Self-Assembly of Nano- and Micro-structured Materials Using Colloidal Engineering

By Elsevier Science

Self-assembly of Nano- and Micro-structured Materials Using Colloidal Engineering, Volume 12, covers the recent breakthroughs in the design and manufacture of functional colloids at the micro- and nanoscale level. In addition, it provides analyses on how these functionalities can be exploited to develop self-assembly pathways towards nano- and micro-structured materials. As we seek increasingly complex functions for colloidal superstructures, in silico design will play a critical role in guiding experimental fabrication by reducing the element of trial-and-error that would otherwise be involved.

In addition to novel experimental approaches, recent developments in computational modelling are also presented, along with an overview of the arsenal of designing tools that are available to the modern materials scientist.

• Focuses on promoting feedback between experiment, theory and computation in this cross-disciplinary research area
• Shows how colloid science plays a crucial role in the bottom-up fabrication of nanostructured materials
• Presents recent developments in computational modelling

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 25, 2019
• ISBN: 9780081023037

Book preview

Preface

Dwaipayan Chakrabarti; Stefano Sacanna

The advances in colloid science through the 20th century paved the way for what we now call colloid engineering. The term refers to the research activities devoted to engineering the interactions between colloidal particles to program their self-assembly into desired structures and/or realize certain functions. Our understanding of the structure–function relationships often sets the targets in the first place. Understanding the interplay of interactions between colloidal particles is crucial to the success of their programmed self-assembly. Traditionally, colloidal particles have been spherical in shape, with a diameter in the range from a few nanometers to a few microns–large enough to be visualized under optical microscope, yet small enough to sustain Brownian motion while dispersed in a fluid medium. Since the turn of the century, we have experienced a surge in the synthesis of colloidal particles beyond this traditional remit, pushing the boundaries of colloidal self-assembly.

A library of exotic colloidal particles is now available, rich in shape and surface chemistry, thanks to a remarkable progress in the synthetic techniques in recent years. A key feature that these exotic colloidal particles often share is the directionality and/or specificity of interactions, paving the way for a wide array of finite supracolloidal structures as well as colloidal crystals beyond the closed-packed architectures conventionally observed. This trend has firmly established the concept of the so-called colloidal molecules beyond the traditional picture of colloidal particles being viewed as big atoms suitable for studying various physical phenomena, such as crystallization, gelation, and glass transition.

This edited volume draws on the contributions from active researchers in the form of six chapters, providing an overview of the state-of-the-art colloid engineering for bottom-up routes to nano- and microstructured materials. The emergence of colloid engineering at the turn of the 21st century and its subsequent growth have immensely benefited from feedback between experiment, theory and simulation, which is captured by these chapters. In addition to providing accounts of recent developments in the synthesis and self-assembly of a variety of colloidal particles, the authors reflect on the challenges and present their outlook on research directions in the future.

While magnetic interactions between colloidal particles have long been studied in the context of ferrofluids as a primitive example of anisotropic interactions, more recent trend in exploiting magnetic interactions in engineering self-assembly of colloidal particles is covered in Chapters 1 and 2 by Rossi and Dobnikar, respectively. In Chapter 1, Rossi discusses recent advances in the synthesis of magnetic colloids and highlights the distinct advantages of employing magnetic interactions, being amenable to an external magnetic field, in the context of programmable self-assembly. Her account is especially focused on exploiting magnetic interactions in the presence of shape anisotropy, and thus the interplay between the two anisotropy attributes. Dobnikar presents an account in Chapter 2, focusing on dynamic self-assembly of magnetic nanocolloids. This chapter discusses a number of recent studies of mostly superparamagnetic colloids and highlights emergent complexity out of equilibrium, driven by time-dependent magnetic fields.

Chapters 3 and 4 focus on patchy particles, which offer highly directional interactions, relying on their rich surface chemistry. Since the inception of the concept of patchy particles, an extensive body of work has been undertaken to study the self-assembly of patchy colloidal particles into a variety of structures, both finite and periodic. In Chapter 3, Bianchi presents a theoretical and numerical perspective of patchy colloids, giving a brief historical account of the evolution of the class of patchy-particle models. She also highlights the prospects of these particles to serve as the building blocks of self-assembled materials, with a focus on the so-called inverse patchy colloids. In Chapter 4, Chen and coworkers discuss synthetic methods for Janus and multipatch colloidal particles and their self-assembly behavior, investigated both in experimental and computational studies. These studies are largely targeted for open crystals, which are known to have appealing applications as photonic crystals as well as phononic and mechanical metamaterials.

In Chapter 5, Angioletti-Uberti offers a perspective of DNA-coated colloids as building blocks for programmable self-assembly due to their highly selective interactions arising from DNA hybridization. In particular, a variety of theoretical and computational approaches are described to study DNA-coated colloids, with a focus on the understanding of the physics of multivalent binding to enable colloid engineering. Finally, in Chapter 6, Royall and coworkers review the use of microfluidics to produce particles on the colloidal length scale. In addition to conventional production techniques, newer methods, which can produce oil droplets on the colloidal length scale, are discussed, along with the applications of these droplets as colloidal model systems.

While the present volume was not intended to provide a comprehensive picture of the current state of the art of colloid engineering, we do hope that the overview presented here captures the current excitement of this vibrant area of research. This would not have been possible without the contributions from the authors, to whom we would like to express our sincere gratitude. Our endeavor would be deemed successful only if this excitement is conveyed to the readers.

Chapter 1

Magnetic Colloids as Building Blocks for Complex Structures: Preparation and Assembly

Laura Rossi¹    Department of Chemical Engineering, Delft University of Technology, Delft, The Netherlands

¹ Corresponding author: email address: [e-mail]

Abstract

Assembling complex architectures with novel geometries and tailored properties requires the development of suitably designed colloidal building blocks that assemble through specific and directional interactions. Magnetic colloids have the potential to provide the necessary tools to obtain programmable building blocks due to the innate directionality of magnetic interactions. Magnetic dipoles can be permanently embedded into particles and they allow for remote control and manipulation via external fields independently of the chemical and physical composition of their dispersion medium. These properties put magnetic colloids at an advantage point over other currently available systems. In this chapter, we discuss recent advances on the synthesis of magnetic model colloids and their role in the preparation of rational structures via self-assembly.

Keywords

Colloids; Magnetic particles; Magnetic patches; Self-assembly; Complex colloids

1.1 Introduction

Spontaneous assembly of complex structures from colloidal building blocks is not only relevant to understand the underlying mechanisms driving self-assembly processes from the molecular to the macroscopic length scale, but it is also important in material science where new structures can lead to the preparation of novel materials with unique properties. To achieve higher levels of structural complexity, such as low-coordination architectures or metamaterials, it is necessary to induce specific oriented attachment of the building blocks [1,2]. However, programming colloidal building blocks to obtain complex structures is no easy task. Reverse-engineering of target structures can be realized only when colloidal building blocks possess key anisotropy attributes in shape and/or interaction, requiring accurate control over particle design. Several approaches are currently being investigated for the development of colloids for self-assembly [2–9], as will become clear in later chapters. One possible strategy consists of using inherently directional magnetic interactions to design programmable building blocks [10]. This approach has already been used to drive paramagnetic colloid assembly with an external magnetic field [11–14] and, more recently, theoretical and computational works have demonstrated the potential to assemble particles into novel hierarchical structures using permanently magnetized colloids [15–17]. Magnetic interactions are very promising as they allow for control and manipulation of the building blocks and their assemblies independently of the chemical and physical properties of the medium in which they are dispersed, a great advantage over other surface-mediated assembly methods. Another benefit is that, contrary to electric dipoles, magnetic dipoles can be permanently embedded into particles. This feature allows to effectively program the building blocks, promoting spontaneous assembly without the help of complex external fields.

In spite of all these benefits, magnetic colloids are only just recently finding direct application in colloidal assembly [18]. Nowadays, not only new synthesis methods are being developed, but also older preparation techniques are being revisited giving new life to colloidal systems that have been disregarded for many years in the context of programmed assembly. One striking example, as we see later, is of micron-sized hematite colloids [19–21].

Magnetic colloids can be prepared from iron, cobalt, manganese, chromium, and nickel [22] usually in the form of oxides, although the majority of the commonly used synthesis methods rely on iron and cobalt compounds. Most of the magnetic metal oxide colloids available at the moment are in the form of nanoparticles and have been mostly used to develop ferrofluids or as model systems to understand dipolar fluids and other fundamental phenomena [23,24]. They have also been widely used for the preparation of larger composite colloids, in which dipolar nanoparticles are embedded in the particle matrix, usually a polymer, with random dipolar orientations. These composite particles do not possess a permanent dipole moment, but they can be magnetized and manipulated with an applied magnetic field [14,25]. When exposed to uniform magnetic fields, magnetizable particles tend to form dipolar chains, similarly to polarizable particles responding to an applied electric field [26], with the advantage that no special consideration to the physicochemical properties of the particles and the solvent needs to be taken into account, making paramagnetic colloids extremely versatile. Since dipolar interactions are anisotropic by nature, designing magnetic colloids with more complex structures (e.g., shape anisotropy) enables the preparation of building blocks in which the coexistence of steric (driven by the particle shape) and dipolar (driven by the direction of the dipole moment) interactions drives the self-assembly of structures with novel geometries as recent experiments are starting to show [18].

In this chapter, we survey the preparation of magnetic colloids and their assembly into complex structures. We describe recent advances in the design and manufacture of magnetic colloids (Section 1.2) and explore and analyze their programmed assembly into nano- and microstructured materials (Section 1.3), including a brief overview of computer simulations. We finish the chapter by providing a short perspective on the advantages of using magnetic interaction in colloidal self-assembly and by drawing some conclusions (Section 1.4).

1.2 Preparation of Magnetic Colloids

The preparation of novel colloidal particles is a fast evolving area of soft matter. Magnetic colloids appear only in a small portion of the available literature on colloidal synthesis; however, as novel techniques are being developed, an increasing number of publications are appearing on the topic. Here, we are going to review the most common and newly developed techniques available for the preparation of magnetic colloids focusing on systems with well-defined shapes and properties that are most useful for assembly purposes.

We first cover the preparation of uniform magnetic particles, particles in which the magnetic properties are uniformly distributed among their volume or surface, focusing our attention on the synthesis of superparamagnetic nanoparticles, and of magnetizable latex and silica beads. We then describe synthetic methods for the preparation of magnetic patchy particles, particles in which the magnetic properties are localized in specific areas.

1.2.1 Uniform Magnetic Particles

One of the simplest methods for the preparation of magnetic colloids is the coprecipitation of iron salts into iron oxide nanoparticles by addition of a base [27,28]. These syntheses are carried out in aqueous environments; however, surface functionalizations with, for instance, surfactants or fatty acids enable the dispersion of magnetic nanoparticles in organic media [29]. While synthesis parameters such as salts concentration, temperature, and pH can be tuned to influence the end result, usually these syntheses yield nanoparticles that are fairly polydisperse in both size and shape. Similar procedures can also be employed for the preparation of manganese and cobalt ferrites [30].

A certain control over the size distribution of the magnetic nanoparticles can be obtained, for instance, by precipitating magnetic materials inside microemulsion droplets [31]. Furthermore, monodispersed magnetic nanoparticles can be obtained with more laborious synthesis methods which include the thermal decomposition of organometallic precursors (e.g., iron pentacarbonyl) at high temperatures, a single-step process in which nucleation and growth occur at different stages [22,32,33]. These methods allow for the preparation of particles with a better control over size and morphology and can also lead to high reaction yields [32]. Additionally, monodisperse dipolar particles can be obtained by multistep synthesis procedures where nucleation and growth steps are well defined as reported for magnetite colloids of different sizes [24,34]. Tuning the crystal shape of the magnetic nanoparticles using the appropriate surfactant as stabilizer during synthesis can promote the formation of magnetic nanoparticles with, for instance, cubic shapes [35,36].

Besides their use in many technological applications [37], magnetic nanoparticles can be employed in the preparation of larger, more complex, composite colloids where they are embedded in amorphous, nonmagnetic matrices such as organic polymers or silica. These particles can be relatively simple, when their magnetic content is homogeneously distributed over the whole particle volume or surface. However, more complex colloids can be obtained, when the magnetic content is localized to give rise to what are now known as magnetic patches. These particles rarely possess a permanent dipole moment, nonetheless they find a variety of applications since their assembly can be easily manipulated and controlled with applied external magnetic fields. Monodisperse magnetizable polymeric and silica microspheres with various sizes are available for purchase from several suppliers and most assembly studies are carried out using such particles. Synthesis techniques that are found in the literature for the preparation of magnetic spheres are mostly dedicated to their applications in catalysis [38–40], biomedicine, and biotechnology [41–44]. This is because magnetic spheres are often used as carriers of catalysts and drugs. Nonetheless, many of such syntheses yield monodisperse magnetizable particles that are also suitable for self-assembly studies. In Ref. [45], Claesson and Philipse reported the preparation of magnetizable silica spheres of about 200 nm in diameter with tunable dipolar interactions by depositing different amounts of maghemite and cobalt ferrite onto silica particles via surface functionalization with a silane coupling agent 3-mercaptopropyl(trimethoxy)silane. These composite particles were further coated with a silica layer to improve stability in different solvents, tune the magnetic interactions, and promote surface functionalization. A similar technique has also been used by Salgueiriño-Maceira et al. [46] to coat silica with maghemite and magnetite without surface functionalization. In this case the authors performed an additional coating with gold to provide the particles with specific optical properties. In the work by Fang et al. [47], silica particles with sizes of about 120–700 nm were loaded with magnetite nanoparticles by performing the traditional Stöber method [48] in the presence of magnetite nanoparticles. The particles’ magnetic content was adjusted by changing the ratio between the magnetic particles and the silica precursor in the Stöber reaction mixture. Further modification of the silica surface after loading with magnetic particles allows for the preparation of more complex magnetic particles. For instance, growing a thermosensitive poly(N-isopropylacrylamide) layer on magnetic silica particles allowed Luo et al. [49] to prepare magnetic spheres whose size is sensitive to temperature variations [50]. Magnetic hollow silica rough [51] or smooth [52] particles can also be prepared using polystyrene particles as a template.

Magnetic lattices are synthesized using different techniques such as seed emulsion polymerization [53,54], miniemulsion polymerization [55–57], and double miniemulsion polymerization [58]. Another technique involves using magnetic particles to stabilize Pickering emulsions [59] of a polymerizable oil [60]. Here, after polymerization, magnetic nanoparticles remain on the surface, allowing the composite lattices to respond to an external magnetic field [61,62]. The same technique was later used by the same authors in a different work to promote the formation of monodisperse magnetic core–shell colloids with a size of 200–500 nm in a single-step synthesis. In brief, magnetic particles (magnetite or cobalt ferrite), a polymerizable oil (methacryloxy-propyltrimethoxysilane, TPM), and methyl methacrylate monomers were mixed in water where the magnetic nanoparticles can stabilize emulsion droplets of the TPM oil [60]. When radical polymerization starts, not only the TPM droplets polymerize, but a poly(methyl methacrylate) shell is simultaneously grown on the surface of the TPM particles [63]. In the same work the authors also show how magnetic TPM particles can be coated with a silica shell.

Micron-sized magnetically responsive nonspherical particles can be prepared using various lithographic techniques [64,65]. Examples include molding a UV-curable monomer loaded with magnetic nanoparticles into preformed PDMS wells with different shapes [66] and incorporation of magnetic nanoparticles in microgels prepared by stop-flow lithography [67].

Composite magnetic colloids as described here, while responsive to an external magnetic field, do not generally possess a permanent dipole moment, even when the nanoparticles used for their preparation do. This is due to the random orientations that the dipole moments of the single nanoparticles adopt in the matrix, giving the overall particle a zero net dipole moment. In certain cases, it has been shown that such particles can be permanently magnetized at room temperature [68,69]; however, their magnetic dipole moment is usually very low and often not sufficient to drive self-assembly.

Increasing the size of magnetic colloids from common magnetic materials, such as magnetite, is also not an option. This is because while the spontaneous magnetization of magnetite is quite big, its magnetic domain size, a material-specific volume in which the magnetization is in a uniform direction, is in the order of 100 nm. Therefore, above the 100 nm size range magnetite nanoparticles become multidomain and, while they might still show some remanence magnetization, they possess a much lower, usually negligible, volume magnetization compared to the single domain case. This behavior is prevalent for most of the common magnetic materials used for the preparation of magnetic nanoparticles with the exception of hematite.

Hematite is an iron(III) oxide commonly found in rocks and soils. Its synthesis in colloidal form was introduced in the 1970s by Matijević and Scheiner [70] where a method for the preparation of micron-sized hematite colloids with different shapes was reported. The magnetic properties of hematite originate from a canted antiferromagnetism [71] and therefore its spontaneous magnetization is much lower than that of common magnetic materials like magnetite. However, because its domain size is of the order of a few micrometers [72], hematite colloidal particles with a permanent dipole moment can be prepared over a large size range. Fig. 1 shows examples of magnetic hematite particles of different shapes. These particles find many applications as model systems owing to their unique shapes and interesting magnetic properties [17,19–21,74,75]. Yet, some of the most interesting applications rely on their use as magnetic components for the preparation of more complex colloidal building blocks [10,76].

Fig. 1 Scanning (A), transmission (B), and (C) electron microscope images of hematite particles with different shapes prepared following the recipe by Sugimoto et al. [73].

1.2.2 Magnetic Patchy Particles

Magnetic patchy particles are colloids in which the magnetic content is anisotropically distributed among their volume or surface as schematically shown in Fig. 2. With a higher degree of complexity compared to uniform magnetic colloids, colloids with magnetic patches deserve special attention as they provide a novel pathway to controlled assembly. Because the magnetic material is localized only in very specific parts of the particles, the patches, self-assembly can drive the formation of complex structures that can, in principle, be programmed a priori as reported by recent computer simulation studies [77,78]. Despite their significant potential for the assembly of novel structures, there are only a few techniques available for the preparation of magnetic patchy particles, most of which rely on the deposition of magnetic material on colloidal monolayers [79].

Fig. 2 Schematic representation of a library of colloids with single and multiple magnetic patches from spheres (A), (B), and (E) and rod-like particles (C) and (F). Dumbbell particles (D) can be used to prepare more complex patchy colloids. In these schematics, magnetic patches are indicated by the purple areas ( dark gray in the print version). When half of the particle is functionalized with magnetic material as in panels (E) and (F), the particles are usually referred to as magnetic Janus