9 research outputs found
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-NH<sub>2</sub>) 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
TiO<sub>2</sub>. 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 ZnÂ(C<sub>3</sub>N<sub>2</sub>H<sub>2</sub>–CH<sub>3</sub>)<sub>2</sub>·<sup>1</sup>/<sub>6</sub>PEG-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