Publications

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We describe a novel approach to fabricate high-aspect-ratio membranes in microchannels by direct laser scanning, and demonstrate >10-fold improvement in sample preconcentration speed by achieving lower fM detection of proteins within 5 minutes. The integrated device can be used for continuous sample preparation, injection, preconcentration, and biochemical binding/reaction applications. © 2008 CBMS.

We have demonstrated purification of proteins in a simple aqueous two-phase extraction process in a microfluidic device. The laminar flows inherent to microchannels allows us to perform a binary split of a complex cell lysate sample, in an open channel with no chromatography support and no moving parts. This mild process allows recovery of functional proteins with a modest increase in purity. Aromatic-rich fusion tags are used to drive partitioning of enzymes in a generic PEG-salt two-phase system. Addition of affinity ligands to the PEG phase allows us to exploit other popular fusion tags, such as polyhistidine tags and GST-tags. © 2008 CBMS.

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Towards developing rapid and portable diagnostics for detecting zoonotic diseases, we have developed microchip-based electrophoretic immunoassays for sensitive and rapid detection of viruses. Two types of microchip-based electrophoretic immunoassays were developed. The initial assay used open channel electrophoresis and laser-induced fluorescence detection with a labeled antibody to detect influenza virus. However, this assay did not have adequate sensitivity to detect viruses at relevant concentrations for diagnostic applications. Hence, a novel assay was developed that allows simultaneous concentration and detection of viruses using a microfluidic chip with an integrated nanoporous membrane. The size-exclusion properties of the in situ polymerized polyacrylamide membrane are exploited to simultaneously concentrate viral particles and separate the virus/fluorescent antibody complex from the unbound antibody. The assay is performed in two simple steps-addition of fluorescently labeled antibodies to the sample, followed by concentration of antibody-virus complexes on a porous membrane. Excess antibodies are removed by electrophoresis through the membrane and the complex is then detected downstream of the membrane. This new assay detected inactivated swine influenza virus at a concentration four times lower than that of the open-channel electrophoresis assay. The total assay time, including device regeneration, is six minutes and requires <50 μl of sample. The filtration effect of the polymer membrane eliminates the need for washing, commonly required with surface-based immunoassays, increasing the speed of the assay. This assay is intended to form the core of a portable device for the diagnosis of high-consequence animal pathogens such as foot-and-mouth disease. The electrophoretic immunoassay format is rapid and simple while providing the necessary sensitivity for diagnosis of the illness state. This would allow the development of a portable, cost-effective, on-site diagnostic system for rapid screening of large populations of livestock, including sheep, pigs, cattle, and potentially birds. © The Royal Society of Chemistry.

We demonstrate the power of our technique for establishing and immobilizing well-defined polymer gradients in microchannels by fabricating two miniaturized analytical platforms: microscale immobilized pH gradients (μIPGs) for rapid and high resolution isoelectric focusing (IEF) applications, and polyacrylamide porosity gradients to achieve microscale pore limit electrophoresis (μPLE) in which species are separated based on molecular size by driving them toward the pore size at which migration ceases. Both separation techniques represent the first microscale implementation of their respective methodologies.

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Desulfovibrio vulgaris Hildenborough belongs to a class of sulfate-reducing bacteria (SRB) and is found ubiquitously in nature. Given the importance of SRB-mediated reduction for bioremediation of metal ion contaminants, ongoing research on D. vulgaris has been in the direction of elucidating regulatory mechanisms for this organism under a variety of stress conditions. This work presents a global view of this organism's response to elevated growth temperature using whole-cell transcriptomics and proteomics tools. Transcriptional response (1.7-fold change or greater; Z ≥ 1.5) ranged from 1,135 genes at 15 min to 1,463 genes at 120 min for a temperature up-shift of 13°C from a growth temperature of 37°C for this organism and suggested both direct and indirect modes of heat sensing. Clusters of orthologous group categories that were significantly affected included posttranslational modifications; protein turnover and chaperones (up-regulated); energy production and conversion (down-regulated), nucleotide transport, metabolism (down-regulated), and translation; ribosomal structure; and biogenesis (down-regulated). Analysis of the genome sequence revealed the presence of features of both negative and positive regulation which included the CIRCE element and promoter sequences corresponding to the alternate sigma factors σ32 and σ54. While mechanisms of heat shock control for some genes appeared to coincide with those established for Escherichia coli and Bacillus subtilis, the presence of unique control schemes for several other genes was also evident. Analysis of protein expression levels using differential in-gel electrophoresis suggested good agreement with transcriptional profiles of several heat shock proteins, including DnaK (DVU0811), HtpG (DVU2643), HtrA (DVU1468), and AhpC (DVU2247). The proteomics study also suggested the possibility of posttranslational modifications in the chaperones DnaK, AhpC, GroES (DVU1977), and GroEL (DVU1976) and also several periplasmic ABC transporters. Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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We report a novel means of fractionating proteins based on their voltage-dependent electromigration through nanopores of a polymer membrane. The nanoporous membranes were fabricated in situ in channels of a microchip using photopolymerization. The pores (1-10 nm) are small enough that proteins are excluded from passage with low applied electric fields, but increasing the field enables proteins to pass through. The magnitude of field required for a change in exclusion behavior is protein-specific with a correlation to protein size. Passage of proteins through the pores at higher field strengths could be attributed to partial unfolding or deformation of proteins due to the driving force of the applied field. The field-dependent exclusion mechanism could be useful as a multifaceted fractionation tool with single membranes or a network of membranes. Another exciting possibility is characterizing protein conformation, folding and stability based on field-dependent transport through nanopores. © 2006 Society for Chemistry and Micro-Nano Systems.

Goal of this study was to develop and characterize novel polymeric materials as pseudostationary phases in electrokinetic chromatography. Fundamental studies have characterized the chromatographic selectivity of the materials as a function of chemical structure and molecular conformation. The selectivities of the polymers has been studied extensively, resulting in a large body of fundamental knowledge regarding the performance and selectivity of polymeric pseudostationary phases. Two polymers have also been used for amino acid and peptide separations, and with laser induced fluorescence detection. The polymers performed well for the separation of derivatized amino acids, and provided some significant differences in selectivity relative to a commonly used micellar pseudostationary phase. The polymers did not perform well for peptide separations. The polymers were compatible with laser induced fluorescence detection, indicating that they should also be compatible with chip-based separations.