Metal-organic Framework Films Research Articles

Liquid-Phase Molecular-Ionic Layer Deposition of Surface-Supported Metal-Organic Frameworks of Cu3(BTC)2 HKUST-1 for Electrochemical Sensors

Metal-organic frameworks (MOFs) are traditionally developed as effective gas separation/storage materials, however, recently they are receiving increasing attention as multifunctional layers in electronic devices [1]. MOFs can possess unique combinations of properties derived from complex interaction between inorganic and organic constituents that can provide better functionality in comparison to conventional materials. Recently, applications of electrochemically active MOFs for small molecules sensing, such as glucose [2], ethanol [3], NADH [4] and H2O2 [2, 5] have been demonstrated. However, MOFs are still rarely used in electrochemical sensors, because fabrication of thin films of MOFs and their integration into electronic devices is challenging due to incompatibility of MOFs with conventional physical or chemical vacuum deposition methods, while conventional solvent-assisted methods, such as spin-coating and die-coating, or solvothermal methods are difficult to realize in controllable manner due to extremely low solubility of MOFs. From the other hand, the low solubility of MOFs becomes an advantage in the case of liquid-phase molecular ionic layer deposition (LP-MILD) method, which belongs to the family of layer-by-layer deposition methods similar to the atomic layer deposition (ALD). In the LP-MILD method the growing film is sequentially exposed to solutions of reactive precursors mediated by washing steps that remove excess of precursors and reaction byproducts, and effectively confine the reaction space to only one surface monolayer of the growing film. As a result, the film grows in steps of a half monolayer per process cycle, which allows easy control over film thickness and composition, including heteroepitaxy and/or doping. The LP-MILD method can be technically realized in different flavors, where sequential substrate exposures to precursor solutions and washing steps are facilitated by dip-coating [6], spray-coating [7] or spin-coating [8].In this work we demonstrate that spin-coting facilitated LP-MILD can be effectively used to fabricate layers of surface supported metal organic framework (SURMOF) of Cu2(BTC)3, also known as HKUST-1, on fluorine-doped tin oxide (FTO) transparent conductive layers. Glass/FTO substrates cleaned by consecutive sonication in detergent, water, acetone and IPA and then treated in oxygen plasma were placed on a vacuum chuck of spin-coater and sequentially exposed to (1) solution of copper (II) acetate in ethanol (2 mM, 1000 rpm, 10 s), (2) ethanol (500 rpm, 20 s), (3) 1,3,5-benzenetricarboxylic acid (BTC) in methanol (5 mM, 1000 rpm, 10 s) and (4) methanol (500 rpm, 20 s). The precursor solutions and the substrate were kept at the same temperature during the LP-MILD process. We have found that the growth per cycle (GPC) for the deposition at 25 oC is larger than should be in the expected half-layer GPC mechanism, but nevertheless smaller than the full-layer GPC. This is already better result than in the previously reported works [7, 8], where the growth rate significantly exceeds even the full-layer GPC, which can be explained by entrapment of precursors in the cages of the growing SURMOF due to insufficient rinsing. We have found that the deposition at 40 oC approaches the ideal half-layer GPC mechanism, which indicates effective rinsing at elevated temperatures.In this presentation we are going to compare various fabrication methods of HKUST-1 SURMOFs and demonstrate advantages of our LP-MILD method supported by morphological, structural and electrochemical characterizations of our HKUST-1 SURMOF layers as active layers for electrochemical sensors. References I. Stassen et al., Chem. Soc. Rev. 46, 3185, 2017.D. Zhang et al., Talanta 144, 1176, 2015.L. Yang et al., Angew. Chem., Int. Ed. 49, 5348, 2010.Y. Zhang et al., Chem. Commun. 49, 6885, 2013.L. Wang et al., Electrochim. Acta 213, 691, 2016.C. Munuera et al., Phys. Chem. Chem. Phys., 10, 7257, 2008.H. K. Arslan et al. Adv. Funct. Mater. 21, 4228, 2011.V. Chernikova et al., ACS Appl. Mater. Interfaces 8, 20459, 2016.

Automatic Deposition of Oriented Copper Hydroxide Nanobelt Films for the Heteroepitaxial Growth of Metal-Organic Frameworks

One of the key challenges that the field of metal-organic frameworks (MOFs) faces today is the possibility of processing these materials as thin films to enable their integration into existing technologies [1-4]. This is particularly important for devices that exploit anisotropic properties obtained from highly oriented materials. Based on a previously described procedure to obtain centimetre-scale oriented MOF films [5], we hereby report the optimization and extension of this procedure into an automated, operator-independent method that affords higher yields without compromising the long-range orientation. The procedure is based on the deposition of crystalline copper hydroxide nanobelts that can be aligned to obtain substrates with preferential orientation. The influence of different deposition parameters on the coverage and orientation of the films was studied and tuned in order to obtain reproducible samples with higher yields. With applications in fields such as gas storage, catalysis and light harvesting, among others, these oriented ceramic substrates can be converted on demand to different MOFs by exploiting a rapid heteroepitaxial growth [6]. This automatic method introduces the attractive prospect of the potential implementation of MOFs in large-scale thin-film processing. Figure 1: Comparison between manual and automated procedures for the deposition of oriented copper hydroxide films on silicon substrates. Left: Azimuthal angle dependence of intensity profiles of the (020) reflection of Cu(OH)2 at a diffraction angle of 23.8°corresponding to a typical sample obtained by each method, showing the superior alignment of the automatic procedure. Right: Optical images showing the surface coverage and long range (centimetre-scale, top) and micro alignment of the substrate (by polarized-light optical microscopy, bottom).

Enhancing photovoltages at p-type semiconductors through a redox-active metal-organic framework surface coating

Surface modification of semiconductors can improve photoelectrochemical performance by promoting efficient interfacial charge transfer. We show that metal-organic frameworks (MOFs) are viable surface coatings for enhancing cathodic photovoltages. Under 1-sun illumination, no photovoltage is observed for p-type Si(111) functionalized with a naphthalene diimide derivative until the monolayer is expanded in three dimensions in a MOF. The surface-grown MOF thin film at Si promotes reduction of the molecular linkers at formal potentials >300 mV positive of their thermodynamic potentials. The photocurrent is governed by charge diffusion through the film, and the MOF film is sufficiently conductive to power reductive transformations. When grown on GaP(100), the reductions of the MOF linkers are shifted anodically by >700 mV compared to those of the same MOF on conductive substrates. This photovoltage, among the highest reported for GaP in photoelectrochemical applications, illustrates the power of MOF films to enhance photocathodic operation.

Electrochemically-assisted deposition of toluidine blue-functionalized metal-organic framework films for electrochemical immunosensing of Indole-3-acetic acid

A simple and rapid electrochemical synthesis strategy for in-situ fabrication of toluidine blue (TB)-functionalized metal organic framework ([email protected]) thin films via one-step reduction approach was reported. The [email protected] thin layer has been synthesized on the working electrode surface in different times at room temperature, and its morphology and thickness was characterized by varied instruments, including scanning electron microscope, transmission electron microscope, and scanning ion conductance microscope. The prepared [email protected] shows exceptional electrochemical activity and stability under mild conditions, and its electrochemical response is dependent on the amount of TB molecules which are encapsulated in the MOFs. Based on the [email protected] sensing platform, a sensitive label-free electrochemical immunosensor for the detection of Indole-3-acetic acid (IAA, a plant hormone) was developed with a wide linear range (2.5 pg mL−1 - 5 ng mL−1) and a low detection limit (1.4 pg mL−1). Moreover, this immune sensor shows good sensitivity and selectivity in real samples for IAA detection.

Hierarchical Metal-Organic Framework Films with Controllable Meso/Macroporosity.

The structuring of the metal‐organic framework material ZIF‐8 as films and membranes through the vapor‐phase conversion of ZnO fractal nanoparticle networks is reported. The extrinsic porosity of the resulting materials can be tuned from 4% to 66%, and the film thickness can be controlled from 80 nm to 0.23 mm, for areas >100 cm2. Freestanding and pure metal‐organic frameworks (MOF) membranes prepared this way are showcased as separators that minimize capacity fading in model Li‐S batteries.

Low-cost inkjet printing of metal–organic frameworks patterns on different substrates and their applications in ammonia sensing

Due to their vivid material properties, metal-organic frameworks (MOFs) have been successfully proven useful in a wide variety of applications. In recent years, MOFs have been explored in electrical, electronic, and optical devices as well. The fabrication of such devices requires a controlled growth of thin films of MOFs on diverse substrates. In the present work, we demonstrate a simple way of inkjet printing for the patterning of thin films of transition metals and Terbium (Tb) MOFs on substrates like paper and flexible PET (polyethylene terephthalate). In particular, inkjet printing of Cr-MIL-101, Mn-BDC, Fe-MIL-101, Co-MOF-71, and Ni-BDC MOFs is one of the novel aspects of this work. The printed patterns have been characterized with different instrumental techniques. The studies have confirmed that the inkjet printing technique can conveniently facilitate the growth of homogeneous MOF thin film patterns with intact structural and functional characteristics. The experiments have also proven that the basic characteristics of the patterned MOFs remains stable for months. Two printed patterns, i.e., one Mn- and one Tb- based MOF films, have been demonstrated as colorimetric/fluorescent sensing strips for easy detection of ammonia vapor with reasonable sensitivity. The films have facilitated the detection of ammonia over 5–80 ppm with a low limit of detection of 0.3 ppm.

Two-Dimensional COF-Three-Dimensional MOF Dual-Layer Membranes with Unprecedentedly High H2/CO2 Selectivity and Ultrahigh Gas Permeabilities.

Composite membranes embodying multilayered architecture have been on an uptrend to tap the synergy between different materials to attain new heights in gas separation performance. In the light of sustainable materials research, covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) have emerged as cutting-edge platforms for molecular-sieving membranes owing to their phenomenal surface areas, ultrahigh porosities, and precise control over chemical functionalities. In this study, we report for the first time a three-dimensional (3D) MOF-mediated strategy where a specially designed MOF film provides the binding sites along the vertical direction to anchor the two-dimensional (2D) COF structural building units. The strong chemical bonding between the 3D MOF and 2D COF provides a new outlook to fabricate 2D COF-based composite membranes. The π-stacked columns of 2D H2P-DHPh COF that can contribute to direct pathways for gas transport render the resulting membrane incredibly promising for high-flux gas separation. Besides, the chemical synergy between the MOF and COF endows the thus-developed H2P-DHPh COF-UiO-66 composite membrane with unprecedented H2/CO2 gas mixture selectivity (32.9) as well as ultrahigh H2 (108 341.3 Barrer) and CO2 permeabilities, which significantly outperform the present Robeson upper bound and polymer membranes hitherto reported.

Preparation of MOF Film/Aerogel Composite Catalysts via Substrate-Seeding Secondary-Growth for the Oxygen Evolution Reaction and CO2 Cycloaddition.

Substrate-supported metal-organic frameworks (MOFs) films are desired to realize their potential in practical applications. Herein, a novel substrate-seeding secondary-growth strategy is developed to prepare composites of uniform MOFs films on aerogel walls. Briefly, the organic ligand is "pre-seeded" onto the aerogel walls, and then a small amount of metal-ion solution is sprayed onto the prepared aerogel. The sprayed solution diffuses along the aerogel walls to form a continuous thin layer, which confines the nucleation reaction, promoting the formation of uniform MOFs films on the aerogel walls. The whole process is simple in operation, highly efficient, and eco-friendly. The resulting hierarchical MOFs/aerogel composites have abundant accessible active sites and enable excellent mass transfer, which endows the composite with outstanding catalytic activity and stability in both liquid-phase CO2 cycloaddition and electrochemical oxygen evolution reaction (OER) process.

Preparation of MOF Film/Aerogel Composite Catalysts via Substrate‐Seeding Secondary‐Growth for the Oxygen Evolution Reaction and CO2 Cycloaddition

AbstractSubstrate‐supported metal–organic frameworks (MOFs) films are desired to realize their potential in practical applications. Herein, a novel substrate‐seeding secondary‐growth strategy is developed to prepare composites of uniform MOFs films on aerogel walls. Briefly, the organic ligand is “pre‐seeded” onto the aerogel walls, and then a small amount of metal‐ion solution is sprayed onto the prepared aerogel. The sprayed solution diffuses along the aerogel walls to form a continuous thin layer, which confines the nucleation reaction, promoting the formation of uniform MOFs films on the aerogel walls. The whole process is simple in operation, highly efficient, and eco‐friendly. The resulting hierarchical MOFs/aerogel composites have abundant accessible active sites and enable excellent mass transfer, which endows the composite with outstanding catalytic activity and stability in both liquid‐phase CO2 cycloaddition and electrochemical oxygen evolution reaction (OER) process.

Structural and electronic switching of a single crystal 2D metal-organic framework prepared by chemical vapor deposition

The incorporation of metal-organic frameworks into advanced devices remains a desirable goal, but progress is hindered by difficulties in preparing large crystalline metal-organic framework films with suitable electronic performance. We demonstrate the direct growth of large-area, high quality, and phase pure single metal-organic framework crystals through chemical vapor deposition of a dimolybdenum paddlewheel precursor, Mo2(INA)4. These exceptionally uniform, high quality crystals cover areas up to 8600 µm2 and can be grown down to thicknesses of 30 nm. Moreover, scanning tunneling microscopy indicates that the Mo2(INA)4 clusters assemble into a two-dimensional, single-layer framework. Devices are readily fabricated from single vapor-phase grown crystals and exhibit reversible 8-fold changes in conductivity upon illumination at modest powers. Moreover, we identify vapor-induced single crystal transitions that are reversible and responsible for 30-fold changes in conductivity of the metal-organic framework as monitored by in situ device measurements. Gas-phase methods, including chemical vapor deposition, show broader promise for the preparation of high-quality molecular frameworks, and may enable their integration into devices, including detectors and actuators.

Electro-reduction of carbon dioxide at low over-potential at a metal\u2013organic framework decorated cathode

Electrochemical reduction of carbon dioxide is a clean and highly attractive strategy for the production of organic products. However, this is hindered severely by the high negative potential required to activate carbon dioxide. Here, we report the preparation of a copper-electrode onto which the porous metal–organic framework [Cu2(L)] [H4L = 4,4′,4″,4′′′-(1,4-phenylenebis(pyridine-4,2,6-triyl))tetrabenzoic acid] can be deposited by electro-synthesis templated by an ionic liquid. This decorated electrode shows a remarkable onset potential for reduction of carbon dioxide to formic acid at −1.45 V vs. Ag/Ag+, representing a low value for electro-reduction of carbon dioxide in an organic electrolyte. A current density of 65.8 mA·cm−2 at −1.8 V vs. Ag/Ag+ is observed with a Faradaic efficiency to formic acid of 90.5%. Electron paramagnetic resonance spectroscopy confirms that the templated electro-synthesis affords structural defects in the metal–organic framework film comprising uncoupled Cu(II) centres homogenously distributed throughout. These active sites promote catalytic performance as confirmed by computational modelling.

Crystalline assembly of perylene in metal–organic framework thin film: J-aggregate or excimer? Insight into the electronic structure

The spatial orientation of chromophores defines the photophysical and optoelectronic properties of a material and serves as the main tunable parameter for tailoring functionality. Controlled assembly for achieving a predefined spatial orientation of chromophores is rather challenging. Metal–organic frameworks (MOFs) are an attractive platform for exploring the virtually unlimited chemical space of organic components and their self-assembly for device optimization. Here, we demonstrate the impact of interchromophore interactions on the photophysical properties of a surface-anchored MOF (SURMOF) based on 3,9-perylenedicarboxylicacid linkers. We predict the structural assembly of the perylene molecules in the MOF via robust periodic density functional theory calculations and discuss the impact of unit topology and π–π interaction patterns on spectroscopic and semiconducting properties of the MOF films. We explain the dual nature of excited states in the perylene MOF, where strong temperature-modulated excimer emission, enhanced by the formation of perylene J-aggregates, and low stable monomer emission are observed. We use band-like and hopping transport mechanisms to predict semiconducting properties of perylene SURMOF-2 films as a function of inter-linker interactions, demonstrating both p-type and n-type conduction mechanisms. Hole carrier mobility up to 7.34 cm2 Vs−1 is predicted for the perylene SURMOF-2. The results show a promising pathway towards controlling excimer photophysics in a MOF while controlling charge carrier mobility on the basis of a predictive model.

(Keynote) Electromagnetic Field Stimulated MOFs

Electromagnetic, external stimuli can be used to switch Metal-Organic Framework (MOF) materials and to achieve control over the pores of these materials on different ways. We designed MOFs with switchable moieties in their side-chains, such as azobenzene, to be switchable under irradiation of UV/Vis light. Additionally, we developed devices to be able to measure the impact of electromagnetic fields in-situ. We focused on surface anchored MOF (SURMOF) thin films, deposited on asymmetric, porous α-Al2O3 supports as gas separation membrane layers. It is possible to control the gas transport properties of the materials using, but also using electric fields, and measure changes gas separation (permeability and selectivity) of binary mixtures in-situ, using the Wicke-Kallenbach technique. The light switchable systems were investigated on their CO2 separation performance: By switching the azobenzene between cis and trans isomers inside the pore windows, it was possible to switch dipolar moments, thus changing the adsorption properties towards the quadrupolar moment of CO2. Using electric fields of 500 V/mm we could show on the prominent example of ZIF-8, a zeolitic imidazolate framework, that it has a strong impact on the gas transport properties, especially looking at the molecular sieving. The mechanistic of the electric field influence on the ZIF is as follows: The electric field distorts the lattice of the MOF, shifting the Zn2+ ions, lowering the symmetry of the unit cell. This effect leads to polarized polymophs of the structure, sharpening the molecular sieving through a stiffening effect. The purification of Propylene/Propane, a highly desired separation for the plastics industry, could be increased through the electric field stimuli by 33%.References Knebel, A. et al. Defibrillation of soft porous metal-organic frameworks with electric fields. Science 358, 347–351; 10.1126/science.aal2456 (2017).Wang, Z. et al. Tunable molecular separation by nanoporous membranes. Nat. Commun. 7, 13872; 10.1038/ncomms13872 (2016).Müller, K. et al. Switching Thin Films of Azobenzene-Containing Metal-Organic Frameworks with Visible Light. Chem. Eur. J. 23, 5434–5438; 10.1002/chem.201700989 (2017).Knebel, A. et al. Azobenzene Guest Molecules as Light-Switchable CO 2 Valves in an Ultrathin UiO-67 Membrane. Chem. Mater. 29, 3111–3117; 10.1021/acs.chemmater.7b00147 (2017).Tan, N. Y. et al. Investigation of the terahertz vibrational modes of ZIF-8 and ZIF-90 with terahertz time-domain spectroscopy. Chem Commun 51, 16037–16040; 10.1039/c5cc06455d (2015). Figure 1

(Invited) The Interplay of Conformation and Electronic Structure in Metal Organic Frameworks

Metal organic frameworks (MOFs) have emerged as a versatile class of designer crystals with tunable properties. Due to their vast chemical composition space and key importance of molecular structure and organization on their electronic properties, modelling and simulation are essential to understand and to optimize MOF functionality. Here, we show that multiscale models that combine structural simulations with electronic structure calculations can be used to explain the origin of the photophysical and conducting behavior of MOF films as the function of their nano/microscale morphology and to tune the specific property by predictive crystal engineering.Using the density functional theory formalism and our in-house ab initio method, we investigate efficient excited-state transport properties of porphyrin SURMOF [1] and photoconduction of MOF with embedded acceptors like C60 [2]. We predict the charge transport and mobility in polycyclic aromatic hydrocarbons based on the surface-anchored metal-organic frameworks (SURMOF) as a function of the type of the organic linker. We show the reason for the improved conductivity as derived from the spatially ordered structure of a material and confirm the charge transfer anisotropy through the detailed analysis of the transfer rates for electron and hole transport.To achieve unprecedented PL quantum yields for crystalline core-substituted naphthalenediimides (cNDIs) MOF, we determined the optimal alignment of chromophoric linkers to yield highly emissive J-aggregates [3]. We tuned the molecular alignment of linkers by introducing adjustable “steric control units” (SCUs) controlling the excitonic coupling and computationally screened the large library of handle-equipped MOF chromophoric linkers and for the best SCUs.Together with experimental observations, we prove that ab initio calculations enable valuable predictions of new promising candidates for emitting and/or semiconducting organic materials made in spatially ordered fashion in the SURMOF. Our data demonstrates the feasibility of MOF-based crystal engineering approaches that can be universally applied to tailor the materials properties.

Formation and Antibacterial Performance of Metal–Organic Framework Films via Dopamine-Mediated Fast Assembly under Visible Light

Metal–organic frameworks (MOFs) have recently attracted intensive interest toward a variety of potential applications due to their very high porosity. However, the preparation of MOF films is tedio...

Growth of ZIF-8 MOF Films with Tunable Porosity by using Poly (1-vinylimidazole) Brushes as 3D Primers.

This work reports on a novel and versatile approach to control the structure of metal-organic framework (MOFs) films by using polymeric brushes as 3D primers, suitable for triggering heterogeneous MOF nucleation. As a proof-of-concept, this work explores the use of poly(1-vinylimidazole) brushes primer obtained via surface-initiated atom transfer radical polymerization (SI-ATRP) for the synthesis of Zn-based ZIF-8 MOF films. By modifying the grafting density of the brushes, smooth porous films were obtained featuring inherently hydrophobic microporosity arising from ZIF-8 structure, and an additional constructional interparticle mesoporosity, which can be employed for differential adsorption of targeted adsorbates. It was found that the grafting density modulates the constructional porosity of the films obtained; higher grafting densities result in more compact structures, while lower grafting density generates increasingly inhomogeneous films with a higher proportion of interparticle constructional porosity.

Iontronics Using V2CTx MXene-Derived Metal-Organic Framework Solid Electrolytes.

Electronic applications of porous metal–organic frameworks (MOFs) have recently emerged as an important research area. However, there is still no report on using MOF solid electrolytes in iontronics, which could take advantage of the porous feature of MOFs in the ionic transport. In this article, MXene-derived two-dimensional porphyrinic MOF (MX-MOF) films are demonstrated as an electronic-grade proton-conducting electrolyte. Meanwhile, the MX-MOF film shows high quality, chemical stability, and capability of standard device patterning processes (e.g., dry etching and optical and electron beam lithography). Using the commercialized nanofabrication processes, an electric double-layer (EDL) transistor is demonstrated using the MX-MOF film (derived from V2CTx MXene) as an ionic gate and MoS2 film as a semiconducting channel layer. The EDL transistor, operated by applying an electric field to control the interaction between ions and electrons, is the core device platform in the emerging iontronics field. Therefore, The MX-MOF, confirmed as a solid electrolyte for EDL transistor devices, could have a significant impact on iontronics research and development.

On-Chip Template-Directed Conversion of Metal Hydroxides to Metal-Organic Framework Films with Enhanced Adhesion.

Interfacial compatibility between metal-organic framework (MOF) films and the underlying substrates determines the integrity of MOF films and their associated functions, and thus it has been gaining growing attention. Herein, we present a comparison of adhesion properties at the chip level of two disparate nickel (Ni)-MOF films, respectively, obtained by direct hydro/solvothermal growth and template-directed conversion approaches. We demonstrate that the on-chip delamination/corrugation of the films obtained by the direct growth approach can be circumvented by adopting the template-directed approach, which enables delicate dissolution of primarily grown nanoflaked nickel hydroxide (Ni(OH)2) films and thus triggers the controllable formation of Ni-MOF films. Successful on-chip conversions of Ni(OH)2 layers to different Ni-MOF thin films with good homogeneity, compactness, and appreciable affinity to the substrates are verified by multiple microscopic and spectroscopic techniques. Notably, the resultant Ni-MOF films do not show delamination even after activation with additional treatments, such as solvent soaking, nitrogen (N2) blowing for 1 h, and scotch-tape tests. As a demonstration of the application of MOF films, a Ni-NDC (NDC stands for 2,6-naphthalenedicarboxylate) MOF-coated sensor exhibits selective detection toward benzene vapor. This study highlights the importance of interfaces between MOF films and substrates and provides new perspectives for integrating MOF films onto microelectronic devices with robust adhesion for practical applications.

Growth of Crystalline Bimetallic Metal-Organic Framework Films via Transmetalation.

Crystalline films of the Cu3(BTC)2 (BTC3- = 1,3,5-benzenetricarboxylate) metal-organic framework (MOF) have been grown by dip-coating an alumina/Si(111) substrate in solutions of Cu(II) acetate and the organic linker H3BTC. Atomic force microscopy (AFM) experiments demonstrate that the substrate is completely covered by the MOF film, while grazing incidence wide-angle X-ray scattering (GIWAXS) establishes the crystallinity of the films. Forty cycles of dip-coating results in a film that is ∼70 nm thick with a root mean squared roughness of 25 nm and crystallites ranging from 50-160 nm in height. Co2+ ions were exchanged into the MOF framework by immersing the Cu3(BTC)2 films in solutions of CoCl2. By varying the temperature and exchange times, different concentrations of Co were incorporated into the films, as determined by X-ray photoelectron spectroscopy experiments. AFM studies showed that morphologies of the bimetallic films were largely unchanged after transmetalation, and GIWAXS indicated that the bimetallic films retained their crystallinity.

Advancing Metal‐Organic Frameworks toward Smart Sensing: Enhanced Fluorescence by a Photonic Metal‐Organic Framework for Organic Vapor Sensing

AbstractPolymer‐based colloidal crystals are promising materials for the enhancement of fluorescent emissions, however, due to limited porosity, they are not yet readily deployed in chemical vapor sensing. On the contrary, metal‐organic frameworks (MOFs) are highly porous with an exceptional ability to detect a wide range of toxic chemicals through fluorescence. Here, it is shown for the first time how enhanced fluorescence of a fluorophore can be achieved by incorporating a fluorescent dye into a template‐free periodically structured MOF film. A 200‐fold fluorescent intensity amplification is observed when Nile red (NR) dye is adsorbed on zeolitic imidazolate framework‐8 (ZIF‐8) particles and is then assembled into a colloidal crystal film (NR∼ccZIF‐8) when compared to the disordered film (NR∼ZIF‐8). The NR∼ccZIF‐8 film shows a response to 100 ppm of acetone vapor at room temperature with a detection limit of 60 ppm. The NR∼ccZIF‐8 film shows more than 40 times higher sensitivity to toluene vapor at 770 ppm than NR adsorbed on polystyrene particles forming a colloidal crystal film. From this study, it is expected that MOFs will play a role in the future development of smart sensors for chemical and biomedical applications.