Hierarchical structuring of metal-organic framework thin-films on quartz crystal microbalance (QCM) substrates for selective adsorption applications

Continuous stepwise liquid-phase epitaxial (LPE) growth is one of the most effective procedures for structuring metal-organic frameworks (MOFs) as two-dimensional superstructures, such as thin-films. Alternation of the building block precursors between the individual LPE growth cycles (i.e. from one linker to the other) allows heterostructured MOF films consisting of two different MOFs with different structural or chemical properties to be synthesized with a precise control of the growth sequence. Here, we employ the LPE growth strategy for the preparation of highly functional, hierarchically structured core-shell architectures consisting of [Zn4O(3,5-dialkylcarboxypyrazolate)(3)](n)-based frameworks. Specifically, the small-pore [Zn4O(3-methyl-5-isopropyl-4-carboxypyrazolate)(3)](n) (Zn-MI) and [Zn4O(3,5-diethyl- 4-carboxypyrazolate)(3)](n) (Zn-DE) frameworks are respectively deposited as a size selective layer upon larger-pore [Zn4O(3,5-dimethyl-4-carboxypyrazolate)(3)](n) (Zn-DM) and [Zn4O(3-methyl-5-ethyl-4-carboxypyrazolate)(3)](n) (Zn-ME) layers. Direct growth of the MOF layers on the Au surfaces of quartz crystal microbalance (QCM) sensors allowed the adsorption properties of the heterostructures to be probed in real-time. Multiple-component adsorption experiments in an environment-controlled QCM apparatus revealed size selectivity with respect to the adsorption of alcohols, as well as the molecular recognition of methanol over water. These properties stem from the positioning of the small-pore Zn-MI (or Zn-DE) layer on the larger-pore Zn-DM (or Zn-ME) layer, facilitating attractive synergistic properties for adsorptive selectivity and providing a possibility for further development in MOF-based sensing applications.