Mamdouh E. Abdelsalam

Optical properties of nanostructured metal films

Nanostructured metal films of platinum, gold and silver up to 675 nm thick we prepared by electrochemical deposition through templates of 700 nm diameter polystyrene spheres assembled as hexagonal close packed monolayer on an evaporated gold surface followed by removal of the template by dissolution in tetrahydrofuran. The reflection spectra of the films at normal incidence were recorded as a function of film thickness and the spectra correlated with the local visual appearance of the film and the surface structure from SEM. For thin films, below one quarter sphere height, the spectra show a single reflectivity dip at a wavelength just below the sphere diameter consistent with surface-plasmon grating-like behaviour. For the thicker films several reflectivity dips are observed which move towards longer wavelength with increasing film thickness. This behaviour is shown to be consistent with a model in which light reflected from the top of the structure interferes with light reflected from within the spherical segment cavities in the film.

Sculpted substrates for SERS.

Sculpted SERS-active substrates are prepared by assembling a closed packed monolayer of uniform polystyrene colloidal particles (diameter 350 to 800 nm) onto an evaporated gold surface and then electrodepositing gold through this template to produce films with controlled thicknesses, measured as fractions of the sphere diameter, d. The resulting surfaces consist of a regular hexagonal array of interconnected spherical cross-section dishes. The role of localised plasmons in determining the SERS enhancement factor obtained for benzene thiol adsorbed onto the surfaces is then investigated by correlation of the UV-visible reflectance spectra, 400 to 900 nm, measured at the same positions on the substrate surfaces, with the SERS spectra. The results are interpreted in terms of the relative contributions of plasmons that are free to propagate across the top surface and those trapped within the dishes of the sculpted surface.

Quantitative electrochemical SERS of flavin at a structured silver surface.

In situ electrochemical surface enhanced Raman spectra (SERS) for an immobilized monolayer of a flavin analogue (isoalloxazine) at nanostructured silver surfaces are reported. Unique in the present study, the flavin is not directly adsorbed at the Ag surface but is attached through a chemical reaction between cysteamine adsorbed on the Ag surface and methylformylisoalloxazine. Even though the flavin is held away from direct contact with the metal, strong surface enhancements are observed. The nanostructured silver surfaces are produced by electrodeposition through colloidal templates to produce thin (<1 microm) films containing close-packed hexagonal arrays of uniform 900 nm sphere segment voids. The sphere segment void (SSV) structured silver surfaces are shown to be ideally suited to in situ electrochemical SERS studies at 633 nm, giving stable, reproducible surface enhancements at a range of electrode potentials, and we show that the SER spectra are sensitive to subfemtomole quantities of immobilized flavin. Studies of the SER spectra as a function of the electrode potential show clear evidence for the formation of the flavin semiquinone at the electrode surface at cathodic potentials.

Strong coupling of light to flat metals via a buried nanovoid lattice: the interplay of localized and free plasmons

We study the optical plasmonic properties of metal surfaces which have a periodic lattice of voids buried immediately beneath their flat upper surface. Light reflection spectra calculated in the framework of a self-consistent electromagnetic multiple-scattering layer-KKR approach exhibit two types of plasmon resonances originating from the excitation of different plasmon modes: surface plasmon-polaritons propagating on the planar surface of metal and Mie plasmons localized in the buried voids. Coupling between these two types of plasma oscillation leads to an enhancement of the surface plasmon-polariton resonances even for close-packed void lattices. Our theoretical model quantitatively agrees with experimental results, demonstrating that planar surfaces can exhibit strong plasmonic field enhancements.

A study investigating the sonoelectrochemical degradation of an organic compound employing Fenton's reagent

The degradation of an organic dye molecule (specifically meldola blue, MDB) is reported under the influence of power ultrasound in combination with electrochemically-generated hydrogen peroxide. A novel flow system is employed to measure the degradation as a function of time while minimising the disturbance to the acoustics of the sonoelectrochemical reactor employed. The effect of adding Fe2+ to the rate of dye degradation is measured and demonstrated to be significant. Under optimum conditions the rate constant for dye degradation was found to reach a maximum value of (23.7±0.35)×10−3 min−1 assuming pseudo-first order kinetics. The rate constant for the complete destruction of MDB, determined by chemical oxygen demand, was found to be significantly slower at (10.2±2.6)×10−3 min−1.

Electrodeposition of highly ordered macroporous iridium oxide through self- assembled colloidal templates

Iridium oxide electrodeposited through a self-assembled colloidal template has an inverse opal structure. Monolayers present long range hexagonal arrangements of hemispherical nanocavities while multilayers present 3D honeycomb structures with spherical voids. The films are amorphous, have several electroactive redox states and are electrochromic. The nanostructure modifies their reflectivity thus indicating that these films could be used as tunable photonic devices.

Detection of hydroxide ions in aqueous solutions by steady-state voltammetry

The electrochemical oxidation of hydroxide ions on microdisk electrodes was studied to find the conditions where the steady-state current can be used for analytical applications. Results are presented for the dependence of the voltammetric response on the microdisk radius, the electrode material and the hydroxide concentration. Well-defined waves for hydroxide oxidation were obtained under steady-state conditions with all microdisks employed. However, the steady-state wave splits for some hydroxide concentrations and radii, but the overall current is still proportional to the concentration. The usefulness of the wave for analytical applications depends on the electrode material. Although the wave is seen on nickel, platinum and gold the latter is the most promising for the amperometric detection of basicity of aqueous solutions. On platinum, the wave can be used but the plateau is not so well defined. On nickel, oxide formation makes measurements difficult. Under steady-state conditions a well-defined wave attributed to the direct oxidation of hydroxide ions was also observed on a gold rotating disk electrode. The wave height was found to be proportional to the square root of the rotation speed, thus indicating that the process was under diffusion control. The limiting current was also found to be proportional to the concentration of hydroxide ions over the range 0.05-10 mM.

Calibrationless determination of cadmium, lead and copper in rain samples by stripping voltammetry at mercury microelectrodes: Effect of natural convection on the deposition step

Abstract Mercury microelectrodes were prepared by ex situ deposition of Hg onto Pt microdiscs. By exploiting the known properties of microelectrodes in stripping analysis, an absolute method based on a simple equation derived from the stripping charge and the microelectrode steady-state current was assessed for the simultaneous quantification of Cd 2+ , Pb 2+ and Cu 2+ concentrations. The method was tested with synthetic solutions containing known amounts of Cd 2+ , Pb 2+ and Cu 2+ . Then, it was used to determine the labile and total fractions of these metal ions in rain samples. The labile fractions were measured from samples at their natural pH while the total concentrations were determined from samples at pH=2.

Electrochemical, luminescent and photographic characterisation of cavitation

The characterisation of a small sonochemical reactor has been performed using electrochemical, luminescent and photographic techniques. The electrochemical experiments have employed a novel flow system to determine the formation of sonochemical products (in this case hydrogen peroxide) in semi-real time with high sensitivity. The rate of production of hydrogen peroxide is reported as a function of driving pressure amplitude. The degradation of an organic molecule, specifically the organic dye amaranth, within the sonochemical cell is also reported.

Strong coupling between plasmons and excitons in organic molecular J-aggregates on plasmonic crystals

By mapping optical absorption spectra from nanostructured plasmonic crystals coated with organic molecular J-aggregate films of cyanine dyes, strong coupling between J-aggregate excitons and two types of localized and delocalized plasmons is observed.