Self-assembled Colloidal Crystals Research Articles

DNA Base Pair Stacking Crystallization of Gold Colloids

We describe a DNA base pair (bp) stacking driven 3D crystallization of 70-80 nm gold nanospheres (Au NSs) into a large-area, face-centered-cubic (FCC) lattice. Although great advances have been achieved over the past decade, DNA nanoparticle (NP) crystallization has relied solely on the base complementary binding. This limits the accessible crystal size particularly for the larger and heavier Au NPs (>50 nm). In this work, we argue that the use of DNA bp-stacking (so-called blunt-end stacking) instead of complementary binding can widen the scope of controlled interparticle interactions used to assemble larger Au colloids into a larger-area crystal. Through the optimization of the melting transition, relatively large Au NSs (e.g., 75 nm) with nearly ideal roundness can be crystallized into FCC crystals with the area of up to approximately 1400 μm2. A strong metallodielectric stopband is experimentally observed in the visible range, confirming the high quality of our self-assembled Au colloidal crystals.

Effect of Defective Microstructure and Film Thickness on the Reflective Structural Color of Self-Assembled Colloidal Crystals.

Structural color arises from geometric diffraction; it has potential applications in optical materials because it is more resistant to environmental degradation than coloration mechanisms that are of chemical origin. Structural color can be produced from self-assembled films of colloidal size particles. While the relationship between the crystal structure and structural color reflection peak wavelength is well studied, the connection between assembly quality and the degree of reflective structural color is less understood. Here, we study this connection by investigating the structural color reflection peak intensity and width as a function of defect density and film thickness using a combined experimental and computational approach. Polystyrene microspheres are self-assembled into defective colloidal crystals via solvent evaporation. Colloidal crystal growth via sedimentation is simulated with molecular dynamics, and the reflection spectra of simulated structures are calculated by using the finite-difference time-domain algorithm. We examine the impact of commonly observed defect types (vacancies, stacking fault tetrahedra, planar faults, and microcracks) on structural color peak intensity. We find that the reduction in peak intensity scales with increased defect density. The reduction is less sensitive to the type of defect than to its volume. In addition, the reflectance of structural color increases as a function of the crystal thickness, until a plateau is reached at thicknesses greater than about 9.0 μm. The maximum reflection is 78.8 ± 0.9%; this value is significantly less than the 100% reflectivity predicted for a fully crystalline, defect-free material. Furthermore, we find that colloidal crystal films with small quantities of defects may be approximated as multilayer reflective materials. These findings can guide the design of optical materials with variable structural color intensity.

Preparation and self-assembly of NaYF4:Eu3+ nanocrystals-PNIPAm complex nanogels

In this work, NaYF4: Eu3+ nanocrystals were prepared by hydrothermal method, and then they were decorated by 3-butenylamine hydrochloride to obtain functionalized nanocrystals. After that, by a “graft-to” route of free radical polymerization of N-isopropylacrylamide (NIPAm), a complex nanogels of NaYF4:Eu3+-PNIPAm were synthesized. Finally, 3D self-assembled colloidal crystals (CCs) were fabricated by centrifugation. The microstructure of the nanogels was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and their thermo-sensitive performance was investigated by differential scanning calorimeter (DSC) and dynamic light scattering (DLS). Moreover, the self-assembly behavior of the nanogels was studied by Ultraviolet–visible absorption spectrum (UV–Vis) and photoluminescence (PL) respectively. The results indicated that concentration of the nanogels and environmental temperatures have significant effect on their self-assembly, due to volume variation of the 3D assemblies induced by environmental temperatures, its PL emission intensity changed drastically.

Laser Emission from Self-Assembled Colloidal Crystals of Conjugated Polymer Particles in a Metal-Halide Perovskite Matrix

Here, we present a hybrid organic/inorganic photonic composite, which generates laser emission from the organic material after pumping the inorganic component. The composite consists of a methylammonium lead-halide perovskite matrix CH3NH3Pb(BrxCl(1–x))3 and monodisperse poly(fluorene-co-divinylbenzene) particles, which have excellent optical feedback and gain. Micrometer-sized conjugated polymer particles (CPPs) are deposited together with the perovskite precursor from solution using a single-step vertical deposition method. The particles self-assemble into a photonic crystal and the perovskite forms an inorganic matrix in the interstitial space. Energy transfer to the polymer particles after optically pumping the metal-halide perovskite is studied in two systems with different halide ratios in the perovskite (Br to Cl: 1/9 and 4/6) to control the overlap of the perovskite emission energy with the absorption of the particles. From time-resolved photoluminescence experiments, we observe nonradiative energ...

自组装胶体晶体在微纳光学领域的研究进展

自组装胶体晶体在微纳光学领域的研究进展

Binary Colloidal Crystal Layers as Platforms for Surface Patterning of Puroindoline-Based Antimicrobial Peptides.

The ability of bacteria to form biofilms and the emergence of antibiotic-resistant strains have prompted the need to develop the next generation of antibacterial coatings. Antimicrobial peptides (AMPs) are showing promise as molecules that can address these issues, especially if used when immobilized as a surface coating. We present a method that explores how surface patterns together with the selective immobilization of an AMP called PuroA (FPVTWRWWKWWKG-NH2) can be used to both kill bacteria and also as a tool to study bacterial attachment mechanisms. Surface patterning is achieved using stabilized self-assembled binary colloidal crystal (BCC) layers, allowing selective PuroA immobilization to carboxylated particles using N-(3-dimethylaminopropyl)-N'-ethyl carbodiimide (EDC) hydrochloride/N-hydroxysuccinimide (NHS) coupling chemistry. Covalent immobilization of PuroA was compared with physical adsorption (i.e., without the addition of EDC/NHS). The AMP-functionalized colloids and BCC layers were characterized by X-ray photoelectron spectroscopy, ζ potentials, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Surface antimicrobial activity was assessed by viability assays using Escherichia coli. MALDI-TOF MS analysis revealed that although not all of PuroA was successfully covalently immobilized, a relatively low density of PuroA (1.93 × 1013 molecules/cm2 and 7.14 × 1012 molecules/cm2 for covalent and physical immobilization, respectively) was found to be sufficient at significantly decreasing the viability of E. coli by 70% when compared to that of control samples. The findings provide a proof of concept that BCC layers are a suitable platform for the patterned immobilization of AMPs and the importance of ascertaining the success of small-molecule grafting reactions using surface-MALDI, something that is often assumed to be successful in the field.

Engineered magneto-optical response of cobalt ferrite thin films deposited on self-assembled colloidal crystal

Oriented growth of patterned cobalt ferrite magnetic thin films on polymeric colloidal crystal films has been carried out employing a room-temperature pulse laser deposition method. The series of colloidal crystal films containing uniform polystyrene particles (180–420nm) were prepared by self-assembly. Scanning electron microscopy and optical reflectometry were used to confirm the assembly of well-arranged colloidal crystal films. Magnetic films of different thicknesses were deposited. X-ray diffraction measurements confirmed the nano-crystalline structure of cobalt ferrite grown on the spherical-shaped surface. Magneto-optical effects of the patterned thin films were examined and the results illustrated that the increase in film thickness leads to the decrease in coercivity, while the saturation magnetization increases. Furthermore, the magnetic origin of the coercivity variation dependence on the colloidal diameter and film thickness are discussed. Our results suggest that the preparation conditions of the deposited films should be tailored to obtain perpendicular anisotropy in cobalt ferrite films. The adjustable magnetic anisotropy was obtained due to substrate-induced magnetic shape anisotropy.

Self-Assembled Colloidal Photonic Crystal on the Fiber Optic Tip as a Sensing Probe

This paper presents an effective and efficient method to fabricate novel fiber optic sensing probes. The new, simple, and low cost approach is based on a 3-D photonic crystal dielectric structure directly deposited on the tip of a multimode optical fiber through the self-assembly of colloidal crystals (CCs) via a vertical deposition technique. Here, the CC is made of polystyrene nanospheres with 200 nm diameter, and the optical fiber is a UV–vis fiber with a core diameter of 200 μ m. The obtained fiber probes exhibit a resonant peak at 480 nm and an amplitude enhancement of 3.7 with respect to the bare fiber; these results are highly repeatable. A numerical tool based on a finite element method analysis has been developed to study and analyze the 3-D subwavelength structures. Numerical results are in good agreement with the observed experimental spectra. Moreover, refractive index measurements have been carried out, revealing a sensitivity of up to 445 nm/RIU in the 1.33–1.36 values range. The achieved performances, which have been obtained by using very small active areas and an easy and reliable fabrication procedure, demonstrate the future perspectives of these fiber-optic probes for chemical and biological sensing applications.

Morphological and Structural Control of Organic Monolayer Colloidal Crystal Based on Plasma Etching and Its Application in Fabrication of Ordered Gold Nanostructured Arrays

The organic monolayer colloidal crystals, which are usually prepared by self-assembling, could be used as templates, due to their interstitial geometry, for the periodically arranged nanostructured arrays, which have important applications in many fields, such as photonic crystals, information storage, super-hydrophobicity, biological and chemical sensing. Obviously, the structures of the obtained arrays mainly depend on those of the templates. However, the self-assembled monolayer colloidal crystal is exclusive in structure and for its hexagonal close-packed colloidal arrangement, leading to the limitation of the monolayer colloidal crystal as the template for the nanostructured arrays. Therefore, structural diversity is important in order for colloidal crystals to be used as the templates for various nanostructured arrays. Recently, there have been some reports on the morphological and structural manipulation of the organic monolayer colloidal crystals. In this review article, we focus on the recent progress in morphological and structural manipulation of polystyrene monolayer colloidal crystals based on plasma etching, and its application in the fabrication of the ordered gold nanostructured arrays with different structures, mainly including close-packed monolayer colloidal crystal and its transferrable property; structural manipulation based on plasma etching; and fabrication of gold nanostructured arrays based on varied monolayer colloidal crystals as template.

Bioinspired broadband midwavelength infrared antireflection coatings on silicon

Silicon has been extensively used in manufacturing refractive infrared optics due to its high refractive index and excellent transmission over a very broad range of infrared wavebands. However, the high refractive index of silicon leads to large reflection loss which greatly limits the performance of the final optoelectronic devices. Here, the authors report a simple and scalable templating nanofabrication technology for making subwavelength-structured, broadband antireflection coatings on crystalline silicon wafers, targeting the midwavelength infrared (MWIR) waveband (3–8 μm), which has important implications for various civilian and military purposes. Periodic arrays of silicon nanopillars with tapered shapes, which mimic the microstructured cornea of nocturnal moths, can be patterned on both surfaces of silicon wafers using self-assembled monolayer silica colloidal crystals as structural templates. The resultant moth-eye gratings can greatly enhance optical transmission for the entire MWIR region. Finite-difference time-domain simulations have also been performed and the theoretical predictions agree reasonably well with the experimental optical measurements.

Suppressing the Rayleigh-Plateau Instability in Field-Directed Colloidal Assembly.

Suspensions of superparamagnetic colloids that equilibrate in a toggled magnetic field undergo a Rayleigh-Plateau instability with a characteristic wavelength λ = 600 μm for the toggle frequency ν = 0.66 Hz. The instability is suppressed when the chamber length L in the field direction is less than 2λ. The final size of the magnetic domains perpendicular to the field, D, follows a power law relation of D ∼ L(0.71±0.07). These results demonstrate the structural differences of field-directed suspensions when confined to lengths scale set by the phase separation process and can potentially be used to create self-assembled colloidal crystals with well-defined size and shape.

Direct Writing of Three-Dimensional Macroporous Photonic Crystals on Pressure-Responsive Shape Memory Polymers.

Here we report a single-step direct writing technology for making three-dimensional (3D) macroporous photonic crystal patterns on a new type of pressure-responsive shape memory polymer (SMP). This approach integrates two disparate fields that do not typically intersect: the well-established templating nanofabrication and shape memory materials. Periodic arrays of polymer macropores templated from self-assembled colloidal crystals are squeezed into disordered arrays in an unusual shape memory "cold" programming process. The recovery of the original macroporous photonic crystal lattices can be triggered by direct writing at ambient conditions using both macroscopic and nanoscopic tools, like a pencil or a nanoindenter. Interestingly, this shape memory disorder-order transition is reversible and the photonic crystal patterns can be erased and regenerated hundreds of times, promising the making of reconfigurable/rewritable nanooptical devices. Quantitative insights into the shape memory recovery of collapsed macropores induced by the lateral shear stresses in direct writing are gained through fundamental investigations on important process parameters, including the tip material, the critical pressure and writing speed for triggering the recovery of the deformed macropores, and the minimal feature size that can be directly written on the SMP membranes. Besides straightforward applications in photonic crystal devices, these smart mechanochromic SMPs that are sensitive to various mechanical stresses could render important technological applications ranging from chromogenic stress and impact sensors to rewritable high-density optical data storage media.

Exploration and exploitation of water in colloidal crystals.

Water on solid surfaces is ubiquitously found in nature, in most cases due to mere adsorption from ambient moisture. Because porous structures have large surfaces, water may significantly affect their characteristics. This is particularly obvious in systems formed by separate particles, whose interactions are strongly influenced by small amounts of liquid. Water/solid phenomena, like adsorption, condensation, capillary forces, or interparticle cohesion, have typically been studied at relatively large scales down to the microscale, like in wet granular media. However, much less is known about how water is confined and acts at the nanoscale, for example, in the interstices of divided systems, something of utmost importance in many areas of materials science nowadays. With novel approaches, in-depth investigations as to where and how water is placed in the nanometer-sized pores of self-assembled colloidal crystals have been made, which are employed as a well-defined, versatile model system with useful optical properties. In this Progress Report, knowledge gained in the last few years about water distribution in such nanoconfinements is gathered, along with how it can be controlled and the consequences it brings about to extract new or enhance existing material functionalities. New methods developed and new capabilities of standard techniques are described, and the water interplay with the optical, chemical, and mechanical properties of the ensemble are discussed. Some lines for applicability are also highlighted and aspects to be addressed in the near future are critically summarized.

Structural Colors: Self-Assembled Coffee-Ring Colloidal Crystals for Structurally Colored Contact Lenses (Small 8/2015)

On page 926, Z. Y. Xie, Z. Z. Gu, and co-workers demonstrate the fabrication of structurally colored circular contact lenses from colloidal crystal templates. The structurally colored circular contact lenses display variable brilliant color under light illumination, without requiring the addition of any colorants to the hydrogel lenses, hence preventing the potential hazards from traditionally colored contact lenses.

Self-assembled binary colloidal crystal monolayers as cell culture substrates.

This study investigated the formation of self-assembled binary colloidal crystal (BCC) monolayers using evaporation induced confined area assembly (EICAA), and fabricated a family of various BCCs for cell culture. A library of various BCCs with different structures was established and it was demonstrated that after stabilisation the BCCs have potential to be used as substrates with well-ordered surface topographies and chemistries for manipulating cell-surface interactions. Three cell types including MG63 osteoblasts, L929 fibroblasts, and primary human adipose-derived stem cells (hADSCs) show different responses on selected surfaces (either between BCCs or BCCs vs. flat controls). In general, cell spreading was inhibited on BCCs due to surface topography. However, the chemical composition presented on the BCCs can compensate the topographic effect depending on what combination was used. The ordered topographic and heterogeneous chemical patterns provide a complexity of surface properties and have potential to be selectively modified with desired biomolecules for controlling biointerface interactions.

Lateral capillary interactions between colloids beneath an oil-water interface that are driven by out-of-plane electrostatic double-layer interactions.

We study the lateral capillary interactions between colloids beneath an oil-water interface that lead to closely packed two-dimensional self-assembled colloidal crystals. These capillary forces are caused by the overlap of deformed interfaces above colloids on a solid substrate. The interface deformation is due to the electrostatic disjoining pressure between the charged particles and the charged oil-water interface. It is notable that the short-range (i.e., on the nanometer scale) and out-of-plane electrostatic double-layer interactions, which occur through an aqueous phase, can generate the long-range lateral capillary attraction (i.e., on the micrometer scale).

Structural Color for Security Printing: Patterned Robust Colloidal Crystals

Structural color is a unique optical property of nano- or microstructured materials with periodic dimensions similar to the wavelength of visible light. Characteristic color is produced through reflectance, interference, scattering, diffraction or refraction of incident light of a specific wavelength in visible spectrum, thereby producing a specific color or so-called structural color. Structural color is seen throughout nature, notably Morpho butterflies and opals present a striking sheen and accompanying vivid hue. Prior analysis of minerals and organisms that produce these distinctive colors allows for continuing analysis of color developing mechanisms [1]. Recently, with improvements in micro-fabrication, trial reproduction of these naturally occurring structural colors is proceeding across the field of Biomimetics [2]. Mono-dispersed micro-particles, structured in an ordered arrangement, are called colloidal crystals. Through Bragg reflection they exhibit structural color with a vivid hue. Colloidal crystals are relatively easy to produce by self-organization, and since dielectric periodic structures can be used as a photonic crystal for controlling electromagnetic waves, it has been extensively studied as a structural color element and photonic crystal. Some of the methods for manufacturing colloidal crystals include capillary method, electric electrophoresis, spin coating, spray coating, and substrate raising [3 - 6]. The color developing mechanism for structural color differs from pigment and does not absorb a specific wavelength, therefore in principle, fading from UV rays should be reduced. These materials can be used in applications that require pigments with excellent light-resistance and durability. In addition, since these colored micro-particle aggregates are non-toxic with a high level of safety, they show promise for wide use in printed materials. Here we present research results involving the formation of a pattern of stable colloidal crystals on various substrates and exploring their suitability for use in printed materials. Monodisperse silica nano-particles with a diameter of 300 nm were dispersed in a highly polar solvent. As the solvent evaporates the particles self-assemble into colloidal crystals. By depositing this dispersed suspension as an ink with a direct write printer (aerosol jet) we confirmed the ability to print colloidal crystals with structural color in any pattern. The cohesion of self-assembled colloidal crystals relies on electrostatic attraction making them extremely brittle with limited practical use. As such, a polymer was added to the suspension to reinforce the colloidal crystal structure with limited influence on the self-assembly mechanism. Thus, a strong coating is produced while also producing the desired color effect. Printed colloidal crystals produced by this method are difficult to reproduce by color copying and traditional printing methods considering the color and gloss change depending on the viewing angle. Figure 1 shows a feature printed using the colloidal crystal ink under various lighting/viewing conditions. Also, since it is possible to direct print any pattern, the method is amenable to variable data printing, providing a tamper prevention effect. Thus, these materials are attractive for use in security printing applications.

Preparation of poly(BMA-co-MMA) particles by soap-free emulsion polymerization and its optical properties as photonic crystals.

Narrowly dispersed poly(BMA-co-MMA) and PBMA latices with particle diameters ranging within 216-435 nm were synthesized successfully by surfactant-free emulsion polymerization with KPS and AIBA. The average particle diameter and particle size distribution, average molecular weight and its distribution, glass transition temperature, reflectance spectra in visible wavelength, and refractive indices for the respective poly(BMA-co-MMA) latices and their photonic crystals were systematically investigated in terms of BMA/MMA ratio, BMA content, polymerization temperature, and DVB effect. The rate of polymerization increased with increasing MMA concentration in BMA/MMA ratio. The particle diameter increased with BMA concentration in BMA/MMA ratio. The molecular weight increased with BMA concentration in BMA/MMA ratio and monomer concentration. The drying of the latices offered self-assembled shiny colloidal crystal films showing the characteristic structural colors in visible wavelength. All the poly(BMA-co-MMA) latices prepared in the study were fallen within the range of photonic grade microspheres. The reflectance measurement on the colloidal photonic crystals having different particle diameters clearly exhibited narrow stopbands. The reflection maxima (λ(max)) measured in this study were well close to the λ(max) calculated, derived from the Bragg's equation. The refractive indices of poly(BMA-co-MMA) photonic crystals were found to be almost same as the theoretical values and increased proportionally from 1.50 to 1.57 with BMA content in BMA/MMA ratios. It was, thus, found that the optical reflectance properties of the poly(BMA-co-MMA) colloidal photonic crystals can be controlled easily by adjusting the reaction conditions and BMA/MMA ratio in soap-free emulsion copolymerization of BMA and MMA.

Self-Assembled Coffee-Ring Colloidal Crystals for Structurally Colored Contact Lenses

A circlular structural-colored contact lens is reported, which is fabricated by replicating self-assembled colloidal photonic crystal templates. The structural-colored contact lenses not only display variable and brilliant color under light illumination, but also avoid the addition of any colorants to the hydrogel lenses and prevent the potential harm posed by traditional colored contact lenses.

Fabrication and optical properties of novel plasmonic cone-shell crystal

Tweaking the shape, size, and material of metallic nanostructure enables researchers to engineer the scattering properties. In particular, by breaking the symmetry of the nanostructures, additional degrees of freedom to manipulate the optical properties can be obtained. In this work, we fabricate a novel large-area plasmonic cone-shell crystal (PCSC) via two-process physical deposition of dielectric and metallic materials on the self-assembled monolayer colloidal crystal (CC). In comparison with the enhanced optical transmission (EOT) spectrum of the conventional plasmonic semi-shell array, multiple plasmon resonances within a remarkably broadened spectrum are observed in this PCSC, which are the main results of the plasmon coupling and hybridization effects based on the increased symmetry-breaking structure. Our findings could develop the investigation on the plasmonic shell structures and hold potential applications in light modulation, surface-enhanced spectroscopy and optoelectronic detection.