NANO LETTERS
Iron Oxide Particles Are the Active Sites for Hydrogen Peroxide Sensing at Multiwalled Carbon Nanotube Modified Electrodes
2006 Vol. 6, No. 7 1556-1558
Biljana Sˇ ljukic´, Craig E. Banks, and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom Received February 17, 2006; Revised Manuscript Received April 26, 2006
ABSTRACT We demonstrate that the “electrocatalytic” hydrogen peroxide detection reported at multiwalled carbon nanotube modified electrodes is due to iron oxide particles arising from the chemical vapor deposition nanotube fabrication process rather than due to intrinsic catalysis attributable to the carbon nanotubes arising, for example, from edge plane-like sites/defects.
The discovery of carbon nanotubes has had a profound impact on many diverse areas of science and technology, including that of electrochemistrysnotably in the areas of electrocatalysis and electroanalysis.1-5 The properties and applications of carbon nanotubes have been well summarized in the literature with many reviews having an electrochemical emphasis.1 Carbon nanotubes (CNTs) are interesting electrode materials due to their special geometry and electronic, mechanical, chemical, and thermal properties.1 They have good electrical conductivity and mechanical strength, as well as relative chemical inertness in most electrolyte solutions, high surface activity, and a wide operational potential window.2-5 Due to these properties, important advantages have been claimed for CNTs for applications in electroanalysis over other carbon materials, namely, a decrease in overpotentials, increase in peak currents, and thus lower detection limits providing high sensitivity and selectivity in analytical sensing.6 Electrochemists have successfully taken advantage of CNTs for accelerating the electron transfer reaction involving a wide range of biomolecules and environmentally significant compounds such as proteins, nucleic acids, NADH, neurotransmitters, cytochrome c, cysteine, homocysteine, and hydrazine compounds.4 The advantages of carbon nanotubes paste electrodes (CNTPE) on the electrochemical behavior of dopamine, ascorbic acid, DOPAC, uric acid, hydrogen peroxide, guanine, adenine, and nucleic acids have also been reported;7,8 of all these observations, the ability to detect hydrogen peroxide is perhaps the * To whom correspondence
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most significant and has attracted the most attention within the electroanalysis community probably because of the pivotal role of hydrogen peroxide detection in many biosensors.7,8 The claimed superior behavior of carbon nanotube modified electrodes raises the question of the origin of such high electrochemical reactivity of CNT-modified electrodes. With the increasingly important role of CNTs in electrochemistry, it is a fundamental requirement to understand the factors determining their electrochemical properties and analytical performance. Structurally, CNTs can be described as ‘‘rolled-up” sheets of graphite; either a single graphite sheet rolled in the case of single walled (SWCNTs) or concentric tubes fitted one inside the other in the case of multiwalled carbon nanotubes (MWCNTs). The way in which the graphite sheets are rolled up influences the electronic properties of the CNTs. It is believed that CNTs have two distinct possible reactive sites: basal plane sites which occur on the side walls of the CNTs and edge-plane-like sites/defects which occur at the ends of the tubes and, in the case of MWCNTs, along the tube axis. The terms edge plane-like and basal plane-like arise from comparison with the structure of highly ordered pyrolytic graphite.9 Various morphological variations of the MWCNTs can be distinguished6; specifically, there are ‘‘hollowtube”, ‘‘herringbone”, or ‘‘bamboo-like” MWCNTs. The difference between different types of MWCNTs is that a proportionally higher number of edge-plane-like defect sites occur on herringbone and bamboo variants than hollow-tube MWCNTs; this is because in these two cases the plane of the graphite sheets are at an angle to the axis of the tube
requiring a high proportion of the graphite sheets to terminate at the surface of the tube. It has been demonstrated that it is these edge-plane-like sites that are responsible for much of the chemical and electrochemical activity of the CNTs.5,6,9 However, we have shown proof of the concept via voltammetric and spectroscopic characterization for the specific and niche case of the electrochemical oxidation of hydrazine at multiwalled carbon nanotubes the electrochemical reactivity is due to iron oxide impurities which are left behind from the chemical vapor deposition (CVD) fabrication process. It was also found that washing the MWCNTs using conventional reported procedures10 does not remove these particles; they are likely partially coated with graphite shells.11 The present work seeks to explore the generality or otherwise of this type of inadverted nanotube electrocatalysis. In this paper, we challenge some of the general beliefs about the electrocatalytic properties of carbon nanotubes and demonstrate the origin of their activity for the electrochemical reduction of hydrogen peroxide at “bamboo” MWCNTs supported on basal plane pyrolytic graphite (BPPG) electrodes noting that the purported CNT “electrocatalysis” of hydrogen peroxide reduction is perhaps the classic and most widely appreciated example of this claimed phenomenon. It is well-known that the sensitivity of carbon toward peroxide detection is low. However, literature reports show that reduction of hydrogen peroxide is accelerated at CNTs.10 Recent studies demonstrated improved electrochemical behavior of hydrogen peroxide at SWCNTs and MWCNTs.10,12 A significant lowering of the hydrogen peroxide overpotential with improvement of the reversibility of its voltammetry was observed,10 and these reports are pivotal to the assertion of intrinsic CNT electrocatalysis. Accelerated electron transfer for reduction of hydrogen peroxide at CNT-modified electrodes is of a great importance for amperometric biosensors as hydrogen peroxide is a compound widely involved in enzymatic reactions of interest. A MWCNT-modified basal plane electrode was first formed by abrasively attaching carbon nanotubes to the surface of the basal plane electrode by gently rubbing a freshly prepared basal plane pyrolytic graphite (BPPG) electrode on a fine quality filter paper containing the carbon nanotubes. A BPPG electrode was prepared for modification by renewing the electrode surface with cellophane tape.13 This procedure involves polishing a BPPG electrode surface on carborundum paper, pressing cellophane tape on the cleaned BPPG surface, and then removing the cellophane tape, along with several surface layers of graphite. This is repeated several times to attain the final surface. Prior to use, the electrode is cleaned in acetone to remove any adhesive. The MWCNT abrasively modified BPPG electrode was then immersed into a degassed 5 mM solution of hydrogen peroxide in pH 7.4 phosphate buffer. The voltammetric response of the MWCNT-modified BPPG electrode was investigated with a large reduction current obtained as can be seen in Figure 1. To validate the assumption that high electrochemical reactivity of CNTs is due to the existence of edge plane sites, the reduction of hydrogen peroxide was Nano Lett., Vol. 6, No. 7, 2006
Figure 1. Cyclic voltammetric responses for the electrochemical reduction of 5 mM hydrogen peroxide in pH 7.4 phosphate buffer.
next examined at both unmodified basal plane and edge plane pyrolytic graphite electrodes. The BPPG electrode was prepared as described above, and the response of reduction of hydrogen peroxide in 5 mM solution was recorded. No significant reduction wave is observed at bare BPPG electrode in the potential range studied. Before use, the edge plane pyrolytic graphite (EPPG) electrode was polished to a mirror-like finish with 1.0 and 0.3 µm alumina slurry followed with careful rinsing with water to remove any alumina residue. Yet again, no voltammetric signal is observed. The strong electrocatalytic activity of carbon nanotubes toward the reduction of hydrogen peroxide compared with the bare BPPG and EPPG electrodes is observed. A great enhancement in the response obtained at the MWCNT-modified BPPG electrode indicates that edge plane sites at the MWCNTs are not sole responsible for their electrocatalytic activity and that there are other factors that contribute to the high electrocatalytic activity of CNTs. Next, iron (III) oxide, Fe2O3, was abrasively immobilized onto a BPPG electrode by gently rubbing the electrode surface on a fine quality filter paper containing iron (III) oxide. The modified BPPG electrode was immersed in the 5 mM solution of hydrogen peroxide, and the electrochemical reduction was explored. The reduction current obtained at iron(III)oxide-modified BPPG electrode was analogous to the one observed at MWCNT-modified BPPG electrode, as shown in Figure 1. This suggests that iron (III) oxide particles present in MWCNTs are responsible for their electrochemical activity for the electrochemical reduction of hydrogen peroxide. The same procedure was repeated for preparation of iron(II)oxide-modified BPPG electrode and the reduction of hydrogen peroxide examined at the prepared electrode under the same conditions as in case of MWCNT- and iron(III)oxide-modified BPPG. The response was similar to the one obtained at iron(III)oxide-modified BPPG electrode. (Note that the nature of the immobilization procedure for abrasively modified electrode makes the absolute quantification of the amount of iron oxide on the surface difficult.14 However, we emphasize that it is the peak potential which is characteristic of the electrocatalysis not the magnitude of the peak current.) 1557
Multiwalled carbon nanotubes (MWCNTs) used in the experiments were “bamboo-like” MWCNTs manufactured by NanoLab (Brighton, MA) of 30 ( 10 nm diameter and 5-20 µm length and purity greater than 95%. Our previous work has demonstrated via X-ray photoelectron spectroscopy (XPS) that unwashed “bamboo” MWCNTs contain 98.9% atomic carbon, 1.0% atomic oxygen, 0.1% atomic iron, and a trace (less than 0.1%) of atomic copper and sulfur.11 Note that the same nanotubes are used throughout this study. We have also shown that iron present on CNTs is likely iron(III) oxide with other oxidation states of iron impurities likely existing on the MWCNTs. This has indicated that the electrocatalytic behavior of CNTs can be partially attributed to iron oxide impurities present in the material which remain from the fabrication process and are partially over-coated with graphite shells and so cannot be removed even after acid pretreatment. The manufacturer, NanoLab, has performed EDX investigation of their carbon nanotubes and found that their carbon nanotubes normally contain ca. 1% weight iron along with 0.1 wt % of sulfur. In general, CNTs are prepared by two main methods: the carbon ARC discharge process or chemical vapor deposition (CVD), with the CNTs prepared by CVD displaying a higher electrocatalytic activity.12 CVD involves the use of different metal catalysts such as iron/ graphite, cobalt/graphite, and iron/silica catalysts. The electrochemical properties of the CNT-modified electrode, both the response, that is, reaction kinetics, and background current, that is, capacitance, are influenced by the method of production determining defect density or presence of impurities. Hydrogen peroxide however remains one of the most important and intriguing analytes ever determined with carbon nanotubes leading to the claimed superior sensing properties of CNTs over other carbon-based substrates; it is
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clear from the work reported in this note that the observed catalytic behavior may arise more from iron oxide nanoparticles simply electrically “wired” with nanotubes rather than from any intrinsic properties of the latter. In summary, we have pinpointed that the electrochemical activity of carbon nanotubes for the electrochemical reduction of hydrogen peroxide can be attributed to metal iron oxide “impurities” present in the CNT material. Acknowledgment. B.Sˇ . would like to thank the Clarendon Fund for partial funding. Supporting Information Available: A continuation of the experimental methods is described. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Wildgoose, G. G.; Banks, C. E.; Leventis, H. C.; Compton, R. G. Microchim. Acta 2006, 152, 187. (2) Gooding, J. J. Electrochim. Acta 2005, 50, 3049. (3) Lin, Y.; Yantasee, W.; Wang, J. Front. Biosci. 2005, 10, 492. (4) Wang, J. Electroanalysis 2005, 17, 7. (5) Banks, C. E.; Compton, R. G. Analyst 2006, 131, 15. (6) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 7, 829. (7) Rubianes, M. D.; Rivas, G. A. Electrochem. Commun. 2003, 5, 689. (8) Rubianes, M. D.; Rivas, G. A. Electroanalysis 2005, 17, 73. (9) Banks, C. E.; Moore, R. R.; Davies, T. J.; Compton, R. G. Chem. Commun. 2004, 16, 1804. (10) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (11) Banks, C. E.; Crossley, A.; Salter, C.; Wilkins, S. J.; Compton, R. G. Angew. Chem., Int. Ed. 2006, 45, 2533. (12) Lawrence, N. S.; Deo, R. P.; Wang, J. Electroanalysis 2005, 17, 64. (13) Moore, R. R.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 2677. (14) Scholz, F.; Schroder, U.; Gulaboski, R.; Schrvder, U. Electrochemistry of Immobilized Particles and Droplets, 1st ed; Springer: New York, 2005.
NL060366V
Nano Lett., Vol. 6, No. 7, 2006
