We present the fabrication of one-dimensional photonic crystals (Bragg stacks) based on a microporous metal-organic framework material and mesoporous titanium dioxide. The Bragg stack heterostructures were obtained using two complementary synthesis approaches utilizing the bottom-up assembly of heterogeneous, i.e. two-component photonic crystal multilayer structures. Zeolitic imidazolate framework ZIF-8 and mesoporous titanium dioxide were chosen as functional components with different refractive indices. While ZIF-8 is intended to impart molecular selectivity, mesoporous TiO 2 is used to ensure high refractive index contrast and to guarantee molecular diffusion within the Bragg stack. The combination of micro-and mesoporosity within one scaffold endows the 1D-MOF PC with characteristic adsorption properties upon exposure to various organic vapors. In this context, the sorption behavior of the photonic material was studied as a function of partial pressure of organic vapors. The results show that the multilayered photonic heterostructures are sensitive and selective towards a series of chemically similar solvent vapors. It is thus anticipated that the concept of multilayer heterogeneous photonic structures will provide a versatile platform for future selective, label-free optical sensors.

The detection, differentiation and visualization of compounds such as gases, liquids or ions are key challenges for the design of selective optical chemosensors. Optical chemical sensors employ a transduction mechanism that converts a specific analyte recognition event into an optical signal. Here we report a novel concept for fluoride ion sensing where a porous crystalline framework serves as a host for a fluorescent reporter molecule. The detection is based on the decomposition of the host scaffold which induces the release of the fluorescent dye molecule. Specifically, the hybrid composite of the metal-organic framework NH2-MIL-101(Al) and fluorescein acting as reporter shows an exceptional turn-on fluorescence in aqueous fluoride-containing solutions. Using this novel strategy, the optical detection of fluoride is extremely sensitive and highly selective in the presence of many other anions.

Coordinatively unsaturated metal sites (CUS) are used to create dye-functionalized metal-organic frameworks (MOFs). The quenching of dye fluorescence through interactions with the CUS can be utilised for chemical sensing of Lewis bases that displace the dye from the CUS, resulting in a triggered turn-on fluorescence signal.

The oriented growth of the amino-functionalized metal-organic framework [Al4(OH)2(OCH3)4(H2N-bdc)3].xH2O (named CAU-1; CAU = Christian-Albrechts-University) on different self-assembled monolayers (SAMs) is reported. Gold substrates were modified with both 16-mercaptohexadecanol and 16-mercaptohexadecanoic acid SAMs and immersed in a pre-treated crystallization solution. The different terminal functionalities and synthesis parameters result in the direct surface growth of CAU-1 crystals along preferred crystallographic orientations. While for the OH-terminated SAMs a preferential attachment of the CAU-1 crystals either in [011] or [002] direction is observed (depending on the reaction conditions), highly oriented crystal growth in [002] orientation was obtained on the carboxylic acid functionalized SAMs.

PapersSupramolecular isomers of metal-organic frameworks: the role of a new mixed donor imidazolate-carboxylate tetradentate ligand