Opals, self-assembled

Another nice example of nanostructuring an MIP layer is the work published by Wu et al. [138, 139] who developed a label-free optical sensor based on molecularly imprinted photonic polymers. Photonic crystals were prepared by self-assembly of silica nanospheres. The space between the spheres was then filled with MIP precursor solution. After polymerization, the silica was dissolved, leaving an MIP in the form of a 3D-ordered interconnected macroporous inverse polymer opal (Fig. 15). The authors were able to detect traces of the herbicide atrazine at low concentrations in aqueous solution [139]. Analyte adsorption into the binding sites resulted in a change in Bragg diffraction of the polymer characterized by a color modification (Fig. 15). 

The SERS spectra from an organic analyte deposited on the substrate were found to show a enhancement factor of 104, which is comparable to the enhancement obtained from two-dimensional silver gratings produced by electron beam lithography by Kahl et al. [39]. Like the self-assembled three-dimensional opal gratings, the templated gratings provide clear practical advantages over ordered substrates produced by more complex and expensive methods. The results reported are summarized in Table 10.2. From the... 

Another important method for photonic crystal fabrication employs colloidal particle self-assembly. A colloidal system consists of two separate phases a dispersed phase and a continuous phase (dispersion medium). The dispersed phase particles are small solid nanoparticles with a typical size of 1-1000 nanometers. Colloidal crystals are three-dimensional periodic lattices assembled from monodispersed spherical colloids. The opals are a natural example of colloidal photonic crystals that diffract light in the visible and near-infrared (IR) spectral regions due to periodic modulation of the refractive index between the ordered monodispersed silica spheres and the surrounding matrix. 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

The opals obtained by self-assembly are mechanically unstable because there is only Van der Waals force between spheres. The subsequent infiltration process could easily destroy the ordered colloid arrays. So we annealed the opals of polymer sphere to increase their stability. As a result, there would form interconnections between spheres, which come from the slight melting of the sphere surfoces. These necks can provide the opal with necessary mechanical stability. In addition, they are important for producing inverse opal structure. After infiltration, when the samples are treated with calcinations, these necks can act as channels for the transport of the products formed during calcination like CO2. 

Figure 2. Self-assembled artificial opals fabricated with different methods (A) sedimentation, (B) filtration, (C) evaporation, and (D) flow-controlled evaporation. 

Moreover, the isolation of self-assembled LiNbOj powders using this route has added credibility to the methodology they were otherwise prepared by templating colloidal crystals of polyelectrolyte-coated spheres. The interest in LiNb03 inverse opals stems from the fact that they have a constant refractive index, but a spatially periodic second-order nonlinear susceptibility. Such nonlinear periodic structures allow for efficient qnasi-phase-matched second-order harmonic generation, which conld find applications where simultaneous conversion of multiple wavelengths is reqnired. Thns, in this chapter we will focus our... 

FIGURE 6.7 Scanning electron micrographs of (a) LiNbOj powder calcined at 500°C and (b-d) self-assembled inverse opals of LiNbO, powders calcined at 600°C... 

Crystallization of monodispersed spherical colloids into long-range ordered lattices (opals and inverse opals) using confined self-assembly... 

FIGURE 2.19 Self-assembly of opal structures onto electrode substrates viamonodispersed colloidal dispersions. (With permission from V. Misoska, Ph.D. thesis, University of Wollongong, 2002.)... 

Fig. 14 a Schematic illustration of engineering air-core line defects within self-assembled inverse opals b SEM images of the actual samples at various stages [146]... 

Sumida T, Wada Y, Kitamura T, Yanagida S (2001) Macroporous ZnO films electro-chemically prepared by templating of opal films. Chem Lett 30 38 Bartlett PN, Birkin PR, Ghanem MA, Toh CS (2001) Electrochemical syntheses of highly ordered macroporous conducting polymers grown around self-assembled colloidal templates. J Mater Chem 11 849... 

Ge HL, Wang GJ, He YN, Wang XG, Song YL, Jiang L, Zhua DB (2006) Photoswitched wettability on inverse opal modified by a self-assembled azobenzene monolayer. Chem Phys Chem 7 575... 

Crystalline colloidal arrays (CCA) are mesoscopically periodic fluid materials which efficiently diffract light meeting the Bragg condtion (/-. These materials consist of arrays of colloidal particles which self assemble in solution into BCC or FCC crystalline arrays (7,5) (Figure 1) with lattice constants in the mesoscale size range (50 to 500 nm). Just as atomic crystals diffract x-rays that meet the Bragg condition, CCA diffract UV, visible, and near IR light (2-4) the diffraction phenomena resemble that of opals, which are close-packed arrays of monodisperse silica spheres (6). 

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