Pierre F. Brevet

Finite element simulation of the chronoamperometric response of recessed and protruding microdisc electrodes

The chronoamperometric and steady-state responses of both recessed and protruding microdisc electrodes have been evaluated by numerical simulation using a commercial finite element software package.

Second Harmonic Generation of Glucose Oxidase at the Air/Water Interface

We present a study of the adsorption of the glucose oxidase enzyme (GOx) at the air/water interface, using the nonlinear optical technique of surface second harmonic generation (SSHG). Resonant SSHG experiments were achieved by probing the pi-pi* transition of the flavin adenine dinucleotide (FAD) chromophores embedded in the GOx protein. Because of the subsequent resonance enhancement of the signal, the second harmonic (SH) wave arising from the GOx entities adsorbed at the interface was detectable for protein bulk aqueous concentrations as low as 70 nM. The protein adsorption was followed, and, at high GOx coverage, a change in the orientation of the FAD chromophore was observed, indicating either a rearrangement or a reorientation of the protein at the interface. Inasmuch as GOx is negatively charged at the biological pH of 7, its interactions with charged surfactants were also investigated. As expected, spreading positively charged surfactants onto a partial protein monolayer was found to increase the GOx surface concentration, whereas in the case of negatively charged surfactants, the GOx surface concentration decreased until the SH signal went back to the pure buffer solution response level. With the increasing GOx surface concentration, the rearrangement or reorientation of the protein was also observed.

Resonant-surface second-harmonic generation studies of phenol derivatives at air/water and hexane/water interfaces

Resonant-surface second-harmonic generation and surface tension measurements have been used to study the adsorption of p-nitrophenol, phenol and p-propylphenol at the air/water and hexane/water interfaces at full monolayer coverage. The determining forces defining the angle of orientation of the moieties are the solute–solvent interactions at both interfaces. In particular, we observed the screening effect of the hexane phase on the solute–water interactions. Surface tension measurements performed on these interfaces showed that the number of molecules per unit surface at full coverage follows from the angle of orientation. From a wavelength analysis of the SH response of the monolayers, we are led to the conclusion that partial hydration of the aromatic rings of phenol and p-propylphenol occurs at both the air/water and the hexane/water interfaces, whereas p-nitrophenol remains largely within the water phase at both interfaces.

Surface second-harmonic generation at air/solvent and solvent/solvent interfaces

The origin of the optical surface second-harmonic response at neat air/liquid and liquid /liquid interfaces is still a matter of debate. To elucidate this question, we first set out the basic equations describing the problem, essentially introducing, in an effective surface susceptibility tensor, the two contributions arising from the response of the bulk : the discontinuity of the normal component of the fundamental electromagnetic field and the molecular correlation in the vicinity of the surface. Then we discuss the difference in the magnitude and the ratio of the components of the tensor for the two types of interface, raising the problem of the strength of the field discontinuity at the boundary and the molecular ordering disparities near the interface. From our results, we conclude that in the case of the neat liquid/liquid interfaces, the main source of non-linearity arises from the surface itself rather than from the adjacent bulk solution.

Solar Energy Conversion Using Dye Sensitised Liquid/Liquid Interfaces

Abstract A novel approach to solar energy conversion is presented on the basis of photoinduced heterogeneous electron transfer at polarised interfaces between two immiscible electrolyte solutions. Preliminary studies at the water|1,2-dichloroethane interface sensitised by water-soluble zinc porphyrin heterodimers exhibited conversion efficiencies around 0.1%, corresponding to a photocurrent to absorbed photon flux ratio close to unity. The conversion efficiency is increased in the presence of redox couples acting as supersensitiser in the aqueous phase. The characteristic power relationship for these interfaces is also presented, and the possibility of solar cells where no solid electrodes are directly involved in the photogeneration and transport of charge carriers is envisaged.

Hyper-Rayleigh scattering of gold nanorods and their relationship with linear assemblies of gold nanospheres

The surface plasmon enhanced hyper-Rayleigh scattering light collected from an aqueous solution of gold nanorods is reported. A non negligible part of the signal is attributed to a photoluminescence background attributed to the electron hole recombination following multiphoton excitation of d-valence band electrons into the sp-conduction band. This radiative relaxation process is likely favored by the presence of the organic species adsorbed at the surface of the nanorods. The absolute value for the hyperpolarisability of nanorods is also compared by the external reference method to that of para-nitroaniline and found to be rather large although an absolute value cannot be given because the exact number density of the gold nanorods is unaccessible. This value is however compared with values reported for linear assemblies of gold spherical nanoparticles and further support the simple model of gold metal ellipsoids to describe the hyper-Rayleigh light intensities. The polarisation analysis of the hyper-Rayleigh scattering light is also determined for gold nanorods and compared to the expected one for gold nanospheres. For the latter spheres, the weakness of the signal intensities precludes a definite comparison with the model. On the opposite, for the nanorods, the polarisation dependence of the hyper-Rayleigh scattered light clearly deviates from the one expected for nanospheres.

Selective structure changes of core-shell gold-silver nanoparticles by laser irradiation: homogeneisation vs. silver removal

A method involving laser irradiation is shown to lead to selective modification of core–shell gold–silver nanoparticles.

Finite element simulation of the amperometric response of recessed and protruding microband electrodes in flow channels

The convective mass transfer for recessed and protruding microband electrodes, two geometries found in practical devices, is determined by the finite element method. The problem is solved by a two-step method, first solving the Navier-Stokes equation to compute the real hydrodynamic flow, then solving the convective diffusion equation to calculate the electrochemical response of the systems. In this work, we focus on the chronoamperometric response of recessed and protruding microband electrodes, emphasizing the role of the edge effects and convection streamlines, in particular the role of stagnant recirculating eddies due to these particular geometries.

Surface plasmon enhanced non-linear optical response of gold nanoparticles at the air/toluene interface

Colloidal gold nanoparticles in toluene solution, synthesised using a two-phase liquid/liquid system, display second harmonic generation, wavelength analysis of which indicates the narrow frequency band of the gold surface plasmon for nanoparticles present at the air/toluene interface.

Potential Modulated Reflectance Spectroscopy of the Methyl Orange Transfer Across the Water/1,2-Dichloroethane Interface

The kinetics of methyl orange transfer across the water1,2-dichloroethane interface is studied by potential-modulated reflectance (PMR) and ac-impedance. The frequency dependence analysis of both techniques provide complementary information about charge transfer kinetics and the potential distribution across the interface. The PMR signal is obtained under total internal reflection from the liquidliquid interface. It is concluded that the PMR responses are originated from the ac flux of ion crossing the interface, and therefore provide a direct analysis of the faradaic responses. This technique is also less sensitive to uncompensated resistance effects in comparison to the admittance analysis. In the case of methyl orange transfer, a standard apparent rate constant of (4.590.7) 10 2 cm s 1 and a transfer coefficient of 0.409 0.03 were estimated from the frequency dependence of the PMR.