Building microstructures on nano-sand
There is a popular truism that implores the architect not to build his house on sand. But, like so much else in science, what is true for the macro scale does not necessarily hold true on the nano scale. In this case, sand (or silica, to be technical) is an extremely versatile surface on which to anchor and build complex microstructures.
My PhD research has concentrated on using a technique called Living Radical Polymerization (LRP) to build novel nano structures out of polymers (plastics). We can use LRP to make polymer chains linear, branched, hyper branched, circular or a whole raft of other shapes. Control over the architecture of the polymer chains allows us to control the properties of the final material; for example, I recently made some polymers that self-assemble into hairy nano-spheres when immersed in water (see Figure 1).
Figure 1: Self-assembly of poly(butyl acrylate)-poly(ethylene glycol) chains in water
Using LRP in conjunction with silica opens up a range of possibilities for advanced nano-engineering. We start with silica nanoparticles about one thousandth the diameter of the human hair. Using an anchoring LRP group, we can start growing polymers on the surface of the particles – pretty much like growing hair on a perfectly bald head. The polymers can be functionalised: for example we can make the hair anti-bacterial or we can make it change shape when it gets too warm or when it gets wet. In short we can form a microstructure on the silica nanoparticle with properties of our choosing.
The chemistry is so precise that the polymers grow in a very controlled fashion and we can decide how long we want the individual hairs to be. The level of control means that the microstructures arrange themselves in a very uniform pattern (see Figure 2). Because they are so tiny, they can trap light and appear translucent – like a synthetic opal. We were quite excited to see this effect and are investigating the use of our silica-polymer hybrids as photonic crystals that could one day form the basis of a new type of optical computer or fibre-optic cable. It’s not quite a house built on sand, but the possibilities of this type of nano-engineering are still quite exciting.
Figure 2: Pattern formed by the self-assembly of silica-polystyrene hybrid microstructures