A. Caprani

Study of the adsorption of albumin on a platinum rotating disk electrode using impedance measurements

Abstract The study of the adsorption of albumin on a platinum rotating disk electrode was performed using impedance measurements. The kinetics of the adsorption was investigated by recording, during 15 min after protein addition, the variation with time of the electrochemical double layer capacitance, Cd . The resulting curve was computed and a model of two consecutive reactions occuring at the interface was proposed to describe the adsorption reaction. The corresponding theoretical equation used to fit the data showed five parameters: three capacitance (of the bare surface, of the surface covered with the reversibly adsorbed proteins, and of the surface covered with the irreversibly adsorbed proteins) and two time constants. The influence of the shear rate on these parameters was studied within the range 0–1728s t- and the two time constants, corrected for the mass transfer contribution, showed a shear dependence. Different possibilities were considered for the decrease in capacitance that goes with the adsorbed protein denaturation reaction.

Kinetics of albumin and fibrinogen adsorption onto a rotating disk electrode: Effect of temperature, concentration and material

Abstract In a previous paper, the kinetics of albumin adsorption onto a platinum rotating disk electrode polarized at 75 mV/SCE were reported as measured by the variations of the double layer capacitance C d . The adsorption was found to consist of two steps. In the present work, a better characterization of these steps was attempted through a study of the concentration and temperature dependence of the adsorption of albumin and fibrinogen onto platinum and of fibrinogen onto carbon. As expected from the model, each of the two steps of the adsorption process displayed a specific reaction order and activation energy. The reaction orders of the first and second steps were found to be, respectively, 0.36 and 0 for albumin onto platinum, 0.44 and 0 for fibrinogen onto platinum, and 0.81 and 0.61 for fibrinogen onto carbon. In the case of albumin adsorption onto platinum, activation energies of 14.6 and 10.9 kJ mol −1 were calculated for the first and second step, respectively. The area of close contact between the adsorbed protein and the electrode was found to be significantly larger on carbon than platinum. Concerning albumin adsorption onto platinum, the bulk concentration dependence of the variation of C d suggests that proteins adsorb in a different conformation depending on their concentration in solution.

Analysis and physical significancy of the kinetic parameters associated with albumin adsorption onto glassy carbon obtained by electrochemical impedance measurements

Abstract The kinetics of albumin adsorption onto a glassy carbon rotating disk electrode in a phosphate buffer, is reported from the time variations of the double-layer capacitance Cd, of the charge transfer resistance Rt and of the Tafel coefficient of the electrochemical reaction b: these three electrical quantities are determined by electrochemical impedance and faradaic current I measurements. The variations of Cd, 1/Rt, b  (RtI)−1 and I can be written under the form: α0 + α1 e−t/τ1 + α2 e−t/τ2, where α0, α1, α2, τ1, τ2 characterize a given electrical quantity. We demonstrate that this type of variation validates a two-step adsorption mechanism already proposed, starting from simply taking into account Cd(t). We analyze and discuss comprehensively various possibilities of interpreting the shifts between the time-constants associated with the various quantities as well as the correlations between these constants and those associated with physical adsorption phenomena.

On the anodic dissolution of titanium between 15°C and 100°C in deaerated 2 m hydrochloric acid

Abstract We have demonstrated in previous investigations, under usual conditions of temperatures and pressures, that titanium dissolution, in media including chloride acid, is consistent with a reaction model having a “bifurcation”, and involves six or nine kinetic parameters. This result was obtained mainly via the utilization of polarization curves and an optimization calculation. In this paper, we suggest extending the study in order to include temperatures up to 100°C and pressures up to 70 atm. A specific apparatus was developed to ensure a single phase medium. Calculation of activation energies will be used to investigate the influence of temperature on the kinetics of dissolution-passivation of titanium.

Responses to shear stress and to histamine addition of a cultured vascular endothelium—kinetic study using electrical impedance measurements

Electrical properties of a layer of human vascular endothelial cells are investigated in vitro, under hydrodynamic stress. The measurements of the electrical impedance of vascular endothelial cells cultured at an electrode provides information on the interfacial structure. We give new results which confirm the interpretation of an electrical equivalent network, consisting of three RC circuits in series, proposed in a previous paper: this electrical network corresponds to the junction resistance and the cell capacitance, to the extracellular matrix structure and to that of the macromolecular layer adsorbed on the luminal side of the cell. The modifications of these structures are followed vs time after histamine addition, or after shear rate variations. After histamine addition, there is an increase of the junction permeability to small species and to macromolecules. The two phenomena depend on histamine concentration in different ways. During and after shear rate steps, we follow the modifications generated by the cell response on all the structures. The junction permeability to macromolecules increases, and the capacitance related to cell membranes is modified; both processes have long time constants (several hours). The extracellular matrix structure is slightly modified and quickly reverses to its initial structure. The adsorbed layer undergoes more important modifications, probably involving molecular exchanges with the culture medium, related to changes in the cell surface potential.

Effect of the Electrode Roughness on the Kinetics of Albumin Adsorption at a Rotating Disk Electrode Investigated by Double‐Layer Capacitance Measurements

The roughness effect on the kinetics of the adsorption of bovine serum albumin on a platinum or glassy carbon RDE is investigated by analyzing the roughness dependence on adjustable parameters involved in the equation of the calculated curve which fits the experimental curve. This experimental variation satisfies a two‐exponential law, which is consistent with a two‐step irreversible mechanism: . Thus, the adsorption kinetics can be quantified by the time‐constants of the two steps, and the adsorbed states are characterized by the double‐layer capacitance values of the interface covered with a monolayer of each adsorbate. Whatever the material investigated, a change of the electrode roughness, at constant potential, markedly affects the kinetics of both steps. In addition, the roughness conditions the adsorption mechanism, i.e., the structure of the two adsorbates. Two distinct mechanisms are indeed determined, the one occurring on smooth interfaces, and the other on rough interfaces, the former being slower and leading to stronger interactions between the electrode and the protein layer. Moreover, the role of roughness per se, investigated at constant electrical charge, corroborates that the kinetics are generally faster and the interactions weaker on rough interfaces.

Adsorption of bovine serum albumin onto glassy carbon in a couette flow. Effect of shear rate on the adsorption kinetics and on the structure of the adsorbed proteic layer

Adsorption of bovine serum albumin onto glassy carbon is investigated by analysing the time-variation of the double-layer capacitance recorded during the adsorption process. The effect of shear rate is investigated under laminar conditions in a Couette flow. Stationary and sinusoidally modulated values of the shear rate are imposed over the (0-200 s-1) range. The flow conditions are shown to play an essential role by markedly modifying the rate of all the steps (three at most) involved in the adsorption mechanism. Moreover, the structure of the adsorbed layers in the intermediate and final states are also strongly modified, an increase of the shear rate increasing the interaction between the protein and the electrode. Piezoelectric properties of albumin are invoked to account for the experimental results.

Electrical impedance measurements of endothelial cells grown at an electrode under hemodynamic stress

Abstract Measurements of differential electrical impedance of human endothelial cells cultured on an electrode were performed under hydrodynamic stress. These measurements were achieved in situ within the 1 mHz to 1 MHz frequency range. Three reproducible capacitive time constants characterizing the biological material and sensitive to numerous physicochemical parameters were found.

Novel tools for determining mechanically induced modifications of membrane structure: Electrochemical and fluorescence methods

Abstract Two methods for investigating the effects of mechanical strain on the structure of biological membranes are proposed and illustrated in the case of the erythrocytic membrane. Firstly, we show that the diffusion limiting current measured at a rotating disc electrode for a biological suspension allows one to follow the variation of the adhesion of the cells to the wall with the hydrodynamic conditions, provided that the results be interpreted in the framework of partially blocked electrodes. Secondly, we propose an entirely novel method, which consists in following in a Couette flow, the variation of the light intensity of a fluorescent probe (for instance a membrane potential probe) with the shear rate. Original results concerning the erythrocytic membrane are provided both under stationary and non-stationary conditions.