Since first extracted in the 5th century, Gold has been regarded as one of the most important matters in the world. When divided into smaller fragment sizes, way below 100nm it becomes even more ‘precious’. The optical, catalytical and electronic properties of Gold nanoparticles differ greatly from those of their bulk counterparts. This is mainly due to the large surface area-to-volume ratio as well as the spatial confinement of the free electrons of Gold nanocyrstals.
Gold at Bulk scale
Noble metals such as Gold, silver and platinum exhibit plasmonic properties. When the surface of a noble metal is hit with incident light, electrons which are situated on the surface begin to oscillate. This is known as the surface plasmonic resonance (SPR), as illustrated in figured 1.
Metals are able to absorb and reflect light with great efficiency to their SPR. It is because of these plasmonic properties that noble metals such as Gold, silver and platinum are widely used in jewellery. Moreover being highly reflective metals makes them very appealing to the eye. Plasmonic properties arise due to noble metals having delocalised electrons on the surface. An atom consists of protons, electrons and neutrons. The nucleus is made up of protons and neutrons and the electrons spin around the nucleus in different orbitals. There are many orbitals in metals which overlap and form metallic bonding between the atoms. In the bulk form, there are many delocalised electrons within the metal that cause metallic bonding. This in turn allows the electrons to flow freely between the nuclei. Good electrical and thermal conductivity is due to the delocalised electrons
Gold at Nanoscale
Reflection does not occur in particles which are smaller than the wavelength of the incident light; however there is still an interaction between the nanoparticles and the light. The two main interactions are light scattering and light absorption. Electrons tend to oscillate at the same frequency as the light which was absorbed causing a dipole moment around the nanoparticles where all the electrons on the surface of the nanoparticle are oscillating. The oscillating dipole is known as a localised surface plasmon resonance (LSPR). Electromagnetic radiation is released when these electrons are oscillated. This can be seen in figure 2. The secondary electromagnetic radiation released by the nanoparticles is called scattered light. The electromagnetic radiation that is released has the same frequency
History
The synthesis of colloidal gold, or nanogold as it is now called, has been known to man since the ancient times. Although the process was not fully understood, synthesis of colloidal gold was crucial to the 4th century Lycurgus cup. The Lycurgus cup was known to change colour depending on the direction of light. Later it was used as a method for staining glass.
A potion made from gold, which was also known as an Elixir of Life was discussed, and may also have been manufactured, in ancient times. It was not until the 16th century that the alchemist Paracelsus, claimed that he had created a potion called Aurum Potabile. (latin: potable gold).
It was in the 17th century that the glass-colouring process was refined byAndreus CassiusandJohann Kunckel, allowing them to produce a deep-ruby coloured form of glass. However modern scientific evaluation was first made by Michael Faraday in the 1850s. Faraday is said to have been inspired by previous work done by Paracelsus. In 1857 Faraday prepared the first pure sample of colloidal gold, which he called ‘activated gold’, in 1857. Phosphorus was used to reduce a solution of Gold chloride.
For a very long time chemists were unclear about the composition of the Cassius ruby-gold. Several chemists suspected Cassius ruby-gold to be a gold tin compound due to its preparation, However it was Faraday who was the first to recognize that the colour was actually due to the minute size of the gold particles
The first colloidal gold in solution was first prepared in 1898 by Richard A. Zsigmondy.
Shape and size tuning
Fine tuning of shape and size in a controlled environment is one of greatest challenges faced by material scientists. These factors are not only very important in the rational design of nanomaterials, but are also equally as important for their applications. This is because many of their catalytical, optical and electronic properties of nanomaterials depend greatly on their size and shape. In Gold nanorods the longitudinal plasmon wavelength exhibits a nearly linear dependence on their aspect ratio, making it one of the most intriguing properties possessed by Gold nanorods. Moreover, even when looking at Gold nanorods with the same aspect ratio, the plasmon resonance properties are strongly dependent on the shape of their head.
Over the past five years, the ability to finely tune the shape and size of Gold nanorods, has made huge progress.
Seed mediated growth
Gold nanospheres
Changing the diameter of the sphere can easily and effectively tune the optical properties of gold nanospheres. This can easily be shown using the Mie Theory, which has been successfully shown to model the light scattering and absorption properties of spherical particles on a nanometer scale.

The intense peak in the spectrum for each particle can be seen in figure 9. This peak is caused by the oscillating electrons which in turn produce a single dipole. Increasing the diameter of the gold nanosphere results in shifting the peak to a higher wavelength, this indicates the interaction between the light and particles is changing. In comparison to bulk gold (figure 4), the optical properties of gold nanospheres differ immensely.
The local medium can also change the optical properties of the particles. This is due to the LSPR of the particle interacting with the medium. A change in the medium can result in a measurable change in the optical properties of the particles. In addition shape of the gold nanoparticles can significantly alter the optical properties.
Gold nanorods
Gold nanorods are of great interest due to their biocompatibility and NIR ( near infra-red) optical properties. The shape of the gold nanorod is what determines its NIR properties. The non-spherical shape of the particle causes two different dipoles to form when interacting with light. The transverse dipole (diameter) and the longitudinal dipole (length) account for the oscillating electrons throughout the particle. The dipole interactions affect the optical properties of the particles.

GNR LSP illustration
Optical properties of GNRs

The aspect ratio (A.R) is defined as the length over the diameter of the Gold nanorod (GNR). The shape of the particle causes the absorption and light scattering spectrum to have two peaks: longitudinal and transverse. The interaction between these two dipoles causes the longitudinal peak to be observed in the visible to NIR region. The longitudinal peak is much more intense than the transverse peak and can be tuned by changing the aspect ratio of the gold nanorods. As the A.R of the gold nanorods is increased, the longitudinal peak shifts into the NIR region.
The optical properties of GNRs have been successfully modelled using Gans theory and Discrete Dipole Approximation (DDA). Two equations were derived using the two theories to output the longitudinal peak wavelength by inputting a given aspect ratio.

GANS THEORY
DDA

The two equations can predict the A.R of GNRs using absorption data collected from the UV-Vis-NIR spectrophotometer. The equations can give preliminary estimation of the A.R before observing the particles on a TEM. Figure 11 displays the absorption data, estimated A.R from theory, and a visual representation for a set of GNRs with different aspect ratios. The tunable optical properties of the GNRs make them very desirable for a wide range of applications. The visible representation shows a colour shift as the A.R changes.

Visual representation of different shapes and sizes of GNRs
Normalised absorption of the particles