Nanoparticle assembly: from fundamentals to applications: concluding remarks

Abstract

Nanoparticles, due to their broadly tunable functions, are major building blocks for generating new materials. However, building such materials for practical applications by self-assembly is quite challenging. Following the Faraday Discussion on “Nanoparticle Assembly: from Fundamentals to Applications” we discuss here the current trends in the field of self-assembly, including: understanding the unique interplay of molecular and nanoscale effects, a development of novel approaches for the creation of targeted nanoparticle architectures, advances in controlling dynamic behavior of systems and enabling new functions through specifically formed structures.

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Introduction

The great advances in nano-synthesis methods of the last two decades have permitted a fabrication of new functional blocks, nanoparticles (NP), whose properties can be significantly expanded beyond the atomic or macroscale forms of materials with the same chemical compositions. Despite our ability to generate such individual nanoscale blocks, there is an increasing need to establish methods for generating materials and devices that incorporate nanoparticles of different kinds in a unified manner. Moreover, since a material’s properties are directly related to the underlying structure, it is essential to control the NP arrangements in these new types of materials. Approaches based on self-assembly offer tremendous cost-advantages and an ease of manufacturing compared to lithographic methods. Furthermore, self-assembly addresses tasks that are intrinsically challenging for conventional lithography processes, such as creating three-dimensional architectures or structures containing pre-fabricated nano-objects of multiple types. On the other hand, the methods of self-assembly typically have significant limitations: they form relatively simple structures from the same type of components, and they rarely permit the rational design of systems. Developing principles and establishing fabrication approaches for organizing nano-components into targeted materials with desired functions is a primary goal of nanoparticle self-assembly. The Faraday Discussion covered the range of topics from synthesis of nanoparticles and their assemblies to advancing theoretical methods for predicting the structure formation, and from structural properties of nanocomposites (inorganic particle and organic material) to novel functionalities of particle systems. The broad range of nanoscale materials, progress in experimental and theoretical approaches, and potential applications were discussed.

Driving forces of nanoparticle self-assembly

Despite a variety of scales and driving forces the general principles of the spontaneous formation of ordered organizations, i.e. self-assembly, remain similar: the requirement to comply with the interactions and entropic constraints of the components results in the realization of certain structural arrangements. Sphere packing is one of the earliest and the simplest examples of how objects can be arranged into ordered arrays based on their intrinsic properties, in this case, shape. The question of the ‘best’ arrangements of simple spheres can be traced back to Sanskrit texts (499 CE), and it is typically well known as the Kepler conjecture: face-centered cubic and hexagonal close lattices of equal spheres exhibit the highest packing density. The mathematical proof of the Kepler conjecture was shown only very recently by Thomas Hales.¹ The notion of forming an ordered structure based on the properties of objects defines the objective of self-assembly.

In the realm of physical systems, even non-interacting objects can form ordered phases from the disordered liquid state. As was shown more than half a century ago by Onsager for rods and by Wood and Alder for hard spheres, entropy increases for crystallized systems due to the larger number of available microscopic states. The phases can be significantly diversified as soon as more complex blocks are introduced. For example, assembling binary mixtures of spherical particles of dissimilar sizes results in a rather rich set of crystallographic symmetries. As soon as an object shape becomes anisotropic the complexity of organization increases drastically. A recent comprehensive simulation study on the assembly of solid shaped blocks showed the richness of phases, the formation of ordered, plastic and quasi-crystalline organizations.^2,3 Real physical systems rarely obey the idealized conditions of non-interacting objects, however the richness of these interactions is a scientists' playground. The interparticle interactions can significantly modulate the assembly behavior leading to phases beyond those determined by shape considerations. For example, many studies have explored how van der Waals,⁴ electrostatic,⁵ dipolar,^6,7 and magnetic forces⁵ between particles affect the structure formation. Moreover, the idealized representation of a particle as a solid block is also far from reality. In this respect, the structure of shells formed by ligands and polymers^8–13 around a particle core has a profound effect on the phase behavior. These effects become more important as soon as we move to nanoscale blocks where particle and molecular dimensions become comparable.

Nanoparticles: the intersection of molecules and colloids

In contrast to typical micron-sized colloidal particles, the sizes of nanoscale colloids, or nanoparticles, approach those of molecules. Since coating a nanoparticle with ligands is needed for particle stabilization and functionality, the organic shells are an integral part of the particle structure. Therefore, the idealized representation of nanoscale objects as describing them only by shape is far from complete. In fact, those organic shells play a crucial role in the way NPs interact and assemble. However, to deduct the relationship between particle assembly behavior and shell properties is often difficult, mainly due to the limited methods for revealing the NP shell structure. Indeed, commonly used electron microscopy and X-ray scattering methods are well positioned to extract information about inorganic particle cores, but not shells.

There are several aspects of organic shells that lead to their special role for nanoparticles. First of all, organic molecules introduce a certain degree of ‘softness’ to particles and provide interactions through chemical groups and/or charges. Further, novel effects can arise for molecules attached to a surface with a high (and not necessarily constant) curvature. For example, due to entropic reasons molecules can distribute heterogeneously on the surface of a shaped nanoparticle, thus, leading to the formation of a shell, which is not conformal with the underlying core shape. Additionally, a relatively small number of molecules on a nanoparticle might manifest in a discrete character of shells, and that can result in the anisotropic interactions. Hence, soft shells can significantly modulate particle shape, influence interparticle binding motifs, and even contribute to establishing anisotropic interactions. Accordingly, the resulting NP arrays can be quite different from expectations based only on geometrical considerations.^8,15,16

The directionality provided by organic shells can be used to regulate nanoscale assembly, for instance, via electrostatics, steric effects, and molecular bonds established between organic tethers. In addition, related effects can occur when nanoparticles are imbedded in the polymer matrix, and the structure of polymers can be significantly affected by the particles. Apart from the fascinating structural effects, the molecular arrangements can have significant influence on the mechanical properties of nanoparticle-based materials. For example, Michael Bockstaller of Carnegie Mellon University discussed the relationships between the structure of a polymer brush grafted to a nanoparticle, the degree of order in the system and mechanical response of these hybrid materials (DOI: 10.1039/C5FD00121H). Also, Jacques Jestin of Laboratoire Léon Brillouin CEA-CNRS presented a study on the interplay of polymer chain conformations and particle assembly behavior, relevant to industrial problems (DOI: 10.1039/C5FD00130G). As was shown by Andrea Tao from the University of California, San Diego, the polymer distribution on cubic nanoparticles can play an important role in particle placement relative to each other, and consequently, that is reflected in the plasmonic properties (DOI: 10.1039/C5FD00134J).¹⁴

Complex morphologies through self-assembly

The majority of self-assembly studies in the field are typically concerned with the question: what is the origin of assembly behavior and how can one track back the particle characteristics and interparticle forces to explain the observed phases? These important questions drove the field and resulted in great advances in our understanding of assembly phenomena. However, the demand for transferring self-assembly effects into the territory of concrete material fabrication methods requires answering a different question: how to assemble a particular targeted organization from those specific particles? Given the fact that nanoparticle blocks carry the functions and the final material requires a certain organization of those blocks, the minimum modification of the blocks would be highly beneficial from a practical perspective. On a fundamental level, this required addressing an inverse problem in self-assembly: rather than asking why the structure has formed, one should ask how to drive the system into formation of the targeted structure.

A number of approaches have been proposed in recent years to address that question. For example, an ability to modulate the interactions between particles in an arbitrary way permits inducing the desired organization, as has been shown in computational studies.¹⁵ However, in order to achieve such complex structures long-range interactions (several interparticle distances) with tailored potential shapes are needed. Unfortunately, it is quite challenging to implement these ideas experimentally.

Alternative approaches for controlling particle structures have been considered using anisotropic interactions. For example, one of the broadly used models, known as patchy particles, requires a precise arrangement of binding patches on a particle surface, and that restricts interparticle connections. Thus, through the geometrical arrangement of the patches one can, in principal, control the formation of assembled clusters and lattices. Extensive studies were conducted^2,17–23 and interesting organizations were observed both theoretically and experimentally. However, the formation of ordered lattices from patchy particles might be hampered by defects due to particle orientation freedom and such imperfections are difficult to heal.^24,25 Besides, that approach requires massive high-fidelity particle fabrication, which is quite challenging at the present time, particularly at the nanoscale.^21,26–28

Recently, a new idea was demonstrated: instead of tailoring the particle anisotropic properties, it was proposed to use an anisotropic connecting block as the connector between spherical particles.^29–31 In this approach, the directionality of interactions is determined by the block's anisotropic characteristics, thus, simply by introducing different types of blocks, different types of organization can be induced without changing the spherical particles. The demonstration of this approach for blocks that interact with spheres via facets and via vertices was shown experimentally, while DNA was used to regulate the local binding properties. For example, cubic and octahedral NPs were used to coordinate, by their faces, spherical particles into clusters and lattices.²⁹ Also, using precisely designed and shaped molecular constructs, based on DNA, it was possible to coordinate particles through attachment to the vertices of the constructs and that resulted in the formation of a variety of NP lattice types, including cubic, body-centred-tetragonal, diamond and others, using polyhedral DNA frames as topological interparticle connectors and particle cages.^30,31 While the effect of block shapes and the placement of binding sites on the final lattice symmetry still have to be understood, the approach opens exciting opportunities for building periodic structures in a predictable way.

The idea of anisotropic interactions can be significantly advanced towards the fabrication of complex shaped morphologies if a particle possesses not only anisotropic binding properties but also addressable interactions, as was demonstrated by a computational study by Daan Frenkel of Cambridge University (DOI: 10.1039/C5FD00135H). In this case using DNA addressability it is possible to differentiate different bonds, and such “coloring” of bonds provides a means to connect a large number of particles with different types of coloring in quite unique architectures.^32–34 Remarkably, the formed structures have programmed structural elements with dimensions of many particle sizes. Erica Eiser of Cambridge University showed the experimental example of these DNA-based methods (DOI: 10.1039/C5FD00120J). It is perceived that approaches based on so-called programmable interactions, for example using DNA specificity of interparticle interactions, might potentially allow for the practical realization of self-assembly by design. An interesting question that arises for the assembly of complex non-periodic architectures is how to provide information about the desired structure to be assembled into the self-assembly process?

In addition to controlling local intercalations alternative approaches are based on inducing particle morphologies, for example, through the media in order to promote the assembly process. Guruswamy Kumaraswamy from CSIR-National Chemical Laboratory at Pune demonstrated the use of ice crystallization for particle aggregation between crystalline ice domains, so-called ice templating (DOI: 10.1039/C5FD00125K). The possibility of using lipid membranes to mediate the formation of complex organizations of curved particles was discussed by Mohamed Laradji of the University of Memphis (DOI: 10.1039/C5FD00144G).

Dynamic self-organization processes

One of the long-standing challenges in self-assembly is the ability to simultaneously control structure formation on different length-scales and its transformation in time or on demand.³⁵ The structural plasticity of organic molecules forming NP shells and their sensitivity to environmental parameters and molecular stimuli makes NP systems highly suitable for post-assembly transformations. The change in interparticle distance is quite important for modulating functional materials made from NPs and a number of approaches have been demonstrated. On the other side it is even more exciting to regulate a change in the entire type of structural organization on demand. Recent studies show such implementations by employing switchable DNA bonds.^36,37

Conventional self-assembly approaches typically rely on a system evolving towards its equilibrium state. However, even energetically favorable states can be difficult to realize, owing to the complexity of evolution pathways. It is important to stress that self-assembly, as a process, is non-equilibrium. Depending on the system details, the process might be thermodynamic or driven by external forces. Regardless of the process details, it is never guaranteed that a system will reach a well-ordered structural organization, either because of metastable states or due to the intrinsic characteristics of the system. In fact, a plethora of theoretical and experimental studies has shown that ordered systems only form under well-controlled conditions in which a balance of interactions and mobility of components is achieved. The ‘successful’ (i.e. producing order) assembly is realized only when the assembly and disassembly continuously occur. That implies that interactions are strong enough to drive components toward each other but, at the same time, the components have the ability to explore microstates, which can only occur if there is no permanent binding. While for thermodynamically driven assembly the interaction energy during assembly should be on the order of kT, for systems driven by external forces much stronger interactions can be allowed.

For instance, arrested states can be overcome more easily for systems far from equilibrium. In non-equilibrium scenarios, additional control is achieved via regulated kinetic effects, which often allow the control of structural organization on several scales. A number of experimental approaches have been developed for enhancing order in a system using local heating and mechanical methods. Alamgir Karim of the University of Akron discussed the use of shear forces to promote nanoparticle organization in string-like structures in polymer films (DOI: 10.1039/C5FD00141B). The transient, far from equilibrium, states often have more profound effects on improving the order. The interplay of kinetic and equilibrium effects in this respect is far from being understood. An important related question pertains the characteristic length of a particle's motion and its timescale during the assembly and domain re-arrangements. Obtaining a better understanding of particle transport in highly confined environments is important in this respect. Lynn Walker of Carnegie Mellon University demonstrated NP diffusion in the matrix formed by block copolymer micelles (DOI: 10.1039/C5FD00122F).

Another aspect of dynamic self-assembly is using external fields to induce the athermal mobility of a system's components or alter the interactions between them. It was shown by Steve Granick of the BS Center for Soft and Living Matter in Korea and the University of Illinois at Urbana-Champaign that complex collective motion can be induced, for example on Janus magnetic particles in magnetic fields,³⁸ and the field controls the character of the motion and dynamic structure (DOI: 10.1039/C6FD00024J). Also, plasmofluidic fields can be efficiently used to drive plasmonic NPs, as was shown by Pavan Kumar of IISER Pune (DOI: 10.1039/C5FD00127G).

Outlook

In recent years, a variety of powerful approaches for particle assembly have been developed using ligands, polymers, biomolecules, and particle shaping. The approaches rely either on a final balance of interactions and entropic factors, on a more deliberate encoding of components, or on the use of patterns and templates for particle positioning. Much progress has been achieved in the creation of clusters and lattices using particles of different sizes, shapes, and materials. While we are far from the complete understanding of these systems, both experimental and theoretical efforts demonstrate significant progress in our ability to predict and to explain the formation of ordered phases. In the near future we may witness a rapid development of approaches that bring the science of self-assembly to a new level; new strategies might allow for the rational self-assembly of desired architectures with and without periodic organization, and with the ability to reconfigure these structures on demand. These advances can be translated into the practical fabrication of novel types of materials from functional particles and such systems might enable the engineering of desired optical, magnetic and bio-related functions.³⁹ For example, Nick Kotov of the University of Michigan demonstrated that chiroptical activity can be fully engineered into particle clusters and those systems can be applied for gas detection (DOI: 10.1039/C5FD00138B). Rajdip Bandyopadhyaya from IIT Bombay showed a catalytic activity enhancement by NPs imbedded into a silica matrix (DOI: 10.1039/C5FD00126A). The notion of limited practical applicability of self-assembled systems is of high concern, as was discussed by many participants. Thus, there is an increasing need to transfer the development of self-assembly methods into the fabrication of functional materials and devices.

In the upcoming years self-assembly will tackle the range of problems that should establish it as a versatile manufacturing platform at the nanoscale. Moreover, it might enable the creation of materials that mimic biological systems in their adaptive and energy-processing properties, but will still take advantage of inorganic components.

These potentially interesting developments might address the following challenges in building complex systems:

• Reverse engineering of multicomponent self-assembled structures: from arbitrarily prescribed lattices to non-periodic designed morphologies, and to supra-nanoscale architectures.

• Error correction and structure healing in self-assembled structures.

• Systems with regulated dynamic behavior and evolution pathways.

• Incorporation of active components into assembly schemes to overcome the limitations of thermal systems.

• An ability to deliver information and instructions about required processes and structuring to individual components and to a system on a global level.

Merging such capabilities of self-assembly methods with the functions of nanoscale building blocks will open tremendous prospects for creating materials from blueprints, establishing materials with dynamic performance, and inventing material systems that can autonomously manage energy flows for the function-required reorganizations.

Acknowledgements

Research, carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.

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