Characterization of dense arrays of chemiresistor vapor sensors with submicrometer features and patterned nanoparticle interface layers

The performance of arrays of small, densely integrated chemiresistor (CR) vapor sensors with electron-beam patterned interface layers of thiolate-monolayer-protected gold nanoparticles (MPNs) is explored. Each CR in the array consists of a 100-μm(2) interdigital electrode separated from adjacent devices by 4 μm. Initial studies involved four separate arrays, each containing four CRs coated with one of four different MPNs, which were calibrated with five vapors before and after MPN-film patterning. MPNs derived from n-octanethiol (C8), 4-(phenylethynyl)-benzenethiol (DPA), 6-phenoxyhexane-1-thiol (OPH), and methyl-6-mercaptohexanoate (HME) were tested. Parallel calibrations of MPN-coated thickness-shear-mode resonators (TSMR) were used to derive partition coefficients of unpatterned films and to assess transducer-dependent factors affecting responses. A 600-μm(2) 4-CR array with four different patterned MPN interface layers, in which the MPN derived from 7-hydroxy-7,7-bis(trifluoro-methyl)heptane-1-thiol (HFA) was substituted for HME, was then characterized. This is the smallest multi-MPN array yet reported. Reductions in the diversity of the collective response patterns are observed with the patterned films, but projected vapor discrimination rates remain high. The use of such arrays as ultralow-dead-volume detectors in microscale gas chromatographic analyzers is discussed.     

Formation of nanoparticle arrays on S-layer protein lattices

Crystalline bacterial cell surface layers (S-layers) composed of identical protein units have been used as binding templates for well-organized arrangements of nanoparticles. Isolated S-layer proteins were recrystallized into monomolecular arrays on solid substrates (such as silicon wafers and SiO2-coated grids) and in suspension forming so-called self-assembly products. These S-layer assemblies were studied by atomic force microscopy and transmission electron microscopy (TEM). The orientation of the S-layer lattice, exhibiting anisotropic surface properties, on the solid surface and on the self-assembly products, was compared with the orientation on the bacterial cell. On both bacterial cells and SiO2 surfaces the outer face of the S-layer protein was exposed. On the self-assembly products occasionally the inner face was also visible. Metal- and semiconductor nanoparticles 2 to 10 nm in mean diameter were covalently or electrostatically bound to the solid-supported S-layers and self-assembly products. TEM studies reveal that upon activation of carboxyl groups in the S-layer lattice with 1-ethyl-3,3'(dimethylaminopropyl)carbodiimide (EDC), a close-packed monolayer of 4-nm amino-functionalized CdSe nanoparticles could be covalently established on the S-layer lattice. Because of electrostatic interactions, anionic citrate-stabilized Au nanoparticles (5 nm in diameter) formed a superlattice at those sites where the inner face of the S-layer lattice was exposed. In contrast, cationic semiconductor nanoparticles (such as amino-functionalized CdSe particles) formed arrays on the outer face of the solid-supported S-layer lattices.     

Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection

Nanowires of various materials and configurations have been shown to be highly effective in the detection of chemical and biological species. In this paper, we report a novel, nanosphere-enabled approach to fabricating highly sensitive gas sensors based on ordered arrays of vertically aligned silicon nanowires topped with a periodically porous top electrode. The vertical array configuration helps to greatly increase the sensitivity of the sensor while the pores in the top electrode layer significantly improve sensing response times by allowing analyte gases to pass through freely. Herein, we show highly sensitive detection to both nitrogen dioxide (NO(2)) and ammonia (NH(3)) in humidified air. NO(2) detection down to 10 parts per billion (ppb) is demonstrated and an order-of-magnitude improvement in sensor response time is shown in the detection of NH(3).     

Scalable arrays of chemical vapor sensors based on DNA-decorated graphene

Arrays of chemical vapor sensors based on graphene field effect transistors functionalized with single-stranded DNA have been demonstrated. Standard photolithographic processing was adapted for use on large-area graphene by including a metal protection layer, which protected the graphene from contamination and enabled fabrication of high quality field-effect transistors （GFETs）. Processed graphene devices had hole mobilities of 1,640 ± 250 cm2.V-1.s-1 and Dirac voltages of 15 ± 10 V under ambient conditions. Atomic force microscopy was used to verify that the graphene surface remained uncontaminated and therefore suitable for controlled chemical functionalization. Single-stranded DNA was chosen as the functionalization layer due to its affinity to a wide range of target molecules and π-π stacking interaction with graphene, which led to minimal degradation of device characteristics. The resulting sensor arrays showed analyte- and DNA sequence-dependent responses down to parts-per-billion concentrations. DNA/GFET sensors were able to differentiate among chemically similar analytes, including a series of carboxylic acids, and structural isomers of carboxylic acids and pinene. Evidence for the important role of electrostatic chemical gating was provided by the observation of understandable differences in the sensor response to two compounds that differed only by the replacement of a （deprotonating） hydroxyl group by a neutral methyl group. Finally, target analytes were detected without loss of sensitivity in a large background of a chemically similar, volatile compound. These results motivate further development of the DNA/graphene sensor family for use in an electronic olfaction system.     

Selective growth of organic 1-D structures on au nanoparticle arrays

We demonstrate that the growth of F16CuPc 1-D nanostructures can be directed by templates of gold nanoparticles. The growth occurs via vapor-phase transport, whereby the gold nanoparticles act as nucleation sites for F16CuPc molecules and promote their anisotropic growth. The F16CuPc 1-D structures adopt diameters of approximately 15-30 nm independent of the nanoparticle size. This approach enables a technologically simple and inexpensive fabrication of very uniform organic 1-D structures (aspect ratio of approximately 30) and precise control of their location and packing density.     

Effect of polycyclic aromatic hydrocarbons on detection sensitivity of ultratrace nitroaromatic compounds

Polycyclic aromatic hydrocarbons (PAHs) with different numbers of pi-electrons and geometric symmetry of pi-systems, including anthracene, phenanthrene, pyrene, triphenylene, perylene, benzo[ghi]perylene, and coronene, were chosen to modify glassy carbon electrodes (GCEs) by self-assembling. The self-assembled monolayer of PAHs was investigated by STM and was used in the electrochemical detection of nitroaromatic compounds (NACs). The results indicate that PAH-modified GCE shows higher sensitivity to NACs than an unmodified one. Among the seven different PAHs, coronene-modified GCE exhibits the highest sensitivity to 2,4,6-trinitrotoluene and 1,3,5-trinitrobenzene.     

Impact of Pd nanoparticle loading method on SnO2 surface for natural gas detection in humid atmosphere

To improve the sensor response to low concentrations of methane (CH₄) at low operating temperatures in humid atmospheres, we prepared Pd-loaded SnO₂ (Pd-SnO₂) nanoparticles via two different Pd-loading processes: (i) a general impregnation method and (ii) a new loading method using poly(N-vinyl-2-pyrrolidone) (PVP) as a protective agent for Pd receptor particles. According to the measured electric resistances, the Pd particles limited the hydroxyl-poisoning of the SnO₂ particle surface. Because Pd is oxidized to PdO, a p–n junction is formed at the interface between PdO and SnO₂, and such interface gives the enlargement of the electron depletion layer. Therefore, Pd further improved the resistance against hydroxyl poisoning of the SnO₂ surface in humid air. In addition, although the sensor based on neat SnO₂ did not respond to low-concentration CH₄ at 200–400 °C, both the sensors based on the Pd-loaded SnO₂ samples exhibited high sensor response to 200 ppm CH₄ in a humid atmosphere. The Pd-SnO₂ obtained by the new loading method exhibited a higher response to CH₄ at lower concentrations in the lower operating temperature range (200–250 °C). This improvement in the sensor response is probably due to the catalytic activity of the larger Pd nanoparticles. According to high-resolution transmission electron microscopy–energy-dispersive X-ray spectroscopy images, the new loading method successfully provided Pd-loaded SnO₂ nanoparticles with Pd nanoparticles dispersed uniformly on the SnO₂ particle surface. The average particle size of Pd nanoparticles loaded on the surface of SnO₂ by the new loading method was slightly larger than that of the Pd nanoparticles loaded by the impregnation method. As the Pd particle size increases, it is thought that crystalline PdO particles are formed more easily, thereby improving the combustion activity of CH₄ under humid conditions. These results are of great significance for further decreasing the energy consumption of the CH₄ sensor and increasing its sensor response in humid atmospheres.

     

Growth of two-dimensional arrays of uncapped gold nanoparticles on silicon substrates

A method of preparing large area patterned 2D arrays of uncapped gold (Au) nanoparticles has been developed. The pattern has been formed using self-assembly of uncapped Au nanoparticles. The Au nanoparticles were synthesized via toluene/water two phase systems using a reducing agent and colloidal solution of Au nanoparticles was produced. These nanoparticles have been prepared without using any kind of capping agent. Analysis by TEM showed discrete Au nanoparticles of 4 nm average diameter. AFM analysis also showed similar result. The TEM studies showed that these nanoparticles formed self-assembled coherent patterns with dimensions exceeding 500 nm. Spin coating on silicon substrate by suitably adjusting the speed can self-assemble these nanoparticles to lengths exceeding 1 μm.     

Functionalized periodic Au@MOFs nanoparticle arrays as biosensors for dual-channel detection through the complementary effect of SPR and diffraction peaks

A facile and low-cost method to prepare periodic Au@metal-organic framework (MOF) (MIL-100(Fe)) nanoparticle arrays was developed.The arrays were fabricated in situ using monolayer colloidal crystals as templates,followed by Au deposition on substrates,and annealing.MIL-100(Fe) coatings were applied on the nanospheres using a simple solvent thermal process.The prepared periodic Au@MIL-100(Fe) nanoparticle (NP) arrays were characterized by two peaks in the visible spectra.The first peak represented the surface plasmon resonance (SPR) of the Au nanospheres,and the other peak,or the diffraction peak,originated from the periodic structure in the NP array.After modification with 3-aminophenylboronic acid hemisulfate (PBA),the Au@MIL-100(Fe) NP arrays exhibited sensitive responses to different glucose concentrations with good selectivity.These responses could be due to the strong interaction between PBA and glucose molecules.The diffraction peak was sensitive at low glucose concentrations (less than 12 mM),whereas the SPR peak rapidly responded at high concentrations.The peaks thus demonstrated satisfactory complementary sensitivity for glucose detection in different concentration regions.These results can be used to develop a dual-channel biosensor.We also created a standard diagram,which can be used to efficiently monitor blood glucose levels.The proposed strategy can be extended to develop different dual-channel sensors using Au@MIL-100(Fe) NP arrays functionalized with different recognition agents.     

2-Methyl-2,4-pentanediol gas sensor properties of nano-SnO2 flat-type coplanar gas sensing arrays at low detection limit

Nano-SnO₂ flat-type coplanar 2-Methyl-2,4-pentanediol (MPD) gas sensor arrays were fabricated by a screen-printing technique based on nano-SnO₂ powders prepared by a hydrothermal method. The results show that the fabricated gas sensor arrays have good MPD gas sensing characteristics, such as good selectivity and response-recovery characteristics. Especially, they can be used for detecting the concentration of MPD gas as low as 1 ppm which is much lower than the legal concentration of 20 ppm or 25 ppm. The good sensing properties indicate that the SnO₂ gas sensor arrays have great potential for on-line or portable monitoring of MPD gas in practical environments.     

聚醚砜-纳米二氧化硅气体分离复合膜的性能

研究了纳米二氧化硅(SiO2)颗粒的添加对聚醚砜纳米二氧化硅气体分离复合膜气体渗透性能的影响.采用SEM、机械性能和气体分离性能等评价方法分别表征膜的微观结构、力学性能及分离性能.结果表明:随着SiO2添加量的增加,杨氏模量增大,H2、N2、O2、CH4和CO2气体的渗透通量增大,但O2/N2,CO2/CH4和H2/N2的分离因子有所降低;中空纤维气体分离复合膜在放置一段时间后,性能趋于稳定;制备好的膜以乙醇和正己烷为交换剂,在烘箱中烘干作为后处理方式能够使膜具有较好的分离性能.     

Two-dimensional nanoparticle arrays formed by dewetting of thin gold films deposited on pre-patterned substrates

We demonstrate the formation of accurate 2D gold nanoparticle arrays via solid-state dewetting on a pre-patterned substrate. The annealing-induced dewetting of Au film on both flat and pre-patterned SiO₂ substrates is investigated. The pre-patterned structures affect clearly the formation of nanoparticles, and there is a depth effect of the pre-patterned grooves on the formation of nanoparticles during dewetting. Especially in pre-patterned areas with deep grid grooves (depth 150 nm) there is almost one single particle formed in the flat areas of every unit square, thus resulting in a very periodic 2D structure of gold nanoparticles.     

Implementation of the near-field signal redundancy phase-aberration correction algorithm on two-dimensional arrays

Near-field signal-redundancy (NFSR) algorithms for phase-aberration correction have been proposed and experimentally tested for linear and phased one-dimensional arrays. In this paper the performance of an all-row-plus-two-column, two-dimensional algorithm has been analyzed and tested with simulated data sets. This algorithm applies the NFSR algorithm for one-dimensional arrays to all the rows as well as the first and last columns of the array. The results from the two column measurements are used to derive a linear term for each row measurement result. These linear terms then are incorporated into the row results to obtain a two-dimensional phase aberration profile. The ambiguity phase aberration profile, which is the difference between the true and the derived phase aberration profiles, of this algorithm is not linear. Two methods, a trial-and-error method and a diagonal-measurement method, are proposed to linearize the ambiguity profile. The performance of these algorithms is analyzed and tested with simulated data sets.     

Portable gas chromatograph with tunable retention and sensor array detection for determination of complex vapor mixtures

A prototype portable gas chromatograph that combines a multiadsorbent preconcentrator/focuser, a tandem-column separation stage with individual column temperature control and junction point pressure modulation, and a detector consisting of an integrated array of polymer-coated surface acoustic wave microsensors is described. Using scheduled first-column stop-flow intervals and independent temperature programming of the two columns, it is possible to adjust the retention of eluting analyte vapors to maximize vapor recognition with the microsensor array and minimize the time of analysis. A retention window approach is combined with Monte Carlo simulations to guide retention tuning requirements and facilitate pattern recognition analyses. The determination of a 30-vapor mixture of common indoor air contaminants in < 10 min is demonstrated using ambient air as the carrier gas. Detection limits of < 10 ppb are achieved for the majority of compounds from a 1-L air sample on the basis of the most sensitive sensor in the array. Performance is assessed in the context of near-real-time indoor air quality monitoring applications.     

Effect of sintered grain growth on chemical ordering in binary FePt/Cu nanoparticle arrays

Recent studies have shown a strong correlation between grain growth and chemical ordering in chemically synthesized FePt nanoparticles. In order to study this effect, we have prepared a series of samples in which 3.5 nm FePt nanoparticles are dispersed in a matrix of Cu nanoparticles. The samples were annealed at 600 degrees C and at 800 degrees C. Grain size was determined by XRD Scherrer analysis and time-dependent remanent coercivity measurements were made to determine the intrinsic remanent coercivity, Hcr0. For samples annealed at 600 degrees C, Hcr0 increases strongly with grain size up to approximately 5 nm and increases weakly with additional grain growth. By contrast, after annealing at 800 degrees C, Hcr0 appears nearly independent of grain size. The results suggest that isolated 3.5 nm FePt nanoparticles can be weakly ordered when annealed at 600 degrees C and sintering is necessary for significant chemical ordering.     

Catalyst-free synthesis of macro-scale ZnO nanonail arrays on Si substrate by simple physical vapor deposition

Macro-scale ZnO nanonail arrays have been synthesized on silicon wafer by a simple physical vapor deposition approach without any catalyst. These synthesized ZnO nanonails grow vertically on the substrate with their caps upside. This probably results from the crowding effect. Each ZnO nanonail has a large hexagonal cap and a thinner shaft of several microns in length. Most of the nanonails are perfect single crystals with wurtzite structure and their preferred growth orientation is along [001] direction. The growth mechanism is VS mechanism and the detailed growth process is also proposed. The macro-scale nanonail arrays on Si substrate could offer novel opportunities for both fundamental research and technological applications.     

新型二维材料MXene的气敏性能研究进展

MXene是一种新型二维过渡金属碳化物/氮化物。作为二维材料,MXene具有大的比表面积和丰富的表面官能团,表面容易吸附气体分子,且吸附的气体分子会影响材料的导电性能。因此,MXene可以用来作为新型气敏材料。从理论到实验的角度综述各种MXene（Ti₃C₂ MXene、V₂C MXene、Mo₂C MXene等）的气敏性能以及气敏应用,归纳不同MXenes对气体的响应特性,分析MXene的气敏机理,总结MXene作为气敏材料的优势和缺点,展望MXene在气体传感器领域的未来应用前景。     

Fabrication of microwell arrays based on two-dimensional ordered polystyrene microspheres for high-throughput single-cell analysis

This paper describes a method of fabricating rounded bottom microwell arrays (MA) in poly(dimethylsiloxane) (PDMS) by molding a monolayer of ordered polystyrene (PS) microspheres. PS microspheres were self-assembled on a glass slide and partially melted mainly from the bottom at 240 °C to increase adhesive force with the substrate. The partially melted PS arrays were used as master to generate MA. Microwell sizes are tunable in the 10-20 μm range with rounded bottoms; such a 3D structure is not readily obtainable through conventional soft lithography. Both adherent and nonadherent cell types can be retained in the microwells with high efficiency. As a demonstration of the advantage of real-time cell screening with this MA, single cell enzyme kinetic analysis was also carried out on trapped single cells. The PDMS MA may find applications in high-throughput drug screening, guided formation of cell clusters, and multicellular communication.     

Sensitive DNA hybridization detection based on the deposition of gold nanoparticle on lyophobic surface

An assay for the detection of DNA hybridization was developed using Ellipsometric Scanner (ES) as a detection method. The different deposition trends of Au nanoparticle on glutaraldehyde-covered Si substrate caused by the different electrostatic properties of Au nanoparticles after interaction with ssDNA and dsDNA respectively can be observed by measuring the ellipsometric parameter y. At the same time, the average height of the y with respect to the cross-sectional contour c can be described by y = 0.0920 + 2.43c in the range of 5.4 to 27 nM (r = 0.943, c is target concentration, the unit is μM). This assay using ES imaging could successfully detect the total amounts of the target as low as 5.4 fmol. The proposed assay system has the advantage of allowing label-free detection, high sensitivity, and operational simplicity.