Prem S. Thapa

Directional growth of polypyrrole and polythiophene wires

This work establishes an innovative electrochemical approach to the template-free growth of conducting polypyrrole and polythiophene wires along predictable interelectrode paths up to 30 μm in length. These wires have knobby structures with diameters as small as 98 nm. The conductivity of the polypyrrole wires is 0.5±0.3 S cm−1; that of the polythiophene wires is 7.6±0.8 S cm−1. Controlling the growth path enables fabrication of electrode-wire-target assemblies where the target is a biological cell in the interelectrode gap. Such assemblies are of potential use in cell stimulation studies.

Physical Characterization and In Vitro Biological Impact of Highly Aggregated Antibodies Separated into Size-Enriched Populations by Fluorescence-Activated Cell Sorting

An IgG2 monoclonal antibody (mAb) solution was subjected to stirring, generating high concentrations of nanometer and subvisible particles, which were then successfully size-enriched into different size bins by low-speed centrifugation or a combination of gravitational sedimentation and fluorescence-activated cell sorting (FACS). The size-fractionated mAb particles were assessed for their ability to elicit the release of cytokines from a population of donor-derived human peripheral blood mononuclear cells (PBMC) at two phases of the immune response. Fractions enriched in nanometer-sized particles showed a lower response than those enriched in micron-sized particles in this assay. Particles of 5-10 μm in size displayed elevated cytokine release profiles compared with other size ranges. Stir-stressed mAb particles had amorphous morphology, contained protein with partially altered secondary structure, elevated surface hydrophobicity (compared with controls), and trace levels of elemental fluorine. FACS size-enriched the mAb particle samples, yet did not notably alter the overall morphology or composition of particles as measured by microflow imaging, transmission electron microscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. The utility and limitations of FACS for size separation of mAb particles and potential of in vitro PBMC studies to rank-order the immunogenic potential of various types of mAb particles are discussed.

Directional growth of metallic and polymeric nanowires

This work delineates the mechanism by which directional nanowire growth occurs in the directed electrochemical nanowire assembly (DENA) technique for growing nanowires on micro-electrode arrays. Indium, polythiophene, and polypyrrole nanowires are the subjects of this study. This technique allows the user to specify the growth path without the use of a mechanical template. Nanowire growth from a user-selected electrode to within +/- 3 microm of the straight line path to a second electrode lying within a approximately 140 degrees angular range and a approximately 100 microm radius of the selected electrode is demonstrated. Theory for one-dimensional electrochemical diffusion in the inter-electrode region reveals that screening of the applied voltage is incomplete, allowing a long range voltage component to extend from the biased to the grounded electrode. Numerical analysis of two-dimensional multi-electrode arrays shows that a linear ridge of electric field maxima bridges the gap between selected electrodes but decays in all other directions. The presence of this anisotropic, long range voltage defines the wire growth path and suppresses the inherent tip splitting tendency of amorphous polymeric materials. This technology allows polythiophene and polypyrrole to be grown as wires rather than fractal aggregates or films, establishing DENA as an on-chip approach to both crystalline metallic and amorphous polymeric nanowire growth.

Branched Oligopeptides Form Nanocapsules with Lipid Vesicle Characteristics

In a recent article (Gudlur et al. PLOS ONE, 2012, 7 (9) e45374), we described the special properties of a mixed branched peptide assembly in which equimolar bis(FLIVI)-K-KKKK and bis(FLIVIGSII)-K-KKKK self-associate to form bilayer delimited capsules capable of trapping solutes. These polycationic vesicle-like capsules are readily taken up by epithelial cells in culture, escape or evade the endocytic pathway, and accumulate in the perinuclear region where they persist without any apparent degradation. In this report, we examine the lipidlike properties of this system including initial assembly; solute encapsulation and washing; fusion and resizing by membrane extrusion through polycarbonate filters with defined pore sizes. The resized peptide capsules have uniform diameters in nm size ranges. Once resized, the capsules can be maintained at the new size by storing them at 4 °C. Having the ability to prepare stable uniform nanoscale capsules of desired sizes makes them potentially attractive as biocompatible delivery vehicles for various solutes/drugs.

Hexagonal magnetite nanoprisms: preparation, characterization and cellular uptake

The capacity of iron oxide nanocrystals to heat tissue when subjected to an alternating magnetic field (AMF hyperthermia) is shape-selective. Although iron oxide nanostructures with numerous shapes have been synthesized to date, hexagonal Fe3O4 prisms of low toxicity remained elusive. Here, we report the use of a dual ligand system permitting feasible reaction conditions to synthesize nearly perfect hexagonal Fe3O4 nanoplatelet structures, with edge length of 45 ± 5 nm and thickness of 5 to 6 nm. Their Specific Absorption Rate (SAR) is >750 W g(Fe)-1. The Fe3O4 hexagons were coated with a dopamine-based ligand to increase dispersibility in aqueous buffers. The Fe3O4 hexagons were only minimally toxic to RAW264.7 cells, which can be utilized in cell-based cancer targeting approaches.

Carbon dioxide hydrogenation to aromatic hydrocarbons by using an iron/iron oxide nanocatalyst

Summary The quest for renewable and cleaner energy sources to meet the rapid population and economic growth is more urgent than ever before. Being the most abundant carbon source in the atmosphere of Earth, CO2 can be used as an inexpensive C1 building block in the synthesis of aromatic fuels for internal combustion engines. We designed a process capable of synthesizing benzene, toluene, xylenes and mesitylene from CO2 and H2 at modest temperatures (T = 380 to 540 °C) employing Fe/Fe3O4 nanoparticles as catalyst. The synthesis of the catalyst and the mechanism of CO2-hydrogenation will be discussed, as well as further applications of Fe/Fe3O4 nanoparticles in catalysis.

Characterization of an Oncolytic Herpes Simplex Virus Drug Candidate

The structural integrity and conformational stability of a genetically modified live, oncolytic herpes simplex virus (o-HSV) were investigated across a wide pH (5.5-8.0) and temperature (10°C-87.5°C) range. A combination of circular dichroism, intrinsic and extrinsic fluorescence, and static light scattering results was visualized using an empirical phase diagram approach to provide a global assessment of physical stability. Distinct phases were identified including the native state of the virus, an intermediate phase that could represent gradual swelling and/or shedding of the viral envelope, and a highly disrupted, aggregated phase. The nature of these altered forms of the virus was further evaluated by transmission electron microscopy and viral plaque assays. The effect of freeze-thaw (F/T) stress on o-HSV was also examined. After one F/T cycle, a loss of infectious virus titers was observed. In addition, the monomeric virus particle concentration decreased during F/T stress, whereas there was a concurrent increase in larger particles (2-10 μm). The comprehensive biophysical characterization of viral stability conducted in this study identified major degradation events leading to loss of infectivity of o-HSV and represents an important step toward stabilization of the virus against thermal and F/T stresses.

Long reach cantilevers for sub-cellular force measurements

Maneuverable, high aspect ratio poly(3,4-ethylene dioxythiophene) (PEDOT) fibers are fabricated for use as cellular force probes that can interface with individual pseudopod adhesive contact sites without forming unintentional secondary contacts to the cell. The straight fibers have lengths between 5 and 40 μm and spring constants in the 0.07-23.2 nN μm(-1) range. The spring constants of these fibers were measured directly using an atomic force microscope (AFM). These AFM measurements corroborate determinations based on the transverse vibrational resonance frequencies of the fibers, which is a more convenient method. These fibers are employed to characterize the time dependent forces exerted at adhesive contacts between apical pseudopods of highly migratory D. discoideum cells and the PEDOT fibers, finding an average terminal force of 3.1 ± 2.7 nN and lifetime of 23.4 ± 18.5 s to be associated with these contacts.

Forces at individual pseudopod-filament adhesive contacts

On-chip cellular force sensors are fabricated from cantilever poly(3,4-ethylene dioxythiophene) filaments that visibly deflect under forces exerted at individual pseudopod-filament adhesive contacts. The shape of the deflected filaments and their ∼3 nN/μm spring constants are predicted by cantilever rod theory. Pulling forces exerted by Dictyostelium discoideum cells at these contacts are observed to reach ∼20 nN without breaking the contact.

Controlled electrochemical growth of ultra-long gold nanoribbons

This paper describes the electrochemical growth of branchless gold nanoribbons with ∼40 nm × ∼300 nm cross sections and >100 μm lengths (giving length-to-thickness aspect ratios of >103). These structures are useful for opto-electronic studies and as nanoscale electrodes. The 0.75-1.0 V voltage amplitude range is optimal for branchless ribbon growth. Reduced amplitudes induce no growth, possibly due to reversible redox chemistry of gold at reduced amplitudes, whereas elevated amplitudes, or excess electrical noise, induce significant side-branching. The inter-relatedness of voltage-amplitude, noise, and side-branching in electrochemical nanoribbon growth is demonstrated.