Gold and silver nanostructures of controlled shape

Valerie A. Gerard and Yurii K. Gun'ko

DOI: 10.1039/9781849737623-00001

Plasmonic properties of metallic nanoparicles are strongly dependent on the size and shape of these nanomaterials. Therefore significant efforts have been directed towards controlling the synthesis of new shaped and anisotropic metal nanostructures. This review presents some recent years advances in the production and potential applications of shape-controlled gold and silver nanostructures and their assemblies. Many interesting synthetic routes have been developed for a very wide variety of particle morphologies. These include: seed-mediated aqueous synthesis of gold nanoparticles of defined shape; preparation of 1D gold and silver nanostructures (e.g. nanowires); photo-chemical synthesis of silver nanostructures; synthesis of anisotropic gold nanostructures using galvanic replacement; preparation of silver nanostructures by cluster aggregation and some other. We also consider and discuss some recent advances in production and uses of gold and silver based nanoparticle assemblies. One of the point of interest is the fabrication of tailor-made nanostructures for specific applications. SERS is presented as an example for it benefits immensely from optimisation of plasmon resonance through shape-control. Over last few years there also were significant advances in the development of novel chiral plasmonic nanostructures. It is expected that these new chiral nanomaterials could find a range of potential applications in new plasmonic devices and chiral sensing.

1 Introduction

Gold and silver nanoparticles have been widely investigated for their unique optical and electronic properties enabling potential applications in biological sensing, catalysis, drug delivery energy harvesting and optoelectronics amongst others. Their small sizes enable them to achieve electronic confinement, which combined to their metallic nature results in unique properties. In bulk metals, the valence and conduction bands are not separated by any energy gap, a property responsible for their high conductivity but also for the appearance of surface plasmons. These are the collective oscillation of electrons that occur in a metal subjected to an external electromagnetic field. At the interface of the metal with a dielectric medium, surface plasmons can arise from interaction with light and propagate as an evanescent wave, or polariton, along the surface until the energy is dissipated through absorption by the metal or scattering. In the case of metal nanoparticles the surface area is so large that all plasmons can be seen as polaritons as opposed to the free plasmons occurring in bulk metal.

With nanoparticles being much smaller than the wavelength of visible light, it can be considered that all the electrons confined in a given nanoparticle are subjected to the same field at any given time; this is referred to as the quasi-static hypothesis. Displacement of the electron cloud by the electric field results in the creation of surface charges as represented in Fig. 1. The electrons can thus be assimilated to an oscillator which may be described by the Maxwell equations. Consequently, conditions may be found for which the electron cloud can resonate.

That collective oscillation of electrons creates a strong enhancement of the electromagnetic field in the vicinity of the nanoparticles upon photo-excitation. This in turn is the basis for many applications. It has been successfully employed to enhance Raman signals in Surface Enhanced Raman Spectroscopy (SERS). Raman spectroscopy is a characterisation technique based on the inelastic scattering of photons by certain molecules. The photon sets the material into vibration and is scattered with a different frequency, or energy, and that shift is measured. SERS arises from the presence of metal nanoparticles on the substrate. The SPR-induced enhancement of the electromagnetic field in the vicinity of the surface strongly enhances the vibrational modes. A charge-transfer complex may also be formed between the surface and the analyte molecule, with electronic transitions in the visible range. It should be noted that the SERS effect does not occur on smooth surfaces, but only on rough, nanoparticulate ones. Although initially developed with spherical nanoparticles, SERS has been proven to be influenced by the particle size and shape. The optimal signal enhancement is obtained with the appropriate combination of analyte and nanoparticle. Furthermore, there is a growing interest for gold nanoparticles in the fields of drug delivery and biological imaging. They are indeed a biologically inert material which also provides opportunities for functionalization as well as plasmonic imaging. They can also be triggered by an external stimulus to release their cargo or produce heat.

Since the position and intensity of SPR are sensitively dependent on the size and shape of the metallic nanoparticle, most efforts have been directed towards controlling the synthesis of new shaped and anisotropic metal nanostructures. This review will present some recent years advances in the production and potential applications of shape-controlled gold and silver nanostructures and their assemblies.

2 Recent developments in the synthesis gold and silver nanostructures of controlled shape

Nanospheres, nanoplates, nanoprisms, nano-cubes, nanooctohedra, nanopyramids, nanorods and nanowires are some examples of the shapes that have been produced from gold. In the colloidal synthetic route, it is generally accepted that ligands play a crucial role in directing the growth by preferentially binding to one crystal face or another. More control has recently been gained by investigating the influence of other parameters. Also, other synthetic routes such as electrochemical growth and layer-by-layer template synthesis have been developed further.

2.1 Seed-mediated aqueous synthesis of gold nanoparticles of defined shape

Most shapes of gold nanostructures can be obtained via the reduction of a gold salt in the presence of an appropriate surfactant. A variety of geometrically complex high-index faceted structures, such as bipyramids,...