biomolecules such as proteins and DNA. Wang et al. [16] have reported recently on the improved detection of purines, nucleic acids, and DNA hybridization at MWCNT modified glassy carbon electrodes. The multi-wall carbon nanotubes-modified glassy carbon electrodes were also used by Hu and coworkers [17] for the sensitive determination of adenine and guanine, either as free bases or as residues of DNAs.

Recently, we have reported on the advantages of carbon nanotubes paste electrodes (CNTPE) on the electrochemical behavior of dopamine, ascorbic acid, dopac, uric acid and hydrogen peroxide [9]. The work presented here describes the use of CNTPE prepared by dispersion of carbon nanotubes within mineral oil for studying the adsorption and electrooxidation of nucleic acids. The new composite electrode combines the ability of carbon nanotubes to promote the adsorption and electron-transfer reactions with the attractive remarks of composite materials.

In the following sections we demonstrate the advantages of this new composite material over the analogue graphite one on the adsorption and electrooxidation of nucleic acids by chronopotentiometric stripping analysis using the guanine oxidation signal as the analytical signal. Special attention was given to the influence of experimental conditions on the adsorptive behavior and further electrooxidation of nucleic acids, such as the effect of surface pretreatments, paste composition, nucleic acid nature and adsorption potential and time.