Bioinspired stimuli-responsive color-changing systems

Stimuli-responsive colors are a unique characteristic of certain animals, evolved as either a method to hide from enemies and prey or to communicate their presence to rivals or mates. From a material science perspective, the solutions developed by Mother Nature to achieve these effects are a source of inspiration to scientists for decades. Here, an updated overview of the literature on bioinspired stimuli-responsive color-changing systems is provided. Starting from natural systems, which are the source of inspiration, a classification of the different solutions proposed is given, based on the stimuli used to trigger the color-changing effect

Fabrication and characterization of inverse opals with tunable stopbands

Inverse opals, a major type of self-assembled structures, provide good examples of photonic crystals that result from the periodic arrangement of voids. The periodic arrays of voids interfere with the light passing through them and prevent the propagation of certain wavelengths (stopband). The ability to tune the stopband of an inverse opal is important in applications such as photonics and sensing. Inverse opal films can be fabricated by filling the interstitial sites of self-assembled colloids with a precursor solution and then removing the template (assembly of colloids) by means of heat or chemical dissolution. However, the fabrication of inverse opals with long range ordering of voids by using traditional methods is challenging due to the introduction of defects. Co-assembly is an evolving technique that is used to generate inverse opals with minimal defects but the use of experimental conditions to control the defects has not been widely explored. In this study, silica-based inverse opals were fabricated by using co-assembly technique and the quality of the resultant films was evaluated with respect to the colloidal concentration and sol-gel precursor concentration. To tune the stopband, the size of voids was altered by varying the size of colloids. Also, another type of inverse opals was produced from a cross-linked polymer based on 2-hydroxyethyl methacrylate which can result tunable stopbands in response to the external stimuli. The mechanical and dimensional stability of the polymer inverse opals were improved using a poly(dimethylsiloxane) mold. Overall, defect-free inverse opal photonic crystals with tunable stopbands in the visible region of the electromagnetic spectrum have been produced using inexpensive and simple techniques

Nanostructured composite materials based on carbon nanotubes and 3-D photonic crystals

Carbon nanotubes (CNT) and in particular, single-wall carbon nanotubes (SWCNT) have been extensively studied, in large part, due to their unique one-dimensional crystalline structures and related electronic and optical properties. Various polymeric composite materials, which were based on carbon nanotubes, have been also developed in an attempt to combine the properties of polymer and CNT in a single film. Such composites were mainly formed by mixing carbon nanotubes within the polymer without special emphasis on the structure and thereby, the nanoscopic properties of the resultant material. Photonic crystals belong to a class of man-made structures aimed at manipulating the propagation of electromagnetic waves at sub-wavelength dimensions in the visible range. The objective of this research work was to fabricate optical nano-composites from the bottom up: by incorporating carbon nanotubes within nano-structured templates we attempted to achieve novel composites with unique optical properties. Three-dimensional photonic crystals were made by self-assembly using monodisperse suspension of silicon dioxide colloids. Upon sedimentation, this highly ordered crystal, also known as opal, serves as a template for polymeric and polymer/CNT composites. For example, by infiltrating of the templates voids with a desired polymeric solution followed by etching of the silica template away, a three-dimensional inverse polymeric structure is obtained. Single-wall carbon nanotubes (SWCNT) have been directly grown into the template voids (in the range of 20 - 70 nm) by catalytic Chemical Vapor Deposition (CVD) technique with carbon monoxide as the carbon feedstock. The resultant SWCNTs were mostly semiconductive (p-doped). Control over the growth of SWCNT has been obtained by changing the catalyst concentration and the template\u27s void-size. Various techniques were used to characterize the SWCNT and its composites: Scanning Electron Microscope (SEM) has been used to identify the morphology of structures; interactions between polymer and nanotubes have been characterized by Raman spectroscopy; optical properties were studied by linear and nonlinear optical transmission and optical activity measurements; electrical properties were studied using thermoelectric and photoconductivity measurements. These data suggest that selforganized nano-scale templates are a promising route for realizing novel optical composite materials

Templating hydrogels

Templating processes for creating polymerized hydrogels are reviewed. The use of contact photonic crystals and of non-contact colloidal crystalline arrays as templates are described and applications to chemical sensing and device fabrication are illustrated. Emulsion templating is illustrated in the formation of microporous membranes, and templating on reverse emulsions and double emulsions is described. Templating in solutions of macromolecules and micelles is discussed and then various applications of hydrogel templating on surfactant liquid crystalline mesophases are illustrated, including a nanoscale analogue of colloidal crystalline array templating, except that the bead array in this case is a cubic array of nonionic micelles. The use of particles as templates in making core-shell and hollow microgel beads is described, as is the use of membrane pores as another illustration of confinement templating

Macroporous materials: microfluidic fabrication, functionalization and applications

This article provides an up-to-date highly comprehensive overview (594 references) on the state of the art of the synthesis and design of macroporous materials using microfluidics and their applications in different fields

Fabrication, Ordering and Optical Properties of Photonic Crystals Prepared From Crystalline Colloidal Arrays

We developed novel understandings pertaining to the ordering and optical properties of crystalline colloidal array (CCA) materials and applied these understandings to develop novel non-close-packed inverted photonic crystal materials. CCA materials are highly charged electrostatically stabilized colloidal particles in water which readily form face centered cubic or body centered cubic lattice structures. Because their periodicity is on the order of the wavelength of light, CCA materials have the ability to Bragg diffract light in the UV, visible and NIR regions of the electromagnetic spectrum. We utilized time resolved normal incidence reflection spectroscopy to probe the degree and kinetics of CCA ordering during the CCA crystallization process. Bragg diffraction interference fringe intensity is used to qualitatively determine the overall CCA ordering between CCA samples which have incremental additions of added ionic impurity.We defined the physical mechanism for anomalous reflection peaks obtained in the specular reflection direction from photonic crystal materials. We utilize variable angle specular reflection spectroscopy to probe angular ranges about the normal to the (111) planes of an fcc CCA to monitor the dispersion of anomalous reflection peaks. We correlated these reflection peaks to the diffraction from higher order Miller index crystal planes through Bragg's Law. We explain the origin of these peaks as the result of a multiple diffraction process whereby light is first Bragg diffracted into a beam from a set of higher order Miller index planes and consecutively diffracted by the in plane (111) periodicity into the (111) specular reflection direction. We also uncovered a novel use for CCA and PCCA materials allowing us to fabricate a non-close-packed inverted photonic crystal material. Our novel fabrication method consists of an infiltration and condensation of a sol-gel precursor into the hydrogel matrix of a PCCA and then the subsequent removal of the PCCA material. We show that the original high ordering of the CCA is maintained through and in-depth study which examining the (111) in-plane ordering. Tuning the CCA particle number density, prior to the fabrication process provides the ability to readily tune the Bragg diffracted wavelength of the final inverted photonic crystal

Fabrication of Three-Dimensionally Ordered Nanostructured Materials Through Colloidal Crystal Templating

The void spaces in colloidal crystals (opals, three-dimensional (3D) close-packed arrays of silica nanospheres) and their replicas are used as templates in the fabrication of new nanostructured materials. 3D ordered nanomeshes and nanosphere arrays are readily obtained by chemical and/or electrochemical methods. Using silica opal templates, metals or polymers are infiltrated into the interstices between the silica nanospheres. Subsequent dissolution of the opals with HF solution produces open 3D mesh structures. Metal (such as Ni, Co, Fe, Pd, Au, Ag, and Cu) and conductive polymer (such as polyaniline) meshes are obtained by electrochemical deposition approach, while the nonconductive polymer (such as poly(methyl methacrylate) (PMMA)) meshes are synthesized by chemical polymerization method. Some new types of meshes are fabricated by the conversion of metal meshes and polymer meshes. NiO meshes are formed by oxidizing Ni meshes in the air. The NiO meshes exhibit higher volume occupation fraction than Ni meshes and the nanocrystalline sizes of NiO particles can be adjusted by the oxidation temperature. Due to the mechanical flexibility of polymer meshes, the compression of PMMA meshes produces deformed PMMA meshes which contain oblate pores. These meshes can be again served as templates to prepare new types of colloidal crystals (nanosphere arrays) and specific nanocomposites. By the use of poorly conductive NiO mesh or PMMA mesh arrays as templates, 3D periodic metal nanosphere arrays, such as those of Ni, Co, Au and Pd, are readily fabricated by the electrodeposition method. Metal/NiO or Metal/PMMA composites can also be obtained if the templates are left intact. The magnetic behavior of metal (such as Ni and Co) meshes and sphere arrays has been investigated. These nanoscale arrays show significantly enhanced coercivities compared with bulk metals, due to the size effect of the nanometer dimensions of the components in meshes and sphere arrays. Angle-dependent magnetic properties of Ni and Co sphere array membranes exhibit out-of-plane anisotropy

Fabrication and characterization of direct and inverse opals for the manipulation of spontaneous emission in conjugated polymers

In this work, I investigated and improved a technique, known as vertical method, for growing synthetic opals by means of colloidal self-assembly of polymeric microparticles. This enables an unprecedentedly easy tuning of the opal photonic bands and thus enhanced control of the radiative properties of conjugated polymers after their infiltration into such photonic structures. The first part of the thesis is dedicated to direct opals defined as a close-packed array of spherical particles. In particular, I present a study of the optical response of polystyrene opals across an extended energy range (0.8 < a/λ < 1.8, where a is the lattice parameter and λ is the wavelength of light), which I achieved by adopting the aforementioned vertical method technique. Furthermore, in addition to discussing the conventional photonic stop band (PSB), I highlight the high-energy photonic bands associated to the Van Hove singularities which demonstrate the fcc arrangement of the microparticles signalling the high-quality standard of fabricated opals. The second part of the work is focused on inverse opals, namely a close-packed array of spherical voids. This is formed by co-growing a sacrificial opal template with a material of high refractive index (i.e. silica) and the removal of the spheres by calcination. Typically, the central frequency of the PSB is tuned by changing the sphere diameters of the opal template. In contrast, when keeping the sphere diameters fixed, I found that I could blue-shift the PSB by ~100 nm by varying the SiO2/colloidal ratio. To verify the performance of the photonic crystals obtained, I studied the spontaneous emission of luminescent organic polymers (i.e. PDPV, and the conjugated polyrotaxane PDV10Li β-CD) after their infiltration into the opals. I achieved, respectively for direct and inverse opals, a strong reshaping of the photoluminescence and a modification on the value of the radiative constant

Fabrication of Three-Dimensionally Ordered Nanostructured Materials Through Colloidal Crystal Templating

The void spaces in colloidal crystals (opals, three-dimensional (3D) close-packed arrays of silica nanospheres) and their replicas are used as templates in the fabrication of new nanostructured materials. 3D ordered nanomeshes and nanosphere arrays are readily obtained by chemical and/or electrochemical methods. Using silica opal templates, metals or polymers are infiltrated into the interstices between the silica nanospheres. Subsequent dissolution of the opals with HF solution produces open 3D mesh structures. Metal (such as Ni, Co, Fe, Pd, Au, Ag, and Cu) and conductive polymer (such as polyaniline) meshes are obtained by electrochemical deposition approach, while the nonconductive polymer (such as poly(methyl methacrylate) (PMMA)) meshes are synthesized by chemical polymerization method. Some new types of meshes are fabricated by the conversion of metal meshes and polymer meshes. NiO meshes are formed by oxidizing Ni meshes in the air. The NiO meshes exhibit higher volume occupation fraction than Ni meshes and the nanocrystalline sizes of NiO particles can be adjusted by the oxidation temperature. Due to the mechanical flexibility of polymer meshes, the compression of PMMA meshes produces deformed PMMA meshes which contain oblate pores. These meshes can be again served as templates to prepare new types of colloidal crystals (nanosphere arrays) and specific nanocomposites. By the use of poorly conductive NiO mesh or PMMA mesh arrays as templates, 3D periodic metal nanosphere arrays, such as those of Ni, Co, Au and Pd, are readily fabricated by the electrodeposition method. Metal/NiO or Metal/PMMA composites can also be obtained if the templates are left intact. The magnetic behavior of metal (such as Ni and Co) meshes and sphere arrays has been investigated. These nanoscale arrays show significantly enhanced coercivities compared with bulk metals, due to the size effect of the nanometer dimensions of the components in meshes and sphere arrays. Angle-dependent magnetic properties of Ni and Co sphere array membranes exhibit out-of-plane anisotropy