Dispersion studies of carboxyl, amine and thiol-functionalized carbon nanotubes for improving the electrochemical behavior of screen printed electrodes

Abstract

Carbon nanotubes (CNTs) are nanostructures that have been discovered in 1991 but their interest continues growing due to their excellent properties and possible applications. In the field of Electroanalysis, most of those are based on the modification of working electrodes using solutions or dispersions in suitable solvents. However, the dispersing agent could have an important effect on the electrochemical behavior of analytes. In this work, an exhaustive study of dispersion of multi-walled carbon nanotubes (MWCNTs) functionalized with carboxylic (COOH), thiol (SH) and amine (NH₂) groups in various solvents is performed. The electrochemical characterization of gold screen-printed electrodes (Au-SPEs) nanostructured with these dispersions of CNTs is carried out by recording the electrochemical signal from the redox reactions of a model analyte, methylene blue, by different electrochemical techniques (CV, SWV and DPV). Evaluation of the influence of the dispersing agent by itself is also made.

Introduction

Carbon, in various forms, has been long used as the main constituent material of solid electrodes as an alternative to metal electrodes. Carbon nanotubes [1], [2] attracted much interest in different fields of Science, included Analytical Chemistry. Particularly, they showed a great potential in Electroanalysis due to their small size and good electrical properties as well as those mechanical and chemical. The first application in this area was reported by Britto in 1996 [3], but the impact of CNTs in electrochemistry is reflected in the number of published reviews [4], [5], [6], [7], [8], [9]. Even when they have been discovered many years ago, CNTs maintain the interest with time being nowadays one of the most intensely nanostructures studied due to the excellent properties (936 entries in the Web of Knowledge in 2012 with the settings “carbon nanotubes electrochemical”). Moreover, today, the phenomenon of dispersion of carbon nanotubes has become one of the important points to be considered [10].

Most electroanalytical CNTs applications are based on the modification of working electrodes using solutions or dispersions in suitable solvents. Poor solubility and difficult manipulation of carbon nanotubes is reported in most common solvents [11], [12], [13], [14], [15]. Apart from variations (in diameter [16], length [16], chirality [17], closed- [18] or open-ended [19] CNTs) different chemical functionalizations such as COOH [20], OH [21], SH or NH₂ [20] should be considered. Moreover, the morphology of CNTs plays an important role in their reactivity.

Therefore, two different aspects have to be thoroughly considered when an application is going to be performed: the type of CNTs and the effect that the solvent could have on the electrochemical behavior. Functionalization of CNTs with or without self-assembling procedures is highlighted in the fabrication of DNA-sensors [22], [23]. A variety of covalent [24], [25], [26], [27] and non-covalent [25], [28], [29], [30], [31] methods have been proposed to ensure efficient dispersion of CNTs. Since covalent functionalization has been found to impair the intrinsic properties of CNTs, non-covalent functionalization approaches, also known as “supramolecule formation”, have aroused much interest [32] due to the minimization of changes in CNTs electronic and mechanical properties [33]. It seems that non-covalent procedures preserve both the integrity and intrinsic properties of CNTs and can efficiently disaggregate raw CNTs [34]. Dispersion via non-covalent functionalization is based on the direct contact between a CNT and a dispersant molecule [28], [35]. Modification of the CNT surface facilitates the disaggregation of CNT bundles into smaller diameter bundles [35] or even individual CNTs [36], [37] and leads to the stabilization of suspended CNTs via steric or electrostatic repulsion mechanisms or both.

Although some company sell carbon nanotubes in solution, even CNTs-modified electrodes [38], generally, CNTs are presented in the market as a black solid powder. Therefore, in most applications, especially in those where electrode modification is later involved, a pre-solubilization for obtaining a homogeneous suspension is required. However, achieving this step is not easy because CNTs are not easily solubilized in a large number of solvents. In the solubilization step, there are two critical points: the CNTs weight/solvent volume ratio and the dispersion procedure. In this work, and taking into account the relevance of CNTs and solvents, the solubilization of three types of functionalized multi-walled carbon nanotubes, with carboxyl, amine and thiol groups (MWCNT-COOH, SH and NH₂) in different dispersing agents was studied. Among all the three, those carboxylated are the most employed and reported in the bibliography, meanwhile thiolated CNTs seems to be a good option for working with gold electrodes due to the affinity gold-sulphur [39]. The electrochemical behavior of screen-printed electrodes modified with different dispersions is also evaluated, paying special attention to the effect of the dispersant itself on the electrode.