Improving the Loading Capacity of Metal–Organic Framework Thin Films Using Optimized Linkers

The large surface area of metal–organic frameworks (MOFs) sparks great interest for their use in storage applications. While the bulk of MOF applications focuses on incorporation of gases, we demonstrate that these highly porous frameworks are also well-suited for metal ion storage. For well-defined, highly oriented surface-anchored MOF thin films grown on modified gold surfaces using liquid-phase epitaxy (LPE), also referred to as SURMOFs, we determined the loading of two different types of MOF materials with a total of seven types of metal ions (Zn²⁺, Ag⁺, Pd²⁺, Fe³⁺, Cd²⁺, Ni²⁺, and Co²⁺). Measurements using a quartz crystal microbalance (QCM) allowed determination of loading capacities as well as diffusion constants in a quantitative fashion. The adsorption capacities were observed to be highly ion specific; the largest uptake was for Fe³⁺ and Pd²⁺ ions with six and four metal ions per MOF pore, respectively. By comparing results for SURMOFs fabricated from different types of linkers, we demonstrate that S-containing functionalities in particular drastically improve the storage capacity of MOFs for metal ions

Nanoporous Designer Solids with Huge Lattice Constant Gradients: Multiheteroepitaxy of Metal–Organic Frameworks

We demonstrate the realization of hierarchically organized MOF (metal–organic framework) multilayer systems with pronounced differences in the size of the nanoscale pores. Unusually large values for the lattice constant mismatch at the MOF–MOF heterojunctions are made possible by a particular liquid-phase epitaxy process. The multiheteroepitaxy is demonstrated for the isoreticular SURMOF-2 series [Liu et al. Sci. Rep. 2012, 2, 921] by fabricating trilayer systems with lattice constants of 1.12, 1.34, and 1.55 nm. Despite these large (20%) lattice mismatches, highly crystalline, oriented multilayers were obtained. A thorough theoretical analysis of the MOF-on-MOF heterojunction structure and energetics allows us to identify the two main reasons for this unexpected tolerance of large lattice mismatch: the healing of vacancies with acetate groups and the low elastic constant of MOF materials

Post-Synthetic Modification of Metal–Organic Framework Thin Films Using Click Chemistry: The Importance of Strained C–C Triple Bonds

In this work, we demonstrate that strain-promoted azide–alkyne cycloaddition (SPAAC) yields virtually complete conversion in the context of the post-synthetic modification (PSM) of metal–organic frameworks (MOFs). We use surface-anchored MOF (SURMOF) thin films, [Zn₂(N₃-bdc)₂(dabco)], grown on modified Au substrates using liquid-phase epitaxy (LPE) as a model system to first show that, with standard click chemistry, presently, the most popular method for rendering additional functionality to MOFs via PSM, quantitative conversion yields, cannot be reached. In addition, it is virtually impossible to avoid contaminations of the product by the cytotoxic Cu^I ions used as a catalyst, a substantial problem for applications in life sciences. Both problems could be overcome by SPAAC, where a metal catalyst is not needed. After optimization of reaction conditions, conversion yields of nearly 100% could be achieved. The consequences of these results for various applications of PSM-modified SURMOFs in the fields of membranes, optical coatings, catalysis, selective gas separation, and chemical sensing are briefly discussed

Monolithic High Performance Surface Anchored Metal−Organic Framework Bragg Reflector for Optical Sensing

We report the fabrication of monolithic dielectric mirrors by stacking layers of metal–organic frameworks (MOFs) and indium tin oxide (ITO). Such Hybrid Photonic Band-Gap (PBG) Materials exhibit high optical quality (reflectivities of 80%) and are color tunable over the whole visible range. While the ITO deposition is accomplished by using a conventional sputter process, the highly porous MOF layers are deposited using liquid-phase epitaxy (LPE), therefore yielding crystalline, continuous, and monolithic HKUST-1 SURMOF thin films with high optical performance. We demonstrate the optical sensing capabilities of these monolithic and porous Bragg stacks by investigating the chemo-responsive optical properties (PBG shift and modulation of the intensity of the PBG maximum) upon the exposure to different organic solvents

Fabrication of Highly Uniform Gel Coatings by the Conversion of Surface-Anchored Metal–Organic Frameworks

We report the fabrication of 3D, highly porous, covalently bound polymer films of homogeneous thickness. These surface-bound gels combine the advantages of metal–organic framework (MOF) materials, namely, the enormous flexibility and the large size of the maximum pore structures and, in particular, the possibility to grow them epitaxially on modified substrates, with those of covalently connected gel materials, namely, the absence of metal ions in the deposited material, a robust framework consisting of covalent bonds, and, most importantly, pronounced stability under biological conditions. The conversion of a SURMOF (surface-mounted MOF) yields a surface-grafted gel. These SURGELs can be loaded with bioactive compounds and applied as bioactive coatings and provide a drug-release platform in in vitro cell culture studies