Yunfeng Lu

Continuous formation of supported cubic and hexagonal mesoporous films by sol-gel dip-coating

Thin films of surfactant-templated mesoporous materials could find applications in membrane-based separations, selective catalysis and sensors. Above the critical micelle concentration of a bulk silica–surfactant solution, films of mesophases with hexagonally packed one-dimensional channels can be formed at solid–liquid and liquid–vapour interfaces. But this process is slow and the supported films are granular and with the pore channels oriented parallel to the substrate surface, so that transport across the films is not facilitated by the pores. Ogawa has reported a rapid spin-coating procedure for making transparent mesoporous films, but their formation mechanism, microstructure and pore accessibility have not been elucidated. Here we report a sol–gel-based dip-coating method for the rapid synthesis of continuous mesoporous thin films on a solid substrate. The influence of the substrate generates film mesostructures that have no bulk counterparts, such as composites with incipient liquid-crystalline order of the surfactant–silica phase. We are also able to form mesoporous films of the cubic phase, in which the pores are connected in a three-dimensional network that guarantees their accessibility from the film surface. We demonstrate and quantify this accessibility using a surface-acoustic-wave nitrogen-adsorption technique. We use fluorescence depolarization to monitor the evolution of the mesophase in situ, and see a progression through a sequence of lamellar to cubic to hexagonal structures that has not previously been reported.

Continuous self-assembly of organic-inorganic nanocomposite coatings that mimic nacre

Nanocomposite materials are widespread in biological systems. Perhaps the most studied is the nacre of abalone shell, an orientated coating composed of alternating layers of aragonite (CaCO3) and a biopolymer. Its laminated structure simultaneously provides strength, hardness and toughness: containing about 1u2009vol.u2009% polymer, nacre is twice as hard and 1,000 times as tough as its constituent phases. Such remarkable properties have inspired chemists and materials scientists to develop synthetic, ‘biomimetic’ nanocomposite assemblies. Nonetheless, the efficient processing of layered organic–inorganic composites remains an elusive goal. Here we report a rapid, efficient self-assembly process for preparing nanolaminated coatings that mimic the structure of nacre. Beginning with a solution of silica, surfactant and organic monomers, we rely on evaporation during dip-coating to induce the formation of micelles and partitioning of the organic constituents into the micellar interiors. Subsequent self-assembly of the silica–surfactant–monomer micellar species into lyotropic mesophases simultaneously organizes the organic and inorganic precursors into the desired nanolaminated form. Polymerization fixes this structure, completing the nanocomposite assembly process. This approach may be generalized both to other composite architectures and to other materials combinations.

Chemical sensors based on hydrophobic porous sol-gel films and ATR-FTIR spectroscopy

A new chemical sensor based on attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy has been fabricated by coating an ATR crystal with a hydrophobic mesoporous silica film. The highly porous coating can extract hydrophobic analytes, such as benzene, from aqueous solutions and concentrate them inside the film. The enrichment of the analytes inside the film provides the sensor with enhanced sensitivity. The hydrophobic porous film can efficiently exclude water from the region probed by the evanescent wave, thus eliminating the spectral interference due to water absorption bands. Measurement of nitrogen adsorption and desorption isotherms of the films using surface acoustic wave techniques has been used to characterize the pore structure of the films. Relationships between the structure of the porous film and sensor performance are discussed.

Chemical Sensors Based on Surface-Modified Sol-Gel-Coated Infrared Waveguides *

Attenuated total reflectance sampling, or waveguide sampling, is commonly used to obtain infrared spectra of aqueous solutions. The detection limits achieved for the determination of organic analytes in aqueous solutions using waveguide sampling are limited by the strong water absorption features and relatively small effective sample thickness due to the short penetration depth of the evanescent wave into the sample. Sol-gel processing has been used to produce a porous silica film on the surface of an infrared waveguide. The surface of the sol-gel film was modified with a silanizing reagent to produce a hydrophobic surface. The resulting waveguide was used to obtain infrared spectra from samples of benzonitrile in water. The benzonitrile spectral features obtained with the modified waveguide are enhanced by more than a factor of 103 when compared with those obtained with an unmodified waveguide.

Controlling the porosity of microporous silica by sol-gel processing using an organic template approach

The authors use an organic template approach to prepare microporous silica with controlled pore size and narrow pore size distributions. This was accomplished by fabricating relatively dense hybrid silica matrices incorporating organic template ligands by sol-gel synthesis and then removing the organic ligands to create a microporous silica network. Comparison of computer simulation results and experimental data indicated that using this fugitive template approach, pore volume can be controlled by the amount of organic template added to the system, and pore size can be controlled by the size of the organic ligands.