Elsevier

Ultramicroscopy

Volume 109, Issue 8, July 2009, Pages 899-906
Ultramicroscopy

AFM imaging of functionalized double-walled carbon nanotubes

https://doi.org/10.1016/j.ultramic.2009.03.034Get rights and content

Abstract

We present a comparative study of several non-covalent approaches to disperse, debundle and non-covalently functionalize double-walled carbon nanotubes (DWNTs). We investigated the ability of bovine serum albumin (BSA), phospholipids grafted onto amine-terminated polyethylene glycol (PL-PEG2000-NH2), as well as a combination thereof, to coat purified DWNTs. Topographical imaging with the atomic force microscope (AFM) was used to assess the coating of individual DWNTs and the degree of debundling and dispersion. Topographical images showed that functionalized DWNTs are better separated and less aggregated than pristine DWNTs and that the different coating methods differ in their abilities to successfully debundle and disperse DWNTs. Height profiles indicated an increase in the diameter of DWNTs depending on the functionalization method and revealed adsorption of single molecules onto the nanotubes. Biofunctionalization of the DWNT surface was achieved by coating DWNTs with biotinylated BSA, providing for biospecific binding of streptavidin in a simple incubation step. Finally, biotin-BSA-functionalized DWNTs were immobilized on an avidin layer via the specific avidin–biotin interaction.

Introduction

Due to their remarkable electronic, mechanical, chemical and thermal properties CNTs have turned out to be nanodevices with a wide range of potential applications in various composite materials, technical devices [1], [2], as well as medical and pharmaceutical products [3], [4]. Double-walled carbon nanotubes (DWNTs) are of special interest as they bridge the gap between single-walled carbon nanotubes (SWNTs) and the more complex multi-walled carbon nanotubes (MWNTs). DWNTs are comparable to SWNTs with respect to their small diameter, yet their mechanical stability is much higher than that of SWNTs. Moreover, the outer wall can be functionalized without changing the mechanical and electronic properties of the inner nanotube [5]. These properties make them attractive for biological and biomedical applications. However, DWNTs, as all pristine CNTs, have a strong tendency to aggregate and are practically insoluble in any kind of solvent because of substantial van der Waals attractions among them. Solubility in aqueous media, however, is a fundamental prerequisite for potential biological and biomedical applications. Several strategies have been explored to disperse and solubilize CNTs which can be divided into two main categories. One approach is based on covalent CNT functionalization by cutting and oxidizing CNTs. Thus, carboxylic groups are generated which are subsequently derivatized with different types of molecules [6], [7], [8], [9], [10]. The second dispersal method is the non-covalent coating of CNTs [11] with surfactants molecules [12], [13], [14], nucleic acids [15], [16], [17], proteins and peptides [18], [19], [20], [21], or polymers [22], [23], [24], [25], [26], [27]. The non-covalent dispersion procedures usually involve ultrasonication, centrifugation and filtration. They are quick and easy, achieving debundling of CNTs, dispersion in water and biocompatibility in one step.

In this study we present a comparative investigation of non-covalent approaches to debundle, disperse and functionalize DWNTs. We used bovine serum albumin (BSA) and a phospholipid-linked polyethylene glycol chain with a terminal amino group (PL-PEG-NH2), as well as a combination thereof, to coat purified DWNTs. Topographical atomic force microscope (AFM) imaging was used for the direct assessment of a successful functionalization procedure and proved that functionalized DWNTs are better separated and less aggregated than pristine DWNTs. We demonstrate the ability of the non-covalent functionalization scheme to provide DWNTs for specific recognition by, and binding of biomolecules, using the well-studied biotin/avidin and biotin/streptavidin interaction [28]. We present high-resolution topographical images of functionalized DWNTs captured onto a dense avidin layer in aqueous buffer solution. To our knowledge, this is the first successful attempt to image single functionalized carbon nanotubes in liquid under physiological conditions using AFM.

Section snippets

Preparation and purification of double-walled carbon nanotubes

Synthesis and purification of double-walled carbon nanotubes used for the experiments are explained in detail elsewhere [5]. Briefly, DWNTs were synthesised by catalytic chemical vapour deposition (CCVD), the catalyst being an Mg1−xCoxO solid solution containing added Mo oxide. The MgO-based catalyst was removed by thorough washing with HCl solution. The extracted powder contained only clean carbon nanotubes of which ∼80% are DWNTs and the rest being SWNTs (15%) and triple-walled CNTs (5%).

Topographical imaging of functionalized DWNTs

Fig. 1a shows a survey image of non-functionalized DWNTs after immersion in the organic solvent DCE and subsequent evaporation revealing aggregates and long bundles of different height and length. Cross-sectional analysis (Fig. 1b and d) revealed that most DWNTs are present as bundles ranging from 2 to 130 nm in height and few micrometers in length. From cross-section profiles taken on non-functionalized DWNTs the height variance was observed to be ∼0.5 nm (Fig. 1c). Because of their hydrophobic

Conclusion

We have established simple functionalization procedures that provide stable aqueous suspensions of debundled and individual DWNTs using BSA, PL-PEG or a combination thereof. AFM topographical imaging was used for a simple and direct assessment of bundling and aggregation of DWNTs, revealing well dispersed and separated DWNTs after proper functionalization. Biotin-BSA-functionalized DWNTs were shown to specifically bind streptavidin, moreover biotin-BSA DWNTs could be specifically and tightly

Acknowledgements

We thank Tuomas Näreoja for technical discussions, Dr. Ruediger Klingeler for supporting materials, and Elena Heister, Vera Neves, Dr. Vanesa Beltran, Dr. Helen Coley and Prof. Johnjoe McFadden for helpful discussions. This work was supported by EC grant Marie Curie RTN-CT-2006-035616, CARBIO ‘Carbon nanotubes for biomedical applications’.

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