Synthesis of non-spherical gold nanoparticles

Non-spherical gold nanoparticles such as rods (short, long) (1,2), wires, cubes (3), nanocages (4), (multi-)concentric shells (5), triangular prisms (6-7), as well as other more exotic structures such as hollow tubes, capsules (6), even branched nanocrystals (8-9) have garnered significant research attention in the past few years. They exhibit unique and fine-tuned properties which either strongly differ or are more pronounced from those of symmetric, spherical gold nanoparticles. Their unusual optical and electronic properties, improved mechanical properties and specific surface-enhanced spectroscopies make them ideal structures for emerging applications in photonics, electronics, optical sensing and imaging, biomedical labelling and sensing, catalysis and electronic devices among others (10,11,12,13,14,15,16,17,18). Furthermore, some of these anisotropic nanoparticles enable elucidation of the particle growth mechanism, which in turn makes it possible to predict and systematically manipulate the final nanocrystal morphology (8,1920). Finally, these anisotropic gold nanomaterials provide templates for further generation of novel materials (21,22). This article provides an overview of current research in the area of anisotropic gold nanoparticles. We begin by outlining key properties that they possess; we then describe how to control their morphology. Some of the most innovative synthetic strategies are highlighted together with an emphasis on recent results from our laboratories as well as future perspectives for anisotropic gold nanoparticles as novel materials. 1 Interests of anisotropic gold nanoparticles Non-spherical gold nanoparticles are known for their appealing optical properties. One of their most interesting optical features is the presence of multiple absorption bands correlated with their multiple axes (13,14,15,18). Such structures can support both propagating and localized surface plasmons. For instance, gold nanorods possess two different resonance modes (in contrast to the single one developed for spherically symmetric gold particles which is relatively independent of size). These two separate modes are due to electron oscillation across and along the long axis of the nanorod and are termed the transverse and longitudinal modes, respectively. The latter of which is extremely sensitive to the aspect ratio of the rod (resonance shifts by approximately 100 nm for a change in aspect ratio of 1 unit). Additional weaker bands corresponding to quadrupoles may also be observed. Nanoshells composed of dielectric cores coated with thin metal layer(s) have similar properties to spherical gold nanoparticles in the sense that they also exhibit a single surface plasmon resonance (SPR) absorption. Nanoshells, however, allow tuning of the SPR across the visible and infrared region over a range of wavelengths spanning hundreds of nanometers, far exceeding the spectral range of spherical particles. Moreover, for branched nanocrystals, nanocubes or nanocages, SPR absorption is shifted to longer wavelengths relative to the ordinary green 520 nm resonance of spherical particles. For example, triangle-branched nanocrystals exhibit a blue color. The plasmon resonance has three bands, corresponding to the two in-plane or longitudinal surface plasmon absorptions at longer and shorter wavelengths and one out-of-plane or transverse plasmon absorption band. The plasmon bands are very sensitive to the length and sharpness of the tips. Thus, those nanostructures also have the advantage of being tunable in their optical properties. In contrast to nanorods, nanoprisms or nanoshell, however, their plasmon resonances cover a lower spectral range. In summary, a key feature of the non-spherical nanoparticles is that their optical properties vary dramatically with their physical dimensions. The ease of tuning their optical properties gradually with particle size and shape makes them very interesting when compared to traditional organic dye molecules. In contrast to gold nanospheres, their resonance frequency is tuneable over a wide range from blue to nearinfrared and enables one to set the SPR to a wavelength or spectral region specific to a particular application (Figure 1). For instance, the resonance of anisotropic nanoparticles can be positioned in the ‘water window’ in the near infrared (8001300 nm), where absorption by biomatter is low. Together with the high degree of biocompatibility of gold, these structures show potential in a wide variety of biological applications (optical labels for biosensing events and biomedical labelling) (1,10,11,16,17). It has also been Gold Bulletin 2008 • 41/2 195 Gold Bulletin 2008 • 41/2 196 hundredfold at the tip (“lightning rod” effect). The greatest electromagnetic field has been observed at the ends of isolated nanorods or nanowires, compared to spheres, leading to enhanced SERS activity (25,26,27). Another interesting feature of anisotropic nanorods is that their interaction, when assembled, produce huge field enhancement at the nanorod junctions leading to SERS hot spots. Together these features make non-spherical gold nanoparticles promising for all chemical sensing spectroscopies; not only SERS, but also surface-enhanced fluorescence (SEF). The ability to precisely control the interactions between nanoparticles is a powerful tool towards new applications. Other structures, such as tiny supported anisotropic gold nanoparticles, have been found to have outstanding potential as catalysts. Supported ultra-small gold particles play an important role in a number of industrially relevant reactions such as pollution control, chemical processing, sensors and fuel cell technology (12,28). It is well established that their original catalytic properties are greatly dependent on the particle size (< 5 nm). Shape control over gold nanoparticles may also modulate catalytic activities. Anisotropic nanostructures exhibit different crystal surfaces; they have different fractions of atoms located at different corners, edges and at different defects. Thus, the catalytic efficiency is expected to be different in catalyzing the same reaction. Studies on the catalytic activities of different anisotropic platinum nanoparticles showed pronounced differences for the same electron transfer reaction in colloidal solution (29). For gold, investigations regarding shape dependency on activity and selectivity are still in their infancy (30). This speculated that they could play a role in future cancer diagnosis and therapy (4,5). Furthermore, the ability to engineer gold nanoparticles with plasmon resonances in regions of the electromagnetic spectrum appropriate to a particular application also shows potential in fields such as optics, electronics, information technology and in fact all applications which benefit from localized heating (1,18,23). Plasmons indeed play a key role in these different areas. (Self-)organization of such different nanostructures also opens the door to new methods of studying novel chemical and physical properties arising from collective interactions of the constituting units. It might provide new classes of photonic and electronic devices that control and manipulate light at the nanometer scale, as well as generating, transporting and detecting digital information for homeland security, environmental science, high-speed data communication, and computing components (24). In addition to photon absorption, heightened surfaceenhanced Raman scattering (SERS) also arises from anisotropic gold nanoparticles. This technique can detect traces of complex molecules, including hazardous agents with molecular specificity. Most experiments were carried out on rough metal films with poorly defined optical properties. However, with gold nanoshells or nanorods for example, the plasmon resonance can be tuned to the excitation of common laser radiation sources optimizing the electromagnetic enhancement mechanism. Moreover, anisotropic particles have highly curved, sharp surface features, with dimensions less than 100 nm which lead to an increase of the localized electromagnetic field up to one Figure 1 Range of plasmon resonance of gold nanoparticles as a function of their morphology Au nanocube