A method for manufacturing gold nanoparticles could hasten their commercial adoption in the medical and electronics industries.
Nanostructured gold particles and films have found novel uses in drug delivery systems and advanced diagnostics, as well as photovoltaics and traditional electronic devices.
The advantages of the element are that it is relatively inert and biocompatible and it demonstrates interesting optical properties in terms of the propagation of so-called surface plasmons.
However, there is a limit to the size and intricacy of the nanostructures that can be built with gold — imposed by the width of the laser used to etch them.
‘The physical properties of nanoscale objects are intimately tied to their dimensions,’ said Dr Paul Warburton of the London Centre for Nanotechnology.
‘So let’s suppose you want to optically excite this gold particle by shining light on it, well gold particles of a 30nm radius respond at a completely different wavelength of light to gold particles at a 40nm radius. So you can tune your nanoparticle to respond to light of a certain wavelength, which — if you’re trying to target radiation to some cancerous cell or something — could be important.’
The researchers’ solution to the size problem involved focusing a laser beam onto the surface of a thin gold film that was locally melted, causing droplets to bounce up from the surface — akin to a pebble dropped into a pond.
This effect, though, requires a specific geometry of the surface substrate. The team uses rows of gold G-shaped lines measuring 200nm in thickness that have chiral symmetry, allowing them to focus the laser light. It meant that the team could make 40nm gold particles that were 20 times smaller than the wavelength of the laser light, which caused the melting to happen.
‘You’re exciting nanoscale currents in these thin films of gold,’ said Warburton. ‘The laser photons excite these electrons in the gold; that’s essentially what a plasmon is — coupling between the light and electrons — then the electrons flow in the gold and, much the same way as when you put a current through the element in your domestic kettle, it gets hot.’
In the future, the manufactured nanoparticles could be purified and then adapted for use in applications such as drug delivery, or alternatively frozen in place and used to fabricate electronic circuits.
‘One could imagine using a comparatively conventional microfabrication technique to get you down to say a micron-type length scale, then writing with the laser to give you something at a much finer length scale,’ said Warburton.
‘This is a general problem in the electronics sector: how do you write circuits with features that are much smaller than the wavelength of the light you are using to write the circuit in the first place?’