Proton coupled electron transfer of ubiquinone Q2 incorporated in a self-assembled monolayer.

We present a complete study of the reduction of ubiquinone Q(2) (UQ(2)) in simpler aqueous medium, over a pH range of 2.5 to 12.5. The short isoprenic chain ubiquinones (UQ(2)) were incorporated in a self-assembled monolayer. Under these conditions, the global 2e(-) electrochemical reaction can be described on the basis of a nine-member square scheme. The thermodynamic constants of the system were determined. The global 2e(-) process is controlled by the uptake of the second electron. The elementary electrochemical rate constants obtained by fitting of the experimental rate constant were k(s4) = 1.5 s(-1) for QH˙(+)(2)↔ QH(2), k(s5) = 1.5 s(-1) for QH˙↔ QH(-) and k(s6) = 1 s(-1) for Q˙(-)↔ Q(2-). The three electrochemical reactions QH˙(+)(2)↔ QH(2), QH˙↔ QH(-) and Q˙(-)↔ Q(2-) are successively involved when increasing the pH. Protonations can occur or not, before or after the electron uptake and the reaction paths are, from low to high pH: e(-), H(+)e(-), e(-)H(+), H(+)e(-)H(+), H(+)e(-) and e(-)H(+).

Channel microband chronoamperometry: from transient to steady-state regimes.

Chronoamperometric transient regimes were investigated at a single channel microband electrode during chronoamperometric measurements in a microchannel continuously filled by a redox solution. Simulations were performed by spanning a wide range of conditions according to the geometry of microdevices, flow velocity, and time scale of experiments. Boundary conditions were identified and zone diagrams were established showing the predominance areas of transient and steady-state regimes. The predictions were compared to chronoamperometric experiments performed with microdevices of various geometries. The good agreement observed between data validated the predictions.

Theory and experiments of transport at channel microband electrodes under laminar flows. 2. Electrochemical regimes at double microband assemblies under steady state.

The development of any particular analytical or preparative applications using electrochemical techniques in microfluidic devices requires integration of microelectrodes. This involves detailed predictions for optimizing the design of devices and selecting the best hydrodynamic conditions. For this purpose, we undertook a series of works aimed at a precise investigation of mass transport near electrodes with focus on analytical measurements. Part I of this series (Anal. Chem. 2007, 79, 8502-8510) evaluated the common case of a single microband electrode embedded within a microchannel under laminar flow. The present work (Part 2) investigated the case of a pair of microband electrodes operating either in generator-generator or generator-collector modes. The influence of the confining effect and flow velocity on the amperometric responses was examined on the basis of numerical simulations under steady-state regime. Several situations were identified, each of them corresponding to specific interactions taking place between the electrodes. Related conditions were extracted to establish a zone diagram describing all the situations. These predictions were systematically validated by experimental measurements. The results show that amperometric detections within microchannels can be performed at dual electrodes with higher analytical performances than at single ones.