In my first post I mentioned that one of the key advantages to nanomaterials is that their properties depend directly upon the size and shape of the particles. This becomes important in developing new technologies as the material can be tailored to suit a specific task by changing the size and shape of the nanostructures.
Current manufacturing cuts and stamps things to order. We cut and etch silicon wafers for electronics and we mould plastics for casings. In some ways this is like trying to make a house out of a cave. To a large extent it works, you can drill and dig new chambers to make new rooms and put in plumbing for hot showers etc. However, you are always limited by the tools you have to cut and dig with, and the size of the cave. People have always found it easier to build houses from the ground up and changing the size and shape to fit the future occupants (two bedrooms and one bathroom for a one-child family, sixteen rooms for several generations under one roof).
In a very general sense, this is what nanotechnology promises, the means to put molecules together into larger structures or grow particles to specific sizes fit to our needs. The key to achieving this is to understand the processes of self-assembly. This is exactly what it sounds like, putting molecules together in the right conditions so that they may spontaneously arrange themselves into a larger structure or pattern that we can use for various purposes. A larger structure made when individual molecules are put together like building blocks is called a “supramolecular structure.”
A key element of such a structure is that the smaller molecules are NOT chemically bonded together. If you picture a jig-saw puzzle, the pieces all fit together to make your picture of (let’s say) the Houses of Parliament. You’ll notice it is not necessary to glue the pieces together but that the notches and bumps one the sides of each piece allows them to stay connected when fit correctly. In nature, molecules and atoms experience all sorts of interactions and attractions that make them want to stick together without the need for the more extreme process (the transfer of electrons) of forming a chemical bond.
One example of such a connection is the hydrogen “bond“. This is best explained if we take a look at a water molecule as in the image below (original source here).
Most of science can be described by the fact that Mother Nature is lazy. Atoms and molecules always want to be in the state or position of lowest energy. They don’t like to be bouncing off the walls or doing stuff, they don’t want to be promoted at work and have more stress and responsibilities, they want to be chilled out on the couch and doing as little as possible. They will also do anything to try and get into this lowest energy state, which scientists often call the most stable state, maybe because the ‘couch potato state’ sounded bad. In general, atoms form chemical bonds because they find that together they have to do less i.e. have lower energy than on their own.
In the water molecule an oxygen atoms has determined its best chance for stability is to hook up with two hydrogen atoms (hence the chemical name H2O). When the bond is formed the hydrogen atoms share their electrons with the oxygen atoms and this combined pool of electrons are now buzzing about all three atoms like a cloud of bees. However, the negatively charged electrons are attracted to positively charged protons in each atom. The much larger oxygen atom has 8 protons and the hydrogens atoms only one each, in other words the oxygen atom has most of the honey and more bees (electrons) will swarms around the oxygen atoms than the hydrogen atoms. This effect makes the region surrounding the oxygen atom more negative due to the greater number of swarming electrons, and the end where the hydrogen atoms are more positive due to the lack of electrons. The difference is small, but enough to have effects on how the molecules behave, and is shown in the diagram where blue = negative and green = positively charged regions of the cloud.
Hydrogen bonding occurs when several of molecule with this subtle charge imbalance are put close together. Positive attracts negative and so the negatively charged ends of a molecules will be attracted to the positive ends of a neighbouring molecule. In some cases, depending on how fast the molecules are moving and how strong the charge difference is, this attraction may be enough to let the ends of molecules stick to each despite not actually forming a chemical bond the way that the atoms did in forming the molecule. For example the hydrogen ends of a water molecule can ‘stick’ to the oxygen end of another water molecule but not in the same way that the hydrogen atoms are attached to the oxygen atoms within each molecule. In fact, this is what happens when water becomes ice and it determines the shape of ice crystals such a snow flake.
Back the the nanotechnology, the image below is from the website of the nanoscience group at the University of Nottingham. This shows a Scanning Tunnelling Microscope image (another technique that allows scientists to take pictures of individual atoms) of a hexagonal network made from hydrogen-bonded self-assembly. The scientist, Dr Luis Perdigao, put a rod-shaped molecule (the grey ovals) and a triangular molecule onto a flat surface (just visible as darker edges to the grey molecules). He then heated the molecules to give them the energy to skim around the surface and mix themselves up then let it cool. As the molecules cooled they began to get slower and started to feel this hydrogen bonding effect start to stick the molecules together. The end of the rods were attracted to the edges of the triangle to make a joint that was similarly shaped to the letter Y. When lots of these joints stick together they make a pattern resembling a honeycomb (back to the bees again!) . You can try it yourself by drawing a Y and then drawing another one, the same size, next to it with the arms touching. Then draw a third Y, this time with the two arms touching the leg of each of the previous Ys. You should have a hexagonal space in the middle and if you keep going with that pattern, you’ll end up with a honeycomb.
This is a very simple example of self-assembly, which is still some way from being developed into a new technology. However, you can already see the next step which is to use the honeycomb pattern as a template or mould to arrange other molecules. In this case Dr Perdigao used the open spaces to arrange clusters of Buckminster Fullerene molecules (buckyballs, the white spheres in the image). It is possible that patterns of fullerenes like this could be sued to develop new computers. However, it demonstrates the potential for self-assembly. Self-assembled patterns like this can be seen as a foundation allowing us to put other molecules into precise arrangements suited for specific functions. These devices would built layer-by-layer from the ground up, using single molecules as building blocks much like the houses built to fit different families mentioned earlier. This vision of future manufacturing is called ‘bottom-up’ processing.
I hope that provides an adequate introduction to the concept of self-assembly. I hope to add some posts about other possible applications of self-assembled nanostructures, how they can be used in new technologies now and not in 20 years time and how biology got there first.
