Colloidal Synthesis of Plasmonic Nanometals

Luis Liz-Marzán, Luis Liz-Marzán

• Publisher: Jenny Stanford Publishing
• Year: 2020
• ISBN:
  9781000021301
• Edition: 1
• Topic: Medizin
• Subtopic: Biotechnologie in der Medizin

Noble metal nanoparticles have attracted enormous scientific and technological interest because of their unique optical properties, which are related to surface plasmon resonances. The interest in nanosized metal particles dates back to ancient societies, when metals were used in various forms as decorative elements. From the famous Lycurgus cup, made by the Romans in the 4th century AD, through thousands of stained glasses in churches and cathedrals all over medieval Europe, bright-yellow, green, or red colors have been obtained by a touch of metallic additions during glass blowing. This peculiar interaction of light with nanometals can be widely tuned through the morphology and assembly of nanoparticles, thereby expanding the range of potential applications, from energy and information storage to biomedicine, including novel diagnostic and therapeutic methods.

This book compiles recent developments that clearly illustrate the state of the art in this cutting-edge research field. It comprises different review articles written by the teams of Prof. Luis Liz-Marzán, an international leader in chemical nanotechnology who has made seminal contributions to the use of colloid chemistry methods to understand and tailor the growth of metal particles at the nanoscale. Apart from synthesis, the book also describes in detail the plasmonic properties of nanomaterials and illustrates some representative applications. This book will appeal to anyone involved in nanotechnology, nanocrystal growth, nanoplasmonics, and surface-enhanced spectroscopies.

Chapter 1

Nanometals: Formation and Color^*

* Reprinted from Materials Today, 7(2), Luis M Liz-Marzán, Nanometals formation and color, 26–31, Copyright (2004), with permission from Elsevier.

Luis M. Liz-Marzán

CIC biomaGUNE and CIBER-BBN, Paseo de Miramón 182, 20014
Donostia-San Sebastián, Spain
Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
[email protected]

Metal nanoparticles are very attractive because of their size-and shape-dependent properties. From the plethora of existing procedures for the synthesis of metal nanoparticles, the most widely used wet-chemical methods are briefly discussed, which are suitable for the production of both spherical and anisometric (rod-like or prismatic) nanoparticles. The optical properties of these nanoparticles are spectacular and, therefore, have promoted a great deal of excitement during the last few decades. The basics of the origin of such optical properties are described and some of the theoretical methods accounting for them are briefly presented. Examples are shown of the color variations arising from changes in the composition, size, and shape of nanoparticles, as well as from the proximity of other metal nanoparticles.

1.1 Introduction

Nanotechnology, nanoscience, nanostructures, nanoparticles… These are now some of the most widely used terms in materials science literature. But why are nanoscale materials and processes so attractive? From the point of view of the general public, nanotechnology appears to be the fabrication of miniature machines, which will be able to travel through the human body and repair damaged tissues, or supercomputers small enough to fit in a shirt pocket. However, nanostructured materials have potential applications in many more areas, such as biological detection, controlled drug delivery, low-threshold lasers, optical filters, and sensors, among others.

In fact, it is relatively easy to find examples of the use of metal nanoparticles (maybe not deliberately) as decorative pigments since the time of the Romans, such as those contained in the glass of the famous Lycurgus Cup (4^th century AD). The cup can still be seen at the British Museum¹ and possesses the unique feature of changing color depending upon the light in which it is viewed. It appears green when viewed in reflected light, but looks red when a light is shone from inside and is transmitted through the glass. Analysis of the glass reveals that it contains a very small amount of tiny (~70 nm) metal crystals containing Ag and Au in an approximate molar ratio of 14:1. It is the presence of these nanocrystals that gives the Lycurgus Cup its special color display.

It was not until 1857, however, that Michael Faraday reported a systematic study of the synthesis and colors of colloidal gold². 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. All this has led not only to reliable procedures for the preparation of metal nanoparticles of basically any desired size and shape, but also to a deep understanding of many of the physico-chemical features that determine the characteristic behavior of these systems.

One of the most interesting aspects of metal nanoparticles is that their optical properties depend strongly upon the particle size and shape. Bulk Au looks yellowish in reflected light, but thin Au films look blue in transmission. This characteristic blue color steadily changes to orange, through several tones of purple and red, as the particle size is reduced down to ~3 nm. These effects are the result of changes in the so-called surface plasmon resonance³, the frequency at which conduction electrons oscillate in response to the alternating electric field of incident electromagnetic radiation. However, only metals with free electrons (essentially Au, Ag, Cu, and the alkali metals) possess plasmon resonances in the visible spectrum, which give rise to such intense colors. Elongated nanoparticles (ellipsoids and nanorods) display two distinct plasmon bands related to transverse and longitudinal electron oscillations. The longitudinal oscillation is very sensitive to the aspect ratio of the particles⁴, so that slight deviations from spherical geometry can lead to impressive color changes. Apart from single-particle properties, the environment in which the metal particles are dispersed is also of relevance to the ...