Rapid assembly of carbon nanotube-based magnetic composites

https://doi.org/10.1016/j.matchemphys.2011.03.038Get rights and content

Abstract

The rapid assembly of magnetic carbon nanotubes is mediated through the electrostatic attraction of α-haematite nanoparticles to carboxylic groups decorating their outer surface. The system is then stabilised through covalently bonding a silica coat using a 3-aminopropyltriethoxysilane precursor, which creates a thin barrier protecting the α-haematite particles from aggressive pH solutions. The nanocomposites can be effectively dispersed in aqueous solution and can be attracted to an external magnetic field. The proposed method can be used for synthesis of magnetic CNTs suitable for assembling densely packed magnetic arrays, remotely guided drug delivery and organic chemical wastewater remediation with the added benefit of nanomaterial recovery. Therein, p-nitroaniline was demonstrated to still adsorb to uncoated areas of the silica-sheathed magnetic MWCNT composite.

Highlights

► Trapping of α-haemetite on acid-oxidised carbon nanotubes enables magnetic nanocomposites; ► Coating magnetic carbon nanotubes with silica improves chemical stability in low pH solution; ► Silica-coated magnetic carbon nanotube composites still adsorb polycyclic aromatic hydrocarbons; ► Final composites are useful for organic chemical remediation and rapid adsorbate recovery.

Introduction

The functionalisation of carbon nanomaterials can modify their physicochemical properties, making them more suitable for chemical and biological applications. Of particular interest are CNTs with magnetic properties because of their potential applications in biological labelling, drug delivery and magnetic storage media [1], [2], [3].

Various synthetic routes have been developed. Open-ended, template-grown nanotubes were filled with a ferrofluid [4], [5]. Magnetic nanoparticles were also embedded into CNT walls [6]. Correa-Duarte et al. produced a CNT-based magnetic material through a polymer wrapping and layer-by-layer assembly technique [7]. Georgakilas et al. attached magnetic nanoparticles to CNTs via a carboxylic derivative of pyrene immobilised on the surface of the nanotubes [8]. A hydrothermal approach to decorating CNT surfaces with magnetic beads utilising reduction of iron chloride (III) with ethylene glycol, was reported by Jia et al. [9]. However, the methods described above produce composites with magnetic particles loosely bound to CNTs, where they are exposed to the environment, and may either become detached or react directly with the surrounding media. CNTs that are embedded with magnetic particles are strongly bound, but this technique does not prevent the availability of the magnetic nanoparticles to external chemical reactions.

Herein, we report the synthesis of magnetic MWCNT composites through the electrostatic attraction of α-haematite nanoparticles, stably “trapped” within a thin silica shell. The process exploits a set of facile and rapid chemical reactions with acidic groups decorating the CNT surfaces.

Section snippets

Carbon nanotube preparation

MWCNTs (TMSpetsmash, Ukraine, ca. 96% purity) were refluxed in nitric acid (70%) for 8 h at 100 °C in order to remove any residual catalytic particles [10] and surface amorphous carbon and also introduce surface functional groups [10]. The samples were cooled, vacuum-filtered and washed with deionised water until the system reached pH 7 and then dried at 120 °C for 8 h [11]. The acid-oxidised MWCNTs were refluxed in 1.0 M NaOH for 1 h, washed, filtered until a neutral pH in order to remove any fulvic

Results and discussion

The experimental procedure involves three main steps (Fig. 1). First, the treatment of MWCNTs with nitric acid removes catalytic metal impurities within the system and also oxidises the carbon nanotube surface. At this stage it is important to remove the oxidised fragments from the carbon lattice in order to allow covalent bonding directly to the carbon nanotube surface [11]. Second, the immobilisation of α-haematite occurs through electrostatic attraction to the surface carboxylic groups.

Conclusions

Aminosilica-coated magnetic composites of α-Fe2O3 nanoparticles attached to MWCNT surface were successfully prepared via a facile method involving the hydrolysis–polycondensation of gamma-aminopropyltriethoxysilane. The silica coating of the α-haemetite nanoparticles extends partly to the MWCNTs, leaving sites available for surface adsorption of PAH contaminants. Aqueous suspensions of these magnetic composites can be easily separated from the liquid phase using an external magnetic field and

Acknowledgements

The authors acknowledge the RCUK Academic Fellowship (RW) and Dr. Martin Smith from University of Brighton. A.K. acknowledges support of her PhD studentship by the University of Brighton. Y.G. was supported by NSF grant DMR-0945230.

References (28)

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1

Tel.: +44 1273 643986; fax: +44 1273 642674.

2

Tel.: +1 215 895 6446; fax: +1 215 895 1934.

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