We provide an overview of our recent advances in the manipulation of wetting in inverse-opal photonic crystals.
Exploiting photonic crystals with spatially patterned surface chemistry to confine the infiltration of fluids to liquidspecific spatial patterns, we developed a highly selective scheme for colorimetry, where organic liquids are distinguished based on wetting. The high selectivity of wetting, upon-which the sensitivity of the response relies, and the bright iridescent color, which disappears when the pores are filled with liquid, are both a result of the highly symmetric pore structure of our inverse-opal films. The application of horizontally or vertically orientated gradients in the surface chemistry allows a unique response to be tailored to specific liquids. While the generic nature of wetting makes our approach to colorimetry suitable for applications in liquid authentication or identification across a broad range of industries, it also ensures chemical non-specificity. However, we show that chemical specificity can be achieved combinatorially using an array of indicators that each exploits different chemical gradients to cover the same dynamic range of response. Finally, incorporating a photo-responsive polyelectrolyte surface layer into the pores, we are able to dynamically and continuously photo-tune the wetting response, even while the film is immersed in liquid. This in situ optical control of liquid percolation in our photonic-crystal films may also provide an error-free means to tailor indicator response, naturally compensating for batch-to-batch variability in the pore geometry.
Herein we present the characterization of the photomechanical effect of thin films of azobenzene based polymers
PDR1A, PDR13A and PMMA-co-PDR1A using an AFM based sensor. The polymers were coated on silicon and mica
cantilevers, and the cantilever bending and surface stress changes were characterized. We have shown that the
photoisomerization results in fast and significant cantilever bending ranging from 50-313 μm and changes in surface
stresses ranging from 96 to 568 N/m for PDR1A coated mica cantilevers. The photomechanical effect was shown to be
robust and repeatable. Of all the polymers studied, PDR1A exerted the largest forces with variations in solvent presence,
main chain modification and chromophore modification, all reducing the change in surface stress in order of increasing
impact. The utility of this new sensor in the understanding and characterization of the photomechanical effect of
azobenzene based polymers is demonstrated.
The kinetics of electrostatic layer-by-layer adsorption of a weakly charged polycation, poly(allylamine hydrochloride), PAH, and a polyanion containing an azobenzene chromophore, P-Azo, was studied using UV-vis spectroscopy and ellipsometry. The thickness of the multilayer films was first measured over the adsorption pH range of 3 to 11, and the growth of multilayers was examined as function of time and concentration. Films assembled in bath pH near that of their pKa value produced both the thickest films, and displayed remarkably rapid adsorption isotherms. In some PAH/P-Azo films a significantly large thickness was achieved in less than 5 seconds, which is more than two orders of magnitude faster than what is usually observed. We show that this anomalously rapid adsorption is a consequence of the weak acid-base nature of the layers. Self-assembled polymer films containg azobenzene groups are interesting materials as the chromophores can be addressed as light-responsive groups for surface patterning and sensing.
Poly(methyl methacrylate) slab waveguide materials were prepared, incorporating covalently attached azobenzene side groups. Birefringence was rapidly photoinduced in the films with linearly polarized light from an Ar+ laser to define stable channel waveguides, and the irradiated regions were shown to be suitable for multimode guiding of light at 633 nm. This single-step photoinscription process gives a controlled refractive index variation up to 0.012 for step or graded index channels, and can be rapidly modulated or completely erased with irradiation from a circularly polarized Ar+ laser beam. Written waveguides are stable indefinitely. Coupling in and out of the waveguides can be achieved with diffraction gratings photoinscribed in the polymer film using interfering beams from the same Ar+ laser. These high efficiency volume and surface diffraction gratings are stable over time and light exposure at the guiding wavelength. The grating spacing can be controlled by the geometry of the interference pattern, and hence can be optimize for high efficiency in-coupling and out-coupling at any required angle. Gratings with fringe spacings from 350 nm to 2000 nm were photoinscribed, and shown to couple light in and out of birefringent channel waveguides photoinscribed in the same material. The reversibility of the channels allows the guides to be photoaddressed for switching and mode filtering.
Conference Committee Involvement (2)
Molecular and Nano Machines II
11 August 2019 | San Diego, California, United States
21 August 2018 | San Diego, California, United States