Soft, Hard, and Hybrid Janus Structures

The following sections are included:

• Preface
• About the Editors
• Contents

In this chapter, we comprehensively introduce the recent developments on anisotropic Janus particles, including hollow structure, non-spherical, sheets/disks, rods/cylinders, ring, etc. Also, we systematically compare different methodologies and present some emerging application examples. At last, we conclude some issues and trending for next research as well.

The properties of Janus particles (JPs) have been extensively studied in recent years, but their practical applications are still hampered by limitations in the synthetic approach, especially for particles on the nanoscale. Simple and versatile technologies for the well-defined and scalable synthesis of JPs are highly sought after for broad applications. This chapter reviews the main existing approaches for the synthesis of JPs, with particular emphasis on seeded emulsion polymerization. Emulsion-based methods have proven to be effective for the large-scale synthesis of multifunctional JPs with nanometer size range. Important applications of JPs are also reviewed and discussed.

Colloidal inorganic nanocrystals (NCs) constitute an important class of advanced nanomaterials owing to the flexibility with which their dimensionality-dependent physical–chemical properties can be controlled by engineering their compositional, structural, and geometric features in the synthesis stage and the versatility with which they can be exploited in disparate technological fields, spanning from optoelectronics, energy conversion/production to catalysis, and biomedicine. In recent years, building upon knowledge acquired on the thermodynamic and kinetic processes that underlie NC evolution in liquid media, synthetic nanochemistry research has made tremendous advances, opening new possibilities for designing, creating, and mastering increasingly complex NC-based assemblies, in which sections of different materials are grouped together into free-standing, easily processable multifunctional nanocomposite systems. This chapter will provide an overview of this fast-growing research field by illustrating progress achieved in the wet-chemical development of last-generation breeds of so-called hybrid or heterostructured nanocrystals (HNCs) in asymmetric non-core/shell geometries, in which distinct material modules are interconnected in heterodimer, heterooligomer, and anisotropic multidomain architectures via heteroepitaxial bonding interfaces of limited extension. The focus will be on HNCs that incorporate at least one magnetic material component combined with semiconductors and/or plasmonic metals, which hold potential for generating enhanced, unconventional magnetic behavior, on one side, and diversified or even new properties and capabilities, on the other side. Various synthetic strategies, all based on the manipulation of seeded-growth techniques, will be described and rationally interpreted within the framework of the currently understood mechanisms of colloidal heteroepitaxy.

Recent breakthroughs in nanomaterial synthesis have led to continuous development of an expanding library of novel, complex, and shape-controlled structures with unique properties. Such anisotropic Janus particles (JPs) are colloids that provide asymmetry and can impart different chemical and physical properties and directionality within a single particle. The fabrication of such asymmetric particles has attracted tremendous interest amongst research scientists and this interest arises from their fascinating properties and the exciting range of potential application opportunities. JPs are typically divided into three categories, namely, polymeric, inorganic, and polymeric-inorganic. Each of this kind of JPs can be of a range of shapes such as spherical, snowman-shaped, rod-shaped, or cylindrical. Herein, this chapter focuses mainly on JPs with a magnetic component, with emphasis on the fabrication methods and also the unique properties and applications of magnetic JPs in recent years.

Magnetic anisotropic particles, including Janus, patchy and multicompartment particles, have been particularly of great interest due to their magnetic response and anisotropy potential application in a wide variety of fields. Some strategies and techniques have been used to fabricate magnetic anisotropic particles. In this chapter, one of the main intension is to impart information about achievements on fabrication strategies and techniques of magnetic anisotropic particles in the last several years. A special emphasis will be made on several facial and scalable techniques. Some applications of these magnetic anisotropic particles are also introduced.

This chapter reviews recent progress in the emerging field of synthesis, self-assembly, and applications of amphiphilic Janus nanoparticles (JNPs) and triblock JNP analogs formed by the phase separation of ligands on the inorganic nanoparticle (NP) surface. In particular, JNPs, composed of NPs tethered with phase-separated polymer brushes, have recently emerged as a novel class of building blocks for fabricating new functional hybrid materials. This type of JNP shows great promise in the nanoscale design of new hybrid materials with increasing complexity and functionality. On the one hand, the JNPs can be structurally considered as organic molecular analogs that are capable of self-assembly at multiple hierarchical levels. On the other hand, the assembly of JNPs is significantly different from molecular assembly owing to their unique dimensions, hybrid composition, complex geometry, and multiscale interactions throughout the course of assembly. In this chapter, we will summarize the recent progresses on the self-assembly of JNPs into functional materials and their corresponding biological applications.

We review the results of recent numerical and experimental studies of the self-assembly of Janus ellipsoids to examine the role of particle aspect ratio and patch orientation in dictating associated cluster morphologies and properties. Our main emphasis is on the use of simple models in computer simulation to describe the process and structural and thermodynamic consequences of self-assembly. More specifically, we consider both oblate and prolate spheroidal particles and examine the implications of particle geometry on cluster formation. The discussion is then expanded to include the case of patchy ellipsoids in which the size of spheroidal surface patches can be systematically varied. The overarching aim of this review is to highlight the broad range of structures and rich thermodynamic behavior that characterize Janus ellipsoidal systems.

This chapter describes how small-scale devices with a “two-faced” structure, often termed catalytic Janus swimmers, can be used to generate autonomous motion within fluids at small scales. The chapter starts by giving a general introduction to the field of swimming devices, and then surveys the variety of different Janus structures that have been used to produce motion. We then describe the methods that have been employed to synthesize these asymmetrical catalytic swimmers, and review the current understanding of the mechanisms that produce motion from catalytic Janus structures. The remainder of the chapter first focuses on the challenge of accurately quantifying the motion characteristics for catalytic Janus swimmers, and then describes the considerable challenge of controlling the directionality of motion, which is subject to stochastic phenomena. Finally, we survey existing demonstrations of potential applications for Janus catalytic swimmers, and discuss future directions within this research field.

Recent years have witnessed a rapid development of novel Janus structures and exploration of their biomedical applications. Unlike conventional homogeneous particles, the surface or structural anisotropy of Janus particles (JPs) allows different or even incompatible functionalities to be combined within a single particle unit. As a result of such anisotropy, novel biomedical applications were made possible. Recent advances include synergistic drug delivery, cell targeting, multimodal bioimaging, biosensing, self-propelled therapeutic motors, and various combinations of these capacities. These examples, which are highlighted in this chapter, are likely to be just the beginning of a new era of JP-based therapeutics. This chapter also discusses the challenges that must be overcome to realize the many unique opportunities presented by JPs.

Janus particles (JPs), named after the two-faced Roman god, are a unique class among the multifunctional patchy particles, combining two distinct chemical/physical functionalities at their opposite sides. The exclusive chemical and geometrical asymmetry of the JPs provides the possibility to address complex self-assembled architectures and materials inaccessible for homogeneous building blocks. In this chapter, we will highlight the recent developments in the employment of JPs as novel building blocks for active multifunctional materials. First, a short introduction into the synthesis, assembly, and applications of the JPs will be presented, covering a broad variety of the state-of-the-art strategies and techniques. The challenges faced in the topic of JPs and the subsequent opportunities will be discussed. The main focus, however, will be placed on the application of JPs for active surfaces and interfaces, providing a review on the diversity of up-to-date studies aimed at different applications. In terms of active interfaces, the stabilization efficiency of the JPs at interfaces can be combined with the ability of interface structuring/functionalizing. Due to the high interfacial activity of the JPs and their favorable orientation, the inner and outer sides of the stabilized interfaces (e.g., emulsion droplets) can be furnished with active compounds and catalysts, or feature different optical appearances and other properties, yielding active functional interfaces. In terms of active surfaces, JPs can be utilized as advanced building blocks to construct materials with controlled chemical and topographical heterogeneity as well as wettability and adhesion. Especially, combining both chemical and topographical heterogeneity, such materials may produce synergistic effects favorable for specific applications. Finally, our critical view on the applicability and rational design of multifunctional materials based on the JPs will be suggested [153], closing with the future perspectives and visions.

The following section is included:

• Index