Colloidal Assembly Research Articles

Paddlebots: Translation of Rotating Colloidal Assemblies near an Air/Water Interface.

Microbot propulsion requires unique strategies due to the dominance of viscosity and the reversible nature of microscale flows. To address this, swimmers of specific structure that translate in bulk fluid are commonly used; however, another approach is to take advantage of the inherent asymmetry of liquid/solid surfaces for microbots (μbots) to walk or roll. Using this technique, we have previously demonstrated that superparamagnetic colloidal particles can be assembled into small μbots, which can quickly roll along solid surfaces. In an analogous approach, here we show that symmetry can be similarly broken near air/liquid interfaces and μbots propelled at rates comparable to those demonstrated for liquid/solid interfaces.

Modulation of Colloidal Assembly Behavior Enables Printable Low‐Dimensional Perovskite Photovoltaics

AbstractThe multiple quantum wells (QWs) distribution in low‐dimensional perovskite films hinders charge transport due to the fundamental difficulty of controlling crystal growth from precursor solutions, yielding poorly homogeneous low‐dimensional perovskite solar cells (PSCs), especially in upscaling fabrication. Here, efficient low‐dimensional PSCs are realized by modulating the colloidal assembly behavior in the precursor solution to induce intermediate structures. In combination with in situ liquid time‐of‐flight secondary ion mass spectrometry, the assembly behavior of organic cations involved lead iodide‐dominated colloidal soft framework is visualized by investigating the precursor species differences under hydrogen bonding interactions. Subsequently, solid‐state reactions emerge and the formamidine (FA)‐based perovskite films exhibit significantly suppressed multiple QWs distribution. Encouragingly, the FA device (n=9, by meniscus‐assisted coating) achieves a power conversion efficiency (PCE) of 20.28 % for a size of 0.04 cm2 and a PCE of 15.35 % for a mini‐module of 16.94 cm2 with superior stability.

Modulation of Colloidal Assembly Behavior Enables Printable Low-Dimensional Perovskite Photovoltaics.

The multiple quantum wells (QWs) distribution in low-dimensional perovskite films hinders charge transport due to the fundamental difficulty of controlling crystal growth from precursor solutions, yielding poorly homogeneous low-dimensional perovskite solar cells (PSCs), especially in upscaling fabrication. Here, efficient low-dimensional PSCs are realized by modulating the colloidal assembly behavior in the precursor solution to induce intermediate structures. In combination with in situ liquid time-of-flight secondary ion mass spectrometry, the assembly behavior of organic cations involved lead iodide-dominated colloidal soft framework is visualized by investigating the precursor species differences under hydrogen bonding interactions. Subsequently, solid-state reactions emerge and the formamidine (FA)-based perovskite films exhibit significantly suppressed multiple QWs distribution. Encouragingly, the FA device (n = 9, by meniscus-assisted coating) achieves a power conversion efficiency (PCE) of 20.28% for a size of 0.04 cm2 and a PCE of 15.35% for a mini-module of 16.94 cm2 with superior stability.

Cooperative Lithium Sorption in Doped Layered Double Hydroxides Is Modulated by Colloidal (Dis)Assembly

Lithium–aluminum layered double hydroxides (LDHs) selectively sorb lithium from brines, concentrating and purifying this critical element for subsequent conversion to active battery components. Lithium ion partitioning into lattice vacancies within the LDH structure is selectively enhanced with iron doping. However, this process leads to a highly coupled set of intercalation interactions whose mechanisms are challenging to assess in situ. Here, we show that iron modulates the size- and shape-dependent composition of LDHs and imposes a powerful control on lithium sorption processes in complex fluids. We observe fundamental units of LDH layers and aluminum ferrihydrite nanoclusters that (dis)assemble to form at least five distinct particle types that influence LDH lithium capacity and cyclability. Importantly, lithium sorption is controlled by feedbacks arising from the dynamic interconversion of planar stacks and scrolls of LDH layers, which exchange lithium, water, and other species in the process of (un)rolling due to similar energy scales of hydration, sorption, and deformation. Under appropriate iron redox conditions, the cycling efficiency and stability of lithium sorption can be optimized for the range of lithium concentrations found in many natural brines.

Directed colloidal assembly and banding via DC electrokinetics.

Manipulating the transport and assembly of colloidal particles to form segregated bands or ordered supracolloidal structures plays an important role in many aspects of science and technology, from understanding the origin of life to synthesizing new materials for next-generation manufacturing, electronics, and therapeutics. One commonly used method to direct colloidal transport and assembly is the application of electric fields, either AC or DC, due to its feasibility. However, as colloidal segregation and assembly both require active redistribution of colloidal particles across multiple length scales, it is not apparent at first sight how a DC electric field, either externally applied or internally induced, can lead to colloidal structuring. In this Perspective, we briefly review and highlight recent advances and standing challenges in colloidal transport and assembly enabled by DC electrokinetics.

Catalytic-assembly of programmable atom equivalents

Escape from metastable states in self-assembly of colloids is an intractable problem. Unlike the commonly adopted approach of thermal annealing, the recently developed enthalpy-mediated strategy provided a different option to address this dilemma in a dynamically controllable manner at room temperature. However, it required a complex catalytic-assembly DNA strand-displacement circuitry to mediate interaction between multiple components. In this work, we present a simple but effective way to achieve catalytic-assembly of DNA-functionalized colloidal nanoparticles, i.e., programmable atom equivalents, in a far-from-equilibrium system. A removable molecule named "catassembler" that acts as a catalyst was employed to rectify imperfect linkages and help the system escape from metastability without affecting the assembled framework. Notably, catalytic efficiency of the catassembler can be effectively improved by changing the seesaw catassembler in toehold length design or numbers of the repeat units. Leveraging this tractable catalytic-assembly approach, different ordered architectures were easily produced by directly mixing all reactants, as in chemical reactions. By switching bonding identities, solid-solid phase transformations between different colloidal crystals were achieved. This work opens up an avenue for programming colloid assembly in a far-from-equilibrium system.

Collective transport and reconfigurable assembly of nematic colloids by light-driven cooperative molecular reorientations.

Nanomotors in nature have inspired scientists to design synthetic molecular motors to drive the motion of microscale objects by cooperative action. Light-driven molecular motors have been synthesized, but using their cooperative reorganization to control the collective transport of colloids and to realize the reconfiguration of colloidal assembly remains a challenge. In this work, topological vortices are imprinted in the monolayers of azobenzene molecules which further interface with nematic liquid crystals (LCs). The light-driven cooperative reorientations of the azobenzene molecules induce the collective motion of LC molecules and thus the spatiotemporal evolutions of the nematic disclination networks which are defined by the controlled patterns of vortices. Continuum simulations provide physical insight into the morphology change of the disclination networks. When microcolloids are dispersed in the LC medium, the colloidal assembly is not only transported and reconfigured by the collective change of the disclination lines but also controlled by the elastic energy landscape defined by the predesigned orientational patterns. The collective transport and reconfiguration of colloidal assemblies can also be programmed by manipulating the irradiated polarization. This work opens opportunities to design programmable colloidal machines and smart composite materials.

Acoustic tweezers using bisymmetric coherent surface acoustic waves for dynamic and reconfigurable manipulation of particle multimers

HypothesisThe accurate and dynamic manipulation of multiple micro-sized objects has always been a technical challenge in areas of colloid assembly, tissue engineering, and organ regeneration. The hypothesis of this paper is the precise modulation and parallel manipulation of morphology of individual and multiple colloidal multimers can be achieved by customizing acoustic field. ExperimentsHerein, we present a colloidal multimer manipulation method by using acoustic tweezers with bisymmetric coherent surface acoustic waves (SAWs), which enables contactless morphology modulation of individual colloidal multimers and patterning arrays by regulating the shape of acoustic field to specific desired distributions with high accuracy. Rapid switching of multimer patterning arrays, morphology modulation of individual multimers, and controllable rotation can be achieved by regulating coherent wave vector configurations and phase relations in real time. FindingsTo demonstrate the capabilities of this technology, we have firstly achieved eleven patterns of deterministic morphology switching for single hexamer and precise switching between three array modes. In addition, the assembly of multimers with three kinds of specific widths and controllable rotation of single multimers and arrays were demonstrated from 0 to 22.4 rpm (tetramers). Therefore, this technique enables reversible assembly and dynamic manipulation of particles and/or cells in colloid synthesis applications.

Effect of Hydrophobic Moieties on the Assembly of Silica Particles into Colloidal Crystals

To boost the implementation of colloidal crystals (CCs) in separation science, the effects of the most common chromatographic reversed phases, that is, butyl and octadecyl, on the assembly of silica particles into CCs and on the optical properties of CCs are investigated. Interestingly, particle surface modification can cause phase separation during sedimentation because the assembly is highly sensitive to minute changes in surface characteristics. Solvent-induced surface charge generation through acid-base interactions of acidic residual silanol groups with the solvent is enough to promote colloidal crystallization of modified silica particles. In addition, solvation forces at small interparticle distances are also involved in colloidal assembly. The characterization of CCs formed during sedimentation or via evaporative assembly revealed that C4 particles can form CCs more easily than C18 particles because of their low hydrophobicity; the latter can only form CCs in tetrahydrofuran when C18 chains with a high bonding density have extra hydroxyl side groups. These groups can only be hydrolyzed from trifunctional octadecyl silane but not from a monofunctional one. Moreover, after evaporative assembly, CCs formed from particles with different surface moieties exhibit different lattice spacings because their surface hydrophobicity and chemical heterogeneity can modulate interparticle interactions during the two main stages of assembly: the wet stage of crystal growth and the late stage of nano dewetting (evaporation of interparticle solvent bridges). Finally, short, alkyl-modified CCs were effectively assembled inside silica capillaries with a 100 μm inner diameter, laying the foundation for future chromatographic separation using capillary columns.

Highly transparent silanized cellulose aerogels for boosting energy efficiency of glazing in buildings

To maintain comfortable indoor conditions, buildings consume ~40% of the energy generated globally. In terms of passively isolating building interiors from cold or hot outdoors, windows and skylights are the least-efficient parts of the building envelope because achieving simultaneously high transparency and thermal insulation of glazing remains a challenge. Here we describe highly transparent aerogels fabricated from cellulose, an Earth-abundant biopolymer, by utilizing approaches such as colloidal self assembly and procedures compatible with roll-to-roll processing. The aerogels have visible-range light transmission of 97–99% (better than glass), haze of ~1% and thermal conductivity lower than that of still air. These lightweight materials can be used as panes inside multi-pane insulating glass units and to retrofit existing windows. We demonstrate how aerogels boost energy efficiency and may enable advanced technical solutions for insulating glass units, skylights, daylighting and facade glazing, potentially increasing the role of glazing in building envelopes.

Interfacial Colloidal Self-Assembly for Functional Materials

ConspectusSelf-assembly bridges nanoscale and microscale colloidal particles into macroscale functional materials. In particular, self-assembly processes occurring at the liquid/liquid or solid/liquid/air interfaces hold great promise in constructing large-scale two- or three-dimensional (2D or 3D) architectures. Interaction of colloidal particles in the assemblies leads to emergent collective properties not found in individual building blocks, offering a much larger parameter space to tune the material properties. Interfacial self-assembly methods are rapid, cost-effective, scalable, and compatible with existing fabrication technologies, thus promoting widespread interest in a broad range of research fields.Surface chemistry of nanoparticles plays a predominant role in driving the self-assembly of nanoparticles at water/oil interfaces. Amphiphilic nanoparticles coated with mixed polymer brushes or mussel-inspired polydopamine were demonstrated to self-assemble into closely packed thin films, enabling diverse applications from electrochemical sensors and catalysis to surface-enhanced optical properties. Interfacial assemblies of amphiphilic gold nanoparticles were integrated with graphene paper to obtain flexible electrodes in a modular approach. The robust, biocompatible electrodes with exceptional electrocatalytic activities showed excellent sensitivity and reproducibility in biosensing. Recyclable catalysts were prepared by transferring monolayer assemblies of polydopamine-coated nanocatalysts to both hydrophilic and hydrophobic substrates. The immobilized catalysts were easily recovered and recycled without loss of catalytic activity. Plasmonic nanoparticles were self-assembled into a plasmonic substrate for surface-enhanced Raman scattering, metal-enhanced fluorescence, and modulated fluorescence resonance energy transfer (FRET). Strong Raman enhancement was accomplished by rationally directing the Raman probes to the electromagnetic hotspots. Optimal enhancement of fluorescence and FRET was realized by precisely controlling the spacing between the metal surface and the fluorophores and tuning the surface plasmon resonance wavelength of the self-assembled substrate to match the optical properties of the fluorescent dye.At liquid/solid interfaces, infiltration-assisted (IFAST) colloidal self-assembly introduces liquid infiltration in the substrate as a new factor to control the degree of order of the colloidal assemblies. The strong infiltration flow leads to the formation of amorphous colloidal arrays that display noniridescent structural colors. This method is compatible with a broad range of colloidal particle inks, and any solid substrate that is permeable to dispersing liquids but particle-excluding is suitable for IFAST colloidal assembly. Therefore, the IFAST technology offers rapid, scalable fabrication of structural color patterns of diverse colloidal particles with full-spectrum coverage and unprecedented flexibility. Metal-organic framework particles with either spherical or polyhedral morphology were used as ink particles in the Mayer rod coating on wettability patterned photopapers, leading to amorphous photonic structures with vapor-responsive colors. Anticounterfeiting labels have also been developed based on the complex optical features encoded in the photonic structures.Interfacial colloidal self-assembly at the water/oil interface and IFAST assembly at the solid/liquid/air interface have proven to be versatile fabrication platforms to produce functional materials with well-defined properties for diverse applications. These platform technologies are promising in the manufacturing of value-added functional materials.

Conducting polypyrrole-coated leathers

Conducting polymers are promising materials applicable as interfaces in various functional materials given they intrinsically possess electronic and ionic conductivity. Herein, a conducting polymer, polypyrrole (PPy) was explored for surface modification of three different leathers via in-situ oxidation of pyrrole in the absence and the presence of two stabilizing polymers, hydroxypropylcellulose and poly(N-vinylpyrrolidone). Colloidal dispersed PPy assemblies were formed with colloidal stabilizers along with PPy-coated leathers and hampered the formation of PPy precipitates that would otherwise contaminate the leather surface. The surface-modified bio-based composites were characterized by FTIR and Raman spectroscopy, and optical and scanning electron microscopy. PPy was deposited on collagen fibers on both sides of the leather. The thickness of the PPy layer varied between 24 and 200 μm. Mechanical properties of the leathers manifested themselves by the decrease in tensile strength and elongation at break after the modification process. Sheet resistances were in the range of 100–103 kΩ/sq with the highest values observed for materials modified in the presence of the polymeric stabilizers. Different sheet resistances were obtained for the top and bottom sides of the leathers depending on the surface finishing. Furthermore, the modified leathers were subjected to cyclic bending tests and the variation in their resistivity was studied. The conductivity and electroactivity of the modified materials may serve as guidance for the rational design of intelligent leathers in the footwear, as heating elements, and in energy storage.

"Wettability Transition-Induced Multi-Layer Assembly of Colloids at a Water-Air Interface: An Improved Mechanism"

"Wettability Transition-Induced Multi-Layer Assembly of Colloids at a Water-Air Interface: An Improved Mechanism"

Molecular Engineering of Colloidal Atoms.

Creation of architectures with exquisite hierarchies actuates the germination of revolutionized functions and applications across a wide range of fields. Hierarchical self-assembly of colloidal particles holds the promise for materialized realization of structural programing and customizing. This review outlines the general approaches to organize atom-like micro- and nanoparticles into prescribed colloidal analogs of molecules by exploiting diverse interparticle driving motifs involving confining templates, interactive surface ligands, and flexible shape/surface anisotropy. Furthermore, the self-regulated/adaptive co-assembly of simple unvarnished building blocks is discussed to inspire new designs of colloidal assembly strategies.

Metal-Organic Framework-Based Colloidal Particle Synthesis, Assembly, and Application.

Metal-organic frameworks (MOFs) assembled from metal nodes and organic ligands have received significant attention over the past two decades for their fascinating porous properties and broad applications. Colloidal MOFs (CMOFs) not only inherit the intrinsic properties of MOFs, but can also serve as building blocks for self-assembly to make functional materials. Compared to bulk MOFs, the colloidal size of CMOFs facilitates further manipulation of CMOF particles in a single or collective state in a liquid medium. The resulting crystalline order obtained by self-assembly in position and orientation can effectively improve performance. In this review, we summarize the latest developments of CMOFs in synthesis strategies, self-assembly methods, and related applications. Finally, we discuss future challenges and opportunities of CMOFs in synthesis and assembly, by which we hope that CMOFs can be further developed into new areas for a wider range of applications.

Dissipative crystallization of ion-pair receptors

•Crystal presence is dependent on the transient presence of chemical stimuli •Single crystals undergo dissipative crystal growth that is visually observable •Partially degraded single crystals can be rescued back to their original state •This dissipative system is highly modular Temporally controlled assemblies exhibit behaviors associated with living systems. Thus far, these transient assemblies have been studied primarily in soft materials, such as gels, vesicles, and colloidal assemblies. Here, we describe transient single crystals of supramolecular assemblies based on calix[4]pyrrole as an ion-pair receptor. The crystals form transiently upon the addition of a chemical stimulus, terephthalic acid, and disassemble when the chemical stimulus is consumed by alkylation using bromoethane. Single crystals observable by optical microscopy were grown and allowed to degrade repetitively by adding the chemical stimulus multiple times to a solution of the monomers. Partially degraded crystals were rescued and healed by adding more chemical stimulus to the solution before they dissolved completely. These results demonstrate that host-guest chemistry can be leveraged to create high-quality yet dynamic crystals that exhibit dissipative behavior. The modular nature of calix[4]pyrrole ion-pair recognition shows promise for obtaining diverse molecular materials capable of transient assembly. Temporally controlled assemblies exhibit behaviors associated with living systems. Thus far, these transient assemblies have been studied primarily in soft materials, such as gels, vesicles, and colloidal assemblies. Here, we describe transient single crystals of supramolecular assemblies based on calix[4]pyrrole as an ion-pair receptor. The crystals form transiently upon the addition of a chemical stimulus, terephthalic acid, and disassemble when the chemical stimulus is consumed by alkylation using bromoethane. Single crystals observable by optical microscopy were grown and allowed to degrade repetitively by adding the chemical stimulus multiple times to a solution of the monomers. Partially degraded crystals were rescued and healed by adding more chemical stimulus to the solution before they dissolved completely. These results demonstrate that host-guest chemistry can be leveraged to create high-quality yet dynamic crystals that exhibit dissipative behavior. The modular nature of calix[4]pyrrole ion-pair recognition shows promise for obtaining diverse molecular materials capable of transient assembly.

Stable Bassanite Bulk Phase Formed in Aqueous Solution under the Control of Polymer-Mediated Water Activity

Bassanite, as the metastable phase in the family of calcium sulfate minerals, plays an important role in natural and industrial processes, but a stable bulk bassanite can rarely precipitate in nature or be fabricated under ambient conditions. Here, we report the first example of stable bulk bassanite precipitated in solution, which fills in the gap of calcium sulfate phase diagram. By precisely controlling water activity with polymer (polyvinyl pyrrolidone), bulk bassanite is formed, instead of transforming into thermodynamically favorable gypsum. A new thermodynamic model of the non-classical crystallization process is described in terms of water-driven colloidal assembly, and molecular dynamics simulation further demonstrates the role of water molecules in interacting with the polymer ligands to initiate the assembly and growth of bassanite into bulk. These findings not only contribute to a deeper understanding of the role of water activity in the formation of stable bulk bassanite in extreme natural environments but also allow for the fabrication of bassanite materials for industrial applications at room temperature.

Dynamic interfaces for contact-time control of colloidal interactions.

Understanding pairwise interactions between colloidal particles out of equilibrium has a profound impact on dynamical processes such as colloidal self assembly. However, traditional colloidal interactions are effectively quasi-static on colloidal timescales and cannot be modulated out of equilibrium. A mechanism to dynamically tune the interactions during colloidal contacts can provide new avenues for self assembly and material design. In this work, we develop a framework based on polymer-coated colloids and demonstrate that in-plane surface mobility and mechanical relaxation of polymers at colloidal contact interfaces enable an effective, dynamic interaction. Combining analytical theory, simulations, and optical tweezer experiments, we demonstrate precise control of dynamic pair interactions over a range of pico-Newton forces and seconds timescales. Our model helps further the general understanding of out-of-equilibrium colloidal assemblies while providing extensive design freedom via interface modulation and nonequilibrium processing.

Equilibrium state model for surfactants in oils: Colloidal assembly and adsorption

An equilibrium state model addressing the aggregation and adsorption of colloidal assemblies in apolar solvents (oils) via monomer exchange is presented. The model is based on the previously reported step-wise aggregation response of fatty acids and monoglycerides in bio-oils, and captures surface crowding via scaled particle theory (SPT). The sensitivity of key observables - mean aggregation number, adsorbed surfactant amount, and free monomer concentration - to model parameters is demonstrated. Fits to molecular modelling based aggregation and adsorption data of oleic acid and monoolein reveal that the model accurately reproduces chemically specific aggregate exponential distributions in both bulk and surface phases, even outside of its parameterization conditions. A biased state model, where the initial bulk aggregation step (dimer formation) differs from other steps results in a notable improvement in accuracy. Fits to various phospholipid adsorption isotherms demonstrate the applicability of the model to isotherm type experimental data. The fits reveal either monolayer or aggregate like adsorption structures, depending on surfactant head group charge. The presented model provides an easily accessible, computationally feasible means to estimate colloidal assembly and adsorption in oil environments, and enables assessment of surfactant aggregation propensity and adsorption energetics.

Nature-inspired functional porous materials for low-concentration biomarker detection.

Nanostructuration is a promising tool for enhancing the performance of sensors based on electrochemical transduction. Nanostructured materials allow for increasing the surface area of the electrode and improving the limit of detection (LOD). In this regard, inverse opals possess ideal features to be used as substrates for developing sensors, thanks to their homogeneous, interconnected pore structure and the possibility to functionalize their surface. However, overcoming the insulating nature of conventional silica inverse opals fabricated via sol-gel processes is a key challenge for their application as electrode materials. In this work, colloidal assembly, atomic layer deposition and selective surface functionalization are combined to design conductive inverse opals as an electrode material for novel glucose sensing platforms. An insulating inverse opal scaffold is coated with uniform layers of conducting aluminum zinc oxide and platinum, and subsequently functionalized with glucose oxidase embedded in a polypyrrole layer. The final device can sense glucose at concentrations in the nanomolar range and is not affected by the presence of common interferents gluconolactone and pyruvate. This method may also be applied to different conductive materials and enzymes to generate a new class of highly efficient biosensors.