Organization of Chemical Engineering Graduate Students University of Houston

23rd Annual Symposium – 2008


Oral Presentations

Jennifer Knoop, Dr. Willson

Immunoassays are commonly used to detect pathogens, gene expression levels, and contamination in food and drug supplies. Immunoassays use an antibody and a label, typically dyes, enzymes and fluors. The label is what is actually detected, and sensitivity is vital. Another major issue is false positives created by non-specific interactions; a city of 5 million can’t be evacuated on a false positive identification of e.g., anthrax in the air.In this project, instead of utilizing traditional labels, magnetic particles will be used. They are routinely used in sample preparation to capture targeted molecules from complex media, such as blood, stool, saliva, and food. The magnetic properties of the particles are used to discriminate against non-specific interactions.

The detection surface is composed of arrays of microfabricated retroreflector tetrads. A detector tetrad is composed of four retroreflectors; one assay retroreflector with antibodies is surrounded by three reference reflectors without antibodies that are always bright. 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 reflected off of 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. Initial results show that integrating another force component, such as fluid flow, is needed in addition to the magnetic field to remove the non-specifically bound particles from retroreflector surfaces. 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 our assay.

Divesh Bhatia, Dr. M.P. Harold, Dr. V. Balakotaiah

Modeling and experimental studies on model Pt/Al2O3 and Pt/BaO/Al2O3 catalysts are performed to elucidate the kinetics of NO oxidation and NO reduction by H2. A first order dependence of NO reduction on H2 concentration and a negative order dependence on NO concentration is reported. Experiments performed under kinetically controlled regime are used to find the activation energies of formation of various products during the reduction reaction. Selectivities of the reduction reactions to various products have been studied in the presence of H2O and CO2 to maintain conditions closer to that in a real vehicle exhaust. N2O formation is found to occur at lower temperatures, whereas the selectivity to NH3 formation is high at higher temperatures. A high H2/NO ratio favors NH3 formation, and the selectivity to N2 shows a maximum at H2:NO ratio of 1. Experiments have shown that a steady state is never truly achieved during NO oxidation. So, transient experiments on NO oxidation are performed which show a continuous decrease in the reaction rate with time on both the catalysts. NO oxidation reaction performed after catalyst pretreatments with H2, O2 and NO2 showed that NO2 is responsible for the decrease in the NO oxidation reaction rate. The highly oxidizing nature of NO2 results in Pt surface being covered with O either as chemisorbed O or in the form of Pt oxides, which results in a slow poisoning of the catalyst. The global model is unable to predict the NO oxidation behavior under a large temperature range because of the complex surface chemistry of NO2 decomposition, which is not taken into account in the global model.

Existing micro and global reaction kinetic models used for simulation of TWCs are not able to capture some of the key trends of the CO-H2-O2 system. We use a detailed microkinetic mechanism from the literature for the simultaneous oxidation of CO and H2 and modify it to predict the experimental observations. The kinetic scheme proposed in this work correctly predicts H2 light-off at room temperature in the absence of CO, selectivity towards CO oxidation before the simultaneous light-off of CO and H2, the enhancement effect of H2, and the inhibiting effect of CO on light-off in a CO-H2-O2 system. To explain the effect of H2 on light-off temperature, we have proposed that the activation energy of CO desorption decreases with an increase in the coverage of H species on Pt, thus assisting in the creation of more sites for oxygen adsorption. Based on the microkinetic studies and experimental results available in the literature, modifications have also been made to the global reaction kinetics, which currently do not distinguish between CO and H2 oxidation kinetics, and also do not contain the effect of H2 on CO oxidation kinetics. A bifurcation analysis of a short monolith reactor model has been conducted to study the effect of inlet temperature, inlet concentrations and the exposed Pt surface area on the light-off and multiplicity behavior. A comparison of the light-off curves generated using the global model and microkinetic model is done and the effect of rate parameters on the region of multiplicity is investigated.

Shirlaine Koh, Chengfei Yu and Dr. Peter Strasser

As the search for new and better materials that can replace platinum as the best electrocatalysts for proton exchange membrane (PEM) fuel cells continue, the pressure is on to identify highly active, reliably stable and reasonably cost-effective electrocatalysts for the oxygen reduction reaction (ORR). Currently, platinum-based alloy catalysts are recognized to be the most versatile substitute for platinum: catalytic properties of these alloyed materials can be tuned via compositional changes or synthesis routes.

Contrary to the general belief that electrochemical dissolution of base-metal deactivates the electrocatalyst, we report the enhancement of the activities of Pt-Cu alloyed electrocatalysts upon dissolution of Cu from the highly non-corrosion resistant Pt-Cu alloy system in an acidic medium. The de-alloying of the Pt-Cu system left a strained but stable Pt-shell surrounding an almost unleached Pt-Cu core that recorded exceptionally high Pt mass-based activities. In addition, the partially de-alloyed core-shell structure reduces the Pt loading while maintaining the high ORR activities, hence increasing the cost-effectiveness of the fuel cell system.

A variety of laboratory and synchrotron-based x-ray techniques were used to study the structural changes of the electrocatalysts before and after various leaching conditions: X-ray diffraction (XRD) was used to identify the various alloyed phases present in the catalysts; anomalous small angle X-ray scattering (ASAXS) enabled the tracking of elemental specific particle size changes in various electrochemical leaching conditions and transmission electron microscopy (TEM) provided visual abilities of the catalytic particles as well as particle size distribution.

The structural characteristics studied were then correlated with the electrochemical activities obtained from rotating disk electrode for the understanding of the structure-activity-stability relationships of the Pt-M nanoparticle electrocatalysts. The knowledge is crucial in understanding how catalytic activities and stabilities can be manipulated via structural changes in order to identify the next cathodic electrocatalyst for the PEM fuel cells.

References

1. J.P. Simon and O. Lyon Resonant Anomalous X-ray Scattering Theory and Applications, 1994 Elsevier Science B.V.
2. Greeley, J. et. al. Electrochim. Acta 52 (2007) 5829-5836
3. Stamenkovic, V.R. et. al. Nature Mat. 6 (3) (2007) 241-247

Ashok Kumar, Dr. V. Balakotaiah, and Dr. M. P. Harold

NOX Storage and Reduction (NSR) is an emerging technology for NOX emission abatement for lean burn and diesel engines. The NOX removal process involves two stages on a bifunctional catalyst (i.e. lean NOX trap, LNT). The first stage involves storage of NOX on an alkali earth component (Ba, Ca) mediated by precious metal (Pt, Rh). The second stage involves injecting a rich pulse of shorter exposure to reduce the stored NOX.

We employed Temporal Analysis of Products (TAP) experiments and micro-kinetic mechanistic modeling to further the understanding of NSR. TAP experiments are carried out isothermally in the Knudson diffusion regime thereby avoid thermal and mass limitations encountered in atmospheric pressure studies. Transient TAP studies were performed on Pt/Al2O3 and Pt/BaO/Al2O3 catalysts having Pt loading 1.5 wt.% with the BaO loading 16 wt.%. NO storage and NO/H2 pump probe experiments are performed on pre-oxidized, pre-reduced catalysts to study the role of temperature, NO/H2 delay time, NO/H2 ratio etc. on selectivity of NH3 and N2O production.

Micro-kinetic modeling attempt has been made to gain insight for the transport and reaction kinetics during NO storage and NO/H2 pump probe experiments from bench scale to ultra high vacuum condition. The deviations, in transport mechanism from Knudson diffusion, produced by injecting a narrow pulse in vacuum are suitably characterized. Parameter estimation is accomplished by curve fitting between experimental exit flow rate and model exit flow rate. Kinetic parameters were initialized using kinetic theory of gases or taken from literature and modified using optimization technique. The modeling explains all essential trends for N2 peak and its variation with temperature, oxygen poisoning as well as determine state of catalyst with position and time.

Sameer H. Israni , Dr. M. P. Harold

Membranes prepared from palladium and palladium alloys (Pd-Ag, Pd-Cu, Pd-Au) are highly hydrogen selective and these can be used in membrane reactors in order to generate, separate and purify hydrogen in a single step. Due to the removal of hydrogen from the reaction mixture equilibrium limitations or kinetic inhibitions can also be overcome in order to yield higher productivities. The major problems with palladium based membranes are their cost and long term stability. Our lab has developed ‘nanopore Pd’ membranes which involve a novel method for preparing ultra-thin Pd membranes (< 5 micron thickness). These ‘nanopore Pd’ membranes have shown very encouraging results in terms of hydrogen flux, hydrogen to nitrogen separation factor and stability. These ‘nanopore Pd’ membranes have been employed for membrane reactors studies using fuels like ammonia and methanol. This talk will include the ‘nanopore Pd’ membrane synthesis and characterization results along with experimental and modeling results from the membrane reactor studies. The differences in design principles between membrane reactors and conventional reactors will be highlighted. Some scale-up principles which have emerged from these studies will also be discussed.

Anupam Prakash, Dr. Manolis Doxastakis

Epidermal growth factor receptors (ErbB’s) a family of four proteins have a significant role in cell proliferation. Activation of ErbB’s is attributed to ligand binding and receptor dimerization/oligomerization of the proteins in the membrane. The specific contribution of the transmembrane domains to the dimerization process of ErbB’s remains a subject of extensive ongoing research. Based on the well studied dimer of Glycophorin-A (GpA), motifs similar to GxxxG could favor a close packing of the helix interfaces in ErbB transmembrane dimer structure. We performed detailed atomistic simulations of individual model ErbB and GpA transmembrane domains in dipalmitoylphosphatidylcholine (dppc) membranes and compared them with the corresponding coarse grain simulations. The coarse grain model efficiently captures the structural details observed in the atomistic simulations. Simulations starting from two non interacting transmembrane domains far apart in the dppc membrane resulted in a stable homodimer in coarse grain simulations of all the five proteins. The transmembrane dimer structures for ErbB proteins are proposed based on the coarse grain simulations. The structural properties of GpA dimer agreed with the results obtained from the experiments. The ErbB2 transmembrane dimer structure obtained from the simulations is consistent with the experimental structure recently submitted in the protein data bank. Our simulations support experimental evidence that transmembrane domains contribute to the dimerization process.

S.G. Belostotskiy, V.M. Donnelly, D.J. Economou and N. Sadeghi

Laser scattering experiments were performed in high pressure (100s of Torr) DC microdischarges operating in argon or nitrogen. Laser Thomson Scattering (LTS) and Rotational Raman Scattering were employed in a novel, backscattering, confocal configuration to measure important plasma parameters. LTS allows direct and simultaneous measurement of both electron density (ne) and electron temperature (Te). LTS experiments in microdischarges are challenging because of the low signal and excessive stray light. Measurements were performed at the center of the gap of a parallel plate slot-type microdischarge with plate separation of 600 microns. This location corresponded to the positive column of the DC microdischarge. For 50 mA current and over the pressure range of 300 – 700 Torr, measurements yielded Te = 0.9 ± 0.3 eV and ne = (6 ± 3)·1013 cm-3, in reasonable agreement with the predictions of a mathematical model. In order to obtain absolute values of the electron density, calibration of the Thomson scattered intensity was carried out using Raman scattering in nitrogen. This Rotational Raman spectroscopy was also employed to measure the gas temperature (Tg) in nitrogen DC microdischarges. Gas temperatures were determined by matching experimental spectra to synthetic spectra obtained by convolution of theoretical line intensities with the apparatus spectral resolution, with Tg as the adjustable parameter. Measurements were performed for a set of N2 pressures (P = 400 – 600 Torr) and over the current range of 5 – 30 mA. In the center of the interelectrode gap, Tg changed from 450 ± 40 K at 5 mA to 740 ± 40 K at 30 mA. The gas temperature was nearly independent of pressure within the error of the experiment. Advantages and limitations of the laser scattering techniques employed will also be discussed. Laser Thomson Scattering (LTS) and Rotational Raman Scattering were performed in high pressure (100s of Torr) DC microdischarges operating in argon or nitrogen. Measurements were done at the center of the gap (corresponding to the positive column) of a parallel plate slot-type microdischarge with plate separation of 600 microns. For 50 mA current and over the pressure range of 300 – 700 Torr, measurements LTS yielded Te = 0.9 ± 0.3 eV and ne = (6 ± 3)·1013 cm-3, in reasonable agreement with the predictions of a mathematical model. Rotational Raman spectroscopy was employed to measure the gas temperature (Tg) in nitrogen DC microdischarges. Measurements were performed for a set of N2 pressures (P = 400 – 600 Torr) and over the current range of 5 – 30 mA. In the center of the interelectrode gap, Tg changed from 450 ± 40 K at 5 mA to 740 ± 40 K at 30 mA. The gas temperature was nearly independent of pressure within the error of the experiment. Advantages and limitations of the laser scattering techniques employed will also be discussed. Also the diode laser absorption spectroscopy was applied to measure the density of argon metastables (s5 in Paschen notations) in argon microdischarge. From the width of absorption profile the gas temperatures were extracted.

Jyoti Phirani, Dr. Kishore K. Mohanty

Description of material: Large quantities of natural gas hydrate are present in marine sediments along the coastlines of many countries as well as in arctic regions. This work is aimed at assessing production of natural gas from the marine hydrate deposits. We had developed a multiphase, multicomponent, thermal, 3D simulator in the past, which can simulate production of hydrates both in equilibrium and kinetic modes. Four components (hydrate, methane, water and salt) and five phases (hydrate, gas, aqueous-phase, ice and salt precipitate) are considered in the simulator. In this work, we simulate depressurization and warm water flooding for hydrate production in a confined and unconfined hydrate reservoir underlain by a water layer. Water flooding has been studied as a function of injection temperature, injection pressure and production pressure.

Application: In order to produce gas from hydrates economically, efficient production techniques must be developed. Experiments on hydrates are difficult to perform; feasibility of production can be found from simulations. Hydrate reservoirs associated with aquifer beneath are not uncommon. The determination of injection and production conditions for these reservoirs through simulation will help in designing the effective production techniques.

Results: For confined reservoirs, at high injection temperature, gas production rate increases with injection pressure. If the production pressure is low, depressurization is better than warm water injection. For unconfined reservoirs, reservoir depressurization is ineffective; thermal stimulation is necessary for gas production.

Significant new contribution: Production strategies for hydrate reservoirs.

Saurabh Y. Joshi Dr. Michael P. Harold and Dr.Vemuri Balakotaiah

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.

Keywords: catalytic monolith; Liapunov-Schmidt reduction; averaging; convection-diffusion-reaction equation; washcoat diffusion; internal and external mass transfer coefficients

Veselina V. Uzunova, MD and Dr. Vekilov, Peter

Polymerization of hemoglobin S – a pathological variant of the transport protein hemoglobin – is the primary event in the pathogenesis of sickle cell anemia. Understanding of formation of polymers and other condensed phases in hemoglobin solution is important in order to gain insight for the mechanisms of the disease.

The polymerization process starts with nucleation. It has been shown that the nuclei form within metastable clusters of dense liquid. In hemoglobin solutions the dense liquid phase is metastable and exists only as mesoscopic clusters. We show using light scattering that addition of heme to hemoglobin solutions affects the properties and behavior of those clusters.

Heme also lowers the second virial coefficient of the system in a manner consistent with the theory for light scattering from multicomponent systems developed by Kirkwood and Goldberg. In our case we simplify the model up to a single equation and calculate the effective second virial coefficient for the hemoglobin-heme and heme-hemoglobin-heme interactions.

The found enhanced attraction between hemoglobin molecules provides an explanation of the effects of heme on volume of the metastable clusters and the rate of nucleation of the polymers.


Poster Presentations

Mohan Boggara, Dr. Ramanan Krishnamoorti

The location of small drug molecules such as Non-Steroidal Anti–Inflammatory Drugs (NSAIDs) in lipid membranes is an important factor that will give a molecular basis for understanding drug-membrane interactions in more detail. Neutron Diffraction, based on Bragg’s law, was used to study a system of multilamellar lipid membranes with and without NSAIDs to determine the location of drug in the membrane at nanometer resolution. This is then compared with the Molecular Dynamics simulations.

Michael Clark , Dr. Ramanan Krishnamoorti

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 dispersing the nanotubes throughout the host material and fostering nanotube – matrix interactions while ensuring the integrity of the nanotubes. However, strong inter-tubular van der Waals forces facilitate nanotube bundling limiting their solubility in common solvents and ultimately, the host matrix.

We are primarily interested in understanding the fundamental thermodynamics that govern the solution behavior of carbon nanotubes and using that knowledge to successfully generate ceramic nanocomposites with enhanced mechanical and thermal properties. In this pursuit, we have attempted to quantify the surface energetics which lead to nanotube aggregation. In addition, we have undertaken a detailed study of the overall effectiveness of both covalent and non-covalent functionalization schemes useful for solubilizing nanotubes in common processing solvents.

Mai Ha, Ayman Atallah and Ramanan Krishnamoorti

Without nanoparticles, the PS/PMMA 1/1 blend is thermodynamically unfavorable with PS as matrix and PMMA as dispersed phase. In thin films on silicon wafers, size of PS domains reduce with increasing nanoparticle content up to 0.3 wt%. 0.6 wt% of clay in blends converts the blend morphology from discrete to co-continuous. TEM pictures reveal well dispersed clays locating at interphase between small PS domains in PMMA phase. In linear viscoelastic region, rheological tests shown nanoparticles have significant affects on PS at low frequencies, PMMA at high and on PS/PMMA blend at all frequencies tested. Palierne model is used to model the relationship between rheology and dynamics of the blends.

Chengfei Yua, Shirlaine Koha , Michael F Toneyb, and Peter Strassera ,

a Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004 pstrasser@uh.edu

b Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA 94025, United States

Probing and understanding of structure-property-stability relationships of high surface area Pt and Pt alloy nanoparticle electrocatalysts represents one of the biggest challenges in the field of energy-related electrocatalysis. The design of durable Pt alloy nanoparticle catalyst requires knowledge of the structural and compositional changes that occur under electrocatalytic reaction conditions. Our understanding of particle distributions dynamics (particle size and compositon) is still very poor to date. What is needed are reliable methods to study the size, the bulk composition as well the surface composition of alloy particle ensembles in-situ during electrochemical reaction conditions.

We studied the size and composition dynamics of Cu-rich Pt-Cu alloy nanoparticle catalysts using synchrotron-based ex-situ and in situ X-ray diffraction (XRD) as well as Small angle X-ray scattering (SAXS). Our goal was a detailed characterization of the Cu dissolution processes which occur in acidic electrolyte and under elevated electrode potentials.

In-situ XRD provides changes of structural characteristics on the atomic scale during the electrocatalytic process and allowed evaluation of the kinetic parameters of metal dissolution. Small Angle X-ray scattering (SAXS) is a reciprocal-space method that is suited for the investigation of the particle size dynamics in the 1-100 nm diameter range. In addition, performing SAXS in the anomalous mode, that is measuring samples at varying incident X-ray energies, allows accurate background correction and introduces metal-specificity. XRD and SAXS combined are therefore two powerful technique to study changes of the size and composition distribution of nanoparticles during the catalytic reaction.

SAXS results confirmed that particle size increases with annealing temperature. It also showed that particle size increases with increasing base metal content. However, there was a general decrease in particle size upon undergoing electrochemical testing reflecting the dissolution of Cu atoms from the surface of the nnaoparticles. XRD results showed that lattice parameters of Pt-Cu alloy phases was getting bigger over time when the potential was applied consistent with Cu dissolution from Cu rich phases. the rate of lattice parameter change was found to be strongly potential dependent.

Mansour AbdulBaki , Dr. Ramanan Krishnamoorti

Piezoelectric polymers are of great interest for advancing the field of smart and active materials systems. Piezoelectric polymers offer the benefits of low power requirements, high voltage sensitivity, fast electro-mechanical response, mechanical strength, and ease of processing. The right combination of material properties would garner the potential to greatly affect work in the fields of integrated smart structures that would not only provide lightweight mechanical reinforcement, but also shape control as well as embedded sensing and energy harvesting functionality.

Here, we use calorimetric as well as wide and small angle X-ray scattering techniques to observe the effects of different nanoparticles on the crystallization of PVDF in order to understand the impact of such additions on the material’s morphology as part of a larger effort moving toward truly active, multi-functional, smart materials with intelligently tailored properties based on knowledge of material, property, and processing relationships.

Ajay Pratap Singh and Dr. Mike Nikolaou

Designing controller for known time variant systems are well studied and different adaptation schemes has been built. But when disturbances are random and are very large we require identifying the system behavior time to time. However, having a good understanding of disturbance’s behavior, logic based adaptation and identification can be implemented instead of adapting all the time. We have a combustion process where natural gas and waste streams are burnt with Air in rich fuel region. These waste streams are mainly rich with Hydrogen. There are large fluctuations in flow rate of these streams and some time flow rate become so less that process becomes lean in fuel. We have to avoid this state or we have to recover from this lean fuel state. To avoid this we need prior information of disturbance. This can not be done without measuring flow rates and compositions of streams, however we can always bound disturbances and there behavior can be studied. So, with understanding of disturbance behavior, without measuring disturbances, we are able to identify when system is going towards lean state. Once identified, a rule based adaptation scheme can be used for controller tuning and thus we can avoid lean fuel state.

*Yi-Ju Wang1, Dmitri Litvinov1, 2, Richard C. Willson1, 3

1Department of Chemical and Biomolecular Engineering
2Department of Electrical and Computer Engineering
3Department of Biology and Biochemistry

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.

Spin-polarized sensing devices have recently raised great interest, and an active field of research has developed around them. Magnetic tunnel junctions (MTJ) are based on spin-polarized electron tunneling. MTJs employ a thin dielectric tunnel barrier, usually 10-20 Å in thickness, between two ferromagnetic layers, one of which has its magnitude pinned by contact with an antiferromagnetic layer. Changes in the applied field or magnetic environment produce detectable changes in the resistance of the device. It can generate relatively high magneto-resistance in low magnetic fields.

Applying nanomagnetic device engineering to biosensing technology, we developed a tunneling magnetoresistant (TMR) sensor that is extremely sensitive to external magnetic fields. The MTJ was composed of two iron layers of different thickness sandwiching a magnesium oxide tunnel barrier (Fe/MgO/Fe). Magnetic beads are introduced as highly-detectable labels for biomolecules, potentially at the single-molecule level. 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 5’-end modified DNA to create a functionalized substrate, and the magnetic beads were coated with antibodies. The detection mechanism involves the capture of magnetic beads on the chemically modified PMMA. Herein, the fabrication of TMR sensors and the working principles of the biosensing steps are discussed.

Binh V. Vu , Dr. Richard C. Willson

Gold nanoparticles were recently reported to reduce the formation of non-specific products in PCR at remarkably low temperatures. In contrast to these reports, we report that gold nanoparticles do not enhance the specificity of PCR, but rather suppress the amplification of longer products while favoring amplification of shorter products, independent of specificity. Gold nanoparticles bearing a self-assembled monolayer of hexadecanethiol did not affect PCR, suggesting that surface interactions play an essential role. This role was further confirmed by experiments using gold nanoparticles ranging in size from 20 nm to 200 nm, in which a similar effect on PCR was observed for the same total surface area of particles over a 100-fold range of per- particle surface areas. The effect was seen with Taq and Tfl polymerases but not with Vent polymerase, suggesting the involvement of polymerase in nanoparticle effects. The effects of nanoparticles can be reversed by increasing the polymerase concentration or by adding BSA. Transient high-temperature nanoparticle exposure of PCR mix containing polymerase but not template or primers, followed by nanoparticle removal, modified subsequent nanoparticle-free PCR. These results suggest that the surface interactions which are most important are those of the polymerase, rather than with the template or primers. Interaction between polymerase and gold nanoparticles was confirmed by changes in nanoparticle absorption spectrum and electrophoretic mobility in the presence of polymerase. Taken together, these results suggest that the nanoparticles nonspecifically adsorb polymerase, thus effectively reducing polymerase concentration.

Veselina V. Uzunova, Dr. Vekilov, Peter

Polymerization of hemoglobin S – a pathological variant of the transport protein hemoglobin – is the primary event in the pathogenesis of sickle cell anemia. Understanding of formation of polymers and other condensed phases in hemoglobin solution is important in order to gain insight for the mechanisms of the disease.

The polymerization process starts with nucleation. It has been shown that the nuclei form within metastable clusters of dense liquid. In hemoglobin solutions the dense liquid phase is metastable and exists only as mesoscopic clusters. We show using light scattering that addition of heme to hemoglobin solutions affects the properties and behavior of those clusters.

Heme also lowers the second virial coefficient of the system in a manner consistent with the theory for light scattering from multicomponent systems developed by Kirkwood and Goldberg. In our case we simplify the model up to a single equation and calculate the effective second virial coefficient for the hemoglobin-heme and heme-hemoglobin-heme interactions.

The found enhanced attraction between hemoglobin molecules provides an explanation of the effects of heme on volume of the metastable clusters and the rate of nucleation of the polymers.

Joseph Y. Fu1 , Sindhu Balan2, Ajish Potty1, Van Nguyen2, and Richard C. Willson1, 2

1. Department of Chemical and Biomolecular Engineering, University of Houston, 4800 Calhoun, Houston, TX 77204-4004
2. Department of Biology and Biochemistry, University of Houston, 4800 Calhoun, Houston, TX 77204-5001

Ion-exchange chromatography is the most widely used method for purification of biomolecules. Traditionally, charged ligands are introduced into ion-exchange matrices by a process of random chemical derivitization which produces a distribution of charges, such that some regions of the adsorbent display relatively low charge density, while some have higher local density. This produces a broad range of local site affinities, and may also “waste” some charged ligands by stranding them in regions of charge density too low to constitute efficient binding sites. In this work, we examined the possibility of improving ion-exchange adsorbent performance by nanoscale structuring of ligands into clusters of fixed size rather than a random distribution of individual charges. More specifically, we examined the adsorption characteristics of these tailor-made clustered-charge adsorbents for proteins with known charge clusters. The calcium-depleted form of the protein alpha-lactalbumin, which displays a cluster of acidic amino acid residues, showed enhanced adsorption affinity and capacity by equilibrium adsorption isotherms on clustered-charge pentalysinamide and pentaargininamide adsorbents as compared to single-charge lysinamide and argininamide adsorbents of matched total charge. Two equivalent-total-charge, differently-charge-clustered mutants of rat microsomal cytochrome b5, E11Q and E44Q also were well differentiated by clustered-charge adsorbents. Thus, an organized rather than random distribution of charges may produce adsorbents with higher capacity and selectivity, especially for biomolecules with inherent charge clustering.

Steven Kemper

Many current bioanalytical methods rely on labeling of the species of interest and the use of specialized detection equipment to monitor the label. Fluorescence and luminescence are common techniques, but suffer from photobleaching and signal loss from dispersion of the emitted signal light in all directions. Retroreflectors may offer a solution to this problem, as they return a light signal back to the source in a narrow directional beam. Retroreflectors are among the most detectable man-made objects, with corner-cube retro-reflectors having been placed on the lunar surface for use in measuring the distance between the earth and moon. While the high signal fidelity from fully pre-assembled retro-reflectors makes them attractive as taggants or labels, our current work focuses on analyte-responsive modulation of signal from pre-fabricated retro-reflectors. In this approach, a gold corner-cube retro-reflector is fabricated on a silicon wafer prior to functionalization with antibodies specific to the antigen of interest. The surface is then exposed to the sample and any virus will attach to the antibodies. Gold nanoparticles similarly coupled with another antibody are then added to the surface, reducing reflectance due to light scattering if analyte is present. A similar device can be envisioned for in vivo glucose monitoring, wherein glucose competes with a dextran bound to gold nanoparticles for a covalently bound lectin. As glucose level increases, the gold-bound sugar is displaced from the surface and overall reflectance increases. In addition to high detectability, these devices would benefit from being easily producible and robust. Initial studies using microfabricated linear retro-reflectors show promise for these applications.

Jin Xu, Michael P. Harold, and Vemuri Balakotaiah

A steady-state microkinetic model for NO reduction by H2 and NH3 in O2 on alumina supported Pt/BaO is developed based on the measurements from a parallel experimental study (Clayton et al., Appl. Catal. B. Environ., 2008). Kinetic parameters not available from the literatures are estimated to capture the experimental trends and to meet thermodynamic constraints. The kinetic model is incorporated into a short monolith reactor model to simulate the steady state NH3/O2, NH3/NO and NO/NH3/H2 reaction systems. The predicted conversion and product distribution are in excellent qualitative and good quantitative agreement with the experimental data. Among other features, the model predicts for the three reaction systems the nonlinear light-off, and the product selectivity dependencies on temperature and feed composition. The model predicted trends in the species surface coverages with operating conditions help to elucidate the selectivity trends. The effect of external mass transfer on the conversions and product distributions are assessed and discussed