Nanotechnology: the stuff of science fiction or science fact?

Nanotechnology: the stuff of science fiction or science fact?

Small-scale science Nanotechnology: the stuff of science fiction or science fact? Diane Aston ABSTRACT This article discusses nanotechnology as a ro...

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Small-scale science

Nanotechnology: the stuff of science fiction or science fact? Diane Aston

ABSTRACT This article discusses nanotechnology as a route to the production of new materials and provides a brief history of the evolution of this branch of materials science. Properties on the nanoscale are compared with those on the macroscale. The practical application of nanomaterials in industries such as communications, construction, cosmetics, energy, medicine and textiles is discussed.

What is nanotechnology? Nanotechnology (or nanoscience or nanomaterials) is essentially the study of the design, production and application of materials on the nanoscale, that is materials that are on a scale of a few millionths of a millimetre (typically 1–100 nm in size). One nanometre (nm) is 10−9 m and about the same length as ten atoms in a line (a typical atom has a diameter of about 0.1 nm). In contrast, a human hair is about 80 000 nm thick. Rather than being a new subject in its own right, the nano-discipline is simply an extension of existing subjects (physics, chemistry, biology, engineering and materials science) but concentrates on the behaviour and use of substances on a tiny scale. When working on such a small scale, the building blocks are individual atoms and molecules, and macroscale phenomena such as inertia and turbulence are replaced by surface effects as the materials have a dramatically increased surface area to volume ratio. Nanoparticles are governed by van der Waals forces, atomic bonding (ionic, covalent and hydrogen bonding), electronic charge and quantum

effects, which means that they often display very different properties compared with the same materials on the macroscale. Some interesting examples of such contrast are given in Table 1. This is a fascinating area of research and has the potential to make an impact on our everyday lives in many different ways, yet some groups are concerned about the risks involved in working with materials on such a small scale as we know so little about how they will behave.

How long has nanotechnology been around? Nanotechnology isn’t really anything that new. Nanoparticles or nanostructures have existed in nature for millions of years. Enamel on teeth is made up from natural nanocrystals, some crystalline sponges have very efficient nanolenses, and it is the nanoscale forces generated between the minute hairs on a gecko’s feet that allow it to hang upside-down from seemingly smooth surfaces. Other natural nanomaterials include proteins that control and regulate biological systems, spider silk that is stronger than the equivalent-diameter steel wire, and waterrepelling plant leaves.

Table 1  A comparison of some properties of a few well-known materials Material Aluminium Copper Gold Platinum Silicon

Macroscale property Stable Opaque Solid at room temperature Inert Insulator

Nanoscale property Combustible Transparent Liquid at room temperature Reactive Conductor SSR March 2011, 92(340)

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Nanotechnology: the stuff of science fiction or science fact? Aston Scientists and engineers have been working with materials on this small scale for many years too. Stained glass and ceramics dating back to the tenth century have been found to contain coloured pigments based on nanoparticles of gold and silver – gold particles can appear red, blue or gold depending on their size. In more recent times, nanotechnology has been used to build large molecules from much smaller nanomolecules – we commonly call such large molecules ‘polymers’, which are now materials that we all take very much for granted. The microchips at the heart of our computers are made by processing silicon on the nanoscale and this technology has been around for some 50 years. The first time the concept of nanotechnology or nanoscience was distinguished from general or conventional science and engineering was in 1959, when Richard Feynman described how precise tools could be developed to manipulate individual atoms and molecules. However, it was not until 1974 that the term nanotechnology was defined by Norio Taniguchi of the Tokyo University of Science who described nanotechnology as ‘consisting of the processing, separation, consolidation and deformation of materials by one atom or one molecule’. The field really started to develop in the early 1980s with the birth of cluster science and the development of the scanning tunnelling microscope. In 1986, fullerenes such as C60 were discovered (I remember reading about the discovery of buckyballs in New Scientist when I was just 12 years old and I thought the whole concept was astonishing – perhaps that helped to inspire my passion for all things materials) and the synthesis of semiconducting nanocrystals was investigated, leading to the development of quantum dots (see Box 1). Nanotubes, nanowires and nanoparticles have now been developed and are under investigation for possible uses in many everyday areas, including construction, communication, medicine and cosmetics.

How are nanomaterials made? In the late 1800s, it was possible to synthesise nanoscale colloidal materials but nanostructures have only been practically possible since the late 1980s. Making and measuring nanomaterials has only become a reality as tools for working on such a small scale have been developed. The atomic force microscope and the scanning tunnelling microscope have allowed us not only to see but also to work with materials on the nanoscale. 72

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BOX 1  Joining the dots to light up the future Quantum dots can also be called semiconductor nanocrystals or even artificial atoms and they are tiny crystals whose size is on the nanoscale. A quantum dot has a typical diameter of 2–10 nm, or 10–50 atoms, and may contain 100–100 000 atoms in total. The term artificial atom arises because quantum dots have discrete energy levels in much the same way as atoms, and these energy levels can be changed and controlled by varying the size and shape of the quantum dot (i.e. how many actual atoms it contains). When light is shone on a quantum dot, the electrons within it become excited and emit light at a wavelength that depends on its size (Figure 1). The intensity of the fluorescence can be thousands of times brighter than that produced using conventional dyes.

Figure 1  As the quantum dot size increases, the colour of induced fluorescence moves towards the red end of the spectrum, and towards the blue end as the size reduces Quantum dots have excellent transport and optical properties and are being investigated for use in biological sensors, amplifiers and diode lasers. Until the blue quantum dot laser was developed, it was thought that using a blue laser for reading digital information was impossible. However, now such lasers are used in highdefinition Blu-ray players. In the future, quantum dots may allow solid-state quantum computing. It is also likely that quantum dots will have a role to play in medicine, particularly for imaging inside the body and perhaps detecting tumours.

There are two distinct approaches for making nanomaterials. In the ‘top-down’ method a bulk material is reduced in size to a nanoscale, whereas in the ‘bottom-up’ approach large structures are

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Nanotechnology: the stuff of science fiction or science fact?

built up or grown atom by atom to produce the nanomaterial or nanostructure. In top-down processes, scanning probes such as those found in the atomic force microscope and the scanning tunnelling microscope can be used to manipulate nanostructures, but the process is very slow. Nanolithography, electron beam lithography and nanoprint lithography have allowed the process of reducing bulk materials to the nanoscale to be achieved more quickly. Bottom-up technologies, such as chemical synthesis, self-assembly and positional assembly, are very slow as they involve building up large structures one atom or molecule at a time.

Where might nanomaterials be used? If you believe all you read then you may be under the impression that nanotechnologies could revolutionise every aspect of the way we live our lives. Nanomaterials have very distinct properties and their fabrication is becoming increasingly possible on a practical level. Nanomaterials have the potential to benefit many areas of our technology, including their use in the following. Communications

Nanotechnologies have been used in the communications industry for many years, with the materials being processed using top-down techniques. The gate length of transistors in central processing units (CPUs) and dynamic random access memory (D-RAM) devices is already on the nanoscale (50 nm or less). Nanostructures have been used to improve the data storage density of hard discs and to create non-volatile main memory for computers. Optical or optoelectronic devices are increasingly replacing traditional analogue electronic devices. Photonic crystals resemble semiconductors but use light or photons instead of electrons, and quantum dots are being used in the construction of lasers. Carbon nanotubes could be used in field emission displays that work in a similar way to cathode ray tubes but on a much smaller scale and with much lower energy consumption. Construction

Nanomaterials such as carbon nanotubes offer tremendous strength for their size. The use of these in composites instead of carbon fibres could allow much larger, lighter structures to be built. This could include less bulky suspension bridges

that could span larger gaps (for example, joining Europe and Africa across the Strait of Gibraltar). Cosmetics

Probably the most well-known use of nanotechnology in cosmetics is in sunscreen. Traditional chemical ultraviolet filters suffer from poor long-term stability and thus need to be reapplied at regular intervals. Nanoparticles of titanium dioxide show comparable ultraviolet protection to conventional screens, they last longer and they have the advantage that they are transparent so the wearer is not covered in white streaks. Nanoparticles are also used in creams that claim to penetrate deeper into the skin and slowly release vitamins or other agents to reduce the appearance of wrinkles. Energy

Nanotechnology can be used in the efficient production of ‘green’ energy and in reducing our overall energy consumption by increasing efficiency. Light-emitting diodes (LEDs) based on nanomaterials last much longer and are far more efficient than conventional light bulbs, which only convert 5% of the electrical energy to light. You may notice that many new sets of traffic lights use LEDs rather than conventional bulbs: as well as offering a tremendous energy saving, they produce a very bright light and require far less maintenance. The best solar cells currently available are only 30% efficient and commercially available systems are even less efficient, at only about 20%. Specially designed nanostructures have the potential to increase the efficiency of solar collectors dramatically and it may be possible to use coatings made from nanomaterials to turn every rooftop into a solar energy collector. A hot topic in terms of energy production at the moment is the hydrogen fuel cell. These could be used to generate electrical energy for use in the home or to power vehicles or even hand-held electronics gadgets such as mobile phones. One big drawback with these fuel cells is the storage of the hydrogen prior to its use and this is where nanotechnology could offer a practical solution. Nanoporous materials such as nanotubes, zeolites and alanates are possible candidates. In moreconventional engines, nanomaterials can be used as filters or in catalytic converters to remove pollutants from exhaust gases, and nanoparticles can be used as surface catalysts in combustion engines to increase efficiency. SSR March 2011, 92(340)

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Nanotechnology: the stuff of science fiction or science fact? Aston Medicine

Terms such as biomedical nanotechnology or nanomedicine have been used to describe the use of nanotechnology in the medical field. It is thought that nanomaterials will have a number of medical applications as they are of a similar size to most biological molecules. In imaging, nanoparticles can be used as contrast agents or markers. Quantum dots have been used instead of conventional dyes to watch blood flow in the tissues of mice. The images produced were detailed enough to be able to show the walls of the blood vessels rippling with every heartbeat. By adding antibodies or other molecules to the dots, it is possible to target them very specifically and image the results to label and track cells or even identify cancerous tissues. Nanoporous materials, nanoparticles or nanomolecules such as buckyballs or dendrimers may be used to encapsulate drugs and transport them to a particular site within the body. This highly selective approach means that the drugs are only deposited in the required area, reducing overall consumption, potential side effects and cost. Tissue engineering uses artificial materials as a support or scaffold for new tissue to grow on. These scaffolds can be made using suitable nanomaterials that are impregnated with cells that are artificially stimulated to grow. Many hospitals are already using tissue-engineered skin in burn victims as it eliminates the need for a graft that can in itself be very painful and lead to scarring. Tissue engineering is one way of reducing the risks associated with conventional artificial body parts or organ transplantation, but it is not without its own risks or controversy. It is closely linked with the debate over the ethics of stem cell research, as one way of obtaining the tissues which are used with the artificial scaffolds is from stem cells. Outside the body, nanotechnology can be used in medical diagnostics. When nanomaterials are used as tags or labels in biological tests to identify or measure the presence and activity level of a particular substance, the results can be obtained faster and they are more sensitive. Textiles

The innovative stain-repelling, crease-free clothes that are now available (particularly for school uniforms) rely on nanoscale surface coatings. These coatings are simply made from a polymer that has hydrophobic sections which orientate 74

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themselves perpendicularly to the textile surface and repel water, so that a spill will simply form beads on the surface rather than soak in.

What are the risks associated with nanomaterials? As this area of technology is still relatively new, no one really knows exactly what the associated risks are. However, the potential risks can be split broadly into three areas: l risks to our health and environment; l risks associated with molecular manufacturing; l risks to society.

It is the mobility and increased reactivity of nanomaterials that pose a potential risk to us. It is free nanoparticles (those that are not part of another material) that could be dangerous. We do not know what will happen if these are unintentionally inhaled or introduced into the body. Similarly, we do not know how the release of these materials into the environment will affect specific ecosystems. In terms of the risks to society, materials developed for domestic and industrial use will be subject to control at every stage of development. The use of materials for military purposes might be less carefully scrutinised. As with all new areas of research and technology, international bodies are in place to regulate these materials and oversee future applications.

What is the future of nanomaterials? Without a doubt, nanomaterials are a hot topic of research and discussion in the scientific and engineering communities. It is likely that these materials will continue to be introduced slowly and to an extent silently into our everyday lives and, as with most new materials, will improve the technology that we take for granted and rely on every day.

Where can I find out more information? A presentation on nanotechnology is available to schools that are members of the Institute of Materials, Minerals and Mining Schools Affiliate Scheme (see Websites). The Scheme aims to support the teaching of the materials-, mineralsand mining-related topics in the secondary school curriculum through the provision of support literature, newsletters, journals, conferences and interactive presentations. The author can be

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Nanotechnology: the stuff of science fiction or science fact?

contacted for more information, including on how to join the Scheme. There is a vast amount of information about nanomaterials and nanotechnology on the internet. When writing this article, I found the information on Wikipedia to be very useful indeed. You can also find out about nanotechnology on the How Stuff Works website, which has a great list of links to organisations such as NASA (looking at

nanogears). The Institute of Physics website has an excellent article on this topic. If you are looking for a paper resource that you can use in the classroom, the Wellcome Trust has produced a booklet on nanoscience as part of their Big Picture series. The Big Picture booklets are available from the Wellcome Trust website, where you can subscribe to receive one free copy of each issue, or purchase class sets for a small charge.

References

Websites

Feynman, R. (1959) There’s plenty of room at the bottom. Transcript at: www.zyvex.com/nanotech/feynman.html. Taniguchi, R. (1974) On the basic concept of ‘nanotechnology’. In Proceedings of the International Conference on Production Engineering, Tokyo, 1974. Part II. Tokyo: Japan Society of Precision Engineering.

How Stuff Works: howstuffworks.com. Institute of Materials, Minerals and Mining Schools Affiliate Scheme (SAS): www.iom3.org/sas. Institute of Physics: www.iop.org. Wellcome Trust: www.wellcome.ac.uk/bigpicture.

Diane Aston is the education coordinator for the Institute of Materials, Minerals and Mining, where she manages the schools and colleges education activities and runs the Schools Affiliate Scheme. E-mail: [email protected]

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