Shape-anisotropic colloids: Building blocks for complex assemblies

Abstract

Recent breakthroughs in colloidal synthesis allow the control of particle shapes and properties with high precision. This provides us with a constantly expanding library of new anisotropic building blocks, thus opening new avenues to explore colloidal self-assembly at a higher level of complexity. This article reviews the most recent advances in the preparation and self-assembly of colloids with well-defined anisotropic shapes. A particular emphasis is given to solution-based syntheses that provide micron-sized colloids in high yields, and to assembly schemes that exploit the shape anisotropy of the building blocks involved.

Introduction

Self assembly of basic building blocks, such as atoms and molecules, into larger and more complex architectures is a fundamental principle by which nature ‘fabricates’ functional materials. Clearly, this is also the most efficient mechanism to organize colloidal particles into patterns, crystals, or more complex phases that can serve, for example, as photonic band gap structures or templates for advanced materials. However, in contrast to molecular building blocks, whose binding is regulated by highly specific and directional interactions, colloidal building blocks are generally characterized by highly symmetric potentials (e.g., electrostatic and van der Waals), which limit their self assembly to a few simple crystal structures or disordered aggregates. It is not surprising then that over the past decade colloidal self assembly has been one of the major driving themes in materials science and that an enormous amount of work has been devoted to engineering colloidal building blocks with unusual shapes and functionalities [1], [2], [3]. In particular, novel synthetic strategies have been developed to impart colloidal particles with chemical and morphological anisotropy, trying to mimic atoms and molecules in their selective and directional interactions.

Today, one of the most powerful strategies to achieve highly selective interactions between colloidal particles is to functionalize their surface with DNA strands and borrow from biology the binding specificity encoded into the DNA sequences. This creates the ability to use colloidal spheres functionalized with DNA to engineer crystalline assemblies, clusters and potentially self-replicating colloidal structures [4], [5], [6], [7], [8]. Other than selectivity, surface functionalization can also provide colloids with directional interactions. This is realized by inhomogeneous surface coatings that yield chemically patterned Janus and ‘patchy’ particles, whose binding can be regulated by small molecules, click-chemistry [9], [10] or DNA hybridization [7], [11].

Despite this constantly expanding library of exotic colloids, advances in colloidal self-assembly are surprisingly scarce and the corresponding self-assembled structures still remain fairly simple. With the exception of mechanisms based on hydrophobic–hydrophilic interactions [12•], [13], self-assembly strategies that rely solely on particles' surface chemistry are, in fact, complex and not efficiently scalable. Therefore, over the past few years there has been a significant effort to exploit particle shape to simplify chemistry-driven assembly schemes into simpler mechanisms driven by general and more fundamental principles.

Herein we survey the most recent advances concerning the synthesis and self-assembly of nonspherical micro-particles and we highlight how adding shape anisotropy to colloidal building blocks enormously extends their potential to form new and more complex structural motifs.