Synthetic routes toward MOF nanomorphologies.

As metal–organic frameworks (MOFs) are coming of age, their structural diversity, exceptional porosity and inherent functionality need to be transferred into useful applications. Fashioning MOFs into various shapes and at the same time controlling their size constitute an essential step toward MOF-based devices. Moreover, downsizing MOFs to the nanoscale triggers a whole new set of properties distinguishing nanoMOFs from their bulk counterparts. Therefore, dimensionality-controlled miniaturization of MOFs enables the customised use of nanoMOFs for specific applications where suitable size and shape are key prerequisites. In this feature article we survey the burgeoning field of nanoscale MOF synthesis, ranging from classical protocols such as microemulsion synthesis all the way to microfluidic-based techniques and template-directed epitaxial growth schemes. Along these lines, we will fathom the feasibility of rationally designing specific MOF nanomorphologies—zero-, one- and two-dimensional nanostructures—and we will explore more complex “second-generation” nanostructures typically evolving from a high level of interfacial control. As a recurring theme, we will review recent advances made toward the understanding of nucleation and growth processes at the nanoscale, as such insights are expected to further push the borders of nanoMOF science

Additive-mediated size control of MOF nanoparticles

A fast synthesis approach toward sub-60 nm sized MOF nanoparticles was developed by employing auxiliary additives. Control over the size of HKUST-1 and IRMOF-3 particles was gained by adjusting the concentration and type of stabilizers. Colloidal solutions of the MOFs were used for the formation of optically homogeneous thin films by spin-coating

One-dimensional metal-organic framework photonic crystals used as platforms for vapor sorption.

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 TiO2 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

Tandem MOF-Based Photonic Crystals for Enhanced Analyte-Specific Optical Detection

Owing to their structural variability, metal–organic frameworks (MOFs) lend themselves well as chemical sensing materials by providing both high sensitivity and selectivity. Here, we integrate different types of MOFs (ZIF-8, HKUST-1, CAU-1-NH₂) into photonic multilayers referred to as Bragg stacks (BSs), which report on adsorption events through changes in their effective refractive index (RI). The fabrication of photonic multilayers is accomplished by spin-coating colloidal suspensions of MOF nanoparticles and/or the high RI-material TiO₂. While their incorporation in BSs allows for the label-free readout of host–guest interactions, the choice of particular types of MOFs determines the sensing properties of the BS. Here, we present MOF-based BSs with enhanced specificity toward molecular analytes by combining two different MOFs in a single platform. The sensing performance of our BSs is demonstrated by a combined spectroscopic and principal component analysis of their vapor response. Time-dependent measurements reveal fast response times and good recoverability of the multilayers. Moreover, we demonstrate that combinatorial sensing is feasible by arranging different MOF BSs in a basic color pattern, which highlights the potential of MOF-based multilayers in arrayed sensor devices

ZIF‑8 Films Prepared by Femtosecond Pulsed-Laser Deposition

As metal–organic frameworks (MOFs) are coming of age, processing strategies and morphology engineering have gained considerable importance, given the need of thin film geometries for many applications. Using the femtosecond pulsed-laser deposition (femto-PLD) technique, we have fabricated films of the zeolitic imidazolate framework (ZIF) zinc 2-methylimidazolate (ZIF-8) for the first time, thus extending the available film fabrication techniques for MOFs to physical vapor deposition. While deposition of pristine ZIF-8 turned out to be unsuccessful, we demonstrate that hybrid ZIF-8 impregnated with polyethylene glycol 400 as a “vehicle” ablate under ultrahigh vacuum conditions to form films with approximate composition Zn­(C₃N₂H₂–CH₃)₂·¹/₆PEG-400. By washing the films with ethanol, the polyethylene glycol (PEG) additive can be removed, leading to pure ZIF-8 films on sapphire substrates. The target films and powders were comprehensively characterized by diffraction, spectroscopic and microscopic techniques as well as thermogravimetry and Ar physisorption measurements