Xuan
Dai
,
Gregory G.
Wildgoose
and
Richard G.
Compton
*
Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom. E-mail: richard.compton@chem.ox.ac.uk; Fax: +44 (0)1865 275410; Tel: +44 (0)1865 275413
First published on 19th June 2006
The electrocatalytic detection of the anaesthetic halothane on a multiwalled carbon nanotube modified glassy carbon electrode is reported with a low limit of detection of 4.6 µM. A thorough investigation of the underlying cause of this apparent catalytic effect is undertaken by comparing the response of various carbon electrodes including glassy carbon, basal- and edge-plane pyrolytic graphite electrodes (bppg and eppg respectively) to increasing additions of halothane. The reduction of halothane is shifted by 250–300 mV to more negative potentials at an eppg electrode than that observed at the GC-CNT electrode. Therefore the results of this investigation show that, surprisingly, the electrocatalysis is not solely due to the introduction of edge-plane-like defect sites on the carbon nanotubes as is commonly found for many other substrates showing favourable voltammetry at nanotube modified electrodes. Instead, we reveal that in this unusual case the electroactive sites for the reduction of halothane are due to the presence of copper nanoparticles occluded within the carbon nanotubes during their production, which are never completely removed by standard purification techniques such as acid washing. This is only the third known case where apparent electrocatalysis by carbon nanotube modified electrodes is due to occluded metal-related nanoparticles within the nanotube structure, rather than the active sites being the edge-plane-like defect sites on the nanotubes. Furthermore this is the first case where the active sites are nanoparticles of copper metal, rather than metal oxide nanoparticles (namely oxides of iron(II)/(III)) as was found to be the case in the previous examples.
A common trend amongst electroanalysts seeking new electrocatalytic surfaces is to modify a suitable electrode substrate, usually a glassy carbon electrode, with a thin layer or film of CNTs. The number of papers published using such carbon nanotube modified glassy carbon electrodes has exploded into the hundreds since Wang's early work, and the list of new analytes detected using such electrodes is growing on an almost weekly basis. Illustrative examples of the range of different types of analytes studied using CNT modified glassy carbon electrodes or CNT paste electrodes include: dopamine,4 cytochrome c,11 hydrazine,12 trinitrotoluene (the explosive, TNT!)13 NADH,5 hydrogen sulfide,14 morphine,15 galactose,16 glucose,17 nitric oxide,18 cadmium,19 insulin,20 catechol,21 and uric acid22 and perhaps most importantly hydrogen peroxide23,24 (due to the large number of bioanalytical systems that depend upon its detection) to name but a few! Typically for those authors using CNT modified glassy carbon electrodes the claimed benefits of using such electrodes for electroanalysis include low detection limits, increased sensitivity, resistance to surface fouling and decreased overpotentials.9 In particular the decrease in overpotential of CNT modified glassy carbon electrodes compared to bare glassy carbon electrodes is, often reported as evidence for enhanced ‘electrocatalysis’ by the modified electrode surface. Surprisingly, despite the large number of research groups claiming ‘electrocatalytically’ enhanced detection of target analytes on CNTMEs, the question of why the CNTs were ‘electrocatalytic’ appears not to have been asked. Seemingly this was yet another remarkable intrinsic property of CNTs!
This question prompted this research group to examine the cause of the observed electrocatalytic properties of CNTMEs. In a series of publications (see references 10 and 25 and references contained therein) we have systematically examined the so-called electrocatalytic behaviour of CNTMEs to a wide variety of commonly studied analytes, using graphite powder, basal-plane or edge-plane pyrolytic graphite (bppg and eppg respectively) electrodes, which are more robust and less expensive than using CNTMEs. The voltammetry at graphite electrodes, to which the properties and structure of CNTs have more in common than to glassy carbon electrodes, has been shown to be almost solely due to electron transfer occurring at edge-plane or edge-plane-like defect sites on the graphite surface.10,25 The voltammetry of the analyte systems studied at eppg electrodes has been shown to be almost identical to that observed at CNTMEs, with a similar reduction in overpotential as CNTMEs. In fact in many cases eppg electrodes were found to produce lower detection limits and improved sensitivities than those reported for CNTMEs. Thus the beneficial ‘electrocatalytic’ behaviour of CNTMEs has been shown, in almost every case we have studied, to be due to the CNTs introducing a large number of edge-plane-like sites onto the electrode surface compared to the bare electrode substrate.
Two further conclusions can be drawn from this work. First, for many electroanalytical applications studied using CNTs, eppg electrodes are in fact the electrodes of choice.10 Second, that the common practice of comparing CNT modified glassy carbon electrodes to bare glassy carbon electrodes is inappropriate and potentially misleading. We encourage researchers working in this field to conduct a thorough comparison of CNTMEs to other similar electrode substrates such as graphite, and in particular eppg, before any claims of electrocatalytically enhanced detection of target analytes are made.
Until now there were only two notable exception to the rule of edge-plane sites being the active electrocatalytic sites for CNTMEs, one is in the case of the important analyte, hydrogen peroxide and the other is the electrocatalytic oxidation of hydrazine. The electrocatalytic detection of hydrogen peroxide or hydrazine such as that seen at CNTMEs has never been observed using an eppg electrode. However, recent work within this group has shown that, in this instance, it is nanoparticles of iron oxide—that have been occluded between the graphite sheets that make up the walls of the multiwalled carbon nanotubes (MWCNTs) during the MWCNT chemical vapour deposition (CVD) manufacturing process—that are the active sites for the electrocatalytic detection of these analytes.26,27 These iron oxide nanoparticles were found to never be completely removed using the common technique used to clean CNTs of stirring in strong acid, even for excessively long exposure times.26,27
In this report, we provide the third known example of electrocatalytic detection of a target analyte, in this case the important anaesthetic halothane, at a CNT modified glassy carbon electrode where the catalytic active sites are not entirely due to the edge-plane sites on the nanotubes. Instead, we will demonstrate that in this exceptional case the electrocatalytic detection of halothane on CNTs is likely brought about by the presence, not of iron oxide particles, but of copper metal nanoparticles occluded between the walls of the MWCNTs, which cannot be removed by conventional cleaning techniques such as acid washing.
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Fig. 1 The response to increasing additions of halothane (0–5 mM) in 0.1 M NaOH of (a) GC-CNT modified electrode, (b) bare GC electrode, and (c) eppg electrode. |
Therefore we investigated whether the active sites for halothane reduction in the case of using a GC-CNT modified electrode were also due to the presence of occluded metal or metal oxide nanoparticles. To this end microparticles of iron(II) oxide and iron(III) oxide were separately immobilised onto a bppg electrode by rubbing the bppg electrode on a filter paper containing the material under investigation. The iron oxide modified electrode was then immersed in the cell and the voltammetric response to increasing additions of halothane was recorded again using cyclic voltammetry. Surprisingly, the results of these experiments revealed that neither oxide of iron was responsible for the electrocatalysis observed on GC-CNT modified electrodes, as was the case in the detection of hydrogen peroxide on GC-CNT electrodes.
Next we examined whether the electrocatalytic active sites were due to the small amount of copper or copper(II) oxide nanoparticles occluded within the MWCNTs. No electrocatalytic response was observed when copper(II) oxide particles were immobilised on a bppg electrode. However Fig. 2 shows the voltammetric response of metallic copper powder on the surface of a bppg electrode. The voltammetry is remarkably similar to that produced at the GC-CNT modified electrode, with a reduction peak corresponding to the reduction of halothane at identical potentials of ca. −0.79 V vs. SCE. This result in excellent agreement with previous studies of the reduction of halothane at copper electrodes where the reduction potential was found to be −0.795 V.28 Furthermore the reduction potential on different metals is known to vary by several hundred millivolts, and the formation of metal oxides (e.g. copper(II) oxide) was found to inhibit the reduction of halothane, indicating that this reduction potential is quite specific for metallic copper.28
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Fig. 2 The response of a bppg electrode abrasively modified with metallic copper microparticles to increasing halothane additions (0–3 mM) in 0.1 M NaOH. |
In order to check that these copper impurities were not simply present due to incomplete washing we carried out a ‘super washing’ procedure similar to that used in our previous studies of occluded metal nanoparticle electrocatalysis of hydrogen peroxide26,27 where the MWCNTs were washed in 2 M nitric acid for a period of 24 hours. Fig. 3 shows the voltammetric response to halothane of a GC-CNT electrode constructed using these ‘super washed’ MWCNTs. Again a reduction peak at ca −0.79 V vs. SCE can be observed. The magnitude of the peak current is similar to that of Fig. 1a using the unwashed MWCNTs, the slight difference in peak current is likely to be attributable to differences in loading of the CNTs within the film on the electrode surface rather than any loss of copper from the MWCNTs. Thus we can conclude that in the case of the electrocatalytic reduction of halothane by GC-CNT modified electrodes, the electrocatalytic active sites responsible are nanoparticles of metallic copper, which are occluded into the walls of the MWCNT during manufacture and which can not be removed using conventional purification techniques such as acid stirring. For the occluded copper nanoparticles to be electroactive, they must be accessible to the solvent and the halothane molecules diffusing to the electrode surface. This fact indicates that the active metal nanoparticles are probably located at edge-plane-like defect sites on the MWCNTs, otherwise they would remain inaccessibly occluded between the tube walls. Despite the small percentage of copper (ca. 0.1%) within the bamboo-like MWCNTs, the unique structure of these CNTs (discussed in references 10 and 25 in more detail) imparts a great number of edge-plane-like defect sites along the tube length, and therefore the number of exposed copper nanoparticles at these defect sites is likely to be sufficient to produce the voltammetric responses observed in Fig. 1 and 3.
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Fig. 3 The response of a GC electrode modified with a film of ‘super washed’ MWCNTs to increasing halothane additions (0–5 mM) in 0.1 M NaOH. |
The voltammetric response of the modified electrode to smaller concentrations of halothane was investigated using linear sweep voltammetry (LSV) over the range 0–500 µM using 100 µM additions and also over the range from 0 to 50 µM using 10 µM additions, shown in Fig. 4 and a standard addition plot was constructed (shown inset). From this plot it was possible to determine that the electrode had a limit of detection (based on 3σ) of 4.6 µM and a sensitivity similar to that of a silver macroelectrode. This result compares favourably with the current state of the art detection of halothane on arrays of silver nanoparticle modified carbon electrodes which possess a limit of detection of around 60 µM.31
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Fig. 4 The LSV response of a GC-CNT modified electrode to 10 µM additions of halothane (0–50 µM) in 0.1 M NaOH. Inset: the corresponding standard addition plot. |
Bamboo-like multiwalled carbon nanotubes (purity > 95%, diameter 30 ± 15 nm, length 5–20 µm) were purchased from Nanolab (Brighton, MA, USA). All the reagents were used without further purification. Aqueous solutions and subsequent dilutions were prepared using purified water from Vivendi UHQ grade water system (Vivendi, UK) with a resistivity of not less than 18.2 MΩ cm and degassed for a minimum of 30 minutes using N2 (BOC Gases, Guildford, UK) prior to carrying out any electrochemical measurements. Stock solutions of halothane were prepared in methanol.
Electrochemical measurements were recorded using an Autolab PGSTAT 30 computer controlled potentiostat (EcoChemie, Netherlands) with a standard three electrode system. Either a glassy carbon electrode (GC, geometric area of 0.07 cm2, BAS, UK), a multiwalled carbon nanotube film-modified GC electrode (GC-CNT prepared as described below), an edge plane pyrolytic graphite electrode (eppg, geometric area of 0.13 cm2, Le Carbone Ltd, Sussex, UK) or a basal plane pyrolytic graphite electrode modified with metal or metal oxide powders as described below and in the text (bppg, geometric area of 0.20 cm2, Le Carbone Ltd, Sussex, UK) served as the working electrode. A platinum wire (99.99%, Goodfellow, Cambridge, UK) was used as a counter electrode and a saturated calomel electrode (SCE, Radiometer, Copenhagen, Denmark) acted as the reference electrode to complete the cell assembly. The GC and eppg electrode were polished with alumina powder (Micropolish II, Buehler, UK) using decreasing particle sizes from 1 µm to 0.3 µm and then sonicated and finally rinsed in ethanol and then pure water before each experiment. The bppg electrode was preparing by renewing the electrode surface with cellotape. This procedure involves polishing the bppg electrode surface on carborundum paper (P1000C grade) and then pressing cellotape to the fresh bppg surface before removing the cellotape with the attached graphite layers. All experiments were carried at a temperature 20 ± 2 °C.
Abrasive modification of the bppg electrode was carried out as follows: a freshly cleaned bppg electrode was modified with either iron(II) oxide, iron(III) oxide, copper(II) oxide or metallic copper powders by rubbing the surface of the electrode onto a filter paper (Whatman) containing the modifying material.
This example highlights the need for scientific investigators using CNT modified glassy carbon electrodes to carry out rigorous investigations comparing a target analyte's response on graphite electrodes, especially eppg electrodes, and not to simply compare bare and modified glassy carbon electrodes as is often found in the literature. Furthermore, where necessary, one may need to look for other catalytically active sites on the CNTs before any claims of so-called ‘electrocatalytically’ enhanced detection by carbon nanotube modified electrodes should be made.
Footnote |
† Note that the most common misconception about carbon nanotubes is who actually discovered them. Most workers credit Iijima with their discovery in 1991. In fact Oberlin and Endo reported in 1976 that they had prepared carbon fibres with various external shapes that contained a hollow tube with diameters ranging from 20–500 Å along the fibre axis, and parallel stacks of carbon layers arranged as concentric sheets.2 In 1978 Wiles and Abrahamson first mentioned carbon fibres down to 5 nm in diameter found on a graphite electrode: viz ‘nanotubes’, although this term originates from the 1990s after their rediscovery by Iijima. |
This journal is © The Royal Society of Chemistry 2006 |