Article

Surface Engineering of Metal Organic Framework Prepared on Film Bulk Acoustic Resonator for Vapor Detection

Article

Surface Engineering of Metal Organic Framework Prepared on Film Bulk Acoustic Resonator for Vapor Detection

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Abstract

Gravimetric resonators based on Micro/Nanoelectromechanical System (M/NEMS) are potential candidates in developing smaller, less expensive, and higher performance gas sensors. Metal organic frameworks (MOFs), with high surface areas, recently come into focus as advanced nano-porous sensitive materials in micro gravimetric gas sensors. The surface of MOFs on those sensors is critical in offering water stability and varying absorption behaviors. However, the influences of surface on sensing performance are less explored and the strategy to tune surface properties of MOFs mounted on gravimetric resonators is still rare. In this paper, a straightforward strategy to engineer surface properties of MOFs, specifically Cu3(benzenetricarboxylate)2 (known as HKUST-1), is reported and the surface hydrophilicity/hydrophobicity of HKUST-1 is tuned by chemical vapor deposition combined with monolayer self-assembly. It is found out that the hybrid inorganic and organic surface engineering strategy not only preserves the absorption capacity of inner MOFs but also significantly enhances sensor stability toward water.

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The moisture sensitivity of many metal-organic frameworks (MOFs) poses a critical issue for their large-scale real application. One of the most effective methods to solve this problem is to convert the surface of MOFs from hydrophilic to hydrophobic. Herein, we develop a general strategy to modify hydrophobic polydimethysiloxane (PDMS) on the surface of MOF materials to significantly enhance their moisture or water resistance by a facile vapor deposition technique. MOF-5, HKUST-1, and ZnBT as representative vulnerable MOFs were successfully coated by PDMS, and these coated samples well inherited their original crystalline nature and pore characteristics. Strikingly, the surface areas of these MOFs were nearly 100% retained upon PDMS-coating. Such a coating process might render MOFs applicable in the presence of water or humidity in extended fields such as gas sorption and catalysis.
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Metal-Organic Frameworks (MOFs) constitute a new generation of porous crystalline materials, which have recently come into focus as analyte-specific active elements in thin film sensor devices. Cu-BTC - also known as HKUST-1 - is one of the most theoretically and experimentally investigated members of the MOF family. Its capability to selectively adsorb different gas molecules has already been reported and this property renders this material a promising candidate for applications in chemical gas and vapour sensing. Detailed knowledge about the gas adsorption mechanisms of a material used for gas sensing is crucial to achieve good sensor performance, and it governs the choice of the appropriate transducer suitable for reliable gas detection. In this paper mass sensitive and work function based readouts were successfully applied for the detection of low ppm (2 to 50 ppm) concentrations of different alcohols (methanol, ethanol, 1-propanol and 2-propanol) with Cu-BTC. The evaluation of the results allows for the comparison of the amounts of adsorbed vapours and the contribution of each vapour to the changes of the electronic properties of Cu-BTC. The influence of the length of the alcohol chain (C1-C3) and geometry (1-propanol, 2-propanol) on the sensing performance was investigated and different adsorption mechanisms in dry and humid synthetic air are proposed based on the obtained characteristic adsorption curves.
Article
Surface-initiated polymerization has been used to grow thick, uniform poly(methyl methacrylate) (PMMA) films on nanocantilever sensors. Cantilevers with these coatings yielded significantly greater sensitivity relative to bare devices as well as relative to devices that had been coated with dropcast polymer films. The devices with surface-initiated polymer films also demonstrated high selectivity towards polar analytes. Surface-initiated polymerization can therefore provide a straightforward, reproducible method for large-scale functionalization of nanosensors.
Article
These last 10 years, smaller, less expensive, and higher performance sensors are required for gas sensing applications. To date no true detection principle has been recognized as the best candidate for such application. Microsytems or Micro/Nano ElectroMechanical Systems (M/NEMS) used as gravimetric detectors are among the probable candidates. The technology can indeed be manufactured en masse and can provide multi-gas analysing platform. In this paper, we present a comprehensive overview of micro/nano sensors based on the gravimetric effect to detect an absorbed gas on top of their surfaces. The paper provides a comparison between different electromechanical devices (Bulk Acoustic Wave, Surface Acoustic Wave, Capacitive Micro-machined Ultrasonic Transducer, Micro/Nano cantilevers) with an introduction to gas adsorption mechanisms, material selection, detection principles and design guidance useful to researchers or engineers.
Article
The adsorption characteristics of a series of planar (ethylene, benzene, and pyridine) and tetrahedral (methane, chloromethane, dichloromethane, chloroform, and carbon tetrachloride) molecules on a carbon molecular sieve used for air separation (CMS A) were investigated over a range of temperatures as a function of pressure, to study the selective porosity. The size-exclusion characteristics of planar and tetrahedral molecules indicate that the selective porosity behaves as though it has spherical-shaped structural characteristics and, therefore, two minimum dimensions need to be considered in relation to size exclusion. The partial exclusion of the probe molecule adsorptives from the microporous structure for sizes >360 pm allowed the selective and nonselective microporosity and the meso/macroporosity to be quantified. Adsorption kinetics obey a linear-driving-force mass transfer, combined barrier resistance/diffusion, or Fickian kinetic models depending on the adsorptive and experimental conditions. Comparison of the results with previous studies of the adsorption of linear and spherical molecules on CMS A show that a decrease in the adsorption rate constant of 4 orders of magnitude was observed for an increase in molecular size over the range 290−420 pm. The activation energies and preexponential factors for the adsorption kinetics and the isosteric heats of adsorption are discussed in terms of the structural characteristics of the adsorptives and the adsorption mechanism.
Article
An acetylcholinesterase-coated thin film bulk acoustic resonator has been developed for the detection of organophosphorus pesticides. The thin film bulk acoustic resonator acts as a robust mass-sensitive transducer for bio-sensing. This device works in thickness shear mode with a resonance at 1.97GHz. The detection is based on the inhibitory effects of organophosphorus compounds on the enzymatic activity of the acetylcholinesterase immobilized on one of the faces of the acoustic resonator. The enzyme reaction in the substrate solution and the inhibitory effect is observed are real time by measuring the frequency shift. The presence of organophosphorus pesticides can be detected from the diminution of the frequency shift compared with the levels found in their absence. The device exhibits linear responses, good reproducibility, simple operation, portability and a low detection limit of 5.3×10(-11)M for paraoxon. The detection results of organophosphorus pesticide residues in practical samples show that the proposed sensor has the feasibility and sensing accuracy comparable to gas chromatography.
Article
The use of metal-organic framework (MOF) thin films to detect water vapor across a wide concentration range is demonstrated using MOF-functionalized quartz surface acoustic wave (SAW) sensors. A range of 3-14,800 ppmv was obtained with thin films of the MOF Cu(3)(benzenetricarboxylate)(2) (Cu-BTC) deposited by an automated layer-by-layer method. Devices coated by a manual technique demonstrated sensitivity from 0.28 to 14,800 ppmv, the limit of our test system. This exceeds the sensitivity of many commercially available sensors. Cu-BTC layers were covalently bonded directly to the silicon oxide surface, allowing devices to be heated beyond 100 °C to desorb water adsorbed in the pores without decomposition, thereby regenerating the sensors. Sensor response as a function of coating thickness was evaluated, showing that the SAW sensor response is bounded by maximum and minimum layer thicknesses. Computer simulation of H(2)O uptake shows a multistep adsorption isotherm defined by initial adsorption at open Cu-sites, followed by pore-filling and finally full saturation. Modeling and experimental results are consistent. Calculated uptake values suggest an efficient adsorption of H(2)O by Cu-BTC. These results provide the first convincing evidence that MOF functionalization of compact sensing technologies such as SAW devices and microcantilevers can compete with state-of-the art devices.
Article
Wird eine Fremdschicht auf eine zu Dickenscherungsschwingungen angeregte Schwingquarzplatte aufgebracht, so ndert sich die Eigenfrequenz der Platte infolge Vergrerung der schwingenden Masse. Da die Frequenznderung eines Schwingquarzes sehr genau vermessen werden kann, ergibt sich daraus eine sehr empfindliche Methode zur Wgung dnner Schichten.Massenbelegung der Fremdschicht und Frequenznderung sind einander proportional. Die Proportionalittskonstante lt sich aus der Eigenfrequenz des Schwingquarzes berechnen, so da eine empirische Eichung bei der Schichtwgung mit Schwingquarzen entfllt.Die Genauigkeit des Schichtwgeverfahrens ist in erster Linie durch die Temperaturabhngigkeit der Quarzeigenfrequenz begrenzt und betrgt bei 1 C zugelassener Temperaturschwankung etwa 4 10–9 g cm–2. Das entspricht einer mittleren Dicke von 0,4 bei der Dichte =1 g cm–3.Das Verfahren wurde auch zur direkten Wgung einer Masse ausgenutzt (Mikrowgung). Dabei lie sich eine Genauigkeit von 10–10g erreichen.
Article
We synthesized and tested four different monolayer protected gold nanoclusters (MPCs) as chemically selective interfaces for an organic vapor sensor array. The ligands chosen for capping the nano-Au particles and for selective organic vapor sorption were octanethiol, 2-naphthalenethiol, 2-benzothiazolethiol and 4-methoxythiolphenol. The same set of gold nanoclusters were tested on two different types of sensor platforms, a chemiresistor (CR) and a quartz crystal microbalance (QCM). The sensing properties of both sensor arrays were investigated with 10 organic vapors of various functional groups. Vapor sensing selectivity, dominated by the shell ligand structure of MPC, was demonstrated. The sensitivities of MPC coated CR are better than those of QCM sensors coated with the same material. The average CR/QCM amplification factors are range from 1.9 for 4-methoxythiolphenol MPC to 16.9 for octanethiol MPC. These differences in amplification factors indicate the functional group specific mechanisms for each vapor–MPC pair. The shell penetration mechanism of hydrogen-bonding vapor molecules into the 2-benzothiazolethiol capped MPC reduced the CR/QCM amplification factors. Strong attraction between MPC shell ligands can also reduce the magnitude of resistance changes during vapor sorption.
Article
A mass sensor based on thin-film bulk acoustic resonator, intended for biomolecular applications, is presented. The thin film is a (002) AlN membrane, sputtered over Ti/Pt on a (001) Si wafer, and released by surface micromachining of silicon. Two experiments are proposed to test the mass sensing performance of the resonators: (a) distributed loading with a MgF(2) film by means of physical vapor deposition and (b) localized mass growing of a C/Pt/Ga composite using focused-ion-beam-assisted deposition, both on the top electrode. For the distributed and localized cases, the minimum detectable mass changes are 1.58x10(-8) g/cm(2) and 7x10(-15) g, respectively. (c) 2006 American Institute of Physics.
Article
We present an application of film bulk acoustic resonator for trace nerve gas sensing. The resonator consisted of a piezoelectric Au/AlN/Mo stack and a Bragg reflector has a working resonance near 2.35 GHz and a high performance. A self-assembled composite layer of Cu2+/11-mercaptoundecanoic acid is modified on the Au electrode as a specific coating to capture organophosphorus compounds. The experimental results show that the chemical modified film bulk acoustic resonator can yield a rapid, sensitive, reversible and reproducible response to dimethyl methyl phosphonate (a stimulant of nerve agent) vapor. When the sensitive layer absorbs organophosphorus compounds, the mass loading increases and the resonance frequency proportionally goes down. The chemical modified film bulk acoustic resonator sensor is able to detect dimethyl methyl phosphonate vapor at the concentration as low as 100 ppb. The advantages of this sensor, including the simple fabrication process, ease of detection method, rapid response and high sensitivity, make this promising in the early alarm of nerve agents.
Article
Piezoelectric microelectromechanical systems (MEMS) resonant sensors, known for their excellent mass resolution, have been studied for many applications, including DNA hybridization, protein-ligand interactions, and immunosensor development. They have also been explored for detecting antigens, organic gas, toxic ions, and explosives. Most piezoelectric MEMS resonant sensors are acoustic sensors (with specific coating layers) that enable selective and label-free detection of biological events in real time. These label-free technologies have recently garnered significant attention for their sensitive and quantitative multi-parameter analysis of biological systems. Since piezoelectric MEMS resonant sensors do more than transform analyte mass or thickness into an electrical signal (e.g., frequency and impedance), special attention must be paid to their potential beyond microweighing, such as measuring elastic and viscous properties, and several types of sensors currently under development operate at different resonant modes (i.e., thickness extensional mode, thickness shear mode, lateral extensional mode, flexural mode, etc.). In this review, we provide an overview of recent developments in micromachined resonant sensors and activities relating to biochemical interfaces for acoustic sensors.
Article
Innovative approaches of materials and transducers based on selective wireless gas sensing are being developed to solve the problems of sensor selectivity in realistic conditions. The development of sensing materials based on new materials design concepts and new materials fabrication principles will produce a large impact for new sensor system. Significant work conducted on carbon nanotubes and graphene is aimed to meet the challenge of finding a functionalization approach that will suppress humidity effects. Assembly of porphyrins and related molecules into polymers and nanostructures could bring new sensing capabilities due to the possible surpamolecular nature of these assemblies and charge distribution between individual molecules. The importance of studies on improvements of stability of sensing materials will increase because of the need for wireless sensing.
Article
Metal-organic frameworks (MOFs) are a fascinating class of novel inorganic-organic hybrid materials. They are essentially based on classic coordination chemistry and hold much promise for unique applications ranging from gas storage and separation to chemical sensing, catalysis, and drug release. The evolution of the full innovative potential of MOFs, in particular for nanotechnology and device integration, however requires a fundamental understanding of the formation process of MOFs. Also necessary is the ability to control the growth of thin MOF films and the positioning of size- and shape-selected crystals as well as MOF heterostructures on a given surface in a well-defined and oriented fashion. MOFs are solid-state materials typically formed by solvothermal reactions and their crystallization from the liquid phase involves the surface chemistry of their building blocks. This Review brings together various key aspects of the surface chemistry of MOFs.
Article
Metal-organic frameworks (MOFs) represent a new class of hybrid organic-inorganic supramolecular materials comprised of ordered networks formed from organic electron donor linkers and metal cations. They can exhibit extremely high surface areas, as well as tunable pore size and functionality, and can act as hosts for a variety of guest molecules. Since their discovery, MOFs have enjoyed extensive exploration, with applications ranging from gas storage to drug delivery to sensing. This review covers advances in the MOF field from the past three years, focusing on applications, including gas separation, catalysis, drug delivery, optical and electronic applications, and sensing. We also summarize recent work on methods for MOF synthesis and computational modeling.
Article
Miniaturized gas chromatography (GC) systems can provide fast, quantitative analysis of chemical vapors in an ultrasmall package. We describe a chemical sensor technology based on resonant nanoelectromechanical systems (NEMS) mass detectors that provides the speed, sensitivity, specificity, and size required by the microscale GC paradigm. Such NEMS sensors have demonstrated detection of subparts per billion (ppb) concentrations of a phosphonate analyte. By combining two channels of NEMS detection with an ultrafast GC front-end, chromatographic analysis of 13 chemicals was performed within a 5 s time window.
Article
Covalently bonded monolayers of two monofunctional aminosilanes (3-aminopropyldimethylethoxysilane, APDMES, and 3-aminopropyldiisopropylethoxysilane, APDIPES) and one trifunctional aminosilane (3-aminopropyltriethoxysilane, APTES) have been deposited on dehydrated silicon substrates by chemical vapor deposition (CVD) at 150 °C and low pressure (a few Torr) using reproducible equipment. Standard surface analytical techniques such as x-ray photoelectron spectroscopy (XPS), contact angle goniometry, spectroscopic ellipsometry, atomic force microscopy, and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) have been employed to characterize the resulting films. These methods indicate that essentially constant surface coverages are obtained over a wide range of gas phase concentrations of the aminosilanes. XPS data further indicate that the N1s/Si2p ratio is higher after CVD with the trifunctional silane (APTES) compared to the monofunctional ones, with a higher N1s/Si2p ratio for APDMES compared to that for APDIPES. AFM images show an average surface roughness of 0.12- 0.15 nm among all three aminosilane films. Stability tests indicate that APDIPES films retain most of their integrity at pH 10 for several hours and are more stable than APTES or APDMES layers. The films also showed good stability against storage in the laboratory. ToF-SIMS of these samples showed expected peaks, such as CN(-), as well as CNO(-), which may arise from an interaction between monolayer amine groups and silanols. Optical absorption measurements on adsorbed cyanine dye at the surface of the aminosilane films show the formation of dimer aggregates on the surface. This is further supported by ellipsometry measurements. The concentration of dye on each surface appears to be consistent with the density of the amines.
Article
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.
Article
Many volatile organic compounds (VOCs), principally alkanes and benzene derivatives, have been identified in breath from patients with lung cancer. We investigated whether a combination of VOCs could identify such patients. We collected breath samples from 108 patients with an abnormal chest radiograph who were scheduled for bronchoscopy. The samples were collected with a portable apparatus, then assayed by gas chromatography and mass spectroscopy. The alveolar gradient of each breath VOC, the difference between the amount in breath and in air, was calculated. Forward stepwise discriminant analysis was used to identify VOCs that discriminated between patients with and without lung cancer. Lung cancer was confirmed histologically in 60 patients. A combination of 22 breath VOCs, predominantly alkanes, alkane derivatives, and benzene derivatives, discriminated between patients with and without lung cancer, regardless of stage (all p<0.0003). For stage 1 lung cancer, the 22 VOCs had 100% sensitivity and 81.3% specificity. Cross-validation of the combination correctly predicted the diagnosis in 71.7% patients with lung cancer and 66.7% of those without lung cancer. In patients with an abnormal chest radiograph, a combination of 22 VOCs in breath samples distinguished between patients with and without lung cancer. Prospective studies are needed to confirm the usefulness of breath VOCs for detecting lung cancer in the general population.
Article
Acoustic wave sensor arrays for gas-phase chemical sensing are reviewed. For this purpose, papers on acoustic waver sensor arrays are cited according to some given criteria. Original studies and recent progress in polymer-coated acoustic wave sensor arrays are highlighted, with emphasis on issues such as interaction mechanisms, chemical diversity, coating selection approaches, array design, and multivariate data analysis.
Article
Desolvation of Ni(2)(4,4'-bipyridine)(3)(NO(3))(4).2CH(3)OH and Ni(2)(4,4'-bipyridine)(3)(NO(3))(4).2C(2)H(5)OH give flexible metal-organic porous structures M and E, respectively, which have the same stoichiometry, but subtly different structures. This study combines measurements of the thermodynamics and kinetics of carbon dioxide, methanol, and ethanol sorption on adsorbents M and E over a range of temperatures with adsorbent structural characterization at different adsorbate (guest) loadings. The adsorption kinetics for methanol and ethanol adsorption on porous structure E obey a linear driving force (LDF) mass transfer model for adsorption at low surface coverage. The corresponding adsorption kinetics for porous structure M follow a double exponential (DE) model, which is consistent with two different barriers for diffusion through the windows and along the pores in the structure. The former is a high-energy barrier due to the opening of the windows in the structure, required to allow adsorption to occur, while the latter is a lower-energy barrier for diffusion in the pore cavities. X-ray diffraction studies at various methanol and ethanol loadings showed that the host porous structures E and M underwent different scissoring motions, leading to an increase in unit cell volume with the space group remaining unchanged during adsorption. The results are discussed in terms of reversible adsorbate/adsorbent (host/guest) structural changes and the adsorption mechanism involving hydrogen-bonding interactions with specific surface sites for methanol and ethanol adsorption in relation to pore size and extent of filling. This paper contains the first evidence for individual kinetic barriers to diffusion through windows and pore cavities in flexible porous coordination polymer frameworks.