Colloidal Template Research Articles

Accelerated nerve regeneration using induced pluripotent stem cells in chitin–chitosan–gelatin scaffolds with inverted colloidal crystal geometry

Polymeric biomaterials with regular pore structure can generate distinctive properties in various biomedical applications. This study presents scaffolds comprising chitin, chitosan, and gelatin with pore geometry of inverted colloidal crystals (ICC) for guiding differentiation of induced pluripotent stem (iPS) cells toward neurons. Chitin–chitosan–gelatin ICC scaffolds were fabricated by self-assembly, crosslinking, infiltration, dehydration, and particle leaching. The results revealed that ethanol as a dispersion medium yielded a higher regularity of colloidal template than ethylene glycol. For the adhesion of iPS cells, freeform constructs were more efficient than ICC constructs. The reverse was true for the viability of iPS cells. The quantity of stage-specific embryonic surface antigen-1 in cultured freeform construct was larger than that in cultured ICC construct, indicating that the former preserved more phenotypic characteristics of iPS cells than the latter. Moreover, β III tubulin-identified region in ICC construct was larger than that in freeform construct, demonstrating that the differentiation of iPS cells toward neurons in ICC construct was faster than that in freeform construct. ICC topography in chitin–chitosan–gelatin scaffolds can accelerate neuronal differentiation of iPS cells.

Colloidal Occlusion Template Method for Micromanufacturing of Omniphobic Surfaces

AbstractAn efficient strategy to produce forests of aligned nanowires and nail‐like micrometer‐sized structures, whose density can be tuned in a broad range, is reported in this study. It relies on a combination of two template‐assisted nanofabrication/patterning methods: electrochemical growth of metal nanowires in nanoporous sacrificial templates and partial masking of a surface with a self‐assembled colloidal monolayer. A great potential of this novel approach, termed here colloidal occlusion template method, is demonstrated on the example of the fabrication of omniphobic surfaces comprised of nickel micronails whose density is varied to approach highest possible contact angles. After chemical modification to reduce their surface tension, these microstructures with reentrant geometry support the non‐wetting Cassie state for both high‐surface‐tension water and low‐surface‐tension hexadecane. In particular, superhydrophobic behavior (contact angles exceeding 150°) is found for water, while oleophobicity (contact angles approaching 110°) is observed for hexadecane. The proposed approach can be exploited for the fabrication of a large variety of supported high‐aspect‐ratio nano/microstructures in applications where a surface density of features has to be several orders of magnitude lower than can be obtained with conventional template methods.

Fabrication of size-tunable, periodic Si nanohole arrays by plasma modified nanosphere lithography and anisotropic wet etching

In this study, we propose a new, facile and efficient method to define the desired patterns on Si surfaces and to fabricate periodic arrays of size- and position-controllable Si nanoholes on blank- and pre-patterned (001)Si substrates, which is based on the O2 plasma modified nanosphere lithography with an anisotropic wet etching technique. In contrast to earlier approaches, no additional thin-film deposition equipments and metal film hard masks are needed for this method. The Si nanohole arrays exhibit the same hexagonal arrangement as the initial colloidal nanospheres template. The sizes of the Si nanoholes can be tuned from 150nm to 480nm by varying the etching time. The Si substrates with nanohole-textured surfaces exhibit strong antireflection properties. UV–Vis spectroscopic measurements revealed that by increasing the nanohole size, the optical reflectance of the nanohole-structured (001)Si surfaces was gradually decreased down to less than 8%. Since the nanohole sizes, shapes and spacings can be readily controlled by adjusting the diameters of the colloidal nanospheres and anisotropic wet etching conditions, the combined approach presented here provides the capability to fabricate a variety of nanohole array structures on various Si-based substrates without complex lithography.

Hierarchically Structured Porous Cadmium Selenide Polycrystals Using Polystyrene Bilayer Templates

In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.

Mesoporous Protein Particles Through Colloidal CaCO3 Templates

AbstractPorous colloidal particles can be tailored using templating techniques to maximize their effectiveness for a wide range of applications, including separation, catalysis, and drug delivery. However, templating usually involves harsh and complex preparation conditions, thereby complicating the fabrication of sensitive bio‐functionalized particles. Here a simple, yet versatile and mild approach us used to create porous protein particles using mesoporous CaCO3 colloids as sacrificial templates. The three‐step preparation procedure involves infiltrating the colloidal templates with the protein by solvent evaporation, protein crosslinking, and removal of CaCO3. Using this method one can obtain porous particles consisting of virtually any protein. To explore the applicability of the particles for various scenarios particles composed of different proteins are fabricated focusing on hemoglobin and trypsin and particle morphology, porosity, mechanical properties, the protein redox state, and enzymatic activity are determined. The results show that the nanoporous template structure is replicated and that the proteins are fully functional. By varying preparation conditions such as crosslinker concentration and protein content the elastic modulus is adjusted in the range of red blood cells. This ensures high deformability upon flow in microchannels and makes the porous protein particles a versatile platform for biomedical applications.

Surface-enhanced Raman spectroscopy based on ordered nanocap arrays

We fabricated the Ag cap array for surface-enhanced Raman scattering (SERS) by Ag deposition onto two dimensional polystyrene colloid sphere templates, and 4-mercaptopyridine (4-MPy) was used as the probing molecule. When the colloids with different size were chosen as the substrate for 20nm Ag deposition, the film on 100nm colloids gave the significant enhancement. SERS intensity increased with the increase of Ag thickness. When 20nm Ag film was coated by Ta, the SERS signals decreased with the increase of Ta thickness, indicating the main effect from the top of cap structure. When Co layer was added under the Ag film, the SERS intensity decreased with the increase of Co thickness because the Co layer affects electromagnetic and plasmon resonance.

Preparation of Hollow TiO2 Spheres of the Desired Polymorphs by Layer‐by‐Layer Assembly of a Water‐Soluble Titanium Complex and Hydrothermal Treatment

AbstractA facile approach, based on the polyelectrolyte‐mediated electrostatic adsorption of a water‐soluble titanium complex on colloid templates and hydrothermal treatment, is presented for the formation of hollow TiO2 spheres of the desired polymorphs. The core–shell particles were formed by the alternate deposition of titanium–glycolate complex 1 and positively charged polyelectrolyte, poly(allylamine hydrochloride) (PAH) on spherical polystyrene (PS) templates by utilizing electrostatic interactions for shell formation. Hollow spheres formed with the polyionic complex of 1 and PAH were obtained by removal of the PS templates. Formation of the polyionic complex affected the crystallization of TiO2 from 1 by hydrothermal treatment. Hollow spheres of brookite and anatase were obtained by hydrothermal treatment at pH 4 and 10, respectively. The approach presented here can be exploited for the fabrication of novel advanced titania‐based materials with desired structures and polymorphs.

Crystallisation of the orthorhombic form of acetaminophen: Combined effect of surface topography and chemistry

Abstract A new method for the crystallisation of the metastable orthorhombic polymorph of acetaminophen using colloidal templates with specific surface topographies and surface chemistry is reported in this study. The templates were prepared from silica nanoparticles (220 nm) and their surface chemistry was modified by in situ silanisation using different organo-silanes to attach amino, phenyl, dodecyl and trifluoropropyl functional end groups. Cooling crystallisation was performed on the surfaces of novel colloidal templates using a simple single step rapid cooling method, which resulted in selective crystallisation of the metastable orthorhombic form on the surfaces with a specific surface topography and surface functional end groups in conditions, which typically yielded monoclinic crystal forms. Here, we report crystallisation of the orthorhombic form of acetaminophen at super-saturation c/csat = 1.5, which is lower as compared to the c/csat = 2.0 reported in the literature. The method reported here has shown potential for surface selective polymorphic control of organic molecules using combined effect of surface topography and surface chemistry.

Preparation of hollow titania and strontium titanate spheres using sol–gel derived silica gel particles as templates

A facile approach, based on polyelectrolyte-mediated electrostatic adsorption of a water-soluble titanium complex on colloidal templates and hydrothermal treatment, is presented for the formation of hollow titania (TiO2) and strontium titanate (SrTiO3) spheres. Monodispersed silica gel particles were prepared by the sol–gel method and adopted as core templates. Deposition of a water-soluble titanium complex, titanium (IV) bis(ammoniumlactato)dihydroxide (TALH), on the silica gel particles was carried out via the layer-by-layer assembly technique. Hollow spheres were successfully formed from the core–shell particles. The silica gel particles used as core templates dissolved during hydrothermal treatment because of the particles’ undeveloped siloxane network. In addition, the hydrothermal treatment induced crystallization of the hollow shells. Therefore, the hydrothermal treatment played two roles; removal of the silica templates and crystallization of the hollow shells. When deionized water was used, hollow TiO2 spheres were obtained. Hollow SrTiO3 spheres could also be formed when an aqueous solution of Sr(OH)2 was used. The approach presented here could be exploited as a novel and sustainable approach for the fabrication of a range of different inorganic hollow spheres.

Inverse Opal Carbons for Counter Electrode of Dye-Sensitized Solar Cells

We investigated the fabrication of inverse opal carbon counter electrodes using a colloidal templating method for DSSCs. Specifically, bare inverse opal carbon, mesopore-incoporated inverse opal carbon, and graphitized inverse opal carbon were synthesized and stably dispersed in ethanol solution for spray coating on a FTO substrate. The thickness of the electrode was controlled by the number of coatings, and the average relative thickness was evaluated by measuring the transmittance spectrum. The effect of the counter electrode thickness on the photovoltaic performance of the DSSCs was investigated and analyzed by interfacial charge transfer resistance (R(CT)) under EIS measurement. The effect of the surface area and conductivity of the inverse opal was also investigated by considering the increase in surface area due to the mesopore in the inverse opal carbon and conductivity by graphitization of the carbon matrix. The results showed that the FF and thereby the efficiency of DSSCs were increased as the electrode thickness increased. Consequently, the larger FF and thereby the greater efficiency of the DSSCs were achieved for mIOC and gIOC compared to IOC, which was attributed to the lower R(CT). Finally, compared to a conventional Pt counter electrode, the inverse opal-based carbon showed a comparable efficiency upon application to DSSCs.

Dually tunable inverse opal hydrogel colorimetric sensor with fast and reversible color changes

Through opal-templated photopolymerization of copolymeric hydrogel consisting of 2-hydroxyethyl methacrylate, N-isopropylacrylamide, acrylic acid, and N,N′-methylenebis(acrylamide), a dually tunable inverse opal sensor by temperature and pH has been successfully synthesized. Temperature drop or pH increase induced hydrogel swelling and subsequent color changes of the sensor respectively due to lower critical solution temperature behavior of N-isopropylacrylamide and the altered Donnan potential by acrylic acid units. The color change driven by hydrogel swelling could be quantitatively measured by reflectance spectra. Increased content of N-isopropylacrylamide in the hydrogel resulted in a larger volume change under the same T-drop experiment. On the contrary, higher crosslinker content led to a less swollen hydrogel. From the kinetic investigations, response time for temperature change was measured to be much faster than that for pH variation (0.5s vs. 28s). An exceptionally rapid temperature response could be explained by fast water influx to the thin layers at the top of inverse opal hydrogel after an instantaneous temperature change. A cyclic pH/temperature changes (pH jump–temperature-drop–pH drop–temperature-jump) on a dually tunable sensor showed a perfect restoration of diffraction wavelength as well as the rapid response times for all stimuli. This work facilitates the fabrication of a variety of multi-responsive inverse opal sensors exhibiting rapid response kinetics by simply copolymerizing the individual sensing monomers using the optimized photo-polymerization and the colloidal templating route.

Highly Ordered Large-Area Colloid Templates for Nanostructured TiO<SUB>2</SUB> Thin Film Gas Sensors

We report a novel process that uses highly ordered colloidal templating to fabricate nanostructured TiO2 thin film gas sensors. An O2 plasma treatment is used to decrease the contact angle of a water droplet on a SiO2/Si substrate from 46 degrees to 3 degrees. The formation of this hydrophilic surface enhances the adhesion of polystyrene microspheres to the substrate during the spin coating of the colloidal solution, leading to a large-area colloid template of closely packed monolayer microspheres on the substrates. Embossed TiO2 thin film gas sensors fabricated through highly ordered colloidal templating using O2 plasma exhibit substantially enhanced gas sensing performance as compared to those without surface treatments prior to colloidal templating.

초임계 증착법을 통한 실리카와 타이타니아 역 오팔의 제조

정보 전달의 수단으로의 빛을 제어하는 기술에 대한 연구가 활발한 가운데, 최근 가장 각광 받고 있는 것이 광결정(Photonic crystals)을 갖는 물질이다. 이를 합성하는 다양한 방법 중에서 초임계 증착법(Supercritical deposition)을 사용하면, 복잡한 내부 구조물까지의 반응물의 침투가 용이하여 신속하고 효율적인 공정이 가능하다. 본 논문에서는 비독성, 비인화성 등으로 친환경 초임계 용매인 초임계 이산화탄소(Supercritical carbon dioxide) 분위기 하에서 고분자 콜로이드 주형에 금속 알콕사이드를 코팅하는 방법으로 매크로 기공을 갖는 역 오팔(Inverse opals) 구조물을 합성하였다. 반응온도와 압력은 <TEX>$40^{\circ}C$</TEX>와 80 bar로 고정하였으며, 반응 시간과 반응물의 비율을 조절하여 역 오팔의 구조의 변화를 살펴보았다. 실험에는 금속 알콕사이드로써 Tetraethyl orthosilicate (TEOS)와 Titanium (IV) isopropoxide (TTIP)가 사용되었으며, 각각을 통하여 실리카와 타이타니아 역 오팔을 합성할 수 있었다. Photonic crystals (PCs) are highly ordered porous materials which have been much attention because of its potential for controlling the light sauces. There are many methods for synthesizing this kind of materials among them we chose the supercritical deposition. With this method the reactants can easily infiltrate into the complex structure. In this paper, supercritical carbon dioxide (<TEX>$scCO_2$</TEX>) was used as a reaction medium, which is known as a sustainable solvent due to its nontoxic and noninflammable characteristics. We coated the colloidal template with metal alkoxide by using <TEX>$scCO_2$</TEX> and then obtained macro-porous inverse opals. The reaction was carried out at <TEX>$40^{\circ}C$</TEX> and 80 bar. We synthesized two different inverse opals which called silica and titania inverse opals by use of tetraethyl orthosilicate (TEOS) and titanium isopropoxide (TTIP) as a precursor, respectively.

Patterning Hierarchy in Direct and Inverse Opal Crystals

Biological strategies for bottom-up synthesis of inorganic crystalline and amorphous materials within topographic templates have recently become an attractive approach for fabricating complex synthetic structures. Inspired by these strategies, herein the synthesis of multi-layered, hierarchical inverse colloidal crystal films formed directly on topographically patterned substrates via evaporative deposition, or "co-assembly", of polymeric spheres with a silicate sol-gel precursor solution and subsequent removal of the colloidal template, is described. The response of this growing composite colloid-silica system to artificially imposed 3D spatial constraints of various geometries is systematically studied, and compared with that of direct colloidal crystal assembly on the same template. Substrates designed with arrays of rectangular, triangular, and hexagonal prisms and cylinders are shown to control crystallographic domain nucleation and orientation of the direct and inverse opals. With this bottom-up topographical approach, it is demonstrated that the system can be manipulated to either form large patterned single crystals, or crystals with a fine-tuned extent of disorder, and to nucleate distinct colloidal domains of a defined size, location, and orientation in a wide range of length-scales. The resulting ordered, quasi-ordered, and disordered colloidal crystal films show distinct optical properties. Therefore, this method provides a means of controlling bottom-up synthesis of complex, hierarchical direct and inverse opal structures designed for altering optical properties and increased functionality.

Label-free specific detection of immunoglobulin G antibody using nanoporous hydrogel photonic crystals

Photonic crystal (PC) based biosensors have been given attention because of their obvious advantages such as sensitivity, simple and low-cost realization. Many kinds of PC based biosensors have been reported, for example, holographic PCs, porous silicon PCs, PC fibers and colloidal crystal film. Among these PC based sensors, we have interested on an inverse opal structure film in order to apply to specific and label-free detection of proteins. This label-free PC bioassay is more attractive than labeling detection because target molecules can be detected in their nature forms, truly portable point-of-care testing is possible, and quantitative measurement of molecule interaction is allowable. In this study, we propose a specific and label-free detection method for immunoglobulin G (IgG) antibody using nanoporous hydrogel photonic crystals with simple process and cost-effective way. The hydrogels used as a backbone of PC in this study, which provide rather homogeneous water surrounding, are ideal substrates for biomolecular screening. Also, three-dimensional hydrogel structures show high capacity for biomolecule immobilization, greater probability of interacting with the target ligand, high S/N, and sensitivity. Finally, the film type biosensor was fabricated using photo-curable hydrogels with colloidal templating methods. The reflective color of the fabricated sample is changed from green to dark orange with the bandgap shift of 50nm, when the IgG concentration was 10mg/ml. Furthermore, our device showed different bandgap shifts according to the level of antibody concentration, so that it can be used to measure quantitative molecular interactions. This designed label free detection system has great potential for next generation of bioassay platform for diagnosis of pathogens such as virus, bacteria, and fungi.

Generation of Highly Ordered Porous Antimony-Doped Tin Oxide Film by A Simple Coating Method with Colloidal Template

An ordered porous antimony-doped tin oxide (ATO) film was successfully prepared using a simple coating process with colloidal templates. The facile production was effective when a combination of 16-nm ATO (as a model of an inorganic nanoparticle) and polystyrene (PS) spheres (as a model of the template) weresimply coated to produce a composite ATO/PS film. Heat treatment was then used to remove the PS and produce the porous film. The porous film with a spherical pore shape and a highly ordered porous structure could be obtained. A potential way for the control of pore size could be also achieved by changing initial template size. The theoretical explanation and mechanism of porous formation were also added, which would be important for the scaling-up prediction and estimation.

Observation of unconventional optical diffraction in α-Fe2O3/Si two-dimensional photonic crystal slab fabricated by sol-gel method

The α-Fe2O3/Si two-dimensional (2D) photonic crystal (PC) with hexagonal honeycomb structure was fabricated by self-assembly and solution-dipping method based on colloidal monolayer template with 780 and 470 nm diameter polystyrene (PS) spheres. Optical diffraction measurement was carried out under wavelengths of 800, 532 and 400 nm. An interesting dispersion phenomenon with a hexagonal diffraction pattern composed of six elliptical spots is observed under specific wavelengths. Both theoretical and experimental calculations reveal that the diffraction can only occur when the incident wavelength is smaller than \(\sqrt{3}/2\) of lattice constant, and both the shape and distribution of these spots can be modified by the PC lattice constant and the incident wavelength. All these results reveal that optical properties of these 2D PC slabs are different from conventional diffractive devices.

Hollow Polypyrrole Films: Applications for Energy Storage Devices

Hexagonally arrayed structures of colloidal crystals with uniform particle size are good candidates for templates for synthesizing porous nanostructures. Herein, we present the first reported use of three-dimensionally (3D) ordered porous structures to increase the energy storage abilities of conducting polymer based energy storage devices. A robust 3D colloidal crystal template was prepared using a colloidal suspension. Polypyrrole (PPy) was deposited into the interstices between the colloidal spheres by electrochemical infiltration. PPy inverse nanoporous structures were obtained by dissolving the colloidal template in a solvent (99.5% Benzene). The morphologies of nanoporous PPy structures and their electrochemical characteristics were investigated.

Colloidal templating: seeded emulsion polymerization of a soluble shell with a controlled alkyne surface density

A general methodology for producing ca. 100 nm core–shell colloidal particles in which the shell has an elevated alkyne functionality and yet remains thermoplastic is presented. The availability of accessible alkyne groups on the surface of the aqueous-phase particles allows for the in situ surface modification of the particles through a copper(I) catalyzed Huisgen 1,3-dipolar cycloaddition with an azide-terminated surface agent. The core is an extensively crosslinked polymer which can be easily removed by dispersing the particles in a solvent and centrifuging and collecting the cores, leaving the solubilized shells. This allows for the complete characterization of the colloidal surface reactions in the absence of the volumetrically dominant core. The technique is demonstrated with a core–shell colloid composed of a 135 nm crosslinked polystyrene (PS) core coated with a ca. 10 nm thick uncrosslinked poly(methyl acrylate-co-propargyl acrylate) shell. Due to the applicability of this technique for generating particles useful in biomedical imaging or drug delivery applications, the core–shell particles are surface modified with a variety of azide-terminated poly(ethylene glycol) (PEG) derivatives, including a poloxamer which was terminated on either end by an azide and a naphthalimide chromophore. The resulting fluorescent particles had an absorbance at 413 nm and peak emission at 525 nm. The PEG derivatives could be attached to the particles at a grafting density of ca. 0.2–0.3 groups/nm2.

High-fidelity fabrication of Au–polymer Janus nanoparticles using a solution template approach

Janus particles have attracted significant attention in recent years, due to their potential in nanomanufacturing. Their asymmetric features impart unique physical and chemical properties, which can be tuned and utilized to control their solution-state assembly. While several examples of Janus nanostructures have been reported in the literature, the scientific community continues to pursue synthetic routes which are less time and resource intensive. Herein, we describe a facile method to synthesize Janus nanoparticles in which colloidal micelles template the in situ formation of Au nanoparticles in the shell layer. The resulting morphologies of the hybrid Au–polymer nanoparticles can be adjusted widely by controlling the polymer and reducing agent (HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)) concentrations. High-fidelity populations of Au–micelle Janus nanoparticles are obtained when the polymer concentration is high (≥1.0 × 10−6 mmol L−1) and the HEPES concentration is low (≤0.3 mol L−1). Conversely, when the HEPES concentration is high (≥0.5 mol L−1) and the polymer concentration is low (≤2.4 × 10−7 mmol L−1), raspberry-like clusters are formed, where each micelle encumbers several Au nanocrystals. Our method is attractive in that the number of Au nanoparticles on each Au–micelle entity can be controlled using a scalable, aqueous solution process. It also has significant potential for the directed assembly of Au nanoparticle superstructures, as the nature and geometry of the polymer precursors can be varied.