Catherine Sella

Retention behaviour of polar compounds using porous graphitic carbon with water-rich mobile phases

The retention factors of polar compounds (mono-, di- and trisubstituted aromatic derivatives) were measured on porous graphitic carbon (PGC), alkyl-modified silicas and an apolar copolymer (PRP-1) with water-methanol mobile phases. It was first shown that the mobile phase effects were similar with the three sorbents and that the comparison of retention factors extrapolated to aqueous mobile phases (k′w) could give information on stationary phase-solute interactions. The functional group contribution was examined. For aromatic derivatives containing hydrophobic substituents (alkyl and chloro groups), correlations with octanol-water partition coefficients were obtained for the three sorbents. For aromatic derivatives containing polar substituents, these correlations were obtained only for alkylsilicas and PRP-1. On PGC, the retention factor increased with increase in the number of polar substituents and was shown to depend on both the field and the mutual resonance effects of the different substituents on the aromatic ring. The results indicate that electronic interactions are more important than hydrophobic interactions in the retention mechanism of polar compounds. The parametrization of the polarity of the solutes, taking into account field and resonance effects, was carried out using local dipolar moments and the overall electron-excess charge density. The analyte retention factors could be predicted through correlation between log k′w and the electron-excess charge density.

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.

General Concept of High-Performance Amperometric Detector for Microfluidic (Bio)Analytical Chips

In this work, we established theoretically that amperometric detector arrays consisting of a series of parallel band microelectrodes placed on the wall of a microchannel may offer excellent analytical detection performances when implemented onto microfluidic (bio)analytical devices after the separative stages. In combination with the concentration imprinting strategies reported in a previous work, these exceptional performances may be extended to nonelectroactive or poorly diffusing analytes. Using an array of electrodes instead of a large single band allows the whole core of the channel to be probed though keeping an excellent time resolution. Thus, analytes with close retention times may be characterized individually with a resolution which eventually outpaces that of spectroscopic detections. Such important advantages may be obtained only through a complete understanding of the complex coupling between diffusional and convective transport of molecules in microfluidic solutions near an electrochemical detector. As a consequence, the conditions underlying the theoretical data presented in this work have been selected after optimizing procedures rooted on previous theoretical analyses. They will be fully disclosed in a series of further works that will also establish the experimental performances of such amperometric detectors and validate the present concept.

Direct electroanalytical method for alternative assessment of global antioxidant capacity using microchannel electrodes.

A new electroanalytical method for the characterization of global antioxidant capacity is proposed based on chronoamperometric responses monitored at microchannel band electrodes. This approach does not require any titrating species, biological elements, or precalibration curves. A thin-layer regime is established at the working electrode according to the geometry of the device and hydrodynamic flow rate. Under these conditions, the currents are directly proportional to the total concentration of antioxidants and do not depend on their respective diffusion coefficients. Measurements were performed with synthetic solutions and mixtures of four antioxidants used as sample tests: trolox, ascorbic acid, gallic acid, and caffeic acid. Operating potentials were selected at the formal potentials of some reactive oxygen species to simulate their oxidative attacks. The very good agreement obtained between simulations and experimental data validated this new electroanalytical procedure. These results pave the way for the concept of innovative sensor-type microfluidic devices for alternative determination of antioxidant capacity.

Mass Transport at Microband Electrodes: Transient, Quasi-Steady-State, and Convective Regimes

Mass transport at microband electrodes is investigated theoretically and experimentally in unstirred solutions by chronoamperometry and cyclic voltammetry. Because natural convection limits the convection-free domain up to which diffusion layers may only expand, several regimes of mass transport are identified through simulation by means of a previous model. A zone diagram is established which allows all relative contributions to mass transport to be delineated according to the electrode dimension, timescale of experiment, and amplitude of natural convection. In opposition to the quasi-steady-state regime usually expected at microband electrodes under diffusion control, a steady-state regime always occurs at long enough times. By comparison to microdisk electrodes, a greater influence of natural convection is predicted. These results are validated experimentally by monitoring current responses and mapping steady-state concentration profiles at microband electrodes.

Theory and experiments of transport at channel microband electrodes under laminar flow. 3. Electrochemical detection at electrode arrays under steady state.

Microband arrays improve the analytical performance and information content of electrochemical detection in flow channel relative to single-electrode configurations. However, exploiting their full advantages requires a detailed understanding of the properties of arrays, which depend on their geometry and on the hydrodynamic regimes established inside the microfluidic channel. This paper investigates the influence of two main operating situations (sequential and coupling regimes) on steady-state amperometric responses of microband arrays performing under laminar flow conditions. Simulations and experimental measurements showed that the resulting properties of the arrays are a function of the number of electrodes and average ratio between gaps and electrode widths, whether the layout of the arrays is regular or not. Since the contribution of each electrode can be finely tailored, this allows the arrays to be designed and adapted to a wide variety of experimental demands.

Effects of chemical environment on diffusivities within thin Nafion® films as monitored from chronoamperometric responses of generator-collector double microband assemblies

Abstract The possibility of monitoring in situ the modifications of a polymeric thin film in relation to its chemical environment by means of measurement of diffusive properties and assessment of the volume available to diffusion is investigated. These measurements are performed electrochemically through the monitoring of the currents at paired-microband electrodes operated in a generator–collector mode. The experimental results reported here were primarily focused to assess the validity of the principle. This involved the evaluation of the effects of the chemical environment of the polymer on the diffusional cross-talk of Fe III redox species incorporated into a Nafion ® micrometric film coating the electrode assembly. Modification of the chemical environment was performed by adding different amounts of organic compound such as methanol, ethanol, ethyleneglycol, propanol and dimethylformamide (DMF) (from 0 to 20% v/v) into the 0.1 mol l −1 H 2 SO 4 electrolyte in which the assembly is placed. An procedure that could be automated based on a theoretical diffusional model developed previously was used to analyze the generator–collector chronoamperometric responses. This allowed the determination of the concentration c o and the diffusion coefficients of Fe III and Fe II species within Nafion ® films as well as the film thickness h . This showed a concentration dependence of the Fe III diffusion coefficient suggesting that the rate of physical diffusion of iron centers governs mainly the charge transport within Nafion ® thin films under the conditions investigated. The presence of an organic compound in the surrounding electrolyte caused a decrease of the steady-state generator–collector current, the effect being the most significant for DMF. The magnitude of this decrease was observed to depend on the nature and the concentration of the organic compound added to the electrolyte. Detailed chronoamperometric analyses established that the current decrease was related principally to the decrease in diffusion coefficient and to a moderate change of the amount of iron species available to diffusional transport, without significant change of the film thickness. This suggested that the presence of organic compound modified the plasticization of the polymer matrix.

Mass transport at infinite regular arrays of microband electrodes submitted to natural convection: theory and experiments.

Mass transport at infinite regular arrays of microband electrodes was investigated theoretically and experimentally in unstirred solutions. Even in the absence of forced hydrodynamics, natural convection limits the convection-free domain up to which diffusion layers may expand. Hence, several regimes of mass transport may take place according to the electrode size, gap between electrodes, time scale of the experiment, and amplitude of natural convection. They were identified through simulation by establishing zone diagrams that allowed all relative contributions to mass transport to be delineated. Dynamic and steady-state regimes were compared to those achieved at single microband electrodes. These results were validated experimentally by monitoring the chronoamperometric responses of arrays with different ratios of electrode width to gap distance and by mapping steady-state concentration profiles above their surface through scanning electrochemical microscopy.

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.

Downstream Simultaneous Electrochemical Detection of Primary Reactive Oxygen and Nitrogen Species Released by Cell Populations in an Integrated Microfluidic Device

An innovative microfluidic platform was designed to monitor electrochemically four primary reactive oxygen (ROS) and reactive nitrogen species (RNS) released by aerobic cells. Taking advantage of the space confinement and electrode performances under flow conditions, only a few experiments were sufficient to directly provide significant statistical data relative to the average behavior of cells during oxidative-stress bursts. The microfluidic platform comprised an upstream microchamber for cell culture and four parallel microchannels located downstream for separately detecting H2O2, ONOO-, NO·, and NO2-. Amperometric measurements were performed at highly sensitive Pt-black electrodes implemented in the microchannels. RAW 264.7 macrophage secretions triggered by a calcium ionophore were used as a way to assess the performance, sensitivity, and specificity of the integrated microfluidic device. In comparison with some previous evaluations achieved from single-cell measurements, reproducible and relevant determinations validated the proof of concept of this microfluidic platform for analyzing statistically significant oxidative-stress responses of various cell types.