Redox Control of Particle Deposition from Drying Drops.

The coffee-ring effect (CRE), which denotes the accumulation of nonvolatile compounds at the periphery of a pinned sessile drying drop, is a universal and ubiquitous yet complex phenomenon. It is crucial to better understand and control it, either to avoid its various deleterious consequences in many processes requiring homogeneous deposition or to exploit it for applications ranging from controlled particle patterning to low cost diagnostics. Here, we report for the first time the use of a reduction-oxidation (redox) stimulus to cancel the CRE or harness it, leading to a robust and tunable control of particle deposition in drying sessile drops. This is achieved by implementing redox-sensitive ferrocenyl cationic surfactants of different chain lengths in drying drops containing anionic colloids. Varying surfactant hydrophobicity, concentration, and redox state allows us not only to control the overall distribution of deposited particles, including the possibility to fully cancel the CRE, but also to modify the microscopic organization of particles inside the deposit. Notably, with all other parameters being fixed, this method allows the adjustment of the deposited particle patterns, from polycrystalline rings to uniform disks, as a function of the oxidation rate. We show that the redox control can be achieved either chemically by the addition of oxidants or electrochemically by applying a potential for additive-free and reversible actuation in a closed system. This correlation between the redox state and the particle pattern opens a perspective for both redox-programmable particle patterning and original diagnostic applications based on the visual determination of a redox state. It also contributes to clarify the role of surfactant charge and its amphiphilic character in directing particle deposition from drying suspensions.

Electrochemical assessments of droplet contents in microfluidic channels. Application to the titration of heterogeneous droplets.

Series of aqueous droplets containing redox species were generated on-demand in a microfluidic channel and detected downstream by an electrochemical cell. Depending on the cell geometry, amperometric detections were performed to simultaneously determine the velocity, volume and content of circulating droplets in oil. Volumes and velocities were estimated from specific transition times on the chronoamperometric responses, while charge were evaluated from current integration. The results showed that the total charge within droplets was controlled by the geometry of the electrochemical cell and droplet velocity, leading to accurate determinations of droplet content under specific operating conditions. An active merging of droplets with titrating solutions was tested for analytical purposes. The results demonstrated that even if the mixing was not complete during detection, the assessment of droplet content was still valid. The performance of electrochemical detection was thus evidenced to determine the content of heterogeneous droplets. This property is pertinent since the design of sophisticated circuits is no longer required to fully homogenize the droplet content before characterization, opening broader perspectives in droplet-based microfluidics.

Electrochemical Generation of Steady-State Linear Concentration Gradients within Microfluidic Channels Perpendicular to the Flow Field.

A new design of microelectrode was introduced to generate electrochemically steady-state linear concentration gradients perpendicular to the flow direction throughout the cross section of microchannels. The shape and geometry of the electrode were established based on operating regimes at microchannel electrodes. Before implementation, optimal conditions were preliminary delineated by numerical simulations according to the flow velocity and microchannel dimensions. To assess experimentally these predictions, a specific microfluidic platform was developed with optimized geometry to simultaneously allow the generation of linear concentration gradients and the mapping of concentration profiles by confocal fluorescence microscopy. As a model, the electrochemical reduction of a quinone in the presence of fluorescein was selected to both generate and monitor a linear proton gradient. A good agreement was observed between theoretical and experimental data, establishing the proof of concept. These results should broaden the performance and applications of electrochemical platforms, particularly in the field of active control of microenvironments in biology, biochemistry, and analytical chemistry.

Electrochemical Generation and Detection of Transient Concentration Gradients in Microfluidic Channels. Theoretical and Experimental Investigations.

Transient concentration gradients generated and detected electrochemically in continuous flow microchannels were investigated by numerical simulations and amperometric measurements. Operating conditions including device geometry and hydrodynamic regime were theoretically delineated for producing gradients of various profiles with tunable characteristics. Experiments were carried out with microfluidic devices incorporating a dual-channel-electrode configuration. Under these conditions, high electrochemical performance was achieved both to generate concentration gradients and to monitor their dynamics along linear microchannels. Good agreement was observed between simulated and experimental data validating predictions between gradient properties and generation conditions. These results demonstrated the capability of electrochemical microdevices to produce in situ tunable concentration gradients with real-time monitoring. This approach is versatile for the active control in microfluidics of microenvironments or chemical gradients with high spatiotemporal resolution.

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.

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.

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.

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 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.

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.

Theory and experiments of transport at channel microband electrodes under laminar flows. 1. Steady-state regimes at a single electrode.

Integration of microelectrodes in microfluidic devices has attracted significant attention during the past years, in particular for analytical detections performed by direct or indirect electrochemical techniques. In contrast there is a lack of general theoretical treatments of the difficult diffusion-convection problems which are borne by such devices. In this context, we investigated the influence of the confining effect and hydrodynamic conditions on the steady-state amperometric responses monitored at a microband electrode embedded within a microchannel. Several convective-diffusive mass transport regimes were thus identified under laminar flow on the basis of numerical simulations performed as a function of geometrical and hydrodynamic parameters. A rationalization of these results has been proposed by establishing a zone diagram describing all the limiting and intermediate regimes. Concentration profiles generated by the electrode across the microchannel section were also simulated according to the experimental conditions. Their investigation allowed us to evaluate the thickness of the diffusive-convective layer probed by the electrode as well as the distance downstream from which the solution becomes again homogeneous across the whole microchannel section. Experimental checks of the theoretical principles delineated here have validated the present results. Experiments were performed at microband electrodes integrated in microchannels with aqueous solutions of ferrocene methanol under pressure-driven flow.