How small is a gold nanorod and what are they good for?
Gold nanorods are single crystals of gold that are typically about 15 nm in width and 50 nm in length. For a better visualization, Figure 1 compares the thickness of a human hair (20 micrometer thick) to gold nanorods that are about 300 times smaller in length. These nanocrystals are typically made in water, and can be considered as colloidal particles.
At this size regime, gold nanocrystals can interact with the electromagnetic radiation (light) to absorb and scatter the incoming light. That’s why their colloidal solutions are colorful. Absorption and scattering of light by nanorods have made them excellent optical materials for:
1- Photothermal cancer therapy
2- Optical tomography
3- Obscurant materials
4- Optical coatings for better energy management
Origin of their color: Plasmon resonance
Like other metals, gold has a large number of electrons in its conduction band. The collection of these electrons, in crystals smaller than 100 nm, is susceptible to an external oscillating electric field such as the electromagnetic rays in the visible and infrared. The electron cloud can initiate a coherent oscillation in a nanorod with a certain frequency, which is also called a plasmon resonance frequency. This means a nanorod is polarizable in its two different directions upon an optical excitation resulting in two resonance frequencies (a.k.a. absorption wavelengths) in the visible or infrared. Interestingly enough, for a spherical gold nanoparticle, since their dimensions are about the same, as seen in Figure 2, there is only one plasmon resonance frequency in their spectrum; while for nanorods a second peak appears at a longer wavelength and a stronger magnitude. The wavelength of the lower energy band in nanorods can be customized by tailoring its dimensions.
Light absorption and heat release
The rapid oscillation of electrons in a nanorod results in a number of collisions such as electrons to electrons or electrons to the gold lattice. These events result in an energy transfer of electrons to the lattice causing an increase in its heat content, which then is followed by heat dissipation of gold lattice to the surrounding. Therefore nanorods convert part of the absorbed light to heat.
Because of their extraordinary large electron polarizability, nanorods have become the nanocrystals of choice in photothermal therapies where heat release is used to annihilate the ill cells of a tumor.
Light scattering and electric-field enhancement
The rapid oscillation of electrons also results in an electric filed on the surface of the gold nanorods. In this case they act like small antennas and transmit photons with energy similar to those of the incoming light (Rayleigh scattering). This property in gold nanorods and nanospheres has been used in chemical sensing to probe the chemical composition of their immediate vicinity. The generated strong electric field on the surface of nanorods has also been proposed for enhancing the light absorption in photonic devices or solar cells.