Organization of Chemical Engineering Graduate Students University of Houston

24th Annual Symposium – 2009

Keynote lecture

“Sustainable Transportation Fuels – The Transition from Lab to Commercial Scale”

Oral Presentations

Jennifer D. Knoop1, Steven M. Kemper1, Tim Sherlock2, Eliedonna Cacao1, Paul Ruchhoeft2, Richard C. Willson1
1 University of Houston Department of Chemical and Biomolecular Engineering 2 University of Houston Department of Electrical and Computer Engineering

Immunoassays are commonly used in diagnostic assays, pathogen detection and in the detection of contamination of food supplies. Immunoassays use an antibody for specificity and for readout they employ a label, typically a dye, enzyme or fluor. The label is what is actually detected, and label detection sensitivity limits the assay sensitivity.

This work introduces magnetic particles as labels in assays based on micro-fabricated retroreflectors. Magnetic particles are routinely used for sample preparation in complex media, such as blood, stool, saliva, and food. The magnetic properties of the particles can also be used to discriminate against non-specific interactions.

The detection surface is composed of arrays of microfabricated retroreflector tetrads. Retroreflectors return light directly to its source and are readily detectable with inexpensive optics. A detector tetrad is composed of four retroreflectors; one assay retroreflector decorated with antibodies is surrounded by three always-on reference reflectors without antibodies. Magnetic particles are used in sample preparation to capture the target and then exposed to the detector. When the target is present the magnetic particles will bind to the region in front of the assay reflector and scatter some of the light, reducing the amount returned to CCD camera.

The light intensity returned by the assay reflector is compared to that of the three reference reflectors to determine the concentration of the magnetic beads without needing to optically calibrate the device. The assay can easily detect the presence of a single particle bound to the surface. A horizontal magnetic field can remove the non-specifically bound particles from a flat gold surface. Integrating another force component, such as fluid flow, is useful to more completely remove the non-specifically bound particles from retroreflector surfaces. Initial experiments have begun to incorporate the assay into a microfluidic chip. Results also show that one or two Rickettsia conorii bacteria can hold down a 1.0 µm magnetic particle coated with rabbit polyclonal anti-Rickettsia antibodies, supporting the possibility of detecting a single target pathogen with this assay.

Figure 1: (Left) 1.0 μm magnetic particles held down by R. conorii. (Middle) Printed array of micro-retroreflector tetrads with 2.8 μm mag from CCD camera. (Right) Optical output collected from CCD camera, attenuated signals are from retroreflectors with single beads.

K. Chen, K.S. Martirosyan and D. Luss
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

IR imaging technique was used to identify the dynamic features of particulate matter regeneration in the diesel particulate filter (DPF). The experiments revealed some rather surprising novel features about the evolution and dynamics of the soot combustion. The combustion may proceed either uniformly all over the surface or with a moving temperature front depending on the operating conditions. The maximum temperature of the moving fronts exceeded by 100 oC those obtained during the uniform combustion. The peak temperature and velocity varied as the temperature fronts propagated on the surface. A sudden decrease in the feed temperature by 100 oC can lead to sudden temperature rise (wrong-way behavior) of about 50 oC above that obtained under stationary operation with the original temperature. The transient temperature rise highly depended on the position where the temperature shift was initiated, i.e., the time that the moving temperature front stayed in the DPF before exiting it. When the reaction front was moving upstream the temperature excursion near the end of DPF was much higher than the temperature rise in middle or near the entrance. This suggests a rapid shift of a diesel car driving mode from fast to idle may lead to a transient temperature rise, much higher than those, encountered under either the original or final operating conditions. This transient temperature rise may explain the failure of a DPF material integrity during uncontrolled regeneration.

Ajay Pratap Singh and Dr. Michael Nikolaou
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

It is well known that no linear controller with integral action can stabilize a plant for which the sign of the steady-state gain may change sign. A nonlinear control scheme must be used, such as nonlinear model-based control or adaptive control. In this work, we focus on an instance of this problem, in which the sign of the steady-state gain may change as a result of large unmeasured external disturbances entering a process with input multiplicities. The study is motivated by a specific gaseous emissions treatment unit in a chemical plant. External disturbances include large changes in the flow rate of hydrogen feed, which itself is combusted, thus potentially changing the air-to-fuel ratio in the process from lean to rich or vice versa. The objective of this paper is to explain the idiosyncrasies of the dynamic behavior of the controlled process, suggest potential control system design strategies, and demonstrate some results via computer simulations. In particular, we demonstrate that simple linear control can be effective for a wide range of operating conditions, if designed correctly. The key for linear controller design is that (a) control has to be tight enough (i.e. the controller gain should be large enough) to ensure that the process does not escape far from the desired set-point trajectory and reversal of the steady-state gain is not realized, and (b) control must not be too tight (i.e the controller gain should not be too high) to avoid potential problems that are well understood in linear control theory (such as instability, noise amplification, or input saturation). Novel theoretical analysis based on nonlinear operator theory is used to provide controller design guidelines and suggest the anticipated closed-loop behavior. Numerical simulations using a dynamic model calibrated on plant data are used to illustrate the proposed controller design approach. Finally, future investigations are suggested for the development of nonlinear and/or adaptive controller design methods.

Michael Clark and Dr. R. Krishnamoorti
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

Carbon nanotubes are unique materials, which are at the forefront of the nanotechnology revolution due to their extraordinary mechanical, thermal, and electrical properties. They are amongst the strongest materials known to man making them ideal candidates as reinforcing elements in ceramic-based materials. Their use in this capacity is contingent on successfully dispersing the nanotubes throughout the host matrix and fostering nanotube-matrix interactions while ensuring the integrity of the nanotubes. However, strong inter-tubular forces facilitate nanotube bundling limiting their solubility in common solvents and ultimately, dispersability within the host matrix.

In this talk, the fundamental thermodynamics that govern nanotube solubilization are examined through quantification of nanotube surface energetics. The effectiveness of enhancing solubility through covalently binding alkyl chains to the nanotube surface is explored by varying both chain length and grafting density. From these results, a theory is presented to explain the complex interactions between the nanotube surface, functional group, and surrounding solvent which are paramount in determining nanotube dispersability. Finally, to ensure nanotube integrity during ceramic processing, two low temperature routes for synthesizing silicon carbide are examined: namely, a polymeric precursor and sol-gel chemistry. Preliminary results using these two synthetic methods will be presented.

Saurabh Y. Joshi, Michael P. Harold, and Vemuri Balakotaiah
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

We present accurate low-dimensional models for real time simulation, control and optimization of monolithic catalytic converters used in automobile exhaust treatment. These are derived rigorously using the Liapunov-Schmidt (LS) technique of bifurcation theory and are expressed in terms of three concentration and two temperature modes. They include washcoat diffusional effects without using the concept of effectiveness factor and reduce to the classical two-phase models under steady-state conditions and when the washcoat thickness is very small. The models are validated by comparing the solutions with the exact solution of the detailed convection-diffusion-reaction equations. The usefulness of these new models is illustrated by simulating the transient behavior of the three-way converter and comparing the predictions with detailed solution. It is shown that these new models are robust and accurate with practically acceptable error, speed up the computations by orders of magnitude, and can be used with confidence for the real time simulation and control of monolithic and other catalytic reactors.

Sameer Israni, Michael P. Harold, Vemuri Balakotaiah
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

Pd membrane based reactors have the potential to generate high purity H2 in a single unit for stationary and mobile applications spanning power stations, soldier-power, and vehicles. Previous studies [1,2] from our group have analyzed in some detail the use of methanol reforming in membrane reactors as a way of intensifying the reaction and separation/purification into a single unit. In the current study methanol steam reforming was carried out in both a regular packed bed reactor (PBR) and a number of packed bed membrane reactor (PBMR). Pd-Ag membranes (3.1 micron thickness nanopore membrane) were used in the PBMR to separate the hydrogen from the reaction mixture. The catalyst used in this study was Cu/ZnO/Al2O3. Since our previous studies [3,4] have highlighted that slow radial diffusion of hydrogen is one of the major factors limiting productivity & utilization in the membrane reactor, the reactor diameters of the PBMRs were varied to further study this effect. The methanol conversion, hydrogen productivity, hydrogen utilization and outlet CO/CO2 ratio for the PBR and the PBMRs were compared at different pressures and temperatures. Separate studies were carried out to investigate the effect of the reactants and products (methanol, water, CO, CO2) on the H2 flux through the Pd-Ag membrane. A 2-dimensional model was also developed to simulate the results and to elucidate the rate limiting processes. The results of these experiments and simulations were then used to develop a 3-dimensional model for multi-fiber PBMR in which various aspects of the design were explored, including spacing between the membranes.

References
1. Harold, M.P., B. Nair, and G. Kolios, “Hydrogen Generation in a Pd Membrane Fuel Processor: Assessment of Methanol-Based Reaction Systems,” Chemical Engineering Science,58, 2551-2571 (2003).
2. Lattner, J.R., and M.P. Harold, “Comparison of Methanol Based Fuel Processors for PEM Fuel Cell Systems,” Appl. Catalysis B. Environmental, 56, 149-169 (2005).
3. Nair, B., and M.P. Harold, “Experiments and Modeling of Transport in Composite Pd and Pd/Ag Coated Alumina Hollow Fibers,” J. Membrane Sci , 311, 53-67 (2008).
4. Israni, S.H., B. Nair and M. P. Harold, “Hydrogen Generation and Purification in a Composite Pd Hollow Fiber Membrane Reactor: Experiments and Modeling,” Catalysis Today,139, 299-311 (2008).

Divesh Bhatia, Michael P. Harold, Vemuri Balakotaiah
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

A crystallite-scale model is incorporated into a reactor-scale model to study the effect of dispersion and temperature during the regeneration of a lean NOx trap (LNT), based on a parallel experimental study [R.D. Clayton, M.P. Harold, V. Balakotaiah, C.Z. Wan, Appl. Catal., B 90 (2009) 662.]. It is shown that for a fixed Pt loading, an increase in the Pt dispersion results in an increase in the interfacial perimeter between Pt and Ba, where the reduction of NOx takes place. The rate determining process during the regeneration is found to be the diffusion of stored NOx within the Ba phase towards the Pt/Ba interface. The transient product distribution for three catalysts having varied Pt dispersions (3.2%, 8% and 50%) is explained by the localized stored NOx gradients in the Ba phase. Temperature-dependent NOx diffusivities in the Ba phase are used to predict the breakthrough profiles of H2, N2 and NH3 over a range of catalyst temperatures. Finite gradients in the stored NOx concentration are predicted in the Ba phase, thus showing that the nitrate ions are not very mobile. The model predicts that the highest amount of NH3 is produced by the low dispersion catalyst (3.2% dispersion) at high temperatures, by the high dispersion catalyst (50% dispersion) at low temperatures, and by the medium dispersion catalyst (8% dispersion) at intermediate temperatures, which is consistent with the experimental studies. The model considers the consumption of chemisorbed oxygen on Pt by H2, which is used to predict the low effluent N2 concentration for the 50% dispersion catalyst as compared to the 8% dispersion catalyst. Finally, a novel design is proposed to maximize the amount of NH3 in the effluent of a LNT, which can be used as a feed to a selective catalytic reduction (SCR) unit placed downstream of the LNT.

¹Pankaj Kumar, ²Mathew Franchek, ²Karlos Grigoriadis, ¹Vemuri Balakotaiah
¹Department of Chemical and Biomolecular engineering, University of Houston,TX-77204
²Department of Mechanical engineering, University of Houston,TX-77204

A four-mode (two concentration and two temperature) low-dimensional model for in-cylinder combustion process that includes all the relevant physics and chemistry occurring at different time and length scales is developed. The lumped parameter ordinary differential equation model is based on two mixing times that capture the fuel and air mixing limitations. The first mixing time captures the reactant diffusional limitations inside the cylinder, while the second one accounts for the mixing limitations caused by reactant input and exit stream distribution. For a given fuel inlet conditions, the model predicts the exhaust composition of regulated gases (total unburned HC’s, CO, and NOx) as well as the in-cylinder pressure and temperature. The model is able to capture the qualitative trends observed with change in fuel composition (gasoline and ethanol blending), air/fuel ratio, and spark timing. The preliminary results show good qualitative and fair quantitative agreement with the experimental results published in the literature. Improvements and extensions to the model are discussed.

Sergey G. Belostotskiy, Tola Ouk, Vincent M. Donnelly, Demetre J. Economou
Plasma Processing Laboratory, Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004, USA
Nader Sadeghi, Laboratoire de Spectrométrie Physique (UMR C5588), Université J. Fourier de Grenoble, B P 87, F-38402 Saint-Martin d’Hères Cedex, France

Optical Emission Spectroscopy was employed to study a high pressure (100s of Torr) DC microdischarge in argon, with traces of N2 and H2 present and acting as optical tracers. Spatially resolved measurements of gas temperature across the 600 μm slot-type discharge were obtained from analysis of the rotational structure of two transitions of the first positive band of N2: B3Πg(v=4) → A3Σu+(v=1) and B3Πg(v=5) → A3Σu+(v=2). Gas temperature profiles peaked at the cathode side of the discharge and slowly decreased towards the anode. Such behavior is consistent with the physics of DC discharges, where most of the power dissipation occurs in the cathode layer. The gas temperature increased with increasing current, reaching a maximum of Tg = 1200 K at I = 30 mA and P = 600 Torr. Electron densities were extracted from the spectral profile of the Hβ line. The profile was fit with a Voigt function, which included Doppler, pressure, instrumental and Stark broadening. The electron density was estimated from the contribution of Stark broadening. The spatial profile of electron density was found to have a maximum in the cathode sheath edge region, followed by a minimum in the bulk plasma, and then a maximum some distance from the anode. This spatial distribution was explained by the non-homogeneous structure of the microdischarge, having a highly contracted positive column. The electron density near the sheath edge increased with both pressure and current reaching ne = 1.7∙1014 cm-3 at I = 30 mA and P = 600 Torr.

Mai Ha, Dr. R. Krishnamoorti
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

The influence of an organically modified layered silicate on the compatibility and dynamics of an immiscible blend of atactic polystyrene (PS) and poly(methyl methacrylate) (PMMA) is studied. Bulk samples of equal volume fraction blends of PS and PMMA, examined here, form large phase separated discrete phases of PMMA in a continuous PS matrix. The addition of the organoclay nanoparticles at a concentration of 0.6 wt % results in a significant alteration of the morphology with co-continuous domains of PS and PMMA including discrete and small domains of the other component dispersed in these continuous domains. This drastic change in morphology is reflected in the melt-state rheological behavior of the nanocomposites when compared to the blend. The nanocomposite with 0.6 wt% layered silicate exhibits longer relaxation time and higher modulus at low-frequencies. Bulk morphology of different blend ratio and various silicate content suggest the nanoparticles prefer to segregate at the interface between small PS and large PMMA domains. The Palierne dynamic model is used to quantify the observed relationship between the melt-state rheological behavior and the observed morphology for the blend nanocomposite.

Poster Presentations

Divesh Bhatia, Robert D. Clayton, Michael P. Harold, Vemuri Balakotaiah
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

A global kinetic model for NOx storage and reduction for the case of anaerobic regeneration with hydrogen is developed, based on parallel experimental studies [1,2]. The existence of two different types of BaO storage sites on the catalyst is proposed, which differ in their storage as well as regeneration activity. The two-site model explains the close to complete NOx storage at the start of the storage phase and the gradual emergence of NO and NO2 during later storage times. The effluent concentrations and concentration fronts of the reactants and products within the monolith are predicted by the model, providing insight into the mechanisms of regeneration and storage. The H2 front velocities are predicted to increase as the H2 front propagates down the length of the monolith, thus showing the presence of more stored NOx in the front of the reactor. The simulations show that even though regeneration is fast, H2 concentration fronts are not very steep, which is attributed to the lower regeneration activity of the “slow” sites. The model captures the formation of NH3 and the NH3 concentration fronts, which reveal the reaction of NH3 formed upstream with the stored NOx downstream of the H2 front. The lower diffusivity of NH3 as compared to H2 is shown to be responsible for the wider width of the NH3 front and earlier appearance of NH3 in the effluent than H2.

References
[1] R.D. Clayton, M.P. Harold and V. Balakotaiah, Appl. Catal., B, 84 (2008) 616.
[2] R.D. Clayton, M.P. Harold, V. Balakotaiah, C.Z. Wan, Appl. Catal. B, 90 (2009) 662.

Sameer Israni, Dr. Harold
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

Methanol steam reforming has been widely studied in packed bed membrane reactors for its utility in hydrogen generation. The various reactants and products of this reaction (methanol, steam, carbon dioxide, and carbon monoxide) can severely decrease the H2 flux through the membrane. The purpose of the current study is to understand the causes of this decrease in flux due to these contaminants. A 3.4 micron Pd-Ag (23 wt% Ag) nanopore membrane was used in a membrane separator apparatus. Initially pure H2 was introduced into the shell side of the separator containing the Pd-Ag membrane and the flux through the membrane was measured at temperatures between 225 – 300 oC and retentate pressures of 3 and 5 bars. Then various concentrations of methanol, water, carbon dioxide and carbon monoxide were introduced along with the H2. The decrease in H2 flux was noted for each case. The three main causes for this decrease are lowering of the retentate side hydrogen partial pressure, concentration polarization and surface coverage effects (i.e. decrease in the surface area of the membrane). In order to model the surface coverage effect it was assumed that Langmuir type adsorption was occurring at the surface of the Pd-Ag membrane. A 2-dimensional model was developed to simulate the results. This model along with the experimental results was used to estimate the coefficients of adsorption of the various gases on the membrane surface. These coefficients were then verified using independent experimental data. The coefficients were then used in a model of a packed bed membrane reactor carrying out methanol steam reforming.

¹Pankaj Kumar, ²Mathew Franchek, ²Karlos Grigoriadis, ¹Vemuri Balakotaiah
¹Department of Chemical and Biomolecular engineering, University of Houston,TX-77204
²Department of Mechanical engineering, University of Houston,TX-77204

A four-mode (two concentration and two temperature) low-dimensional model for in-cylinder combustion process that includes all the relevant physics and chemistry occurring at different time and length scales is developed. The lumped parameter ordinary differential equation model is based on two mixing times that capture the fuel and air mixing limitations. The first mixing time captures the reactant diffusional limitations inside the cylinder, while the second one accounts for the mixing limitations caused by reactant input and exit stream distribution. For a given fuel inlet conditions, the model predicts the exhaust composition of regulated gases (total unburned HC’s, CO, and NOx) as well as the in-cylinder pressure and temperature. The model is able to capture the qualitative trends observed with change in fuel composition (gasoline and ethanol blending), air/fuel ratio, and spark timing. The preliminary results show good qualitative and fair quantitative agreement with the experimental results published in the literature. Improvements and extensions to the model are discussed.

Ram R. Ratnakar and Vemuri Balakotaiah
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

We use the Liapunov-Schmidt technique of classical bifurcation theory to derive a low-dimensional model for describing the time evolution of a non-reactive tracer in laminar flow through a tubular reactor. Unlike the other averaging techniques, the Liapunov-Schmidt formalism leads to an exact averaged model which is valid for all times and converges for all values of parameters with any arbitrary initial or inlet conditions, including points sources. This model is consistent with the physics of system as the single mode combined model is parabolic in terms of cross-sectionally averaged concentration and hyperbolic in terms of cup-mixing concentration after truncating up to the second order spatial and temporal derivatives. We also analyze the temporal evolution of spatial moments for the general initial release of a tracer and show that it does not have the centroid displacement or variance deficits predicted by other methods. Extensions of the low-dimensional models to include more commonly encountered physical situations are also discussed.

Keywords: Taylor-Aris dispersion, Liapunov-Schmidt method, Linear Algebra, Moment Analysis, Non-linear Analysis.

L. Wang, K. S .Martirosyan, D. Luss.
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

Major progress has been made during the past two decades in developing energetic materials that can rapidly release temperature and pressure waves and have extensive potential applications. Nanoscale particles have a significantly higher surface area to volume ratio than microparticles, providing a closer contact between solid particles in a mixture. The small size of nanoparticles increases the homogeneity of the reactant mixture and improves the uniformity of a propagating reaction front. Nanoenergetic materials have various potential military applications and are likely to become the next-generation explosive materials such as aircraft fuels, rocket propellants, explosives, and primers.

Our experiments showed that the combustion of Al-Bi2O3 nanoparticles mixture generated the highest pressure pulse among common nanothermite reactions and can potentially be used as a Nanoenergetic Gas Generator (NGG). The combustion front propagation velocity and rate of energy release increased by up to 3 orders of magnitude when the particles size of both aluminum and the oxidizer were reduced to a nanosize range. We developed a novel one step (metal nitrate-glycine) combustion synthesis of nanostructured amorphous-like and highly crystalline bismuth trioxide nanoparticles. Increasing the crystallinity of the bismuth oxide nanoparticles increased the peak gas pressure. The maximum PV (Pressure x Volume, 862 Pa m3) value obtained at m=0.1 g with our synthesized Bi2O3 was much higher than that reported in literature (33 Pa m3) for the same sample mass. Addition of small amount of boron to the thermite systems increased the peak pressure.

Eliedonna E. Cacao1, Steven Kemper1, Tim Sherlock2, Azeem Nasrullah2, Katerina Kourentzi2, Jennifer Knoop2, Paul Ruchhoeft2, Richard Willson1
1 University of Houston (UH) Department of Chemical and Biomolecular Engineering, 2 UH Department of Electrical and Computer Engineering

In most diagnostics and immunoassays, quantification of the amount of analyte relies on detection of a label, rather than the analyte itself. Commonly used labels, such as fluorophores and luminophores, suffer signal loss due to isotropic dispersion of emitted light. Moreover, these techniques often require the use of delicate and specialized equipment to detect the label. Utilization of micro-fabricated retroreflectors in immunoassays and biosensors potentially offers an attractive and economical solution to these problems.

Retroreflectors are designed to return the reflected light in directions close to (or nearly the same as) the incident light. This makes them ideal for assay applications as there is minimal signal loss and no label decay over time. Micron-scale linear retroreflectors are readily detectable; signal can be easily measured with simple, cheap optics. Standard lithographic techniques, which provide a cost-effective method for large scale production, are used to fabricate the retroreflectors.

This work introduces an antigen-detection diagnostic assay based on micro-retroreflectors, where brightness is modulated by the formation of insoluble precipitates by an enzymatic catalysis. Alkaline phosphatase, an enzyme widely used in histochemical and cytochemical methods, was used for the detection process, together with its substrate, bromochloroindolyl phosphate-nitroblue tetrazolium (BCIP-NBT). The good localization property of the insoluble products of this enzyme-substrate system suggested their utility in reporting antigen-mediated enzyme binding on localized assay structures. Other labels, such as gold nanoparticles, can also be used to modulate the brightness of micro-retroreflectors in the presence of the desired antigens. The signal provided by this label can be further amplified by silver staining using chemistry similar to black and white photography.

Functionalizing a surface with a variety of capture antibodies for different antigens (such as viruses, bacteria and toxins) could be used to evaluate the viability of a “syndrome chip” for use in clinical diagnostics. Such system would allow a physician to test a patient suffering from common symptoms for a large range of causative agents to manage the correct treatment. If a serious public health or bioterrorism concern is found, the proper response can be implemented immediately.

J. Nick Taylor§*, Qusai Darugar§, Katerina Kourentzi†, Christy Landes§ and Richard C. Willson†
§ Department of Chemistry, University of Houston, Houston, TX 77204-5003, * Present Address: Department of Chemistry, Rice University, Houston, TX 77005
† Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004

Single molecule fluorescence resonance energy transfer (SMFRET) is used to study Mg2+ -dependent conformational dynamics of an anti-VEGF (aV) DNA aptamer as well as its interaction with its binding target, the dimeric form of vascular endothelial growth factor (VEGF). Although the overall equilibrium shows the closed form of the aptamer to be favored, the aV aptamer has fast conformational dynamics that vary inversely to the Mg2+ concentration. These dynamics remain in the presence of VEGF, but upon interaction with VEGF, the aptamer equilibrium shifts toward a more open conformation. The observed kinetics of fluctuation shows two processes that characterize the aptamer’s return to the closed conformation in the presence of VEGF. A fast transition corresponds to unbound aptamer dynamics, while a slow transition corresponds to the release of VEGF and the return of the aptamer to the closed conformation.

YI-JU WANG*1, DMITRI LITVINOV1,2, RICHARD C. WILLSON1
1Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4005
2Department of Electrical and Computer Engineering, University of Houston, Houston, TX 77204-4005

Biomolecular sensing in medical diagnosis is a significant focus of recent research. Sensors must be tailored for the purpose of the analyses, be robust under a variety of conditions, and be stable over a large number of assays. A continuing challenge in many clinical applications is the extremely small size of many biopsy samples, requiring great sensitivity for detection of analytes such as DNA, RNA, and proteins.

Giant magnetoresistant (GMR) phenomenon is expressed when the resistance of a material decreases dramatically upon the application of an external magnetic field. The structure of GMR thin films includes ferromagnetic layers with alternating magnetization which are separated by a non-magnetic layer and is tens of nanometers thick. Changes in the applied field or magnetic environment produce detectable changes in the resistance of the device. This suggests that GMR devices might be a good candidate for constructing low field sensors as the basis of magnetic particle-based assays.

Applying nanomagnetic device engineering to biosensing technology, we developed a GMR sensor that is extremely sensitive to external magnetic fields. Electron beam lithography was used to fabricate devices on the sub-micron scale. The fabricated biosensors were coated by poly(methyl meth- acrylate) (PMMA) thin films to electrically insulate the device from corrosive biological media. The PMMA surface was covalently immobilized with modified DNA to create a functionalized substrate, and the magnetic beads were coated with antibodies specific to RNA/DNA hybrids for microRNA detection. The detection mechanism involves the capture of magnetic beads on the chemically modified PMMA. Herein, we describe the fabrication of GMR sensors and the working principles of biosensing steps.

Zuze Mu, Ramanan Pitchumani, Vemuri Balakotaiah , Akhil Bidani
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

Respiratory burst, sometimes also called oxidative burst, is the rapid release of reactive oxygen species by professional phagocytes, which always associates with a sudden increase of the oxygen consumption. Respiratory burst plays important role in mammalian immune system. NADPH oxidase (NOX) is a multi-component enzyme which is responsible to mediate the respiratory burst. In this work, we specifically target at macrophages. We decompose the complex biophysical and biochemical process of superoxide production into a sequence of unit processes, then a mathematical model is developed to simulate in the process. In the model, NADPH oxidase, major ion transporters on the plasma membrane, as well as the cell geometry of macrophages, membrane potential, kinetics of the formation of superoxide and the assembly of the NADPH oxidase are all included. The model results are validated with our experimental data. The production of superoxide is measured by luminol chemiluminescence. The membrane potential changes are measured by fluorescent microscopy.

Zhiying Chen, Vincent M. Donnelly and Demetre J. Economou
Plasma Processing Laboratory, Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004
Lee Chen, Merritt Funk and Radha Sundararajan
Tokyo Electron America, Austin, TX 78741

Dual-frequency capacitively coupled plasmas (2f-CCP) used in the fabrication of modern integrated circuits may provide quasi-independent control of ion flux and energy. Accurate determination of the electron temperature (Te) and the electron energy distribution function (EEDF) are important for understanding plasma behavior and optimizing plasma processes in 2f-CCPs. In this study, measurements of Te and EEDF in CF4/O2 plasmas generated in a 2f-CCP etcher were performed by using trace rare gases-optical emission spectroscopy (TRG-OES). TRG-OES is a nonintrusive method based on a comparison of atomic emission intensities from trace amounts of rare gases (a mixture of Ne, Ar, Kr, and Xe) added to the plasma, with intensities calculated from a model. The parallel plate etcher was powered by a high frequency (60 MHz) “source” top electrode, and a low frequency (13.56 MHz) “substrate” bottom electrode.

The electron temperature was determined as a function of pressure (4-200 mTorr) at different applied top (500 and 1000W) and bottom (0-500 W) RF powers and two oxygen contents (10% and 20% by volume) by using trace rare gases optical emission spectroscopy (TRG-OES). Below 20 mTorr, the electron temperature Te increased with increasing pressure. The dependence of plasma electronegativity on pressure may be responsible for this behavior. Te decreased rapidly with increasing pressure in the 20-60 mTorr range, and then slowly decreased with further increases in pressure to 200 mTorr. Increasing the applied bottom RF power resulted in higher electron temperature. In the whole pressure range investigated, the Te in 90%CF4+10%O2 plasmas was very similar to that in 80%CF4+20%O2 plasmas.

Approximate EEDF for 80%CF4+20%O2 plasmas as a function of pressure were also constructed. The high energy tail of the EEDF was obtained by selecting rare gas emission lines originating mostly from ground states. The EEDF exhibited a bi-Maxwellian character with enhanced high energy tail, especially above 20 mTorr.

R. Khare, L. Stafford, J. Guha, and V. M. Donnelly
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

Previously, we studied recombination reaction of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In those studies, Cl and O atoms formed in chlorine or oxygen plasmas impinged on a cylindrical substrate that was rapidly rotated such that points on the surface were exposed to the plasma and then to a differentially-pumped analysis chamber equipped with either an Auger electron spectrometer or a mass spectrometer. Langmuir Hinshelwood (LH) recombination was observed by monitoring desorption of Cl2 and O2 with the mass spectrometer or through a pressure rise. In these previous experiments, however, Eley Rideal (ER) recombination (if it occurs) could not be detected because it would take place instantaneously in the presence of atom flux, and hence would cease as soon as the sample left the plasma. To observe the ER component, as well as to isolate LH recombination in plasmas with multiple radical species (i.e. most plasmas), a separate radical beam source is needed in combination with the plasma and spinning substrate. In this context, a surface-wave chlorine plasma operating at 2.45 GHz was sustained in a 8 mm quartz tube using a gap-type wave launcher, namely a surfatron. Using trace rare gas optical emission spectroscopy, we measured the Cl and Cl2 densities and the electron temperature, Te, at 50 mTorr as a function of distance with respect to the wave launcher. Measurements of the Cl2(306 nm)-to-Xe(828 nm) intensity ratio showed that Cl2 is almost completely dissociated (97 %) near the gap, with a smaller degree of dissociation (89 %) near the end of the plasma column. A much stronger decrease was obtained using the same Xe level and the Cl line at 792.4 nm. This can be attributed to a position-dependant (i.e. Te-dependant) actinometry constant corresponding to electron-impact excitation of Cl and Xe. By selecting Ne, Ar, Kr, and Xe lines excited from the ground state which are characteristic of the high energy portion of the electron energy distribution function, we found that Te increased from 5 to 10 eV as we moved away from the wave launcher. On the other hand, a constant value of Te = 3.1 ± 0.6 eV was obtained using rare gas lines excited to a significant extent through electron-impact excitation from the metastable levels. Possible heating mechanisms leading to the presence of such high-energy tails in surface-wave plasmas will be discussed.

Suchanun Moungthai, Gila Stein
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas, USA, 77204

The objective of this project is to nanopattern thin films of a p-type polymer semiconductor for applications in solar cells. Electron beam lithography is used to pattern dense arrays of 50 nm lines and dots. Exposure to radiation cross-links the polymer to prevent dissolution during subsequent solution-based processing.