DOI: 10.1002/chem.201103266

Redox-Active Nickel in Carbon Nanotubes and Its Direct Determination

Adriano Ambrosi and Martin Pumera*[a]

Introduction

The increasing manufacture and use of nanomaterials for in-dustrial and biomedical applications as well as in consumerproducts has raised serious concerns over their safety to-wards human health and the environment.[1–4]

In particular, carbon nanotubes (CNTs) have attracted in-terest from both academia and industry owing to their out-standing physical, chemical, and mechanical properties.CNTs are widely used for the production of many consumerproducts, such as sporting goods, biomedical and sensing de-vices, as well as for construction materials.[5–7]

The fast development of new products and the exponen-tial increase in the worldwide production of CNTs has gen-erated concern over their potential toxicity.[8] Many toxico-logical assays of different types of CNTs have been per-formed, but surprisingly, the results are often contradictory,specific, and valid only for a certain CNT sample or for aparticular batch used in each study.[9–11] However, it iswidely accepted that the toxicological effects of CNTs canmainly be attributed to metallic impurities found in the sam-ples.[12,13] These impurities are inherently present in CNTsamples because CNTs are typically synthesized by usingmetal (Fe, Ni, Co, Mo)-catalyst nanoparticles. CNT samplesmay contain up to 30 % (wt) of residual metallic impuritiesafter production and up to 10 % (wt) after purification pro-cesses.[14–17]

Bioavailability is defined as “the degree of activity oramount of an administered drug or other substance that be-comes available for activity in the target tissue”.[18] From atoxicological point of view, it is very important to distinguishbetween the total and the accessible/mobilizable content ofmetallic impurities because only the bioavailable portioncan interact adversely with biological systems.[16,19–21] Themetallic impurities in CNTs can participate in the redoxchemistry of various compounds, such as hydrogen perox-ide,[22] glucose,[23] halothane,[24] hydrazine,[25] and aminoacids,[26] and they may also cause DNA damage.[27] It shouldalso be noted that there is strong batch-to-batch variation insuch participation.[28] Several techniques are available toquantify the content of metallic impurities within CNTs,such as transmission-electron-microscopy/scanning-electron-microscopy–energy-dispersive-X-ray-spectroscopy (TEM/SEM-EDX), X-ray photoelectron spectroscopy (XPS), ther-mogravimetric analysis (TGA), magnetic susceptibility,[14]

neutron-activation analysis, and inductively coupled plasma–atomic-emission-spectroscopy/mass-spectrometry (ICP-AES/MS).[15] Although these methods can determine the totalcontent of impurities in CNTs, they are not able to distin-guish between the total and the bioavailable portion. It isimportant to note that metallic nanoparticles can sometimesremain trapped between several graphene layers in thenanotube structure and therefore do not interact with thesurrounding solution.[22,25, 28] In this situation, these “sheath-ed” nanoparticles are unlikely to participate in toxicologicalevents. To quantify the bioavailable portion of impurities,other methods have been proposed, but these methods typi-cally require long incubation periods to allow the metals toleach out of the CNT samples.[16,19, 20] Herein, we report acompletely new approach based on solid-state electrochem-istry for the rapid and specific determination of redox-active(bioavailable) Ni in CNT samples.

The electrochemistry of Ni electrodes in alkaline solutionhas been well-known for more than forty years, owing to

Abstract: The presence of residualmetal-catalyst impurities in carbonnanotubes is responsible for their toxic-ity. It is important to differentiate be-tween the total amount of impuritiesand the redox-active (bioavailable)amount of such impurities becauseonly the bioavailable impurities exhibittoxic effects. Herein, we report a

simple and specific method for quanti-fying the amount of redox-active Nipresent in various commercial samplesof CNTs. It is based on the electro-

chemical oxidation of Ni(OH)2 that isformed in alkaline solutions when Niimpurities are opened to the surround-ing environment. Metallic Ni impuritiesplay an extremely active role in toxico-logical assays as well as in undesiredcatalytic processes, and thus a methodto rapidly quantify the amount ofredox-active Ni is of great importance.

Keywords: bioavailability · electro-chemistry · nanotubes · nickel · re-dox chemistry

[a] Dr. A. Ambrosi, Prof. M. PumeraDivision of Chemistry & Biological ChemistrySchool of Physical and Mathematical SciencesNanyang Technological UniversitySingapore 637371 (Singapore)Fax: (+65) 6791-1961E-mail : pumera@ntu.edu.sg

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201103266.

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their applications in batteries,[29] fuel cells,[30–32] and superca-pacitors.[33]

The catalytic features of Ni electrodes are directly relatedto the electrochemistry of Ni(OH)2 film layers that formspontaneously on the Ni surface.[34] Ni electrodes, or in situgenerated Ni-nanoparticle-modified electrodes, have beenpreviously used for the analysis of methanol,[35] sulfide,[36]

and adenine[37] by exploiting the catalytic features of theNi(OH)2/NiOOH redox couple. It is surprising that such cat-alytic effects have not been explored in correlation with theNi-based metallic impurities present in CNTs. Herein, we in-vestigate the specific electrochemistry of Ni impurities pres-ent in several CNT samples in alkaline conditions and sub-sequently propose a method to correlate the generated sig-nals from the electrooxidation of Ni(OH)2 into NiOOHwith the amount of redox-active mobilizable Ni that is freeto interact with the surrounding environment. The “sheath-ed” portion of metallic impurities within the graphenelayers has no access to the solution and therefore does notgive rise to a voltammetric response. This method is simpleand offers a unique possibility to rapidly quantify the por-tion of redox-active Ni from the total amount present in sev-eral commercial CNT samples.

Results and Discussion

We have characterized and investigated the presence of met-allic impurities in eight different commercially availableCNT samples. Using ICP-AEP, we quantified the totalamount of metallic impurities present in these samples,paying particular attention to impurities based on Fe, Co,Mo, and Ni, which are widely recognized to be responsiblefor the adverse toxicological effects as well as for the unde-sired catalytic properties of CNTs: CNT-A (Fe 0.09, Ni22.2 wt %); CNT-B (Fe 0.04, Ni 2.8 wt %); CNT-C (Co 0.5,Fe 0.1, Mo 0.2, Ni 17.2 wt %); CNT-D (Fe 0.09, Ni19.1 wt %); CNT-E (Fe 0.2 Ni 0.7 wt. %); CNT-F (impuri-ties<detection limit); CNT-G (Co 0.4, Fe 1.7, Mo0.07 wt %); CNT-H (Fe 1.25, Mo 0.22 wt %). CNT-A–CNT-E contained different combinations of metallic impuritieswith different amount of Ni. CNT-F–CNT-H were impurity-free or contained metallic impurities other than Ni, andtherefore were used in this work as control samples.

TEM was used to gain information on the morphology ofeach CNT sample and to investigate the presence of nano-meter-sized metallic particles as impurities. Figure 1 showsrepresentative TEM micrographs of the eight CNT sampleswith their corresponding EDX spectra. Metallic impuritieswere present in significant amounts in all of the samples asdispersed residues or shielded within the graphene layers ofthe CNT structures. EDX analysis revealed that CNT-Acontained Ni/Fe-based impurities, CNT-B contained anabundance of Ni-based impurities, CNT-C contained Ni/Mo/Fe impurities, CNT-D contained Ni/Y/Mo impurities, andCNT-E contains mainly Ni/Fe impurities. CNT-F–CNT-Hdid not show any Ni signals. In particular, CNT-F showed

extremely low signals for all metals, CNT-G exhibited Feimpurities, and CNT-H contained Fe/Mo impurities. The Cusignal was present in each case owing to the Cu-based TEMspecimen holder. EDX analysis confirmed the ICP-AESdata.

Next, we investigated the electrochemical behavior of theCNT samples in alkaline media. It is well-established that anickel electrode immersed in aqueous alkaline hydroxidesolutions is spontaneously covered by layers of Ni(OH)2.Therefore, the electrochemistry of Ni electrodes is relatedto the redox features of Ni(OH)2, which have a very well-defined electrochemical behavior. Knowing that CNT sam-ples can sometimes contain up to 30 % (wt) of Ni impuritiesin the form of nanoparticles with different sizes and distri-butions, we decided to first investigate the voltammetric be-havior of Ni and NiO nanoparticles in a 0.1 m NaOH solu-tion.

Figure 2 shows the cyclic voltammograms of Ni- and NiO-modified glassy carbon electrodes in 0.1 m NaOH (pH 13)and phosphate-buffered solutions (pH 7.2). At neutral pH(Figure 2 a, c), no significant oxidation or reduction signalswere recorded, even after 100 voltammetric scans, whichshowed that both Ni and NiO were very stable and did notundergo any oxidative or reductive conversion at this pHvalue. At pH 13, the Ni-modified electrode showed no sig-nificant oxidation or reduction peak during the first voltam-metric scan. However, from the second scan onwards, an ox-idation signal appeared at about 0.45 V with a maximum atabout 0.5 V. The oxidation signal grew progressively withthe number of scans, and reached a stable contour after ap-proximately the 80th scan. Similarly, for the cathodic scans,a reduction peak only appeared from the second scan on-wards (starting at 0.5 V with a maximum at 0.4 V), whichprogressively grew with the number of scans (Figure 2 b).The NiO-modified electrode showed a similar behavior,with oxidation and reduction peaks that grew in intensitywith the number of scans; again, this behavior was only ob-served at pH 13, and at very similar potentials to the Ni-modified electrodes (Figure 2 d). The charge under the oxi-dative peak was almost coincident with that of the reductionscan, thus demonstrating the reversibility of the process. Formore-sensitive evaluations, DPV signals were recorded after100 potential cycles (see the Supporting Information, Fig-ure S1). An intense peak was only observed (at about 0.5 V)when NaOH (pH 13) was used as the supporting electrolytefor both the Ni- and NiO-modified electrodes.

The oxidation and reduction signals recorded at pH 13were attributed to the oxidation of Ni(OH)2 into NiOOHand successive reduction back to Ni(OH)2, according to thereaction given in Equation (1).[38]

NiðOHÞ2 ! NiOOHþHþþe� ð1Þ

A plethora of publications have explained in detail themechanisms involved during oxidation and reduction usingNi-based electrodes in alkaline media.[34,39–45] For the pur-pose of this work, it is sufficient to know that under alkaline

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conditions, Ni or Ni/NiO nanoparticles spontaneously gener-ate a thin film of Ni(OH)2 on their surfaces. Potential cy-cling between the redox peaks increases the thickness of theNi(OH)2 layer, and consequently the intensity of the redoxsignals, until the thickness is such that it impedes the availa-bility of Ni for further oxidation.[38]

The area under the oxidation (or reduction) peak gives ameasure of the charge required to oxidize Ni(OH)2 intoNiOOH (or reduce NiOOH into Ni(OH)2), and can be usedto quantify the amount of Ni that is present in its hydroxideform. This process could be an excellent indicator of theamount of redox-active (bioavailable) Ni in the totalamount present in the CNT samples.

Therefore, we proceeded to investigate the electrochemis-try of the CNT samples in the presence of 0.1 m NaOH. TheCNT-modified GC electrodes were prepared as explained inthe Experimental Section; their preparation was similar to

that of the Ni and NiO-modified electrodes. Figure 3 showsthe cyclic voltammograms recorded using GC electrodesmodified with CNT-A–CNT-D. These four samples con-tained significant amounts of Ni impurities (between 2.8 and22.2 wt %). There were striking similarities with the voltam-mograms recorded using Ni- and NiO-modified electrodes.As such, it was evident that a portion of the Ni content inthe CNTs was readily available to interact with the sur-rounding environment.

We decided to use the oxidation peak to measure thecharge required to oxidize the Ni(OH)2 layer and to corre-late it to the amount of redox-active Ni by using the Faradaylaw. The area of the oxidation peak at each potential cyclewas measured to make sure that the final quantification wasdone after reaching saturation. These measurements areshown as insets in each Figure. Signal saturation wasreached after 100 scans for CNT-A, CNT-B, and CNT-C,

Figure 1. Representative TEM images of CNT samples used in this work and their corresponding EDX spectra, which show the metallic impurities:A) CNT-A, B) CNT-B, C) CNT-C, D) CNT-D, E) CNT-E, F) CNT-F, G) CNT-G, H) CNT-H.

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M. Pumera and A. Ambrosi

Figure 2. Electrochemical behavior of Ni- and NiO nanoparticles at dif-ferent pH values over 100 voltammetric scans. Cyclic voltammograms ofNi nanoparticles at a) pH 7.2 and b) pH 13. Cyclic voltammograms ofNiO nanoparticles at c) pH 7.2 and d) pH 13. For clarity, only the 1st,20th, 40th, 60th, 80th, and 100th CVs are represented. Arrows indicatethe increasing number of scans. Background electrolyte: 0.05 m phosphatebuffer solution (pH 7.2) and 0.1 m NaOH solution (pH 13); scan rate:0.1 Vs�1 vs. Ag/AgCl.

Figure 3. Repetitive cyclic voltammograms in 0.1 m NaOH solutions ofGC electrodes modified with: a) CNT-A, b) CNT-B, c) CNT-C, andd) CNT-D. Inset: Plot of total charge under the oxidation peak vs.number of voltammetric scans. For clarity, only the 10th, 20th, 40th, 60th,80th, and 100th CVs (a–c), and the 10th, 20th, 40th, 60th, 80th, 100th,120th, 140th, 160th, 180th, and 200th CVs (d) are shown. Arrows indicatethe increasing number of scans. Scan rate: 0.1 V s�1; reference electrode:Ag/AgCl.

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whilst for CNT-D it was reached only after 200 scans. Aftermeasuring the charge required to oxidize Ni(OH)2 followingsaturation, we extrapolated the amounts of Ni to be:5.7 wt % for CNT-A, 0.2 wt % for CNT-B, 4.2 wt % for CNT-C, and 8.5 wt % for CNT-D, which corresponded to 25.6, 8.5,24.5, and 44.8 wt % of the total Ni present, respectively.Therefore, redox-active (bioavailable) Ni was only a portionof the total amount of Ni present. It should be stressed thatthe calculated amount of redox-active Ni differed signifi-cantly from the starting total content, and therefore itshould always be measured, because it cannot be predictedas a fixed portion of the total Ni amount. For a more thor-ough evaluation of the sensitivity of this method, we testeda CNT sample that contained a small total amount of Ni im-purities. Figure 4 shows the voltammograms recorded usinga CNT-E-modified electrode. CNT-E contained only0.7 wt % of Ni-based impurities. The oxidation peak was notpresent during the first 60 scans, whilst a small oxidationpeak became apparent between the 100th and 250th scansbefore it reached saturation (Figure 4, inset). By measuringthe charge required for oxidation, 0.02 wt % of redox-activeNi was deduced, which corresponded to 3.2 wt % of thetotal Ni present in this particular CNT sample. This tech-nique was able to detect a very small amount of Ni when itwas accessible from a CNT sample (for a summary of all thedata, see the Supporting Information, Table S1).

As control experiments, we performed cyclic voltammetryscans on CNT samples containing either no impurities at all(CNT-F) or impurities other than Ni (CNT-G, CNT-H).Figure 5 shows the voltammetric signals recorded for CNT-F(pure), CNT-G (Fe/Co impurities), and CNT-H (Fe/Mo im-purities) over 300 scans. No oxidation or reduction peaks

appeared in any of these cases. To further confirm that thevoltammetric signals recorded for CNT-A–CNT-E were ex-clusively due to the presence of Ni, we subjected other rele-vant impurities in CNT samples, such Fe3O4, Co, and Monanoparticles to the same experimental conditions (see theSupporting Information, Figure S2). Fe3O4 and Co nanopar-ticles showed an absence of voltammetric peaks, even after300 scans (see the Supporting Information, Figure S2 a, b).An intense oxidative wave was recorded for a Mo-modifiedelectrode during only the first voltammetric scan, reaching amaximum at 0.2 V (see the Supporting Information, Fig-ure S2c). This signal disappeared during the second voltam-metric scan, and therefore could not interfere with the Nidetection which was performed over 100–300 scans and wascharacterized by an oxidative peak at a more-anodic poten-tial. In any case, the presence of a signal from Mo nanopar-ticles was not surprising because we have previously opti-mized the rapid electrochemical detection of Mo nanoparti-cles in a neutral buffered solution.[46,47] The possibility of de-tecting Mo and Ni during the same experimental procedureis the subject of current investigation in our lab. The Sup-porting Information, Figure S3, summarizes representativeDPV signals recorded for all CNT samples in 0.1m NaOH

Figure 4. Repetitive cyclic voltammograms in 0.1m NaOH solution of aGC electrode modified with CNT-E. Inset: Plot of total charge under theoxidation peak vs. number of voltammetric scans. For clarity, only the40th, 80th, 160th, 220th, and 300th CVs are represented. Arrows indicatethe increasing number of scans. Scan rate: 0.1 V s�1; reference electrode:Ag/AgCl.

Figure 5. Repetitive cyclic voltammograms in 0.1 m NaOH solution of GCelectrodes modified with: a) CNT-F, b) CNT-G, c) CNT-H. Scan rate:0.1 Vs�1; reference electrode: Ag/AgCl.

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M. Pumera and A. Ambrosi

immediately after the cyclic voltammetric process. ClearDPV peaks at about 0.5 V were recorded for CNT-A–CNT-E, whilst no signals were observed for CNT-F–CNT-H.

Conclusion

We have demonstrated that Ni impurities within commer-cially available CNT samples can become extremely activein alkaline media, showing well-defined and specific electro-chemical behavior. It is very important to distinguish theportion of redox-active (bioavailable) Ni (that interacts withthe surrounding environment) from the total amount, be-cause only mobilizable Ni can be responsible for the catalyt-ic and adverse toxicological effects. We have proposed asimple method to quantify the redox-active Ni by exploitingthe well-known electrochemistry of Ni(OH)2. This method ishighly specific to Ni, does not suffer from interference fromother metallic impurities within CNT samples, and alsoshows excellent sensitivity. This method is able to detect theamount of redox-active Ni in a CNT sample with a total Nicontent of only 0.7 wt %.

Experimental Section

Materials : The following CNT samples were obtained from Sigma–Al-drich and consisted of the following dimensions: CNT-A: diameter 1.2–1.5 nm, length 2–5 mm; CNT-B: diameter 4–5 nm, length 0.5–1.5 mm;CNT-C: diameter 1.2–1.5 nm, length 2–5 mm; CNT-D: diameter 2–10 nm,length 1–5 mm; CNT-F: outer diameter (O.D.) 90–110 nm, length 7 mm;CNT-G: O.D. 10–15 nm, internal diameter (I.D.) 2–6 nm, length 0.1–10 mm; CNT-H: O.D. 3–10 nm, I.D. 1–3 nm, length 0.1–10 mm. CNT-Ewas purchase from Bucky (USA) and consisted of a diameter of 5–15 nmand a length of 1–5 mm. DMF, NaOH, potassium phosphate dibasic,sodium phosphate monobasic, molybdenum nanoparticles, nickel nano-particles, nickel(II) oxide nanoparticles, cobalt nanoparticles (all with di-ameter<100 nm), and iron ACHTUNGTRENNUNG(II/III) oxide nanoparticles (diameter<50 nm)were purchased from Sigma–Aldrich (Singapore). Glassy carbon electro-des (diameter 3 mm) were obtained from CH Instruments (TX, USA).

Apparatus : A JEM-2100F field-emission transmission electron micro-scope (JEOL, Japan) operating at 200 kV was used to acquire TEMimages in a scanning TEM mode (spot size, 0.7 nm). TEM/EDX spectrawere collected using a JEM-2100F microscope equipped with an energy-dispersive X-ray spectrometer with an ultrathin window (JEOL). All vol-tammetric experiments were performed on a mAutolab type III electro-chemical analyzer (Eco Chemie, The Netherlands) connected to a per-sonal computer and controlled by General Purpose Electrochemical Sys-tems Version 4.9 software (Eco Chemie). Electrochemical experimentswere performed in a 5 mL voltammetric cell at RT by using a three-elec-trode configuration. A platinum electrode (Autolab) served as an auxili-ary electrode, whilst an Ag/AgCl electrode (CH instruments) served as areference electrode. All electrochemical potentials are stated versus aAg/AgCl reference electrode.

Procedures : Dispersions of carbon nanotubes in DMF were prepared ata concentration of 5 mg mL�1. The suspension was then placed into anultrasonic bath for 30 min, after which 1 mL of the suspension was pipet-ted onto the surface of the glassy carbon (GC) electrode. The suspensionwas allowed to evaporate at RT, thereby creating a randomly distributedCNT film on the surface of the GC electrode. Nanoparticle-modified GCelectrodes were prepared in a similar fashion by dispersing nanoparticlesin DMF (5 mg mL�1) with subsequent deposition of 1 mL of the dispersedNPs on the GC surface. Cyclic voltammetry experiments were performed

at a scan rate of 100 mV s�1 using 50 mm phosphate buffer (pH 7.2) or0.1m NaOH solution. For TEM measurements, a suspension of CNT(1 mL, 0.5 mg mL�1) was dropped onto a copper TEM grid and allowed todry in air.

Calculations : The peak area of the signal for Ni(OH)2 oxidation wasmeasured after reaching saturation when the maximum amount ofNi(OH)2 had formed. Considering Faraday�s law (Q =nF when one elec-tron is involved in the redox event), the mass of Ni (mg) was extrapolatedby dividing the total charge (mC) required to oxidize Ni(OH)2 intoNiOOH for the Faraday constant (96 486 C mol�1) and multiplying for theatomic weight of Ni (58.69). This value was then related to the mass ofthe CNT sample deposited on the electrode surface for each measure-ment (2.5 mg). The Supporting Information, Table S1, summarizes all theresults, including total Ni quantification (determined by ICP-AES analy-sis).

Acknowledgements

This work was supported by an NAP start-up-fund grant (no.M58110066) provided by NTU.

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Received: October 17, 2011Published online: February 3, 2012

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M. Pumera and A. Ambrosi