Microchip gas chromatography column using magnetic beads coated with polydimethylsiloxane and metal organic frameworks.

Micro gas chromatography (µGC) using microfabricated silicon columns has been developed in response to the requirement for portable on-site gas analysis. Although different stationary phases have been developed, repeatable and reliable surface coatings in these rather small microcolumns remains a challenge. Herein, a new stationary phase coating strategy using magnetic beads (MBs) as carriers for micro column is presented. MBs modified with organopolysiloxane (MBs@OV-1) and a metal organic framework (MBs@HKUST-1) are deposited in on-chip microcolumns assisted with a magnetic field with an optimized modification process. MBs@OV-1 column showed a minimum HETP of 0.074 cm (1351 plates/m) of 62 cm/s. Mixtures of volatile organic compounds are successfully separated using MBs carried stationary phase which demonstrates that this technique has good chromatographic column efficiency. This method not only provides a novel coating process, washing and characterization of the stationary phases but also establishes a straightforward strategy for testing new absorbent materials for µGC systems.

On-Chip Monolithic Integrated Multimode Carbon Nanotube Sensor for a Gas Chromatography Detector.

Carbon nanotube (CNT)-based chemiresistors are promising gas detectors for gas chromatography (GC) due to their intrinsic nanoscale porosity and excellent electrical conductivity. However, fabrication reproducibility, long desorption time, limited sensitivity, and low dynamic range limit their usage in real applications. This paper reports a novel on-chip monolithic integrated multimode CNT sensor, where a micro-electro-mechanical system-based bulk acoustic wave (BAW) resonator is embedded underneath a CNT chemiresistor. The device fabrication repeatability was improved by on-site monitoring of CNT deposition using BAW. We found that the acoustic stimulation can accelerate the gas desorption rate from the CNT surface, which solves the slow desorption issue. Due to the different sensing mechanisms, the multimode CNT sensor provides complementary responses to targets with improved sensitivity and dynamic range compared to a single mode detector. A prototype of a chromatographic system using the multimode CNT sensor was prepared by dedicated design of the connection between the device and the separation column. Such a GC system is used for the quantitative identification of a gas mixture at different GC conditions, which proves the feasibility of the multimode CNT detector for chromatographic analysis. The as-developed CMOS compatible multimode CNT sensor offers high sensing performance, miniaturized size, and low power consumption, which are critical for developing portable GC.

In-Line Detection with Microfluidic Bulk Acoustic Wave Resonator Gas Sensor for Gas Chromatography.

A microfluidic film bulk acoustic wave resonator gas sensor (mFBAR) adapted specifically as an in-line detector in gas chromatography was described. This miniaturized vapor sensor was a non-destructive detector with very low dead volume (0.02 µL). It was prepared by enclosing the resonator in a microfluidic channel on a chip with dimensions of only 15 mm × 15 mm × 1 mm. The device with polymer coating showed satisfactory performance in the detection of organophosphorus compound, demonstrating a very low detection limit (a dozen parts per billion) with relatively short response time (about fifteen seconds) toward the simulant of chemical warfare agent, dimethyl methylphosphonate. The in-line detection of the mFBAR sensor with FID was constructed and employed to directly measure the concentration profile on the solid surface by the mFBAR with the controlled concentration profile in the mobile phase at the same time. The difference of peak-maximum position between mobile phase and solid phase could be a convenient indicator to measure mass transfer rate. With the response of the mFBAR and FID obtained in one injection, an injection mass-independent parameter can be calculated and used to identify the analyte of interest.

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

Gravimetric resonators based on micro/nanoelectromechanical systems (M/NEMS) are potential candidates in developing smaller, less expensive, and higher-performance gas sensors. Metal-organic frameworks (MOFs) with high surface areas have recently come into focus as advanced nanoporous sensitive materials in microgravimetric gas sensors. The surface of MOFs on those sensors is critical in offering water stability and varying absorption behaviors. However, the influences of the 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 was found that the hybrid inorganic and organic surface engineering strategy not only preserves the absorption capacity of inner MOFs but also significantly enhances the sensor's stability toward water.

A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated by Soft Nano-Lithography.

Flexible ammonia (NH3) sensors based on one-dimensional nanostructures have attracted great attention due to their high flexibility and low power consumption. However, it is still challenging to reliably and cost-effectively fabricate ordered nanostructure-based flexible sensors. Herein, a smartphone-enabled fully integrated system based on a flexible nanowire sensor was developed for real-time NH3 monitoring. Highly aligned, sub-100 nm nanowires on a flexible substrate fabricated by facile and low-cost soft lithography were used as sensitive elements to produce impedance response. The detection signals were sent to a smartphone and displayed on the screen in real time. This nanowire-based sensor exhibited robust flexibility and mechanical durability. Moreover, the integrated NH3 sensing system presented enhanced performance with a detection limit of 100 ppb, as well as high selectivity and reproducibility. The power consumption of the flexible nanowire sensor was as low as 3 µW. By using this system, measurements were carried out to obtain reliable information about the spoilage of foods. This smartphone-enabled integrated system based on a flexible nanowire sensor provided a portable and efficient way to detect NH3 in daily life.

Hierarchical assembly of gold nanorod stripe patterns for sensing and cells alignment.

Hierarchical assemblies of nanomaterial superstructures with controlled orientation affords a multitude of novel properties of plasmonics and broad applications. Yet constructing multi-functional superstructures with nanoparticles positioned in desired locations remains challenging. Herein, gold nanorods (GNRs) assembled in stripe patterns with controlled orientation and structures in millimeter scale for versatile application have been achieved. Applications of patterned GNRs in sensing enhancement and engineering mammalian cells alignment are investigated experimentally. The performance of patterned GNRs in surface enhanced Raman scattering (SERS) and electrical sensing are found in orientational dependence. The SERS signals of vertically arranged GNR arrays exhibit double the folder intensity than those horizontally arranged. In contrast, the horizontally arranged GNRs exhibit twice as much electrical conductivity. The system is further explored to pattern mammalian cells. For the first time, we reveal the nanostructured topography of GNR confined cells to a specific region, and direct the adhesion and extension of living cells, which opens up broad applications in tissue engineering and biosensing.

A chemiresistive sensor array from conductive polymer nanowires fabricated by nanoscale soft lithography.

One-dimensional organic nanostructures are essential building blocks for high performance gas sensors. Constructing an e-nose type sensor array is the current golden standard in developing portable systems for the detection of gas mixtures. However, facile fabrication of nanoscale sensor arrays is still challenging due to the high cost of the conventional nanofabrication techniques. In this work, we fabricate a chemiresistive gas sensor array composed of well-ordered sub-100 nm wide conducting polymer nanowires using cost-effective nanoscale soft lithography. Poly(3,4-ethylene-dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) nanowires functionalized with different self-assembled monolayers (SAMs) are capable of identifying volatile organic compounds (VOCs) at a low concentration range. The side chains and functional groups of the SAMs introduce different sensitivities and selectivities to the targeted analytes. The distinct response pattern of each chemical is subjected to pattern recognition protocols, which leads to a clear separation towards ten VOCs, including ketones, alcohols, alkanes, aromatics and amines. These results of the chemiresistive gas sensor array demonstrate that nanoscale soft lithography is a reliable approach for fabricating nanoscale devices and has the potential of mass producibility.

Dual-Mode Gas Sensor Composed of a Silicon Nanoribbon Field Effect Transistor and a Bulk Acoustic Wave Resonator: A Case Study in Freons.

In this paper, we develop a novel dual-mode gas sensor system which comprises a silicon nanoribbon field effect transistor (Si-NR FET) and a film bulk acoustic resonator (FBAR). We investigate their sensing characteristics using polar and nonpolar organic compounds, and demonstrate that polarity has a significant effect on the response of the Si-NR FET sensor, and only a minor effect on the FBAR sensor. In this dual-mode system, qualitative discrimination can be achieved by analyzing polarity with the Si-NR FET and quantitative concentration information can be obtained using a polymer-coated FBAR with a detection limit at the ppm level. The complementary performance of the sensing elements provides higher analytical efficiency. Additionally, a dual mixture of two types of freons (CFC-113 and HCFC-141b) is further analyzed with the dual-mode gas sensor. Owing to the small size and complementary metal-oxide semiconductor (CMOS)-compatibility of the system, the dual-mode gas sensor shows potential as a portable integrated sensing system for the analysis of gas mixtures in the future.

Conductive polymer nanowire gas sensor fabricated by nanoscale soft lithography.

Resistive devices composed of one-dimensional nanostructures are promising candidates for the next generation of gas sensors. However, the large-scale fabrication of nanowires is still challenging, which restricts the commercialization of such devices. Here, we report a highly efficient and facile approach to fabricating poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) nanowire chemiresistive gas sensors by nanoscale soft lithography. Well-defined sub-100 nm nanowires are fabricated on silicon substrate, which facilitates device integration. The nanowire chemiresistive gas sensor is demonstrated for NH3 and NO2 detection at room temperature and shows a limit of detection at ppb level, which is compatible with nanoscale PEDOT:PSS gas sensors fabricated with the conventional lithography technique. In comparison with PEDOT:PSS thin-film gas sensors, the nanowire gas sensor exhibits higher sensitivity and a much faster response to gas molecules.

Smartphone-Enabled Colorimetric Trinitrotoluene Detection Using Amine-Trapped Polydimethylsiloxane Membranes.

A smartphone-enabled platform for easy and portably colorimetric analysis of 2,4,6-trinitrotoluene (TNT) using amine-trapped PDMS is designed and implemented. The amine-trapped polydimethylsiloxane (PDMS) is simply prepared by immersing the cured PDMS in aminosilane solutions forming an amine-containing polymer. After contacting with TNT-containing solutions, the colorless PDMS showed a rapid colorimetric change which can be easily identified by the naked eye. The amine-trapped PDMS was carefully optimized to achieve visible detection of TNT at concentrations as low as 1 µM. Using an integrated camera in the smartphone, pictures of colored PDMS membranes can be analyzed by a home-developed mobile application. Thus, the TNT amount can be precisely quantified. Direct TNT detection in real samples (e.g., drinking, tap, and lake waters) is demonstrated as well. The smartphone-enabled colorimetric method using amine-trapped PDMS membranes realizes a convenient and efficient approach toward a portable system for field TNT detections.

Detection of Volatile Organic Compounds by Self-assembled Monolayer Coated Sensor Array with Concentration-independent Fingerprints.

In this paper, we have modeled and analyzed affinities and kinetics of volatile organic compounds (VOCs) adsorption (and desorption) on various surface chemical groups using multiple self-assembled monolayers (SAMs) functionalized film bulk acoustic resonator (FBAR) array. The high-frequency and micro-scale resonator provides improved sensitivity in the detections of VOCs at trace levels. With the study of affinities and kinetics, three concentration-independent intrinsic parameters (monolayer adsorption capacity, adsorption energy constant and desorption rate) of gas-surface interactions are obtained to contribute to a multi-parameter fingerprint library of VOC analytes. Effects of functional group's properties on gas-surface interactions are also discussed. The proposed sensor array with concentration-independent fingerprint library shows potential as a portable electronic nose (e-nose) system for VOCs discrimination and gas-sensitive materials selections.

Detection of Volatile Organic Compounds Using Microfabricated Resonator Array Functionalized with Supramolecular Monolayers.

This paper describes the detection of volatile organic compounds (VOCs) using an e-nose type integrated microfabricated sensor array, in which each resonator is coated with different supramolecular monolayers: p-tert-butyl calix[8]arene (Calix[8]arene), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (Porphyrin), ß-cyclodextrin (ß-CD), and cucurbit[8]uril (CB[8]). Supramolecular monolayers fabricated by Langmuir-Blodgett techniques work as specific sensing interface for different VOCs recognition which increase the sensor selectivity. Microfabricated ultrahigh working frequency film bulk acoustic resonator (FBAR) transducers (4.4 GHz) enable their high sensitivity toward monolayer gas sensing which facilitate the analyses of VOCs adsorption isotherms and kinetics. Two affinity constants (K1, K2) are obtained for each VOC, which indicate the gas molecule adsorption happen inside and outside of the supramolecular cavities. Additional kinetic information on adsorption and desorption rate constants (ka, kd) are obtained as well from exponential fitting results. The five parameters, one from the conventional frequency shift signals of mass transducers and the other four from the indirect analyses of monolayer adsorption behaviors, thus enrich the sensing matrix (Δf, K1, K2, ka, kd) which can be used as multiparameter fingerprint patterns for highly selective detection and discrimination of VOCs.

Tuning the resonant frequency of resonators using molecular surface self-assembly approach.

In this work, a new method to tune the resonant frequency of microfabricated resonator using molecular layer-by-layer (LbL) self-assembly approach is demonstrated. By simply controlling the polymer concentration and the number of layers deposited, precisely tuning the frequency of microfabricated resonators is realized. Due to its selective deposition through specific molecular recognitions, such technique avoids the high-cost and complex steps of conventional semiconductor fabrications and is able to tune individual diced device. Briefly, film bulk acoustic resonator (FBAR) is used to demonstrate the tuning process and two types of LbL deposition methods are compared. The film thickness and morphology have been characterized by UV-vis reflection spectra, ellipsometer and AFM. As a result, the maximum resonant frequency shift of FBAR reaches more than 20 MHz, meaning 1.4% tunability at least. The minimum frequency shift is nearly 10 kHZ per bilayer, indicating 7 ppm tuning resolution. Pressure cooker test (PCT) is performed to evaluate the reliability of LbL coated FBAR. Furthermore, applications for wireless broadband communication and chemical sensors of LbL coated FBAR have been demonstrated.

6,13-dibromopentacene [2,3:9,10]-bis(dicarboximide): a versatile building block for stable pentacene derivatives.

6,13-Dibromopentacene [2,3:9,10]-bis(dicarboximide) (1) was synthesized for the first time by using in situ generated benzo[1,2-c:4,5-c']difuran as a key intermediate. Compound 1 exhibits good photostability in comparison to other pentacene derivatives and it can be further functionalized by Pd-catalyzed coupling reactions to give a series of soluble and stable functional pentacenes.

H-shaped oligothiophenes with low band gaps and amphoteric redox properties.

H-shaped bridged oligothiophenes HT-1 and HT-2 were synthesized by two different approaches. Different from normal oligothiophenes, HT-1 and HT-2 showed low band gaps and amphoteric redox behaviors due to intramolecular charge transfer, which is further supported by time-dependent DFT calculations.

Fused bispentacenequinone and its unexpected Michael addition.

Fused bispentacenequinone 2 was synthesized by photocyclization of bispentacenequinone 1. Unusual regioselective Michael addition was observed for 2 when excess aryl Grignard reagent was used. Subsequent acidification and oxidation in air gave diaryl-substituted bispentacenequinone 3. Tetra-aryl-substituted fused bispentacenequinone 4 was obtained from 3 after the second Michael addition followed by oxidation in air.

A stable heptacene derivative substituted with electron-deficient trifluoromethylphenyl and triisopropylsilylethynyl groups.

A heptacene derivative 1 substituted with four electron-deficient trifluoromethylphenyl and two triisopropylsilylethynyl (TIPSE) groups was prepared by a new synthetic strategy. Photo-oxidative resistance studies showed that this newly developed heptacene compound persisted 47 h in solution under ambient light and air conditions, and it represents the most stable heptacene derivative reported to date.

Electron-deficient triphenylene and trinaphthylene carboximides.

A series of triphenylene and trinaphthylene carboximides were synthesized mainly through Diels-Alder reactions between the in situ generated radialenes and N-alkylmaleimides. High electron affinity was obtained due to the attachment of electron-withdrawing carboximide groups. These disklike molecules can assemble into one-dimensional columnar structures in solid state and their mesophase properties were characterized by DSC, POM, and X-ray diffraction. Long-range ordered columnar phase was observed, qualifying them as potential n-type semiconductors in electronic devices.