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

Dwaipayan Chakrabarti, Stefano Sacanna

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780081023020
• eBook ISBN: 9780081023037
• Topic: Physical Sciences
• Subtopic: Nanoscience

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

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: [email protected]

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 p...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Preface
• Chapter 1: Magnetic Colloids as Building Blocks for Complex Structures: Preparation and Assembly
• Chapter 2: Dynamic Assembly of Magnetic Nanocolloids
• Chapter 3: Patchy Colloids: A Theoretical and Numerical Perspective on Functionalized Units for Self-Assembly
• Chapter 4: Synthesis and Self-Assembly of Janus and Triblock Patchy Particles
• Chapter 5: Understanding the Self-Assembly of DNA-Coated Colloids via Theory and Simulations
• Chapter 6: Colloidal Microfluidics
• Index