Metal Nanoparticles and Clusters

Technological requirements to operate at increasingly smaller scales have brought nanoscience to develop chemical procedures that allow producing particles of subnanometric scale with well-controlled sizes. The properties of small nanoparticles (NPs) and large and small atomic quantum clusters (AQCs) are an illustration of how an incremental confinement produces a natural transition between macroscopic and quantum behaviour. The properties of the same metal particle change in such a way as its size becomes reduced that even its definition as a metal system ceases to make sense. NPs and metal AQCs are not only the basic components of a new technology but the illustration of how the laws of physics naturally combine into the pieces of matter architecture.

This chapter provides a short overview of synthetic routes to nanostructured metals of various shapes, compositions and sizes with an emphasis on topical methods. No attempt has however been made to be comprehensive. Instead, a short overview with detailed highlights of particular reagents and certain schemes is provided. It is hoped that such a structure would enable the reader to get to grips with the twists and turn of a few specific methods as well provide an at a glance summary of the myriad synthetic schemes.

Metal nanoparticles represent a bridge between single atoms and bulk materials, presenting peculiar chemical and optical properties. Under irradiation with an appropriate electromagnetic wave, the conduction electrons do not oscillate freely, because they are trapped in the nanometric size of the metal particles, which exhibit collective excitations called “localized plasmons.” These latter are needed to promote enhancements for both the Raman signal and the fluorescence emission of molecules adhering to the metal surface, when the exciting radiation wavelengths match those of the plasmon bands. Hence, Raman enhancements up to 10⁷ factors are generally observed for molecules adsorbed on silver or gold nanoparticles in the SERS (surface-enhanced Raman scattering) measurements. When, instead, metal particles have sizes below about 2 nm, they do not have metallic properties owing to the existence of discrete electronic energy levels and the loss of overlapping electronic bands. These metal clusters exhibit a typical quantum size behavior, with optical and electronic properties different from those relative to plasmons. In this work, the spectroscopic properties of silver and gold nanoparticles and clusters, capped with organic ligands, are investigated by Raman scattering, absorption, and fluorescence measurements and interpreted by different computational approaches.

Surface-enhanced Raman spectroscopy (SERS) is a spectroscopic technique that simultaneously combines fingerprint recognition capabilities, typical of vibrational spectroscopies, and very high sensitivity (down to single molecule), owing to the enhancement provided by plasmonic effects. Its discovery dates back to the 1970s, and since then, SERS has gained a lot of interest in the scientific community, as witnessed by the quick raise in the percentage of publications involving SERS, especially in the last two decades. In this book chapter, we would like to provide the reader with an overview of SERS, going from the illustration of its basic principles to the description of a wide selection of its applications. At first, the physical phenomena responsible for the electromagnetic and chemical SERS enhancements are described; thereafter, two key features of SERS, namely, its distance dependence and the concept of hot spot, are discussed, as well as the near- vs. far-field properties in plasmonic systems. Two sections are then dedicated to the materials that are more often used in SERS and to the strategies adopted to fabricate efficient SERS substrates. The last section illustrates the applications of SERS in several fields of sensing, like the detection of chemical warfare agents, environmental pollutants, food contaminants, and illicit drugs; the use of SERS in art preservation, forensic science, and medical diagnosis is also described, with specific and relevant examples from the most recent literature.

The importance of surface structure in catalysis is well documented by a large volume of surface science studies carried out on single crystal surfaces. Recent years has seen rapid strides in the synthesis of structured nanomaterials with varying morphologies and architecture. There is a growing interest in utilizing model nanoparticles like morphology-controlled nanostructures and core–shell-like bimetallic nanoparticles in catalysis. Apart from showing unprecedented reactivity, they serve as a model surfaces to answer many fundamental question in catalysis and also to arrive at structure vs activity correlations in heterogeneous catalysis. This chapter gives an introduction to such nanomaterials and recent advances in utilizing these materials for catalytic applications.

The use of computational methods to characterise and describe different properties in the nanoscale has increased considerably in the recent decades. Catalysis has risen as one of the major focuses in different technological fields since the use of nanostructured materials becomes more common in many industrial processes. Different computational methods have been developed to complement the experimental effort in the design of novel nanocatalysts. To date, density functional (DFT), kinetic Monte Carlo (KMC) and classical molecular dynamics (CMD) simulations allow one to describe catalytic activity for a wide diversity of reactions in different materials. Computational simulations could provide a theoretical guideline for the choice of conditions and nanomaterials to improve a specific catalytic reaction. In this work, we review the most common computational methods used to describe catalytic activity highlighting their applicability and failures. We also examine different cases in which the combination of methods improves the accuracy of the simulations. We also provide a study case in which highly active catalytic nanoparticles can be produced by using CMD simulations.

Metal Nanoparticles and Clusters are an outstanding set of nanomaterials that attract great attention due to a plethora of applications. However suitable characterization methods that can provide an atomistic picture are essential to tailor their design for exploring unique properties and novel applications. In this chapter the recent developments in advanced electron microscopic techniques that are now readily available and which can provide a complete understanding is described in detail. A brief survey of Transmission Electron Microscopy (TEM) techniques along with their principles are initially outlined, followed by specific case studies where these techniques are employed for the characterization of metallic, bimetallic, multimetallic and supported nanoparticles/clusters. The physical and chemical properties of metal NPs on various suitable supports are dependent on their unique and specific interaction between the metal and the support. Hence understanding the exact nature and atomic structure of the nanoparticle-support is essential for various important catalytic reactions. While conventional high resolution TEM (HRTEM) can enable characterization of ultra-small nanoparticles or nanoclusters, High Angle Annular Dark Field mode in STEM (HAADF-STEM) especially on an aberration-corrected instrument offers some unique advantages. In addition to catalytic nanoparticles, there is also increasing interest in few-atom nanoclusters, or indeed single atoms, as powerful catalysts and with the recent advances in aberration corrected imaging, ultrafine clusters or even isolated atoms can nowadays be characterized. Although scanning/transmission electron microscopy (STEM) is a suitable high-resolution imaging technique, in order to ascertain the complete 3D morphology of such nanoparticles, electron tomography is a viable solution. In combination with spectroscopy, it is now possible to carry out 3D–Tomography-Spectroscopy characterization; an important techniques of choice in recent times which provides the structure-morphology-chemical composition analysis in 3D. With the advent of In situ liquid cell holders it is possible to probe nucleation and growth mechanisms from the initially formed pre-nuclei. The availability of In situ gas-cell holders makes it is possible to simulate catalytic reactions very close to realistic conditions for several important reactions/transformations of industrial importance. These key advances are highlighted in the chapter with several recent examples.

Computational simulations and numerical methods have become an essential tool in the study of metal nanostructures. Given the complexity of the energy landscape, finding the most energetically favorable configuration of even the smallest metal clusters is a task that requires the use of efficient search algorithms and robust interaction potentials, and the study of structural and dynamical properties of metal nanoparticles at finite temperature makes necessary an appropriate scan of the phase space, either by stochastic methods or by solving a large set of equations of motion, performed on a model system sophisticated enough to maintain the main features of the real systems being modeled. This chapter is aimed to present some examples in the use of these models and algorithms, making special emphasis in the use of molecular dynamics simulations to describe the behavior of metal nanoparticles on the melting transition, the sintering of nanoparticles, the proposal of phase diagrams, and the role of substrates and other forms of confinement. Connection with experimental results is also discussed, with several examples on the simulation of scanning transmission electron micrographs and the mechanical manipulation of metal nanowires.

A high percentage of deaths per year worldwide are caused by environmental pollution. It is well known that excessive usage of toxic chemicals including heavy metal ions, pesticides, other food toxins, etc. leads to adverse effect to living organisms and contributing to biodiversity losses and severe damage to the environment. Thus, the detection of toxic compounds with high sensitivity and specificity in real time is essential nowadays. For the past few decades, the development of chemical sensors have received much attention due to the sensitivity and selectivity achieved, possibility of in situ monitorization with rapid response, low cost, simple instrumental setup, etc. Traditionally, the use of organic dyes such as cyanine, fluorescein, etc. or more recently the use of semiconductor quantum dots and upconverting nanoparticles has been employed as fluorophores in order to generate optical sensors. However, these fluorophores have certain limitations such as poor photostability, large particle size, or poor water solubility. On the other hand, metal nanoclusters (NCs) and nanodots (NDs) show strong luminescence with high photostability, large Stokes shifts, and good aqueous solubility and biocompatibility. It is well known that the size of metal nanoclusters is comparable to the Fermi wavelength of electrons (∼0.7 nm), giving rise to molecular-like properties and size-dependent fluorescence from visible to near-infrared range. These novel properties have been exploited in the field of chemical and biochemical sensing, bioimaging, electronic device fabrication, clean energy storage, etc.

In this chapter, we briefly summarize the most common synthesis procedures and recent progress of luminescent Ag/AuNCs and NDs. Their application for chemical and biochemical sensing is also collected, paying special attention to the detection of toxic heavy metals (including mercury, lead, copper, chromium, arsenic, etc.), toxic ions (such as cyanide, sulfide, etc.), biological compounds (cysteine, tyrosine, cysteamine, glutathione, glucose, H₂O₂, etc.), drugs (mercaptopurine, penicillamine, clioquinol, antibiotics, etc.) and some other interesting molecules (salicylaldehyde, poly diallyldimethyl ammonium chloride, sodium dodecyl sulfate), toxic contaminants (tea polyphenols, melamine, bisphenol A, etc.), and pathogenic bacteria.

After AIDS, cancer is the second most lethal disease in the world. According to the World Health Organization (WHO), worldwide 8.8 million deaths in 2015 are due to cancer disease. Therefore, this is an essential issue nowadays to develop effective diagnostic and treatment modalities for cancer to save human life from its deathtrap. Chemotherapy is a well-known cancer treatment tool and used for the bio-distribution of anticancer drugs to specific cancer target. Photothermal therapy (PTT) is an alternative to chemotherapy and considered to be a novel technique for treating a variety of cancers, because of their high efficacy, convenience, and minimal damage to the normal cells. Near-infrared (NIR) light-sensitive gold nanomaterials are considered as noninvasive candidate for PTT, because of their highly efficient production of heat from light. In recent decades, the combined chemo and PTT (chemophotothermal therapy) has been gaining momentum. Generally, the non-specific anticancer drug release can produce some toxic side effects. To overcome this problem, chemophotothermal therapy has been used, and it enhances the anticancer drug release rate.

In this book chapter, I have highlighted the synthesis and properties of novel plasmonic NIR light-sensitive nanomaterials (gold nanorods, gold nanoshells, gold nanocages, hollow gold nanospheres, gold nanostars, and gold nanoclusters) and their PTT and chemotherapeutic applications. Further, this book chapter will provide a recent progress of chemophotothermal therapy-based combined treatments for various tumor cells and modification protocol of these nanomaterials and address specifically on tumor target, drug delivery, etc. The aim of this book chapter is to provide a summary of synthesis of different plasmonic gold nanomaterials and their efficiency in PTT and chemotherapy.