Nanochemistry: Chemistry of Nanoparticle Formation and Interactions

By Anna Klinkova and Héloïse Thérien-Aubin

Nanochemistry: Chemistry of Nanoparticle Formation and Interactions provides an overview of the chemistry aspects of nanoparticle science, including nanoparticle synthesis, chemical properties, stability, applications and self-assembly behavior. The critical concepts discussed in this book represent the necessary toolbox for enabling the rational design of nanoparticle-based materials for target applications. After an introduction to standard analytical techniques used for nanoparticle characterization, four separate chapters cover inorganic, organic, polymer nanoparticles, and carbon nanostructures to highlight the synthetic protocols, structural intricacies, and chemical properties specific to each of these material classes.

Finally, physicochemical phenomena governing self-assembly behavior of nanoparticles are also discussed in detail separately. This book is intended for senior undergraduate, graduate and postgraduate students and research scientists in nanoscience and nanotechnology, material science, chemistry, physics, biomedical sciences and relevant engineering fields that want to develop a deeper understanding of the governing chemical principles on the nanoscale.

• Provides an up-to-date text reflecting the latest changes in the field, acting as a fully restructured successor text to Nanochemistry, 2nd Edition (Elsevier, 2013) by Klabunde and Sergeev
• Leads the reader through the fundamental concepts and illustrative examples of inorganic, organic, and polymer nanoparticle formation, discussing, in detail, the aspects of synthetic geometry control, surface chemistry, and nanoparticle stability
• Provides in-depth coverage of nanoparticle self-assembly behavior, including the self-assembly driving forces and approaches to control this process through nanoparticle design and environmental cues

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 4, 2023
• ISBN: 9780443214486

Anna Klinkova

Anna Klinkova is Assistant Professor of Chemistry at the Waterloo Institute for Nanotechnology, based at the University of Waterloo, Canada. After completing degrees at Saint Petersburg State University, Russia, and Bowling Green State University, USA, she went on to obtain her doctorate at the University of Toronto. Professor Klinkova has been with the University of Waterloo since 2017, where her research concentrates on the synthesis and self-assembly of inorganic and soft-matter nanomaterials, heterogeneous catalysis, electrochemistry and plasmonics. She teaches various courses in the undergraduate Nanotechnology Engineering program and in the interdisciplinary graduate Nanotechnology program.

Book preview

Preface

Nanotechnology is a rapidly growing field dealing with the properties, behaviors, and applications of materials at the nanoscale. Many recent technological breakthroughs from materials for green energy production to those used to fight against pathogens have relied heavily on advances in nanotechnology. One of the reasons these small materials are able to have such a large impact is due to the highly interdisciplinary nature of nanotechnology. This broad field of research combines elements of chemistry, physics, materials science, and engineering. The combination of perspectives, approaches, and points-of-view from these disciplines confers a unique vitality to the field.

The existing books on nanomaterials typically cover a wide range of topics, including their synthesis, properties, and applications. In addition to synthesis typically not being the major focus of such texts, they tend to encompass both cleanroom techniques and wet-lab approaches. This makes the scope of study of synthetic foundational knowledge quite wide and difficult to master. Cleanroom techniques for nanomaterial fabrication such as nanolithography and nanodeposition are more systematically covered in designated literature, while wet-lab approaches are often described more in a cookbook style, where the how-to is more important than the why. Thus researchers have had to gather dispersed information from different sources or primarily rely on review articles and direct training by senior peers in place of study and research work. Consequently, synthesizing complexly shaped nanoparticles and performing their self-assembly appears to many to be more of an artisan skill guided by nanochemistry intuition than a systematic synthetic framework akin to organic synthesis.

This textbook aims to fill that gap. It focuses predominantly on the recent body of data on nanoparticle synthesis and presents it in a systematic way for those entering the field. The content is grouped by particle nature to facilitate an understanding of the formation mechanisms of inorganic, carbon, organic, and polymer nanoparticles, and, therefore, of any of their hybrid forms. Furthermore, a designated chapter on self-assembly is introduced to provide a comprehensive basis for understanding how and why nanoparticles can self-organize and what triggers can be used to induce and control this process. Characterization techniques are discussed only in the context of analyzing the particular nature of nanoparticles. Solid-state chemistry and surface and interface chemistry characterization techniques are left outside the scope of this text, as they are comprehensively covered in the corresponding literature that addresses these chemistries at the macro, micro, nano, and atomic scales. Chapters 1, 3, 4 and 5 were authored by Anna Klinkova, and Chapters 2, 6, and 7 were authored by Heloise Therien-Aubin. The authors hope that the resulting text will fill the existing need for a basic reference text focused on the chemical approaches to nanoparticle synthesis and self-assembly. It will help those new to the field of wet-lab nanochemistry effectively select, troubleshoot, and optimize synthetic procedures for nanoparticle synthesis and self-assembly and be more successful and productive in their research.

Chapter 1

Introduction

Abstract

The general objects of study of nanochemistry are nanomaterials, i.e., materials with nanoscale features in at least one dimension. The subject of nanochemistry is the chemistry associated with these materials, including their composition and structure, formation and transformations, chemical interactions with various environments and chemical reactivity. Nanomaterials include nanoparticles, materials constructed from such particles, continuous bulk materials with nanoscale pores or channels, and hybrid bulk materials that have distinct nanoscale domains. A plethora of natural and synthetic materials fall under the definition of a nanomaterial, making the scope of nanochemistry very broad. One way to narrow down the scope in order to build a basic understanding of nanochemistry is to select a subset of nanomaterials for discussion. Nanoparticles are good candidates for this purpose, as they are often seen to be at the core of nanotechnology due to the many unique physical properties that they offer for a variety of applications. Furthermore, many fundamental aspects of the chemical behaviour of nanoparticles are relevant to the understanding of the chemistry of other types of nanomaterials, although those certainly have specific chemical characteristics that are not discussed in the context of nanoparticles. This book focuses on the chemistry of nanoparticle formation and interactions. The aspects of nanochemistry pertaining to continuous two- and three-dimensional materials are not covered here. Instead, here we cover specifically the mechanisms of nanoparticle formation and the interactions of nanoparticles with each other, as well as with the environment in the context of stability. This book provides a focused general guide to understanding the foundations of nanoparticle formation and behaviour from the chemistry standpoint.

Keywords

Colloidal chemisty; nanoengineering; nanoscience; nanochemistry, nanotechnology, nanoparticle, nanoparticle formation, nanoparticle interaction

1.1 The subject of the book

Nanochemistry is a subdiscipline of nanoscience, which is an area of science that studies nanoscale physical objects and associated phenomena. Nanoscience is focused on the nature of these objects and phenomena, on the understanding of why and how things work the way they do in the world of nano. The progress in nanoscience goes hand in hand with the development of nanoengineering and nanotechnology, which are the application of scientific knowledge to designing useful nanomaterials for various tangible purposes and the technological outcomes of such design, respectively. While nanoscience, nanoengineering, and nanotechnology mutually stimulate developments in all three of these fields via multiple positive feedback loops, nanoscience is foundational and essential to nanoengineering, as it enables rational design of new technologies based on the deep understanding of the nature of nanomaterials and their properties.

In 1959 Richard Feynman gave a lecture at the American Physical Society titled There’s plenty of room at the bottom: an invitation to enter a new field of physics, proposing to focus on manipulating matter on atomic- or nano- scale to build devices from bottom up. It went largely unnoticed then, but since the 1980s the lecture has been regarded as an inception point of nanotechnology and nanoscience. To systematize and categorize the areas of studies at the bottom, plenty of nanoterminology has been introduced since, with just a few examples named below. Various physical effects on the nanoscale, including electronic, optical, and photonic properties of nanomaterials are studied by respective branches of physics, and sometimes they are collectively referred to as nanophysics. Many of the physical properties of nanomaterials originate in quantum confinement effects and are studied by quantum nanoscience. The field of nanobiology and nanobiotechnology explores the biological aspects of nanoscience; the latter are often used interchangeably due to the highly applied focus of these studies. Nanotoxicology investigates the toxicity of nanomaterials. Nanochemistry covers multiple aspects of nanoscience that complement or facilitate the progress in the above fields, including the mechanisms of nanomaterials formation, their stability, and complex physicochemical interactions with various environments. It also encompasses a distinctly chemical aspect of nanomaterials pertaining to thermal, photo-, and electrocatalysis, often collectively referred to as nanocatalysis.

The relatively new field of nanochemistry has been developing from the convergence of the parts of many mature chemistry subfields relevant to nanomaterials, including, but not limited to, colloid and interface chemistry, surface chemistry, inorganic, organic, polymer, physical chemistry, and material chemistry. As a result of this amalgamation of many branches of chemistry to attempt to fully grasp all the chemical phenomena occurring on the nanoscale, nanochemistry has become a complex and multifaceted field of study with blurred boundaries with other chemical subdisciplines.

The general objects of study of nanochemistry are nanomaterials, that is, materials with nanoscale (the scale between 1 and 100 nm) features in at least one dimension. The subject of nanochemistry is the chemistry associated with these materials, including their composition and structure, formation and transformations, chemical interactions with various environments, and chemical reactivity. Nanomaterials include nanoparticles, which are objects with at least one dimension on the 1–100 nm scale (although more strictly, all three dimensions should be on that scale), materials constructed from such particles (including bulk materials, films, and powders), continuous bulk materials with nanoscale pores or channels, and hybrid bulk materials that have distinct nanoscale domains. A plethora of natural and synthetic materials fall under the definition of a nanomaterial, making the scope of nanochemistry very broad. One way to narrow down the scope to build a basic understanding of nanochemistry is to select a subset of nanomaterials for discussion. Nanoparticles are good candidates for this purpose, as they are often seen to be at the core of nanotechnology due to the many unique physical properties that they offer for a variety of applications. Furthermore, many fundamental aspects of the chemical behavior of nanoparticles are relevant to the understanding of the chemistry of other types of nanomaterials, although those certainly have specific chemical characteristics that are not discussed in the context of nanoparticles (e.g., mass transport phenomena in nanoporous materials).

This book focuses on the chemistry of nanoparticle formation and interactions. As a result, the aspects of nanochemistry pertaining to continuous two-dimensional and three-dimensional materials are not covered here. Furthermore, the nanocatalysis discussion is also excluded from this edition, as it is primarily concerned with the chemical reactions at the surfaces of the nanoparticles. Instead, here we cover specifically the mechanisms of nanoparticle formation and the interactions of nanoparticles with each other, as well as with the environment in the context of stability. By excluding other areas of nanochemistry mentioned earlier that are covered in other specialized texts, we wish to provide a focused general guide to understanding the foundations of nanoparticle formation and behavior from the chemistry standpoint. The discussion is primarily aimed at students and researchers with basic general chemistry backgrounds. This book will be of interest to chemists and those studying and working in various areas of material science and nanotechnology engineering.

1.2 Naturally occurring and synthetic nanoparticles

Due to the rapid advancements in nanotechnology and relative novelty of the terms nanomaterials and nanoparticles, the public perception is that these materials are predominantly synthetic and anthropogenic in origin. However, Nature itself generates a wide range of micro- and nanoparticles in the environment, such as carbon nanoparticles in ash and soot, mineral nanoparticles as a result of volcanic activity, sulfur and selenium nanoparticles produced by many bacteria and yeasts, and a plethora of organic and polymer nanoparticles commonly occurring as a result of vital functions of plants and animals, for example, polysaccharide nanoparticles produced by corn and rice (phytoglycogen) or protein micro- and nanospheres in milk that make nutrients more available for us to absorb (casein micelles). Moreover, a significant fraction of interplanetary dust that is still falling on the Earth at the rate of thousands of tons per year is composed of nanoparticles, and the same is true about atmospheric dust particles.

Human activity throughout millennia has also included the use of nanoparticles without knowing about their size until recently. Examples include creating micro- and nanoscale micelles when using soap or emulsifying ingredients to make sauces, producing gold nanoparticle-based inorganic dyes for glass and ceramics staining back in ancient Rome and China, or using carbon nanoparticle-based pigments for tattooing since, at least, the copper age.

Over the recent decades, many more sophisticated nanoparticle-based products have become omnipresent, forming an integral part of commercially available materials we use, such as polymer nanoparticles in paints and coatings, carbon nanoparticles in inkjet printers, titanium dioxide nanoparticles in sunscreens, lip balms and other cosmetics, iron nanoparticles in MRI contrast agents, silver nanoparticles in deodorants and clothing or in conductive inks for repairing circuit boards, and so on. With the increasing variety and complexity of nanoparticles that can be produced on a large scale, there are countless emerging applications of these materials in various areas of energy capture and conversion, electronics, biomedicine, food, agriculture, environmental remediation, aerospace, and more.

Analogous to organic chemistry, focused on the study and synthesis of 10 million carbon-based compounds existing in living organisms and more produced synthetically, the chemistry of nanoparticles covers a myriad of nanoscale objects varying in their composition and structure. This nanoparticle diversity is in part associated with the intrinsic multicomponent complexity of nanoparticles: even simple spherical single-component nanoparticles can exist in many forms based on the chemical functionalization of their surface and vary significantly in their properties as a result. Besides carrying a positive or negative surface charge, nanoparticle surface functionalization includes any type, number, and combination of molecular species chemically or physically tethered to the particle surface, which also can be distributed in different manners on the surface and either remain on that surface permanently or exist in dynamic equilibrium with the surrounding environment. In addition to the basic spherical shape, nanoparticles can exist in many geometries, such as various platonic shapes, straight and twisted rods and wires of various aspect ratios, platelets, tubes, cages, and other void-containing geometries, geometries with protruded features, for example, stars and urchins, and other more complex shapes, some of which can also exhibit chirality (i.e., be not superimposable with their mirror image). Once we consider multicomponent nanoparticles with various distributions and positioning of the components within the structure, it becomes apparent that synthetic nanoparticles have an unprecedented number of degrees of freedom compared to synthetic inorganic and organic compounds or bulk materials.

Considering this vast diversity and resulting tunability of properties in synthetic nanoparticles, there is a great promise that some of these materials will help solve technological, environmental, socioeconomic, and other challenges humankind faces. Moreover, further technological innovation and progress, such as in space exploration, are also highly dependent on new materials with unusual properties, which may arise from developments in synthetic nanoparticle chemistry and nanoscience in general. Therefore it is of significant importance to understand how these nanoobjects form and behave chemically to navigate this highly promising and rapidly developing area of research.

1.3 Chemistry of nanoparticles: colloidal chemistry and beyond

One of the main reasons for the rapid progress in nanoscience and especially nanochemistry was the developments in electron microscopy in the 20th century that enabled seeing objects with a scale below the limit of traditional optical microscopy, which is around 200 nm. Although it was possible to detect and analyze nanoparticles by various indirect techniques before the availability of electron microscopy, the information obtained by such means remained limited. Present transmission and scanning electron microscopes offer sub-nm resolution and are equipped with various detectors that make it possible to not only see the morphology and topology of nanoscale objects, but also perform compositional analysis and map the distribution of elements within these objects. The ability to perform these types of analyses allowed researchers to study the effects of various chemical parameters on the outcome of synthesis or chemical modification in the most direct way, providing the means to develop a deeper understanding of the chemical mechanisms associated with nanoparticle formation and interactions. However, the foundations of nanoparticle chemistry, such as the stability of nanoparticle dispersions in liquids and many aspects of their behavior were developed, before the rise of nanoscience and nanotechnology, within the field of colloidal chemistry.

Colloidal chemistry studies dispersions of submicron particles (1–1000 nm) in continuous medium and interfacial phenomena associated with these systems. The origins of this branch of chemistry trace back to the work of Francesco Selmi in 1845–50, who studied the behavior of silver chloride, sulfur, and Prussian blue in water; while what he obtained looked like their solutions, he suggested that these compounds could not exist in the form of molecules or ions and, therefore, could not be true solutions. In 1857 Michael Faraday concluded that ruby-red aqueous gold, or aurum potabile (drinkable gold in Latin) described by alchemists, contained gold in the form of minuscule particles. The term colloid was introduced by Thomas Graham, who performed the first systematic studies of colloidal systems in the 1860s, and the field continued developing through the rest of the 19th and 20th centuries.

To explain why small solid particles remain suspended in a liquid medium and do not combine to form larger pieces of the material and, eventually, a precipitate, a physicochemical theory of colloidal stability was developed by Boris Derjaguin and Lev Landau in 1941 and independently by Evert Verwey and Theodoor Overbeek in 1948, which resulted in the theory being called the DLVO theory. This theory considers two charged surfaces interacting through a liquid medium and mathematically demonstrates that the combined effect of van der Waals attraction and electrostatic repulsion due to the presence of a double layer of counterions at the surface results in a net repulsion force between the surfaces when their double layers begin to overlap. The resulting repulsion provides stability of the dispersed particles against aggregation, in other words, colloidal stability. This balance between the attractive and repulsive forces can be modulated by changing the surface charge and the ionic force of the solution. While the DLVO theory has limitations in terms of its predictive power due to the simplifications in the mathematical model, it is fundamental to the understanding of both the colloidal stability of charged nanoparticles during and after their formation, as well as their self-assembly behavior. However, this theory does not explain why polymer- or surfactant-stabilized nanoparticles with little or no charge and dispersed in a nonpolar medium also remain colloidally stable. Their stability can be explained by the concept of steric stabilization, which considers that bringing the polymer or surfactant molecules together is entropically unfavorable (due to the loss of possible molecular conformations), and increasing the concentration of these species between the particles induces osmotic repulsion.

The particles studied by the basic colloidal chemistry are simplified objects; for example, in terms of the repulsion forces, the DLVO theory focuses on the surface of the particles and, as a result, considers an infinite flat solid surface with a uniform surface charge, while in other contexts, such as when describing Brownian motion, the particles are considered as point objects or spheres. Thus one must go beyond the traditional scope of colloidal chemistry to understand the complexities of nanoparticle chemistry. First of all, depending on the chemical nature of the nanoparticles, it is important to understand and consider the basic knowledge of the associated synthetic inorganic, organic, or polymer chemistry. As the nanoparticles exist in the solid phase, the knowledge of solid-state chemistry (sometimes referred to as materials chemistry) becomes relevant, such as the crystallinity of the material and associated properties. However, considering the high surface-to-volume ratio in nanoparticles, the interfacial effects dominate or alter the behavior of the solid phase. As a result, the understanding of the aspects of surface science relevant to nanoparticle surface chemistry becomes particularly important to integrate into the study of nanoparticle chemistry. Specifically, the atomic scale considerations of the particle surface are of great importance, especially when dealing with inorganic and carbon nanoparticles, as the particle curvature, whether spherical or of another shape, determines the arrangements of the atoms on the surface and associated chemistry. For example, the growth rate and the affinity of a ligand for a specific facet of a complexly shaped crystalline nanoparticle is a function of the surface atomic arrangement on that facet.

By building on the strong foundation of colloidal chemistry and integrating it with the relevant aspects from other chemistry subfields, nanochemistry has established a solid framework for studying the formation and behavior of nanoparticles. Moreover, on the basis of this framework, nanochemistry researchers have been systematizing and making sense of the vast body of experimental data on various nanoparticle synthesis and their self-assembly reported to date. There is still plenty of room for systematic development and many unanswered questions in the field of nanoparticle chemistry, and as new structures and synthetic procedures are being discovered, it will continue to remain a very fruitful and exciting area of study that will continue to mature.

1.4 Book organization

The next chapter of this book discusses several instrumental techniques used to characterize nanoparticles. While many aspects of nanomaterials can be analyzed using a traditional chemistry toolbox, when it comes to nanoparticle size and geometry, specialized instrumentation and analytical techniques are required to gain this nanoscale information. The studies of nanoparticle synthesis and behavior strongly rely on this set of techniques, and the following discussion in this book requires a basic understanding of their operation principles and attainable information. Thus, before diving into the discussion about nanoparticle types, we begin this book by covering different microscopy and scattering techniques and some specialized techniques of interest in this context. The microscopy discussion includes optical microscopy and its application on the nanoscale, transmission and scanning electron microscopy, and scanning probe microscopy. The scattering techniques include light scattering, small-angle X-ray scattering, and small-angle neutron scattering. Analytical ultracentrifugation, field-flow fractionation, and tuneable resistance pulse sensing for nanoparticle size analysis are discussed as well, followed by a comparison between different techniques, including attainable information and size and sample state limitations.

Chapters 3–6 cover different types of nanoparticles. Nanoparticles can be divided into the following basic classes: (1) inorganic nanoparticles, including metal, ceramic, and semiconductor nanoparticles, (2) carbon-based nanoparticles such as carbon nanotubes, fullerenes, graphene scrolls, and carbon black, (3) organic nanoparticles, including micelles, lipid nanoparticles, dendrimers, and cellulose nanocrystals, and (4) polymer nanoparticles (which is technically a very large subcategory of organic nanoparticles with particular chemical specificity). A vast variety of composite, or hybrid, nanoparticles comprising several of these types can be understood in terms of their chemical synthesis and properties based on the chemistry of the individual types; thus, this basic categorization is fundamental to understanding the formation and interactions of any of the existing nanoparticles in general. In this book, we primarily focus on the chemistry of the nanoparticles formation and interactions and the mechanistic understanding of these processes, whereas the discussion of nanoparticle applications mostly appears for illustration purposes or to provide some tangible context to the reader and is not intended to be complete.

Inorganic nanoparticles are discussed in Chapter 3, Inorganic nanoparticles. First, this chapter covers the fundamentals of general wet chemistry approaches, including chemical reduction, sol–gel synthesis and thermal decomposition, and physically assisted methods, including photolysis, sonochemical synthesis, laser ablation to obtain nanoparticles in liquid media, microwave synthesis, and mechanochemical synthesis that has recently been gaining increasing interest as a sustainable alternative to more traditional techniques. We then discuss in great detail how the shape control, including platonic and more complex shapes (such as platelets, rods, and branched geometries), can be achieved, including the mechanisms of specific shape formation. Multicomponent inorganic structures and synthetic approaches to forming them are discussed separately. Different approaches to surface stabilization and functionalization of inorganic nanoparticles, including in situ during synthesis as well as ligand stripping and ligand exchange are covered next, followed by the discussion about ligand segregation and surface patterning of nanoparticles. The inorganic nanoparticle chapter concludes with the mechanistic discussion of the structural stability of nanoparticles in different contexts, including processes that can be used for deliberate alteration of nanoparticles, such as Ostwald and digestive ripening and oxidative dissolution of nanoparticles, and processes of nanoparticle degradation under high temperature, in electrochemical applications, and under physiologically relevant conditions (we note that nanotoxicology is beyond the scope of this book).

Chapter 4, Carbon nanostructures, is dedicated to carbon-based nanostructures, including carbon black, graphene sheets, and wrapped-up graphene nanostructures, which cover fullerenes, carbon nano onions, carbon nanotubes, and carbon nanoscrolls, followed by carbon quantum dots and nanodiamonds. The focus of this chapter is on the approaches to obtain these structures, their chemical forms, and functionalization. We conclude this chapter with a discussion of top-down approaches to organic nanoparticles, with a particular focus on cellulose nanocrystals, covering their origin, structure, and chemical modification.

Chapter 5, Organic nanoparticles, focuses on organic nanoparticles, in which in contrast to inorganic and carbon nanoparticles the interactions within the organic molecules comprising these particles are far more diverse and complex. We begin the discussion with organic nanoparticles where organic molecules are organized into well-defined three-dimensional aggregates, including micelles, vesicles, and various emulsions. Then we discuss the chemistry of dendrimers, which are nanoscale branched macromolecules.

Chapter 6, Polymer nanoparticles, is dedicated to the methods and mechanisms of polymer nanoparticles synthesis, including heterogeneous polymerization, microemulsion and miniemulsion approaches, dispersion polymerization, forming nanoparticles from presynthesized polymers, nanoprecipitation, and emulsion solvent evaporation. The architecture of polymer nanoparticles is then discussed in detail, covering core–shell and multidomain polymer nanoparticles, nanocapsules, nonspherical polymer nanoparticles, and polymer nanoparticle-based hybrids.

Finally, Chapter 7, Self-assembly, is dedicated to nanoparticle self-assembly. The main type of interactions present between the nanoparticles is discussed in detail, as they strongly influence the type of assembly observed in different nanoparticulate systems. Strategies used to influence the balance of interaction and the directionality of the force field to guide the self-assembly process in the formation of different types of self-assembled structures are also