Ronald J. T. Houk

Stress-induced chemical detection using flexible metal-organic frameworks.

In this work we demonstrate the concept of stress-induced chemical detection using metal-organic frameworks (MOFs) by integrating a thin film of the MOF HKUST-1 with a microcantilever surface. The results show that the energy of molecular adsorption, which causes slight distortions in the MOF crystal structure, can be converted to mechanical energy to create a highly responsive, reversible, and selective sensor. This sensor responds to water, methanol, and ethanol vapors, but yields no response to either N2 or O2. The magnitude of the signal, which is measured by a built-in piezoresistor, is correlated with the concentration and can be fitted to a Langmuir isotherm. Furthermore, we show that the hydration state of the MOF layer can be used to impart selectivity to CO2. Finally, we report the first use of surface-enhanced Raman spectroscopy to characterize the structure of a MOF film. We conclude that the synthetic versatility of these nanoporous materials holds great promise for creating recognition chemistries to enable selective detection of a wide range of analytes.

Silver Cluster Formation, Dynamics, and Chemistry in Metal−Organic Frameworks

Synthetic methods used to produce metal nanoparticles typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with the potential for both steric and chemical stabilization. We report a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions < or =1 nm, with a significant fraction existing as Ag(3) clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible.

Ordered metal nanostructure self-assembly using metal–organic frameworks as templates

We demonstrate that nanoporous metal–organic frameworks (MOFs) loaded with silver can serve as templates for ordered nanostructures comprising either silver nanoparticles or nanowires. Exposure to an electron beam breaks down the template, leading to rapid silver coalescence. The geometric and chemical structure of the MOF, as well as the extent of metal loading, determine whether nanoparticles or nanowires are formed and define their size and orientation. Nanowires with diameters as small as 4 nm and aspect ratios >125 can be formed, overcoming the limitations of existing templating methods. This method is relatively simple, compatible with many materials, and proceeds by a distinct template-directed growth mechanism. Since MOFs offer an unprecedented level of synthetic flexibility, combined with highly uniform porosity as a result of their crystalline structure, this approach opens a promising new route for synthesis of self-assembled, ordered nanostructures.

Electrophilic Coordination Catalysis : A Summary of Previous Thought and a New Angle of Analysis

One of the most common, and yet least well understood, enzymatic transformations is proton abstraction from activated carbon acids such as carbonyls. Understanding the mechanism for these proton abstractions is basic to a good understanding of enzyme function. Significant controversy has arisen over the means by which a given enzyme might facilitate these deprotonations. Creating small molecule mimics of enzymes and physical organic studies that model enzymes are good approaches to probing mechanistic enzymology. This Account details a number of molecular recognition and physical organic studies, both from our laboratory and others, dealing with the elucidation of this quandary. Our analysis launches from an examination of the active sites and proposed mechanism of several enzyme-catalyzed deprotonations of carbon acids. This analysis highlights the geometries of the hydrogen bonds found at the enzyme active sites. We find evidence to support pi-oriented hydrogen bonding, rather than lone pair oriented hydrogen bonding. Our observations prompted us to study the stereochemistry of hydrogen bonding that activates carbonyl alpha-carbons to deprotonation. The results from our own thermodynamic, kinetics, and computational studies, all of which are reviewed herein, suggest that an unanticipated level of intermediate stabilization occurs via an electrophilic interaction through the pi-molecular orbital as opposed to traditional lone pair directed coordination. We do not postulate that hydrogen bonding to pi-systems is intrinsically stronger than to lone pairs, but rather that there is a greater change in bond strength during deprotonation when the hydrogen bonds are oriented at the pi-system. Through these studies, we conclude that many enzymes preferentially activate their carbon acid substrates through an electrophilic coordination directed towards the pi-bond of the carbonyl rather than the conventional lone pair directed model.

Characterization of HKUST-1 Crystals and Their Application to MEMS Microcantilever Array Sensors

Metal organic frameworks (MOFs) are a new class of crystalline nanoporous materials with salient features such as tailorable nanoporosity, high surface area and analyte specific adsorption. Greater part of the research till now has focused on determining the adsorption isotherms of MOFs in a bid to use them in gas separation and chemical sensor applications. However there is modest data available on the thermodynamic properties of MOFs which are essential for these applications. In this paper, thermodynamic characterization of the well known MOF, HKUST-1 has been conducted using Quartz Crystal Microbalance (QCM). MOF coated stress induced microcantilever sensors have been successfully demonstrated for sensor application. HKUST-1 has the structure of formula Cu3(BTC)2(H2O) comprises a binuclear Cu2 paddlewheel [1]. Its structure consists of two types of “cages” and two types of “windows” separating these cages. Large cages (13.2 and 11.1 A° in diameter) are interconnected by 9 A° windows of square cross section. The large cages are also connected to tetrahedral shaped side pockets of roughly 6 A° through triangular shaped windows of about 4.6 A° (3.5 A° in the hydrated form). The thermodynamic properties of HKUST-1 such as heat of adsorption, adsorption capacity, adsorption affinity constant, diffusion coefficient and diffusion activation energy are determined using the QCM technique. The shift in resonant frequency of the crystal was used to calculate the adsorption isotherms from which the thermodynamic properties were determined. The resonant frequency shift was expressed in terms of mass change due to analyte adsorption using the saurbrey’s equation. In this paper, the MOFs were deposited on the QCM by dropcasting a suspension of the MOF crystals in aprotic solvent like acetone and characterization was carried out for CO2 and other gases. The experimental set up of the QCM based measurement cell is illustrated in Figure 1. Prior to the measurement, the sample was heated to 170°C under vacuum in the cell to remove moisture from the pores. N-doped piezoresistive microcantilever array sensors were fabricated using microfabrication technologies with dimensions 230 µm in length and 100 µm in width [2]. The response of the sensor for H2O and CO2 was measured in a custom designed gas test cell. Dry nitrogen was used as carrier gas and H2O was regulated using hydrator. The stress response was obtained by the subtraction of the uncoated reference microcantilever response from the MOF-coated cantilever response using a Wheatstone bridge. The microcantilever array chip was mounted in a stereolithography package for testing. Figure 2 shows typical response to CO2. The substantial resistance changes with MOFs exhibited a strong and completely reversible response for adsorption and desorption.

Electron beam synthesis of metal and semiconductor nanoparticles using metal?organic frameworks as ordered precursors

We demonstrate a versatile, bottom-up method of forming metal and semiconducting nanoparticles by exposing precursor metal-organic frameworks (MOFs) to an electron beam. Using a transmission electron microscope to initiate and observe growth, we show that the composition, size, and morphology of the nanoparticles are determined by the chemistry and structure of the MOF, as well as the electron beam properties. Zinc oxide, metallic indium and copper particles were produced with narrow and tunable size distributions comparable to those obtained from state-of-the-art methods. This method represents a first step toward the fabrication of nanoscale heterostructures using the highly controlled environment of the MOF pores as a scaffold or template.

Luminescent Metal-Organic Frameworks: A Nanolaboratory for Probing Energy Transfer via Interchromophore Interactions

Metal organic frameworks consist of three dimensional lattices of organic linkers and metal ions. They are being studied for diverse applications ranging from gas storage and separation, to catalysis, drug delivery, and sensing (1). While some of these applications have enjoyed exte platforms for luminescent sensors remains largely in its formative stages (2). Recently, our group reported the synthesis and photophysical properties of luminescent MOFs containing stilbene linkers (3). Studies of the fluorescence emission and ion-beam-induced luminescence spectra (4) of these materials reveal significant spectral changes that can be correlated with interchromophore distances and orientation within the MOF structure. As tuning the optical properties of MOFs is necessary for their use in sensor applications, we are probing the underlying relationships between MOF structure and luminescence to build a fundamental understanding of luminescence phenomena in MOFs.

Luminescent assays for ketones and aldehydes employing catalytic signal amplification

Herein we report the first use of transition metal catalytic signal enhancement for the analysis of small organic analytes. Two assays using Sonogashira and Suzuki cross-couplings have been used in the detection of ketones and aldehydes produce highly luminescent markers. The latter analysis utilizing the Suzuki coupling demonstrates the first use of peroxyoxalate initiated chemiluminescence in a sensing application. Chemiluminescent measurement revealed much higher sensitivity than fluorescence.

Scintillating Metal Organic Frameworks: A New Class of Radiation Detection Materials

The detection and identification of subatomic particles is an important scientific problem with implications for medical devices, radiography, biochemical analysis, particle physics, and astrophysics. In addition, the development of efficient detectors of neutrons generated by fissile material is a pressing need for nuclear nonproliferation and counterterrorism efforts. A critical objective in the field of radiation detection is to develop the physical insight necessary to rationally design new scintillation materials for specific applications. However, none of the material types currently used in has sufficient synthetic versatility to exert systematic control over the factors controlling the light output and its dynamics. Here we describe a spectroscopic investigation of two stilbene-based metal-organic frameworks (MOFs) we synthesized, demonstrating that they emit light in response to ionizing radiation, creating the first completely new class of scintillation materials since the advent of plastic scintillators in 1950. This highly novel and unexpected property of MOFs opens a new route to rational design of radiation detection materials, since the spectroscopy shows that both the luminescence spectrum and its timing can be varied by altering the local environment of the chromophore within the MOF. Therefore, the inherent synthetic flexibility of MOFs, which enables both the chromophore structure and its local environment to be systematically varied, suggests that this class of materials can serve as a controlled “nanolaboratory” for probing a broad range of photophysical and radiation detection phenomena. In this presentation we report on the time-dependent fluorescence and radioluminescence of these MOFs and related structures. Multiple decay characteristics have been observed for some materials under study, including fast (ns) exponential and slow (microsecond) non-exponential components. We interpret the results in terms of the electronic states, crystal structures, intermolecular interactions, and transport effects mediating the luminescence. The potential for particle discrimination schemes and large scale production of MOFs and will be discussed.

The Metal Organic Films for Tailorable Chemical Sensing on Microcantilevers

A new generation of versatile microcantilever sensors with tailorable nanoporous metal-organic frameworks (MOFs) has been successfully demonstrated. Two metal-organic frameworks (MOFs), HKUST-1 and MOF-508 were characterized to demonstrate new stress-induced sensing mechanism. The microcantilever sensors were fabricated on silicon-on-insulator (SOI) wafers with 1 µm thickness of the final beam. The results exhibited extraordinary sensitivity, and a selective reversible response for chemical detection (1). Metal-organic frameworks (MOFs) are attracting attention as the ideal material for sensing applications because of their nanoporosity, ultrahigh surface area, and the fact that they can be tailored for adsorption of specific analytes (1). In this work, HKUST-1 was prepared and characterized to demonstrate that different MOFs can detect multiple analytes with microcantilever array (2). The elastic properties of MOFs were measured by nanoindentation test (MTS Nano-Indentor XP) and Raman microscope (Renishaw) was used to characterize coated MOFs-thin films on gold compared with MOFs powders.