Hydroquinone electrochemistry on carbon nanotubes is accelerated by nanographite impurities.


Hydroquinone plays a key role in a number of biological processes. Herein, we report that nanographite impurities present within carbon nanotubes are responsible for the “electrocatalytic” oxidation of this chemically and biologically important compound. Carbon nanotubes (CNT) are at the forefront of chemistry, physics, and materials science research. They play an important role as active surfaces in sensors and biosensors, as well as in energy-storage devices. The use of carbon-nanotube-modified electrodes provides electrochemists with a number of benefits, including increased voltammetric currents, decreased overpotentials, and enhanced heterogeneous electron-transfer rates for the oxidation or reduction of a variety of important compounds. It has been shown that the reported exceptional “electrocatalytic” activity of CNTs is not actually inherent to CNT materials, but is instead caused by impurities contained within the nanotubes. Carbon nanotubes contain considerable amounts of carbon-based and residual catalyst metallic impurities. Such impurities are sheathed by several sheets of graphene, thus making the attainment of a CNT sample that is free from impurities close to impossible as purification procedures are ineffective against the graphene layers. Compton and co-workers, the pioneering group in this area, documented that metallic impurities from residual catalysts govern the electrochemical oxidation or reduction of a number of important redox compounds at carbon-nanotubemodified electrodes, including halothane, hydrazine, hydrogen peroxide, and glucose. We have previously demonstrated that metallic impurities also electrocatalyze the oxidation of amino acids and peptides, as well as the reduction of organic peroxides. We have also reported the influence of carbon-based impurities on the electrochemical activity of CNTs by studying the ferrocyanide/ferricyanide redox couple and the reduction of the azo group in methyl orange. Hydroquinone and its derivatives are highly important biologically active compounds. They play a role in many biological processes and have been used as mediators in a variety of biosensory systems. There have been several reports on the “electrocatalytic” determination of hydroquinone on carbon nanotubes. The reasons for such “electrocatalysis” of hydroquinone on CNTs have never been properly addressed or explained. Given the importance of hydroquinone, we set ourselves the challenge of establishing the reasons as to why the “electrocatalytic” activity of pure CNTs toward the electrochemistry of hydroquinone is very similar to that of glassy carbon, and why deviation from this behavior only occurs when nanographite impurities are present within CNT samples. Cyclic voltammetry was employed to explore the electrochemical oxidation of hydroquinone on double-walled CNTs (DWCNTs; Figure 1, grey thick line). The oxidation of hydroquinone at a DWCNT-modified GC electrode originates at 41 mV, reaches a first maximum at +185 mV, and reaches a second maximum at +323 mV. The voltammetric behavior of the bare GC electrode towards the oxidation of hydroquinone (Figure 1, black thick dotted line), gave rise to a peak that begins at +51 mV and reaches a maximum at +390 mV. The response of pure CNTs, carbon nanotubes that are free from both metallic and carbon impurities (Figure 1, black thick line), is analogous to that observed for the bare GC electrode. Comparison of the profiles for each of these modified and unmodified electrodes indicated that the second oxidation wave observed in the voltammetric response at DWCNTs is most likely caused by the underlying GC electrode. Therefore, further investigation was required to determine what is responsible for the initial oxidation peak observed in the DWCNT profile. Initially, we considered that the metallic impurities within DWCNTs might be the cause of this observed “electrocata[a] E. J. E. Stuart, Prof. M. Pumera Division of Chemistry & Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 (Singapore) Fax: (+65)6791-1961 E-mail : pumera@ntu.edu.sg [] On leave from the University of Southampton (UK)


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