21296
J. Phys. Chem. C 2010, 114, 21296–21298
Electrochemistry of a Whole Group of Compounds Affected by Metallic Impurities within Carbon Nanotubes Emma J. E. Stuart† and Martin Pumera* DiVision of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, Singapore 637371 ReceiVed: September 15, 2010; ReVised Manuscript ReceiVed: October 19, 2010
In this article, we demonstrate that iron oxide nanoparticles are responsible for the “electrocatalytic” effect of carbon nanotubes toward the reduction of organic peroxides. Taking into account earlier findings that hydrogen peroxide is susceptible to the same effect, we suggest that the electrochemistry of the entire class of compounds containing the peroxide group is affected by the presence of impurities within CNTs. Introduction
SCHEME 1: Organic Peroxides Studied
Carbon nanotubes (CNTs) are a group of materials that have had a considerable impact on the field of electrochemistry. It has transpired that the preference for CNT-based materials as electrodes for electrochemical systems over existing carbon materials offers a number of advantages, including enhanced electron transfer rates, insignificant surface fouling, and decreased overpotentials for many important redox compounds (the so-called “electrocatalytic” effect).1,2 The apparent “electrocatalytic” effect toward redox compounds demonstrated by CNTs is reportedly due to their highly heterogeneous composition. CNTs contain both residual metal3 and carbon-based4 impurities. The elimination of such impurities is close to impossible.3,4 Compton et al. first discovered that residual catalyst metallic impurities are responsible for the electrocatalytic activity of CNT materials toward the oxidation of hydrazine.5 The same finding has since been reported for a number of additional redox compounds, including halothane,6 amino acids,7 peptides,8 glucose,9 and hydrogen peroxide.10 It has also been found that nanographitic impurities within CNTs are responsible for reduction of the azo group in methyl orange11 and that oxygen-containing groups on the surface of CNTs are electrocatalytic toward oxidation of endiols.12 Despite the fact that the redox properties of several compounds are affected by the presence of metallic impurities within CNTs, this serious issue is very often downplayed by many electrochemists as “only some compounds are affected”. For example, recent secondary literature on the electrochemistry of CNTs13 fails to mention any information on impurities and their effect on CNT electrochemistry. When one looks at the above list of compounds whose electrochemistry is affected by impurities, it is clear that only simple compounds have been studied and that there is only one compound, usually of the simplest structure, belonging to each functional group category, such as hydrogen peroxide, hydrazine, etc. We aim to prove that the electrochemistry of whole functional group categories is affected by metallic impurities and that the above-mentioned chemicals are indeed not “black sheep”, but rather, common representatives of the class of compounds carrying a specific group. * Corresponding author. Fax: (65) 6791-1961. E-mail:
[email protected]. † On leave from the University of Southampton.
Peroxide group is highlighted.
Herein, we show that organic peroxides with different substituents are prone to electrocatalytic reduction on carbonnanotube-based electrodes solely due to the presence of iron oxide impurities. Hence, the electrochemistry of the whole category of compounds containing the peroxide group is affected by the presence of impurities within CNTs. Before moving on to discuss the results, we should mention that Compton et al. first reported that iron oxide nanoparticles contained within CNTs are responsible for the electrochemical reduction of hydrogen peroxide.10 We later reported that even trace iron oxide impurities within residual Co/Mo catalyst nanoparticles in CNTs (on the order of 10 Fe atoms per nanoparticle) are responsible for this effect.14 Such studies into “electrocatalytic” hydrogen peroxide sensing have generated a considerable amount of interest because of the importance of hydrogen peroxide as a biomarker. Organic peroxides are involved in a number of environmental, industrial, and natural processes.15 For this reason, we have selected cumene hydroperoxide and tert-butyl hydroperoxide (Scheme 1) as representatives for the peroxide group of compounds. Experimental Procedures Apparatus. All voltammetric experiments were conducted using an electrochemical analyzer µAutolabIII (Ecochemie, Utrecht, The Netherlands). Electrochemical measurements were carried out in a 5 mL voltammetric cell at room temperature (25 °C) using a three-electrode configuration. A platinum electrode served as an auxiliary electrode, and an Ag/AgCl electrode, as a reference electrode. Materials. Phosphate buffer (pH 7.4), double-walled carbon nanotubes (DWCNTs), pure CNTs, and all metal and metal oxide nanoparticles (
