Apparent ‘electrocatalytic’ activity of multiwalled carbon nanotubes in the detection of the anaesthetic halothane: occluded copper nanoparticles

Abstract

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.

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1. Introduction

Carbon nanotubes (CNTs), first discovered in 1978 by Wiles and Abrahamson¹† and then rediscovered thirteen years later by Iijima,³ have had a profound impact on many diverse areas of scientific and technological research, not least that of electrochemistry. In 1996 Britto et al. first used carbon nanotubes in an electrochemical experiment⁴ and the CNT modified electrode (CNTME) was born. However it was not until 2002 and the pioneering work of Wang et al. that the use of CNTME in electroanalysis received wide spread attention.⁵ Since then CNT modified electrodes have affected many aspects of electrochemistry, most notably in the areas of electroanalysis and electrocatalysis.^6–10

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 peroxide^23,24 (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.

2. Results and discussion

2.1 The electrocatalytic reduction of halothane on a MWCNT film-modified glassy carbon electrode and comparison to other carbon electrodes

To investigate the response of a glassy carbon electrode modified with a film of MWCNTs (GC-CNT electrode) to increasing amounts of halothane over the concentration range 0–5 mM, we performed experiments using cyclic voltammetry with 0.1 M NaOH (pH 13.0) as the supporting electrolyte, the results of which are shown in Fig. 1a. Halothane is normally detected using metal electrodes, notably silver and gold^28–31 although it has been detected on freshly polished glassy carbon electrodes; the observed voltammetry was poor and rapidly diminished with prolonged exposure or repeated cycling.³⁰ However using the GC-CNT electrode we observed a stable response with well defined voltammetry and a reduction peak potential at ca. −0.79 V vs. SCE. Fig. 1b shows the response of a bare GC electrode to increasing concentrations of halothane (0–5 mM), showing that no voltammetry corresponding to the reduction of halothane is observed in the absence of MWCNTs. By comparison with other studies of various analytes using GC-CNT electrodes, this result alone would normally be taken to indicate that the MWCNTs are electrocatalytic towards the detection of halothane; but, as we have stressed in the introduction, this comparison alone is an insufficient diagnostic criterion. Therefore, we also diligently compared the response of an eppg electrode to increasing additions of halothane to determine whether the observed electrocatalytic response of the GC-CNT electrode was due to the edge-plane-like defect sites on the MWCNTs. The resulting voltammetry is shown in Fig. 1c. A voltammetric wave corresponding to the reduction of halothane can clearly be observed at ca. −1.1 to −1.14 V vs. SCE and the peak current can be observed to increase with increasing halothane concentration. However 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. Clearly the electrocatalytic behaviour of the MWCNTs towards halothane can not be attributed to the edge-plane-like defect sites on the MWCNTs being the active sites as is usually found to be the case.^10,25

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.

2.2 The role of metal impurities as the active sites for the electrocatalytic reduction of halothane on GC-CNT modified electrodes

To explain this unusual phenomenon we note that it has been recently shown that the only two other known examples of where electrocatalysis is not due to edge-plane-like defect sites on MWCNTs are where the active sites on the CNTs have been found to be due to the presence of occluded metal oxide nanoparticles of both iron(II) and iron(III) within the walls of the CNTs.^26,27 In these previous reports the MWCNTs, which are identical to those used here, were found to contain ca. 1% iron in the form of Fe(III) as well as ca. 0.1% Cu and trace amounts of sulfur using X-ray photoelectron spectroscopy (XPS) coupled with energy and wavelength dispersive X-ray analysis techniques (EDX and WDX respectively).²⁶ These metal oxide nanoparticle impurities are introduced during the manufacturing process which is carried out using the chemical vapour deposition technique with CNT growth ‘seeded’ using nickel, cobalt or more commonly iron nanoparticle catalysts. After fabrication the MWCNTs are purified using an acid treatment that removes most of the metal nanoparticles to impart a purity of 95% or greater on the MWCNTs.^32,33 It was also found that standard techniques of removing impurities from MWCNTs such as washing in concentrated nitric acid for various times ranging from 1–24 hours were ineffective at removing these metal impurities from MWCNTs that were already 95% pure.²⁶

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

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 peroxide^26,27 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.

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.

2.3 The analytical detection of halothane at a GC-CNT modified electrode

Having developed an understanding of the underlying physical processes responsible for the voltammetric response of halothane at the GC-CNT electrode we next sought to optimise the analytical performance of this electrode to the detection of halothane. The response of the GC-CNT electrode to relatively large (1 mM) concentrations of halothane was examined as a function of pH over the range pH 11–14 by varying the concentration of NaOH used as the supporting electrolyte (note that the ionic strength was maintained constant at 0.1 M by using the appropriate amount of NaNO₃). The optimum response was found to occur when the 0.1 M NaOH electrolyte (pH 13.0) was used, and this was used throughout the following experiments.

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

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.

3 Experimental

3.1 Reagents and equipment

All chemical reagents were of the highest commercially available grade and were purchased from Aldrich (Gillingham, UK) with the exception of: sodium hydroxide pellets (Acros Organics, Geel, Belgium); copper powder (99.9%, metal basis, APS 3.25–4.75 micron, Alfa Aesar, Heysham, UK); 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane, 99%, Sigma-Aldrich, Gillingham, UK); methanol (AnalaR grade, BDH, UK).

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.

3.2 Electrode modification

The construction of films consisting of MWCNTs (GC-CNT) on the surface of glassy carbon macroelectrodes were performed as follows: 5 mg multiwalled carbon nanotubes were suspended in 1 cm³ of dimethylformamide (DMF) to form a ‘casting’ suspension. The casting suspension was then briefly sonicated for 30 seconds in order to disperse the MWCNTs. 20 µL of this suspension was then pippeted onto the surface of a freshly polished glassy carbon electrode. The DMF solvent was evaporated off by placing the electrode in an oven at ca. 90 °C for 10 minutes.

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.

4 Conclusion

In this report we have demonstrated the third known example where the electrocatalytic detection of a target analyte at a CNT modified glassy carbon electrode was not due to edge-plane-like defect sites on the CNTs being the active sites. Instead we have conclusively shown that the active sites for the reduction of halothane are due to copper metal nanoparticles occluded within the walls of MWCNTs. This is the first known example where occluded nanoparticles of metallic copper, rather than nanoparticluate oxides of iron have been found to be the likely active sites for the electrocatalytic detection of a target analyte.

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.

Acknowledgements

XD and GGW thank the Clarendon Fund and the BBSRC respectively for funding.

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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.

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