Highly controlled multiplex electrospinning.

Applications of electrospinning (ES) range from fabrication of biomedical devices and tissue regeneration scaffolds to light manipulation and energy conversion, and even to deposition of materials that act as growth platforms for nanoscale catalysis. One major limitation to wide adoption of ES is stochastic fiber deposition resulting from the chaotic motion of the polymer stream as is approaches the deposition surface. In the past, fabrication of structures or materials with precisely determined mesoscale morphology has been accomplished through modification of electrode shape, use of multi-dimensional electrodes or pins, deposition onto weaving looms, hand-held electrospinning devices that allow the user to guide deposition, or electric field manipulation by lensing elements or apertures. In this work, we demonstrate an ES system that contains multiple high voltage power supplies that are independently controlled through a control algorithm implemented in LabVIEW. The end result is what we term "multiplex ES" where multiple independently controlled high-voltage signals are combined by the ES fiber to result in unique deposition control. COMSOL Multiphysics® software was used to model the electric field produced in this novel ES system. Using the multi-power supply system, we demonstrate fabrication of woven fiber materials that do not require complex deposition surfaces. Time-varied sinusoidal wave inputs were used to create electrospun torus shapes. The outer diameter of the tori was found, through parametric analysis, to be rather insensitive to frequency used during deposition, while inner diameter was inversely related to frequency, resulting in overall width of the tori increasing with frequency. Multiplex ES has a high-frequency cutoff based on the time response of the high voltage electrical circuit. These time constants were measured and minimized through the addition of parallel resistors that decreased impedance of the system and improved the high-frequency cutoff by up to 63%.

Ultrafast Charge Injection in Silver-Modified Graphitic Carbon Nitride.

Graphitic carbon nitride (gCN) is a promising organic platform for driving light-activated charge-transfer reactions in a number of valuable photocatalytic cycles. A primary limitation of gCN as a photocatalyst is its short excited-state lifetime, which is mediated by a high density of trap and defect sites that result in rapid excited-state decay and low photocatalytic efficiency. To enhance the catalytic activity, gCN is often functionalized with a metal co-catalyst; however, the mechanism by which metal co-catalysts enhance the reactivity has not been clearly established. In this work, the excited-state dynamics of gCN and silver-modified gCN are compared using ultrafast transient absorption and time-resolved photoluminescence spectroscopies. In silver-modified gCN, an ultrafast spectral shift in the silver plasmon resonance provides direct spectral evidence of electron transfer from gCN to the silver nanoparticles. The electron-transfer rate is competitive with other non-radiative relaxation pathways, with electron-transfer yields approaching 50%, thus providing an effective strategy for mitigating losses associated with defects and trap sites.

Carbon Nanoparticle-Induced Changes to Lipid Monolayer Structure at Water-Air Interfaces.

Surface specific vibrational spectroscopy experiments together with surface tension measurements and spectroscopic ellipsometry data were used to characterize the effects of soluble carbon particulates on compressed and partially compressed lipid monolayers adsorbed to the water-air interface. The lipid monolayers consisted of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DPPC), and measurements were made for both tightly packed monolayers (40 Å2/molecule) and monolayers in their liquid condensed state (55 Å2/molecule). Langmuir trough data show that very small amounts of PHF (0.0075 mg/mL or 6.4 × 10-6 M) decrease lipid film compressibility. This finding supports a cooperative adsorption mechanism whereby the soluble PHFs are drawn to the surface and associate with the insoluble DPPC monolayer. Excess free energies (ΔGmixE) were positive, consistent with the cooperative adsorption mechanism, and although the excess free energies are small (≤1 kJ/mol), adsorbed PHF has measurable effects on monolayer structure. Further support for the cooperative adsorption mechanism at the water-air interface comes from vibrational sum frequency generation (VSFG) experiments. Low PHF concentrations (≤0.06 mg/mL) increase DPPC acyl chain ordering in liquid condensed lipid films and decrease DPPC acyl chain ordering and film thickness in tightly packed lipid films.

Photocatalytic Reduction of Aqueous Nitrate with Hybrid Ag/g-C3N4 under Ultraviolet and Visible Light.

The concentration of nitrate in natural surface waters by agricultural runoff remains a challenging problem in environmental chemistry. One promising denitrification strategy is to utilize photocatalysts, whose light-driven excited states are capable of reducing nitrate to nitrogen gas. We have synthesized and characterized pristine and silver-loaded graphitic carbon nitrides and assessed their activity for photocatalytic nitrate reduction at neutral pH. While nitrate reduction does occur on the pristine material, the silver cocatalyst greatly enhances product yields. Kinetic studies performed in batch photoreactors under both UV and visible excitation suggest that nitrate reduction to produce aqueous nitrite, ammonium, and nitrogen gas proceeds via a cooperative water reduction on the silver metal domains to produce adsorbed H atoms. By varying the percentage of silver loading onto the g-C3N4, the density of metal domains can be adjusted, which in turn tunes the reduction selectivity toward various products.

Well-Adhered Copper Nanocubes on Electrospun Polymeric Fibers.

Electrospun polymer fibers can be used as templates for the stabilization of metallic nanostructures, but metallic species and polymer macromolecules generally exhibit weak interfacial adhesion. We have investigated the adhesion of model copper nanocubes on chemically treated aligned electrospun polyacrylonitrile (PAN) fibers based on the introduction of interfacial shear strains through mechanical deformation. The composite structures were subjected to distinct macroscopic tensile strain levels of 7%, 11%, and 14%. The fibers exhibited peculiar deformation behaviors that underscored their disparate strain transfer mechanisms depending on fiber size; nanofibers exhibited multiple necking phenomena, while microfiber deformation proceeded through localized dilatation that resulted in craze (and microcrack) formation. The copper nanocubes exhibited strong adhesion on both fibrous structures at all strain levels tested. Raman spectroscopy suggests chemisorption as the main adhesion mechanism. The interfacial adhesion energy of Cu on these treated PAN nanofibers was estimated using the Gibbs-Wulff-Kaischew shape theory giving a first order approximation of about 1 J/m2. A lower bound for the system's adhesion strength, based on limited measurements of interfacial separation between PAN and Cu using mechanically applied strain, is 0.48 J/m2.

Influence of Iron-Doped Apatite Nanoparticles on Viral Infection Examined in Bacterial Versus Algal Systems.

The Centers for Disease Control and Prevention have estimated that each year, two million people in the United States become infected with antibiotic-resistant bacteria, of which, approximately 23000 die as a direct result of these infections. Phage therapy, or the treatment of bacterial infection by specific, antagonistic viruses, provides one alternative to traditional antibiotics. Bacteriophages, or phages, are bacteria-specific viruses that possess biological traits that allow for not only the removal of bacterial infection, but also the evasion of bacterial resistance, which renders antibiotics ineffective. Previous research has shown the addition of iron-doped apatite nanoparticles (IDANPs) to bacteria prior to phage exposure results in increased bacterial plaques in vitro. Coupled with the biocompatible nature of apatite, these results provide promise for future use of IDANPs as adjuvants to phage therapy along with anti-bacterial applications yet to be explored. Although IDANP enhancement of phage infection has been replicated many times in gram-positive and gram-negative prokaryotic hosts as well as with the utilization of both RNA and DNA viruses, the specific mechanisms involved remain elusive. To further understand increased phage infections in a prokaryotic system, and to evaluate the safety of IDANPs as a treatment used in a eukaryotic system, we have replicated plaque assay experiments in an algal system using Chlorella variabilis NC64A and its virus, Paramecium bursaria chlorella virus 1 (PBCV-1). Statistical modeling was used to evaluate alteration in numbers of plaques observed after viral introduction in IDANP-exposed versus non-IDANP-exposed bacterial and algal cell cultures. While IDANPs synthesized between 25°C-45°C and doped with 30% iron have been shown to influence dramatic increases in phage-induced bacterial death, experiments replicated in an algal system indicated viral infections do not increase when C. variabilis cells are pre-exposed to IDANPs. It is essential to potential use of IDANPs as an antibacterial adjuvant that IDANPs do not increase viral infection of eukaryotic host cells during treatment.

A MEMS light modulator based on diffractive nanohole gratings.

We present the design, fabrication, and testing of a microelectromechanical systems (MEMS) light modulator based on pixels patterned with periodic nanohole arrays. Flexure-suspended silicon pixels are patterned with a two dimensional array of 150 nm diameter nanoholes using nanoimprint lithography. A top glass plate assembled above the pixel array is used to provide a counter electrode for electrostatic actuation. The nanohole pattern is designed so that normally-incident light is coupled into an in-plane grating resonance, resulting in an optical stop-band at a desired wavelength. When the pixel is switched into contact with the top plate, the pixel becomes highly reflective. A 3:1 contrast ratio at the resonant wavelength is demonstrated for gratings patterned on bulk Si substrates. The switching time is 0.08 ms and the switching voltage is less than 15V.