Ron Hester is an emeritus professor of chemistry at the University of York. In addition to his research work on a wide range of applications of vibrational spectroscopy, he has been actively involved in environmental chemistry and was a founder member of the Royal Society of Chemistry’s Environment Group. His current activities are mainly as an editor and as an external examiner and assessor on courses, individual promotions, and departmental/subject area evaluations both in the UK and abroad.

Roy Harrison OBE is Queen Elizabeth II Birmingham Centenary Professor of Environmental Health at the University of Birmingham. In 2004 he was appointed OBE for services to environmental science. Professor Harrison’s research interests lie in the field of environment and human health. His main specialism is in air pollution, from emissions through atmospheric chemical and physical transformations to exposure and effects on human health. Much of this work is designed to inform the development of policy.

Nanotechnology

Consequences for Human Health and the Environment

By R. E. Hester, R. M. Harrison

The Royal Society of Chemistry

Copyright © 2007 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-216-6

Contents

Current and Future Applications of Nanotechnology Barry Park,
Nanoparticles in the Aquatic and Terrestrial Environments Jamie Lead,
Nanoparticles in the Atmosphere Roy Harrison,
Toxicological Properties of Nanoparticles and Nanotubes Ken Donaldson and Vicki Stone,
Human Effects of Nanoparticle Exposure Lang Tran, Rob Aitken, Jon Ayres, Ken Donaldson and Fintan Hurley,
Nanoparticle Safety – A Perspective from the United States Andrew D. Maynard,
Subject Index, 133,

CHAPTER 1

Current and Future Applications of Nanotechnology

BARRY PARK

1 Introduction

1.1 History

Physicist Richard P. Feynman first described the concept of nanoscience in 1959 in a lecture to the American Physical Society and the term nanotechnology was coined in 1974 by the Japanese researcher Norio Taniguchi to describe precision engineering with tolerances of a micron or less. In the mid 1980s, Eric Drexler brought nanotechnology into the public domain with his book Engines of Creation.

1.2 Definitions

As part of a major report commissioned by the UK Government from the Royal Society and the Royal Academy of Engineering in the UK, entitled “Nanoscience and nanotechnologies: opportunities and uncertainties”, the following definitions were used:

Nanoscience is the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a larger scale.

Nanotechnologies are the design, characterisation, production and application of structures, devices and systems by controlling shape and size at nanometre scale.

The NASA website provides an interesting definition of nanotechnology: “The creation of functional materials, devices and systems through control of matter on the nanometre scale (1–100 nm) and exploitation of novel phenomena and properties (physical, chemical, biological) at that length scale.”

The Oxford English Dictionary defines nanotechnology as “technology on an atomic scale, concerned with dimensions of less than 100 nanometres”.

The prefix nano- derives from the Greek word for dwarf and one nanometre is equal to one billionth of a metre i.e. 10-9 m. Nanomaterials are therefore regarded as those that have at least one dimension of size less than 100 nm.

1.3 Investment

Nanotechnology has received very significant investment over the past ten years with national governments providing the bulk of this investment with estimates ranging as high as $18 billion for investment between 1997 and 2005. There has recently been a four-way split with similar investment in each of USA, Europe, Japan and the rest of the world with approximately $3 billion spent by governments in 2003 alone. In the USA, for example, the National Nanotechnology Initiative (NNI) is a federal R&D program to coordinate the multi-agency efforts in nanoscale science, engineering and technology.

The President’s 2007 budget provides over $1.2 billion for the Initiative, bringing the total investment since the NNI was established in 2001 to over $6.5 billion and nearly tripling the annual investment of the first year of the Initiative. With this investment has come a large number of products, some of which are already on the market, that are based on nanotechnology or contain nanomaterials.

2 Technology

2.1 Nanomaterials

There had already been exploitation of products of particle size falling within the definition of a nanomaterial prior to these developments, but the products were simply referred to as ultrafme or superfine. These products, mainly comprising metal or metalloid oxides and carbon blacks, were primarily additives for the plastics industry in its various guises and these will be considered in some detail as they comprise the greatest body of current applications of nanotechnology. Alongside these products that have considerable sales value are many novel products, which are currently available from a range of new companies and generally started from work originating from research studies in a university. Applications of these products are wide and again these will be considered.

Nanomaterials can be considered under the following three headings:

(i) Natural

(ii) Anthropogenic (adventitious)

(iii) Engineered

Natural nanomaterials comprise those created independently of man and include a wide range of materials that contain a nanocomponent and may be found in the atmosphere such as sea salt resulting from the evaporation of water from sea spray, soil dust, volcanic dust, sulfates from biogenic gases, organics from biogenic gases and nitrates from NOx. The actual content of any one or a combination of these nanomaterials in the atmosphere is dependent on geography.

Anthropogenic (adventitious) nanomaterials are those created as a result of action by man with the main example of this type of nanomaterial being soot resulting from the combustion of fossil fuels. Other anthropogenic nano-materials include welding fume and particulates resulting from the oxidation of gases such as sulfates and nitrates.

These two types of nanomaterials comprise many examples, some of which have been studied in great depth especially to minimise damage to health from exposure to these materials.

The subject of this paper falls largely in the third category, i.e. engineered nanomaterials, which have been designed and manufactured by man. These have been synthesised for a specific purpose and may be found in one of several different shapes. As denned above, the term nano describes the size in at least one dimension so nanomaterials may have nano characteristics in one, two or three dimensions. These correspond to platelet-like, wire-like and spheroidal structures respectively. The engineered nanomaterials may be further subdivided into organic and inorganic types, with the former including carbon itself and polymeric structures with specific nano characteristics. Inorganics include metals, metal and metalloid oxides, clays and a specific subset of compounds known as quantum dots.

2.2 Manufacturing Processes

Nanoparticles can be produced by a variety of methods. These include combustion synthesis, plasma synthesis, wet-phase processing, chemical precipitation, sol-gel processing, mechanical processing, mechanicochemical synthesis, high-energy ball-milling, chemical vapour deposition and laser ablation.

2.3 Product Characteristics

In summary, the key characteristics of nanomaterials that define their potential applications include the following:

• High surface area

• High activity

• Catalytic surface

• Adsorbent

• Prone to agglomeration

• Range of chemistries

• Natural and synthetic

• Wide range of applications

3 Types of Nanomaterials

3.1 Carbon

3.1.1 Carbon Black. Carbon black accounts for the largest tonnage of engineered nanomaterial and carbon blacks are used in a wide variety of applications, including printing inks, toners, coatings, plastics, paper and building products. Dependent on the size and chemistry of the particles, carbon-black-containing plastics can be electrically conducting or insulating and have significant reinforcing characteristics.

Carbon black is a very fine particulate form of elemental carbon and was first produced more than 2000 years ago by the ancient Chinese and Egyptians for use as a colourant. Although carbon black is still valued today for its colouring attributes, it is primarily used to provide reinforcement and other properties, especially to rubber articles. All carbon black is produced either by incomplete combustion or thermal decomposition of a hydrocarbon feedstock.

Two important characteristics of carbon black are surface area, an indirect measure of particle size, and structure, a measure of the degree of particle aggregation or chaining. Surface areas of carbon blacks can range from c. 10 m2 g-1, for use as reinforcing fillers, up to c. 1100 m2 g-1, for use as electrically conductive fillers. Surface area and structure are dependent on the type of process to manufacture the carbon black and they define the performance of the carbon black in its application.

The mass production of carbon blacks started in the first half of the twentieth century in the wake of the expanding tyre industry. Carbon blacks were used as reinforcing fillers to optimise the physical properties of tyres and make them more durable. Even today the tyre industry uses at least 70% of the carbon blacks manufactured worldwide. The remainder finds use in a range of applications. Carbon blacks are now widely used for plastics masterbatch applications for use in conductive packaging, films, fibres, mouldings, pipes and semiconductive cable jackets. They are also used as toners for printers and in printing inks. Carbon blacks can provide pigmentation, conductivity and UV protection for a number of coating applications including marine, aerospace and industrial. In at least some of these applications the coating requires UV curing and specific formulations have to be employed to overcome the inherent UV protection given by the carbon black during this process.

The global market for carbon blacks is forecast to rise 4% per year through 2008 to 9.6 million metric tonnes. The smaller non-tyre segment will show strongest gains. This segment also commands the highest prices with applications such as conductive fillers showing greatest growth prospects. Applications for plastics containing conductive fillers include antistatic surfaces and coatings.

3.1.2 Graphite. One-dimensional carbon is classically graphite, which has sub-nano thickness layers and nano-size spacing between layers leading to use as a lubricant, where advantage can be taken of the ability of these layers to slide across one another reducing friction between two surfaces coated with this material. This spacing is being considered for use as a hydrogen store with potential application in hydrogen fuel cells. Mono-layer graphite, or graphene, has been demonstrated as having novel magnetic properties.

Graphene has a unique electronic structure and theory suggests that novel magnetic properties may be dependent on this structure. The graphene magnetic susceptibility is temperature dependent and increases with the amount of defects in the structure. Work has been done to confirm such novel properties although there has been no commercialisation of this property at present.

Recent work has calculated that graphene spaced between 6 and 7 angstroms apart can store hydrogen at room temperature and moderate pressures. The amount of hydrogen stored comes close to a practical goal of 62 kg per cubic metre set by the US Department of Energy. Another advantage of this form of graphite is that the hydrogen gas can be released by moderate warming. The current challenge is to synthesise graphenes with the appropriate interplanar spacing for maximum hydrogen absorption. If this can be achieved then graphene could be a strong contender for practical hydrogen storage. It has been reported that “tuneable” graphite nanostructures could be created with different hydrogen storage properties by interposing space molecules between the graphite layers. These spacers would have the added advantage of keeping out contaminants such as nitrogen and carbon monoxide, which can reduce hydrogen storage capacity.

3.1.3 Carbon Nanotubes. Carbon nanotubes are fullerene-related structures that consist of graphene cylinders closed at either end with caps containing pentagonal rings. They exhibit extraordinary strength and unique electrical properties and are efficient conductors of heat along their length. They exist in single-wall and multi-wall forms. They have been used as composite fibres in polymers and concrete to improve the mechanical, thermal and electrical properties of the bulk product. They have also been used as brushes for electrical motors. Inorganic variants have also been produced.

A nanotube is cylindrical with at least one end typically capped with a hemisphere of the buckyball structure. There are two main types of nanotube: single-wall nanotubes (SWNTs) and multi-wall nanotubes (MWNTs). Single-wall nanotubes have a diameter of c. 1 nm and a length that can be many thousands of times larger i.e. to the order of centimetres. Single-wall nanotubes exhibit electric properties not shared by the multi-wall variants. They are therefore the most likely candidates for miniaturising electronics past the microelectromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire and SWNTs can be excellent conductors.

Carbon nanotubes are among the strongest materials known to man, in terms of both tensile strength and elastic modulus, and since carbon nanotubes have relatively low density, the strength to weight ratio is truly exceptional. They will bend to surprisingly large angles before they start to ripple and buckle and they finally develop kinks as well. These definitions are elastic, i.e. they all disappear completely when the load is removed. They have already been used as composite fibres in polymers and concrete to improve the mechanical, thermal and electrical properties of the bulk product. Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors. The carbon nanotubes permit reduced carbon in the brush.

Multi-wall nanotubes precisely nested within one another exhibit interesting properties whereby an inner nanotube may slide within its outer nanotube shell creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology. Already this property has been utilised to create the world’s smallest rotational motor and a rheostat. Future applications are likely to include conductive and high-strength composites, energy storage and energy conversion devices, sensors, field emission displays and radiation sources, hydrogen storage media, semiconductor devices, probes and interconnects. Some of these are already products while others are in an early to advanced stage of development.

3.1.4 Carbon “Buckyballs”. Fullerenes are the classic three-dimensional carbon nanomaterials. They have a unique structure comprising 60 carbon atoms in the shape reminiscent of a geodesic dome and are often referred to as “Buckyballs” or “Buckminsterfullerene”, after the American architect R. Buckminster Fuller who designed the geodesic dome with the same fundamental symmetry. These C60 molecules comprise the same combination of hexagonal and pentagonal rings, and the name therefore has seemed appropriate. These spherical molecules were discovered in 1985 and considerable work has gone into their study. However, potential applications have been limited and include catalysts, drug delivery systems, optical devices, chemical sensors and chemical separation devices. The molecule can absorb hydrogen with enhanced absorption when transition metals are bound to the buckyballs, leading to potential use in hydrogen storage.

3.2 Inorganic Nanotubes

Combinations of elements that can form stable two-dimensional sheets can be considered suitable to produce inorganic nanotubes and a number of inorganic chemists have been focusing on such structures. Although the investment devoted to inorganic nanotubes lags behind that of carbon nanotubes, a number of reviews suggest that inorganic nanotube research is increasing rapidly. Examples include tungsten sulfide and boron nitride, which may find uses where their inertness and high durability and conductivity can be exploited. Tungsten sulfide and molybdenum sulfide may have attractive lubricating properties.

Tenne was the first to report the synthesis of inorganic nanotubes and has suggested a list of possible technologies that could use the unique properties of inorganic nanotubes. These include bullet-proof materials, high-performance sporting goods, specialised chemical sensors, catalysts and rechargeable batteries. As examples, titanium dioxide nanotubes have been shown to have potential as a hydrogen sensor and in water photolysis.

3.3 Metals

The simplest inorganic nanomaterials are metallic with a wide range of metals already produced in nano form. These include aluminium, copper, nickel, cobalt, iron, silver and gold with a wide range of potential applications including land remediation, batteries and explosives. Metal nanoparticles have been prepared for some time, but several have found significant commercial application. These include aluminium, iron, cobalt and silver.

3.3.1 Aluminium. Aluminium nanoparticles have been used for their pyrophoric characteristics in explosives. Aluminium is a highly reactive metal when produced as a nanopowder and when in formulations such as metastable intermolecular composites (MIC) reacts to produce a large amount of heat energy. Aluminium powder is air stable due to a thin oxide shell that forms during production and protects the inner core from further oxidation.

3.3.2 Iron. Nanoscale iron particles have large surface areas and high surface reactivity and research has shown that these particles are very effective for the transformation and detoxification of a wide variety of contaminants, such as chlorinated solvents, organochloric pesticides and polychlorinated biphenyls. Thus they have been used for remediation of soil and groundwater, which contains such contaminants.

3.3.3 Cobalt. Cobalt nanoparticles exhibit magnetic behaviour, which may find application in medical imaging.

3.3.4 Silver. Silver nanoparticles, which demonstrate antimicrobial and anti-bacterial activity, have been used in a number of applications including medical dressings and non-smelling socks!

3.3.5 General. Special shaped metal nanometals hold promise for the mini aturisation of electronics, optics and sensors where, for example, studies have shown that the conductance of copper nanowires is determined by the absorption of organic molecules. Electrochemical deposition of palladium nano-structured films has led to potential application as calorimetric gas sensors for combustible gases. In the biological sciences, many applications for metal nanoparticles are being explored, including biosensors, labels for cells and biomolecules and cancer therapeutics.

3.4 Metal Oxides

The largest group of inorganic nanomaterials comprises metal oxides with titanium dioxide, zinc oxide and silicon dioxide as the largest volume materials. Copper oxide, cerium oxide, zirconium oxide, aluminium oxide and nickel oxide have also been produced commercially and are available in bulk.

(Continues…)Excerpted from Nanotechnology by R. E. Hester, R. M. Harrison. Copyright © 2007 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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