CHAPTER 1
INTRODUCTION

R. Tantra, J. C. Jarman and K. N. Robinson

Quantitative Surface Chemical Spectroscopy Group, Analytical Science, National Physical Laboratory, Teddington, TW11 0LW, UK

1.1 OVERVIEW

Over the course of the past few decades, the word "nanomaterial" started to shine in reporting and publishing; nanomaterial thus became the new buzzword, giving the impression of a new type of technology. In fact, nanomaterials are not new at all and can be found in everyday lives, with most people not being aware of their existence. Nanomaterials exist in nature, for example, in volcanic ashes, sea sprays and smoke [1]. In relation to manufactured nanomaterials, they have existed as early as the 4th century. The Lycurgus Cup, a glass cup made with tiny proportions of gold and silver nanoparticles is an example of Roman era nanotechnology. The use of nanoparticles for beautiful art continued ever since, and by 1600s it is not uncommon for alchemists to create gold nanoparticles for stained glass windows. These days, there are far more uses; nanomaterials thus represent a growing class of material already introduced into multiple business sectors. For example, in early 20th century, tire companies used carbon black in car tires, primarily for physical reinforcement (e.g., abrasion resistance, tensile strength) and thermal conductivity to help spread heat load. Although nanomaterials have been around for a long time, it was only the invention of the scanning tunneling microscope in 1981 and the discovery of fullerenes in 1986 that really marked the beginning of the current nanoscience revolution. This led nanoscientists to conduct research, to study their behavior, so as to control their properties and harness their power.

Over the past few decades, research activity on nanomaterial has gained considerable press coverage. The use of nanomaterials has meant that consumer products can be made lighter, stronger, more aesthetically pleasing, and less expensive. The huge impact of nanomaterials to improve quality of life is clear, resulting in faster computers, cleaner energy production, target driven pharmaceuticals, and better construction materials [2, 3]. In particular, carbon nanotubes (CNTs) have been hailed as the wonder nanomaterial of the 21st century. CNTs are composed entirely of carbon and classed as high-aspect-ratio nanomaterial. They can be visualized as a single layer of carbon atoms in a hexagonal lattice called graphene and subsequently rolled to form a seamless cylinder/s. CNTs are classed as either single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs). As the name suggests, the former are in the form of a single tube, whereas the latter consist of multiple rolled layer or concentric tubes. CNTs typically have a diameter of 1-20 nm and a length that can be many millions of times longer. MWCNTs are normally thicker than SWCNTs, with a maximum diameter exceeding 100 nm.

According to the National Science Foundation's National Nanotechnology Initiative (NNI), the global nanotechnology market could be worth $1.2 trillion by 2020 [4]. There is huge demand for CNTs alone, with a worldwide commercial interest being reflected in its production capacity, estimated in 2011 to be 4.5 kt/year [5]. This represents a huge growth from the production of around 0.25 kt/year in 2006. Bulk, purified MWCNTs sell for approximately $1 per gram, between 1 and 10 times as expensive as carbon fibers. SWCNTs, in contrast, are currently several orders of magnitude more expensive than MWCNTs [5].

Most commercial applications of CNTs involved incorporating the powders to produce composite material with special properties, for example, electrically conductive plastics and lithium-ion batteries in laptops. A more recent exploitation of CNTs is when they are used as materials for sporting equipment. For example, CNT-based frame was used in a bicycle that won the Tour de France in 2005. The incorporation of CNTs into the material improved stiffness and fracture toughness without compromising other properties. The result is a bicycle that features minimal weight and maximal strength.

Although it is clear that nanomaterial holds great potential to form the basis of new products with novel or improved properties, concerns surrounding their potential harmful effects on health and the environment have been the topic of much debate. In over a decade, a scientific discipline called nanotoxicology [6] has emerged, which aims at understanding potential hazards posed by nanomaterials and subsequent risk implications, should, for example, they enter the human body through inhalation, ingestion, skin uptake, or injection. The field is thus interdisciplinary in nature and at the interface of biology, chemistry, and material science.

Undoubtedly, nanomaterial research spans across different disciplines, from material science to nanotoxicology. Common to all of these disciplines, however, is the need to measure physicochemical properties of nanomaterials. As mentioned in the Preface section, the goal of the book is to lay a common foundation, giving an introduction to nanomaterial characterization, thus allowing the reader to build background knowledge on this topic. This chapter gives an overview and focuses on generic topics/issues of relevance to nanomaterial characterization. It is sub-divided into four parts. The first part discusses why nanomaterials are unique in relation to their physicochemical properties. The second part presents the relevant terminology, such as the definition on what constitute a nanomaterial and what the different properties actually mean. Terminology is important as it avoids misunderstandings and ensures that the correct term is being used among stakeholders such as researchers, manufacturers, and regulators. The third part of this chapter focuses on good measurement practices; like any other research there is a need to generate reliable and robust data. In order to promote an integrated approach to quality assurance in the data being generated, topics such as method validation and standardization are covered. The last part of the chapter presents some of the common practices that are carried out in nanomaterial research, such as sub-sampling and dispersion. Although this chapter is intended to give a general overview for readers coming from different disciplines, many of the specific examples presented are of relevance to nanotoxicology.

1.2 PROPERTIES UNIQUE TO NANOMATERIALS

Undoubtedly, nanomaterials can exhibit unique physical and chemical properties not seen in their bulk counterparts. An important characteristic that distinguishes nanomaterial from bulk is associated with reduction of scale, which results in materials having unique properties arising from their nanoscale dimensions.

The most obvious effect associated with reduction of scale is the much larger specific surface area or surface area per unit mass [7]. An increase in surface area implies the existence of more surface atoms. As surface atoms have fewer neighbors than atoms in bulk, an increase in surface area will result in more atoms having lower coordination and unsatisfied bonds. Such surface atoms are overall less stable than bulk atoms, which means that the surface of nanomaterials is more reactive than their bulk counterparts [8].

Note that increase in specific surface area due to a reduction in size is an example of what is termed as scalable property. Scalable properties are those that can change continuously and smoothly with size, with no size limit associated with a sudden change in the properties. In addition to scalable properties, nanomaterials can also exhibit non-scalable properties; by this we refer to those properties that can change drastically when a certain size limit is reached. In this respect, nanomaterials cannot be simply thought of as another step in miniaturization. An example of non-scalable property is quantum confinement effects [9], which can be exemplified by some nanomaterials such as quantum dots. Quantum dots are semiconducting nanoparticles, for example, PbSe, CdSe, and CdS, with particle sizes usually smaller than ~10 nm [10]. Similar to all semiconductors, quantum dots possess a band gap; a band gap is an energy gap between valence and conduction bands in which electrons cannot occupy. In the corresponding bulk material and when at room temperature, electronic transitions across the band gap are the main mechanism by which semiconducting materials absorb or emit photons. These transitions are excited by photons of specific wavelengths, which correspond to the energy of the band gap and generate an excited electron in the conduction band and a hole in the valence band. Photons can be emitted by the recombination of these electron-hole pairs across the band gap, in which the wavelength and hence color of the emitted light will depend on the size of the gap. If not recombined, the electron-hole pairs exist in a bound state, forming quasiparticles called excitons. In quantum dots, the particle size is usually 2-10 nm, thus approaching Bohr exciton radius. The reduction in size thus results in the quantum confinement effect, in which the edges of the nanoparticle confine the excitons in three dimensions. This has the effect of increasing band-gap energy as the particle is made smaller, causing the previously continuous valence and conduction bands to split into a set of discrete energy levels, similar to those present in atomic orbitals. This is why quantum dots are sometimes called "artificial atoms." Hence, in quantum dots, band-gap energy can be tuned simply by changing the particle size. The color of...