Hien Duy Tong

Arrays of dual nanomechanical resonators for selective biological detection.

Arrays of small nanomechanical resonators with dual geometry have been fabricated for sensitive biological detection. The arrays consist of silicon nitride resonating 100 nm thick cantilevers with sensing gold areas alternately placed on the free and fixed cantilever ends. The Au areas act as sensing regions as can be functionalized by means of thiol chemistry. The nanomechanical arrays provide a double flavor of the adsorbed molecules: the added mass reported by the cantilevers with the Au area at the tip and the nanoscale elasticity reported by the cantilevers with the Au area at the clamp. The devices were applied for DNA detection based on Watson-Crick pairing rules. The proposed design for nanomechanical resonators provides higher specificity for DNA sensing in comparison with conventional single cantilevers. The nanoscale elasticity induced by the DNA hybridization arises from the intermolecular interactions between the adsorbates bound to the cantilever and the surface stress.

Fabrication and characterization of dual sputtered Pd-Cu alloy films for hydrogen separation membranes

this paper, submicron thin Pd–Cu alloy films are deposited using a dual sputtering technique, which allows a high composition control of the layer. The composition, surface morphology and phase structure of the sputtered layers are investigated by energy-dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractiometry (XRD). For example, the XRD data prove that the Pd–Cu layers are an alloy of Pd and Cu. Subsequently, the characterized Pd–Cu alloy layers are deposited on a silicon support structure to create a 750-nm thin Pd–Cu membrane for hydrogen separation. The reported membrane obtained a high flux of 1.6 mol H2/m2 s at a temperature of 725 K, while the selectivity is at least 500 for H2/He.

Microfabrication of palladium-silver alloy membranes for hydrogen separation

In this paper, a process for the microfabrication of a wafer-scale palladium-silver alloy membrane (Pd-Ag) is presented. Pd-Ag alloy films containing 23 wt% Ag were prepared by co-sputtering from pure Pd and Ag targets. The films were deposited on the unetched side of a -oriented silicon wafer in which deep grooves were etched in a concentrated KOH solution, leaving silicon membranes with a thickness of ca. 50 /spl mu/m. After alloy deposition, the silicon membranes were removed by etching, leaving Pd-Ag membranes. Anodic bonding of thick glass plates (containing powder blasted flow channels) to both sides of the silicon substrate was used to package the membranes and create a robust module. The hydrogen permeability of the Pd-Ag membranes was determined to be typically 0.5 mol H/sub 2//m/sup 2//spl middot/s with a minimal selectivity of 550 for H/sub 2/ with respect to He. The mechanical strength of the membrane was found to be adequate, pressures of up to 4 bars at room temperature did not break the membrane. The results indicate that the membranes are suitable for application in hydrogen purification or in dehydrogenation reactors. The presented fabrication method allows the development of a module for industrial applications that consists of a stack of a large number of glass/membrane plates.

Microsieve supporting palladium-silver alloy membrane and application to hydrogen separation

A submicron thick and defect-free palladium-silver (Pd-Ag) alloy membrane is fabricated on a supporting microsieve by using microfabrication techniques. The microfabrication process also creates a robust wafer-scale membrane module, which can easily be inserted into a membrane holder to have gas-tight connections to the outer world. The microfabricated membrane demonstrated high separation fluxes of up to 4 mol H/sub 2//m/sup 2//spl middot/s with a minimal selectivity of 1500 for hydrogen over helium (H/sub 2//He) at 450/spl deg/C and 83 kPa H/sub 2/ retentate pressure. The present membrane has great potential for hydrogen purification and in applications like dehydrogenation chemistry. In addition, the presented technology can be used to fabricate other kinds of ultrathin but strong and defect-free membranes to set up new applications.

Shedding Light on Axial Stress Effect on Resonance Frequencies of Nanocantilevers

The detection back-action phenomenon has received little attention in physical, chemical, and biological sensors based on nanomechanical systems. We show that this effect is very significant in ultrathin bimetallic cantilevers, in which the laser beam that probes the picometer scale vibration largely modifies the resonant frequencies of the system. The light back-action effect is nonlinear, and some resonant frequencies can even be reduced to a half with laser power intensities of 2 mW. We demonstrate that this effect arises from the stress and strain generated by the laser heating. The experiments are explained by two-dimensional nonlinear elasticity theory and supported by finite element simulations. The found phenomenology is intimately connected to the old unsolved problem about the effect of surface stress on the resonance frequency of singly clamped beams. The results indicate that to achieve the ultimate detection limits with nanomechanical resonators one must consider the uncertainty due to the detection back-action.

High-flux palladium-silver alloy membranes fabricated by microsystem technology

In this study, hydrogen selective membranes have been fabricated using microsystem technology. A 750 nm dense layer of Pd (77 wt%) and Ag (23 wt%) is deposited on a non-porous 1 mm thick silicon nitride layer by cosputtering of a Pd and a Ag target. After sputtering, openings of 5 μm are made in the silicon nitride layer to create a clear passage to the Pd/Ag surface. As a result of the production method, these membranes are pinhole free and have a low resistance to mass transfer in the gas phase, as virtually no support layer is present. The membranes have been tested in a gas permeation system to determine the hydrogen permeability as a function of temperature, gas flow rate, and feed composition. In addition, the hydrogen selectivity over helium has been determined, which appears to be above 1500. At 0.2 bar partial hydrogen pressure in the feed, the hydrogen permeability of the membranes has been found to range from 0.02 to 0.95 mol.H2/m2×s at 350 and 450°C, respectively. It is expected that by improving the hydrodynamics and increasing the operation temperature, substantially higher fluxes will be attainable.

A generic microfluidic biosensor of G protein-coupled receptor activation – impedance measurements of reversible morphological changes of reverse transfected HEK293 cells on microelectrodes

Impedance spectroscopy of cell lines on interdigitated electrodes (IDEs) is an established method of monitoring receptor-specific cell shape changes in response to certain analytes. Normally, assays are done in multiwells making it a bulky, static and single use procedure. Here, we present a biosensor allowing sequential application of biological test samples with an automated microfluidic system. It is capable of monitoring relative changes in impedance using castellated IDEs of 250–500 μm diameter, covered with stable or reverse transfected HEK293 cells. Reversible activation of the Neurokinin 1 (NK1) receptor in stable cell lines was observed in response to a series of 5 minute exposures from 1 pM–10 nM of the specific ligand Substance P (SP) using impedance measurements at 10 mV and 15 kHz. An optimal flow speed of 10 μl min−1 was chosen for the 10 μl flow cell. The EC50 of ∼10 pM was about 10 times lower than the EC50 based on measuring changes in the calcium ion concentration. The method was also shown to work with reverse transfected cells. Plasmid DNA encoding the NK1 gene was spotted onto the electrodes and pre-incubated with a transfection agent. The overlaid HEK293 cells were subsequently transfected by the underlying DNA. After challenge with SP, the cells induced an activation response similar to the stable cell line. The microfluidic micro-electrode reverse transfection system opens up possibilities to perform parallel measurements on IDE arrays with distinct receptors per IDE in a single flow channel.

Fabrication and characterization of MEMS based wafer-scale palladium-silver alloy membranes for hydrogen separation and hydrogenation/dehydrogenation reactions

In this paper, a MEMS based wafer-scale palladium-silver alloy membrane (MWSPdAgM) is presented. This membrane has the potential to be used for hydrogen purification and other applications. A palladium-silver alloy layer (Pd-Ag) was prepared by co-sputtering. The thin Pd-Ag alloy has high hydrogen selectivity, high permeation rate as well as high mechanical and chemical stability. Typical flow rates of 0.5 mol H/sub 2//m/sup 2/.s have been measured with a minimal selectivity of 550 for H/sub 2//N/sub 2/. Anodic bonding of thick glass to silicon was used to package the membrane and create a very robust module. The membrane has high mechanical strength and can withstand pressures up to 4 bars at room temperature. The presented fabrication method allows the development of a module for industrial applications that consists of a stack with a large number of glass/membrane plates.

Simple Technique for Direct Patterning of Nanowires using a Nanoslit Shadow-Mask

Nanowires of various lengths and widths have been fabricated using a wafer-scale shadow mask with deposition windows, or nanoslits, created with focused ion beam machining. Metallic nanowires with widths down to 50 nm and lengths up to 100 mum have been realized. Measurements of electrical I- V characteristics show linear behavior of nanowires with widths and thickness each around 50 nm.