Periodic 3-D nanoparticle arrays by protein crystallization

Very small particles with diameters of only a few nanometres (a nanometre is a unit of length 1000 000 000 times smaller than a metre) can have properties quite different from bulk materials. If particles of this size are assembled to form a periodic array, that is one in which a simple arrangement of particles is repeated many times, electrical and magnetic interactions between the particles can further change the properties. This is what makes periodic arrays of nanometre-sized particles, known as nanoparticles for short, interesting.This project is to develop a new way of making 3-dimensional periodic arrays of nanoparticles, with novel and useful magnetic and optical properties. Many ways have been found to make 2-dimensional periodic arrays of nanoparticles, but making a truly 3-dimensional array, more than a few nanoparticles thick, is much more difficult. The approach we propose promises to be faster and more flexible than the current alternative, which is a purely chemical technique known as colloidal crystallization. Our method introduces elements of biology as well as chemistry, because we will synthesize nanoparticles inside proteins, then crystallize the protein. Since a protein crystal is a periodic array of molecules, and each molecule contains a nanoparticle, the result will be the desired 3-dimensional periodic array of nanoparticles.We will start by using the protein ferritin to make nanoparticles. The ferritin molecule is shaped like a hollow sphere and cells use it to store iron. We will synthesize magnetic metal and oxide nanoparticles for magnetic studies, and other metal nanoparticles for optical studies. We will find the best conditions for growing large crystals of ferritin with nanoparticles inside, and use very sensitive techniques such as scanning probe microscopy and dynamic light scattering to study the very earliest stages of array growth. We will change the symmetry of our 3-dimensional periodic arrays by changing the crystallization conditions and we will also study other proteins, including Dps, which has a similar structure to ferritin, but is used to protect DNA.We will measure the magnetic properties of our nanoparticle arrays at different temperatures and compare the results with computer simulations. This will help us gain a deeper understanding of how the magnetic fields of all the individual particles interact to determine the magnetic behaviour of the array as a whole. Understanding magnetic interactions is important for developing new materials for magnetic data recording as well as being of interest in itself.We will also measure how light is transmitted through and reflected from 3-dimensional periodic arrays of silver, gold and alloy nanoparticles. The fact that the period of the array will be much smaller than the wavelength of light makes these systems particularly interesting. Their study will contribute to the future development of exotic optical devices such as perfect lenses or shields that can make an object invisible.