Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Dynamic Nanoparticle Assemblies for Biomedical Applications

Designed synthesis and assembly of nanoparticles assisted by their surface ligands can create "smart" materials with programmed responses to external stimuli for biomedical applications. These assemblies can be designed to respond either exogenously (for example, to magnetic field, temperature, ultrasound, light, or electric pulses) or endogenously (to pH, enzymatic activity, or redox gradients) and play an increasingly important role in a diverse range of biomedical applications, such as biosensors, drug delivery, molecular imaging, and novel theranostic systems. In this review, the recent advances and challenges in the development of stimuli-responsive nanoparticle assemblies are summarized; in particular, the application-driven design of surface ligands for stimuli-responsive nanoparticle assemblies that are capable of sensing small changes in the disease microenvironment, which induce the related changes in their physico-chemical properties, is described. Finally, possible future research directions and problems that have to be addressed are briefly discussed.

Hierarchical Materials Design by Pattern Transfer Printing of Self-Assembled Binary Nanocrystal Superlattices

We demonstrate the fabrication of hierarchical materials by controlling the structure of highly ordered binary nanocrystal superlattices (BNSLs) on multiple length scales. Combinations of magnetic, plasmonic, semiconducting, and insulating colloidal nanocrystal (NC) building blocks are self-assembled into BNSL membranes via the liquid-interfacial assembly technique. Free-standing BNSL membranes are transferred onto topographically structured poly(dimethylsiloxane) molds via the Langmuir-Schaefer technique and then deposited in patterns onto substrates via transfer printing. BNSLs with different structural motifs are successfully patterned into various meso- and microstructures such as lines, circles, and even three-dimensional grids across large-area substrates. A combination of electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) measurements confirm the ordering of NC building blocks in meso- and micropatterned BNSLs. This technique demonstrates structural diversity in the design of hierarchical materials by assembling BNSLs from NC building blocks of different composition and size by patterning BNSLs into various size and shape superstructures of interest for a broad range of applications.

Efficient Enrichment and Self-Assembly of Hybrid Nanoparticles into Removable and Magnetic SERS Substrates for Sensitive Detection of Environmental Pollutants

A structure consisting of a low surface energy substrate and low surface tension liquid is designed and prepared by taking advantage of perfluorinated fluid infusion into the porous Teflon membrane. This slippery platform allows efficient enrichment and self-assembly of hybrid nanoparticles and the assembled structure can be detached from the membrane. A macroscale superlattice array of Au nanorods doped with magnetic Fe₃O₄ nanoparticles is obtained by suppressing the outward capillary flow and coffee-ring effect during evaporative self-assembly. In SERS (surface enhanced Raman scattering) detection of environmental pollutants including thiram, diquat and polycyclic aromatic hydrocarbons, the removable plasmonic superlattice array with magnetic properties enables rapid separation of analytes from the solution resulting in excellent sensitivity and detection limits down to the nanomolar level. The self-assembly strategy shows great potential in the fabrication of removable 3D plasmonic superlattice arrays for SERS detections.

Arrangement and SERS Applications of Nanoparticle Clusters Using Liquid Crystalline Template

Manipulation of nanomaterials such as nanoparticles (NPs) and nanorods (NRs) to make clusters is of significant interest in material science and nanotechnology due to the unusual collective opto-electric properties in such structures that cannot be found in the individual NPs. This work demonstrates an effective way to arrange NP clusters (NPCs) to make the desired arrays based on removable and NP-guidable liquid crystalline template using sublimation and reconstruction phenomenon. The position of the NPCs is precisely controlled by the defect structure of the liquid crystal (LC), namely toric focal conic domains (TFCDs), during thermal annealing to construct the LC and corresponding NPC structures. As a proof of concept, the surface-enhanced Raman scattering (SERS) activity of a fabricated array of gold nanorod (GNR) clusters is measured and shown to have highly sensitive detection characteristics essential for potential sensing applications.

Interactions and Attachment Pathways between Functionalized Gold Nanorods

Nanoparticle (NP) self-assembly has been recognized as an important technological process for forming ordered nanostructures. However, the detailed dynamics of the assembly processes remain poorly understood. Using in situ liquid cell transmission electron microscopy, we describe the assembly modes of gold (Au) nanorods (NRs) in solution mediated by hydrogen bonding between NR-bound cysteamine linker molecules. Our observations reveal that by tuning the linker concentration, two different NR assembly modes can be achieved. These assembly modes proceed via the (1) end-to-end and (2) side-to-side attachment of NRs at low and high linker concentrations in solution, respectively. In addition, our time-resolved observations reveal that the side-to-side NR assemblies can occur through two different pathways: (i) prealigned attachment, where two Au NRs prealign to be parallel prior to assembly, and (ii) postattachment alignment, where two Au NRs first undergo end-to-end attachment and pivot around the attachment point to form the side-to-side assembly. We attributed the observed assembly modes to the distribution of linkers on the NR surfaces and the electrostatic interactions between the NRs. The intermediate steps in the assembly reported here reveal how the shape and surface functionalities of NPs drive their self-assembly, which is important for the rational design of hierarchical nanostructures.

Controlling the Structure of Supraballs by pH-Responsive Particle Assembly

Supraballs of various sizes and compositions can be fabricated via drying of drops of aqueous colloidal dispersions on super-liquid-repellent surfaces with no chemical waste and energy consumption. A "supraball" is a particle composed of colloids. Many properties, such as mechanical strength and porosity, are determined by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aqueous dispersions of polystyrene microparticles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mechanical properties could be controlled by pH, which allows broader uses of supraballs.

Phase diagram of elastic spheres

Experiments show that polymeric nanoparticles often self-assemble into several non-close-packed lattices in addition to the face-centered cubic lattice. Here, we explore theoretically the possibility that the observed phase sequences may be associated with the softness of the particles, which are modeled as elastic spheres interacting upon contact. The spheres are described by two finite-deformation theories of elasticity, the modified Saint-Venant-Kirchhoff model and the neo-Hookean model. We determine the range of indentations where the repulsion between the spheres is pairwise additive and agrees with the Hertz theory. By computing the elastic energies of nine trial crystal lattices at densities far beyond the Hertzian range, we construct the phase diagram and find the face- and body-centered cubic lattices as well as the A15 lattice and the simple hexagonal lattice, with the last two being stable at large densities where the spheres are completely faceted. These results are qualitatively consistent with observations, suggesting that deformability may indeed be viewed as a generic property that determines the phase behavior in nanocolloidal suspensions.

Effective pair potential between charged nanoparticles at high volume fractions

Simulations of charged colloidal dispersions are technically challenging. One possible workaround consists in reducing the system to the colloids only, whose interactions are described through an effective pair potential, w^f. Still, the determination of w^f is difficult mainly because it depends on the colloidal density, ϕ. Here we propose to calculate w^f from simulations of a pair of colloids placed in a cubic box with periodic boundary conditions. The variation in ϕ is mimicked by an appropriate change in the concentration of counterions neutralized by an homogeneous background charge. The method is tested at the level of the primitive model. A good description of the structure of the colloidal dispersion is obtained in the low and high coupling regimes, even at high ϕ (≈30%). Furthermore, the method can easily be used in popular molecular simulation program packages and extended to non-spherical objects.

Shape-dependent ordering of gold nanocrystals into large-scale superlattices

Self-assembly of individual building blocks into highly ordered structures, analogous to spontaneous growth of crystals from atoms, is a promising approach to realize the collective properties of nanocrystals. Yet the ability to reliably produce macroscopic assemblies is unavailable and key factors determining assembly quality/yield are not understood. Here we report the formation of highly ordered superlattice films, with single crystalline domains of up to half a millimetre in two dimensions and thickness of up to several microns from nanocrystals with tens of nanometres in diameter. Combining experimental and computational results for gold nanocrystals in the shapes of spheres, cubes, octahedra and rhombic dodecahedra, we investigate the entire self-assembly process from disordered suspensions to large-scale ordered superlattices induced by nanocrystal sedimentation and eventual solvent evaporation. Our findings reveal that the ultimate coherence length of superlattices strongly depends on nanocrystal shape. Factors inhibiting the formation of high-quality large-scale superlattices are explored in detail.

Following the Assembly of Iron Oxide Nanocubes by Video Microscopy and Quartz Crystal Microbalance with Dissipation Monitoring

We have studied the growth of ordered arrays by evaporation-induced self-assembly of iron oxide nanocubes with edge lengths of 6.8 and 10.1 nm using video microscopy (VM) and quartz crystal microbalance with dissipation monitoring (QCM-D). Ex situ electron diffraction of the ordered arrays demonstrates that the crystal axes of the nanocubes are coaligned and confirms that the ordered arrays are mesocrystals. Time-resolved video microscopy shows that growth of the highly ordered arrays at slow solvent evaporation is controlled by particle diffusion and can be described by a simple growth model. The growth of each mesocrystal depends only on the number of nanoparticles within the accessible region irrespective of the relative time of formation. The mass of the dried mesocrystals estimated from the analysis of the bandwidth-shift-to-frequency-shift ratio correlates well with the total mass of the oleate-coated nanoparticles in the deposited dispersion drop.

Photo-electrochemical properties of quantum rods studied by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is used for studying the intrinsic photo-electrochemical properties of CdSe/CdS quantum rods.

Coating agent-induced mechanical behavior of 3D self-assembled nanocrystals

The Young's modulus of three-dimensional self-assembled Ag nanocrystals, as so-called supracrystals, is correlated with the type of coating agent as well as the nanocrystal morphology.

Interface control of electronic and optical properties in IV–VI and II–VI core/shell colloidal quantum dots: a review

Core/shell heterostructures provide controlled optical properties, tuneable electronic structure, and chemical stability due to an appropriate interface design.

Bottom-up engineering of thermoelectric nanomaterials and devices from solution-processed nanoparticle building blocks

Nanoparticle-based bottom-up engineered nanomaterials are extremely appealing for the direct solid-state conversion between heat and electricity.

A facile water-stable MOF-based “off–on” fluorescent switch for label-free detection of dopamine in biological fluid

An Abtz–CdI₂–MOF was developed as an “off–on” fluorescent switch for label-free detection of dopamine without any surface modification or functionalization.

Polymer ligand-induced autonomous sorting and reversible phase separation in binary particle blends

The tethering of ligands to nanoparticles has emerged as an important strategy to control interactions and organization in particle assembly structures. We demonstrate that ligand interactions in mixtures of polymer-tethered nanoparticles (which are modified with distinct types of polymer chains) can impart upper or lower critical solution temperature (UCST/LCST)-type phase behavior on binary particle mixtures in analogy to the phase behavior of the corresponding linear polymer blends. Therefore, cooling (or heating) of polymer-tethered particle blends with appropriate architecture to temperatures below (or above) the UCST (or LCST) results in the organization of the individual particle constituents into monotype microdomain structures. The shape (bicontinuous or island-type) and lengthscale of particle microdomains can be tuned by variation of the composition and thermal process conditions. Thermal cycling of LCST particle brush blends through the critical temperature enables the reversible growth and dissolution of monoparticle domain structures. The ability to autonomously and reversibly organize multicomponent particle mixtures into monotype microdomain structures could enable transformative advances in the high-throughput fabrication of solid films with tailored and mutable structures and properties that play an important role in a range of nanoparticle-based material technologies.

Optical phase retrieval using conical refraction in structured media

We propose a method of optical phase retrieval based on the conical refraction imaging in structured media. We show that a multilayered dielectric photonic crystal functioning as a conically refractive flat lens can be used to reconstruct phase information of complex optical signals. Our method enables a single simultaneous measurement of multiple images on the same image plane and allows a rapid stable recovery of the optical phase. The planar geometry of the proposed device is compatible with current nano-fabrication techniques and, therefore, can find broad applications in optical signal processing and imaging.

Gold nanoparticle superlattices: structure and cavities studied by GISAXS and PALS

In this study the sizes of cavities formed within the ligand matrix of gold nanoparticle superlattices were probed using positron annihilation lifetime spectroscopy.

DNA assembled metal nanoclusters: synthesis to novel applications

In this review, we have discussed the emergence of promising environmental-benign DNA assembled fluorescent metal nanoclusters and their unique electronic structures, unusual physical and chemical properties.