Self-assembly of nanoparticles: a snapshot

Seeing the Supracolloidal Assemblies in 3D: Unraveling High-Resolution Structures Using Electron Tomography

Transmission electron microscopy (TEM) imaging has revolutionized modern materials science, nanotechnology, and structural biology. Its ability to provide information about materials' structure, composition, and properties at atomic-level resolution has enabled groundbreaking discoveries and the development of innovative materials with precision and accuracy. Electron tomography, single particle reconstruction, and microcrystal electron diffraction techniques have paved the way for the three-dimensional (3D) reconstruction of biological samples, synthetic materials, and hybrid nanostructures at near atomic-level resolution. TEM tomography using a series of two-dimensional (2D) projections has been used extensively in biological science, but in recent years it has become an important method in synthetic nanomaterials and soft matter research. TEM tomography offers unprecedented morphological details of 3D objects, internal structures, packing patterns, growth mechanisms, and self-assembly pathways of self-assembled colloidal systems. It complements other analytical tools, including small-angle X-ray scattering, and provides valuable data for computational simulations for predictive design and reverse engineering of nanomaterials with the desired structure and properties. In this perspective, I will discuss the importance of TEM tomography in the structural understanding and engineering of self-assembled nanostructures with specific emphasis on colloidal capsids, composite cages, biohybrid superlattices with complex geometries, polymer assemblies, and self-assembled protein-based superstructures.

Green Topochemical Esterification Effects on the Supramolecular Structure of Chitin Nanocrystals: Implications for Highly Stable Pickering Emulsions

In nature, chitin is organized in hierarchical structures composed of nanoscale building blocks that show outstanding mechanical and optical properties attractive for nanomaterial design. For applications that benefit from a maximized interface such as nanocomposites and Pickering emulsions, individualized chitin nanocrystals (ChNCs) are of interest. However, when extracted in water suspension, their individualization is affected by ChNC self-assembly, requiring a large amount of water (above 90%) for ChNC transport and stock, which limits their widespread use. To master their individualization upon drying and after regeneration, we herein report a waterborne topochemical one-pot acid hydrolysis/Fischer esterification to extract ChNCs from chitin and simultaneously decorate their surface with lactate or butyrate moieties. Controlled reaction conditions were designed to obtain nanocrystals of a comparable aspect ratio of about 30 and a degree of modification of about 30% of the ChNC surface, under the rationale to assess the only effect of the topochemistry on ChNC supramolecular organization. The rheological analysis coupled with polarized light imaging shows how the nematic structuring is hindered by both surface ester moieties. The increased viscosity and elasticity of the modified ChNC colloids indicate a gel-like phase, where typical ChNC clusters of liquid crystalline phases are disrupted. Pickering emulsions have been prepared from lyophilized nanocrystals as a proof of concept. Our results demonstrate that only the emulsions stabilized by the modified ChNCs have excellent stability over time, highlighting that their individualization can be regenerated from the dry state.

Self-Assembled Metal Nanoclusters: Driving Forces and Structural Correlation with Optical Properties

Studies on self-assembly of metal nanoclusters (MNCs) are an emerging field of research owing to their significant optical properties and potential applications in many areas. Fabricating the desired self-assembly structure for specific implementation has always been challenging in nanotechnology. The building blocks organize themselves into a hierarchical structure with a high order of directional control in the self-assembly process. An overview of the recent achievements in the self-assembly chemistry of MNCs is summarized in this review article. Here, we investigate the underlying mechanism for the self-assembly structures, and analysis reveals that van der Waals forces, electrostatic interaction, metallophilic interaction, and amphiphilicity are the crucial parameters. In addition, we discuss the principles of template-mediated interaction and the effect of external stimuli on assembly formation in detail. We also focus on the structural correlation of the assemblies with their photophysical properties. A deep perception of the self-assembly mechanism and the degree of interactions on the excited state dynamics is provided for the future synthesis of customizable MNCs with promising applications.

Viscoelasticity Investigation of Semiconductor NP (CdS and PbS) Controlled Biomimetic Nanoparticle Hydrogels

The viscoelastic properties of colloidal nanoparticles (NPs) make opportunities to construct novel compounds in many different fields. The interparticle forces of inorganic particles on colloidal NPs are important for forming a mechanically stable particulate network especially the NP-based soft matter in the self-assembly process. Here, by capping with the same surface ligand L-glutathione (GSH), two semiconductor NP (CdS and PbS) controlled biomimetic nanoparticle hydrogels were obtained, namely, CdS@GSH and PbS@GSH. The dependence of viscoelasticity of colloidal suspensions on NP sizes, concentrations, and pH value has been investigated. The results show that viscoelastic properties of CdS@GSH are stronger than those of PbS@GSH because of stronger surface bonding ability of inorganic particles and GSH. The hydrogels formed by the smaller NPs demonstrate the higher stiffness due to the drastic change of GSH configurations. Unlike the CdS@GSH hydrogel system, the changes of NP concentrations and pH value had great influence on the PbS@GSH hydrogel system. The higher the proportion of water in the small particle size PbS@GSH hydrogel system, the greater the mechanical properties. The stronger the alkalinity in the large particle size PbS@GSH hydrogel system, the greater the hardness and storage modulus. Solution˗state nuclear magnetic resonance (NMR) indicated that the ligand GSH forms surface layers with different thickness varying from different coordination modes which are induced by different semiconductor NPs. Moreover, increasing the pH value of the PbS@GSH hydrogel system will dissociate the surface GSH molecules to form Pb²⁺ and GSH complexes which could enhance the viscoelastic properties.

Self-limiting self-assembly of supraparticles for potential biological applications

Nanotechnology has largely spurred the development of biological systems by taking advantage of the unique chemical, physical, optical, magnetic, and electrical properties of nanostructures. Self-limiting self-assembly of supraparticles produce new nanostructures and display great potential to create biomimicking nanostructures with desired functionalities. In this minireview, we summarize the recent developments and outstanding achievements of colloidal supraparticles, such as the driving forces for self-limiting self-assembly of supraparticles and properties of constructed supraparticles. Their application values in biological systems have also been illustrated.

Enhanced thermal effect of plasmonic nanostructures confined in discoidal porous silicon particles

The design of plasmonic nanostructures could have many exciting applications since it enhances or provides valuable control over efficient energy conversion. A three-dimensional (3D) space is a realistic hotspot matrix harvesting a wide conversion that has been shown in zero-dimensional nanoparticles, one-dimensional linear structures, or two-dimensional films. A novel 3D plasmonic nanostructure platform consisting of plasmonic metal nanoparticles in discoidal porous silicon particles is used in this study. Plasmonic gold nanoparticles are anchored inside the discoidal porous silicon (DPS) particles by in situ reduction synthesis. The novel plasmonic nanostructures can tailor the plasmon band and overcome the instability of photothermal materials. The “trapping well” for the anchored nanoparticles in 3D space can result in a huge change of plasmonic band of metal nanoparticles to the near-IR region in a novel 3D geometry. A multifunctional scaffold, Au–DPS particle, composed of doxorubicin conjugated to poly-(l-glutamic acid) (pDOX), was developed for combinatorial chemo-photothermal cancer therapy. The therapeutic efficacy was examined in treatment of the A549 cell line under near-IR laser irradiation. The highly efficient photothermal conversion can also be demonstrated in the laser desorption/ionization time-of-flight mass spectrometry detection analysis. The limit of detection was obviously improved in the detection of angiotensin II, P14R, and ACTH fragments 18-39 peptides. Overall, we envision that Au–DPS particles may be used in ultrasensitive theranostics in the future.

A 3D plasmonic nanostructure with a tunable plasmon resonance band to the near IR region enabled ultrasensitive theranostics for enhanced thermal effect.

Modulating the hierarchical fibrous assembly of Au nanoparticles with atomic precision

The ability to modulate nanoparticle (NP) assemblies with atomic precision is still lacking, which hinders us from creating hierarchical NP organizations with desired properties. In this work, a hierarchical fibrous (1D to 3D) assembly of Au NPs (21-gold atom, Au21) is realized and further modulated with atomic precision via site-specific tailoring of the surface hook (composed of four phenyl-containing ligands with a counteranion). Interestingly, tailoring of the associated counterion significantly changes the electrical transport properties of the NP-assembled solids by two orders of magnitude due to the altered configuration of the interacting π-π pairs of the surface hooks. Overall, our success in atomic-level modulation of the hierarchical NP assembly directly evidences how the NP ligands and associated counterions can function to guide the 1D, 2D, and 3D hierarchical self-assembly of NPs in a delicate manner. This work expands nanochemists' skills in rationally programming the hierarchical NP assemblies with controllable structures and properties.

Tailoring Interfacial Nanoparticle Organization through Entropy

The ability to tailor the interfacial behaviors of nanoparticles (NPs) is crucial not only for the design of novel nanostructured materials with superior properties and of interest for many promising applications such as water purification, enhanced oil recovery, and innovative energy transduction, but also for a better insight into many biological systems where nanoscale particles such as proteins or viruses can interact and organize at certain interfaces. As a class of emerging building blocks, Janus NPs consisting of two compartments of different chemistry or polarity are ideal candidates to generate tunable and stable interfacial nanostructures because of the asymmetric nature. However, precise control over such interfacial nanostructures toward a controllable order and even responses to various external stimuli still remains a great challenge as the interfaces do not simply serve as a scaffold but rather induce complex enthalpic and entropic interactions. In this Account, we focus on our efforts on exploiting entropy strategies based on computational design to tailor the spatial distribution and ordering of NPs at the interfaces of various systems. First, we introduce the physical principle of entropic ordering, being the theoretical basis of entropy-directed interfacial self-assembly. The typical types of entropy, which have been harnessed to manipulate the interfacial NP organization, are then summarized, including conformational entropy, shape entropy, and rotational and vibrational entropy. Next, we describe the emerging pathways in the development of novel environmentally responsive systems which involve the use of entropy to access the stimuli-responsive behaviors of interfacial nanostructures. Taking one step further, how molecular architectures can be tailored to tune the entropic contributions to the interfacial self-assembly is demonstrated, through identifying the effects of various intrinsic properties of block segments, such as chain length and stiffness, on entropy-governed precise organization of Janus NPs at block copolymer interfaces. Finally, we detail some key factors for tailoring interfacial organization through entropy. In summary, entropy strategies offer a promising and abundant framework for precisely programming the structural organization of NPs at interfaces. We discuss future directions to signify the framework in tailoring the interfacial organization of NPs. We hope that this Account will promote further efforts toward fundamental research and the wide applications of designed interfacial assemblies in new types of functional nanomaterials and beyond.

Large Area Nanoparticle Alignment by Chemical Lift-Off Lithography

Nanoparticle alignment on the substrate attracts considerable attention due to its wide application in different fields, such as mechanical control, small size electronics, bio/chemical sensing, molecular manipulation, and energy harvesting. However, precise nanoparticle positioning and deposition control with high fidelity are still challenging. Herein, a straightforward strategy for high quality nanoparticle-alignment by chemical lift-off lithography (CLL) is demonstrated. This technique creates high resolution self-assembled monolayer (SAM) chemical patterns on gold substrates, enabling nanoparticle-selective deposition and precise alignment. The fabricated nanoparticle arrangement geometries and dimensions are well-controllable in a large area. With proper nanoparticle surface functionality control and adequate substrate molecular manipulation, well-defined nanoparticle arrays with single-particle-wide alignment resolution are achieved.

Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles via Block Copolymers: From Synthesis to Energy Storage Property

Advances in liquid-phase exfoliation and surfactant-directed anisotropic growth of two-dimensional (2D) nanosheets have enabled their rapid development. However, it remains challenging to develop assembly strategies that lead to the construction of 2D nanomaterials with well-defined geometry and functional nanoarchitectures that are tailored to specific applications. Here we report a facile self-assembly method leading to the controlled synthesis of 2D transition metal oxide (TMO) nanosheets containing a high density of holes. We utilize graphene oxide sheets as a sacrificial template and Pluronic copolymers as surfactants. By using ZnFe₂O₄ (ZFO) nanoparticles as a model material, we demonstrate that by tuning the molecular weight of the Pluronic copolymers we can incorporate the ZFO particles and tune the size of the holes in the sheets. The resulting 2D ZFO nanosheets offer synergistic characteristics including increased electrochemically active surface areas, shortened ion diffusion paths, and strong inherent mechanical properties, leading to enhanced lithium-ion storage properties. Postcycling characterization confirms that the samples maintain structural integrity after electrochemical cycling. Our findings demonstrate that this template-assisted self-assembly method is a useful bottom-up route for controlled synthesis of 2D nanoarchitectures, and these holey 2D nanoarchitectures are promising for improving the electrochemical performance of next-generation lithium-ion batteries.

Programmed Self-Assembly of Branched Nanocrystals with an Amphiphilic Surface Pattern

Site-selective surface modification on the shape-controlled nanocrystals is a key approach in the programmed self-assembly of inorganic colloidal materials. This study demonstrates a simple methodology to gain self-assemblies of semiconductor nanocrystals with branched shapes through tip-to-tip attachment. Short-chained water-soluble cationic thiols are employed as a surface ligand for CdSe tetrapods and CdSe/CdS core/shell octapods. Because of the less affinity of arm-tip to the surface ligands compared to the arm-side wall, the tip-surface becomes uncapped to give a hydrophobic nature, affording an amphiphilic surface pattern. The amphiphilic tetrapods aggregated into porous agglomerates through tip-to-tip connection in water, while they afforded a hexagonally arranged Kagome-like two-dimensional (2D) assembly by the simple casting of aqueous dispersion with the aid of a convective self-assembly mechanism. A 2D net-like assembly was similarly obtained from amphiphilic octapods. A dissipative particle dynamics simulation using a planar tripod model with an amphiphilic surface pattern reproduced the formation of the Kagome-like assembly in a 2D confined space, demonstrating that the lateral diffusion of nanoparticles and the firm contacts between the hydrophobic tips play crucial roles in the self-assembly.

Coordination Polymer Nanoglue: Robust Adhesion Based on Collective Lamellar Stacking of Nanoplates

Despite a continuously growing interest in the integration of coordination polymer (CP) colloids toward functional materials, collective properties of the CP colloids have rarely been addressed mainly due to the difficulty in assembling pure CP colloids into superstructures with impressive mechanical strength. We demonstrated that CP nanoplates could stack together spontaneously upon drying the slurry of the nanoplates. The stacked CP nanoplates could work like polymeric adhesives. Versatile articles could be glued when the CP nanoplates were sandwiched between two substrates. In addition, the CP nanoplates themselves could form well-defined bulk structures without using any additional adhesives. The anisotropic shape together with the lamellar stacking way of the CP nanoplates were found to be the key points in leading to the adhesion and cohesion effect. The reasonable adhesion strength of the CP nanoglues can allow the exploration of further applications of integrated CP colloids in the future.

Hierarchical organization and molecular diffusion in gold nanorod/silica supercrystal nanocomposites

Hierarchical organization of gold nanorods was previously obtained on a substrate, allowing precise control over the morphology of the assemblies and macroscale spatial arrangement. Herein, a thorough description of these gold nanorod assemblies and their orientation within supercrystals is presented together with a sol-gel technique to protect the supercrystals with mesoporous silica films. The internal organization of the nanorods in the supercrystals was characterized by combining focused ion beam ablation and scanning electron microscopy. A mesoporous silica layer is grown both over the supercrystals and between the individual lamellae of gold nanorods inside the structure. This not only prevented the detachment of the supercrystal from the substrate in water, but also allowed small molecule analytes to infiltrate the structure. These nanocomposite substrates show superior Raman enhancement in comparison with gold supercrystals without silica owing to improved accessibility of the plasmonic hot spots to analytes. The patterned supercrystal arrays with enhanced optical and mechanical properties obtained in this work show potential for the practical implementation of nanostructured devices in spatially resolved ultradetection of biomarkers and other analytes.

Quantitative 3D analysis of huge nanoparticle assemblies

Nanoparticle assemblies can be investigated in 3 dimensions using electron tomography. However, it is not straightforward to obtain quantitative information such as the number of particles or their relative position. This becomes particularly difficult when the number of particles increases. We propose a novel approach in which prior information on the shape of the individual particles is exploited. It improves the quality of the reconstruction of these complex assemblies significantly. Moreover, this quantitative Sparse Sphere Reconstruction approach yields directly the number of particles and their position as an output of the reconstruction technique, enabling a detailed 3D analysis of assemblies with as many as 10,000 particles. The approach can also be used to reconstruct objects based on a very limited number of projections, which opens up possibilities to investigate beam sensitive assemblies where previous reconstructions with the available electron tomography techniques failed.

End-to-End Self-Assembly of Semiconductor Nanorods in Water by Using an Amphiphilic Surface Design

One-dimensional (1D) self-assemblies of nanocrystals are of interest because of their vectorial and polymer-like dynamic properties. Herein, we report a simple method to prepare elongated assemblies of semiconductor nanorods (NRs) through end-to-end self-assembly. Short-chained water-soluble thiols were employed as surface ligands for CdSe NRs having a wurtzite crystal structure. The site-specific capping of NRs with these ligands rendered the surface of the NRs amphiphilic. The amphiphilic CdSe NRs self-assembled to form elongated wires by end-to-end attachment driven by the hydrophobic effect operating between uncapped NR ends. The end-to-end assembly technique was further applied to CdS NRs and CdSe tetrapods (TPs) with a wurtzite structure.

Charge distribution and Fermi level in bimetallic nanoparticles

Fermi level equilibration driven charge redistribution and electric dipole formation was quantified using a simple nanocapacitor model for bimetallic nanoparticles.

Mechanism of Nanorod Formation by Wormlike Micelle-Assisted Assembly of Nanospheres

Hierarchical self-assembly is an elegant and energy-efficient bottom-up method for the structuring of complex materials. We demonstrate the synthesis of maghemite nanorods via directed self-assembly, assisted by wormlike micelles, under controlled shear. The experimental data are analyzed by formulating a "slithering snake" mechanism and simulating it on a cubic lattice, using a coarse-grained Monte Carlo framework. The influence of shear rate, precursor concentration, and length of Kuhn segment on the morphology of the nanorods is examined. Experiments indicate that the shear is necessary for the formation of nanorods, although diameter and length of the nanorods are insensitive to the shear rate, within the range of shear rates investigated. The model adequately captures the features of directional aggregation of particles, and the computed length and diameter correspond to the typical dimensions of the nanorods obtained experimentally. The protocol has considerable potential for producing nanorods of several materials simply by changing the precursors.

Gold Nanoparticle Internal Structure and Symmetry Probed by Unified Small-Angle X-ray Scattering and X-ray Diffraction Coupled with Molecular Dynamics Analysis

Shape and size are known to determine a nanoparticle's properties. Hardly ever studied in synthesis, the internal crystal structure (i.e., particle defects, crystallinity, and symmetry) is just as critical as shape and size since it directly impacts catalytic efficiency, plasmon resonance, and orients anisotropic growth of metallic nanoparticles. Hence, its control cannot be ignored any longer in today's research and applications in nanotechnology. This study implemented an unprecedented reliable measurement combining these three structural aspects. The unified small-angle X-ray scattering and diffraction measurement (SAXS/XRD) was coupled with molecular dynamics to allow simultaneous determination of nanoparticles' shape, size, and crystallinity at the atomic scale. Symmetry distribution (icosahedra-Ih, decahedra-Dh, and truncated octahedra-TOh) of 2-6 nm colloidal gold nanoparticles synthesized in organic solvents was quantified. Nanoparticle number density showed the predominance of Ih, followed by Dh, and little, if any, TOh. This result contradicts some theoretical predictions and highlights the strong effect of the synthesis environment on structure stability. We foresee that this unified SAXS/XRD analysis, yielding both statistical and quantitative counts of nanoparticles' symmetry distribution, will provide new insights into nanoparticle formation, growth, and assembly.

Au nanoparticle clusters from deposition of a coalescing emulsion

HYPOTHESIS

Nanoparticle adsorption at the oil-water interface in an unstable, coalescing emulsion leads to cluster formation.

EXPERIMENTS

Stable suspensions of clusters are prepared using a facile, two-step procedure involving few reagents and neither thiolated compounds nor chlorinated solvents. First, colloidal gold nanoparticles are assembled at the aqueous-hexanol interface in an emulsion that rapidly coalesces and spontaneously deposits a film on the interior surface of the glass container. The film is dissolved in ethanol with sonication to disperse the clusters. The film and clusters are characterized by transmission electron and atomic force microscopies as well as ultraviolet-visible spectrometry.

FINDINGS

Clusters are observed to contain as few as 8 to as many as 24 Au nanoparticles. The clusters are anisotropic and can also be formed from larger nanoparticles. Hydrophobic and hydrophilic interactions are implicated in the formation of these clusters within the interfacial tension gradients of a coalescing emulsion. The clusters can be re-suspended in ethanol and water, maximizing the utility of these clusters with an extinction band in the near-Infrared region of the electromagnetic spectrum.

Temperature-induced structural transitions in self-assembling magnetic nanocolloids

With the help of a unique combination of density functional theory and computer simulations, we discover two possible scenarios, depending on concentration, for the hierarchical self-assembly of magnetic nanoparticles on cooling.

Entropy-mediated mechanical response of the interfacial nanoparticle patterning

The precise organization of nano-objects into well-defined patterns at interfaces is an outstanding challenge in the field of nanocomposites toward technologically important materials and devices. Herein, by means of computer simulations we show novel mechanomutable nanocomposites designed by binary mixtures of tethered Janus nanoparticles at the interface of a binary fluid mixture under mechanical pressure. Our simulations demonstrate that the nanoparticle organization in the systems undergo reversible transition between random state and long-ranged intercalation state, controlled by various structural parameters of the tethered chains and the applied pressure. The dynamical mechanism during the transition is explored through examining the diffusion trajectories of the nanoparticles confined at the interfaces. We provide a theoretical analysis of the lateral pressure induced by the tethered chains, which is fully supported by simulation data and reveals that the compression-induced transition is fundamentally attributed to the entropic effect from the tethered chains. Our study leads to a class of interface-reactive nanomaterials in which the transfer and recovery of interfacial nanopatterning presents precise and tunable mechanical responses.

Exploiting Hydrophobic Interactions at the Nanoscale

Hydrophobic effects are ubiquitous and manifest themselves in everyday processes such as solubilizing oil, precipitating molecules, and formation of particles or foam. Although this phenomenon is often intuitively recognized, it is not straightforward to predict it and, in particular, to control it experimentally. Hydrophobic effects are however progressively gaining recognition as an important tool providing control at the nanoscale, which may ultimately lead to the design of responsive metamaterials with unprecedented functionalities under nonequilibrium conditions.