Three-dimensional Colloidal Crystals Research Articles

Controlled creation of point defects in three-dimensional colloidal crystals.

Crystal defects crucially influence the properties of crystalline materials and have been extensively studied. Even for the simplest type of defect-the point defect-however, basic properties such as their diffusive behavior, and their interactions, remain elusive on the atomic scale. Here, we demonstrate in situ control over the creation of isolated point defects in a three-dimensional (3D) colloidal crystal allowing insight on a single-particle level. Our system consists of thermoresponsive microgel particles embedded in a crystal of nonresponsive colloids. Heating this mixed-particle system triggers the shrinking of the embedded microgels, which then vacate their lattice positions, creating vacancy-interstitial pairs. We use temperature-controlled confocal laser scanning microscopy to verify and visualize the formation of the point defects. In addition, by reswelling the microgels we quantify the local lattice distortion around an interstitial defect. Our experimental model system provides a unique opportunity to shed light on the interplay between point defects, on the mechanisms of their diffusion, on their interactions, and on collective dynamics.

PH-sensitive colloidal crystals built from monodisperse poly(St-MMA-SPMAP-AA) microspheres via air-liquid interface assembly

By changing the amount of acrylic acid, different monodisperse poly(styrene-co-methyl methacrylate-co-3-sulfopropyl methacrylate potassium salt-co-acrylic acid) (P(St-MMA-SPMAP-AA)) microspheres are synthesized by soap-free emulsion polymerization. Two-dimensional or three-dimensional PSMSSA colloidal crystals are successfully prepared by the gas-liquid interface assembly method. A noticeable shift of the bandgap of the PSMSSA colloidal crystals is observed at different pH, after the addition of diluted HCl aqueous, a structure color of amaranth is found for the PSMSSA colloidal crystals, and a structure color of blue-green is formed under neutral conditions. By using PSMSSA colloidal crystals on the liquid interface, the colloidal voids are filled with liquid, and its optical and pH-responsive properties can be directly measured without re-transfer, realizing the simple and efficient preparation of pH-responsive colloidal crystals sensors. Compared with the traditional pH sensor, the gas-liquid interface self-assembled colloidal crystals sensor is not only economical and economical, but also a reversible sensor visible to the naked eye.

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications.

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

A Smart Magnetic Responsive Microfiltration Membrane Based on Three-Dimensionally Inverse Colloidal Crystal

In this article, a magnetic-responsive microfiltration membrane based on three-dimensional (3 D) inverse colloidal crystal (ICC) was prepared with relatively simple approach. We designed the ICC membrane using self-assembly of Fe3O4@SiO2 colloidal crystal into a colloidal crystal template. The resulting close-packed colloidal crystal was infiltrated with a reactive monomer solution. After polymerization, the silica spheres were etched away, resulting in an ICC membrane with Fe3O4 nanoparticles inside. The typical ICC membrane was characterized by SEM and XRD. Meanwhile, We used the KCl diffusion to test the permeability of the membrane. The results illustrated that the highly ordered permeable polymer membrane was successfully prepared by using Fe3O4@SiO2 colloidal crystals as the templates. Furthermore, the functional ICC membrane exhibited excellent magnetic-responsive gating property to separation. The smart environment-responsive gating membranes have potential applications in filtration, sensors and other applications.

Fabrication of Coatings with Structural Color on a Wood Surface

A facile method for the fabrication of colloidal photonic crystal coatings with tunable structural color on a wood surface was presented. The photonic crystal coatings were formed from monodisperse latex spheres composed of poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)). The latex spheres with a hard PSt core and elastomeric P(MMA-AA) shell were prepared using the emulsion polymerization method. The sessile drop method, a rapid single-step self-assembly method through simple evaporation of emulsion, was used to form three-dimensional colloidal crystals. Coatings with brilliant colors and uniform Bragg’s diffraction covering the entire visible region were fabricated by controlling the sphere size. This simple method provided new insight into the development of wood color embellishment.

Preparation and properties of porous P(St-MMA-AA) microsphere anti-glare film

Monodisperse nanoparticles can form two-dimensional or three-dimensional colloidal crystals with periodic ordered structure by self-assembly. They are widely application in optical devices, sensors, electronic devices and drug delivery. Photonic crystals consisting of spherical colloidal particles periodically have various properties and strong designability. Its performance and application have great research value. In order to ensure the transmittance of anti-static, anti-glare and anti-radiation (three-proof) nano-material films, the mist is often not ideal, which limits the practical application of anti-glare films. P(St-MMA-AA) microspheres with different pore sizes were prepared by adjusting the content of acrylic acid. The effects of particle size, concentration and pore structure of microspheres on the transmittance and mist of colloidal crystal coatings were investigated. It was found that increasing the pore structure of microspheres could improve the transmittance of colloidal crystal coatings and keep the mist constant. The experimental results show that the porous colloidal film can reach the standard transmittance when the concentration is 0.01%. It has good anti-glare and acid resistance. The application of porous nanoparticles in high transmittance anti-glare coating is further broadened. This colloidal crystal coating deposits polymer microspheres on the substrate by self-assembly. The preparation method has the advantages of simple equipment, good repeatability, easy operation, non-toxicity and easy synthesis, low cost of raw materials, high yield and environmental friendliness.

Two-Dimensional Nonclose-Packed Colloidal Crystals by the Electrostatic Adsorption of Three-Dimensional Charged Colloidal Crystals.

We demonstrate that nonclose-packed two-dimensional (2D) colloidal crystals fixed on flat solid surfaces can be obtained by the electrostatic adsorption of three-dimensional (3D) charged colloidal crystals onto oppositely charged substrates. 3D colloidal crystals of negatively charged polystyrene (diameter d = 500 nm) and silica (d = 510 and 550 nm) particles were formed in their aqueous dispersions. Then, a single layer of the 3D crystals (the particle volume fraction = ∼0.07-0.3) was adsorbed onto a glass surface, which was earlier modified with 3-aminopropyltriethoxysilane (APTES), a cationic silane coupling reagent. Under salt-free conditions, the lowermost layer of the 3D crystals, which was oriented parallel to the substrate, was adsorbed onto the substrate surface, forming 2D crystals. Centimeter-sized, large 2D silica crystals were produced by combining a unidirectional 3D crystallization of the silica colloid under a base concentration gradient and a unidirectional adsorption under an acidic concentration gradient, which allowed tuning of the charge number on the APTES-modified substrate. The interparticle separations of the resulted 2D crystals did not vary greatly (within 5%) over a large area (length: 2 cm); however, the separations were smaller than the initial value because of gravitational sedimentation. We also produced 2D crystals of gold particles (d = 250 nm), which we expect to be applicable as plasmonic materials. The present study will provide a facile strategy to produce nonclose-packed 2D colloidal crystals of various types of particles, including large and high-density particles.

Sensitive biological detection based on Langmuir–Blodgett films prepared fromdiluted silica suspensions

Self-assembled three-dimensional (3-D) colloidal crystals have been extensively utilized in nanolithography for fabricating a wide spectrum of functional periodic nanostructures for important technological applications. Here a simple and versatile way is reported using silica colloids to prepare close-packed particle multilayers by the Langmuir– Blodgett technique to create highly ordered 3-D colloidal crystals with unusual nanoarrays on a large variety of substrates. Successive deposition of monolayers using amphiphilic molecules in this approach allows control of the layer thickness, particle size, and especially packing structure of them in each layer. It has been demonstrated that the Langmuir–Blodgett film’s transparency and high antiglare optical properties are preserved even after multiple colloidal particle transfers on both straight and curved surfaces. The surface prepared by the Langmuir–Blodgett technique shows remarkable sensitivity for potential biodetection applications. These colloidal templated films have an optical response time of less than 40 s against essential bioproduct vapors. It is shown that the results are consistent with the Kelvin equation (ln P/Po = 􀀀2 Vl /rRT) when the liquid water layer is getting thicker.

Formation of three-dimensional colloidal crystals in a nematic liquid crystal.

We investigate the possible structures of three-dimensional colloidal crystals formed when these spherical particles are dispersed in a liquid crystal. The case of a strong homeotropic boundary condition is considered here. Their corresponding defect structures in the space-filler nematic liquid crystal are induced by the presence of the spherical surface of the colloids and produce an attraction between colloidal particles. Here, a standard Landau-de Gennes free energy model for a spatially inhomogeneous liquid crystal is numerically minimized to yield an optimal configuration of both spherical colloids and the orientational field. The stable and metastable structures obtained in this work are described and analyzed according to the type of periodic liquid-crystal defect lines that couple the colloidal spheres together. A large range of the spherical size is covered in this study, corresponding to a 5CB-liquid-crystal comparison for assembling micron- to nano-sized colloidal spheres. Multiple configurations are found for each given particle size and the most stable state is determined by a comparison of the free energies. From large to small colloidal particles, a sequence of structures, which range from quasi-one-dimensional (columnar), to quasi-two-dimensional (planar), and to truly three-dimensional, are found to exist.

A smart thermo- and pH-responsive microfiltration membrane based on three-dimensional inverse colloidal crystals

In this paper, a thermo- and pH-responsive microfiltration membrane was prepared based on three-dimensional (3D) inverse colloidal crystals (ICC). To manufacture the smart ICC membrane, the typical thermo-responsive N-isopropylacrylamide (NIPAM) and pH-responsive methacrylic acid (MAA) were polymerized inside silica colloidal crystals. The smart ICC membranes were characterized by SEM, IR and contact angle measurements. Moreover, the permeability of smart microfiltration membrane was carried out by the KCl diffusion tests. The result showed that effective diameter of the polymer ICC membrane can be reversible tuned by temperature and pH. Besides, the functional ICC membrane showed outstanding temperature- and pH-responsive gating property, which was applied to separate particles of different sizes. The savvy environment-responsive gating membranes have potential uses in filtration, separation, purification, sensor and other applications.

Intregrating metallic wiring with three-dimensional polystyrene colloidal crystals using electron-beam lithography and three-dimensional laser lithography

We demonstrate a method to fabricate narrow, down to a few micron wide metallic leads on top of a three-dimensional (3D) colloidal crystal self-assembled from polystyrene (PS) nanospheres of diameter 260 nm, using electron-beam lithography. This fabrication is not straightforward due to the fact that PS nanospheres cannot usually survive the harsh chemical treatments required in the development and lift-off steps of electron-beam lithography. We solve this problem by increasing the chemical resistance of the PS nanospheres using an additional electron-beam irradiation step, which allows the spheres to retain their shape and their self-assembled structure, even after baking to a temperature of 160 °C, the exposure to the resist developer and the exposure to acetone, all of which are required for the electron-beam lithography step. Moreover, we show that by depositing an aluminum oxide capping layer on top of the colloidal crystal after the e-beam irradiation, the surface is smooth enough so that continuous metal wiring can be deposited by the electron-beam lithography. Finally, we also demonstrate a way to self-assemble PS colloidal crystals into a microscale container, which was fabricated using direct-write 3D laser-lithography. Metallic wiring was also successfully integrated with the combination of a container structure and a PS colloidal crystal. Our goal is to make a device for studies of thermal transport in 3D phononic crystals, but other phononic or photonic crystal applications could also be envisioned.

Size-Controlled 3D Colloidal Crystals Formed in an Aqueous Suspension of Polystyrene/Polyglycidol Microspheres with Covalently Bound l-DOPA.

Stable three-dimensional colloidal crystals were fabricated in an aqueous suspension of Tris buffer at pH > 8. The basic building blocks of the crystals were submicron-sized polystyrene-polyglycidol core-shell particles (Dn(SEM) = 270 ± 18 nm) with covalently bound 3,4-dihydroxyphenylalanine (l-DOPA). The growth of the crystals was triggered by a thermodynamically favorable arrangement of particles leading to their close packing and by the formation of covalent cross-links between the individual particles. Under alkaline conditions, molecules of l-DOPA are oxidized, which allows their participation in cross-linking, necessary for the stabilization of the formed colloidal crystals. The average size of the fabricated colloidal crystals is determined by their weight, density of the suspending medium, and the energy of their Brownian motion. Crystals generated during the suspension of particles fall down after reaching the critical weight. Therefore, crystals of similar dimensions are deposited at the bottom of the vessel. The described system is the first example of the formation of stable colloidal crystals in a suspension.

Large Area 2D and 3D Colloidal Photonic Crystals Fabricated by a Roll-to-Roll Langmuir-Blodgett Method.

We present our results on the fabrication of large area colloidal photonic crystals on flexible poly(ethylene terephthalate) (PET) film using a roll-to-roll Langmuir-Blodgett technique. Two-dimensional (2D) and three-dimensional (3D) colloidal photonic crystals from silica nanospheres (250 and 550 nm diameter) with a total area of up to 340 cm(2) have been fabricated in a continuous manner compatible with high volume manufacturing. In addition, the antireflective properties and structural integrity of the films have been enhanced via the use of a second roll-to-roll process, employing a slot-die coating of an optical adhesive over the photonic crystal films. Scanning electron microscopy images, atomic force microscopy images, and UV-vis optical transmission and reflection spectra of the fabricated photonic crystals are analyzed. This analysis confirms the high quality of the 2D and 3D photonic crystals fabricated by the roll-to-roll LB technique. Potential device applications of the large area 2D and 3D colloidal photonic crystals on flexible PET film are briefly reviewed.

An Ultra-High Fluorescence Enhancement and High Throughput Assay for Revealing Expression and Internalization of Chemokine Receptor CXCR4.

Revealing chemokine receptor CXCR4 expression, distribution, and internalization levels in different cancers helps to evaluate cancer progression or prognosis and to set personalized treatment strategy. We here describe a sensitive and high-throughput immunoassay for determining CXCR4 expression and distribution in cancer cells. The assay is accessible to a wide range of users in an ordinary lab only by dip-coating poly(styrene-co-N-isopropylacrylamide) spheres on the glass substrate. The self- assembled spheres form three-dimensional photonic colloidal crystals which enhance the fluorescence of CF647 and Alexa Fluor 647 by a factor of up to 1000. CXCR4 in cells is detected by using the sandwich immunoassay, where the primary antibody recognizes CXCR4 and the secondary antibody is labeled with CF647. With the newly established assay, we quantified the total expression of CXCR4, its distribution on the cell membrane and cytoplasm, and revealed their internalization level upon SDF-1α activation in various cancer cells, even for those with extremely low expression level.

Removing grain boundaries from three-dimensional colloidal crystals using active dopants.

Using computer simulations we explore how grain boundaries can be removed from three-dimensional colloidal crystals by doping with a small fraction of active colloids. We show that for sufficient self-propulsion, the system is driven into a crystal-fluid coexistence. In this phase separated regime, the active dopants become mobile and spontaneously gather at the grain boundaries. The resulting surface melting and recrystallization of domains result in the motion of the grain boundaries over time and lead to the formation of a large single crystal. However, when the self-propulsion is too low to cause a phase separation, we observe no significant enhancement of grain growth.

Plasmon-free SERS detection of environmental CO2 on TiO2 surfaces.

SiO2/TiO2 core/shell beads (T-rex) were designed, fabricated and tested for Raman detection of environmental CO2 under real-working conditions, as those encountered, for example, in solar-to-fuel conversion reactions. The exploitation of light trapping and morphology dependent resonances was crucial for extending the limit of detection of CO2 adsorbed on TiO2 surfaces. T-rex beads allowed for achieving surface enhanced Raman scattering (SERS) without using plasmonic metals showing high-efficiency, fast response and reproducibility in CO2 detection in both air and solvents. The dependence of SERS activity on Mie-type resonances was investigated through a systematic comparison of experimental data and numerical simulations, demonstrating that T-rex beads can be tailored for the detection of gaseous environmental pollutants on the basis of simple, Mie-scattering based calculations. Three-dimensional T-rex colloidal crystals were also successfully tested in precise, in situ, real time detection of CO2 as a function of different temperature-sweep cycles.

Anomalous Fluorescence Enhancement from Double Heterostructure 3D Colloidal Photonic Crystals--A Multifunctional Fluorescence-Based Sensor Platform.

Augmenting fluorescence intensity is of vital importance to the development of chemical and biochemical sensing, imaging and miniature light sources. Here we report an unprecedented fluorescence enhancement with a novel architecture of multilayer three-dimensional colloidal photonic crystals self-assembled from polystyrene spheres. The new technique uses a double heterostructure, which comprises a top and a bottom layer with a periodicity overlapping the excitation wavelength (E) of the emitters, and a middle layer with a periodicity matching the fluorescence wavelength (F) and a thickness that supports constructive interference for the excitation wavelength. This E-F-E double heterostructure displays direction-dependent light trapping for both excitation and fluorescence, coupling the modes of photonic crystal with multiple-beam interference. The E-F-E double heterostructure renders an additional 5-fold enhancement to the extraordinary FL amplification of Rhodamine B in monolithic E CPhCs, and 4.3-fold acceleration of emission dynamics. Such a self-assembled double heterostructue CPhCs may find significant applications in illumination, laser, chemical/biochemical sensing, and solar energy harvesting. We further demonstrate the multi-functionality of the E-F-E double heterostructure CPhCs in Hg (II) sensing.

Nano-contact microscopy of supracrystals.

Background: Highly ordered three-dimensional colloidal crystals (supracrystals) comprised of 7.4 nm diameter Au nanocrystals (with a 5% size dispersion) have been imaged and analysed using a combination of scanning tunnelling microscopy and dynamic force microscopy.Results: By exploring the evolution of both the force and tunnel current with respect to tip–sample separation, we arrive at the surprising finding that single nanocrystal resolution is readily obtained in tunnelling microscopy images acquired more than 1 nm into the repulsive (i.e., positive force) regime of the probe–nanocrystal interaction potential. Constant height force microscopy has been used to map tip–sample interactions in this regime, revealing inhomogeneities which arise from the convolution of the tip structure with the ligand distribution at the nanocrystal surface.Conclusion: Our combined STM–AFM measurements show that the contrast mechanism underpinning high resolution imaging of nanoparticle supracrystals involves a form of nanoscale contact imaging, rather than the through-vacuum tunnelling which underpins traditional tunnelling microscopy and spectroscopy.

Direct observation of liquid nucleus growth in homogeneous melting of colloidal crystals.

The growth behaviour of liquid nucleus is crucial for crystal melting, but its kinetics is difficult to predict and remains challenging in experiment. Here we directly observed the growth of individual liquid nuclei in homogeneous melting of three-dimensional superheated colloidal crystals with single-particle dynamics by video microscopy. The growth rate of nucleus at weak superheating is well fitted by generalizing the Wilson–Frenkel law of crystallization to melting and including the surface tension effects and non-spherical-shape effects. As the degree of superheating increases, the growth rate is enhanced by nucleus shape fluctuation, nuclei coalescence and multimer attachment. The results provide new guidance for the refinement of nucleation theory, especially for the poorly understood strong-superheating regime. The universal Lindemann parameter observed at the superheat limit and solid–liquid interfaces indicates a connection between homogeneous and heterogeneous melting.

Silica core–polystyrene shell nanoparticle synthesis and assembly in three dimensions

Monodisperse silica nanoparticles (SiNPs) grafted with well-defined and highly dense polystyrene brushes are used as building blocks for the formation of three-dimensional (3D) colloidal crystals. By adjusting the refractive indices and the density of the hybrid particles with those of mixed solvents, iridescent microcrystals were formed throughout the entire suspension which were characterised by confocal laser microscopy. These core-shell hybrid particles are not charged and the driving force of the crystallization relies on repulsive forces between the polymer brushes with high grafting density. The interparticle distance is correlated to Bragg's Law and can be controlled by manipulating the grafting density and the length of the polymer brushes. Finally, the uniformity of these unique core-shell particles was exploited to generate 3D assemblies by a rapid and simple process based on centrifugation.