| The prefix of nanoscience and nanotechnology derives from the unit of length, the nanometer, and in their broadest definitions these terms refer to the science and technology that derives from being able to assemble, manipulate, observe and control matter on length scales from one nanometre up to 100 nanometres or so. One nanometer is a billionth of a metre or one thousandth of a micrometre, sometimes called a micron, which in turn is one thousandth of a millimetre. It is abbreviated to 1 nm. These numbers can be put into context by observing that a medium-size atom has a size of a fraction of a nm, a small molecule is perhaps 1 nm, and a biological macromolecule such as a protein is about 10 nm. A bacterial cell might be up to a few thousand nanometers in size. The smallest line width in a modern integrated circuit, such as would be found in a fast home computer, is a few hundred nm. The Difference Between Nanoscience and Nanotechnology We should distinguish between nanoscience, which is here now and flourishing, and nanotechnology, which is still in its infancy. Nanoscience is a convergence of physics, chemistry, materials science and biology, which deals with the manipulation and characterisation of matter on length scales between the molecular and the micron size. Nanotechnology is an emerging engineering discipline that applies methods from nanoscience to create products. What is Special about Nanoscience? The laws of physics operate in unfamiliar ways on these length scales, and this is important to appreciate for two reasons. The peculiarities in behaviour imposed by the nanoscale impose strong constraints on what is possible to design and make on this scale. But the very different behaviour of matter on the nanoscale also offers opportunities for structures and devices that operate on radically different principles from those that underlie the operation of familiar macroscopic objects and devices. For example, the importance of quantum effects could lead to highly novel computer architectures - quantum computing - while the importance of Brownian motion and surface forces leads to an entirely different principle for constructing structures and devices - self-assembly. Differences in the Way Physics Operates at the Nanoscale Key differences in the way physics operates at the nanoscale include: Quantum Physics On small length scales matter behaves in a way that respects the laws of quantum mechanics, rather than the familiar Newtonian mechanics that operates in the macroscopic world. These effects are particularly important for electrons. One example arises from Heisenberg’s uncertainty principle, which states that we cannot know accurately and simultaneously the position and momentum of a particle. If we confine an electron by reducing the dimensions of a metal or semiconductor particle, then its energy has to increase, in effect to compensate for its spatial localisation. This means that confinement can be used to modify the energy levels of electrons in semiconductors, to create novel materials whose optoelectronic properties can be designed to order. Brownian Motion Submicron particles and structures immersed in water are subject to continuous bombardment from the molecules around them, causing them to move about and internally flex in a random and uncontrollable way. If we expect nanomachines to work according to the principles of macroscopic engineering, Brownian motion imposes strong constraints on the stiffness of the component materials and the operating temperatures of the device. In the view of many scientists this renders impractical some radical proposals for nanodevices which consist of assemblies of molecular-scale cogs and gears. On the other hand, some biological nanodevices, like molecular motors, are clearly not subject to these constraints, because their mode of operation actually depends in a deep way on Brownian motion. Surface Forces Surfaces and interfaces play an increasingly important role for particles or structures as they are made smaller. A variety of physical mechanisms underlie the forces that act at surfaces (at a macroscopic scale, the surface tension that allows a water beetle to walk on water is an example of one of these), but the overall effect is simple; small objects have a very strong tendency to stick together. This stickiness at the nanoscale, and the accompanying strong friction that occurs when parts are made to move against each other, are an important factor limiting the degree to which microelectronic mechanical systems (MEMS) technologies can be scaled down to the nanoscale. These phenomena also underlie the almost universal tendency of protein molecules to stick to any surface immersed within the body, with important consequences for the design of biomedical nanodevices. All About Self-Assembly Although the combination of Brownian motion and strong surface forces is sometimes thought of as a problem that nanotechnology must overcome, these features of the nanoworld in fact combine to offer a remarkable opportunity to exploit an approach to fabricating devices peculiar to the nanoscale. If molecules are synthesised with a certain pattern of sticky and non-sticky patches, the agitation provided by Brownian motion can lead to the molecules sticking together in well-defined ways to make rather complex nanoscale structures. The key to understanding this mode of assembly – known as self-assembly – is that all the information necessary to specify the structure is encoded in the structure of the molecules themselves. This is in contrast to the methods of directed assembly that we are familiar with at the macroscale, in which the object is built, whether by a tool-using human being or by a machine, according to some externally defined plan or blueprint. The attraction of self-assembly as a route to creating nanostructures is that it is parallel and scalable - the number of structures created is limited only by how many molecules are put in. This is in contrast to the serial processes that are familiar at the macroscale, in which objects are created one at a time. Bottom-Up Production Self-assembly is an example of an approach to making nanostructures which is often referred to as ‘bottom-up’ nanotechnology. This term indicates approaches which start with small components – almost always individual molecules – which are assembled to make the desired structure. Bottom-up nanotechnology does not necessarily involve self-assembly. An alternative, but much less well developed, realisation of a bottom-up approach uses scanning probe microscopes to position reactive molecules at the desired position on surfaces. Top-Down Production In the opposite approach – ‘top-down’ nanotechnology – one starts with a larger block of material and by physical methods carves out the desired nanostructure, as you would make a statue from a block of marble. Top-down nanotechnology is a natural extension of current methods of microelectronics, in which structures of very limited dimensions are created by laying down thin layers of material and etching away those parts of each layer that are unwanted. Bottom-up Production Techniques in Cell Biology The epitome of bottom-up processing technologies is provided by biology. Nanoscience is thought of as a physical science, but cell biology operates on exactly these length scales. The nanoscale devices that carry out the functions of living cells – the ribosomes that synthesise new proteins according to the blueprint provided by DNA, the chloroplasts that harvest the energy of light and convert it into chemical fuel, the molecular motors that move components around within cells and which in combination allow whole cells and indeed whole multicellular organisms to move around – are all precisely the kinds of machines imagined by nanotechnologists. Cell biology offers a proof that at least one kind of nanotechnology is possible. What interactions, then, are possible between nanoscale science and technology and biology? Synthetic Molecular Devices in Biology Biology can provide lessons for nanotechnology. Long eons of evolution have allowed the perfection of devices optimised for working in the unfamiliar conditions that prevail at the nanoscale, and careful study of the mechanisms by which they work should suggest designs for synthetic analogues. This may lead to the design of synthetic molecular motors, selective valves and pores, and pumps that can move molecules around against concentration gradients. Nanotechnology Will Provide New Tools and Methods for Biology Nanoscience and nanotechnology will also make substantial contributions to biology by providing new tools and methods. This has already started to happen, with single molecule methods allowing the properties of biological macromolecules to be probed one at a time, and the use of fluorescent nanoparticles to tag and track the motion of particular macromolecules and structures. There will be an increasing demand for these sorts of tools. When the complete genome of an organism is known, and one knows the complete set of proteins present in it (the proteome), then to disentangle the complex webs of interaction that convert a sack of chemicals into a living organism will become the major challenge. There will also be a demand for cheaper and faster ways of characterising organisms – a physically based instrument for directly reading the sequence of a strand of DNA would be very valuable, and is likely to be one of the outcomes of nanotechnology as applied to biology. Combining Synthetic and Natural Components to Make New Structures Biological components could themselves be incorporated into man-made nanoscale structures and devices. It is already feasible to incorporate biological molecular motors into artificial structures, and the light harvesting complexes of plants or photosynthesising bacteria can be incorporated into synthetic membranes. It is easy to imagine building up complex nanomachines by combining synthetic and natural components, an approach referred to as bionanotechnology. |