A Chemiluminescent Metal-Organic Framework.

The synthesis and characterization of a chemiluminescent metal-organic framework with high porosity is reported. It consists of Zr6 O6 (OH)4 nodes connected by 4,4'-(anthracene-9,10-diyl)dibenzoate as the linker and luminophore. It shows the topology known for UiO-66 and is therefore denoted PAP-UiO. The MOF was not only obtained as bulk material but also as a thin film. Exposure of PAP-UiO as bulk or film to a mixture of bis-(2,4,6-trichlorophenyl) oxalate, hydrogen peroxide, and sodium salicylate in a mixture of dimethyl and dibutyl phthalate evoked strong and long lasting chemiluminescence of the PAP-UiO crystals. Time dependent fluorescence spectroscopy on bulk PAP-UiO and, for comparison, on dimethyl 4,4'-(anthracene-9,10-diyl)dibenzoate provided evidence that the chemiluminescence originates from luminophores being part of the PAP-UiO, including the luminophores inside the crystals.

Turn-on fluorescence triggered by selective internal dye replacement in MOFs.

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.

Highly sensitive and selective fluoride detection in water through fluorophore release from a metal-organic framework.

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.

Conformational flexibility of the pentasaccharide LNF-2 deduced from NMR spectroscopy and molecular dynamics simulations.

Human milk oligosaccharides (HMOs) are important as prebiotics since they stimulate the growth of beneficial bacteria in the intestine and act as receptor analogues that can inhibit the binding of pathogens. The conformation and dynamics of the HMO Lacto-N-fucopentaose 2 (LNF-2), α-L-Fucp-(1 → 4)[ß-D-Galp-(1 → 3)]-ß-D-GlcpNAc-(1 → 3)-ß-D-Galp-(1 → 4)-D-Glcp, having a Lewis A epitope, has been investigated employing NMR spectroscopy and molecular dynamics (MD) computer simulations. 1D (1)H,(1)H-NOESY experiments were used to obtain proton-proton cross-relaxation rates from which effective distances were deduced and 2D J-HMBC and 1D long-range experiments were utilized to measure trans-glycosidic (3)J(CH) coupling constants. The MD simulations using the PARM22/SU01 force field for carbohydrates were carried out for 600 ns with explicit water as solvent which resulted in excellent sampling for flexible glycosidic torsion angles. In addition, in vacuo MD simulations were performed using an MM3-2000 force field, but the agreement was less satisfactory based on an analysis of heteronuclear trans-glycosidic coupling constants. LNF-2 has a conformationally well-defined region consisting of the terminal branched part of the pentasaccharide, i.e., the Lewis A epitope, and a flexible ß-D-GlcpNAc-(1 → 3)-ß-D-Galp-linkage towards the lactose unit, which is situated at the reducing end. For this ß-(1 → 3)-linkage a negative ψ torsion angle is favored, when experimental NMR data is combined with the MD simulation in the analysis. In addition, flexibility on a similar time scale, i.e., on the order of the global overall molecular reorientation, may also be present for the ϕ torsion angle of the ß-D-Galp-(1 → 4)-D-Glcp-linkage as suggested by the simulation. It was further observed from a temperature variation study that some (1)H NMR chemical shifts of LNF-2 were highly sensitive and this study indicates that Δδ/ΔT may be an additional tool for revealing conformational dynamics of oligosaccharides.

Highly oriented surface-growth and covalent dye labeling of mesoporous metal-organic frameworks.

Mesoporous amino-functionalized metal-organic framework thin films with the UiO-68 topology were grown in a highly oriented fashion on two different self-assembled monolayers on gold. The oriented MOF films were covalently modified with the fluorescent dye Rhodamine B inside the pore system, as demonstrated with size-selective fluorescence quenching studies. Our study suggests that mesoporous metal-organic frameworks are promising hosts for the covalent attachment of numerous functional moieties in a molecularly designed crystalline environment.

Oriented growth of the functionalized metal-organic framework CAU-1 on -OH- and -COOH-terminated self-assembled monolayers.

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.