Nanoparticles: What are They?

1.1 Introduction

Although it was seldom reported in much of the literature before the 1990s, nanotechnology has recently taken the world of science by storm. Today, nanotechnology has taken a multi-faceted scientific route, with exploration undertaken in a breadth of disciplines in science ranging from, but not limited to, material science, chemistry, biology and physics to name but a few. The capability of synthesising and manipulating materials at the nanoscale interests the scientific community, the modern technological world and even the press. This burgeoning interest is amplified by the unique physical and chemical properties that nanomaterials exhibit and the promise they hold for use in future technologies such as those requiring unique optical, electrical and magnetic properties and, the focus of this book, anti-microbial activity.

1.2 Nano, the Beginning to the Present

One of the earliest reports of nanoscale particles related to gold. Faraday demonstrated the preparation of colloidal gold, which he named 'divided metals'. His account, dated 2nd April 1856, called the particles he made 'the divided state of gold', solutions of which, remarkably, can still be found in the Royal Institution in Mayfair, London, UK.

Later in 1890, the early German microbiologist Robert Koch proved that compounds incorporating gold inhibited the growth of bacteria, the discovery of which led to his Nobel Prize for medicine in 1905. Indeed the use of gold in medicine is not new and indications of the use of gold for medicinal purposes can be found throughout history. In India, for example, gold has been prepared for memory prescriptions known as Sarawatharishtam. In China, a gold coin was used in cooking rice, a practice said to help replenish a deficit of gold in the body.

However, the science of nanoscale objects was not discussed until much later in the history books, not until Richard Feynman gave a talk entitled There's Plenty of Room at the Bottom in 1959 at an American Physical Society lecture. During his talk he stated, 'The principles of physics, as far as I can see, do not speak against the possibility of manoeuvring things atom by atom'. This, in a way, was the first suggestion of a bottom-up approach to nanomaterial synthesis. Richard Feynman went on to state '... it is interesting that it would be, in principle, possible for a physicist to synthesize any chemical substance that the chemist writes down. Give the orders and the physicist synthesizes it. How? Put atoms down where the chemists say, and so you make the substance. The problems of chemistry and biology can be greatly helped if our ability to see what we are doing, and to do the things on an atomic level, is ultimately developed – a development which I think cannot be avoided'.

However, it was not until 1981 that tools became available for probing such a hypothesis, with the advent of the scanning tunnelling microscope (STM). This tool enabled unprecedented visualisation and manipulation of materials at the nanoscale. Ultimately, such ability has led to the current interest and growth of nanotechnology research, which is starting to come to fruition as new nanotechnological products reach the marketplace and consumer in the immediate future.

1.3 Defining the Nanodimension

For the purpose of this book we limit the discussion of nanomaterials defined by a minimum of two dimensions less than 100 nm. A current trend of material within this scope can be principally traced to work by Luis Brus in the 1980s in which he postulated that the band gap of a simple direct band gap semiconductor should be dependent on its size once its dimensions were smaller than the Bohr radius.

As a final point, as the research field of nanotechnology and nanomaterials has evolved rapidly with much ambiguity with regard to terminology, it is prudent to introduce some fundamental definitions:

• Colloid – a stable liquid phase containing particles in the range of 1–1000 nm. Particles in the 1–1000 nm range have the ability to be colloidal particles, as shown in Figure 1.1;

• Nanoparticle – a solid particle in the 1–1000 nm range which may either refer to a non-crystalline particle, an aggregate of crystallites or even a single crystallite;

• Quantum dot – a particle that exhibits a size quantisation effect in at least one dimension;

• Nanomaterial – any solid material that has a nanometre dimension. In summary, three dimensions = particles, two dimensions = thin films, one dimension = nanowire.

1.4 Physical Chemistry of Nanoparticles

A predominant feature of nanomaterials relates to the disproportionate influence of the surface area to volume ratio as materials enter the nanoscale dimension. Nanomaterials typically exhibit a high surface area to volume ratio, which has many interesting effects on the subsequent behaviour of these materials. As the radius r of a spherical particle decreases, the surface/volume ratio 3/r and the proportion of the constituent atoms at the surface both increase. The stable interatomic bonding that exists in large crystals is not satisfied for atoms at the surface and these therefore become more mobile and reactive, so that the overall properties of the nanomaterial become dominated by surface chemistry. The nanoparticles discussed below will be based on a metallic model, but most of the characteristics described will also apply to non- metal based materials. This will be addressed in the following contexts:

1. in aqueous environments;

2. in a two-phase mixture (i.e. polymer-metal or ceramic metal mixture);

3. on an insulating substrate surface, as a discontinuous (island) metal thin film (DMTF) of nanoparticles.

In cases 1 and 2, the nanoparticles are usually modelled on spherical particles, but particle shape can interject for DMTF. A classical nucleation theory will form the basis of formation and growth of nanoparticles but prediction of critical nucleus sizes in the sub-nanometre range is clearly inconsistent with the classical model's use of bulk thermodynamic properties. For nuclei containing only a few atoms, as is typically used in all three of these systems, the atomistic nucleation theory is necessary.

1.4.1 Structure

Nanoparticles can exist as perfect crystals since impurities and lattice defects can migrate to the surface in relatively short periods of time. Debye-Scherrer broadening of electron diffraction patterns provides a means of determining nanocrystallite sizes, with their radii giving specific lattice spacing, discussed later in this book....