Colloidal Crystals Research Articles

Potential effects on effective attraction between probes diffusing in colloidal crystal

Potential effects on effective attraction between probes diffusing in colloidal crystal

Curvature-directed anchoring and defect structure of colloidal smectic liquid crystals in confinement

Curvature-directed anchoring and defect structure of colloidal smectic liquid crystals in confinement

Polymerization-induced ultrafast and massive growth of colloidal photonic crystals on hydrogel surface

Structural color materials based on colloidal photonic crystals (CPCs) have been extensively investigated since they can be prepared efficiently. Commonly, photonic hydrogels are prepared by a polymerizing pre-gel solution, in which colloids are self-assembled by passive methods. Except for magnetic colloids, the process of forming colloidal photonic crystals in a pre-gel solution requires hours to days, and it is almost impossible to assemble the colloids directly in hydrogels. Here, we present a mild strategy to prepare hydrogels with surface structural color, ordered colloidal arrays are formed at the air-hydrogel interface actively during the polymerization process, and bright structural color forms in minutes on a large scale. Polymer exclusion is the key driving force for the assembly of the colloids, other factors, such as evaporation flux, can also promote the colloids to assemble. Significantly, heterogeneous structural color patterns can be produced easily by exposing the hydrogel surface to air sequentially using a mask during the polymerization process. The simple and fast method provides a general route for preparing structural colored surfaces, which may be suitable for various applications.

Strong Impact of Particle Size Polydispersity on the Thermal Conductivity of Yukawa Crystals

Control of thermal transport in colloidal crystals plays an important role in modern technologies. A deeper understanding of the governing heat transport processes in various systems, such as polydisperse colloidal crystals, is required. This study shows how strongly the particle size polydispersity of a model colloidal crystal influences the thermal conductivity. The thermal conductivity of model colloidal crystals has been calculated using molecular dynamics simulations. The model crystals created by particles interacting through Yukawa (screened-Coulomb) interaction are assumed to have a face-centered cubic structure. The influence of the Debye screening length, contact potential, and particle size polydispersity on the thermal conductivity of Yukawa crystals was investigated. It was found that an increase in particle size polydispersity causes a strong—almost fivefold—decrease in the thermal conductivity of Yukawa crystals. In addition, the obtained results showed that the effect of the particle size polydispersity on reducing the thermal conductivity of Yukawa crystals is stronger than changes in values of the Debye screening length or the contact potential.

Reconfigurable Antireflection Coatings Enabled by PDMS Oligomer Infusion in Templated Nanoporous Polymer Films.

The diffusion of uncured polydimethylsiloxane (PDMS) oligomers out of bulk PDMS elastomers is usually detrimental to many biomedical and microfluidic applications due to the inevitable contamination of the contacting fluids and substrates. Here, we transform this detrimental process into an enabling technology for achieving novel reconfigurable antireflection (AR) coatings, which are of great technological importance in the development of new nano-optical and optoelectronic applications. Self-assembled monolayer silica colloidal crystals are first used as sacrificial templates in fabricating nanoporous polymer AR coatings. When air in the templated nanopores is replaced with infused PDMS oligomers simply by pressing a PDMS stamp on a nanoporous AR film, the original antireflection conditions are lost, and the coating transforms from a low-reflection configuration to a high-reflection state. The original antireflection performance can be fully recovered by dissolving the infused oligomers in the appropriate solvents (e.g., hexane). This novel tuning mechanism for achieving reconfigurable AR properties has been confirmed by systematic investigations using various microscopes, optical spectroscopy, nanoindentation, thermomechanical tests, and X-ray photoelectron spectroscopy. Complex micropatterns with micrometer-scale spatial resolution and drastically different AR performances can be easily printed on nanoporous AR films by using a soft lithography-based microcontact printing process. Numerical finite-difference time-domain simulations match well with experimental antireflection measurements and reveal a linear relationship between the optical transmission and the amount of infused PDMS oligomers in nanopores.

High Temperature, Isothermal Growth Promotes Close Packing and Thermal Stability in DNA-Engineered Colloidal Crystals.

We report a strategy to accelerate the synthesis and increase the crystallinity of colloidal crystals (CCs) engineered with DNA. Specifically, by holding the DNA-modified Au particle building blocks above the Tm of the DNA bonding elements (i.e., free from the particles), but slightly below the Tm of the anticipated CC during the assembly process, crystallinity is increased, and enthalpically favored phases with high degrees of facet registration are observed. We studied the utility of this approach with systems for which the commonly adopted slow-cooling approach yielded primarily amorphous aggregates. In particular, we used it to synthesize high-volume fraction CCs from large (80 nm) anisotropic nanoparticles (cubes and rhombic dodecahedra) with short (<14 nm) DNA designed to restrict the degrees of freedom for the DNA bonds and maintain the anisotropy of the particle building block. Small-angle X-ray scattering and electron microscopy studies show that the crystalline phases synthesized via this method are more thermally stable than their corresponding aggregate phases, likely due to an increased number of DNA-DNA bonds between particles. Crystal size tunability (between 0.5 and 15 μm edge lengths) and epitaxial growth were demonstrated using this strategy by modulating the NaCl concentration in tandem with previously synthesized CC nuclei. Taken together, this isothermal strategy demonstrates how to deliberately crystallize a wide variety of anisotropic colloidal materials and expands the phase space accessible to nanoparticles modified with DNA.

Active Fréedericksz Transition in Active Nematic Droplets

Active nematic liquid crystals have the remarkable ability to spontaneously deform and flow in the absence of any external driving force. While living materials with orientational order, such as the mitotic spindle, can self-assemble in quiescent active phases, reconstituted active systems often display chaotic, periodic, or circulating flows under confinement. Quiescent active nematics are, therefore, quite rare, despite the prediction from active hydrodynamic theory that confinement between two parallel plates can suppress flows. This spontaneous flow transition—named the active Fréedericksz transition by analogy with the conventional Fréedericksz transition in passive nematic liquid crystals under a magnetic field—has been a cornerstone of the field of active matter. Here, we report experimental evidence that confinement in spherical droplets can stabilize the otherwise chaotic dynamics of a 3D extensile active nematics, giving rise to a quiescent—yet still out-of-equilibrium—nematic liquid crystal. The active nematics spontaneously flow when confined in larger droplets. The composite nature of our model system composed of extensile bundles of microtubules and molecular motors dispersed in a passive colloidal liquid crystal allows us to demonstrate how the interplay of activity, nematic elasticity, and confinement impacts the spontaneous flow transition. The critical diameter increases when motor concentration decreases or nematic elasticity increases. Experiments and simulations also demonstrate that the critical confinement depends on the confining geometry, with the critical diameter in droplets being larger than the critical width in channels. Biochemical assays reveal that neither confinement nor nematic elasticity impacts the energy-consumption rate, confirming that the quiescent active phase is the stable out-of-equilibrium phase predicted theoretically. Further experiments in dense arrays of monodisperse droplets show that fluctuations in the droplet composition can smooth the flow transition close to the critical diameter. In conclusion, our work provides experimental validation of the active Fréedericksz transition in 3D active nematics, with potential applications in human health, ecology, and soft robotics. Published by the American Physical Society 2024

Engineering Anisotropy into Organized Nanoscale Matter.

Programming the organization of discrete building blocks into periodic and quasi-periodic arrays is challenging. Methods for organizing materials are particularly important at the nanoscale, where the time required for organization processes is practically manageable in experiments, and the resulting structures are of interest for applications spanning catalysis, optics, and plasmonics. While the assembly of isotropic nanoscale objects has been extensively studied and described by empirical design rules, recent synthetic advances have allowed anisotropy to be programmed into macroscopic assemblies made from nanoscale building blocks, opening new opportunities to engineer periodic materials and even quasicrystals with unnatural properties. In this review, we define guidelines for leveraging anisotropy of individual building blocks to direct the organization of nanoscale matter. First, the nature and spatial distribution of local interactions are considered and three design rules that guide particle organization are derived. Subsequently, recent examples from the literature are examined in the context of these design rules. Within the discussion of each rule, we delineate the examples according to the dimensionality (0D-3D) of the building blocks. Finally, we use geometric considerations to propose a general inverse design-based construction strategy that will enable the engineering of colloidal crystals with unprecedented structural control.

Investigation of the regulatory mechanism of monolayer colloidal crystals prepared by gas-liquid-solid three-phase interfacial self-assembly method

In recent years, the gas-liquid interface self-assembly method has emerged as a recognized and efficient technique for preparing two-dimensional monolayer colloidal crystals. However, challenges such as limited preparation area, dependence on complex equipment, and high costs persist in the conventional preparation process. This paper proposes a straightforward, rapid, and cost-effective method to implement a gas-liquid-solid three-phase interfacial self-assembly strategy. This approach enhances the gas-liquid interfacial self-assembly method, allowing for the preparation of large-area, high-quality single-layer colloidal crystals (>10 cm2) without using any surfactant. The gas-liquid-solid three-phase interfacial self-assembly process was analyzed in detail, and it was found that the main factors affecting the quality of the monolayer colloidal crystals were the volume ratio of PS colloidal solution to ethanol and the self-assembly temperature. It was shown that the optimal self-assembly conditions were achieved after dispersion by ultrasonic dispersion in a water bath at 32 °C–30 °C at a volume ratio of 1:2.5 of PS colloidal solution to ethanol. In addition, particles of various sizes can be prepared by this method to produce tightly arranged monolayer films that can be transferred to a variety of different substrates with great versatility. The demonstrated efficiency of this self-assembly method underscores its significant potential for application in large-scale manufacturing and practical industrialization.

Bio-inspired colloidal photonic crystal pattern with multiple optical variable images

Colloidal photonic crystals (CPCs) are highly significant for display and anti-counterfeiting applications due to their vibrant structural colors and light-manipulating properties. Efficient and extensive CPC patterns is required for practical applications. Additionally, it is essential to incorporate complex optical elements or responsive qualities to the CPC patterns in order to enhance their performances for security applications. Herein, we proposed CPC patterns with alterable images revealed by changes in viewing angle or exposure to certain vapors. The CPC patterns were prepared by inkjet printing method. Through substrate modification to achieve varying wettability, angle-dependent optical adaptable images could potentially be shown by the patterns. The mesoporous silica nanoparticles (MSNs) were employed as building blocks to impart vapor-chromic characteristics to the CPC patterns. The CPC complex patterns responsive optical characteristics rendered them suitable for use in public authentication and anti-counterfeiting applications.

Influence of Sodium Ions and Carbon Black on the Formation and Structural Color of Photonic Balls by Silica Particles.

In this study, photonic balls─spherical aggregates of submicrometer-sized silica particles with uniform particle size─were investigated as structural colored materials. The structural color of these photonic balls is influenced by the ordered arrangement of the silica particles. The research focused on how the addition of electrolytes, specifically NaCl, affects the formation of photonic balls to achieve the desired structural color. Without NaCl, the photonic balls formed onion-shaped colloidal crystals. At NaCl concentrations above 0.006 mol/L, the particles aggregated into short-range ordered structures. When the concentration exceeded 0.05 mol/L, the aggregates lost their spherical shape. The study also explored the addition of carbon black (CB), a water-dispersible material due to its surface charge. The findings revealed that NaCl induced the phase separation between the charged silica particles and CB, resulting in Janus-shaped photonic balls─one side exhibiting structural color and the other side appearing black due to the presence of CB. Changing the silica particle size altered the hues of these Janus-shaped photonic balls, though they appeared uniformly colored to the naked eye. While this study did not specifically examine the applications of Janus-shaped photonic balls composed of silica particles and CB, CB is known for its ability to absorb near-infrared radiation and convert it into heat as well as its conductive properties. Silica, on the other hand, has a low thermal conductivity and acts as an electrical insulator. The structurally colored Janus-shaped photonic balls created in this study may serve as pigments in applications requiring anisotropic heat generation and electrical conduction. Additionally, the study's findings suggest the potential for creating various types of Janus-shaped photonic balls from materials with differing densities.

Enhanced Colorimetric Detection of Volatile Organic Compounds Using a Dye-Incorporated Photonic Crystal-Based Sensor Array.

Chemoresponsive dyes offer the potential to selectively detect volatile organic compounds (VOCs) unique to certain disease states. Among different VOC sensing techniques, colorimetric sensing offers the advantage of facile recognition. However, it is often challenging to discern the color changes by the naked eye. Here, highly sensitive colorimetric VOC sensor arrays from dye-incorporated colloidal photonic crystals (dye-cPhCs) are reported. cPhCs are scalably fabricated on a 4-inch wafer by spin-coating of silica nanoparticles (NPs) dispersed in a photo-cross-linkable monomer, where the gradient shear flow along the film thickness creates densely-packed square arrays of NPs in the top layers, whereas the bulk is quasi-amorphous with larger periodicities. The broadened reflection peak allows for augmented dye absorption originating from the overlap between the photonic bandgap edge of the cPhC and the dye absorption peak, leading to a more noticeable color change upon exposure to VOCs. The sensor array generates distinct color difference maps for acetaldehyde, acetone, and acetic acid, respectively, without any data amplification. The limit of detection for acetaldehyde, acetone, and acetic acid is 1, 0.1, and 0.02ppm, respectively. Moreover, VOC can be diagonalized by visually intuitive pattern recognition, and principal component analysis at reduced dimensionality is demonstrated.

Co3O4-modified 3DOM LaFe0.6Mg0.4O3 perovskite as robust catalysts for soot oxidation

In this study, 3DOM LaFe0.6Mg0.4O3 perovskite was prepared using a colloidal crystal template technique and subsequently decorated with Co3O4 for soot combustion. The results indicated that the LaFe0.6Mg0.4O3 perovskite with three-dimensional ordered macropores structure could facilitate the dispersion of Co3O4, boost the number of active sites for soot combustion, and significantly enhance the contact efficiency between the catalyst and soot. The interaction between Co3O4 and perovskite created additional oxygen vacancies and enhanced the redox ability of the catalysts. Meanwhile, the presence of Co3O4 facilitated the adsorption and activation of O2, which could accelerate the oxidation of soot particles. Notably, the 2.5 %Co/LaFe0.6Mg0.4O3 catalyst exhibited the highest performance, achieving a T50 of 407 °C for soot combustion in the presence of 500 ppm NO and 5 % O2. The catalyst also demonstrated excellent stability and water resistance, making it a promising candidate for practical applications.

Liquid-Crystalline Photonic Sandwich: Electroresponsive Colloids of Clay Nanosheets Loading Photofunctional Dyes.

Colloidal clay nanosheets obtained by the delamination of layered crystals of smectite-type clay minerals in water form liquid crystals because of their shape anisotropy. Loading of organic dyes onto the liquid crystalline clay nanosheets will enable novel photonic materials, where photofunctions of the loaded dye are controlled by the liquid crystallinity of the clay nanosheets. However, adsorption of organic dyes onto the nanosheets renders the nanosheet surfaces hydrophobic, and consequently, colloidal stability of the nanosheets is lost. In this study, this drawback is overcome by sandwiching cationic stilbazolium dyes between a pair of synthetic fluorohectorite nanosheets. This is realized by the preparation of stilbazolium-clay second-stage intercalation compounds characterized by intercalation of dye cations into every other interlayer space of the hectorite clay, where nonintercalated interlayer spaces are occupied by Na+ ions. The second-stage intercalation compounds are obtained by partial ion exchange of mother clay mineral incorporating Na+ ions in all of the interlayer spaces and delaminated from the Na+-containing interlayer spaces to form clay nanosheets sandwiching the dye molecules. Aqueous colloids of the dye-sandwiching clay nanosheets form colloidal liquid crystals, and the dye-sandwiching liquid crystalline clay nanosheets respond to an applied AC electric field to be aligned parallel to the electric field. The assembled structure of the dye-sandwiching clay nanosheets under the electric field is characterized by aligned discrete clay platelets, which is somewhat different from that of a colloidal liquid crystal of clay nanosheets without dye loading characterized by macroscopic liquid crystalline domains up to submillimeters. The electric alignment of the clay nanosheets induces alteration of light absorption of the sandwiched stilbazolium molecules, which verifies a strategy of constructing stimuli-responsive photonic materials of clay-organic hybrids.

Well-Defined Homopolymer Nanoparticles with Uniaxial Molecular Orientation by Synchronized Polymerization and Self-Assembly.

Synthesizing anisotropic polymeric nanoparticles (NPs) with well-defined shapes, dimensions, and molecular orientations is a very challenging task. Herein, we report the synthesis of surprisingly highly uniform shape-anisotropic polymer NPs with uniaxial internal molecular orientation. Keys to our method are synchronized polymerization and self-assembly (SPSA), which can even be realized by regular dispersion polymerization. This is demonstrated using a monomer containing a rigid 4-nitroazobenzene (NAB) side group. The short nucleation period, the completion of microphase separation before molecular motion is frozen, and sufficient low particle/solvent interfacial tension are shown to be the origins of the highly uniform dimensions, single liquid crystal domains, and well-defined anisotropic shape of particles. The liquid crystallization ability of the polymers, control of molecular weight distribution, and the polymerization kinetics are identified as three key factors controlling the NP formation. The uniformity of these NPs facilitates their SA formation into colloidal crystals. The particles exhibit optically anisotropic properties depending on orientations and, in particular, show intriguing photoswitchable LC-glass (order-disorder) transition, which can be used for the detection of ultraviolet (UV) light and allows the fabrication of photoreversible colloidal films.

Colorimetric IPN hydrogels embedded with colloidal photonic crystals: A novel approach for the detection of ethanol and Ba2+ ions in water

A critical bottleneck in sensor technology is the rapid and precise detection of specific analytes in complex matrices, hindering advancements in environmental monitoring, healthcare, and industrial process control. This study addresses this challenge by introducing a novel composite hydrogel sensor designed for rapid and selective detection of ethanol and barium ions (Ba2+) in aqueous environments. The sensor integrates interpenetrating network (IPN) hydrogels with embedded colloidal photonic crystals (CPCs), synthesized via a solution-based polymerization approach. This innovative configuration allows CPCs to dynamically adjust their photonic bandgap in response to environmental changes, manifesting as a visible, colorimetric shift. This response stems from the synergy between the mechanical properties of the IPN hydrogel and the optical sensitivity of CPCs. Upon exposure to analytes such as ethanol and Ba2+, the sensor exhibits a rapid and reversible color transition that is directly proportional to their concentration. Notably, ethanol (0 vol%–80 vol%) and Ba2+ (5–17.5 mM) induce a distinct blueshift in the photonic bandgap and trigger a color change from red–orange to green due to the alteration in the swelling behavior of the IPN hydrogel, affecting its lattice constant. The IPN hydrogel–CPC composite demonstrates exceptional operational stability and facilitates rapid detection, making it ideal for on-site applications without the need for complex equipment. These characteristics make the composite hydrogel sensor a promising candidate for environmental monitoring, industrial process control, and public health diagnostics, paving the way for the development of next-generation responsive sensor materials.

Multiple temperatures and melting of a colloidal active crystal

Thermal fluctuations constantly excite all relaxation modes in an equilibrium crystal. As the temperature rises, these fluctuations promote the formation of defects and eventually melting. In active solids, the self-propulsion of “atomic” units provides an additional source of non-equilibrium fluctuations whose effect on the melting scenario is still largely unexplored. Here we show that when a colloidal crystal is activated by a bath of swimming bacteria, solvent temperature and active temperature cooperate to define dynamic and thermodynamic properties. Our system consists of repulsive paramagnetic particles confined in two dimensions and immersed in a bath of light-driven E. coli. The relative balance between fluctuations and interactions can be adjusted in two ways: by changing the strength of the magnetic field and by tuning activity with light. When the persistence time of active fluctuations is short, a single effective temperature controls both the amplitudes of relaxation modes and the melting transition. For more persistent active noise, energy equipartition is broken and multiple temperatures emerge, whereas melting occurs before the Lindemann parameter reaches its equilibrium critical value. We show that this phenomenology is fully confirmed by numerical simulations and framed within a minimal model of a single active particle in a periodic potential.

Development of a Methodology Based on Optical Interferometry for Measuring Fibrinolytic Activity.

Fibrinolytic activity assay is particularly important for the detection, diagnosis, and treatment of cardiovascular disease and the development of fibrinolytic drugs. A novel efficacious strategy for real-time and label-free dynamic detection of fibrinolytic activity based on ordered porous layer interferometry (OPLI) was developed. Fibrin or a mixture of fibrin and plasminogen (Plg) was loaded into the highly ordered silica colloidal crystal (SCC) film scaffold to construct a fibrinolytic response interference layer to measure fibrinolytic activity with different mechanisms of action. Fibrinolytic enzyme-triggered fibrinolysis led to the migration of interference fringes in the interferogram, which could be represented by optical thickness changes (ΔOT) tracked in real time by the OPLI system. The morphology and optical property of the fibrinolytic response interference layer were characterized, and the Plg content in the fibrinolytic response interference layer and experimental parameters of the system were optimized. The method showed adequate sensitivity for the fibrinolytic activity of lumbrokinase and streptokinase, with wide linear ranges of 12-6000 and 10-2000 U/mL, respectively. Compared with the traditional fibrin plate method, it has a lower detection limit and higher linearity. The whole kinetic process of fibrinolysis by these two fibrinolytic drug models was recorded in real time, and the Michaelis constant and apparent kinetic parameters were calculated. Importantly, some other blood proteins were less interfering with this system, and it showed reliability in fibrin activity detection in real whole blood samples. This study established a better and more targeted research method of in vitro fibrinolysis and provided dynamic monitoring data for the analysis of fibrinolytic activity of whole blood.

Structural Color Colloidal Photonic Crystals for Biomedical Applications.

Photonic crystals are a new class of optical microstructure materials characterized by a dielectric constant that varies periodically with space and features a photonic bandgap. Inspired by natural photonic crystals such as butterfly scales, a series of artificial photonic crystals are developed for use in integrated photonic platforms, biosensing, communication, and other fields. Among them, colloidal photonic crystals (CPCs) have gained widespread attention due to their excellent optical properties and advantages, such as ease of preparation and functionalization. This work reviews the classification and self-assembly principles of CPCs, details some of the latest biomedical applications of large-area, high-quality CPCs prepared using advanced self-assembly methods, summarizes the existing challenges in CPC construction and application, and anticipates future development directions and optimization strategy. With further advancements, CPCs are expected to play a more critical role in biosensors, drug delivery, cell research, and other fields, bringing significant benefits to biomedical research and clinical practice.

Photothermally Reconfigurable Biomimetic Photonic Structures Prepared via Crystallization of Polydopamine‐Incorporated Core–Shell Nanoparticles

AbstractSome creatures have periodic nanostructures and chromophores in their flexible organs that allow them to control the color and shape of their skin. Artificial photonic structures that can mimic the aforementioned ability of these natural structures are researched extensively. In this study, polydopamine‐incorporated core–interlayer–shell (CIS‐PDA) nanoparticles capable of shape reconfiguration and expressing vivid structural colors are designed. These nanoparticles can be readily assembled to produce colloidal photonic crystals (CPhCs) and can exhibit vivid structural colors by virtue of their light‐absorbing capability. Soft CPhC films prepared with these nanoparticles undergo stable elastic deformation at 15% strain and exhibit mechanochromic properties. Specifically, the PDA in the CIS nanoparticles can absorb broadband light and exhibit a photothermal effect resembling that of melanosomes in living organisms, which enables self‐healing between multiple CPhC films. Due to their favorable properties, the CPhC films prepared in this study represent promising photothermally reconfigurable photonic materials. Moreover, arbitrarily shaped 2D or 3D surfaces are conformally coated with the films via microimprinting or vacuum wrapping, courtesy of the reconfigurability of these films. These findings suggest that CPhC films prepared with CIS‐PDA nanoparticles can be used as form‐free mechanochromic strain sensors and nonfading optical films for industrial products.