What amount of metallic impurities in carbon nanotubes is small enough not to dominate their redox properties?

Abstract

As carbon nanotubes become a part of everyday life, the paramount questions about their toxicity persist. While it has been proven that a significant part of the toxicity of carbon nanotubes is due to the redox activity of the residual metallic impurities present within them, there is still no response to the fundamental question of where the borderline is that would render the impurities ‘redox invisible’. Herein we investigate the electrochemical response of carbon nanotubes containing different amounts of impurities towards the reduction of an important biomarker , hydrogen peroxide, and the oxidation of an important impurity marker, hydrazine. We found that the borderline between being redox active/inactive for iron-based impurities lies in the middle-ppm range.

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Introduction

As carbon nanotubes (CNTs) leave research labs and enter our ‘ordinary’ world where they enhance the properties of a wide range of products such as components of sporting goods, biomedical devices, and aeroplanes, one often hears the question “are these nanotubes really safe?” It is well known that CNTs contain significant amounts of residual metallic catalyst impurities in surprisingly large amounts—on the order of 1–10%.^1,2 While sp² carbon itself is quite a chemically inert material, and the damage to cells and tissues only occurs through mechanical disturbance, metallic impurities, in contrast, strongly participate in the redox properties of biomarkers and metabolic intermediates.^2–5 This is true even in cases where the metallic nanoparticles (NP) are sheathed within CNTs, via intercalation of biomarkers into the CNT lattice.^2–4 It has been shown that apparently encapsulated nickel and iron impurities within single-wall carbon nanotubes (SWCNTs) are bioavailable and strongly interact with biomolecules present in lysosomes, causing e.g., depletion of ascorbate or induction of single-strand breaks in plasmidDNA.^6,7 Large numbers of toxicological studies of CNTs have been carried out to reveal potential risks associated with the use of CNTs, but the results are surprisingly contradictory.^8–10 It has been noted that the uncertainty in the level of toxicity of CNTs comes from the large variation in the types and concentrations of residual metal impurities in the different CNTs studied.^11–14 This should not be surprising, given the fact that the impurities consist of toxic metals such as nickel, copper, cobalt, molybdenum, and iron.

A whole spectrum of methods for purifying CNTs is available. However, they are often not very effective and CNTs containing 1% or less of residual metallic impurities are considered ‘high quality’.¹⁵ Such a large amount of impurities is not acceptable for release into the environment. For example, just 1 ton of CNTs with 1 wt% of metallic impurities would include 10 kg of toxic metallic nanoparticles. Given the fact that it is expected that millions of tons of CNTs will be produced worldwide,¹⁶ the presence of toxic metallic impurities within CNTs is a problem of paramount importance. Therefore, one might ask an important question—“what amount of metallic impurities in carbon nanotubes is small enough to not dominate their redox properties?”.

We investigated the involvement of metallic impurities in the redox properties of CNTs by electrochemistry. Before discussing the experiments any further, it is important to make the following remarks. The electrochemistry of CNTs resembles that of graphite—heterogeneous electron transfer is fast at the ends of the CNTs and at their defect sites (similar to the edge plane of graphite) while pristine CNT walls (similar to the basal plane of graphite) show very low rates of heterogeneous electron transfer.^17–22 It has also been shown that residual metallic catalyst impurities in CNTs are, in some cases, responsible for the ‘exceptional electrocatalysis’ of CNTs. It has been demonstrated that iron-based impurities are responsible for the ‘electrocatalysis’ of hydrogen peroxide^23,24 and hydrazine²⁵ while copper nanoparticle impurities occluded within multi-wall carbon nanotubes (MWCNTs) are responsible for the ‘electrocatalysis’ of halothane.²⁶ We have shown that multicomponent (Mo/Co/Fe) metallic impurities within CNTs catalyze the electrochemical oxidation of hydrazine.²⁷ Recently, we have demonstrated that the issue is even more complex, and that even traces of iron within non-catalytic cobalt impurities can catalyze the reduction of hydrogen peroxide.⁴ Furthermore, we showed that the bi-component nickel and iron residual catalyst impurities within SWCNTs are responsible for the electrochemical oxidation of arginine, where Fe and Ni participate, and histidine, where only Fe participates.²⁸

In this article, we investigated this paramount question “what amount of metallic impurities in carbon nanotubes is small enough to not dominate their redox properties?” using examples of the reduction of hydrogen peroxide and the oxidation of hydrazine on CNTs containing different amounts of residual metallic impurities. Hydrogen peroxide is one of the most important biomarkers and thus naturally one of the most important analytes for biosensing. In addition, electrochemical oxidation of hydrazine is very sensitive to small amounts of metallic impurities and therefore we used hydrazine for the confirmation of redox accessible impurities within CNTs.^29,30 Because there is no available standard reference for CNT material,^31,33 we chose five CNT samples with distinctively different (on the order of several magnitudes) amounts of impurities to determine the threshold below which impurities do not play a significant role in the redox properties of CNTs.

Materials and methods

Materials

CNT-A: Double-walled carbon nanotubes (outer diameter (O.D.) 5 nm; inner diameter (I.D.) 1.3–2.0 nm; length 5–15 µm; consisting of 50% double-walled CNTs and the remaining carbon content triple- and quadruple-walled CNTs; carbon content 93.20 wt%; impurities: Co 33300; Mo 11800; and Fe 450 ppm). CNT-B: Few-walled carbon nanotubes (I.D. 2–5 nm, O.D. 6–10 nm, consisting of 30% double-walled, 50% triple-walled, and about 10% quadruple-walled CNTs; carbon content 96.98 wt%; impurities: Co 11000; Mo 5000; and Fe 100 ppm). CNT-C: Multi-walled carbon nanotubes (O.D. 80–100 nm; I.D. 40–50 nm; length 5 µm; herringbone structure; impurities: Fe 10200 ppm). CNT-D: Multi-walled carbon nanotubes (O.D. 80–100 nm; I.D. 40–50 nm; length 5 µm; herringbone structure; impurities: Fe 10 ppm. These CNT-D samples were heat-treatment purified CNT-C samples). CNT-E: Multi-walled carbon nanotubes (O.D. 90–110 nm; length 7 µm; carbon content >99.9 wt%; impurities: Fe 3 ppm). CNT-A and E were obtained from Sigma-Aldrich; CNT-C and D were obtained from GSI Creos (Japan) and CNT-B was synthesized in-house. N,N-dimethyl formamide (DMF), nitric acid, potassium phosphate dibasic, sodium phosphate monobasic, Fe₃O₄ nanoparticles (NPs, diameter <50 nm), hydrogen peroxide, and hydrazine were purchased from Sigma-Aldrich, Japan. Basal plane pyrolytic graphite (BPPG) electrodes with a diameter of 3 mm were obtained from Autolab, Japan.

Apparatus

A JEM 2100F field-emission transmission electron microscope (JEOL, Japan) operating at 200 kV was used to acquire TEM images in scanning TEM mode (spot size, 0.7 nm). TEM images and EDX spectra were collected using the JEM 2100F equipped with an energy dispersive X-ray spectrometer with an ultrathin window (JEOL). All voltammetric experiments were performed using an Autolab 302 electrochemical analyzer (Eco Chemie, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems v. 4.9 software (Eco Chemie). Electrochemical experiments were performed in a 5 mL voltammetric cell at room temperature (25 °C) using a three-electrode configuration. A platinum electrode served as an auxiliary electrode and a Ag/AgCl electrode as a reference electrode. All electrochemical potentials in this paper are stated vs. Ag/AgCl.

Procedures

As-received CNT materials were stirred in concentrated nitric acid (6 M) for 36 h at 80 °C.³² The acid/CNT mixture was subsequently washed with distilled water and centrifuged several times until the aqueous solution reached neutral pH. The washed CNTs were then filtered through a 0.2 µm membrane (Nuclepore Track-Etched Membrane, Whatman, UK) under vacuum. Such-treated CNTs were used for all experiments in this article and are referred to as ‘washed’ CNTs. The BPPG electrodes were renewed by polishing with 0.05 µm alumina particles and consequently by the repetitive ‘cellophane tape’ method to remove the top layer of graphite. The resulting freshly prepared BPPG electrodes were washed in acetone for 30 s to remove any residual glue. For the electrochemical measurements, the CNTs were cast onto a freshly prepared BPPG electrode surface. The CNTs were first dispersed in DMF at a concentration of 1 mg mL⁻¹. The suspension was then placed into an ultrasonic bath for 5 min, after which 5 µL of the suspension was pipetted onto the BPPG electrode surface. The suspension was allowed to evaporate at room temperature, creating a randomly distributed CNT film on the BPPG electrode surface. Nanoparticle-modified BPPG electrodes were prepared in a similar fashion by dispersing nanoparticles in DMF (1 mg mL⁻¹) and subsequent deposition of 5 µL of the dispersed NPs on the BPPG surface. Cyclic voltammetric experiments were performed at a scan rate of 100 mV s⁻¹ using 50 mM phosphate buffer (pH 7.4). For TEM measurements, 1 µL of a 0.5 mg mL⁻¹ suspension of CNTs was dropped onto a copper TEM grid and left to dry in air.

Results and discussion

The selected samples of CNTs contained the following residual metallic impurities: CNT-A (impurities: Co 33300; Mo 11800; and Fe 450 ppm), CNT-B (impurities: Co 11000; Mo 5000; and Fe 100 ppm), CNT-C (impurities: Fe 10200 ppm), CNT-D (impurities: Fe 10 ppm), and CNT-E (impurities: Fe 3 ppm) based on in-house analysis by ICP-AES and magnetic susceptibility measurements.^1,33

A transmission electron microscope equipped with an energy dispersive X-ray analyzer (TEM/EDX) was used to observe the structure of the CNTs and ascertain the presence of impurities. For example, Fig. 1 shows TEM micrographs of CNT-A, B, C, D, and E. As can be seen, the CNTs have profoundly different structures. This plays an important role in their electrochemistry, as we show later. CNT-A and B have diameters of approximately 5–10 nm and consist of a mixture of two-, three-, and four-walled nanotubes (generally called few-walled CNTs, FWCNTs) while CNT-C, and D have a herringbone structure with open ends, an outer diameter of 80–100 nm and contain an inner channel with a diameter of about 40–50 nm. CNT-E consists of hundreds of concentric graphene tubes. It is clear from the TEM micrographs of CNT-A, B, and C that these CNTs contain a significant number of embedded metallic impurities (dark contrast dots; see arrows in Fig. 1). In contrast, CNT-D and E contain no observable metallic impurities (note that the observable high-contrast parts of the walls of CNT-C, D and E result from electron diffraction by the CNT lattice and these are not related to metallic impurities).³⁴TEM/EDX analysis of the impurities in samples CNT-A, B, and C confirmed that they contain Mo, Co, and Fe in the cases of CNT-A and B (see the selected TEM images and EDX spectra in Fig. 1) in different ratios. CNT-C contains only Fe nanoparticles. TEM/EDX analysis of the residual metallic nanoparticles within CNT-A, B, and C showed that the average composition of the metallic impurities is as follows. In CNT-A: Co 94.28; Mo 4.70; and Fe 0.52 atm%. (n = 28); in CNT-B: Co 71.53; Mo 25.68 ; and Fe 1.48 atm% (n = 8); and in CNT-C: Fe 100% (n = 9). It should be noted here that TEM/EDX is not a statistically representative method. It is used here as an additional analytical method. ICP-AES and magnetic susceptibility measurements provide statistically reliable values. CNT-D and E are virtually impurity-free and it was not possible to localize/analyze any impurities under TEM and TEM/EDX.

Fig. 1 TEM images (left) and EDX spectra (right) of metallic impurities in ‘washed’ carbon nanotubes. (A) CNT-A, (B) CNT-B, (C) CNT-C, (D) CNT-D, and (E) CNT-E. Scale bar in TEM images is 10 nm (A, B) and 100 nm (C, D, and E).

In order to establish the influence of metallic impurities on the electrochemical redox behavior of CNTs, we selected two important markers, hydrogen peroxide and hydrazine. Hydrogen peroxide is a highly important biomarker , which is prone to being electrocatalyzed on Fe particles but not on other metals.⁴Hydrazine is an important marker of the presence of impurities because a wide variety of metals is able to electrocatalyze its oxidation.²⁹

We first started with the evaluation of the redox response of hydrogen peroxide (H₂O₂). Fig. 2 exhibits the voltammetric responses related to the electrochemical reduction of 5 mM H₂O₂ on CNT-A, CNT-B, CNT-C, CNT-D, CNT-E, and Fe₃O₄NP-modified BPPG electrodes. A corresponding voltammogram of the reduction of H₂O₂ on a bare BPPG electrode is also shown. The significant reduction of H₂O₂ on the CNT-A modified BPPG electrode (blue line) starts at −0.02 V and reaches a maximum around −0.70 V. The reduction of H₂O₂ on the CNT-B modified BPPG electrode (purple line) starts at −0.10 V and reaches a maximum at −0.48 V. The reduction of H₂O₂ on the CNT-C modified BPPG electrode (orange line) starts at −0.24 V and reaches a maximum at −0.49 V. These potentials are similar to those observed previously using MWCNTs with Fe(III) impurities^4,23 and in agreement with the reduction of H₂O₂ on an Fe₃O₄NP-modified BPPG electrode (green line), which starts around −0.10 V. In contrast, the CNT-D and E modified electrodes (magenta and red lines, respectively) display negligible reduction current starting at around −0.45 V. The reduction of H₂O₂ on a bare BPPG electrode (black line) also starts at around a potential of −0.45 V. Thus, it is clear that the electrochemistry of CNT-D and E is dominated by edge-plane defects and ends of CNTs (similar to the electrochemistry of bare BPPG, which is governed by edge-like defect sites) and not by Fe-based impurities.

Fig. 2 Cyclic voltammograms resulting from the electrochemical reduction of 5 mM H₂O₂ at BPPG electrodes modified with CNT-A (blue line), CNT-B (purple line), CNT-C (orange line), CNT-D (magenta line), CNT-E (red line), Fe₃O₄NPs (green line), and at bare BPPG (black line). Conditions: scan rate, 100 mV s⁻¹; background electrolyte, 50 mM phosphate buffer, pH 7.4.

Next, we investigated the influence of metallic impurities upon the oxidation of hydrazine because, as mentioned above, hydrazine (N₂H₄) is very sensitive to the presence of metallic impurities in general. Fig. 3 displays cyclic voltammograms corresponding to the electrochemical oxidation of 5 mM N₂H₄ at CNT-A, B, C, D, and E and Fe₃O₄NP-modified BPPG electrodes. We also show the corresponding control voltammogram of bare BPPG. The significant wave of the oxidation of N₂H₄ at CNT-A (blue line) starts around −0.02 V and reaches a maximum at +0.42 V. The oxidation of N₂H₄ at a CNT-B modified BPPG electrode (purple line) starts at +0.14 V while of a CNT-C modified BPPG electrode (orange line) at +0.18 V. The oxidation of N₂H₄ on an Fe₃O₄NP-modified electrode (green line) starts at +0.15 V. In contrast, the oxidation of N₂H₄ on a CNT-D modified BPPG electrode (magenta line) starts around +0.45 V while that on a CNT-E modified BPPG electrode (red line) displays negligible oxidation current for the oxidation of N₂H₄, starting slowly at +0.90 V. This is similar to the response of bare BPPG where negligible oxidation current is also observed around +0.90 V. The difference between the behaviors of CNT-D and E is attributed to their different structures, where the ‘herringbone’ CNT-D exhibits significantly larger numbers of edge-like sites on its surface, thus enhancing the heterogeneous electron transfer rates, while CNT-E has a tubular structure and its ‘edge-like’ defects are limited to nanotube ends and surface defects. The density of such defects in the case of CNT-E is lower than that in the case of CNT-D.³⁵ This electrochemical behavior is analogous to the oxidation of N₂H₄ on BPPG and EPPG.²⁷ Thus, we can conclude that the electrochemistry of CNT-D and E is dominated by the edge-like defects and ends of the CNTs and not by metallic impurities. The observed data are in good agreement with a previous report of the oxidation of N₂H₄ at MWCNTs containing iron-based impurities and at bare BPPG.^25,27

Fig. 3 Cyclic voltammograms resulting from the electrochemical oxidation of 5 mM N₂H₄ at BPPG electrodes modified with CNT-A (blue line), CNT-B (purple line), CNT-C (orange line), CNT-D (magenta line), CNT-E (red line), Fe₃O₄NPs (green line), and at bare BPPG (black line). Conditions: scan rate, 100 mV s⁻¹; background electrolyte, 50 mM phosphate buffer, pH 7.4.

The comparison of the response of the CNT-A, B, and C and Fe₃O₄NP-modified BPPG electrodes led us to the conclusion that iron oxide impurities (in the case of hydrogen peroxide) and metallic impurities (in the case of hydrazine) are responsible for the observed redox behavior of CNT-A, B, and C. The reason for this is that minute amounts of electroactive nanoparticles with small random array coverage might exhibit behavior similar to that of related macroelectrodes because of greatly overlapping diffusion layers, especially when interconnected with CNT nanowires.^36–38 It should be noted that the average thickness of the diffusion layer (according to the conditions of our voltammetric experiments) is around 128 and 115 µm for hydrogen peroxide and hydrazine, respectively.^4,27TEM images show that the distance between the residual metal catalyst nanoparticles, which perform as metallic nanoelectrodes, is on the order of magnitude from the tens to the hundreds of nanometres. Therefore, their diffusion layers overlap a great deal and the metal nanoparticles are responsible for the observed redox action of CNT-A, B and C.

In order to respond to our principal question “what amount of metallic impurities in carbon nanotubes is small enough to not dominate their redox properties?” the following items should be considered. First, we should emphasize that hydrogen peroxide could ‘see’ only iron-based impurities and none of the other metals present in the CNTs used in this study.⁴ In contrast, hydrazine is prone to electrocatalytic oxidation on any metal surface that is present in the CNT sample as an impurity. Therefore, the electrochemically visible content of impurities for the reduction of hydrogen peroxide is 450 ppm for CNT-A, 100 ppm for CNT-B, 14000 ppm for CNT-C, 10 ppm for CNT-D, and 3 ppm for CNT-E. The electrochemically noticeable content of impurities for hydrazine is 45550 ppm for CNT-A, 15100 ppm for CNT-B, 14000 ppm for CNT-C, 10 ppm for CNT-D, and 3 ppm for CNT-E. Interestingly, it is clear that even 100 ppm of Fe impurities within a CNT sample (such as in the case of CNT-B), which is significantly lower than the detection limits of the SEM/EDX or XPS methods and close to the limit of detection of ICP-AES,^1,33 still dominate the redox properties of hydrogen peroxide. This is very disturbing for real-world applications of CNTs because the amount of metallic impurities is usually not estimated with such trace analysis methods. One could thus say that the borderline where iron-based impurities stop dominating the redox properties of CNTs lies somewhere between 100 and 10 ppm. However, the potential influence of the structure of the CNTs should also be considered. Both CNT-A and CNT-B are FWCNTs with up to four walls and a small diameter (about 5–10 nm). The CNT-C, D, and E samples contain large nanotubes with diameters of about 90 nm. CNT-C and D exhibit open ends and large inner channels with diameters of about 50 nm and wall thicknesses of about 25 nm. The mechanism of interaction of the molecules with biomarkers is thought to be via intercalation into the defects of the nanotubes or via metal release from the CNTs.^2,3,6,7 The maximum proven depth of inclusion of metallic impurities within CNTs in which they still participate in the electrochemistry is 13 nm.² If one limits the depth of such intercalation or release in the timescales presented here to 13 nm, then the amount of electrochemically accessible iron-based impurities in the CNT-C and D samples is 100% (because of the large inner channel). However, the electrochemically accessible amount of impurities in the CNT-E sample would be only about 46% of the total amount (assuming homogeneous distribution of the impurities in the nanotube in the form of ‘atomic doping’, as described in ref. 1). Thus, the electrochemically accessible concentration of impurities within the CNT-E sample might be only 1.38 ppm. Therefore, it can be established that the borderline for the amount being “small enough to not dominate the redox properties of CNTs” lies between 100 and 10 ppm for hydrogen peroxide and somewhere between 15100 and 10 ppm for hydrazine. Although the range of the borderline for hydrazine might seem too wide, one should note that from a biomedical point of view, the borderline for hydrogen peroxide is much more important.

Conclusions

In conclusion, we have demonstrated that the borderline of participation of metallic impurities on the redox properties of CNTs towards reduction of the important biomarker hydrogen peroxide lies in the mid-ppm level, whereas at the high-ppm levels the metallic impurities still dominate the electrochemistry of the CNTs and at the low-ppm levels the electrochemistry is dominated by the ‘edge-plane’ electrochemistry of the CNTs, which is inherent in sp² carbon materials. This knowledge is important for the further development of applications of CNTs in the real-world environment. Because there is not yet any reference material or standard material in CNT science, it is very challenging to judge this borderline more accurately. However, we are working in this direction. Future studies will include CNTs with the same morphology and different metallic impurities contents. It is expected that toxicological studies of nanomaterials will gain paramount significance in the future and cheap, fast, high-throughput and reliable methods to fulfil these future needs will be required. We wish to propose that screening of potential toxic activity of CNTs due to the presence of metallic impurities should first be done by electrochemical methods, which are fast and inexpensive. Only if the electrochemical results are negative, should the CNT samples undergo laborious and expensive biomedical testing.

Acknowledgements

This work was supported in part by the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan. The authors are indebted to the Materials Analysis Station (Namiki, NIMS) for ICP-AES measurements, to GSI Creos (Japan) for donation of the CNT-C and D samples, to Dr J. Tang (1D Nanomaterials Group, NIMS) for donation of the CNT-B sample and to Dr T. Kolodiazhnyi (NIMS) for valuable discussions.

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