Carbon nanotubes as templates for one-dimensional nanoparticle assemblies

Abstract

We present a simple, generally applicable procedure for the assembly of nanoparticles on carbon nanotubes in aqueous solution. The method makes use of polyelectrolytes for wrapping carbon nanotubes and providing them with adsorption sites for electrostatically driven nanoparticle deposition. The method is exemplified by the assembly of gold nanoparticles which results in single, optically labelled carbon nanotubes.

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Miguel A. Correa-Duarte

Miguel A. Correa-Duarte obtained a PhD degree in Chemistry from the University of Vigo with Luis Liz-Marzán and since then had postdoctoral position in CAESAR (Bonn) and in Arizona State University (USA). He currently holds a researcher position at the University of Vigo. His current interests include carbon nanotube functionalization and composites.

Luis M. Liz-Marzán

Luis M. Liz-Marzán obtained a PhD degree in Chemistry from the University of Santiago de Compostela and worked at Utrecht University as a postdoctoral fellow until 1995, when he joined the University of Vigo as an Assistant Professor, and secured a permanent Professor position in 1999. He is a member of the International Advisory Editorial Boards of Journal of Materials Chemistry and Langmuir. His current interests include nanoparticle synthesis and assembly, and the optical properties of nanomaterials and nanocomposites.

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The controlled assembly of nanoparticles has been proposed as one of the most promising ways to achieve the targets of Nanotechnology, i.e., manipulation at the nanoscale,¹ as an alternative to top-down methods such as various kinds of lithography. Once we are capable of such control, the creation of components and devices which can exploit the fascinating, size-dependent properties of nanomaterials will be possible. Quite a few different pathways have been developed over the past ten years toward the creation of tailored nanoparticle assemblies, each focusing on a particular interaction force between nanoparticles, and many with restricted applications toward certain materials. One of the techniques offering very broad applicability is the so-called layer-by-layer (LBL) assembly, which is mainly based on the use of polyelectrolytes as molecular glue between the substrate and nanoparticles or between successive nanoparticle layers.² This technique has become extremely popular, not only for the assembly of nanoparticle mono- and multilayers on macroscopic surfaces, but also for the production of micropatterns³ and assembly on mesoscopic⁴ and microscopic substrates (such as colloidal particles).⁵ All these developments provide improved capabilities for the creation of useful nanostructured components. Still, the uniform assembly of nanoparticles into one-dimensional arrays over relatively long distances is still a challenge. Linear arrays would have extremely interesting applications, for instance, in the construction of nanoelectronic circuits,⁶ but could also be used in photonics, since wave guiding via surface plasmons has been predicted for metal nanoparticle chains, for example.⁷

Carbon nanotubes (CNTs), the paradigm material when talking about Nanotechnology, not only possess unique structure dependent electronic, mechanical, optical and magnetic properties,^8–11 but can also reach very high aspect ratios and surface areas. For these reasons, CNTs can be considered as ideal templates for the formation of one-dimensional nanoparticle assemblies. Despite the relative chemical inertness of CNTs, several strategies have been followed for the preparation of CNT–nanoparticle composites, either through in situ nanoparticle synthesis¹² or the assembly of pre-formed nanoparticles. In either case, surface modification is required, which can involve the chemical development of defect-sites and subsequent covalent functionalization¹³ or non-covalent adsorption of macromolecules on the side walls.¹⁴ Non-covalent adsorption is of particular interest because it enables functionalization of the CNTs while still preserving their electronic structure, since the sp²-hybridized carbon structure and conjugation remain unaltered (which on the other hand provides the CNT surface with an inert and hydrophobic character, thus complicating the adsorption of hydrophilic macromolecules).

The challenge of non-covalent functionalization has been recently accomplished^15,16 for gold nanoparticles, by means of a combination of polymer wrapping¹⁴ and layer-by-layer assembly.² The polymer wrapping technique was developed by Smalley and co-workers^14a for the preparation of individual, well-redispersed CNTs in aqueous solution, and comprises the functionalization of the CNTs through non-covalent attachment of macromolecules. This stabilization is based on the thermodynamic preference of CNT–polymer interactions with respect to CNT–water interactions, hiding the hydrophobic surface of the CNTs. Even when mixing with polyelectrolytes, the energy balance favours CNT wrapping, yielding a high density of charged surface sites and thereby serving as a good starting point for the layer-by-layer alternating monolayer adsorption of oppositely charged components, driven by electrostatic and van der Waals interactions.¹⁷ As schematically outlined in Fig. 1, upon CNT wrapping with a negatively charged polyelectrolyte, a positively charged monolayer can then be deposited, which serves as the real template for nanoparticle adsorption. If a uniform, dense distribution of positive charges is achieved over the CNT surface, one-dimensional nanoparticle assemblies should be obtained.

Fig. 1 Schematic illustration of a non-covalent functionalization of CNTs comprising (1) polymer wrapping using poly(sodium 4-styrenesulfonate) (PSS), (2) self-assembly of poly(diallyldimethylammonium chloride) (PDDA) and (3) nanoparticle deposition.

The effectiveness of this method has been first demonstrated through the assembly of silica-coated gold nanoparticles (Au@SiO₂) onto multi-wall CNTs.¹⁵ The use of silica-coated nanoparticles was motivated by our prior experience of their assembly on spherical colloids.¹⁸ Various examples of the resulting composites are shown in Fig. 2, for varying CNT diameters and number of monolayers. In all cases, compact layers have been obtained around the CNT template, which evidences the versatility that can be obtained through this procedure.

Fig. 2 SEM image (top) of one monolayer of Au@SiO₂ nanoparticles assembled onto a carbon nanotube; TEM micrographs (bottom) of thick (a) and thin (b–c) MWNTs (50 and 15 nm diameter, respectively), coated with 1 (a–b) and 2 (c) monolayers of Au@SiO₂ nanoparticles.

Because of the plasmon absorption of Au nanoparticles, such composite nanowires are optically labelled and, therefore, have the potential to be used as components of nanoelectronic circuits and waveguides. Additionally, the use of silica-coated nanoparticles provides a further control parameter, since the distance between neighbouring gold nanoparticles can be adjusted through the thickness of the silica shells, which allows tailoring of the optical response through the control of dipolar interactions.^18,19 An example of the optical effects (red-shift and broadening) upon assembly of Au@SiO₂ on CNTs is shown in Fig. 3.

Fig. 3 UV/Vis spectra of Au spheres in water (solid) and assembled on CNTs (Au@SiO₂, dashed; Au-PVP, dotted) and nanorods with two different aspect ratios (2.6 and 3.3) in water (solid) and on CNTs (dashed).

Even stronger optical effects have been observed when Au nanorods were assembled onto CNTs by means of a similar experimental procedure.¹⁶ In this case, only broadening and the red-shift of the longitudinal plasmon band²⁰ was observed (Fig. 3), which suggests the preferential formation of nanorod linear assemblies with a head-to-tail, stringlike alignment.

This was actually confirmed by TEM, as exemplified in Fig. 4a, which shows the (unexpected) general tendency of gold nanorods to align in stripes on opposite sides of the MWNTs, rather than uniformly adsorbing on the surface to achieve maximum coverage. While the reason for alignment of the MWNTs can be related to the anisotropic surface potential of the nanorods,²¹ the incomplete coverage may have to do with the polymer (polyvinylpyrrolidone, PVP) used to stabilize the nanorods prior to assembly. Recent results showed that the assembly of PVP-protected Au spheres also leads to similar arrangements (see Fig. 4b). The spectrum for these assemblies is shown in Fig. 3 for comparison, and clearly shows a larger shift as compared to Au@SiO₂–CNT composites because of the closer contact between Au spheres.

Fig. 4 TEM images of (a) Au nanorods (average aspect ratio 2.94) and (b) Au spheres (average diameter 15 nm) assembled onto MWNTs (average diameter 30 nm).

Although so far we have only discussed the attachment of nanoparticles on CNTs in solution, several possible ways of integrating these systems in real devices have also been demonstrated. On one hand, Au@SiO₂ have been assembled on aligned CNTs grown perpendicular to a Si substrate¹⁵ onto which ordered Ni nanodots were previously patterned using the nanosphere lithography technique.²² Optical studies of these ordered assemblies are still to be performed, but the coverage of the CNTs was very homogeneous, thus providing a high dielectric constant to these unique structures, so that they can be considered as new members within the family of metallodielectric photonic crystals.²³ On the other hand, CNT–nanorod composites have been embedded within polymer thin films and shown to serve for monitoring of the alignment of CNTs themselves upon stretching of the polymer film. Such sensing of orientation is based on the selective excitation of either the longitudinal or transverse surface plamon modes of aligned Au nanorods when polarized light is used.^20,24 Since the nanorods are preferentially oriented on the surface of the CNTs, illumination with light polarized parallel to the long axis will selectively excite the longitudinal plasmon mode, while using perpendicular polarization only the transverse mode will be excited. Apart from obtaining knowledge on the alignment of CNTs within polymers (of high relevance for reinforcement applications)²⁵ such nanocomposites may find applications in wave guiding or conducting films.

In summary, CNTs have been shown to constitute ideal templates for the formation of one-dimensional strings of metal nanoparticles, with potential uses as waveguides that would allow the miniaturization of devices below the diffraction limit²⁶ and as catalytic motors.²⁷ Given the universality of the LBL process, the procedure outlined here for metal nanoparticles has the potential to be applied for the creation of a large variety of one-dimensional nanoparticle assemblies. Depending on the composition and morphology of the nanoparticles used, the properties of these linear nanocomposites will also differ and thereby also the nature of their potential applications. To name a few, the use of magnetic nanoparticles should allow to control the alignment of CNTs in solution or on surfaces just by using external magnetic films, while the assembly of semiconductor quantum dots may be useful for the fabrication of novel solar cells or active displays. In fact, the ability to control the size, shape, loading, and dispersion of nanocrystals onto conductive nanotubes will be extremely relevant for addressing some of the fundamental issues in many practical applications including fuel cell catalysis, nanodevices, quantum wires, ultrahigh-strength engineering fibers, sensors, and catalyst supports.

Acknowledgements

This work was supported by the Spanish Ministerio de Educación y Ciencia and FEDER (Project No. MAT2004-02991). Contributions to the initial work by Prof. M. Giersig and Dr J. Pérez-Juste are acknowledged.

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