Enhancement of antibacterial properties of Ag nanorods by electric field

Abstract

The effect of an electric field on the antibacterial activity of columnar aligned silver nanorods was investigated. Silver nanorods with a polygonal cross section, a width of 20–60 nm and a length of 260–550 nm, were grown on a titanium interlayer by applying an electric field perpendicular to the surface of a Ag/Ti/Si(100) thin film during its heat treatment at 700 °C in an Ar+H₂ environment. The optical absorption spectrum of the silver nanorods exhibited two peaks at wavelengths of 350 and 395 nm corresponding to the main surface plasmon resonance bands of the one-dimensional silver nanostructures. It was found that the silver nanorods with an fcc structure were bounded mainly by {100} facets. The antibacterial activity of the silver nanorods against Escherichia coli bacteria was evaluated at various electric fields applied in the direction of the nanorods without any electrical connection between the nanorods and the capacitor plates producing the electric field. Increasing the electric field from 0 to 50 V cm^?1 resulted in an exponential increase in the relative rate of reduction of the bacteria from 3.9×10^?2 to 10.5×10^?2 min^?1. This indicates that the antibacterial activity of silver nanorods can be enhanced by applying an electric field, for application in medical and food-preserving fields.     

Probe Mössbauer Spectroscopy of BiNi<Subscript>0.96</Subscript><Superscript>57</Superscript>Fe<Subscript>0.04</Subscript>O<Subscript>3</Subscript>

This paper presents results of a ⁵⁷Fe probe Mössbauer spectroscopy study of the BiNi_0.96⁵⁷Fe_0.04O₃ nickelate. The spectra measured above its TN demonstrate that Fe³⁺ cations heterovalently substitute for Ni²⁺ nickel (←Fe³⁺), being stabilized on four sites of the nickel sublattice in the structure of BiNiO₃. Calculations in an ionic model with allowance for monopole and dipole contributions to the electric field gradient indicate that the parameters of electric hyperfine interactions between ⁵⁷Fe probe atom nuclei reflect the specifics of the local environment of the nickel in the structure of the unsubstituted BiNiO₃ nickelate. Below T^N, Mössbauer spectra transform into a complex Zeeman structure, which is analyzed in terms of first-order perturbation theory with allowance for electric quadrupole interactions as a small perturbation of the Zeeman levels of the ⁵⁷Fe hyperfine structure, as well as for specific features of the magnetic ordering of the Ni²⁺ cations in the nickelate studied.     

Non-linear dielectric properties of aluminium/monomolecular layers of calcium behenate/aluminium structures

We studied the non-linear dielectric properties of aluminium/monomolecular layers of calcium behenate/aluminium structures at high electric induction (D > 10^?3 C m^?2). These properties generally arise from the motion of electric charges that are trapped in both the alumina at the surface of the electrodes and in the monolayers and result in a hyperpolarizability. However, in some particular cases in which dipolar relaxation mechanisms were present we observed a decrease in the polarizability. This decrease is due to a Langevin-Debye-type saturation at high electric field. Since the mechanical compressibility of the monomolecular layers is small (1.6 × 10^?10 N m^?2), electrostatic compressibility and the electrostriction phenomena are of minor importance.The introduction of small quantities of water produces a marked increase in the non-linear phenomena which may mask all the other phenomena.     

Enhancement of antibacterial properties of Ag nanorods by electric field

The effect of an electric field on the antibacterial activity of columnar aligned silver nanorods was investigated. Silver nanorods with a polygonal cross section, a width of 20–60 nm and a length of 260–550 nm, were grown on a titanium interlayer by applying an electric field perpendicular to the surface of a Ag/Ti/Si(100) thin film during its heat treatment at 700 °C in an Ar+H₂ environment. The optical absorption spectrum of the silver nanorods exhibited two peaks at wavelengths of 350 and 395 nm corresponding to the main surface plasmon resonance bands of the one-dimensional silver nanostructures. It was found that the silver nanorods with an fcc structure were bounded mainly by {100} facets. The antibacterial activity of the silver nanorods against Escherichia coli bacteria was evaluated at various electric fields applied in the direction of the nanorods without any electrical connection between the nanorods and the capacitor plates producing the electric field. Increasing the electric field from 0 to 50 V cm⁻¹ resulted in an exponential increase in the relative rate of reduction of the bacteria from 3.9×10⁻² to 10.5×10⁻² min⁻¹. This indicates that the antibacterial activity of silver nanorods can be enhanced by applying an electric field, for application in medical and food-preserving fields.     

The electrical conductivity of copper and tin thin films vacuum deposited in a lateral electric field

Copper and tin thin films of different thicknesses in the coalescence thickness range (160, 210 and 260 Å) were grown by vacuum deposition onto clean glass substrates at room temperature under a pressure of 2 × 10^?5 Torr in different electric fields between 0 and 200 V cm^-1 aligned parallel to the substrates. The electrical conductivities of the films were measured immediately after formation both in situ and after letting air into the system.We found that the application of an electric field of less than 60 V cm^-1 decreased the resistance of the films. However, the application of stronger electric fields produced an increase in the film resistance. This is attributed to a disturbance of the electrostatic forces between the growing islands by the redistribution of charges in the presence of the applied electric field which causes an increased rate of coalescence of the islands in the film.     

Optical piston in glasses

Abstract

Light-driven electron transitions between different traps in the direction opposite to an electric force eE (e is the electron charge and E is the static electric field) are studied. We show that in Ge-doped silica fibres the electrons are transfered predominantly between different Ge centres at concentrations ≥ 10 mol.% and dominate over the ordinary electron mobility in the direction of the force eE. Light acts as an optical piston, moving electrons against the acting force and therefore resulting in positive feedback in response to static electric field. This feedback amplifies the weak initial electric field up to magnitudes of the order of 10⁵?10⁶ Vcm^?1. This strong field breaks the initial inversion symmetry and allows effective second harmonic generation.     

Nonequilibrium Electron Cooling by NIS Tunnel Junctions

We discuss the theoretical framework to describe quasiparticle electric and heat currents in NIS tunnel junctions in the dirty limit. The approach is based on quasiclassical Keldysh-Usadel equations. We apply this theory to diffusive NIS^′S tunnel junctions. Here N and S are respectively a normal and a superconducting reservoirs, I is an insulator layer and S^′ is a nonequilibrium superconducting lead. We calculate the quasiparticle electric and heat currents in such structures and consider the effect of inelastic relaxation in the S^′ lead. We find that in the absence of strong relaxation the electric current and the cooling power for voltages eV<Δ are suppressed. The value of this suppression scales with the diffusive transparency parameter. We ascribe this suppression to the effect of backtunneling of nonequilibrium quasiparticles into the normal metal.     

Macroporous hydrogels based on 2-hydroxyethyl methacrylate. Part 4: Growth of rat bone marrow stromal cells in three-dimensional hydrogels with positive and negative surface charges and in polyelectrolyte complexes

The growth of bone marrow stromal cells was assessed in vitro in macroporous hydrogels based on 2-hydro- xyethyl methacrylate (HEMA) copolymers with different electric charges. Copolymers of HEMA with sodium methacrylate (MA⁻) carried a negative electric charge, copolymers of HEMA with [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MOETA⁻) carried a positive electric charge and terpolymers of HEMA, MA⁻ and MOETA⁺ carried both, positive and negative electric charges. The charges in the polyelectrolyte complexes were shielded by counter-ions. The hydrogels had similar porosities, based on a comparison of their diffusion parameters for small cations as measured by the real-time tetramethylammonium iontophoretic method of diffusion analysis. The cell growth was studied in the peripheral and central regions of the hydrogels at 2 hours and 2, 7, 14 and 28 days after cell seeding. Image analysis revealed the highest cellular density in the HEMA-MOETA⁺ copolymers; most of the cells were present in the peripheral region of the hydrogels. A lower density of cells but no difference between the peripheral and central regions was observed in the HEMA-MA⁻ copolymers and in polyelectrolyte complexes. This study showed that positively charged functional groups promote the adhesion of cells.     

Transient charging currents in annealed bulk polyethylene

Transient charging currents resulting from the application of a low constant voltage, up to 2×10⁴ V cm^–1, are investigated in melt crystallized samples of high density polyethylene. Three distinct regions can be clearly distinguished in the current-field characteristic curves at a given time after application of a voltage: (a) a linear region at a low electric field, (b) a negative resistance region in which the slope is negative at a medium electric field, (c) a superlinear region at a high electric field. At low electric fields and for charging times less than 10³ sec the law of dielectric response, l t ^–n , is obeyed. It is shown that the type of cooling process, after annealing at temperatures near 75° C, markedly influences the current decay. Annealing at this temperature is associated with a partial rearrangement of the lamellar microstructure. The current is enhanced after quenching while it decreases after slow cooling. Various mechanisms to explain the obtained data are considered.     

Electric current and heat induced acoustic emission in cadmium sulfide

Acoustic emission (AE) induced by the electric current passing through a cadmium sulfide (CdS) single crystal was studied. In the temperature range T=300–450 K, the intensity of the AE signals excited in the CdS crystal exposed to a constant electric field increases with the current density. It is suggested that AE in CdS is due to the dislocations breaking off and moving under the action of direct electric current and thermoelastic stresses. The activation energy for this process (E _a=0.35±0.5 eV) was estimated for a current density in the range of j=(1−7)×10⁵ A/m².     

Charge carrier drift mobility in polycrystalline tetracene layers

The charge carrier drift mobility in tetracene layers was measured using the time-of-flight method. Free carriers were generated by short electron pulses. At electric fields E > 2 × 10⁴V cm^?1 the drift velocity of holes is constant, i.e. the drift mobility is proportional to the reciprocal electric field: μ_h ∝ E^?1.     

Estimating the energy of electric breakdown in air gap between electrolyte and metal electrode

The influence of induced electric charge (localized on the surface of a suspended copper rod) on the formation of a protrusion (Taylor cone) on the inducing liquid (aqueous solution) surface is considered. At an applied voltage of U ≤ 12 kV, the protrusion height in the interval of pre-breakdown voltages (U < U _P) is limited by the electric field strength. At U > U _P, the growth of protrusion is terminated by an electric discharge, which drives the liquid to oscillate in a broad range of applied voltages U at almost constant multiple frequencies f = f ₀ n, which are resonantly switched at certain fixed U values. By measuring the amount of evaporated liquid, the energy (27.8 × 10⁻³ J) and current (64.9 A) of single discharge were evaluated and the electric capacitance (7.6 × 10⁻¹⁰ F) of a system comprising the water surface and suspended copper electrode was estimated. Serial connection of an additional capacitor (100 μF) to the copper electrode with induced electric charge leads to a threefold increase in these parameters.     

Refractive index of polymethylmethacrylate oriented by fluid temperature under electrical field

We prepared micron and submicron polymethylmethacrylate (PMMA) layers by the spin-coating method. We investigated the possibility to orientate polymer dipoles in electric field in the glass transition area (T _g) and the fluid temperature of PMMA with the aim to increase its refractive index (n) after the layer is cooled below T _g. We have studied the effect of electric field (up to 12 kV cm⁻¹) on change of surface morphology of the layer, dependence of n and contact angle (surface wettability) on the field and dependence of layers orientation on orientation of electric field. The surface morphology was examined using atomic force microscopy (AFM), contact angles were measured by goniometer, film thickness was measured by profilometer, refractive index of films was determined using refractometer. The change of refractive index as dependent on the PMMA layer orientation in electric field depends on temperature and electric field. The highest change in n was found for electric field 11 kV cm⁻¹. The change in contact angle (wettability) on surface of an orientated PMMA layer confirms the dipoles orientation in electric field unambiguously. The orientation of layers causes a “slight” change in their morphology and a “slight” increase of surface roughness only for one direction of field effect. Change in colour for oriented layers does not depend on orientation of electric field.     

Decomposition of radioactive methyl iodide in an electric discharge field

The possibility of using electric discharge for removing radioactive iodine in the form of volatile organic compounds from air-gas streams was examined. In an electric discharge field, at the methyl iodide concentration in the range from 0.9 to 19 mg l^?1, relative humidity within 48–90%, and linear flow velocity of 0.5–3.5 cm s^?1, the decontamination factor characterizing the methyl iodide decomposition is ≥1.4 × 10⁵.     

Preparation and characterization of sulfonated poly(1,3,4-oxadiazole)/multi-walled carbon nanotube composite films with high performance in electric heating and thermal stability

For manufacturing thermally stable electric heating composite films, a sulfonated poly(1,3,4-oxadiazole) (sPOD) was synthesized and it was composited with pristine MWCNT of 0.1–10.0 wt% by an ultrasonicated solution mixing and casting. SEM images revealed that the pristine MWCNTs were dispersed well in the composite matrix via π–π interaction between the MWCNTs and the aromatic rings of sPOD backbone. The electrical resistivity of the composite films decreased considerably from ∼10⁹ Ω cm to ∼10⁰ Ω cm with the increment of the MWCNT content by forming a percolation threshold at ∼0.026 wt%. The composite films with 5.0–10.0 wt% MWCNT contents, which had sufficiently low electrical resistivity of ∼10³–10⁰ Ω cm, exhibited excellent electric heating performance by attaining high maximum temperatures as well as electric energy efficiency. Since the dominant thermal decomposition of the composite films took place at ∼500 °C, sPOD/MWCNT composite films with low electrical resistivity could be used for high performance electric heating materials for advanced applications.     

Thermally activated plastic flow of metals and ceramics with an electric field or current

The effects of high density electric current pulses (10³–10⁶ A cm⁻²) on the flow stress of metals at low homologous temperatures and of a modest external electric field on the flow stress of fine-grained oxides at high temperatures is presented. The results in both cases are evaluated in terms of thermally-activated plastic deformation processes. In the case of the metals, the influence of an electron wind on each of the parameters in the equation for the thermally-activated motion of dislocations was determined, the largest effect being on the pre-exponential. The derived electron wind push coefficient was one or more orders of magnitude larger than the value normally accepted for the electron drag coefficient. In the case of the oxides, the substantial effect of an applied electric field on the flow stress was evaluated in terms of its influence on the electrochemical potential of vacancies in the space-charge cloud adjacent to the grain boundaries. Both the derived space-charge cloud width and the electric potential/stress parameter Δ∅/Δσ are in reasonable accord with theoretical predictions.     

Structure transition and multiferroic properties of Mn-doped BiFeO3 thin films

Pure BiFeO₃ (BFO) and Mn-doped BiFe_1?yMn_yO₃ thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel method. The effects of Mn-doping on the structure and electric properties of the BFO thin films were studied. The X-ray diffraction (XRD) analysis reveals a structure transition in the Mn-doped BiFe_0.96Mn_0.04O₃ (BFMO) thin film. The Rietveld refined XRD patterns conform the trigonal (R3c: H) and tetragonal (P422) symmetry for the BFO and BFMO thin films, respectively. The structure transition and the mixed valences of Mn ions substantially improve the electric properties of the BFMO thin film. The remnant polarization (P _r) of the BFMO thin film was 105.86 μC/cm² at 1 kHz in the applied electric field of 865 kV/cm. At an applied electric field of 150 kV/cm, the leakage current density of BFMO thin film is 1.42 × 10^?5 A/cm². It is about two orders of magnitude lower than that of the pure BFO thin film (1.25 × 10^?3 A/cm²). And the enhanced saturated magnetization of the BFMO thin film is 4.45 emu/cm³.     

Optical nonlinearity of metal nanoparticle composites fabricated by negative ion implantation

Optical absorption and nonlinear absorption were studied for Ag nanoparticle composite. Negative Ag⁻ ion with 60 keV were applied for implanting into amorphous-SiO₂ at a flux 3 μA/cm² to total fluences ranging from 3 × 10¹⁶ to 1 × 10¹⁷ ions/cm². Absorption spectra of Ag-implanted amorphous-SiO₂ showed a surface plasmon peak resulting from formation of nanoparticles. The strength of the resonance reflected from the local electric field inside the nanoparticle induced by an external electric field. Nonlinear optical constants were evaluated by the z-scan method with a tunable femtosecond laser system. The strength of the nonlinearity also reflected from the local electric field. Nonlinear absorption coefficients exhibit a maximal value of −3.6 × 10³ m/GW for Ag:SiO₂ at 420 nm (2.95 eV), around the surface plasmon resonance.     

Acoustic emission in dislocation-containing silicon exposed to current and thermal influences

An investigation was made of acoustic emission in silicon single crystals during passage of an electric current. It was observed that in the temperature range studied (T=300−450K) acoustic emission signals whose intensity increases with increasing dislocation density are excited in a static electric field. The acoustic emission of silicon single crystals with and without dislocations is compared. It is assumed that the acoustic emission in silicon is caused by the unpinning and migration of dislocations under the influence of the direct electric current and thermoelastic stresses. The activation energy of this process is estimated as E=0.53±0.05 eV during passage of a direct current of density j=2.8×10⁵ A/m². Pis’ma Zh. Tekh. Fiz. 25, 28–32 (February 12, 1999)     

Electrical studies on sputtered CuCl thin films

CuCl is a wide-direct band gap semiconductor, lattice matched to Si and it possesses excellent ultra violet (UV) emission properties. It is thus a promising candidate for the next generation Si based UV optoelectronics. CuCl films were deposited using RF magnetron sputtering technique. X-ray diffraction analysis reveals that the grains are strongly <111> oriented. Triangular crystallites of CuCl were observed in the AFM surface topograph. Au–CuCl–Si–Au structures were fabricated and field dependent electrical studies were carried out in the electric field range of 1.25 × 10⁶ to 2.5 × 10⁷ V/m. I–V characteristics show that ohmic conduction prevails in low electric fields up to 2.5 × 10⁶ V/m. In the higher field range, from 2.5 × 10⁶ to 2.5 × 10⁷ V/m, the conduction mechanism was Schottky emission controlled. There was no trap related charge transport observed at higher electric fields. Preliminary electrical studies are reported in this article.