Introductory Aspects of Soft Nanoparticles

JOAN ESTELRICH, MANUEL QUESADA-PÉREZ, JACQUELINE FORCADA AND JOSÉ CALLEJAS-FERNÁNDEZ

1.1 Nanoparticles

Nanotechnology is the science that deals with matter at the scale of 1 billionth of a metre (i.e. 10-9 m = 1 nm) and is also the study of manipulating matter at the atomic and molecular scale. A nanoparticle is the most fundamental component in the fabrication of a nanostructure and is far smaller than the world of everyday objects that are described by Newton's laws of motion, but larger than an atom or a simple molecule that are governed by quantum mechanics.

According to the definition of the International Organization for Standardization (ISO), a nanoparticle is a particle whose size spans the range between 1 and 100 nm. Metallic nanoparticles have different physical and chemical properties from bulk metals (e.g. lower melting points, higher specific surface areas, specific optical properties, mechanical strengths and magnetizations), properties that might prove attractive in various industrial applications. However, how a nanoparticle is viewed and is defined depends very much on the specific application. In this regard, for biomedical applications, structures and objects up to 1000 nm in size are included as nanostructured materials used in medicine.

Of particular importance, optical properties are among the fundamental attractions and characteristics of a nanoparticle. For example, a 20 nm gold nanoparticle has a characteristic wine-red colour, a silver nanoparticle is yellowish grey and platinum and palladium nanoparticles are black. Not surprisingly, the optical characteristics of nanoparticles have been used for centuries in sculptures and paintings even before the fourth century AD. The most famous example is the Lycurgus cup (fourth century AD). This cup, at present in the British Museum in London, is the only complete historical example of a special type of glass, known as dichroic glass, that changes colour when held up to the light. When it is looked at in reflected light or daylight, it appears green. However, when light is shone into the cup and transmitted through the glass, it changes colour to red. This property puzzled scientists for decades and the mystery was not solved until 1990, when researchers in England scrutinized broken fragments under a microscope and discovered that Roman artisans were nanotechnology pioneers: they had impregnated the glass with a very small quantity of minute (~70 nm) colloidal silver and gold in an approximate molar ratio of 14:1, which gives it these unusual optical properties.

Gold suspensions were familiar to alchemists in the Middle Ages and the reputation of soluble gold was based mostly on its fabulous curative powers against various diseases, for example, heart and venereal diseases, dysentery, epilepsy and tumours. Metallic nanoparticles were used in mediaeval stained glasses. The mediaeval artisans trapped gold nanoparticles in the glass matrix in order to generate ruby-red colour in windows. They also trapped silver nanoparticles, which gave the glass a deep-yellow colour. Beautiful examples of these applications can be found in glass windows of many Gothic European cathedrals.

In the seventeenth century, the so-called Purple of Cassius was highly popular. It was a colloid made by reducing a soluble gold salt with stannous chloride. It was used as a colorant and to determine the presence of gold as a chemical test. The first scientific study of gold particles was carried out by Faraday in 1857. He observed that gold suspensions with a ruby-coloured appearance, made by reducing an aqueous solution of chloroaurate (AuCl4-) with phosphorus in CS2 (a two-phase system), changed their colour from red to blue upon heating or addition of salt. Faraday correctly attributed the colour change to an increase in the effective particle size caused by aggregation. Since that pioneering work, thousands of scientific papers have been published on the synthesis, modification, properties and assembly of metal nanoparticles, using a wide variety of solvents and other substrates. Nowadays, the most widely used nanotechnology product in the field of in vitro diagnostics is colloidal gold in lateral flow assays, which is used in rapid tests for pregnancy, ovulation, human immunodeficiency virus (HIV) and other indications. Gold nanoparticles were introduced into these tests in the late 1980s because gold conjugates have particularly high stability, which is critical for avoiding false positives.

In 1959, Richard Feynman gave a talk entitled 'There's plenty of room at the bottom', where he predicted the new things and new opportunities that one could expect in the very small world. Norio Taniguchi of Tokyo University of Science was the first to propose in 1974 the term 'nanotechnology'. The age of nanotechnology had begun.

The activity in the field of the nanotechnology has grown exponentially worldwide during the past three decades, becoming a major interdisciplinary area of research. This growth has been driven to a great extent by the integration of nanotechnology into the field of medical science, since nanostructured materials have unique medical effects. The control of materials in the nanometric range not only results in new medical effects but also requires novel, scientifically demanding chemistry and manufacturing...