AFM imaging of functionalized double-walled carbon nanotubes
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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