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Nanotechnology and Nanoscience The Royal Society

Nanotechnology: views of Scientists and Engineers

Nanomaterials

a) What would you add to/change about the working group’s definition of nanotechnology?

The group were generally happy with both definitions. The following points were made:

Nanoscience:

  • It was noted that colloid science covers a very similar dimension range. Care is needed to distinguish what is new about nanoscience compared to much of chemistry
  • The upper and lower size limits should be fuzzy: for example photonic crystals go up to 300nm; atom manipulation is sub 1 nm nanoscience.
  • The definition does not directly acknowledge sub-atomic or quantum phenomena.

Nanotechnology:

  • This should be expanded to include the word ‘systems’, eg a functional set of components – a combination of nano (possibly more than one aspect) and meso technology will be essential in practice.
  • This definition implies that nanotechnology will always follow on from nanoscience (‘the application of this knowledge’) whilst in practise both nanoscience and nanotechnology will take place simultaneously, and perhaps technology before the ‘science’. This is not unique to nanotechnology: there are many examples of complicated & well-developed technologies where the underpinning science is not well understood.
b) What is the current state of knowledge in the field of your breakout group, and where is research going?

i) Definitions by dimensionality
It is convenient to classify nanomaterials in terms of the dimensionality of the nanostructures involved. Thus
3-D: (confined in 3 dimensions) e.g. quantum dots, particles, precipitates, colloids, catalysts, etc.
2-D: (confined in 2-D, extended in 1-D) e.g. nanotubes, fibres, interconnects/wires, fibrils, etc.
1-D: (confined in 1-D, extended in 2-D) e.g. surface coatings, thin films, device junctions (diodes etc.), interfaces etc.

1-D nanosystems are generally well understood and technologically advanced. Atom scale control is already possible for devices and coatings by various deposition techniques.
2-D systems are moderately understood in terms of properties, but manufacture is much less advanced.
3-D nanosystems provide the greatest challenges in terms of both properties and controlled manufacture. This is also where the dramatic increase in surface area, and hence chemical activity, is most evident.

A general remark – the electrical transport properties across interfaces remain poorly understood in terms of science/predictive capability. This affects all nanomaterials.

ii) Biology related
Considerable synergy exists between biostructures and nanomaterials. This is an active area of opportunity.

For example much research has been undertaken on using cells as scaffolding on which to build clusters – bio- to inorganic templating. Similarly, templated inorganic surfaces affect cellular function.

c) What applications of this technology currently exist, and what can be envisaged in the short and long term?

i) Sunscreens
These typically contain both physical and chemical components. There is currently a limited understanding of which the most important wavelengths are that need to be protected against. Once this understanding increases it is hoped to develop tuned systems.

Little is known about the fate of sunscreens, including their metabolism, adsorption through skin, and where they end up in the environment.
Enhanced DNA repair enzymes are currently being incorporated into cosmetic formulations.

The driver for the use of nanoparticles in sunscreens was actually cosmetic: viscous, white pastes were not popular so the particle size needed to be lowered in order to overcome this. One obvious side effect was the dramatic increase in surface area. Free radicals are thought to damage skin, and are generated by the surface of the nanoparticle components. Two methods have been devised to overcome this problem of particle activity. One involves coating the particles – however, it is difficult to achieve an even coverage. More efficient is the use of p-type (rather than n-type) semiconductor materials. These can be produced by doping the original materials prior to mixing them with the other components of the sunscreens.
Free radical damage has been shown with in vitro models, but it is considerably harder to demonstrate in vivo.

ii) Self-cleaning windows
These involved the use of highly activated titanium dioxide, engineered to be highly hydrophobic, very anti-bacterial, utilising n-type doped materials.

iii) Nanocrystalline alloys
One example of how properties can significantly change on the nanoscale is the strength of materials. For example, nanocrystalline materials become considerably stronger as the particle size decreases. Nanocrystalline nickel is as strong as hardened steel, and nanocrystalline aluminium alloys can be up to twice as strong as their conventional equivalents. The nanostructuring can be made in two main ways: electroplating and consolidation from powder. Electroplating is preferable to consolidation as it does not involve the use of any ultrafine particles. The latter involve processing difficulties and possible hazards.

iv) Micromachined silicon sensors
There are a number of mass produced, cheap, efficient micromachined sensors (such as accelerometers) used in cars, cameras etc. Whilst these do not necessarily do anything completely new, they are smaller allowing either more sensors or freeing up space to allow miniaturisation. They are also easily intergrated with electronics.

v) Field-emission array displays
It was thought that a large electronics company intends to launch a 28”x70” screen on the market in 2004 or 2005 that is based on field-emission arrays rather than quantum dots. They are based on a ‘bucky paste’ that contains a volatile solvent, and nanotubes/particles. During annealing the solvent is evaporated and the nanocomponents self-assemble. The result is a screen with lower power consumption and heat generation.

vi) Films and coatings
There are a large number of coatings and films that are 1-d nanomaterials. Examples include:

  • Multiwell quantum well lasers
  • Smart materials such as self-cleaning surfaces
  • Super hard coatings (some are carbide based)
  • GMR read heads on PC hard drives. Magnetic nanostructures
  • Coatings on fibre optic cables and other devices

vi) Environmental drivers
New alternative sources of energy are one potential driver for nanomaterials. Applications include fuel cells, nanomaterials with better electrical and mechanical properties, low cost photovoltaics and (perhaps) utilising carbon nanotubes to store hydrogen. Fraser Armstrong, (Oxford), has been working on bio-fuel cells.

vii) Biomaterials
Bristol based company Nanomagnetics are currently manufacturing magnetic nanoparticles via a biological route. Research has also been undertaken on using yeast to produce similar materials.

viii) Bioremediation
Another possible application for nanomaterials is bioremediation. The driver would be to design systems capable of fixing heavy metals, PCBs, cyanide and other environmentally damaging materials.

ix) Improving understanding existing technologies
Nanoscience and nanotechnology have provided improved analytical tools that can be used to increase our understanding of existing technologies. For example, it is now possible to study biomolecules by trapping them in nanostructures making x-ray crystallography possible. The probe microscopies (esp. AFM) provide rapid, easy surface characterisation for thin film manufacture.

d) What are the potential hold-ups in turning research into products? What is needed (time, money etc) to enable this process to happen?

i) Hazards
There are three stages during which the hazards of any material need to be assessed:

  • In production
  • In use
  • In disposal

There are a number of hazard-related issues associated with the production of nanomaterials:

  • Very fine powders can be pyrophoric, so good housekeeping is required. Electrodeposition or melt spinning can be used to avoid these problems.
  • Fluid based systems have many advantages, not least that they avoid free fine-scale particles. However, they generate the problem of how to deal with the disposal of the manufacturing process feedstocks. This problem is not unique to nanomaterials, but is still a serious issue.
  • Lessons might be learnt from microbiological aerosol production where there are many well-established practices to deal with fine particles.

ii) Quantum dots
There is currently no reliable method of producing quantum dots of the same size reproducibly, either by lithography or the colloidal route. This is a significant limitation for their wider application. In contrast quantum lasers are a possibility as they require fewer dots to function.

iii) Electron transport across material junctions
The long standing problem of the Schottky barrier / Ohmmic barrier, and understanding exactly how electrons transfer at material junctions, causes difficulties for nano- as for conventional materials. Alloy-based nanomaterials will often be made in a trial and error approach to obtain the required electrical properties. A related issue is a lack of understanding of how charges get into and out of cells, which has huge importance to bio-nanomaterials.

iv) Protein misfolding
It is not known how nanoparticles will interact with proteins. There is the possibility of causing protein misfolding, which is known to sometimes have damaging effects.

v) Empirical understanding of complex many particle systems
Much of our current understanding of multiphase systems is empirical at best. This knowledge has been built up over many years of chemical engineering and physical chemistry studying ‘real’ systems such as food. Surfactant and colloid science has only started to fully utilise DLVO theory on the attractive and repulsive forces between particles in the past 10-15 years although the theory’s key papers were published by Derjaguin and Landau in 1941 and Vervey and Overbeek in 1948.

It was noted that ‘real’ systems are significantly more complicated than model ones, where our understanding is better. Impurities will influence surface properties, even at very low levels. There is clearly a need for nanoscience study of particle surfaces and interfaces.

vi) Toxicity issues
Experience from other materials (e.g. asbestos) shows that it is not necessary to have a complete understanding of precisely how a new material might affect people in order to determine regulation. Rather, it is essential to know what the dose response is. It is not sufficient to assess the properties of the individual chemical components that make up nanomaterials, as the toxicity will depend on the particle size, particle concentration and surface area.

A huge number of fine particles are produced from a wide range of other existing processes (car exhausts, masonry etc) that will/may have an impact on human health. Nanoparticles are not new! The bio-interactions of fine particles are not well-known, for example whether the particles can pass through skin or go into cells. It is also not known how the toxicity will vary from either the bulk or molecularly dispersed material. It can be postulated that nanoparticles might have a greater toxicity than a molecularly dispersed form of the same material due to either the combination of particle shape and size (cf silicosis).
Peter Dobson, University of Oxford, is involved in organising an EU workshop during summer 2004 on the direct absorption of fine particles through the skin.

It is also important to find out where nanoparticles go once they enter the body via different routes, and ultimately where they go inside cells. This might be achieved by studying luminescent tagged systems.

vii) Lessons from asbestos
Whilst there is not a full understanding of the mechanisms that make asbestos toxic, there is sufficient understanding to prevent significant harm to people. It is known that the size and shape of the particles is important, although the influence of exact shape, durability, solubility and surface chemistry, are not well understood.

e) What are the science 'fictions' in this field?

i) Public perceptions
There is a perception that things that are too small to be seen are somehow in the realm of science fiction. It is important for scientists to engage with the public about nanoscience and nanotechnology to inform them about the opportunities and risks

For example, the use of clean rooms in the fabrication of nanomaterials can be a cause of disquiet amongst the public. Clean rooms are often associated (e.g. in movies) with highly infectious diseases and radiation, and not the life improving technologies such as integrated circuit manufacture, where they are mostly actually used.

Smart materials and self-replicating materials are often confused. This distinction is important and needs to be made clear. ‘Self-replicating’ may cause alarm, with erroneous visions of intelligent nano-machines.

ii) Exaggerated proposals from scientists
Scientists making exaggerated claims about their research and in research proposals can do considerable damage. These claims fuel public perceptions that nanotechnology is either ‘science fiction,’ or dramatic and radical, and therefore risky.

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