Influence of Lanthanum on Stern Layer Conductance in the Nanochannel.

In this report, high-frequency electric impedance spectroscopy was performed to investigate ionic transport through nanochannels. Special attention was focused on (i) conductance behaviors depending on the role of cation valence in three background electrolytes (XCln): monovalent 1-1 (K+ and Cl-), divalent 2-1 (Mg2+ and 2Cl-), and trivalent 3-1 (La3+ and 3Cl-), (ii) the effects of proton and bicarbonate ions on bulk and surface conductance, and (iii) the connected microchannel dimension (surface/height ratio aspect) within the nanochannel apparent conductance. The results highlight a net quantitative increase in surface silanol density and a strong decrease in surface ionization degree when lanthanum cations are employed. The results also demonstrate that La3+ strongly interacts with the silica surface, leading to negative values of standard free energy for ion-site interactions and chemical potential for ion-ion correlations in the Stern layer of -0.8 and -10.2 kT, respectively. We ascribed the evolution of surface charge density to the balance between the mole ratios of water molecules and adsorbed cations at equilibrium. We found that La3+ behaves as an acidic cation (Lewis conceptualization) that neutralizes the negative silica surface accompanying water molecule expulsion due to steric hindrance. This study constitutes a new contribution to ion-site interactions and to ion-ion correlation phenomena on the planar silica surface to explain charge inversion observation in micro-nanofluidic devices.

Predicting ion concentration polarization and analyte stacking/focusing at nanofluidic interfaces.

We report on the investigation of electropreconcentration phenomena in micro-/nanofluidic devices integrating 100 µm long nanochannels using 2D COMSOL simulations based on the coupled Poisson-Nernst-Planck and Navier-Stokes system of equations. Our numerical model is used to demonstrate the influence of key governing parameters such as electrolyte concentration, surface charge density, and applied axial electric field on ion concentration polarization (ICP) dynamics in our system. Under sufficiently extreme surface-charge-governed transport conditions, ICP propagation is shown to enable various transient and stationary stacking and counter-flow gradient focusing mechanisms of anionic analytes. We resolve these spatiotemporal dynamics of analytes and electrolyte ICP over disparate time and length scales, and confirm previous findings that the greatest enhancement is observed when a system is tuned for analyte focusing at the charge, excluding microchannel, nanochannel electrical double layer (EDL) interface. Moreover, we demonstrate that such tuning can readily be achieved by including additional nanochannels oriented parallel to the electric field between two microchannels, effectively increasing the overall perm-selectivity and leading to enhanced focusing at the EDL interfaces. This approach shows promise in providing added control over the extent of ICP in electrokinetic systems, particularly under circumstances in which relatively weak ICP effects are observed using only a single channel.

A compact integrated microfluidic oxygenator with high gas exchange efficiency and compatibility for long-lasting endothelialization.

We have developed and tested a novel microfluidic device for blood oxygenation, which exhibits a large surface area of gas exchange and can support long-term sustainable endothelialization of blood microcapillaries, enhancing its hemocompatibility for clinical applications. The architecture of the parallel stacking of the trilayers is based on a central injection for blood and a lateral injection/output for gas which allows significant reduction in shear stress, promoting sustainable endothelialization since cells can be maintained viable for up to 2 weeks after initial seeding in the blood microchannel network. The circular design of curved blood capillaries allows covering a maximal surface area at 4 inch wafer scale, producing high oxygen uptake and carbon dioxide release in each single unit. Since the conventional bonding process based on oxygen plasma cannot be used for surface areas larger than several cm2, a new "wet bonding" process based on soft microprinting has been developed and patented. Using this new protocol, each 4 inch trilayer unit can be sealed without a collapsed membrane even at reduced 15 µm thickness and can support a high blood flow rate. The height of the blood channels has been optimized to reduce pressure drop and enhance gas exchange at a high volumetric blood flow rate up to 15 ml min-1. The simplicity of connecting different units in the stacked architecture is demonstrated for 3- or 5-unit stacked devices that exhibit remarkable performance with low primary volume, high oxygen uptake and carbon dioxide release and high flow rate of up to 80 ml min-1.

Electropreconcentration diagrams to optimize molecular enrichment with low counter pressure in a nanofluidic device.

Concentration polarization (CP)-based focusing electrokinetics nanofluidic devices have been developed in order to simultaneously detect and enrich highly diluted analytes on-a-chip. However, stabilization of focal points over long time under the application of the electric field remains as a technical bottleneck. If pressure-assisted preconcentration methods have been proposed to stabilize propagating modes at low inverse Dukhin number (1/Du≪1) , these recent protocols remain laborious for optimizing experimental parameters. In this paper, "electric field E/counter-pressure P" diagrams have been established during pressure-assisted electro-preconcentration of fluorescein as a model molecule. Such E/P diagram allows direct observation of the region for which the optimal counter-pressure P leads to a stable focusing regime. This region of stable focusing is shown to vary depending of the nanoslit length (100 µm < Lnanoslit < 500 µm) and the nature of the background electrolyte (KCl and NaCl). Longer nanoslits (500 µm) produce stabilization at low counter-pressure P, whereas NaCl offers a narrower region of stable focusing in the E/P diagram compared to KCl. Finally, the ability of such pressure-assisted protocol to concentrate negatively charged proteins has been tested with a more applicative protein, i.e., ovalbumin. The corresponding E/P diagram confirms the existence of the stable focusing regime at both low electric field E (≤20 V) and counter-pressure P (≤0.4 bar). With an enrichment factor as high as 70 after 2 min for ovalbumin at a concentration of 10 µM, such pressure-assisted nanofluidic electro-preconcentration protocol appears very promising to concentrate and detect biomolecules.

Logic digital fluidic in miniaturized functional devices: Perspective to the next generation of microfluidic lab-on-chips.

Microfluidics has emerged following the quest for scale reduction inherent to micro- and nanotechnologies. By definition, microfluidics manipulates fluids in small channels with dimensions of tens to hundreds of micrometers. Recently, microfluidics has been greatly developed and its influence extends not only the domains of chemical synthesis, bioanalysis, and medical researches but also optics and information technology. In this review article, we will shortly discuss an enlightening analogy between electrons transport in electronics and fluids transport in microfluidic channels. This analogy helps to master transport and sorting. We will present some complex microfluidic devices showing that the analogy is going a long way off toward more complex components with impressive similarities between electronics and microfluidics. We will in particular explore the vast manifold of fluidic operations with passive and active fluidic components, respectively, as well as the associated mechanisms and corresponding applications. Finally, some relevant applications and an outlook will be cited and presented.

Localized Mechanical Stress Promotes Microtubule Rescue.

Microtubule dynamics rely on the properties of tubulin and are regulated by microtubule-associated proteins. GTP-tubulin assembles into hollow polymers, which can depolymerize upon GTP hydrolysis. Depolymerizing microtubules may stop shrinking and resume growth. Such rescues are regulated by microtubule-associated proteins like CLIP-170 and the CLASPs [1, 2]. Microtubule domains prone to rescues contain discrete regions (previously termed "GTP islands") that retain a GTP-tubulin-like conformation in the main body of the microtubule [3]. However, the exact nature of these domains and the mechanisms controlling their occurrence and distribution are largely unknown. Here we show that collisions between growing microtubules and mechanical obstacles (including other microtubules) in vitro result in the higher abundance of GTP-like islands in stressed microtubule regions. Furthermore, these islands were found to be efficiently generated by both lateral contacts and mechanical constraints applied to the main body of the microtubules. They were also particularly prominent where shifts in the number of protofilaments occur in the microtubule lattice. GTP-like islands and rescues frequently co-occurred at microtubule intersections in vitro and in living cells, both in crossing and in crossed microtubules. We also observed that CLIP-170 recognizes GTP-like islands in vivo and is retained at microtubule crossings. Therefore, we propose that rescues occur via a two-stage mechanism: (1) lattice defects determine potential rescue-promoting islands in the microtubule structure, and (2) CLIP-170 detects these islands to stimulate microtubule rescue. Our results reveal the interplay between rescue-promoting factors and microtubule architecture and organization to control microtubule dynamics.

Large area graphene nanomesh: an artificial platform for edge-electrochemical biosensing at the sub-attomolar level.

Recent advances in large area graphene growth have led to tremendous applications in a variety of areas. The graphene nanomesh with its tunable band-gap is of great interest for both fundamental research, to explore the effect of edges on both the 2D electrical conduction and its electrochemical behavior, and applications such as nanoelectronic devices or highly sensitive biosensors. Here, we report on the fabrication of a large surface graphene nanomesh by nanoimprint lithography (NIL) to produce controlled artificial edges. The electrochemical response of this high quality single graphene layer imprinted nanomesh shows an enhancement in capacitance associated with faster electron transfer which can be attributed to the high density of edges. The electrochemical performances of this nanomesh graphene platform have been also studied for label-free DNA detection from Hepatitis C virus as a model. We demonstrate that such a nanomesh platform allows direct detection at the sub-attomolar level with more than 90% of molecules located on the imprinted artificial edges. Such a graphene nanomesh electrode will find useful future applications in the field of biosensing.

Determination of the isomeric forms proportion of fluorogenic naphthalene-2,3-dicarboxaldehyde in a binary mixture of water:methanol using electrochemical methods.

The electrochemical response of the fluorogenic label naphthalene-2,3-dicarboxyaldehyde (NDA) in a binary mixture of water/methanol was characterized with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) electrochemical techniques. Naphthalene-2,3-dicarboxyaldehyde does exist in three isomeric forms in aqueous solution: the unhydrated dialdehyde (DA), the acyclic monohydrated (MA) and the cyclic hemiacetal (HAC). The study underlines that the proportion of each of them varies according to the working pH. At low and high pH, the dialdehyde form is in larger proportion than the acyclic monohydrated form. Conversely at intermediate pH, the concentration of the acyclic form is in greater proportion than the dialdehyde form. These results allowed us to determine the optimal pH of 9 for which the labeling of biomolecules could be more efficient due to the base catalyzed regeneration of the unhydrated form. At this pH, the data processing from the analysis of measured currents and estimation of diffusion coefficients of each form according to the semi-empirical models of Wilke-Chang, Scheibel, Reddy-Doraiswamy and Lusis-Ratcliff allowed us to obtain the concentration of dialdehyde (0.28 mM), acyclic monohydrated (0.57 mM) and cyclic hemiacetal monohydrated (0.15 mM) forms starting from 1mM naphthalene-2,3-dicarboxyaldehyde.

Study of Surface Charge Instabilities by EOF Measurements on a Chip: A Real-Time Hysteresis and Peptide Adsorption Based Methodology.

This paper describes the measurement of the electroosmotic mobility (EOF) in a Wheatstone fluidic bridge (µFWB) as a direct probe of the surface instability. The variation of EOF known as one major contribution of the electrokinetic migration has been determined with a real-time measurement platform after different conditionings on chips. We also scan the pH of the background electrolytes with three different ionic strengths to evaluate the dependencies of the EOF as a function of the pH. A hysteresis methodology has been developed for probing the surface charge instabilities. EOF mobility has been recorded during on-a-chip electrophoresis to estimate the effect of such instability on the analytical performance. As expected, our experimental curves show that a decrease in the ionic strength increases the surface charge stability of the hybrid microchip. This result demonstrates that ionic exchanges between the surface and the fluid are clearly involved in the stability of the surface charge. With this original method based on real-time EOF measurement, the surface state can be characterized after hydrodynamic and electrophoresis sequences to mimic any liquid conditioning and separation steps. Finally, as a demonstrative application, isotherms of the adsorption of insulin have been recorded showing the change in surface charge by unspecific adsorption of this biomolecule onto the microfluidic channel's wall. These methodologies and findings could be particularly relevant to investigating various analytical pathways and to understanding the molecular mechanisms at solid/liquid interfaces.

Electrochemiluminescence on-a-chip: towards a hand-held electrically powered optofluidic source.

We report a microfluidic platform that integrates several parallel optical sources based on electrochemiluminescence (ECL) of 9,10-diphenylanthracene (DPA) as luminophore agent. The annihilation of DPA radicals provides a low wavelength emission at λ=430 nm in the blue-visible range. By varying the distance between electrodes for each ECL integrated source, this glass/PDMS/glass platform enabled a systematic investigation of the main electrochemical parameters involved in ECL. These parameters have been studied either in a static mode or in a dynamic one. Even at slow flow rate (~2 µl s(-1)), the renewal of electroactive species could be easily promoted inside the microfluidic channel which gives rise to a stable optical intensity for several minutes. Compared with traditional optically pumped dye sources, this microfluidic system demonstrates that ECL can be easily implemented on chip for producing much compact optofluidic sources. Such simply electrically powered system-on-chip would surely encourage the future of hand-held µTAS devices with integrated fast detection and embedded electronics.

Improved electrochemical detection of a transthyretin synthetic peptide in the nanomolar range with a two-electrode system integrated in a glass/PDMS microchip.

An alternative to a three-electrode set-up for electrochemical detection and analysis in microfluidic chips is described here. The design of the electrochemical sensor consists of the surface of the glass substrate covered with a PDMS block which bears the microfluidic channels. A band microelectrode which acts as a working electrode surrounded by a large counter electrode is obtained at the micrometric level to propose a simple and efficient sensing area for on-a-chip analysis. The counter-electrode with a surface area about 22-fold greater than the working-microelectrode can also be considered as a pseudo reference since its current density is low and thus limits the potential variations around the rest potential. To this purpose, the [Fe(III)(CN)6]³â»/[Fe(II)(CN)6]4⁻ redox couple was used in order to set a reference potential at 0 V since both electrodes were platinum. The electrochemical microchip performance was characterized using differential pulse voltammetric (DPV) detection and quantification of the optically multi-labelled transthyretin synthetic peptide mimicking a tryptic fragment of interest for the diagnosis of familial transthyretin amyloidosis (ATTR). The limit of detection of the peptide by the working microelectrode was 25 nM, a value 100-fold lower than the one reported with conventional capillary electrophoresis coupled with laser-induced fluorescence under the same analytical conditions.

A microdevice for parallelized pulmonary permeability studies.

We describe a compartmentalized microdevice specifically designed to perform permeability studies across a model of lung barrier. Epithelial cell barriers were reproduced by culturing Calu-3 cells at the air-liquid interface (AIC) in 1 mm² microwells made from a perforated glass slide with an embedded porous membrane. We created a single basolateral reservoir for all microwells which eliminated the need to renew the growth medium during the culture growth phase. To perform drug permeability studies on confluent cell layers, the cell culture slide was aligned and joined to a collection platform consisting in 35 µL collection reservoirs connected at the top and bottom with microchannels. The integrity and functionality of the cell barriers were demonstrated by measurement of trans-epithelial electrical resistance (TEER), confocal imaging and permeability assays of ¹4C-sucrose. Micro-cell barriers were able to form confluent layers in 1 week, demonstrating a similar bioelectrical evolution as the Transwell systems used as controls. Tight junctions were observed throughout the cell-cell interfaces, and the low permeability coefficients of ¹4C-sucrose confirmed their functional presence, creating a primary barrier to the diffusion of solutes. This microdevice could facilitate the monitoring of biomolecule transport and the screening of formulations promoting their passage across the pulmonary barrier, in order to select candidates for pulmonary administration to patients.

Investigating of labelling and detection of transthyretin synthetic peptide derivatized with naphthalene-2,3-dicarboxaldehyde.

Labelling and detection of a synthetic peptide (PN) mimicking a tryptic fragment of interest for the diagnosis of familial amyloidal polyneuropathy have been investigated optically and electrochemically. We decided to covalently label naphtalene-2,3-dicarboxyaldehyde (NDA), a fluorogenic and electroactive molecule on PN. First, the optimization of the labelling chemical reaction was performed by capillary electrophoresis coupled with laser induced fluorescence detection (CE-LIF). The analytical parameters such as separation efficiency and peak area were considered to propose this optimized derivatization reaction. The results obtained allowed us to establish the pH and ionic strength of the derivatization buffer, the molar ratio between NDA and PN and the reaction time of the labelling. Optimal conditions are obtained when [NDA]/[PN]=40, buffer pH of 9, buffer ionic strength of 70 mM and reaction time of 15 min. Second, differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were also used to characterize NDA-labelled PN and different electroinactive amino acids (histidine, lysine, serine, threonine) which are in the PN sequence. The electrochemical detection experiments demonstrated that the labelled biomolecules could be also easily detected at low concentration. Moreover, the derivatization reaction could be followed to describe more precisely the labelling process of these biomolecules. Optimal conditions for labelling are obtained when [NDA]total/[CN(-)] ratio =1 and [NDA]total/[amino acid or peptide]=100 with a buffer having a pH=9 on a glassy carbon electrode. In all cases, an obvious oxidation peak for the N-2-substituted-1-cyanobenz-[f]-isoindole derivative (CBI) has been observed at 0.5-0.7 V/SCE. The multi-labelling of PN and lysine were shown with DPV. We presumed this result to occur because of the shouldered shape of the DPV peak shape. These experiments confirm that NDA can be used as a derivative agent for PN, allowing for electrochemical and fluorescence detections with a limit of detection of labelled PN estimated at 0.2 µM and 5 µM, respectively.

Pressure-assisted selective preconcentration in a straight nanochannel.

We investigate the preconcentration profiles of a fluorescein and bovine serum albumin derivatized with this fluorescent tag in a microfluidic chip bearing a nanoslit. A new preconcentration method in which a hydrodynamic pressure is added to both electroosmotic and electrophoretic contributions is proposed to monitor the location of the preconcentration frontline. A simple predictive model of this pressure-assisted electropreconcentration is proposed for the evolution of the flow profile along this micro/nano/microfluidic structure. We show with a small analyte such as fluorescein that the additional hydrostatic pressure mode enables to stabilize the concentration polarization (CP) effect, resulting in better control of the cathodic focusing (CF) peak. For BSA (bovine serum albumin), we exhibit that the variation of the hydrodynamic pressure can have an even more drastic effect on the preconcentration. We show that, depending on this hydrodynamic pressure, the preconcentration can be chosen, either in the cathodic side or in the anodic one. For the first time, we prove here that both anodic focusing (AF) and cathodic focusing (CF) regimes can be reached in the same structures. These results also open new routes for the detection and the quantification of low abundance biomarkers.

Computational study of velocity profile obtained in microfluidic channel bearing a fluidic transistor: toward highly resolved electrophoretic separation.

The present work is a computational study of velocity profiles in microfluidic channels bearing field flow effect transistors (FFET). In particular, this work investigates perturbations and distortions of the sample band during electrophoretic transport in a rectangular separation channel. The EOF heterogeneity and its induced pressure render the predictions of the analytical performances rather complex. In this context, we propose a systematic numerical inquiry that focuses on the distribution of the velocities for several geometries and EOF modulations. We compare the calculated parabolic velocity profiles to the bare glass microchips. Here, the reported parabolic velocity profiles are coherent with recent experimental results that have been published elsewhere. From the presented equations, in such active hybrid microfluidic chip that integrates a FFET gate layer, separation can be optimized by playing on the gate coverage ratio. The flow fields obtained from analytical models allow further investigations about the efficiency and resolution during electrophoresis. The resulting induced pressure gradient and the associated band broadening underline the need to optimize the resolution in the detriment of the efficiency in such active microfluidic chips.

A nanoliter-scale open chemical reactor.

An open chemical reactor is a container that exchanges matter with the exterior. Well-mixed open chemical reactors, called continuous stirred tank reactors (CSTR), have been instrumental for investigating the dynamics of out-of-equilibrium chemical processes, such as oscillations, bistability, and chaos. Here, we introduce a microfluidic CSTR, called µCSTR, that reduces reagent consumption by six orders of magnitude. It consists of an annular reactor with four inlets and one outlet fabricated in PDMS using multi-layer soft lithography. A monolithic peristaltic pump feeds fresh reagents into the reactor through the inlets. After each injection the content of the reactor is continuously mixed with a second peristaltic pump. The efficiency of the µCSTR is experimentally characterized using a bromate, sulfite, ferrocyanide pH oscillator. Simulations accounting for the digital injection process are in agreement with experimental results. The low consumption of the µCSTR will be advantageous for investigating out-of-equilibrium dynamics of chemical processes involving biomolecules. These studies have been scarce so far because a miniaturized version of a CSTR was not available.

Self-assembled titanium calcium oxide nanopatterns as versatile reactive nanomasks for dry etching lithographic transfer with high selectivity.

We report the simple preparation of ultra-thin self-assembled nanoperforated titanium calcium oxide films and their use as reactive nanomasks for selective dry etching of silicon. This novel reactive nanomask is composed of TiO(2) in which up to 50% of Ti was replaced by Ca (Ca(x)Ti(1-x)O(2-x)). The system was prepared by evaporation induced self-assembly of dip-coated solution of CaCl(2), TiCl(4) and poly(butadiene-block-ethylene oxide) followed by 5 min of thermal treatment at 500 °C in air. The mask exhibits enhanced selectivity by forming a CaF(2) protective layer in the presence of a chemically reactive fluorinated plasma. In particular it is demonstrated that ordered nano-arrays of dense Si pillars, or deep cylindrical wells, with high aspect ratio i.e. lateral dimensions as small as 20 nm and height up to 200 nm, can be formed. Both wells and pillars were formed by tuning the morphology and the homogeneity of the deposited mask. The mask preparation is extremely fast and simple, low-cost and easily scalable. Its combination with reactive ion etching constitutes one of the first examples of what can be achieved when sol-gel chemistry is coupled with top-down technologies. The resulting Si nanopatterns and nanostructures are of high interest for applications in many fields of nanotechnology including electronics and optics. This work extends and diversifies the toolbox of nanofabrication methods.

Photonic crystal patterning of luminescent sol-gel films for light extraction.

Structured luminescent thin films are investigated in the context of improved light extraction of phosphors for solid-state-lighting applications. Thin films composed of a sol-gel titania matrix doped with europium chelates are studied as a model system. These films, patterned with a square photonic lattice by soft nanoimprint lithography, are characterized by angle-resolved fluorescence. Modeling of this simple technique is shown to fit well the experimental data, revealing in great detail the guided modes of the film and their extraction parameters. An eightfold extraction enhancement factor of the film emission is measured. To further improve the extraction efficiency, we investigate the role of an additional low-index mesoporous silica underlayer through its influence on the guided modes of different polarizations and their interactions with the photonic crystal. Results obtained on model systems open the way towards the optimization of light-emitting devices, using a strategy of dielectric microstructure engineering using the sol-gel process.

λ³/1000 plasmonic nanocavities for biosensing fabricated by soft UV nanoimprint lithography.

Arrays of plasmonic nanocavities with very low volumes, down to λ(3)/1000, have been fabricated by soft UV nanoimprint lithography. Nearly perfect omnidirectional absorption (3-70°) is demonstrated for the fundamental mode of the cavity (λ ≃ 1.15 µm). The second-order mode exhibits a sharper resonance with strong angular dependence and total optical absorption when the critical coupling condition is fulfilled (45-50°, λ ≃ 750 nm). It leads to high refractive index sensitivity (405 nm/RIU) and figure of merit (∼21) and offers new perspectives for efficient biosensing experiments in ultralow volumes.

Ionic and mass transport in micro-nanofluidic devices: a matter of volumic surface charge.

The shape and the surface charge of microchannels are critical parameters for ionic and mass transport in microfluidic systems. A great number of studies and developments have been carried out in order to optimize these features separately. We propose to consider them together within a new fundamental parameter for microfluidics, that we named the Volumic Surface Charge (VSC), which is the ratio of the surface charge to the section height in planar microchannels. The non-linear effects induced by rapid VSC variations can result in selective preconcentration processes, which can be used for a simultaneous preconcentration and separation of biomolecules within simple straight channels. In this review, we first present 3 different techniques that we developed to tune the VSC either by surface chemical patterning, integration of polarisable interfaces or geometrical constrictions. The proof of concept of the selective preconcentration using VSC variations will be presented on the basis of experimental results obtained with fluorescent probes and numerical simulations.