Letter pubs.acs.org/ac
Carbonaceous Impurities in Carbon Nanotubes are Responsible for Accelerated Electrochemistry of Cytochrome c Lu Wang, Adriano Ambrosi, and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical, Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore S Supporting Information *
ABSTRACT: There have been many reports on the excellent electrocatalytic properties of carbon nanotubes toward many substrates. Here, we wish to address and investigate the apparent “good promotion” of the electron transfer to cytochrome c as previously shown and attributed to the electrocatalytic properties of carbon nanotubes (Wang et al. Anal. Chem. 2002, 74, 1993). We will show here that the observed electrocatalytic effect toward this probe could be mainly attributed to the carbonaceous impurities present within the carbon nanotubes samples, instead of the carbon nanotubes themselves.
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downplayed by a large part of the electrochemistry community with comments that only few compounds are affected.11 Here, we wish to show that this problem of erroneous attribution of electrocatalysis to CNT is much broader and extends into the electrochemistry of proteins. Carbon nanotubes have been reported to act as electrocatalysts toward the reduction of cytochrome c.21 From a careful examination of the effects generated by pure CNT (impurities-free), standard CNT samples (impurities-rich), and individual impurities (metallic or carbonaceous), we have been able to distinguish here the effect coming from each individual constituent. The results clearly show that the major contribution to the catalytic reduction of cytochrome c is from the carbonaceous impurities which promoted a faster electrochemistry for this probe compared to the isolated metallic impurities or pure CNT.
arbon nanotubes (CNT) have attracted significant interest from the academic research community as well as from industries due to their varying and unique properties.1,2 Carbon nanotubes are typically prepared by metallic catalystinduced growth.3,4 During this growth, not only are tubular carbon structures created but also other forms of carbon, such as nanographitic particles or amorphous carbons, precipitate.5−7 As prepared CNT contain large amounts of impurities, ranging from residual metallic nanoparticle impurities to carbonaceous impurities. For example, a typical single-walled CNT sample might contain up to ∼30% in weight worth of metallic impurities and similar amounts of carbonaceous impurities.8,9 It is very difficult to completely remove all traces of metallic impurities from CNT sample,10 and it is even more challenging to remove carbonaceous impurities, as their physical properties are close to those of the CNT.9 There has been a large amount of publications reporting outstanding electrochemical properties of CNT.11 This was debunked by Compton’s group, whose group was the first to show that it was in fact the metallic impurities within CNT that were responsible for the catalytic oxidation or reduction of a variety of compounds, i.e., hydrazine,12 hydrogen peroxide,13 glucose,14 and halothane.15 The same group also demonstrated that carbonaceous impurities in CNT, present in the form of nanoonions, were responsible for the observed electrocatalytic properties toward many analytes, such as ferro/ferricyanide, ruthenium salts, nicotinamide adenosine dinucleotide (NADH), epinephrine, and norepinephrine.16 These findings were later extended to other carbonaceous impurities, such nanographite and amorphous carbon.17−19 These reports were confirmed and followed by others, as summarized in recent reviews.11,20 Despite several reports demonstrating that electrocatalysis was attributed to CNT erroneously, this issue is often © XXXX American Chemical Society
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EXPERIMENTAL SECTION Apparatus. All voltammetric experiments were measured by using an electrochemical analyzer Autolab PGSTAT 101 (Ecochemie, Utrecht, The Netherlands) connected to a personal computer and controlled by NOVA software Version 1.8 (Methrom Autolab B. V.). The electrochemical measurements were performed in a 5 mL voltammetric cell at room temperature (25 °C) by using a three-electrode configuration. The GC electrode functioned as the working electrode, and platinum disk and an Ag/AgCl (saturated) electrodes were used as the auxiliary and reference electrode, respectively. Received: April 11, 2013 Accepted: June 1, 2013
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Materials. Cytochrome c, N,N-dimethylformamide (DMF), phosphate buffer (pH 7.2), MWCNT-i (0.65% (wt) of Co, 0.014% of Fe, 0.07% of Mo, and 0.5% of Ni), pure MWCNT, NiO, Co3O4, Fe3O4, and MoO2 nanoparticles obtained from Sigma-Aldrich. Amorphous carbon black (particle size 29 nm) was received from Asbury Carbons, NJ. Glassy Carbon (GC, 3 mm diameter), Pt, and Ag/AgCl electrodes were purchased from CHInstruments, TX, USA. Procedures. Carbon materials were used as-received without any further purification. All nanoparticles were dispersed in DMF at a total concentration of 5 mg/mL for modification of the working electrode. A 1 μL portion of the sonicated suspension was then cast onto a GC electrode surface that had previously been polished with 0.05 μM alumina particles on a cloth. The deposited nanotube solution was allowed to dry in air at room temperature resulting in a carbon nanotube film on the electrode surface. GC electrodes modified with metal nanoparticles and amorphous carbon were prepared using the same procedure. All measurements were performed in phosphate buffer at pH 7.2.
one electron transfer reaction. The second reduction wave signal observed at MWCNT-i modified electrode originates from the presence of underlying glassy carbon electrode, similarly as in other cases of CNT modified electrodes.23 When performing control experiments using pure MWCNT, no reduction wave appeared at around −150 mV and the cyclic voltammogram exhibited a single reduction signal of cytochrome c only at ∼−420 mV. A similar result originated when the bare glassy carbon electrode was used. Therefore, it is clear that the reduction wave at −160 mV does not originate from the inherent electrochemical properties of MWCNTs. The electrochemical response of the modified electrodes in the blank buffer is shown in Figure S1 (Supporting Information). We then proceeded to investigate the effects of the individual impurities (carbonaceous and metallic). We performed cyclic voltammetry of cytochrome c at glassy carbon electrode modified with metal oxide nanoparticles which were identified as impurities present in MWCNT-i by ICPMS. Figure 2 shows the cyclic voltammograms of
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RESULTS AND DISCUSSION When revisiting ref 21 reporting on the electrocatalytic properties of CNT toward cytochrome c, one can clearly notice the highly contrasted bright spots on the scanning electron micrograph (SEM) presented there (Figure 1, ref 21). It is difficult to assess from the quality of the image whether these spots are metallic or carbonaceous impurities; however, it is clear that they do not structurally resemble nanotubes.21 We therefore examined the electrochemistry of commercial carbon nanotube samples (Sigma-Aldrich; labeled as MWCNT-i) which contained metallic (0.65% (wt) of Co, 0.014% of Fe, 0.07% of Mo, and 0.5% of Ni, according to in house ICPMS analysis) as well as carbonaceous impurities (∼15% of carbonaceous impurities) and compared the results to the electrochemistry of pure MWCNT18,22 (impurities-free), amorphous carbon (representing carbonaceous impurities19), and NiO, Fe3O4, Co3O4, and MoO2 nanoparticles (representing metallic impurities). First, we recorded the cyclic voltammograms to investigate the reduction potential of cytochrome c on MWCNT-modified electrode. It can be seen from Figure 1 that the first reduction wave starts at ∼−50 mV (vs Ag/AgCl reference electrode), reaching a maximum at −160 mV; and a second reduction wave appears at −415 mV. It should be mentioned that electrochemistry of cytochrome c typically provides quasireversible
Figure 2. Cyclic voltammogram of 0.3 mM cytochrome c at amorphous carbon (AC), Co3O4, NiO, MoO2, Fe3O4, and bare GC electrode. Conditions as in Figure 1.
cytochrome c at the Fe3O4, MoO2, NiO, and Co3O4 modified electrode surface, where it can be seen that no catalytic effect was shown with the voltammograms having similar features as the bare glassy carbon electrode. In contrast, the amorphous carbon modified GC electrode exhibited a strong reduction wave for cytochrome c starting at ∼−100 mV and reaching a first reduction peak at −225 mV, with a following increase of current into a poorly defined second reduction wave at ∼−500 mV. It is apparently clear from these experiments that, among the impurities present within the MWCNT-i sample, those of carbonaceous nature were responsible for the observed electrocatalytic effects toward the reduction of cytochrome c, while the metallic impurities have no participation and the CNT material does not play a role in the observed behavior. The electrochemical response of the corresponding modified electrodes in the blank buffer is shown in Figure S2 (Supporting Information). It would appear, therefore, that the previously claimed electrocatalytic effects of CNT modified electrodes for the reduction of cytochrome c were most likely generated by the presence of carbonaceous impurities that are normally found in commercial CNT samples. Thus, no special or particular catalytic properties can be assigned to the CNT material itself.
Figure 1. Cyclic voltammogram of 0.3 mM cytochrome c at MWCNTi, pure MWCNT, and bare GC electrode. Conditions: 50 mM phosphate buffer, pH 7.2; scan rate of 50 mV/s. B
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Letter
(23) Banks, C. E.; Compton, R. G. Analyst 2005, 130, 1232.
CONCLUSION We have managed to revisit and challenged the claims on the extraordinary electrocatalytic properties of carbon nanotubes toward the electrochemistry of cytochrome c as presented in a 2002 article.21 By a careful and systematic investigation on the effect of each of the components present in a commercial CNT sample, which includes the carbonaceous impurities and the metallic impurities, as well as the CNT material itself, we proved that the electrocatalytic properties originate from the carbonaceous impurities, with the carbon nanotubes themselves presenting no contribution to the accelerated electrochemistry observed.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: (65) 6791-1961. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.P. acknowledges a NAP grant (NTU). REFERENCES
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