Comparison of the antimicrobial photocatalytic activities of SiO2 and Au@SiO2 nanostructures in water decontamination

Photocatalytic disinfection of Escherichia coli suspension by silicon dioxide nanoparticles and silicon dioxide/gold nanocomposite in a batch reactor is investigated experimentally and results are compared. Silica nanoparticles were synthesized by Stöber method and pulsed laser ablation method was employed to prepare gold nanoparticles in distilled water. Composition of two nanoparticles species was carried out, using the second harmonic pulse of Nd:YAG laser, whose wavelength is in the absorption spectra of gold nanoparticles. Results confirm a decrease in the bandgap energy of silica nanoparticles after composition. Escherichia coli were selected as an indicator of the microbial water contamination. Disk diffusion method was used to evaluate the antimicrobial potential of SiO2 and Au@SiO2 nanostructures. Photocatalytic activities of both nanostructures were examined in dark, and under the irradiation of UV and visible light. In all conditions, the performance of Au@SiO2 nanocomposites was higher than SiO2 nanoparticles. In dark condition the higher biocidal nature and activity of Au nanoparticles and for the case of UV radiation, decreasing the bandgap energy and recombination rate of SiO2 nanoparticles after composition with Au increased the efficiency. For the case of visible light radiation, surface plasmon resonances effects, and local heat of Au nanoparticles were responsible for increasing the efficiency. RESEARCH HIGHLIGHTS: Doping large bandgap semiconductors nanostructures, such as silica with metal nanoparticles, such as gold will improve their photocatalytic activity to work in visible light. In this mechanism, gold nanoparticles act as effective traps to prevent the recombination of photogenerated electron-hole pairs. Other mechanisms, such as Schottky barrier formation, surface plasmon resonance absorption of gold nanoparticles, and biocidal nature of the gold nanoparticles are effective in increasing the efficiency of Au doped silica nanostructures.

Optical properties of synthesized Au/Ag Nanoparticles using 532 nm and 1064 nm pulsed laser ablation: effect of solution concentration

The primary objective of this experimental research is to introduce the capacity of laser irradiation into the synthesis of bimetallic nanoparticles from noble metals. Gold and silver nanoparticles are produced through the laser ablating gold and silver targets in distilled water. Originally, the samples are synthesized by using Nd:YAG laser with 1064 nm wavelength and 7 ns pulse width. Following this, solutions mixed with different volumetric ratios, are irradiated by the second harmonic of the said laser at 532 nm wavelength. The absorption peak of gold nanoparticles around 530 nm, is used to transfer the laser energy to nanoparticles and synthesize Au/Ag bimetallic nanoparticles. The wavelength and volumetric ratio of solutions are the experiment's variables. The bimetallic nanoparticles are characterized as follows: X-ray diffraction pattern, spectroscopy in the range of UV–Vis-NIR and IR, Photoluminescence spectrum, Dynamic light scattering, and Fourier transform infrared spectroscopy. Additionally, FE-SEM and TEM images are used to study the size and morphology of nanoparticles. One of the aims of the research is to investigate the effects of laser wavelength and different volumetric concentrations on the optical properties of Au/Ag bimetallic nanoparticles. On the other hand, the study revealed that silver concentration and laser wavelength in the synthesis of Au/Ag bimetallic nanoparticles with different structures, cause the formation of crystalline structure, growth of grain size, and therefore silver oxide reduction.

Beam shaping design for coupling high power diode laser stack to fiber

A beam shaping technique that rearranges the beam for improving the beam symmetry and power density of a ten-bar high power diode laser stack is simulated considering a stripe mirror plate and a V-Stack mirror in the beam shaping system. In this technique, the beam of a high power diode laser stack is effectively coupled into a standard 550 μm core diameter and a NA=0.22 fiber. By this technique, compactness, higher efficiency, and lower cost production of the diode are possible.

Changeover in the molecular and atomic fluorine laser transitions

Measurements of molecular and atomic fluorine laser emissions were done from 2?psi (absolute) to 5?atm to determine pressure-dependent spectral lines. While at low pressures, the atomic transitions are dominant; however, both atomic and molecular transitions are detected simultaneously at atmospheric pressures above approximately 2?atm, where a changeover occurs. The excitation mechanisms are strongly dependent on the excited and ionized He species within the gain medium. Initially it affects the atomic transitions of the submanifolds and the corresponding molecular transition takes place via collisional events. The gain competitions among the doublet and quartet atomic transitions have also been investigated.

Radiation from high-intensity ultrashort-laser-pulse and gas-jet magnetized plasma interaction

Using a gas-jet flow, via the interaction between an ultrashort high-intensity laser pulse and plasma in the presence of a perpendicular external dc magnetic field, the short pulse radiation from a magnetized plasma wakefield has been observed. Different nozzles are used in order to generate different densities and gas profiles. The neutral density of the gas-jet flow measured with a Mach-Zehnder interferometer is found to be proportional to back pressure of the gas jet in the range of 1 to 8 atm. Strength of the applied dc magnetic field varies from 0 to 8 kG at the interaction region. The frequency of the emitted radiation with the pulse width of 200 ps (detection limit) is in the millimeter wave range. Polarization and spatial distributions of the experimental data are measured to be in good agreement with the theory based on the V(p)xB radiation scheme, where V(p) is the phase velocity of the electron plasma wave and B is the steady magnetic field intensity. Characteristics of the radiation are extensively studied as a function of plasma density and magnetic field strength. These experiments should contribute to the development of a new kind of millimeter wavelength radiation source that is tunable in frequency, pulse duration, and intensity.