Location-Dependent Local Field Enhancement Along the Surface of the Metal–Dielectric Core–Shell Nanostructure

A theoretical study based on quasi-static approximation is performed to investigate the location-dependent local field enhancement around the dielectric shell-coated gold nanosphere. Our calculation results show that the local field distribution near a gold nanoparticle can be altered greatly by coating with a dielectric shell. Because of the polarizability of the dielectric shell, increasing azimuth angle along the inner surface leads to the increase of the local field, which is opposite to that of the outer surface. Furthermore, the location-dependent local field enhancement and resonance frequency at both the inner and outer surfaces can also be modulated by varying the shell thickness and shell dielectric constant. These calculation results about the location-dependent local field enhancement show the potential of dielectric-coated metallic nanostructure for single-molecule detection based on surface-enhanced Raman scattering and surface enhanced fluorescence.     

Ultra-Uniform Field Distribution in a Finite-Width Metal–Dielectric–Metal Waveguide

We investigate the propagation characteristics of the fundamental surface plasmon polariton (SPP) mode of a finite-width metal–dielectric–metal waveguide. By changing the refractive index or the thickness of the dielectric layer of the waveguide, the SPP mode can be transformed from a mode confined in the dielectric layer into a mode confined around the metal corners. There always exists a condition at which the mode field distribution in the dielectric layer becomes almost perfectly uniform along the direction parallel to the metal layers, and this condition is insensitive to the width of the waveguide. It is also possible to obtain an ultra-uniform field distribution by controlling the refractive index of a different dielectric placed on both sides of the waveguide. The waveguide can be used as a basic structure for the realization of nanosized photonic devices and sensors.     

Nonlinear Dependences of Optical Properties of Spherical Core–Shell Silver–Gold and Gold–Silver Nanoparticles on Their Parameters

Modeling of nonlinear optical properties of spherical core–shell gold–silver and silver–gold nanoparticles (NPs) placed in water was carried out on the base of extended Mie theory. Efficiency cross sections of absorption σ _abs, scattering σ _sca, and extinction σ _ext of radiation with wavelengths λ?=?400 and 532 nm for core–shell NPs with constant core radii r ₀₀?=?5, 10, 20, and 40 nm and in the range of relative radii r ₁/r ₀₀?=?1–8 were calculated (r ₁ is the radius of shell). Dependences of optical properties of gold–silver and silver–gold NPs on increasing of core radius r ₀ in the range 0???r ₁ under condition r ₁?=?const and increasing of r ₀ under r ₁???r ₀?=?const were investigated. Results show the nonlinear behavior of optical properties of core–shell gold–silver and silver–gold NPs on radiation wavelengths (optical indexes of metals), different core and shell radii, and their correlation, on relative NP radii r ₁/r ₀₀. An increase and decrease of absorption, scattering, and extinction efficiency cross sections of core–shell NPs with changing of wavelengths, core and shell radii, and relative NP radii r ₁/r ₀₀ are established. These dependences can be used for experimental investigation of the interesting first stages of shell formation on core and optical determination of core–shell NP parameters.     

Phase Transition and Optical Properties of DNA–Gold Nanoparticle Assemblies

We review recent work on DNA-linked gold nanoparticle assemblies. The synthesis, properties, and phase behavior of such DNA–gold nanoparticle assemblies are described. These nanoparticle assemblies have strong optical extinction in the ultraviolet and visible light regions; hence, the technique is used to study the kinetics and phase transitions of DNA–gold nanoparticle assemblies. The melting transition of DNA–gold nanoparticle assemblies shows unusual trends compared to those of free DNA. The phase transitions are influenced by many parameters, such as nanoparticle size, DNA sequence, DNA grafting density, DNA linker length, interparticle distance, base pairing defects, and disorders. The physics of the DNA–gold nanoparticle assemblies can be understood in terms of the phase behavior of complex fluids, with the colloidal gold interaction potential dominated by DNA hybridization energies.     

Optical Properties of Core–Shell Gold–Silver and Silver–Gold Nanoparticles for Near UV and Visible Radiation Wavelengths

Modeling of optical properties of spherical core–shell gold–silver and silver–gold nanoparticles (NPs) was carried out based on extended Mie theory for radiation wavelengths in the range 300?≤?λ?≤?650 nm. Efficiency factors of absorption, scattering, and extinction of radiation by core–shell NPs in the range of the radii 5–100 nm and in the range of shell thicknesses 0–40 nm were calculated. Results show the nonlinear dependences of optical properties of core–shell gold–silver and silver–gold nanoparticles on radiation wavelengths, core radii, and shell thicknesses. These results can be applied for photonic technologies of nanoparticles.     

Simulation of the Nonlinear Optical Properties of Colloids Containing Metallic Core–Dielectric Shell Nanoparticles

Nonlinear (NL) optical properties of composite materials containing metallic core–dielectric shell nanoparticles in aqueous solution were investigated numerically using the Maxwell–Garnett model and the degenerate electron gas model. Influence of geometry and excitation laser intensity was considered to describe the local field factor and the third-order NL susceptibility.     

Dielectric behavior of DNA in water–organic co-solvent mixtures

The radiowave dielectric dispersions of DNA in different water–organic co-solvent mixtures have been measured in the frequency range from 100 kHz to 100 MHz, where the polarization mechanism is generally attributed to the confinement of counterions within some specific lengths, either along tangential or perpendicular to the polyion chain. The dielectric dispersions have been analyzed on the basis of two partially different dielectric models, a continuum counterion fluctuation model proposed by Mandel and a discrete charged site model, proposed by Minakata. The influence of the quality of the solvent on the dielectric parameters has been investigated in water–methanol and water–glycerol mixtures at different composition, by varying the permittivity ?_m and the viscosity η of the solvent phase. The analysis of the dielectric spectra in solvents where electrostatic and hydrodynamic interactions vary with the solvent composition suggests that both the two models are able, in principle, to account for the observed high-frequency dielectric behavior. However, while some certain assumptions are necessary about the polyion structure within the Mandel model, no structural prerequisite is needed within the Minakata model, where the polarization mechanism invoked considers a radial counterion exchange with the outer medium, which is largely independent of the local polyion conformation.     

Tunable Properties of Surface Plasmon Resonances: The Influence of Core–Shell Thickness and Dielectric Environment

In the present study, we have investigated the extinction spectra of coated sphere (using dipole model) with different core–shell radius, in which the core is TiO₂ and the shell is made up of silver or gold nanoparticles. Nanoparticles exhibit surface plasmon resonance peak; these plasmonic peaks are highly tunable in wavelength range of 300 to 1,100 nm; in fact, the blue and red shifting of resonance peak highly depends on the core–shell thickness. The broadness of resonance peaks are analysed in terms of full width at half maxima (FWHM), and the width of these resonance peaks is also the function of core–shell radius.     

Simulation of the Linear and Nonlinear Optical Properties of Colloids Containing Metallic Core–Dielectric Shell Nanoellipsoids

Linear and nonlinear (NL) optical properties of colloids containing metallic core–dielectric shell nanoellipsoids (NEs) were studied using the Maxwell-Garnett model. Influence of the NE geometry and the linear refraction index of the shell and the host on the linear optical properties and the enhancement factor due to the local field factor of metallic NEs in aqueous solution were analyzed. The expression for the third-order NL susceptibility for this composite material was obtained based on the NL response of aligned NEs. Results show that the plasmon resonance peak (PRP) and the enhancement factors can be tuned changing the NE geometry and the dielectric properties of the shell and the host.     

Cell Polarity: Which Way to Grow in an Electric Field?

Cell polarity can be influenced by an electric field, but the mechanisms behind this response are poorly understood. A new paper shows that fission yeast cells change their direction of growth in an external electric field and suggests mechanisms based on the cortical pH gradient and on electrophoresis of membrane proteins.     

Local–global overlap in diversity informs mechanisms of bacterial biogeography

Spatial variation in environmental conditions and barriers to organism movement are thought to be important factors for generating endemic species, thus enhancing global diversity. Recent microbial ecology research suggested that the entire diversity of bacteria in the global oceans could be recovered at a single site, thus inferring a lack of bacterial endemism. We argue this is not the case in the global ocean, but might be in other bacterial ecosystems with higher dispersal rates and lower global diversity, like the human gut. We quantified the degree to which local and global bacterial diversity overlap in a diverse set of ecosystems. Upon comparison of observed local–global diversity overlap with predictions from a neutral biogeography model, human-associated microbiomes (gut, skin, mouth) behaved much closer to neutral expectations whereas soil, lake and marine communities deviated strongly from the neutral expectations. This is likely a result of differences in dispersal rate among ‘patches'', global diversity of these systems, and local densities of bacterial cells. It appears that overlap of local and global bacterial diversity is surprisingly large (but likely not one-hundred percent), and most importantly this overlap appears to be predictable based upon traditional biogeographic parameters like community size, global diversity, inter-patch environmental heterogeneity and patch connectivity.     

Gender and Age Differences in the Suppressive Effect of a 50 Hz Electric Field on the Immobilization-Induced Increase of Plasma Glucocorticoid in Mice

We developed an experimental system to characterize the suppressive effect of extremely low-frequency (ELF) electric fields (EFs) on the stress response. We assessed differences in the EF effects by age and gender. Control, EF-alone, immobilization-alone, and co-treated groups were subjected to an EF (50 Hz, 10 kV/m). Co-treated mice were exposed to the EF for 60 min, with immobilization during the latter half. Our results indicate that the suppressive effects of ELF EFs on the stress response in immobilized mice occur regardless of gender or age. As stress plays an important role in the onset and progression of various diseases, these findings may have broad implications for understanding the efficacy of EFs in animal, and perhaps human, health. Bioelectromagnetics. 2020;41:156–163. © 2019 Bioelectromagnetics Society.     

Near-Field Optical Properties of Ag x Au1−x Nanoparticle Chains Embedded in a Dielectric Matrix

Plasmonics - We study by the finite-difference time-domain method the near-field optical properties of isolated or coupled Ag x Au1−x alloy nanoparticles shallowly buried inside dielectric...     

Spatial Variability of Soil Chemical Properties of a Prairie–Forest Transition in Louisiana

Woody plant expansion, particularly eastern red cedar (Juniperus virginiana L.), has been a major threat to Louisiana calcareous prairies. Previous studies have shown that woody plant expansion into grasslands is associated with an increase in soil heterogeneity. We studied the within site spatial variability and among site differences of surface (0–15 cm depth) soil pH, electrical conductivity (EC), and Mehlich III extractable Ca, Mg, K, Fe and Mn from three remnant prairie-forest associations in Winn Parish, Louisiana. The prairie soil was consistently basic (pH > 7.0) and the forest soil was acidic (pH < 7.0) while the transition soil was neutral (pH = 7.0). A nonparametric statistical test for the equality of medians among sites showed the median values of the soil attributes differed (α = 0.05) except for soil Ca and Fe. The similarity in Ca concentration among sites was attributed to the calcareous parent material common to the three sites. Geostatistical analysis showed that spatial dependence was expressed over a range of 20–30 m for most of the soil attributes considered. Semivariogram shapes were similar among sites, suggesting the greater control of soil parent material on the observed spatial soil pattern. Shorter range of variation emerged only for soil pH when soil data from the forest and transition were deleted, indicating the scaling characteristics of soil pH and its susceptibility to plant induced changes. It is concluded that soil pH can be used as an index to determine prairie-forest boundary, and to access the impact of eastern red cedar on these and similar sites derived from calcareous parent material. Further, results from this study can be used for designing future ecological studies within the prairie by taking the soil spatial variability into account.     

Radiative Surface Waves in Layered Plasma–Dielectric Structures and Prospects of Their Application in Plasma Microwave Electronics

Plasma Physics Reports - Surface waves in layered systems consisting of material media with different frequency dispersions are considered: dielectric–plasma–vacuum,...     

Effects of Pulsed Electric Field (PEF) Treatment on Enhancing Activity and Conformation of α-Amylase

To explore an efficient, safe, and speedy application of pulsed electric field (PEF) technology for enzymatic modification, effects of PEF treatment on the enzymatic activity, property and kinetic parameters of α-amylase were investigated. Conformational transitions were also studied with the aid of circular dichroism (CD) and fluorescence spectra. The maximum enzymatic activity of α-amylase was obtained under 15 kV/cm electric field intensity and 100 mL/min flow velocity PEF treatment, in which the enzymatic activity increased by 22.13 ± 1.14 % compared with control. The activation effect could last for 18 h at 4 °C. PEF treatment could widen the range of optimum temperature for α-amylase, however, it barely exerted any effect on the optimum pH. On the other hand, α-amylase treated by PEF showed an increase of V _max, t_1/2 and ΔG, whereas a decrease of K _m and k were observed. Furthermore, it can be observed from fluorescence and CD spectra that PEF treatment had increased the number of amino acid residues, especially that of tryptophan, on α-amylase surface with enhanced α-helices by 34.76 % and decreased random coil by 12.04 % on α-amylase when compared with that of untreated. These changes in structure had positive effect on enhancing α-amylase activity and property.     

Phantom Model Testing of Active Implantable Cardiac Devices at 50/60 Hz Electric Field

Exposure to external extremely low-frequency (ELF) electric and magnetic fields induces the development of electric fields inside the human body, with their nature depending on multiple factors including the human body characteristics and frequency, amplitude, and wave shape of the field. The objective of this study was to determine whether active implanted cardiac devices may be perturbed by a 50 or 60 Hz electric field and at which level. A numerical method was used to design the experimental setup. Several configurations including disadvantageous scenarios, 11 implantable cardioverter-defibrillators, and 43 cardiac pacemakers were tested in vitro by an experimental bench test up to 100 kV/m at 50 Hz and 83 kV/m at 60 Hz. No failure was observed for ICNIRP public exposure levels for most configurations (in more than 99% of the clinical cases), except for six pacemakers tested in unipolar mode with maximum sensitivity and atrial sensing. The implants configured with a nominal sensitivity in the bipolar mode were found to be resistant to electric fields exceeding the low action levels, even for the highest action levels, as defined by the Directive 2013/35/EU. Bioelectromagnetics. 2020;41:136–147. © 2020 Bioelectromagnetics Society.