Manufacture and use of nanomaterials: current status in the UK and global trends

Abstract

This paper provides an overview of the production and use of nanomaterials (NMs), particularly in the UK. Currently, relatively few companies in the UK are identifiable as NM manufacturers, the main emphasis being the bulk markets in metals and metal oxides, and some niche markets such as carbon nanotubes and quantum dots. NM manufacturing in the UK does not reflect the global emphasis on fullerenes, nanotubes and fibres. Some assumptions have been made about the types of NM that are likely to be imported into the UK, which currently include fullerenes, modified fullerenes and other carbon-based NMs including nanotubes. Many university departments, spin-offs and private companies have developed processes for the manufacture of NMs but may only be producing small quantities for research and development (R&D) purposes. However, some have the potential to scale up to produce large quantities. The nanotechnology industry in the UK has strong R&D backup from universities and related institutions. This review has covered R&D trends at such institutions, and appropriate information has been added to a searchable database. While several companies are including NMs in their products, only a few (e.g. manufacturers of paints, coatings, cosmetics, catalysts, polymer composites) are using nanoparticles (NPs) in any significant quantities. However, this situation is likely to change rapidly. There is a need to collect more information about exposure to NPs in both manufacturing and user scenarios. As the market grows, and as manufacturers switch from the micro- to the nanoscale, the potential for exposure will increase. More research is required to quantify any risks to workers and consumers.

Background

Nanotechnology is a broad interdisciplinary area of research, development and industrial activity that has been growing rapidly worldwide for the past decade. It is a multidisciplinary grouping of physical, chemical, biological, engineering and electronic processes, materials, applications and concepts in which the defining characteristic is one of size. It involves the manufacture, processing and application of materials that are in the size range of up to 100 nm. Nanotechnology can be considered to comprise the following four broad areas:Owing to their extremely small size, NMs have a much greater surface area than the same mass of materials at the micro-scale. At this scale, quantum effects are more important in determining the properties and characteristics of the material. This has led to the development of novel materials with distinctly different properties compared to their conventional forms. Of particular interest are nanoparticles (NPs), considered to be substances that are <100 nm in size in more than one dimension. They can be spherical, tubular, irregularly shaped and can also exist in fused, aggregated or agglomerated forms.

• nano medicine,

• nano fabrication,

• nano metrology and

• nanomaterials (NMs)/nanoparticles (NPs).

The nanotechnology sector has achieved a multibillion US$ market, and is widely expected to grow to 1 trillion US$ by 2015. NPs are already used in a variety of consumer products, such as TiO₂ in paints and ZnO in sunscreen products. A number of other applications are also on the market or in the pipeline, for example for targeted drug delivery, gene therapy, cosmetics, stain-resistant coatings, self-cleaning glass, industrial lubricants, advanced tyres and semiconductors.

Other more ambitious uses of NPs are also being projected, such as in bioremediation of contaminated environments. The rapid growth of nanotechnology suggests that it will not be long before a variety of new pharmaceutical, electronic and other industrial uses of NMs are found. Such proliferation of nanotechnology has also prompted concerns over the safety of NMs when released into the environment, although at the moment concerns are mainly focused on issues surrounding occupational health and worker safety at manufacturing premises. These concerns have been most clearly expressed in the 2004 review carried out by the Royal Society and Royal Academy of Engineering (RS/RAEng) [1].

Nanoparticles

The development of new NPs is a rapidly progressing science. For example, a special edition of the Journal of Materials Chemistry published 47 papers concerning the development of new NMs including metallic NPs, germanium, ceramic and aluminium oxide nanowires, carbon, silicon and germanium nanotubes, zinc oxide nanocrystals, gold nanowafers and copper oxide nanocubes [2]. A comprehensive overview of the whole science of NPs, including theory, synthesis, properties and applications, has recently been published [3]. NP types have also been reviewed from a health and environment perspective [4]. Particle morphology has been used as basis for categorizing NPs. Table 1 summarizes the main categories of NP according to their morphologies, material composition and the type of application [5].

Fullerenes

Fullerenes are one of four types of naturally occurring forms of carbon, first discovered in the 1980s [6]. The molecules comprise entirely carbon and take the form of a hollow sphere or a tube. They are similar in structure to graphite, which comprises a sheet of hexagonal carbon rings, but contain pentagonal or heptagonal rings that enable three-dimensional structures to be formed. The smallest, the first discovered, is carbon-60 known as buckminsterfullerene and sometimes called buckyballs. Fullerenes are produced in small amounts naturally in fires, but were first observed in the soot resulting from ablation of graphite with a laser. Since then many other processes have been used to produce them including arcing of graphite, combustion of hydrocarbons, thermal and non-thermal plasma pyrolysis of coals and hydrocarbons and thermal decomposition of hydrocarbons [7]. The approach used most commonly for the production of commercial quantities is based on carbon electric arc.

Nanotubes

Carbon nanotubes (CNT) are a particular form of fullerene, first reported by Iijima [8]. They are similar in structure to carbon-60 but are elongated to form tubular structures, 1–2 nm in diameter. They can be produced with very large aspect ratios and can be >1 mm in length. In their simplest form, nanotubes comprise a single layer of carbon atoms arranged in a cylinder, known as single-wall carbon nanotubes (SWCNTs). They can also be formed as multiple concentric tubes (multi-wall carbon nanotubes) with diameters up to 20 nm and length >1 mm. CNTs have great tensile strength and are considered to be 100 times stronger than steel, while being only one-sixth of its weight, making them potentially the strongest, smallest fibre known. They also exhibit high conductivity, high surface area, unique electronic properties and potentially high molecular adsorption capacity [9]. Applications currently being investigated include polymer composites (conductive and structural filler), electromagnetic shielding, electron field emitters (flat panel displays), super capacitors, batteries, hydrogen storage and structural composites.

A major focus of current research on nanotubes is on scaling-up production rates to kilogram (or greater) quantities because many of the applications require bulk quantities. Nanotubes have also been produced from other materials including silicon and germanium.

Nanowires

Nanowires are small conducting or semiconducting NPs with a single crystal structure and a typical diameter of a few tens of nanometres and a large aspect ratio. They are used as interconnectors in nanoelectronic devices. Various metals have been used to fabricate them, including cobalt, gold and copper. Silicon nanowires have also been produced. Most approaches to their fabrication are derived from methods currently used in the semiconductor industry for the fabrication of microchips, typically involving manufacture of a template and deposition of a vapour to fill the template and grow the nanowire.

Quantum dots

Quantum dots are small (2–10 nm) assemblies of metal, metal oxide or semiconductor materials with novel electronic, optical, magnetic and catalytic properties. Quantum dots (sometimes referred to as artificial atoms) are considered to be neither an extended solid structure nor a single molecular entity. Various methods can be employed to manufacture them, the most common being wet chemical colloidal processes. Most research has centred around semiconductor quantum dots, as they exhibit distinct ‘quantum size effects’. The light emitted can be tuned to the desired wavelength by altering the particle size through careful control of the growth steps.

Other NPs

This category includes a wide range of spherical or aggregated forms of NP, for example ultrafine carbon black and fumed silica, which are synthesized in bulk form by flame pyrolysis methods. NPs of this type may be formed from many materials including metals, oxides, ceramics, semiconductors and organic materials. The particles may be composites having, for example, a metal core with an oxide shell, or alloys in which mixtures of metals are present. Many of the production processes involve the direct generation of aerosols through gas-phase synthesis, similar to flame pyrolysis, but other production processes including wet chemistry methods and attrition methods may be used. This group of particles is less well-defined in terms of size and shape, generally larger (although still within what would be considered NP range), and more likely to be produced in larger bulk quantities than other forms of NP.

Manufacture and use of NMs in the UK

A recent study reviewed the available information about companies in the UK that are manufacturing and/or using NMs in their products [10]. The review considered information from a range of published and web-based sources and relevant companies' literature and industry forums, but did not consider companies that were clearly not NM manufacturers or users (i.e. were dealing with larger sized materials), pharmaceutical developers, drug delivery companies, equipment or instrument manufacturers and suppliers, silicon chip manufacturers or consultants providing advice on product development. It used a matrix approach to identify a vast array of applications for each type of NM (and vice versa, NMs for each application), based on many of the present classes of NMs and their current application areas (Table 2). The applications and NMs identified were those generally described in the literature, as exact specific details and the most up to date information for each NM application were not always available.

The study identified a total of 53 companies that were involved in manufacturing, processing and/or using NMs in the UK in 2005. Some of these companies were also undertaking research and development (R&D) activities into the development of new NMs or new applications. The main findings were that:

• The majority of NM manufacturing and use of NMs occur in the United States (49%), with the European Union responsible for 30% and the rest of the world accounting for the remaining 21%. Within the European Union, the UK accounts for nearly one-third of the market.

• Following the global trend in the new technology, UK industry and research institutions have established themselves with substantial investments and funds from government and other initiatives.

• The UK NM manufacturing industry was still very much at the development stage. In 2005, only 19 companies could be identified as NM manufacturers. Another 11 were processing NMs and 22 were using or developing applications for NMs.

• The main emphasis of NM manufacturing industry in the UK has been the bulk markets in metals and metal oxides, as well as some niche markets such as quantum dots, and does not reflect the global emphasis on fullerenes, nanotubes and nanofibres. No commercial manufacturer of fullerenes was identified in the UK. It was likely that fullerenes, nanotubes and fibres were being imported into the UK, but this had not been confirmed.

• The main NMs currently produced in the UK include nanopowders (metals, metal oxides and alloys), magnetic NMs, CNTs, nanoceramics, nanosilica, quantum dots (metal and semiconducting nanocrystals), polymer composites containing NMs and thin films.

• A number of companies were processing or formulating NMs in their products. However, only a few of them (e.g. manufacturers of paints and coatings, cosmetics, catalysts, polymer composites, etc) were using NMs in any large quantities. With more emerging applications and markets for NM-based products, this situation is likely to change in the future.

• The main current applications of NMs in the UK were in catalysts, lubricants and fuel additives, paints, pigments and coatings, conductive inks and printing materials, cosmetics and personal care products (e.g. sunscreens), drug delivery/bionanotechnology, functional coatings (e.g. on glass, textiles), hydrogen storage and fuel cell applications, nanoelectronics and sensor devices, optics and optic devices, security and authentication applications, structural (composite) materials, therapeutics, medical and dental and uv-light absorbers and free-radical scavengers.

Another study identified further potential applications of NMs for the sectors involving construction materials (including coatings inside drinking-water pipes), detergents, food processing, paper manufacturing, agrochemicals and plant protection products, plastics (including food packaging) and weapons and explosives [11].

In addition to commercial manufacture, processing and R&D, a further 55 non-commercial organizations were undertaking nanotechnology-related R&D in the UK. These included university departments, spin-offs and private companies, who had developed processes for the manufacture of NMs but may only have been producing small experimental quantities. However, some of these had the potential to scale up their processes to produce large quantities.

Several cross-departmental (or cross-university)centres have also emerged in the nanotechnology area. From the available information, it is clear that these institutions have enormous R&D capability across the whole range of manufacturing processes. In several cases, this entire capability range was available within a single university. Most of the universities were, however, involved in the development and prototyping of new processes and materials rather than undertaking commercial manufacture.

Selected examples of UK companies involved in NM production or applications include:

• Metal Nanopowders Limited, a spin-off company from the University of Birmingham, who produce a range of nanometallic powders for applications such as catalysis, paints and hydrogen storage.

• Nanoco Technologies, a spin-off company from the University of Manchester, who produce quantum dots for security, authentication and anti-counterfeiting applications.

• Nanomagnetics Limited who produce magnetic media for use in the manufacture of tapes and discs.

• Nanotecture Limited, a spin-off company from the University of Southampton, who produce nanoporous metals and metal oxides, for applications such as in power sources, sensors and filtration.

• Oxonica Limited, a spin-off company from the University of Oxford, and is one of the UK's leading suppliers of nanodots. Their products have a range of applications such as catalysis for diesel fuel, life sciences, environmental sciences, IT and telecommunications, pigments and printing, polymers, ceramics and cosmetics. For example, Oxonica's cerium oxide-based diesel fuel catalyst (Envirox™) has been adopted by Stagecoach New Zealand across its entire bus fleet, and their manganese-doped titanium dioxide-based uv-absorber and free-radical scavenger (Optisol™) are used in sunscreen products.

• Thomas Swan & Co. Limited, who are the UK's leading commercial producer of CNTs, and have scalable industrial level production facilities and capabilities for functionalization of nanotubes.

• QinetiQ Nanomaterials Limited who produce nanopowders (metals, metal oxides) for a wide range of applications in paints, textiles, printing, automotive, electronics, petroleum and defence.

• Pilkington plc who produce self-cleaning glass with a 5-nm coating of microcrystalline titanium oxide, which reacts to daylight and breaks down any grime, which then cleans when in contact with water.

• Tetronics Limited who offers NM manufacture processes based on plasma arc systems for the production of NMs such as nanopowders.

Global trends

Of the definitive NMs that are presently being applied in commercial applications, the nanoscale metal oxides (e.g. TiO₂, iron and aluminium oxides, etc), various nanoscale polymers and polymeric nanocomposite materials are those being manufactured and applied in the greatest quantities (i.e. kilograms to tons). However, rapid scale up of fullerene production has also been reported; the Mitsubishi Corporation has opened a large-scale fullerene production plant in Japan (Frontier Carbon Corporation), which uses a combustion-based process that, according to a recent press release, will increase production of fullerenes from 4 to 40 tons/year by 2005. This claim neither has been verified nor is it presently clear where such vast quantities of these fullerenes will be applied, given the current limited nature of commercial markets for them. A new process called high-pressure carbon monoxide has been developed for multi-kilogram scale production of SWCNTs, and has been licensed to a US-based company, Carbon Nanotechnologies Inc.

While the worldwide NM manufacturing landscape is presently inundated with small technology start up companies, large volume manufacturing of high-quality NMs will most likely occur at larger multinational companies such as DuPont, BASF or Mitsubishi Chemical (Forbes/Wolfe, 2004).

Currently, it is difficult to predict accurately the specific long-term global R&D trends of the nanotechnology industry. What is clear, however, is that since R&D is intrinsically reliant on access to the materials of interest, near- and long-term R&D trends and ultimately successful commercial applications will favour those NMs that can be produced in large quantities—i.e. it is difficult to research or develop new applications for materials that are not readily accessible. For example, one of the most broadly applied NMs, TiO₂, which is used in numerous commercial products from sunscreens to paints, can be readily manufactured on a large-scale. Conversely, CNTs, which are more difficult to manufacture (and even more difficult to replicate from batch to batch or from one manufacturer to the next) have not yet encountered such commercial success, though their commercial potential is enormous. In fact, only in recent years have scientists managed to keep nanotubes from aggregating into clumps in aqueous suspension—a tendency that also has obstructed the utility of many other NMs in commercial applications. Thus, near-term R&D trends will focus on optimizing and refining manufacturing processes for NMs of presently limited production capability.

Manufacturing processes used to fabricate some NMs have matured more rapidly than for other nanoscale materials. In general, the manufacturing of nanoscale metals, polymers, silica, clays and ceramics involves relatively mature processes capable of generating large quantities of materials.

R&D trends are also likely to focus on applications of NMs to socio-political priority areas tied to large-value commercial or public sector markets such as human health, energy, defence and the availability of clean water. Nevertheless, availability of materials, funding, social priorities and market value are all factors that ultimately will work together to dictate direction of near- and long-term R&D worldwide. Recognizing that future trends are reliant on the interaction of many complex factors that are difficult to predict, particularly for such an immature and evolving industry, near-term nanotechnology R&D trends can clearly be expected to emerge in the following areas:These primary areas represent the intersection of several important variables including high market value, high social priority and availability of relevant materials.

• Human health: drug delivery, imaging, cancer therapeutics;

• Defence: energetic materials, lightweight armour composites;

• Energy: hydrogen storage, improved efficiency, catalysis;

• Agriculture: increased crop yields, secure packaging, chem/bio-detection and

• Environment: water filtration, reduced air emissions, remediation, chemical and biological sensing.

One of the most active current trends of the nanotechnology industry is the application of NMs in chemical and biological sensing, due to their high reactivity, good selectivity, small size and high specific surface area [12]. This research has included the use of various nanoscale materials ranging from semiconducting SWCNTs for electrochemical sensing to quantum dot-based fluorescent probes for optical sensing. The near-term intensity of participation in this research has been fuelled by the fact that only relatively small quantities (milligram to gram) of materials are required to explore many sensing applications. More importantly, however, national governments have provided enormous financial resources to develop advanced, nanotechnology-enabled sensing instrumentation to aid the global war on terror. There are also suggestions that, within the next 10–20 years, the best opportunities for broad scale application of NMs are in health care and electronics. However, the energy generation and storage sector are expected to have a more near-term impact in the commercial market-place within 3 years.

Exposure to NPs

As with all chemical manufacturing processes, production of NPs may give rise to exposure by inhalation, through the skin and by ingestion. Exposure may also occur for workers in downstream processes that use these materials and to consumers as these products enter the market-place.

In gas-phase production processes, exposure by inhalation may be caused by direct leakage of the reactants or products into workplace air. For all production methods, product recovery, subsequent processing and cleaning may result in the generation of airborne NPs. It is, however, probable that these downstream activities will not generate discrete NPs, due to the relatively high energies, which would be necessary to break the forces which keep particles agglomerated. However, exposure to agglomerated NPs may also pose a significant risk to health.

All the production processes described could potentially result in dermal exposure, particularly at the powder handling, packaging and bagging stages. It has been postulated that NP exposure to the skin may result in direct penetration, although currently there is little evidence to support this. Several pharmaceutical companies are, however, developing drug delivery systems based on topical application NPs. Dermal exposure is likely to result in ingestion exposure from hand-to-mouth contact.

Typically, airborne exposures in the workplace are assessed in terms of mass concentration. Current evidence suggests that the most appropriate metric for exposure by inhalation for NPs is surface area. This appears to fit best with current toxicological evidence relating to mechanisms of harm. It would also address directly the issue of agglomeration. Ideally a personal sampler should be available which could assess this metric. However, none currently exists. For those NPs that could be considered as fibres, such as CNTs, particle number may be a more appropriate metric than surface area.

In any case, it is also necessary to consider characterizing exposures against aerosol mass and number concentration until further information and improved methods are available. For each of these exposure metrics, but particularly in the case of mass concentration, size-selective sampling will need to be employed to ensure only particles within the relevant size range are sampled.

For dermal exposure, measurements should also be biologically relevant. At this stage, there is insufficient evidence to indicate whether mass, number or surface area is the most appropriate metric. Measurement approaches should ideally also consider the skin area exposed and the duration of exposure. For ingestion exposure too, measurements should also be biologically relevant. There is insufficient evidence to indicate what is the most appropriate metric.

Information about the exposure of workers to NPs is very limited. None has been identified about exposures in the university/research sector or in the new NP companies in the UK. Only one study, carried out in the United States, has evaluated NP exposure, in this case CNTs [9]. Very limited information is available on existing chemical, pharmaceutical and refining companies. That which is available uses either mass or number as an exposure metric, rather than surface area. The number-based estimates are derived from static samplers rather than personal samplers. Information from other powder-handling processes indicates that exposures may be significant.

There is a clear need to collect more information about exposure to NPs in both manufacturing and user scenarios. Production quantities are still relatively small. As products grow in both number and volume, and as manufacturers switch process from the micro- to the nanoscale, the potential for exposure will clearly increase. Much more information is needed to quantify any risks to workers or consumers.

Conflicts of interest

None declared.

The authors are grateful to the Department for Environment, Food and Rural Affairs, the Environment Agency and the Health & Safety Executive for providing funding for the review.

References