Colloidal Assembly Research Articles

Confinement and aggregation of colloidal particles in an ionic liquid microdroplet formed by optical tweezers

Currently, there is considerable interest in applying colloidal assemblies to photonic and plasmonic devices. Optical tweezing enables the preparation of such assemblies at desired positions, but the assembly process occurs only in areas irradiated by laser light. Here, we demonstrate the collection and assembly of colloidal particles in areas beyond the irradiation area. The particles are taken into a microdroplet formed by optical tweezing in a thermo-responsive ionic liquid (IL)/water mixture. The confined particles aggregate as the droplet shrinks. The mechanism of confinement and aggregation of colloidal particles are discussed in view of the surface charge of the particles.

Modeling Structural Colors from Disordered One-Component Colloidal Nanoparticle-Based Supraballs Using Combined Experimental and Simulation Techniques

Bright, saturated structural colors in birds have inspired synthesis of self-assembled, disordered arrays of assembled nanoparticles with varied particle spacings and refractive indices. However, predicting colors of assembled nanoparticles, and thereby guiding their synthesis, remains challenging due to the effects of multiple scattering and strong absorption. Here, we use a computational approach to first reconstruct the nanoparticles' assembled structures from small-angle scattering measurements and then input the reconstructed structures to a finite-difference time-domain method to predict their color and reflectance. This computational approach is successfully validated by comparing its predictions against experimentally measured reflectance and provides a pathway for reverse engineering colloidal assemblies with desired optical and photothermal properties.

Staged Assembly of Colloids Using DNA and Acoustofluidics.

Assembly of DNA-coated colloids (DNACCs) provides a practical route to programming complex self-assembled materials at the micro/nanoscale. So far, the programmability of DNACC assembly has been extensively exploited internally using different DNA sequences or colloid geometry so that the assembly is mainly manipulated with single-particle spatial resolution such as in crystallization. In this Letter, we present an acoustic approach to externally programming the DNACC assembly with control of spatial resolution over larger scales. We demonstrate assembly of the DNACCs under different acoustic frequencies from stage to stage to produce hierarchical structures that are difficult to fabricate when using DNA coating alone. By programming the acoustic wave frequency, amplitude, and phase, colloidal structures with different morphologies can be assembled. The nonspecific driving force based on acoustic radiation forces at each stage allows our approach to be adopted for most colloidal systems without specific requirements on particle or medium properties.

Next-Generation Colloidal Materials for Ultrasound Imaging Applications.

Ultrasound has important applications, predominantly in the field of diagnostic imaging. Presently, colloidal systems such as microbubbles, phase-change emulsion droplets and particle systems with acoustic properties and multiresponsiveness are being developed to address typical issues faced when using commercial ultrasound contrast agents, and to extend the utility of such systems to targeted drug delivery and multimodal imaging. Current technologies and increasing research data on the chemistry, physics and materials science of new colloidal systems are also leading to the development of more complex, novel and application-specific colloidal assemblies with ultrasound contrast enhancement and other properties, which could be beneficial for multiple biomedical applications, especially imaging-guided treatments. In this article, we review recent developments in new colloids with applications that use ultrasound contrast enhancement. This work also highlights the emergence of colloidal materials fabricated from or modified with biologically derived and bio-inspired materials, particularly in the form of biopolymers and biomembranes. Challenges, limitations, potential developments and future directions of these next-generation colloidal systems are also presented and discussed.

Strategy for optimizing experimental settings for studying low atomic number colloidal assemblies using liquid phase scanning transmission electron microscopy

Observing processes of nanoscale materials of low atomic number is possible using liquid phase electron microscopy (LP-EM). However, the achievable spatial resolution (d) is limited by radiation damage. Here, we examine a strategy for optimizing LP-EM experiments based on an analytical model and experimental measurements, and develop a method for quantifying image quality at ultra low electron dose De using scanning transmission electron microscopy (STEM). As experimental test case we study the formation of a colloidal binary system containing 30 nm diameter SiO2 nanoparticles (SiONPs), and 100 nm diameter polystyrene microspheres (PMs). We show that annular dark field (DF) STEM is preferred over bright field (BF) STEM for practical reasons. Precise knowledge of the material's density is crucial for the calculations in order to match experimental data. To calculate the detectability of nano-objects in an image, the Rose criterion for single pixels is expanded to a model of the signal to noise ratio obtained for multiple pixels spanning the image of an object. Using optimized settings, it is possible to visualize the radiation-sensitive, hierarchical low-Z binary structures, and identify both components.

Self-organized lasers from reconfigurable colloidal assemblies

Non-equilibrium assemblies, where units are able to harness available energy to perform tasks, can often self-organize into dynamic materials that uniquely blend structure with functionality and responsiveness to their environment. The integration of similar features in photonic materials remains challenging but is desirable to manufacture active, adaptive and autonomous photonic devices. Here we show the self-organization of programmable random lasers from the reversible out-of-equilibrium self-assembly of colloids. The random lasing originates from the optical amplification of light undergoing multiple scattering within the dissipative colloidal assemblies and therefore depends crucially on their self-organization behaviour. Under external light stimuli, these dynamic random lasers are responsive and present a continuously tuneable laser threshold. They can therefore reconfigure and cooperate by emulating the ever-evolving spatiotemporal relationship between structure and functionality that is typical of many non-equilibrium assemblies. Experiments inspired by the behaviour of active matter show that an external optical stimulus can spatially reconfigure colloidal random lasers and continuously tune their lasing threshold.

Injectable Granular Hydrogels as Colloidal Assembly Microreactors for Customized Structural Colored Objects.

While structural coloration has captured considerable interests across different areas in the past decades, the development of macroscopic objects with tailorable structural colors remains a challenge due to the difficulty of large-scale fabrication of finely ordered nanostructures and poor processability of their constituent materials. In this work, a type of photonic granular hydrogel is developed as a novel printable ink for constructing customized structural colored objects. The magnetochromatic ink exhibits dynamic properties such as shear thinning and self-healing, enabling direct writing of macroscopic structural colored patterns by extrusion 3D printing. Further, the modularity of the photonic ink allows additive color mixing, which obviates the need for arduous nano-synthesis and expands on the color abundance of structural colored materials in a simple yet efficient manner. These characteristics grant novel photonic inks with great applicability to a variety of fields including switchable color displays, sensors, etc.

Injectable Granular Hydrogels as Colloidal Assembly Microreactors for Customized Structural Colored Objects

AbstractWhile structural coloration has captured considerable interests across different areas in the past decades, the development of macroscopic objects with tailorable structural colors remains a challenge due to the difficulty of large‐scale fabrication of finely ordered nanostructures and poor processability of their constituent materials. In this work, a type of photonic granular hydrogel is developed as a novel printable ink for constructing customized structural colored objects. The magnetochromatic ink exhibits dynamic properties such as shear thinning and self‐healing, enabling direct writing of macroscopic structural colored patterns by extrusion 3D printing. Further, the modularity of the photonic ink allows additive color mixing, which obviates the need for arduous nano‐synthesis and expands on the color abundance of structural colored materials in a simple yet efficient manner. These characteristics grant novel photonic inks with great applicability to a variety of fields including switchable color displays, sensors, etc.

Mirau interferometry of fluid interfaces deformed by colloids under the influence of external fields

The interfacial curvature surrounding colloidal particles pinned to fluid interfaces dictates their interparticle capillary interaction and assembly; however, it is a nontrivial function of particle anisotropy, surface roughness, external field conditions, macroscopic interfacial curvature, and the chemistry of each fluid phase. The prospect of dynamically modifying the pinning properties and interfacial organization of colloidal particles adhered to fluid interfaces via these approaches necessitates the development of experimental techniques capable of measuring changes in the interfacial deformation around particles in situ. Here, we describe a modified technique based on phase-shift Mirau interferometry to determine the relative height of the fluid interface surrounding adsorbed colloids while applying external electric fields. The technique is corrected for macroscopic curvature in the interface as well as in-plane motion of the particle in order to isolate the contribution of the particle to the interfacial deformation. Resultant height maps are produced with a maximum resolution of ±1nm along the height axis. The measured topography of the interface is used to identify the contact line where the two fluids meet the particle, along with the maximal interfacial deformation (Δumax) of the undulating contact line and the three-phase contact angle, θc. The technique is calibrated using anisotropic polymer ellipsoids of varying aspect ratio before the effect of external AC electric fields on the pinned particle contact angle is demonstrated. The results show promise for this new technique to measure and quantify dynamic changes in interfacial height deformation, which dictate interparticle capillary energy and assembly of colloids at fluid interfaces.

DMMP sensors based on Au-SnO2 hybrids prepared through colloidal assembly approach: Gas sensing performances and mechanism study

The development of metal oxides-based gas sensors for DMMP detection simultaneously possessing low operating temperature, low limit of detection and high sensitivity remains formidable challenges. In this work, a facile and efficient colloidal assembly approach is proposed to prepare Au modified SnO2 hybrids (Au-SnO2) with perfect metal-semiconductor interface and successfully used in DMMP detection application. After optimization, the best DMMP sensors based on Au-SnO2 hybrids display outstanding sensing performances, including high response (1.88 to 680 ppb DMMP), low operating temperature (200 °C), extremely low limit of detection (34 ppb), as well as short response/recovery time (25 s/72 s). The sensing mechanism analysis reveals that the excellent sensing performances are mainly attributed to the synergy of chemical oxidation of DMMP catalyzed by Au-SnO2 hybrids and physical adsorption of DMMP by hydrogen-bonding interactions. The present work not only provides an insight into the DMMP sensing mechanism, but also offers a promising general approach for development of high-performance gas sensors.

ROLE OF SYMMETRY IN OPTIMIZATION OF FEM SIMULATION CALCULATIONS

At the studying the Mannesmann piercing process, we unify two approaches to the problem solving. Namely, the commercial FEM software procedure and the mathematical model of the process via the mathematical model of the considered FEM-simulation bound by certain unifying symmetries. Such phenomenon seemingly exists only if the FE-mesh is initiated to be physically interpreted. We shortly outline, how to come to the slightly modified Cahn-Hilliard equation as to the mathematical model of the FE-simulation possessing quasi-symmetry given by a lattice of colloidal assembly formed by the chosen FE-mesh. Separation of two cylindrical surfaces of the pierced product together with the inpainting role of the piercing plug are described with respect to the background given by the Navier-Stokes equations related to the flow between the both surfaces. Influence of the involved groups related to the considered quasi-symmetry is illustrated by the convergence/divergence of the Newton-Raphson number in the CPU-time.

Colloidal Multiscale Assembly via Photothermally Driven Convective Flow for Sensitive In-Solution Plasmonic Detections.

The assembly of metal nanoparticles and targets to be detected in a small light probe volume is essential for achieving sensitive in-solution surface-enhanced Raman spectroscopy (SERS). Such assemblies generally require either chemical linkers or templates to overcome the random diffusion of the colloids unless the aqueous sample is dried. Here, a facile method is reported to produce 3D multiscale assemblies of various colloids ranging from molecules and nanoparticles to microparticles for sensitive in-solution SERS detection without chemical linkers and templates by exploiting photothermally driven convective flow. The simulations suggest that colloids sub 100nm in diameter can be assembled by photothermally driven convective flow regardless of density; the assembly of larger colloids up to several micrometers by convective flow is significant only if their density is close to that of water. Consistent with the simulation results,the authors confirm that the photothermally driven convective flow is mainly responsible for the observed coassembly of plasmonic gold nanorods with either smaller molecules or larger microparticles. It is further found that the coassembly with the plasmonic nanoantennae leads to dramatic Raman enhancements of molecules, microplastics, and microbes by up to fivefold of magnitude compared to those measured in solution without the coassembly.

Path-Dependent Anisotropic Colloidal Assembly of Magnetic Nanocomposite-Protein Complexes.

Anisotropic self-assembly of nanoparticles (NPs) stems from the fine-tuning of their surface functionality and NP interaction. Strategies involving ligand interaction, protein interaction, and external stimulus have been developed. However, robust construction of monodispersed magnetic NPs to tens of microns of anisotropically aligned colloidal assembly triggered by adsorbed protein intermolecular interaction is yet to be elucidated. Here, we present the NP-protein interaction, magnetic force, and protein corona intermolecular interaction serially but independently induced path-dependent self-assembly of 100 nm Fe3O4@SiO2 nanocomposites. Dynamic formation of the micron-sized anisotropic magnetic assembly was reproducibly realized in a continuous medium in a controllable manner. Formation of the primary globular clusters upon the unique NP-protein complexes with the help of ions acts as the prerequisite for the anisotropic colloidal assembly, followed by the magnetic force-driven pre-organization and protein intermolecular electrostatic interaction-mediated elongation. The protein concentration rather than the protein original structure plays a more pivotal role in the NP-protein interaction and subsequent colloidal assembly process. Two typical serum proteins fibrinogen and bovine serum albumin enable formation of the anisotropic colloidal assembly but with a different subtle morphology. Furthermore, the obtained micron-sized magnetic colloidal assembly can be dissociated rapidly by adding a negative electrolyte in the medium due to the interference in the NP-protein interaction. However, the self-assembly process can be recycled based on the dissociated colloidal assembly.

Flash Colloidal Gold Nanoparticle Assembly in a Milli Flow System: Implications for Thermoplasmonic and for the Amplification of Optical Signals

The assembly and stabilization of a finite number of nanocrystals in contact in water could maximize the optical absorption per unit of material. Some local plasmonic properties exploited in applications, such as photothermia and optical signal amplification, would also be maximized which is important in the perspective of mass producing nanostructures at a lower cost. The main lock is that bringing charged particles in close contact requires the charges to be screened/suppressed, which leads to the rapid formation of micrometric aggregates. In this article, we show that aggregates containing less than 60 particles in contact can be obtained with a milli-flow system composed of turbulent mixers and flow reactors. This process allows to stop a fast non-equilibrium colloidal aggregation process at millisecond times after the initiation of the aggregation process which allows to control the aggregation number. As a case study, we considered the rapid mixing of citrate coated gold nanoparticles (NP) and AlCl3 in water to initiate a fast aggregation controlled by diffusion. Injecting a solution of polycation using a second mixer allowed us to arrest the aggregation process after a reaction time by formation of overcharged cationic aggregates. We obtained within seconds stable dispersions of a few milliliters composed of particle aggregates. Our main result is to show that it is possible to master the average aggregation number between 2 and 60 NP per aggregate by varying the a reaction time between 10 ms and 1 s.

Interfacial colloidal assembly guided by optical tweezers and tuned via surface charge

HypothesisThe size, shape and dynamics of assemblies of colloidal particles optically-trapped at an air–water interface can be tuned by controlling the optical potential, particle concentration, surface charge density and wettability of the particles and the surface tension of the solution. ExperimentsThe assembly dynamics of different colloidal particle types (silica, polystyrene and carboxyl coated polystyrene particles) at an air–water interface in an optical potential were systematically explored allowing the effect of surface charge on assembly dynamics to be investigated. Additionally, the pH of the solutions were varied in order to modulate surface charge in a controllable fashion. The effect of surface tension on these assemblies was also explored by reducing the surface tension of the supporting solution by mixing ethanol with water. FindingsSilica, polystyrene and carboxyl coated polystyrene particles showed distinct assembly behaviours at the air–water interface that could be rationalised taking into account changes in surface charge (which in addition to being different between the particles could be modified systematically by changing the solution pH). Additionally, this is the first report showing that wettability of the colloidal particles and the surface tension of the solution are critical in determining the resulting assembly at the solution surface.

Bolaform Surfactant‐Induced Au Nanoparticle Assemblies for Reliable Solution‐Based Surface‐Enhanced Raman Scattering Detection

AbstractSolution‐based surface‐enhanced Raman scattering (SERS) detection typically involves the aggregation of citrate‐stabilized Au nanoparticles into colloidal assemblies. Although this sensing methodology offers excellent prospects for sensitivity, portability, and speed, it is still challenging to control the assembly process by a salting‐out effect, which affects the reproducibility of the assemblies and, therefore, the reliability of the analysis. This work presents an alternative approach that uses a bolaform surfactant, B20, to induce the plasmonic assembly. The decrease of the surface charge and the bridging effect, both promoted by the adsorption of B20, are hypothesized as the key points governing the assembly. Furthermore, molecular dynamic simulations supported the bridging effect of the B20 by showing the preferential bridging of surfactant monomers between two adjacent Au(111) slabs. The colloidal assemblies showed excellent SERS capabilities towards the rapid, on‐site detection and quantification of beta‐blockers and analgesic drugs in the nanomolar regime, with a portable Raman device. Interestingly, the application of state‐of‐the‐art convolutional neural networks, such as ResNet, allows a 100% accuracy in classifying the concentration of different binary mixtures. Finally, the colloidal approach was successfully implemented in a millifluidic chip allowing the automation of the whole process, as well as improving the performance of the sensor in terms of speed, reliability, and reusability without affecting its sensitivity.

Light‐Directed Assembly of Colloidal Matter

AbstractThe assembly of colloidal particles into 2D or 3D superstructures is significant as the colloidal assembly exhibits collective behavior beyond the sum of single particles. Technically, colloidal particles can either self‐assemble when thermodynamic equilibrium is reached, or directed into specific assembly under external stimulus, such as electric, magnetic, acoustic, or light field. Specifically, light can be focused locally and manipulated in a precise manner, providing the possibility to tailor the assembly kinetics in different degrees of freedom. The understanding of light–matter interaction during the assembly process is challenging but critical for the design and fabrication of diverse colloidal superstructures. In this review, these particles are treated as artificial atoms at colloidal scale to mimic the organization of matter. From this aspect, the light‐directed assembly process is discussed on the basis of the roles of light, including light‐directed nucleation, diffusion, interparticle bonding, and phase control. Beyond what has been observed at atomic scale, colloidal atoms exhibit diversity in size, shape, and composition, and their bonding force is irrelevant to the electronic state, which enriches the geometric complexity of colloidal matter. Finally, the authors summarize the emerging applications of these colloidal superstructures in nanophotonics, nanocatalysis, and nanomedicine, and outline the major challenge and future development.

Precise Assembly of Highly Crystalline Colloidal Photonic Crystals inside the Polyester Yarns: A Spray Coating Synthesis for Breathable and Durable Fabrics with Saturated Structural Colors

AbstractIt is a great challenge to prepare highly crystalline photonic crystal (PC)‐modified fabrics with saturated colors without sacrificing the wearing experience, such as breathability and softness, at the same time. Here, a spray coating process is developed to precisely assemble the colloidal PCs inside every yarn of the polyester (PET) fabric. High surface tension and high boiling point for the solvent of colloidal solution, as well as low twist angle, large weave cycles, and large tightness for the fabric substrate, are found to be favorable to the formation of high‐quality PC‐modified fabrics. In addition, the as‐prepared PC/PET fabrics have saturated and durable colors thanks to the highly crystalline PC structures and the polyacrylate adhesives. They also possess good softness and high breathability as the precise colloidal assembly in the yarn avoids the blocking of weaving holes and cracks.

Field-Induced Assembly and Propulsion of Colloids.

Electric and magnetic fields have enabled both technological applications and fundamental discoveries in the areas of bottom-up material synthesis, dynamic phase transitions, and biophysics of living matter. Electric and magnetic fields are versatile external sources of energy that power the assembly and self-propulsion of colloidal particles. In this Invited Feature Article, we classify the mechanisms by which external fields impact the structure and dynamics in colloidal dispersions and augment their nonequilibrium behavior. The paper is purposely intended to highlight the similarities between electrically and magnetically actuated phenomena, providing a brief treatment of the origin of the two fields to understand the intrinsic analogies and differences. We survey the progress made in the static and dynamic assembly of colloids and the self-propulsion of active particles. Recent reports of assembly-driven propulsion and propulsion-driven assembly have blurred the conceptual boundaries and suggest an evolution in the research of nonequilibrium colloidal materials. We highlight the emergence of colloids powered by external fields as model systems to understand living matter and provide a perspective on future challenges in the area of field-induced colloidal phenomena.

Ordering hard-sphere particle suspensions by medium crystallization: Effect of size and interaction strength.

While microstructure in soft materials is usually given by the self-assembly of their constituting building blocks, colloidal assembly can also be obtained via templating a morphology in a disordered suspension of particles by solidification of the melt. This order-templating process is applicable to different soft-matter systems with a variety of characteristic length scales, including particle suspensions in water, liquid-crystal materials, and polymer melts. In this work, we numerically investigate the effect of particle size and solvent size in the process of solidification templating by implementing a coarse-grain model of hard-sphere particles at a moving melt-crystal interface. This approach captures the dependence of crystallization templating on speed of crystal growth, showing the existence of a threshold speed for crystallization templating to occur. Results in this work show that the threshold speed changes following a power form as solvent size and particle size change. Furthermore, this work analyzes and reports the effect of particle-crystal interaction strength in combination with size effects. This scaling study from a numerical perspective sets a starting point for the development of hybrid soft materials via structural templating, allowing solidification-driven particle ordering in different systems with a variety of length scales.