I-Ming Hsing

Absorption, Desorption, and Transport of Water in Polymer Electrolyte Membranes for Fuel Cells

The transport of water vapor through a nafion membrane includes absorption of water at one membrane/gas diffusion layer interface, water transport in the membrane, and desorption of water at another interface. based on the structure of the membrane, a model for the water transport through the membrane is presented. it was assumed that the mass-transfer coefficients for the absorption and desorption of water and the water diffusion coefficient were dependent on the volume fraction of water (f(v)) in the membrane. these parameters were determined from steady-state water transport flux through the membranes at different water activity gradients. the results show that the mass-transfer coefficient for the absorption of water (k(a) = 3.53 x 10(-5)fv m/s, 353 k) is much lower than that for the desorption of water (k(d) = 1.42 x 10(-4)fv m/s, 353 k). the parameters k(a) and k(d) describe the nonequilibrium water uptake of the membrane on operating conditions. using these obtained parameters, the simulation results agree well with experimental data under many different conditions, including thickness of the membrane, water vapor/liquid water, temperature, and flow rate.

Conformation-dependent exonuclease III activity mediated by metal ions reshuffling on thymine-rich DNA duplexes for an ultrasensitive electrochemical method for Hg2+ detection.

Hg(2+) is known to bind very strongly with T-T mismatches in DNA duplexes to form T-Hg(2+)-T base pairs, the structure of which is stabilized by covalent N-Hg bonds and exhibits bonding strength higher than hydrogen bonds. In this work, we exploit exonuclease III (Exo III) activity on DNA hybrids containing T-Hg(2+)-T base pairs and our experiments show that Hg(2+) ions could intentionally trigger the activity of Exo III toward a designed thymine-rich DNA oligonucleotide (e-T-rich probe) by the conformational change of the probe. Our sensing strategy utilizes this conformation-dependent activity of Exo III, which is controlled through the cyclical shuffling of Hg(2+) ions between the solution phase and the solid DNA hybrid. This interesting attribute has led to the development of an ultrasensitive detection platform for Hg(2+) ions with a detection limit of 0.2 nM and a total assay time within minutes. This simple detection strategy could be used for the detection of other metal ions which exhibit specific interactions with natural or synthetic bases.

Organic Electrochemical Transistors Integrated in Flexible Microfluidic Systems and Used for Label‐Free DNA Sensing

enzyme sensors, [ 9 ] DNA sensors, [ 10 ] dopamine sensor, [ 11 ] and cell-based biosensors. [ 12 ] A transistor-based sensor is the combination of a sensor and an amplifi er since a small potential change at an interface can induce a substantial variation of the channel current. [ 16–18 ] Therefore such devices are highly sensitive and potentially low cost. More importantly, OECT can be easily miniaturized and fabricated on fl exible substrates, which is essential for some applications in living systems. OECTs have been integrated in microfl uidic channels and used as ion sensors and enzyme sensors. [ 5 , 6 , 9 ] As for fl exible devices, OECTs were fabricated on fi bers and showed excellent transistor performance. [ 19 , 20 ] However, the application of the fi ber-supported devices in biosensors is limited by solid electrolytes used in the devices. Therefore, we study the OECT integrated in a fl exible microfl uidic system and explore its applications in biosensors, such as DNA sensors. Nucleic acid diagnostics has attracted much interest due to its great scientifi c and economic importance, and it has signifi cant applications in gene expression monitoring, viral and bacterial identifi cation, biowarfare and bioterrorism agents detecting, and clinical medicine. [ 21 ] Besides the traditional technique that is based on the confocal fl uorescence microscope, several different label-free technologies have been developed for the analysis of DNA microarrays, including atomic force microscopy, [ 22 ] electrochemical detection, [ 23 ] surface vibration spectroscopy, [ 24 ] scanning Kelvin probe microscopy (SKPM), [ 25 ]

Surfactant stabilized Pt and Pt alloy electrocatalyst for polymer electrolyte fuel cells

Abstract A simple process for the synthesis of carbon supported Pt and Pt/Ru electrocatalysts was investigated. Borrowing from the homogeneous catalyst preparation, this process uses a surfactant as a stabilizer which prevents the metal colloids from aggregation during the reduction process without influencing the deposition of the colloids onto the carbon support. Chemical, morphological and crystallographic properties of the newly prepared electrocatalysts were characterized using various surface techniques including X-ray diffraction (XRD), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). These techniques show that mono-size, well-dispersed metal colloids can be formed and successfully supported on the carbon black. Moreover, the size of metallic colloids prepared by this method can be manipulated by controlling the synthesis temperature and is independent of the catalyst loading. Electrochemical characterizations show that in comparison with commercial E-TEK electrocatalysts, surfactant-based Pt/C electrocatalysts possess similar catalytic activity in terms of oxygen reduction and higher CO tolerance performance can be obtained by the surfactant stabilized Pt,Ru/C.

Electrochemical Impedance Studies of Methanol Electro-oxidation on Pt/C Thin Film Electrode

Anodic processes of methanol oxidation on Pt/C thin film electrodes are carefully examined using electrochemical impedance spectroscopy. The influence of the electrode potential on the impedance pattern is studied and a quantitative explanation for the impedance behavior of methanol oxidation is achieved using a newly derived mathematical model. Theoretical derivations agree well with the experimental results, both of which show that the impedance patterns are strongly dependent on the electrode potential. At potentials higher than 0.30 V (vs. Ag/AgCl), a pseudoinductive behavior is observed. At potentials higher than 0.42 V (vs. Ag/AgCl), the impedance pattern is reversed to the second and third quadrants. The conditions required for the reversing of impedance pattern are delineated with the use of the impedance model.

A DNA biochip for on-the-spot multiplexed pathogen identification

Miniaturized integrated DNA analysis systems have largely been based on a multi-chamber design with microfluidic control to process the sample sequentially from one module to another. This microchip design in connection with optics involved hinders the deployment of this technology for point-of-care applications. In this work, we demonstrate the implementation of sample preparation, DNA amplification, and electrochemical detection in a single silicon and glass-based microchamber and its application for the multiplexed detection of Escherichia coli and Bacillus subtilis cells. The microdevice has a thin-film heater and temperature sensor patterned on the silicon substrate. An array of indium tin oxide (ITO) electrodes was constructed within the microchamber as the transduction element. Oligonucleotide probes specific to the target amplicons are individually positioned at each ITO surface by electrochemical copolymerization of pyrrole and pyrrole−probe conjugate. These immobilized probes were stable to the thermal cycling process and were highly selective. The DNA-based identification of the two model pathogens involved a number of steps including a thermal lysis step, magnetic particle-based isolation of the target genomes, asymmetric PCR, and electrochemical sequence-specific detection using silver-enhanced gold nanoparticles. The microchamber platform described here offers a cost-effective and sample-to-answer technology for on-site monitoring of multiple pathogens.

Microfabricated PCR-electrochemical device for simultaneous DNA amplification and detection

Microfabricated silicon/glass-based devices with functionalities of simultaneous polymerase chain reaction (PCR) target amplification and sequence-specific electrochemical (EC) detection have been successfully developed. The microchip-based device has a reaction chamber (volume of 8 microl) formed in a silicon substrate sealed by bonding to a glass substrate. Electrode materials such as gold and indium tin oxide (ITO) were patterned on the glass substrate and served as EC detection platforms where DNA probes were immobilized. Platinum temperature sensors and heaters were patterned on top of the silicon substrate for real-time, precise and rapid thermal cycling of the reaction chamber as well as for efficient target amplification by PCR. DNA analyses in the integrated PCR-EC microchip start with the asymmetric PCR amplification to produce single-stranded target amplicons, followed by immediate sequence-specific recognition of the PCR product as they hybridize to the probe-modified electrode. Two electrochemistry-based detection techniques including metal complex intercalators and nanogold particles are employed in the microdevice to achieve a sensitive detection of target DNA analytes. With the integrated PCR-EC microdevice, the detection of trace amounts of target DNA (as few as several hundred copies) is demonstrated. The ability to perform DNA amplification and EC sequence-specific product detection simultaneously in a single reaction chamber is a great leap towards the realization of a truly portable and integrated DNA analysis system.

An improved anodic bonding process using pulsed voltage technique

In this study, we report a pulsed-voltage technique that is commonly employed in the electroplating industry to achieve a more efficient Si-glass anodic bonding process than the conventional constant electric field process. This technique features a less stringent voltage requirement and a shortened bonding time without compromising the tensile strength of the bonded structure. A square waveform voltage profile is used to investigate the effects of pulsed-voltage profile on the bonding time. In particular, the effects of magnitude of the base voltage and duration of the peak and base voltages are investigated. With peak and base voltages set to 400 and 300 V, respectively, and the duration of each voltage pulse fixed at 10-30 s, the bonding time is reduced to 30% of that required by a constant field process (400 V). Tensile strength of all completely bonded Si-glass pairs prepared by this technique is greater than 15 MPa. A postulated bonding mechanism based on the experimental results is presented.

Triggering Hairpin-Free Chain-Branching Growth of Fluorescent DNA Dendrimers for Nonlinear Hybridization Chain Reaction

We present a nonlinear hybridization chain reaction (HCR) system in which a trigger DNA initiates self-sustained assembly of quenched double-stranded substrates into fluorescent dendritic nanostructures. During the process, an increasing number of originally sequestered trigger sequences labeled with fluorescent reporters are freed up from quenched substrates, leading to chain-branching growth of the assembled DNA dendrimers and an exponential increase in the fluorescence intensity. The triggered assembly behavior was examined by PAGE analysis, and the morphologies of the grown dendrimers were verified by AFM imaging. The exponential kinetics of the fluorescence accumulation was also confirmed by time-dependent fluorescence spectroscopy. This method adopts a simple sequence design strategy, the concept of which could be adapted to program assembly systems with higher-order growth kinetics.

Two-dimensional simulation of water transport in polymer electrolyte fuel cells

Abstract A two-dimensional finite-element-based model of coupled fluid flow, mass transport, and electrochemistry is derived for a polymer electrolyte fuel cell operating without external humidification of the reactant gases. Transport in the gas channels and gas-diffusion electrodes is modeled using the continuity, potential flow, and Stefan–Maxwell equations. Concentrated solution theory is used to model transport within the membrane. Predictions of the fraction of product water leaving the anode side of the fuel cell are compared to recent experimental studies in the literature. The present model correctly predicts the dependence of product water leaving the anode on hydrogen stoichiometry, oxygen stoichiometry, current density, and cell temperature. The multidimensional nature of transport within such a fuel cell is discussed and plots of streamlines, gas mole fractions, and water content of the membrane are presented.