Controlling morphology and porosity to improve performance of molecularly imprinted sol-gel silica.

The wealth of molecular precursors for organic and inorganic polymers has resulted in an incredible volume of molecular imprinting literature. The vast majority of reports deal with organic polymer systems, and molecular imprinting in silica can still be considered a small niche in the field. In this review, we present key concepts of molecular imprinting, sol-gel processing, and the synthesis of templated mesoporous silica. We take a small fraction of the literature and use it to understand the ways in which molecular imprinting in siliceous materials of controlled morphology has achieved success in the past fifteen years. Using selected case studies rather than a comprehensive review of the entire field, our goal is to illustrate the key aspects of imprinted silica-based materials as demonstrated by judiciously controlled systems, looking first at control on the micrometre scale in bulk phase materials, and then on the nanometre scale in templated mesoporous materials.

Low-k periodic mesoporous organosilica with air walls: POSS-PMO.

Periodic mesoporous organosilica (PMO) with polyhedral oligomeric silsesquioxane (POSS) air pockets integrated into the pore walls has been prepared by a template-directed, evaporation-induced self-assembly spin-coating procedure to create a hybrid POSS-PMO thin film. A 10-fold increase in the porosity of the POSS-PMO film compared to a reference POSS film is achieved by incorporating ∼1.5 nm pores. The increased porosity results in a decrease in the dielectric constant, k, which goes from 2.03 in a reference POSS film to 1.73 in the POSS-PMO film.

Spatially confined redox chemistry in periodic mesoporous hydridosilica-nanosilver grown in reducing nanopores.

Periodic mesoporous hydridosilica, PMHS, is shown for the first time to function as both a host and a mild reducing agent toward noble metal ions. In this archetypical study, PMHS microspheres react with aqueous Ag(I) solutions to form Ag(0) nanoparticles housed in different pore locations of the mesostructure. The dominant reductive nucleation and growth process involves SiH groups located within the pore walls and yields molecular scale Ag(0) nanoclusters trapped and stabilized in the pore walls of the PMHS microspheres that emit orange-red photoluminescence. Lesser processes initiated with pore surface SiH groups produce some larger spherical and worm-shaped Ag(0) nanoparticles within the pore voids and on the outer surfaces of the PMHS microspheres. The intrinsic reducing power demonstrated in this work for the pore walls of PMHS speaks well for a new genre of chemistry that benefits from the mesoscopic confinement of Si-H groups.

Periodic mesoporous hydridosilica--synthesis of an "impossible" material and its thermal transformation into brightly photoluminescent periodic mesoporous nanocrystal silicon-silica composite.

There has always been a fascination with "impossible" compounds, ones that do not break any rules of chemical bonding or valence but whose structures are unstable and do not exist. This instability can usually be rationalized in terms of chemical or physical restrictions associated with valence electron shells, multiple bonding, oxidation states, catenation, and the inert pair effect. In the pursuit of these "impossible" materials, appropriate conditions have sometimes been found to overcome these instabilities and synthesize missing compounds, yet for others these tricks have yet to be uncovered and the materials remain elusive. In the scientifically and technologically important field of periodic mesoporous silicas (PMS), one such "impossible" material is periodic mesoporous hydridosilica (meso-HSiO(1.5)). It is the archetype of a completely interrupted silica open framework material: its pore walls are comprised of a three-connected three-dimensional network that should be so thermodynamically unstable that any mesopores present would immediately collapse upon removal of the mesopore template. In this study we show that meso-HSiO(1.5) can be synthesized by template-directed self-assembly of HSi(OEt)(3) under aqueous acid-catalyzed conditions and after template extraction remains stable to 300 °C. Above this temperature, bond redistribution reactions initiate a metamorphic transformation which eventually yields periodic mesoporous nanocrystalline silicon-silica, meso-ncSi/SiO(2), a nanocomposite material in which brightly photoluminescent silicon nanocrystallites are embedded within a silica matrix throughout the mesostructure. The integration of the properties of silicon nanocrystallinity with silica mesoporosity provides a wealth of new opportunities for emerging nanotechnologies.

Molecularly imprinted mesoporous organosilica.

We have prepared molecularly imprinted mesoporous organosilica (MIMO) using a semicovalent imprinting technique. A thermally reversible covalent bond was used to link a bisphenol A (BPA) imprint molecule to a functional alkoxysilane monomer at two points to generate a covalently bound imprint precursor. This precursor was incorporated into a cross-linked periodic mesoporous silica matrix via a typical acid-catalyzed, triblock copolymer-templated, sol-gel synthesis. Evidence of imprint sites buried in the pore walls was found through careful characterization of the imprinted material and its comparison to similarly prepared non-imprinted mesoporous organosilica (NIMO) and pure periodic mesoporous silica (PMS). After thermal treatment, the imprinted material (MIMO-ir) removed more than 90% of appropriately sized bisphenol species from water, yet showed significantly lower binding for both smaller and larger molecules containing phenol moieties. Identically treated NIMO-ir showed much poorer retention behavior than MIMO-ir for the same bisphenol species and behaved only slightly better than PMS-ir.

Why PMO? Towards functionality and utility of periodic mesoporous organosilicas.

Creative synthetic chemistry has endowed the class of periodic mesoporous organosilica materials, dubbed PMO, with a variety of new and exciting compositions, properties, and functions since its inception a decade ago. Using a handful of recent trendsetting case histories, the multidisciplinary applications of PMO materials in chemistry and physics, materials science and engineering, biology, and medicine are demonstrated in a most powerful way. In doing so, this Review aims to inspire more collaborative and ambitious endeavors in the second decade of PMO research.