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Synthesis and characterization of carbon nanotubes covalently functionalized with amphiphilic polymer coated superparamagnetic nanocrystals
Journal of Colloid and Interface Science 383 (2012) 110–117
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis and characterization of carbon nanotubes covalently functionalized with 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, UK Department of Materials, University of Oxford, South Parks Road, Oxford OX1 3PH, UK School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia d Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK b c
a r t i c l e
i n f o
Article history: Received 8 May 2012 Accepted 13 June 2012 Available online 20 June 2012 Keywords: Superparamagnetic nanoparticles Carbon nanotubes Polymer coating Chemical modification Magnetic nanotubes Voltammetry
a b s t r a c t Herein, we report the synthesis of three covalently linked superparamagnetic nanocrystal-multi-walled carbon nanotube (MWCNT) composites. A generic strategy for amphiphilic polymer coating of nanocrystals and further functionalization for amide bond formation with the MWCNTs is discussed. This approach can in principle allow attachment of any colloidal nanocrystal to the MWCNTs. The materials were 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 immobilization of these materials on an electrode during non-aqueous electrochemistry. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction In this work, we present a universal strategy for attachment of magnetic nanocrystals to multi-walled carbon nanotubes (MWCNTs), which has scope to be applied to other types of nanocrystal (NC). Use of an amphiphilic polymer layer provides the functionality on the particle surface for attachment to the carbon nanotubes (CNTs). This method can potentially be applied to a wide range of nanocrystals as the hydrophobic interaction between the NC ligands and the polymer is common to NCs stabilized with hydrophobic ligands. In this work, we demonstrate the covalent attachment of three different superparamagnetic NCs, Fe3O4, Ni, and MnFe2O4. The amphiphilic polymer passivates the surface of the particle, thus preventing the particle surface participating in any chemistry with the CNT. Recent developments in carbon nanotubes and nanoparticle composite research have yielded a range of nanomaterials with numerous 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 nanotubes follows two main synthetic stratagies: either the nanocrystals are synthesized and are subsequently attached to the nanotubes ⇑ Corresponding author. E-mail address: [email protected] (G.G. Wildgoose). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.028
afterward or the nanocrystal is formed in situ upon the tube [8]. Attachment of ready synthesized nanocrystals has been achieved using covalent bonds [9], eletrostatic interactions [10], p–p stacking [11], and hydrophobic interactions [12]. Covalent strategies of attachment of ready synthesized nanocrystals to carbon nanotubes are attractive as they take advantage of the existing methods to produce high quality nanocrystals and the strength of a covalent bond avoids complications, such as equilibria. The location of attachment is typically carboxylate ‘‘defect sites’’ formed by the pre-treatment of carbon nanotubes. However, many of these methods produce NC–CNT heterostructures where the NCs are uncoated and hence are susceptible to further chemical or electrochemical processes. The lack of protection limits the potential use of the high surface area of CNTs for other applications, for example, as catalyst supports or as (electro) analytical sensing platforms. Other methods have coated the entire NC–CNT structure in a polymer, resulting in the passivation of both NC and CNT [13]. Magnetic NCs represent a class of ferromagnetic and antiferromagnetic nanomaterials, which have tremendous potential as drug delivery 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, and MnFe2O4 have a critical radius above which they possess multiple magnetic domains and are ferromagnetic. Below this critical radius, the magnetic NCs exhibit superparamagnetism, where each
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nanocrystal has a single magnetic domain. The magnetic dipoles of superparamagnetic NCs are randomly aligned in the absence of an applied magnetic field at temperatures above the ‘‘blocking temperature’’ according to a Boltzmann distribution. In the presence of an applied magnetic field, superparamagnetic NCs will align instantly with the magnetic field and behave as a giant paramagnet. They also exhibit little or no hysteresis effects, such that any residual magnetism is immediately lost when they are removed from the magnetic field [19]. Control of the surface chemistry of nanocrystals is an important prerequisite for their use and is a current challenge within the field. Functionalization is used to change nanocrystal solubilization or to chemically attach NCs to other molecules, biological species [20], and other materials. To date, several approaches to control and sculpt the surface chemistry of NCs have been developed such as 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 of interest, as they provide a route for functionalizing and altering the solubility of potentially any colloidal nanocrystals stabilized by hydrophobic ligands. This technique depends on the formation of a hydrophobic bi-layer between the colloidal nanocrystal ligands and the non-polar subunits of the pro-amphiphilic polymer. Once wrapped, the polymer is further reacted to produce a hydrophilic exterior, hence rendering the particles water soluble [33–35]. Amphiphilic polymers have been shown to provide the colloidal nanocrystals with good colloidal stability in aqueous media and can also offer the opportunity to attach them to biomolecules [36] or dyes [37]. The colloidal stability follows the traditional rules of DLVO theory [38] as demonstrated by Lees et al. [34]. We report the formation of three different superparamagnetic NC materials, Fe3O4, Ni, and MnFe2O4, which are coated in a polymer layer, and which are then covalently attached predominantly to the ends and ‘‘edge-plane-like’’ [3] defects on MWCNTs via amide coupling to surface carboxylate groups. By doing this, the superparamagnetic NCs are passivated and play no further role in the chemical properties of the composite except to impart magnetization to the MWCNTs. The majority of the ‘‘magnetic nanotube’s’’ surface (the sidewall) is left unmodified and available for further 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 without further purification unless stated otherwise. All synthetic reactions and manipulations were performed under a dry nitrogen atmosphere. ‘‘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 degassed to remove dissolved oxygen and were either supplied as anhydrous grade or were dried prior to use by distillation over either sodium/ benzophenone (tetrahydrofuran, THF, diethyl ether, DEE) or calcium hydride (dichloromethane, DCM) under a nitrogen atmosphere. Aqueous solutions were prepared using UHQ deionized water with a resistivity of not less than 18.2 MX cm (Millipore). Cyclic voltammetric measurements were recorded using a computer-controlled potentiostat (PGSTAT30, Autolab, Utrecht, The Netherlands) with a standard three-electrode arrangement. A silver wire served as the pseudo-reference electrode and a platinum
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wire as the counter electrode. The magnetic working electrode was fabricated using an in-house design and consisted of a polished graphite rod sealed in PEEK (working electrode diameter 5 mm) that had a neodymium iron borate disk magnet placed 1 mm behind the working electrode face. Non-aqueous voltammetry was performed under an inert nitrogen atmosphere in degassed acetonitrile that had been dried for 24 h prior to use over 3 Å molecular sieves and contained 0.1 M tetrabutylammonium tetrafluoroborate (TBAF) as supporting electrolyte. Transmission electron microscopy (TEM) was performed using a JEOL-JEM 2000EX instrument. Infrared spectra were recorded using a PerkinElmer Spectrum 100 FT-IR spectrometer fitted with a PerkinElmer25 universal attenuated total internal reflectance (ATR) sampling accessory. X-ray photoelectron spectroscopy (XPS) was performed with a VG clam 4 MCD analyzer system using X-ray radiation from the Al Ka band (1486.6 eV). All experiments were recorded with an analyzer energy of 100 eV and a takeoff angle of 90°. Dynamic light scattering (DLS) and zeta potential measurements were performed on a Malvern Zetasizer Nano ZS instrument. Colorimetric testing for the presence of amines made use of the ninhydrin Kaiser test [39]. 3. Synthetic procedures Syntheses of the Fe3O4, Ni, and MnFe2O4 NCs were carried out according to previously published procedures, yielding hydrophobic 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] Differing amounts of PSMA (Aldrich, Mn = 1700) were used to coat the NCs, thus determining the amount of polymer required to transfer the 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 pphenylenediamine The PMSA–Fe3O4, PMSA–Ni, and PMSA–MnFe2O4 NCs were each separately suspended in 10 ml of chloroform to which was added a solution containing 100 mg (excess) of p-phenylenediamine in a further 5 ml of chloroform. After stirring for 5 h, each sample of NCs had formed a turbid gray suspension. The NCs were re-precipitated by adding a small amount of toluene and then centrifuged at 4800 rpm for 5 min. The precipitates were then washed repeatedly with chloroform followed by a further cycle of centrifuging to remove any unreacted p-phenylenediamine. The precipitates were then each re-suspended in a solution of tetramethylammonium hydroxide (15 mg in 2 ml of water). Finally, in order to remove any excess polymer or physisorbed p-phenylenediamine, each of the NC samples was centrifuge-filtered with a MilliporeÒ 10,000 MW centrifuge filter at 5000 rpm for 20 min per cycle until the pH dropped to 7. Distilled water was added to each sample between the filtration steps to reduce the pH and keep a constant volume. The resulting aminated-polymer-coated NCs (denoted NH2–PMSA–Fe3O4, NH2–PMSA–Ni, and NH2–PMSA–MnFe2O4 NCs respectively) were stored as frozen aqueous suspensions (total volume 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 on the surface of the MWCNTs for particle linking via amide bond formation, 100 mg of MWCNTs was oxidized by refluxing in a 3:1
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mixture of concentrated nitric and sulfuric acid at 80 °C for 3 h. The oxidized MWCNT suspension was allowed to cool to room temperature, carefully diluted with 50 ml of deionized water, and filtered under reduced pressure. The retentate was washed with distilled water until the washing ran neutral and then rigorously dried at 60 °C under vacuum overnight. A Schlenk flask was charged with 10 mg of oxidized MWCNTs and a magnetic stirrer bar and sealed under a nitrogen atmosphere. Thionyl chloride (5 ml, 68.55 mmol) was then added under a flush of nitrogen and the reaction mixture stirred at room temperature for 30 min, after which the excess thionyl chloride was removed under vacuum. Amine-functionalized, polymer-coated nanocrystals were dispersed with the aid of sonication in 10 ml of dry DCM and added to the acid-chloride functionalized MWCNTs together with one equivalent of triethylamine (9.56 ml, 68.55 mmol) under a flush of nitrogen. The reaction mixture was stirred overnight at room temperature before the NC-modified MWCNTs were filtered 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 are denoted as Fe3O4–NC–MWCNTs, Ni–NC–MWCNTs, and MnFe2O4– NC–MWCNTs.
6. Results and discussion 6.1. Synthesis and characterization of aminated-polymer-coated superparamagnetic nanocrystals The polymer-coated-superparamagnetic nanocrystal-MWCNT composites were synthesized in three stages. NCs were synthesized using controlled thermal decomposition of organometallic precursors in a non-polar, high boiling point solvent in the presence of surfactants [40,41]. These yielded black, hydrophobic dispersions of NCs with a narrow size distribution around 10 nm, below the critical domain size such that the NCs are superparamagnetic (vide infra). Each of the three superparamagnetic nanoparticulate materials was then coated in a thin layer of a commercially available amphiphilic polymer, poly(styrene-co-maleic anhydride) (PSMA), following the work of Lees et al. [34] (Scheme 1). This was done in order to passivate the surface of the NCs and prevent them interfering with any further chemical or electrochemical processes involved in a given target application. The choice of polymer-coating the NCs with PSMA is advantageous in that it avoids adversely affecting their magnetic properties
Scheme 1. The polymer coating and aminiation of the nanocrystals. Where A is PSMA and B is p-phenylene diamine. The solvent used is chloroform.
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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 of Fe3O4, (d) Ni, and (e) MnFe2O4. The full line represents the uncoated particles, dashed line after polymer coating and dotted line after amination.
as can be the case when superparamagnetic NCs are coated in carbon or silica shells [42]. Choosing to polymer coat the NCs before coupling to the MWCNT surface also prevents the latter being affected by polymer coating, which may be undesirable if the MWCNTs are to be used as a further support material. Finally, the anhydride rings of the polymer coating were opened using p-phenylenediamine, in order to aminate the polymer so as to provide a method of attaching the NCs to carboxyl groups present 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, which were found to extensively cross-link the polymer chains or NCs (vide infra). The materials were characterized at each stage in the process using TEM, DLS, XPS, and FT-IR. 6.2. Characterization of the polymer-coated superparamagnetic nanocrystals by TEM and DLS Fig. 1 shows TEM images of the initial and aminated Fe3O4 NCs and DLS data showing the increase in hydrodynamic radius through the three stages of the synthesis. In the case of the Fe3O4 and Ni NCs, monodisperse samples of almost spherical NCs were observed with a mean core radius of 11 nm and 6 nm, respectively. These values are below the single-domain to multi-domain transition for magnetite and nickel to become superparamagnetic [19]. The
MnFe2O4 NCs were observed to be polydisperse; with size distributions centered around 10 nm and 35 nm. HRTEM and associated EDX spectrum of the Ni NCs are given in the SI. The TEM of the NCs in Fig. 1 after amine functionalization shows that the metal oxide core is unaffected and clearly visible, but the polymer layer is indistinguishable by TEM. From the DLS measurements shown in Fig. 1 for the unmodified, PSMA-coated NCs and aminated NH2– PSMA–NCs, the hydrodynamic radii of the unmodified Fe3O4 and Ni were again found to be monodisperse with mean values of 11.5 nm and 13.6 nm, slightly larger than observed from TEM measurements due to the effect of both solvation, and the presence of oleic acid surfactant ligands on the surface of the NCs, which are not observed in the TEM images. The smaller MnFe2O4 NCs had mean hydrodynamic radii of between 12 and 14 nm. Although DLS measurements only provide a mean hydrodynamic radius, the trend upon adding the polymer layer and then coupling this with the p-phenylenediamine is apparent, with the mean hydrodynamic radius of all particles increasing upon each modification, as expected. DLS measurements suggest that in all cases, the optimized coating 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 MWCNTs covalently via amide formation to the surface carboxyl groups on
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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-phenylene diamine (dashed line).
Fig. 2. IR spectra of aminated Fe3O4 NCs (top), poly(styrene-co-maleic anhydride) (middle), and p-phenylene diamine (bottom).
the nanotubes, it is necessary to aminate the PSMA–NCs with a diamino moiety. However, the amine functionalization of PSMA coated NCs presents two main challenges: prevention of linking between particles during the reaction with the diamine and ensuring the resulting particles are colloidally stable. The use of double-ended amines or other alkyl amines bearing a different nucleophilic group may lead to inter-particulate crosslinking between NCs, as well as internal cross-linking between different polymer chains wrapped around the NC [33]. These potential issues were overcome by use of an excess of diamine. Amination of the PSMA–NCs with p-phenylenediamine (used as the free-base not the hydrochloride salt) was found to be successful. Our choice of an aryl diamine was guided by the relatively poor nucleophilicity of the aryl amine groups cf. their alkyl analogues – which is further reduced after the initial nucleophilic attack by the greater steric rigidity of the aryl ring cf. to an alkyl chain. This was expected to reduce the likelihood of any NC cross-linking. The opening of an anhydride ring with p-phenylenediamine is shown in Scheme 1. The reaction is facile and goes to completion at room temperature. The aminated NCs (NH2–PSMA–NCs) were then precipitated from chloroform and re-dispersed in an aqueous solution of tetramethylammonium hydroxide in order to open any remaining maleic anhydride rings and render the NCs water dispersible. The resulting NH2–PSMA–NCs were characterized using
ATR-FT-IR, zeta-potential measurements and X-ray photoelectron spectroscopy (XPS). A positive result indicating the presence of free amine groups on the polymer-coated NC surface was also observed after the samples were subjected to the colorimetric Kaiser test. Fig. 2 compares the ATR-FT-IR spectra recorded for the PSMA polymer and p-phenylenediamine precursors and NH2–PSMA– Fe3O4 aminated NCs. The important features in the IR spectrum of the PSMA precursor occur at mmax/cm 1: 3030 and 2927 (aryl CAH stretches), 1856 and 1773(ACOAOACOA acid anhydride C@O stretch) and 1494 and 1455 (CAH deformations) and 1218(ACAOACA). In the p-phenylenediamine, IR bands corresponding 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 successful amination of the PSMA-coated NC materials was confirmed by the disappearance of the acid anhydride stretches in the PSMA polymer at 1856 and 1773 cm 1. The spectra of p-phenylene diamine and the amine functionalized NCs show a near identical profile in the region 2180–1978 cm 1 indicative of aromatic ‘‘combination bands’’ and provide further evidence for the successful attachment of p-phenylene diamine to the PSMA polymer. A number of overlapping resonances were observed between 2916 and 2869 cm 1 assigned to ArAH stretches from both the PSMA and the p-phenylenediamine. The intensity of these ArAH stretches has also increased relative to the PSMA precursor alone, which may also be indicative of modification with p-phenylene diamine [43]. To investigate the effect of the amine functionalization on the colloidal stability of the nanocrystals, we performed zeta-potential measurements in pH 7.4 phosphate buffer (0.1 M) of the aminated polymer-coated NCs and also of the PSMA-coated NCs that had been subjected to a ring-opening step by treatment with 0.1 M
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tetramethylammonium hydroxide, but had not been coupled with the p-phenylenediamine, as a control. Fig. 3 shows the resulting zeta-potential distributions for the Ni and the Fe3O4 NC species. In both cases, the net surface charge was always negative, but the aminated NCs exhibited zeta-potentials that had shifted more positively ( 26.3 mV and 38.3 mV for the NH2–PSMA–Fe3O4 and NH2–PSMA–Ni respectively) than the controls (where the particles are stabilized exclusively by carboxylate groups; 63.3 mV and 60.6 mV for the PSMA–Fe3O4 and PSMA–Ni NCs, respectively). This shift to more positive potentials is likely attributable to the amination of the polymer by the p-phenylenediamine, since this would reduce the net negative charge. Finally, XPS analysis of the aminated NCs revealed a spectral peak at ca. 400 eV corresponding to emission from the N1s level in addition to spectral peaks corresponding to the relevant metal or metal oxides in the NC core (Fig. 4). Detailed scans over the N1s region reveal that two peaks are observed with binding energies of 398 eV and 402 eV, with a ratio of peak areas of 1.6:1 corresponding to nitrogen atoms in an amine and amide chemical environment [44]. The deviation from the expected 1:1 ratio is indicative that the washing procedure performed to remove any physisorbed p-phenylenediamine was not perfect, but that the majority of p-phenylenediamine groups present are coupled to the PSMA polymer via the amide, leaving a pendant amino group available for further coupling to the nanotubes. 6.4. Covalent modification of MWCNTs with polymer-coated superparamagnetic nanocrystals The pendent amine groups on the NH2–PSMA–NC surface offer two routes to covalently attach the NCs to MWCNT surfaces: (i) CAC bond formation via diazotization of the amine groups and
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Fig. 4. X-ray photoelectron spectroscopy (XPS) spectrum for the N 1s electron of aminated Fe3O4 NCs. XPS spectrum (thin full line), baseline (dashed line), Guassian fit for amide group (dot and long dash), Gaussian for amine group (dot and short dash), and envelope from the combination of Gaussians (thick full line).
chemical reduction in the presence of MWCNTs [45] or (ii) amide bond formation to surface carboxyl groups present on the edge-plane-like defect sites at the termini of the graphene tubes comprising the MWCNTs, Scheme 2. Both attachment methods were attempted, and the resulting materials were characterized by TEM and by testing their magnetic properties with a neodymium magnet. No evidence for the covalent attachment of any NCs to the MWCNTs was observed using the diazotization approach. However, in the case of attaching the NCs via an amide bond, the resulting composite NC–MWCNT materials exhibited superparamagnetic properties. In the absence of an applied magnetic field, the MWCNTs simply behaved like
Scheme 2. Synthetic strategy to attach aminated NCs to oxidized MWCNTs using thionyl chloride based amide coupling.
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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.
unmodified MWCNTs – they could be dispersed in non-aqueous suspensions, such as in chloroform, with no obvious aggregation expected if the composites were paramagnetic. However, in the presence of a magnetic field (provided by a neodymium bar magnet or magnetic stirrer), the MWCNTs can be dragged instantly through and even out of the suspension (Fig. 5) or can be picked up by the magnet from a filter paper as a dry solid. Once removed from the magnetic field, the MWCNTs once again behaved as if they had no intrinsic magnetization. TEM images of the superparamagnetic NC–MWCNT composites (Fig. 5) revealed a sparse distribution of small agglomerates of NCs decorating the MWCNT surface. The agglomeration is presumably an effect of using a suspension of NCs in DCM during the coupling step – the aminated polymer-coated NCs are hydrophilic and likely to aggregate in non-aqueous solvents – and improved coupling strategies to minimize any agglomeration are part of our ongoing studies. However, even with a sparse coverage of agglomerated particles, the NCs are covalently attached to the MWCNTs and have imparted significant magnetic properties to the composite material. To demonstrate this conclusively, a graphite electrode was constructed that had a neodymium magnet placed 1 mm behind the exposed electrode surface. A 20 ll aliquot of a suspension of the NC-modified MWCNTs (1 mg/mL DCM) was placed on the electrode surface and the solvent allowed to evaporate, leaving a thin film of MWCNTs held in place by the magnetic field. This was then immersed in a non-aqueous acetonitrile solution of 0.1 M tetrabutylammonium tetrafluoroborate (TBAF) electrolyte. Cyclic voltammetry was performed, scanning between 0 and +1.0 V vs. Ag at a scan rate of 100 mV s 1, and the background capacitive charging current (proportional to the area of the electrode and hence the number of MWCNTs on the surface) measured at +0.5 V vs. Ag. The electrolyte was then stirred at 3000 rpm for 2 min intervals over a total period of 20 min, with the background capacitive current measured by cyclic voltammetry after each period of stirring. It is well known that MWCNTs films immobilized via a dropcasting method (described above) manner do not adhere well to electrode surfaces in non-aqueous electrolytes, and often simply
‘‘drop off’’ the electrode into the solution – a common problem for CNT electrochemists. Hence, for comparison, a 20 ll aliquot of a suspension of unmodified MWCNTs was also placed on the clean graphite electrode surface and subjected to the same hydrodynamic cyclic voltammetric experiments. The results obtained using Fe3O4 NC-modified MWCNTs are expressed in Fig. 6 as the ratio of measured capacitive charging current of the MWCNT-modified electrode to the charging current measured at the unmodified graphite electrode. In the case of the unmodified (non-magnetic) MWCNTs, the measured capacitive charging current is almost identical to the bare underlying graphite electrode after the first 2 min of stirring – indicating that almost all the MWCNTs were swept off the electrode surface. However, for the magnetic NC–MWCNTs, there is almost no decrease in the capacitive charging current, and hence almost no MWCNTs have been removed from the electrode even after 20 min of stirring at
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 bare electrode, Ic, bare determined from cyclic voltammetric measurements recorded in MeCN containing 0.1 M TBAF at a voltage scan rate of 100 mV s 1 after varying intervals of stirring at 3000 rpm.
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3000 rpm. Almost identical behavior was observed with the other magnetic NC-modified nanotubes. Clearly, if the NCs were not covalently attached to the MWCNTs and were simply dispersed among them, then the MWCNTs would have eventually been removed from the electrode under the prolonged stirring in non-aqueous electrolyte. Hence, this experiment strongly suggests that the NCs are indeed covalently bound to the MWCNTs, and that, further more, even with only a sparse coverage of NCs on the MWCNTs, they are sufficiently magnetized in the presence of an external field to secure almost all the MWCNTs to the electrode surface even under vigorous stirring conditions. It should also be noted that no redox voltammetric features were observed in either these experiments in non-aqueous electrolyte, or separate studies in aqueous electrolyte. This indicates that the polymer coating is intact and has successfully passivated the metal/metal oxide cores of the nanocrystals such that they take no observable part in any further chemical or electrochemical processes.
7. Conclusions Magnetic nanotubes, formed by covalently attaching polymercoated NCs to edge-plane-like defects on MWCNTs, have been prepared and characterized using a variety of techniques. The size of the NCs is such that they exhibit characteristic superparamagnetic behavior, exhibiting no net magnetization in the absence of a magnetic 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 NCs hydrophilic and allowed the attachment of the required chemical functionality for coupling to the CNTs. Importantly, the amphiphilic polymer coating was sufficient to passivate the NCs such that they were not observed to take part in any redox or other chemistry. Amination of the polymer layer was achieved using p-phenylenediamine, characterized by FT-IR, colorimetric chemical tests and XPS spectroscopy, and provides a facile route to anchor the NCs to the end and defect sites on the MWCNT surface via amide bond formation. Modification in this way leaves the vast majority of the surface area of the MWCNTs available for further modification as recently communicated by Wildgoose et al. [46]. The magnetic NC–MWCNT composites were able to remain attached to the surface of a purpose-built magnetic electrode in nonaqueous electrolytes, under vigorous stirring, where non-magnetic MWCNTs were almost immediately removed under the same conditions. This has important implications allowing the study of chemically-modified CNT materials in non-aqueous electrolytes for sensor and organometallic electrocatalyst development [46]. Conceivably, the covalent attachment of nanocrystals via an amphiphilic polymer to CNTs could be extended to any type of nanocrystal, leading to the development of further NC–CNT composite materials with wide ranging potential applications.
Acknowledgments GGW thanks the Royal Society for support via a University Research Fellowship.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.06.028.
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis and characterization of carbon nanotubes covalently functionalized with 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, UK Department of Materials, University of Oxford, South Parks Road, Oxford OX1 3PH, UK School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia d Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK b c
a r t i c l e
i n f o
Article history: Received 8 May 2012 Accepted 13 June 2012 Available online 20 June 2012 Keywords: Superparamagnetic nanoparticles Carbon nanotubes Polymer coating Chemical modification Magnetic nanotubes Voltammetry
a b s t r a c t Herein, we report the synthesis of three covalently linked superparamagnetic nanocrystal-multi-walled carbon nanotube (MWCNT) composites. A generic strategy for amphiphilic polymer coating of nanocrystals and further functionalization for amide bond formation with the MWCNTs is discussed. This approach can in principle allow attachment of any colloidal nanocrystal to the MWCNTs. The materials were 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 immobilization of these materials on an electrode during non-aqueous electrochemistry. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction In this work, we present a universal strategy for attachment of magnetic nanocrystals to multi-walled carbon nanotubes (MWCNTs), which has scope to be applied to other types of nanocrystal (NC). Use of an amphiphilic polymer layer provides the functionality on the particle surface for attachment to the carbon nanotubes (CNTs). This method can potentially be applied to a wide range of nanocrystals as the hydrophobic interaction between the NC ligands and the polymer is common to NCs stabilized with hydrophobic ligands. In this work, we demonstrate the covalent attachment of three different superparamagnetic NCs, Fe3O4, Ni, and MnFe2O4. The amphiphilic polymer passivates the surface of the particle, thus preventing the particle surface participating in any chemistry with the CNT. Recent developments in carbon nanotubes and nanoparticle composite research have yielded a range of nanomaterials with numerous 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 nanotubes follows two main synthetic stratagies: either the nanocrystals are synthesized and are subsequently attached to the nanotubes ⇑ Corresponding author. E-mail address: [email protected] (G.G. Wildgoose). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.028
afterward or the nanocrystal is formed in situ upon the tube [8]. Attachment of ready synthesized nanocrystals has been achieved using covalent bonds [9], eletrostatic interactions [10], p–p stacking [11], and hydrophobic interactions [12]. Covalent strategies of attachment of ready synthesized nanocrystals to carbon nanotubes are attractive as they take advantage of the existing methods to produce high quality nanocrystals and the strength of a covalent bond avoids complications, such as equilibria. The location of attachment is typically carboxylate ‘‘defect sites’’ formed by the pre-treatment of carbon nanotubes. However, many of these methods produce NC–CNT heterostructures where the NCs are uncoated and hence are susceptible to further chemical or electrochemical processes. The lack of protection limits the potential use of the high surface area of CNTs for other applications, for example, as catalyst supports or as (electro) analytical sensing platforms. Other methods have coated the entire NC–CNT structure in a polymer, resulting in the passivation of both NC and CNT [13]. Magnetic NCs represent a class of ferromagnetic and antiferromagnetic nanomaterials, which have tremendous potential as drug delivery 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, and MnFe2O4 have a critical radius above which they possess multiple magnetic domains and are ferromagnetic. Below this critical radius, the magnetic NCs exhibit superparamagnetism, where each
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nanocrystal has a single magnetic domain. The magnetic dipoles of superparamagnetic NCs are randomly aligned in the absence of an applied magnetic field at temperatures above the ‘‘blocking temperature’’ according to a Boltzmann distribution. In the presence of an applied magnetic field, superparamagnetic NCs will align instantly with the magnetic field and behave as a giant paramagnet. They also exhibit little or no hysteresis effects, such that any residual magnetism is immediately lost when they are removed from the magnetic field [19]. Control of the surface chemistry of nanocrystals is an important prerequisite for their use and is a current challenge within the field. Functionalization is used to change nanocrystal solubilization or to chemically attach NCs to other molecules, biological species [20], and other materials. To date, several approaches to control and sculpt the surface chemistry of NCs have been developed such as 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 of interest, as they provide a route for functionalizing and altering the solubility of potentially any colloidal nanocrystals stabilized by hydrophobic ligands. This technique depends on the formation of a hydrophobic bi-layer between the colloidal nanocrystal ligands and the non-polar subunits of the pro-amphiphilic polymer. Once wrapped, the polymer is further reacted to produce a hydrophilic exterior, hence rendering the particles water soluble [33–35]. Amphiphilic polymers have been shown to provide the colloidal nanocrystals with good colloidal stability in aqueous media and can also offer the opportunity to attach them to biomolecules [36] or dyes [37]. The colloidal stability follows the traditional rules of DLVO theory [38] as demonstrated by Lees et al. [34]. We report the formation of three different superparamagnetic NC materials, Fe3O4, Ni, and MnFe2O4, which are coated in a polymer layer, and which are then covalently attached predominantly to the ends and ‘‘edge-plane-like’’ [3] defects on MWCNTs via amide coupling to surface carboxylate groups. By doing this, the superparamagnetic NCs are passivated and play no further role in the chemical properties of the composite except to impart magnetization to the MWCNTs. The majority of the ‘‘magnetic nanotube’s’’ surface (the sidewall) is left unmodified and available for further 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 without further purification unless stated otherwise. All synthetic reactions and manipulations were performed under a dry nitrogen atmosphere. ‘‘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 degassed to remove dissolved oxygen and were either supplied as anhydrous grade or were dried prior to use by distillation over either sodium/ benzophenone (tetrahydrofuran, THF, diethyl ether, DEE) or calcium hydride (dichloromethane, DCM) under a nitrogen atmosphere. Aqueous solutions were prepared using UHQ deionized water with a resistivity of not less than 18.2 MX cm (Millipore). Cyclic voltammetric measurements were recorded using a computer-controlled potentiostat (PGSTAT30, Autolab, Utrecht, The Netherlands) with a standard three-electrode arrangement. A silver wire served as the pseudo-reference electrode and a platinum
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wire as the counter electrode. The magnetic working electrode was fabricated using an in-house design and consisted of a polished graphite rod sealed in PEEK (working electrode diameter 5 mm) that had a neodymium iron borate disk magnet placed 1 mm behind the working electrode face. Non-aqueous voltammetry was performed under an inert nitrogen atmosphere in degassed acetonitrile that had been dried for 24 h prior to use over 3 Å molecular sieves and contained 0.1 M tetrabutylammonium tetrafluoroborate (TBAF) as supporting electrolyte. Transmission electron microscopy (TEM) was performed using a JEOL-JEM 2000EX instrument. Infrared spectra were recorded using a PerkinElmer Spectrum 100 FT-IR spectrometer fitted with a PerkinElmer25 universal attenuated total internal reflectance (ATR) sampling accessory. X-ray photoelectron spectroscopy (XPS) was performed with a VG clam 4 MCD analyzer system using X-ray radiation from the Al Ka band (1486.6 eV). All experiments were recorded with an analyzer energy of 100 eV and a takeoff angle of 90°. Dynamic light scattering (DLS) and zeta potential measurements were performed on a Malvern Zetasizer Nano ZS instrument. Colorimetric testing for the presence of amines made use of the ninhydrin Kaiser test [39]. 3. Synthetic procedures Syntheses of the Fe3O4, Ni, and MnFe2O4 NCs were carried out according to previously published procedures, yielding hydrophobic 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] Differing amounts of PSMA (Aldrich, Mn = 1700) were used to coat the NCs, thus determining the amount of polymer required to transfer the 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 pphenylenediamine The PMSA–Fe3O4, PMSA–Ni, and PMSA–MnFe2O4 NCs were each separately suspended in 10 ml of chloroform to which was added a solution containing 100 mg (excess) of p-phenylenediamine in a further 5 ml of chloroform. After stirring for 5 h, each sample of NCs had formed a turbid gray suspension. The NCs were re-precipitated by adding a small amount of toluene and then centrifuged at 4800 rpm for 5 min. The precipitates were then washed repeatedly with chloroform followed by a further cycle of centrifuging to remove any unreacted p-phenylenediamine. The precipitates were then each re-suspended in a solution of tetramethylammonium hydroxide (15 mg in 2 ml of water). Finally, in order to remove any excess polymer or physisorbed p-phenylenediamine, each of the NC samples was centrifuge-filtered with a MilliporeÒ 10,000 MW centrifuge filter at 5000 rpm for 20 min per cycle until the pH dropped to 7. Distilled water was added to each sample between the filtration steps to reduce the pH and keep a constant volume. The resulting aminated-polymer-coated NCs (denoted NH2–PMSA–Fe3O4, NH2–PMSA–Ni, and NH2–PMSA–MnFe2O4 NCs respectively) were stored as frozen aqueous suspensions (total volume 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 on the surface of the MWCNTs for particle linking via amide bond formation, 100 mg of MWCNTs was oxidized by refluxing in a 3:1
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mixture of concentrated nitric and sulfuric acid at 80 °C for 3 h. The oxidized MWCNT suspension was allowed to cool to room temperature, carefully diluted with 50 ml of deionized water, and filtered under reduced pressure. The retentate was washed with distilled water until the washing ran neutral and then rigorously dried at 60 °C under vacuum overnight. A Schlenk flask was charged with 10 mg of oxidized MWCNTs and a magnetic stirrer bar and sealed under a nitrogen atmosphere. Thionyl chloride (5 ml, 68.55 mmol) was then added under a flush of nitrogen and the reaction mixture stirred at room temperature for 30 min, after which the excess thionyl chloride was removed under vacuum. Amine-functionalized, polymer-coated nanocrystals were dispersed with the aid of sonication in 10 ml of dry DCM and added to the acid-chloride functionalized MWCNTs together with one equivalent of triethylamine (9.56 ml, 68.55 mmol) under a flush of nitrogen. The reaction mixture was stirred overnight at room temperature before the NC-modified MWCNTs were filtered 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 are denoted as Fe3O4–NC–MWCNTs, Ni–NC–MWCNTs, and MnFe2O4– NC–MWCNTs.
6. Results and discussion 6.1. Synthesis and characterization of aminated-polymer-coated superparamagnetic nanocrystals The polymer-coated-superparamagnetic nanocrystal-MWCNT composites were synthesized in three stages. NCs were synthesized using controlled thermal decomposition of organometallic precursors in a non-polar, high boiling point solvent in the presence of surfactants [40,41]. These yielded black, hydrophobic dispersions of NCs with a narrow size distribution around 10 nm, below the critical domain size such that the NCs are superparamagnetic (vide infra). Each of the three superparamagnetic nanoparticulate materials was then coated in a thin layer of a commercially available amphiphilic polymer, poly(styrene-co-maleic anhydride) (PSMA), following the work of Lees et al. [34] (Scheme 1). This was done in order to passivate the surface of the NCs and prevent them interfering with any further chemical or electrochemical processes involved in a given target application. The choice of polymer-coating the NCs with PSMA is advantageous in that it avoids adversely affecting their magnetic properties
Scheme 1. The polymer coating and aminiation of the nanocrystals. Where A is PSMA and B is p-phenylene diamine. The solvent used is chloroform.
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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 of Fe3O4, (d) Ni, and (e) MnFe2O4. The full line represents the uncoated particles, dashed line after polymer coating and dotted line after amination.
as can be the case when superparamagnetic NCs are coated in carbon or silica shells [42]. Choosing to polymer coat the NCs before coupling to the MWCNT surface also prevents the latter being affected by polymer coating, which may be undesirable if the MWCNTs are to be used as a further support material. Finally, the anhydride rings of the polymer coating were opened using p-phenylenediamine, in order to aminate the polymer so as to provide a method of attaching the NCs to carboxyl groups present 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, which were found to extensively cross-link the polymer chains or NCs (vide infra). The materials were characterized at each stage in the process using TEM, DLS, XPS, and FT-IR. 6.2. Characterization of the polymer-coated superparamagnetic nanocrystals by TEM and DLS Fig. 1 shows TEM images of the initial and aminated Fe3O4 NCs and DLS data showing the increase in hydrodynamic radius through the three stages of the synthesis. In the case of the Fe3O4 and Ni NCs, monodisperse samples of almost spherical NCs were observed with a mean core radius of 11 nm and 6 nm, respectively. These values are below the single-domain to multi-domain transition for magnetite and nickel to become superparamagnetic [19]. The
MnFe2O4 NCs were observed to be polydisperse; with size distributions centered around 10 nm and 35 nm. HRTEM and associated EDX spectrum of the Ni NCs are given in the SI. The TEM of the NCs in Fig. 1 after amine functionalization shows that the metal oxide core is unaffected and clearly visible, but the polymer layer is indistinguishable by TEM. From the DLS measurements shown in Fig. 1 for the unmodified, PSMA-coated NCs and aminated NH2– PSMA–NCs, the hydrodynamic radii of the unmodified Fe3O4 and Ni were again found to be monodisperse with mean values of 11.5 nm and 13.6 nm, slightly larger than observed from TEM measurements due to the effect of both solvation, and the presence of oleic acid surfactant ligands on the surface of the NCs, which are not observed in the TEM images. The smaller MnFe2O4 NCs had mean hydrodynamic radii of between 12 and 14 nm. Although DLS measurements only provide a mean hydrodynamic radius, the trend upon adding the polymer layer and then coupling this with the p-phenylenediamine is apparent, with the mean hydrodynamic radius of all particles increasing upon each modification, as expected. DLS measurements suggest that in all cases, the optimized coating 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 MWCNTs covalently via amide formation to the surface carboxyl groups on
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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-phenylene diamine (dashed line).
Fig. 2. IR spectra of aminated Fe3O4 NCs (top), poly(styrene-co-maleic anhydride) (middle), and p-phenylene diamine (bottom).
the nanotubes, it is necessary to aminate the PSMA–NCs with a diamino moiety. However, the amine functionalization of PSMA coated NCs presents two main challenges: prevention of linking between particles during the reaction with the diamine and ensuring the resulting particles are colloidally stable. The use of double-ended amines or other alkyl amines bearing a different nucleophilic group may lead to inter-particulate crosslinking between NCs, as well as internal cross-linking between different polymer chains wrapped around the NC [33]. These potential issues were overcome by use of an excess of diamine. Amination of the PSMA–NCs with p-phenylenediamine (used as the free-base not the hydrochloride salt) was found to be successful. Our choice of an aryl diamine was guided by the relatively poor nucleophilicity of the aryl amine groups cf. their alkyl analogues – which is further reduced after the initial nucleophilic attack by the greater steric rigidity of the aryl ring cf. to an alkyl chain. This was expected to reduce the likelihood of any NC cross-linking. The opening of an anhydride ring with p-phenylenediamine is shown in Scheme 1. The reaction is facile and goes to completion at room temperature. The aminated NCs (NH2–PSMA–NCs) were then precipitated from chloroform and re-dispersed in an aqueous solution of tetramethylammonium hydroxide in order to open any remaining maleic anhydride rings and render the NCs water dispersible. The resulting NH2–PSMA–NCs were characterized using
ATR-FT-IR, zeta-potential measurements and X-ray photoelectron spectroscopy (XPS). A positive result indicating the presence of free amine groups on the polymer-coated NC surface was also observed after the samples were subjected to the colorimetric Kaiser test. Fig. 2 compares the ATR-FT-IR spectra recorded for the PSMA polymer and p-phenylenediamine precursors and NH2–PSMA– Fe3O4 aminated NCs. The important features in the IR spectrum of the PSMA precursor occur at mmax/cm 1: 3030 and 2927 (aryl CAH stretches), 1856 and 1773(ACOAOACOA acid anhydride C@O stretch) and 1494 and 1455 (CAH deformations) and 1218(ACAOACA). In the p-phenylenediamine, IR bands corresponding 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 successful amination of the PSMA-coated NC materials was confirmed by the disappearance of the acid anhydride stretches in the PSMA polymer at 1856 and 1773 cm 1. The spectra of p-phenylene diamine and the amine functionalized NCs show a near identical profile in the region 2180–1978 cm 1 indicative of aromatic ‘‘combination bands’’ and provide further evidence for the successful attachment of p-phenylene diamine to the PSMA polymer. A number of overlapping resonances were observed between 2916 and 2869 cm 1 assigned to ArAH stretches from both the PSMA and the p-phenylenediamine. The intensity of these ArAH stretches has also increased relative to the PSMA precursor alone, which may also be indicative of modification with p-phenylene diamine [43]. To investigate the effect of the amine functionalization on the colloidal stability of the nanocrystals, we performed zeta-potential measurements in pH 7.4 phosphate buffer (0.1 M) of the aminated polymer-coated NCs and also of the PSMA-coated NCs that had been subjected to a ring-opening step by treatment with 0.1 M
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tetramethylammonium hydroxide, but had not been coupled with the p-phenylenediamine, as a control. Fig. 3 shows the resulting zeta-potential distributions for the Ni and the Fe3O4 NC species. In both cases, the net surface charge was always negative, but the aminated NCs exhibited zeta-potentials that had shifted more positively ( 26.3 mV and 38.3 mV for the NH2–PSMA–Fe3O4 and NH2–PSMA–Ni respectively) than the controls (where the particles are stabilized exclusively by carboxylate groups; 63.3 mV and 60.6 mV for the PSMA–Fe3O4 and PSMA–Ni NCs, respectively). This shift to more positive potentials is likely attributable to the amination of the polymer by the p-phenylenediamine, since this would reduce the net negative charge. Finally, XPS analysis of the aminated NCs revealed a spectral peak at ca. 400 eV corresponding to emission from the N1s level in addition to spectral peaks corresponding to the relevant metal or metal oxides in the NC core (Fig. 4). Detailed scans over the N1s region reveal that two peaks are observed with binding energies of 398 eV and 402 eV, with a ratio of peak areas of 1.6:1 corresponding to nitrogen atoms in an amine and amide chemical environment [44]. The deviation from the expected 1:1 ratio is indicative that the washing procedure performed to remove any physisorbed p-phenylenediamine was not perfect, but that the majority of p-phenylenediamine groups present are coupled to the PSMA polymer via the amide, leaving a pendant amino group available for further coupling to the nanotubes. 6.4. Covalent modification of MWCNTs with polymer-coated superparamagnetic nanocrystals The pendent amine groups on the NH2–PSMA–NC surface offer two routes to covalently attach the NCs to MWCNT surfaces: (i) CAC bond formation via diazotization of the amine groups and
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Fig. 4. X-ray photoelectron spectroscopy (XPS) spectrum for the N 1s electron of aminated Fe3O4 NCs. XPS spectrum (thin full line), baseline (dashed line), Guassian fit for amide group (dot and long dash), Gaussian for amine group (dot and short dash), and envelope from the combination of Gaussians (thick full line).
chemical reduction in the presence of MWCNTs [45] or (ii) amide bond formation to surface carboxyl groups present on the edge-plane-like defect sites at the termini of the graphene tubes comprising the MWCNTs, Scheme 2. Both attachment methods were attempted, and the resulting materials were characterized by TEM and by testing their magnetic properties with a neodymium magnet. No evidence for the covalent attachment of any NCs to the MWCNTs was observed using the diazotization approach. However, in the case of attaching the NCs via an amide bond, the resulting composite NC–MWCNT materials exhibited superparamagnetic properties. In the absence of an applied magnetic field, the MWCNTs simply behaved like
Scheme 2. Synthetic strategy to attach aminated NCs to oxidized MWCNTs using thionyl chloride based amide coupling.
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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.
unmodified MWCNTs – they could be dispersed in non-aqueous suspensions, such as in chloroform, with no obvious aggregation expected if the composites were paramagnetic. However, in the presence of a magnetic field (provided by a neodymium bar magnet or magnetic stirrer), the MWCNTs can be dragged instantly through and even out of the suspension (Fig. 5) or can be picked up by the magnet from a filter paper as a dry solid. Once removed from the magnetic field, the MWCNTs once again behaved as if they had no intrinsic magnetization. TEM images of the superparamagnetic NC–MWCNT composites (Fig. 5) revealed a sparse distribution of small agglomerates of NCs decorating the MWCNT surface. The agglomeration is presumably an effect of using a suspension of NCs in DCM during the coupling step – the aminated polymer-coated NCs are hydrophilic and likely to aggregate in non-aqueous solvents – and improved coupling strategies to minimize any agglomeration are part of our ongoing studies. However, even with a sparse coverage of agglomerated particles, the NCs are covalently attached to the MWCNTs and have imparted significant magnetic properties to the composite material. To demonstrate this conclusively, a graphite electrode was constructed that had a neodymium magnet placed 1 mm behind the exposed electrode surface. A 20 ll aliquot of a suspension of the NC-modified MWCNTs (1 mg/mL DCM) was placed on the electrode surface and the solvent allowed to evaporate, leaving a thin film of MWCNTs held in place by the magnetic field. This was then immersed in a non-aqueous acetonitrile solution of 0.1 M tetrabutylammonium tetrafluoroborate (TBAF) electrolyte. Cyclic voltammetry was performed, scanning between 0 and +1.0 V vs. Ag at a scan rate of 100 mV s 1, and the background capacitive charging current (proportional to the area of the electrode and hence the number of MWCNTs on the surface) measured at +0.5 V vs. Ag. The electrolyte was then stirred at 3000 rpm for 2 min intervals over a total period of 20 min, with the background capacitive current measured by cyclic voltammetry after each period of stirring. It is well known that MWCNTs films immobilized via a dropcasting method (described above) manner do not adhere well to electrode surfaces in non-aqueous electrolytes, and often simply
‘‘drop off’’ the electrode into the solution – a common problem for CNT electrochemists. Hence, for comparison, a 20 ll aliquot of a suspension of unmodified MWCNTs was also placed on the clean graphite electrode surface and subjected to the same hydrodynamic cyclic voltammetric experiments. The results obtained using Fe3O4 NC-modified MWCNTs are expressed in Fig. 6 as the ratio of measured capacitive charging current of the MWCNT-modified electrode to the charging current measured at the unmodified graphite electrode. In the case of the unmodified (non-magnetic) MWCNTs, the measured capacitive charging current is almost identical to the bare underlying graphite electrode after the first 2 min of stirring – indicating that almost all the MWCNTs were swept off the electrode surface. However, for the magnetic NC–MWCNTs, there is almost no decrease in the capacitive charging current, and hence almost no MWCNTs have been removed from the electrode even after 20 min of stirring at
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 bare electrode, Ic, bare determined from cyclic voltammetric measurements recorded in MeCN containing 0.1 M TBAF at a voltage scan rate of 100 mV s 1 after varying intervals of stirring at 3000 rpm.
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3000 rpm. Almost identical behavior was observed with the other magnetic NC-modified nanotubes. Clearly, if the NCs were not covalently attached to the MWCNTs and were simply dispersed among them, then the MWCNTs would have eventually been removed from the electrode under the prolonged stirring in non-aqueous electrolyte. Hence, this experiment strongly suggests that the NCs are indeed covalently bound to the MWCNTs, and that, further more, even with only a sparse coverage of NCs on the MWCNTs, they are sufficiently magnetized in the presence of an external field to secure almost all the MWCNTs to the electrode surface even under vigorous stirring conditions. It should also be noted that no redox voltammetric features were observed in either these experiments in non-aqueous electrolyte, or separate studies in aqueous electrolyte. This indicates that the polymer coating is intact and has successfully passivated the metal/metal oxide cores of the nanocrystals such that they take no observable part in any further chemical or electrochemical processes.
7. Conclusions Magnetic nanotubes, formed by covalently attaching polymercoated NCs to edge-plane-like defects on MWCNTs, have been prepared and characterized using a variety of techniques. The size of the NCs is such that they exhibit characteristic superparamagnetic behavior, exhibiting no net magnetization in the absence of a magnetic 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 NCs hydrophilic and allowed the attachment of the required chemical functionality for coupling to the CNTs. Importantly, the amphiphilic polymer coating was sufficient to passivate the NCs such that they were not observed to take part in any redox or other chemistry. Amination of the polymer layer was achieved using p-phenylenediamine, characterized by FT-IR, colorimetric chemical tests and XPS spectroscopy, and provides a facile route to anchor the NCs to the end and defect sites on the MWCNT surface via amide bond formation. Modification in this way leaves the vast majority of the surface area of the MWCNTs available for further modification as recently communicated by Wildgoose et al. [46]. The magnetic NC–MWCNT composites were able to remain attached to the surface of a purpose-built magnetic electrode in nonaqueous electrolytes, under vigorous stirring, where non-magnetic MWCNTs were almost immediately removed under the same conditions. This has important implications allowing the study of chemically-modified CNT materials in non-aqueous electrolytes for sensor and organometallic electrocatalyst development [46]. Conceivably, the covalent attachment of nanocrystals via an amphiphilic polymer to CNTs could be extended to any type of nanocrystal, leading to the development of further NC–CNT composite materials with wide ranging potential applications.
Acknowledgments GGW thanks the Royal Society for support via a University Research Fellowship.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.06.028.
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