Journal of Colloid and Interface Science 383 (2012) 110–117

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Journal of Colloid and Interface Science

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Synthesis and characterization of carbon nanotubes covalently functionalizedwith amphiphilic polymer coated superparamagnetic nanocrystals

Joseph C. Bear a, Paul D. McNaughter a, Kerstin Jurkschat b, Alison Crossley b, Leigh Aldous c,Richard G. Compton d, Andrew G. Mayes a, Gregory G. Wildgoose a,⇑a School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, UKb Department of Materials, University of Oxford, South Parks Road, Oxford OX1 3PH, UKc School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australiad Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 May 2012Accepted 13 June 2012Available online 20 June 2012

Keywords:Superparamagnetic nanoparticlesCarbon nanotubesPolymer coatingChemical modificationMagnetic nanotubesVoltammetry

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.06.028

⇑ Corresponding author.E-mail address: G.Wildgoose@uea.ac.uk (G.G. Wild

Herein, we report the synthesis of three covalently linked superparamagnetic nanocrystal-multi-walledcarbon nanotube (MWCNT) composites. A generic strategy for amphiphilic polymer coating of nanocrys-tals and further functionalization for amide bond formation with the MWCNTs is discussed. Thisapproach can in principle allow attachment of any colloidal nanocrystal to the MWCNTs. The materialswere characterized at each stage of the syntheses using DLS, zeta-potential measurements, FT-IR, TEM,and XPS techniques. The practicality of this linkage is demonstrated by the reversible magnetic immobi-lization of these materials on an electrode during non-aqueous electrochemistry.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

In this work, we present a universal strategy for attachmentof magnetic nanocrystals to multi-walled carbon nanotubes(MWCNTs), which has scope to be applied to other types of nano-crystal (NC). Use of an amphiphilic polymer layer provides thefunctionality on the particle surface for attachment to the carbonnanotubes (CNTs). This method can potentially be applied to awide range of nanocrystals as the hydrophobic interaction be-tween the NC ligands and the polymer is common to NCs stabilizedwith hydrophobic ligands. In this work, we demonstrate the cova-lent attachment of three different superparamagnetic NCs, Fe3O4,Ni, and MnFe2O4. The amphiphilic polymer passivates the surfaceof the particle, thus preventing the particle surface participatingin any chemistry with the CNT.

Recent developments in carbon nanotubes and nanoparticlecomposite research have yielded a range of nanomaterials withnumerous potential applications in the areas of electrochemistry[1–3], drug delivery [4,5], catalysis [6], and energy materials [7].Thus far, the combination of nanocrystals and carbon nanotubesfollows two main synthetic stratagies: either the nanocrystals aresynthesized and are subsequently attached to the nanotubes

ll rights reserved.

goose).

afterward or the nanocrystal is formed in situ upon the tube [8].Attachment of ready synthesized nanocrystals has been achievedusing covalent bonds [9], eletrostatic interactions [10], p–p stack-ing [11], and hydrophobic interactions [12]. Covalent strategies ofattachment of ready synthesized nanocrystals to carbon nanotubesare attractive as they take advantage of the existing methods toproduce high quality nanocrystals and the strength of a covalentbond avoids complications, such as equilibria. The location ofattachment is typically carboxylate ‘‘defect sites’’ formed by thepre-treatment of carbon nanotubes. However, many of these meth-ods produce NC–CNT heterostructures where the NCs are uncoatedand hence are susceptible to further chemical or electrochemicalprocesses. The lack of protection limits the potential use of the highsurface area of CNTs for other applications, for example, as catalystsupports or as (electro) analytical sensing platforms. Other meth-ods have coated the entire NC–CNT structure in a polymer, result-ing in the passivation of both NC and CNT [13].

Magnetic NCs represent a class of ferromagnetic and antiferro-magnetic nanomaterials, which have tremendous potential as drugdelivery vectors [14], contrast agents for MRI [15], catalysis [16],magnetic hyperthermia [17], and in data storage devices [18].Nanoparticulate ferromagnetic substances such as Co, Fe3O4, andMnFe2O4 have a critical radius above which they possess multiplemagnetic domains and are ferromagnetic. Below this critical ra-dius, the magnetic NCs exhibit superparamagnetism, where each

J.C. Bear et al. / Journal of Colloid and Interface Science 383 (2012) 110–117 111

nanocrystal has a single magnetic domain. The magnetic dipoles ofsuperparamagnetic NCs are randomly aligned in the absence of anapplied magnetic field at temperatures above the ‘‘blocking tem-perature’’ according to a Boltzmann distribution. In the presenceof an applied magnetic field, superparamagnetic NCs will align in-stantly with the magnetic field and behave as a giant paramagnet.They also exhibit little or no hysteresis effects, such that any resid-ual magnetism is immediately lost when they are removed fromthe magnetic field [19].

Control of the surface chemistry of nanocrystals is an importantprerequisite for their use and is a current challenge within thefield. Functionalization is used to change nanocrystal solubilizationor to chemically attach NCs to other molecules, biological species[20], and other materials. To date, several approaches to controland sculpt the surface chemistry of NCs have been developed suchas ligand exchange [21–23], core–shelling [24–26], silica shells[27–30], polymer shells [31,32], or amphiphilic polymer coating[30,33,34].

The use of amphiphilic polymers has received a great deal ofinterest, as they provide a route for functionalizing and alteringthe solubility of potentially any colloidal nanocrystals stabilizedby hydrophobic ligands. This technique depends on the formationof a hydrophobic bi-layer between the colloidal nanocrystal li-gands and the non-polar subunits of the pro-amphiphilic polymer.Once wrapped, the polymer is further reacted to produce ahydrophilic exterior, hence rendering the particles water soluble[33–35]. Amphiphilic polymers have been shown to provide thecolloidal nanocrystals with good colloidal stability in aqueousmedia and can also offer the opportunity to attach them tobiomolecules [36] or dyes [37]. The colloidal stability follows thetraditional rules of DLVO theory [38] as demonstrated by Lees etal. [34].

We report the formation of three different superparamagneticNC materials, Fe3O4, Ni, and MnFe2O4, which are coated in a poly-mer layer, and which are then covalently attached predominantlyto the ends and ‘‘edge-plane-like’’ [3] defects on MWCNTs viaamide coupling to surface carboxylate groups. By doing this, thesuperparamagnetic NCs are passivated and play no further role inthe chemical properties of the composite except to impart magne-tization to the MWCNTs. The majority of the ‘‘magnetic nano-tube’s’’ surface (the sidewall) is left unmodified and available forfurther chemical modification steps as required.

2. Experimental

2.1. Materials

All reagents were purchased from Sigma-Aldrich (Gillingham,UK), were of the highest grade available, and were used withoutfurther purification unless stated otherwise. All synthetic reactionsand manipulations were performed under a dry nitrogen atmo-sphere. ‘‘Bamboo-like’’ multi-walled carbon nanotubes (MWCNTs)were purchased from Nanolab (Brighton, MA, USA; purity >95%,diameter 30 ± 15 nm, length 2–20 lm). All solvents were degassedto remove dissolved oxygen and were either supplied as anhydrousgrade or were dried prior to use by distillation over either sodium/benzophenone (tetrahydrofuran, THF, diethyl ether, DEE) or cal-cium hydride (dichloromethane, DCM) under a nitrogen atmo-sphere. Aqueous solutions were prepared using UHQ deionizedwater with a resistivity of not less than 18.2 MX cm (Millipore).

Cyclic voltammetric measurements were recorded using a com-puter-controlled potentiostat (PGSTAT30, Autolab, Utrecht, TheNetherlands) with a standard three-electrode arrangement. A sil-ver wire served as the pseudo-reference electrode and a platinum

wire as the counter electrode. The magnetic working electrode wasfabricated using an in-house design and consisted of a polishedgraphite rod sealed in PEEK (working electrode diameter 5 mm)that had a neodymium iron borate disk magnet placed 1 mm be-hind the working electrode face. Non-aqueous voltammetry wasperformed under an inert nitrogen atmosphere in degassed aceto-nitrile that had been dried for 24 h prior to use over 3 Å molecularsieves and contained 0.1 M tetrabutylammonium tetrafluoroborate(TBAF) as supporting electrolyte.

Transmission electron microscopy (TEM) was performed usinga JEOL-JEM 2000EX instrument. Infrared spectra were recordedusing a PerkinElmer Spectrum 100 FT-IR spectrometer fitted witha PerkinElmer25 universal attenuated total internal reflectance(ATR) sampling accessory. X-ray photoelectron spectroscopy(XPS) was performed with a VG clam 4 MCD analyzer system usingX-ray radiation from the Al Ka band (1486.6 eV). All experimentswere recorded with an analyzer energy of 100 eV and a takeoffangle of 90�. Dynamic light scattering (DLS) and zeta potentialmeasurements were performed on a Malvern Zetasizer Nano ZSinstrument. Colorimetric testing for the presence of amines madeuse of the ninhydrin Kaiser test [39].

3. Synthetic procedures

Syntheses of the Fe3O4, Ni, and MnFe2O4 NCs were carried outaccording to previously published procedures, yielding hydropho-bic saturated suspensions of NCs [40,41].

NC polymer-coating with poly(styrene-co-maleic anhydride)(PSMA) was based on the method developed by Lees et al. [34] Dif-fering amounts of PSMA (Aldrich, Mn = 1700) were used to coat theNCs, thus determining the amount of polymer required to transferthe nanocrystals to water using ethanolamine (0.12 M solution,Sigma-Aldrich, P99.9%). The NCs are denoted as PMSA–Fe3O4,PMSA–Ni, and PMSA–MnFe2O4 NCs.

4. Functionalization of PSMA coated NCs with p-phenylenediamine

The PMSA–Fe3O4, PMSA–Ni, and PMSA–MnFe2O4 NCs were eachseparately suspended in 10 ml of chloroform to which was added asolution containing 100 mg (excess) of p-phenylenediamine in afurther 5 ml of chloroform. After stirring for 5 h, each sample ofNCs had formed a turbid gray suspension. The NCs were re-precip-itated by adding a small amount of toluene and then centrifuged at4800 rpm for 5 min. The precipitates were then washed repeatedlywith chloroform followed by a further cycle of centrifuging to re-move any unreacted p-phenylenediamine. The precipitates werethen each re-suspended in a solution of tetramethylammoniumhydroxide (15 mg in 2 ml of water). Finally, in order to removeany excess polymer or physisorbed p-phenylenediamine, each ofthe NC samples was centrifuge-filtered with a Millipore�

10,000 MW centrifuge filter at 5000 rpm for 20 min per cycle untilthe pH dropped to 7. Distilled water was added to each samplebetween the filtration steps to reduce the pH and keep a constantvolume. The resulting aminated-polymer-coated NCs (denotedNH2–PMSA–Fe3O4, NH2–PMSA–Ni, and NH2–PMSA–MnFe2O4 NCsrespectively) were stored as frozen aqueous suspensions (totalvolume 4 ml) at �18 �C in the dark until required.

5. Covalent coupling by amide bond formation

In order to increase the number of surface carboxyl groups onthe surface of the MWCNTs for particle linking via amide bond for-mation, 100 mg of MWCNTs was oxidized by refluxing in a 3:1

112 J.C. Bear et al. / Journal of Colloid and Interface Science 383 (2012) 110–117

mixture of concentrated nitric and sulfuric acid at 80 �C for 3 h. Theoxidized MWCNT suspension was allowed to cool to room temper-ature, carefully diluted with 50 ml of deionized water, and filteredunder reduced pressure. The retentate was washed with distilledwater until the washing ran neutral and then rigorously dried at60 �C under vacuum overnight.

A Schlenk flask was charged with 10 mg of oxidized MWCNTsand a magnetic stirrer bar and sealed under a nitrogen atmosphere.Thionyl chloride (5 ml, 68.55 mmol) was then added under a flushof nitrogen and the reaction mixture stirred at room temperaturefor 30 min, after which the excess thionyl chloride was removedunder vacuum. Amine-functionalized, polymer-coated nanocrys-tals were dispersed with the aid of sonication in 10 ml of dryDCM and added to the acid-chloride functionalized MWCNTs to-gether with one equivalent of triethylamine (9.56 ml, 68.55 mmol)under a flush of nitrogen. The reaction mixture was stirred over-night at room temperature before the NC-modified MWCNTs werefiltered off as a black solid, washed with copious quantities(4 � 50 ml) of acetone and dry DCM, and dried under vacuum.The resulting polymer-coated, nanocrystal-modified MWCNTs aredenoted as Fe3O4–NC–MWCNTs, Ni–NC–MWCNTs, and MnFe2O4–NC–MWCNTs.

Scheme 1. The polymer coating and aminiation of the nanocrystals. Where A

6. Results and discussion

6.1. Synthesis and characterization of aminated-polymer-coatedsuperparamagnetic nanocrystals

The polymer-coated-superparamagnetic nanocrystal-MWCNTcomposites were synthesized in three stages. NCs were synthe-sized using controlled thermal decomposition of organometallicprecursors in a non-polar, high boiling point solvent in the pres-ence of surfactants [40,41]. These yielded black, hydrophobic dis-persions of NCs with a narrow size distribution around 10 nm,below the critical domain size such that the NCs are superpara-magnetic (vide infra).

Each of the three superparamagnetic nanoparticulate materialswas then coated in a thin layer of a commercially available amphi-philic polymer, poly(styrene-co-maleic anhydride) (PSMA), follow-ing the work of Lees et al. [34] (Scheme 1). This was done in orderto passivate the surface of the NCs and prevent them interferingwith any further chemical or electrochemical processes involvedin a given target application.

The choice of polymer-coating the NCs with PSMA is advanta-geous in that it avoids adversely affecting their magnetic properties

is PSMA and B is p-phenylene diamine. The solvent used is chloroform.

Fig. 1. (a) TEM micrograph of Fe3O4 NCs before polymer coating, (b) TEM micrograph of Fe3O4 NCs after polymer coating and amination, (c) DLS size distributions for NCs ofFe3O4, (d) Ni, and (e) MnFe2O4. The full line represents the uncoated particles, dashed line after polymer coating and dotted line after amination.

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as can be the case when superparamagnetic NCs are coated in car-bon or silica shells [42]. Choosing to polymer coat the NCs beforecoupling to the MWCNT surface also prevents the latter beingaffected by polymer coating, which may be undesirable if theMWCNTs are to be used as a further support material.

Finally, the anhydride rings of the polymer coating were openedusing p-phenylenediamine, in order to aminate the polymer so asto provide a method of attaching the NCs to carboxyl groups pres-ent on the surface of MWCNTs via the formation of amide bonds.The use of sterically rigid p-phenylenediamine molecules as the‘‘linker’’ was found to be preferable to using alkyl diamines, whichwere found to extensively cross-link the polymer chains or NCs(vide infra). The materials were characterized at each stage in theprocess using TEM, DLS, XPS, and FT-IR.

6.2. Characterization of the polymer-coated superparamagneticnanocrystals by TEM and DLS

Fig. 1 shows TEM images of the initial and aminated Fe3O4 NCsand DLS data showing the increase in hydrodynamic radius throughthe three stages of the synthesis. In the case of the Fe3O4 and Ni NCs,monodisperse samples of almost spherical NCs were observed witha mean core radius of 11 nm and 6 nm, respectively. These valuesare below the single-domain to multi-domain transition formagnetite and nickel to become superparamagnetic [19]. The

MnFe2O4 NCs were observed to be polydisperse; with size distribu-tions centered around 10 nm and 35 nm. HRTEM and associatedEDX spectrum of the Ni NCs are given in the SI. The TEM of theNCs in Fig. 1 after amine functionalization shows that the metaloxide core is unaffected and clearly visible, but the polymer layeris indistinguishable by TEM. From the DLS measurements shownin Fig. 1 for the unmodified, PSMA-coated NCs and aminated NH2–PSMA–NCs, the hydrodynamic radii of the unmodified Fe3O4 andNi were again found to be monodisperse with mean values of11.5 nm and 13.6 nm, slightly larger than observed from TEM mea-surements due to the effect of both solvation, and the presence ofoleic acid surfactant ligands on the surface of the NCs, which arenot observed in the TEM images. The smaller MnFe2O4 NCs hadmean hydrodynamic radii of between 12 and 14 nm. AlthoughDLS measurements only provide a mean hydrodynamic radius, thetrend upon adding the polymer layer and then coupling this withthe p-phenylenediamine is apparent, with the mean hydrodynamicradius of all particles increasing upon each modification, as ex-pected. DLS measurements suggest that in all cases, the optimizedcoating of PSMA increased the hydrodynamic radius by ca. 2–4 nm.

6.3. Amination of the polymer-coated NCs and characterization

In order to attach the polymer-coated NCs to the MWCNTscovalently via amide formation to the surface carboxyl groups on

Fig. 2. IR spectra of aminated Fe3O4 NCs (top), poly(styrene-co-maleic anhydride)(middle), and p-phenylene diamine (bottom).

Fig. 3. Zeta-potential distributions for Fe3O4 NCs (top) and Ni NCs (bottom)showing the anhydride opened with base (solid line) and opened with p-phenylenediamine (dashed line).

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the nanotubes, it is necessary to aminate the PSMA–NCs with a dia-mino moiety. However, the amine functionalization of PSMAcoated NCs presents two main challenges: prevention of linkingbetween particles during the reaction with the diamine and ensur-ing the resulting particles are colloidally stable.

The use of double-ended amines or other alkyl amines bearing adifferent nucleophilic group may lead to inter-particulate cross-linking between NCs, as well as internal cross-linking betweendifferent polymer chains wrapped around the NC [33]. Thesepotential issues were overcome by use of an excess of diamine.Amination of the PSMA–NCs with p-phenylenediamine (used asthe free-base not the hydrochloride salt) was found to be success-ful. Our choice of an aryl diamine was guided by the relatively poornucleophilicity of the aryl amine groups cf. their alkyl analogues –which is further reduced after the initial nucleophilic attack by thegreater steric rigidity of the aryl ring cf. to an alkyl chain. This wasexpected to reduce the likelihood of any NC cross-linking.

The opening of an anhydride ring with p-phenylenediamine isshown in Scheme 1. The reaction is facile and goes to completionat room temperature. The aminated NCs (NH2–PSMA–NCs) werethen precipitated from chloroform and re-dispersed in an aqueoussolution of tetramethylammonium hydroxide in order to open anyremaining maleic anhydride rings and render the NCs water dis-persible. The resulting NH2–PSMA–NCs were characterized using

ATR-FT-IR, zeta-potential measurements and X-ray photoelectronspectroscopy (XPS). A positive result indicating the presence of freeamine groups on the polymer-coated NC surface was also observedafter the samples were subjected to the colorimetric Kaiser test.

Fig. 2 compares the ATR-FT-IR spectra recorded for the PSMApolymer and p-phenylenediamine precursors and NH2–PSMA–Fe3O4 aminated NCs. The important features in the IR spectrumof the PSMA precursor occur at mmax/cm�1: 3030 and 2927 (arylCAH stretches), 1856 and 1773(ACOAOACOA acid anhydrideC@O stretch) and 1494 and 1455 (CAH deformations) and1218(ACAOACA). In the p-phenylenediamine, IR bands corre-sponding to the amine groups were observed at mmax/cm�1: 3372,3304, and 3197 (NAH stretches) 3045-3007 (aryl-H stretches),1627 (NAH bend), 1610 and 1511 (aromatic C@C stretch) 821(Aryl-H, p-substituted benzene ring). Evidence for the successfulamination of the PSMA-coated NC materials was confirmed bythe disappearance of the acid anhydride stretches in the PSMApolymer at 1856 and 1773 cm�1. The spectra of p-phenylene dia-mine and the amine functionalized NCs show a near identical pro-file in the region 2180–1978 cm�1 indicative of aromatic‘‘combination bands’’ and provide further evidence for the success-ful attachment of p-phenylene diamine to the PSMA polymer. Anumber of overlapping resonances were observed between 2916and 2869 cm�1 assigned to ArAH stretches from both the PSMAand the p-phenylenediamine. The intensity of these ArAHstretches has also increased relative to the PSMA precursor alone,which may also be indicative of modification with p-phenylenediamine [43].

To investigate the effect of the amine functionalization on thecolloidal stability of the nanocrystals, we performed zeta-potentialmeasurements in pH 7.4 phosphate buffer (0.1 M) of the aminatedpolymer-coated NCs and also of the PSMA-coated NCs that hadbeen subjected to a ring-opening step by treatment with 0.1 M

Fig. 4. X-ray photoelectron spectroscopy (XPS) spectrum for the N 1s electron ofaminated Fe3O4 NCs. XPS spectrum (thin full line), baseline (dashed line), Guassianfit for amide group (dot and long dash), Gaussian for amine group (dot and shortdash), and envelope from the combination of Gaussians (thick full line).

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tetramethylammonium hydroxide, but had not been coupled withthe p-phenylenediamine, as a control. Fig. 3 shows the resultingzeta-potential distributions for the Ni and the Fe3O4 NC species.In both cases, the net surface charge was always negative, butthe aminated NCs exhibited zeta-potentials that had shifted morepositively (�26.3 mV and �38.3 mV for the NH2–PSMA–Fe3O4

and NH2–PSMA–Ni respectively) than the controls (where the par-ticles are stabilized exclusively by carboxylate groups; �63.3 mVand �60.6 mV for the PSMA–Fe3O4 and PSMA–Ni NCs, respec-tively). This shift to more positive potentials is likely attributableto the amination of the polymer by the p-phenylenediamine, sincethis would reduce the net negative charge.

Finally, XPS analysis of the aminated NCs revealed a spectralpeak at ca. 400 eV corresponding to emission from the N1s levelin addition to spectral peaks corresponding to the relevant metalor metal oxides in the NC core (Fig. 4). Detailed scans over theN1s region reveal that two peaks are observed with binding ener-gies of 398 eV and 402 eV, with a ratio of peak areas of 1.6:1 cor-responding to nitrogen atoms in an amine and amide chemicalenvironment [44]. The deviation from the expected 1:1 ratio isindicative that the washing procedure performed to remove anyphysisorbed p-phenylenediamine was not perfect, but that themajority of p-phenylenediamine groups present are coupled tothe PSMA polymer via the amide, leaving a pendant amino groupavailable for further coupling to the nanotubes.

6.4. Covalent modification of MWCNTs with polymer-coatedsuperparamagnetic nanocrystals

The pendent amine groups on the NH2–PSMA–NC surface offertwo routes to covalently attach the NCs to MWCNT surfaces: (i)CAC bond formation via diazotization of the amine groups and

Scheme 2. Synthetic strategy to attach aminated NCs to oxidiz

chemical reduction in the presence of MWCNTs [45] or (ii) amidebond formation to surface carboxyl groups present on theedge-plane-like defect sites at the termini of the graphene tubescomprising the MWCNTs, Scheme 2.

Both attachment methods were attempted, and the resultingmaterials were characterized by TEM and by testing their magneticproperties with a neodymium magnet. No evidence for the cova-lent attachment of any NCs to the MWCNTs was observed usingthe diazotization approach. However, in the case of attaching theNCs via an amide bond, the resulting composite NC–MWCNTmaterials exhibited superparamagnetic properties. In the absenceof an applied magnetic field, the MWCNTs simply behaved like

ed MWCNTs using thionyl chloride based amide coupling.

Fig. 5. Photograph of NC functionalized CNTs (a) without magnet and (b) with magnet. (c) Shows Fe3O4 NCs attached to MWCNTs after amide coupling. (d) Shows MnFe2O4

NCs attached to MWCNTs.

Fig. 6. The ratio of background charging current, Ic, measured at a potential of+0.5 V vs. Ag, for the MWCNT-modified electrode vs. that of the underlying bareelectrode, Ic, bare determined from cyclic voltammetric measurements recorded inMeCN containing 0.1 M TBAF at a voltage scan rate of 100 mV s�1 after varyingintervals of stirring at 3000 rpm.

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unmodified MWCNTs – they could be dispersed in non-aqueoussuspensions, such as in chloroform, with no obvious aggregationexpected if the composites were paramagnetic. However, in thepresence of a magnetic field (provided by a neodymium bar mag-net or magnetic stirrer), the MWCNTs can be dragged instantlythrough and even out of the suspension (Fig. 5) or can be pickedup by the magnet from a filter paper as a dry solid. Once removedfrom the magnetic field, the MWCNTs once again behaved as ifthey had no intrinsic magnetization. TEM images of the superpara-magnetic NC–MWCNT composites (Fig. 5) revealed a sparse distri-bution of small agglomerates of NCs decorating the MWCNTsurface. The agglomeration is presumably an effect of using a sus-pension of NCs in DCM during the coupling step – the aminatedpolymer-coated NCs are hydrophilic and likely to aggregate innon-aqueous solvents – and improved coupling strategies to min-imize any agglomeration are part of our ongoing studies. However,even with a sparse coverage of agglomerated particles, the NCs arecovalently attached to the MWCNTs and have imparted significantmagnetic properties to the composite material.

To demonstrate this conclusively, a graphite electrode wasconstructed that had a neodymium magnet placed 1 mm behindthe exposed electrode surface. A 20 ll aliquot of a suspension ofthe NC-modified MWCNTs (1 mg/mL DCM) was placed on the elec-trode surface and the solvent allowed to evaporate, leaving a thinfilm of MWCNTs held in place by the magnetic field. This was thenimmersed in a non-aqueous acetonitrile solution of 0.1 M tetrabu-tylammonium tetrafluoroborate (TBAF) electrolyte. Cyclic voltam-metry was performed, scanning between 0 and +1.0 V vs. Ag at ascan rate of 100 mV s�1, and the background capacitive chargingcurrent (proportional to the area of the electrode and hence thenumber of MWCNTs on the surface) measured at +0.5 V vs. Ag.The electrolyte was then stirred at 3000 rpm for 2 min intervalsover a total period of 20 min, with the background capacitive cur-rent measured by cyclic voltammetry after each period of stirring.

It is well known that MWCNTs films immobilized via a drop-casting method (described above) manner do not adhere well toelectrode surfaces in non-aqueous electrolytes, and often simply

‘‘drop off’’ the electrode into the solution – a common problemfor CNT electrochemists. Hence, for comparison, a 20 ll aliquot ofa suspension of unmodified MWCNTs was also placed on the cleangraphite electrode surface and subjected to the same hydrody-namic cyclic voltammetric experiments.

The results obtained using Fe3O4 NC-modified MWCNTs are ex-pressed in Fig. 6 as the ratio of measured capacitive charging cur-rent of the MWCNT-modified electrode to the charging currentmeasured at the unmodified graphite electrode. In the case of theunmodified (non-magnetic) MWCNTs, the measured capacitivecharging current is almost identical to the bare underlying graphiteelectrode after the first 2 min of stirring – indicating that almost allthe MWCNTs were swept off the electrode surface. However, forthe magnetic NC–MWCNTs, there is almost no decrease in thecapacitive charging current, and hence almost no MWCNTs havebeen removed from the electrode even after 20 min of stirring at

J.C. Bear et al. / Journal of Colloid and Interface Science 383 (2012) 110–117 117

3000 rpm. Almost identical behavior was observed with the othermagnetic NC-modified nanotubes.

Clearly, if the NCs were not covalently attached to the MWCNTsand were simply dispersed among them, then the MWCNTs wouldhave eventually been removed from the electrode under the pro-longed stirring in non-aqueous electrolyte. Hence, this experimentstrongly suggests that the NCs are indeed covalently bound to theMWCNTs, and that, further more, even with only a sparse coverageof NCs on the MWCNTs, they are sufficiently magnetized in thepresence of an external field to secure almost all the MWCNTs tothe electrode surface even under vigorous stirring conditions. Itshould also be noted that no redox voltammetric features were ob-served in either these experiments in non-aqueous electrolyte, orseparate studies in aqueous electrolyte. This indicates that thepolymer coating is intact and has successfully passivated the me-tal/metal oxide cores of the nanocrystals such that they take no ob-servable part in any further chemical or electrochemical processes.

7. Conclusions

Magnetic nanotubes, formed by covalently attaching polymer-coated NCs to edge-plane-like defects on MWCNTs, have been pre-pared and characterized using a variety of techniques. The size ofthe NCs is such that they exhibit characteristic superparamagneticbehavior, exhibiting no net magnetization in the absence of a mag-netic field, but impart net paramagnetic properties to the NC–MWCNT composites when an external magnetic field is applied.The polymer coating of the NCs with PSMA rendered the NCshydrophilic and allowed the attachment of the required chemicalfunctionality for coupling to the CNTs. Importantly, the amphi-philic polymer coating was sufficient to passivate the NCs such thatthey were not observed to take part in any redox or other chemis-try. Amination of the polymer layer was achieved using p-phenyl-enediamine, characterized by FT-IR, colorimetric chemical testsand XPS spectroscopy, and provides a facile route to anchor theNCs to the end and defect sites on the MWCNT surface via amidebond formation. Modification in this way leaves the vast majorityof the surface area of the MWCNTs available for further modifica-tion as recently communicated by Wildgoose et al. [46].

The magnetic NC–MWCNT composites were able to remain at-tached to the surface of a purpose-built magnetic electrode in non-aqueous electrolytes, under vigorous stirring, where non-magneticMWCNTs were almost immediately removed under the same con-ditions. This has important implications allowing the study ofchemically-modified CNT materials in non-aqueous electrolytesfor sensor and organometallic electrocatalyst development [46].Conceivably, the covalent attachment of nanocrystals via anamphiphilic polymer to CNTs could be extended to any type ofnanocrystal, leading to the development of further NC–CNT com-posite materials with wide ranging potential applications.

Acknowledgments

GGW thanks the Royal Society for support via a University Re-search Fellowship.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2012.06.028.

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