Monitoring phase transition of aqueous biomass model substrates by high-pressure and high-temperature microfluidics.

Aqueous-Phase Reforming (APR) is a promising hydrogen production method, where biomass is catalytically reformed under high pressure and high temperature reaction conditions. To eventually study APR, in this paper, we report a high-pressure and high-temperature microfluidic platform that can withstand temperatures up to 200°C and pressures up to 30 bar. As a first step, we studied the phase transition of four typical APR biomass model solutions, consisting of 10 wt% of ethylene glycol, glycerol, xylose or xylitol in MilliQ water. After calibration of the set-up using pure MilliQ water, a small increase in boiling point was observed for the ethylene glycol, xylitol and xylose solutions compared to pure water. Phase transition occurred through either explosive or nucleate boiling mechanisms, which was monitored in real-time in our microfluidic device. In case of nucleate boiling, the nucleation site could be controlled by exploiting the pressure drop along the microfluidic channel. Depending on the void fraction, various multiphase flow patterns were observed simultaneously. Altogether, this study will not only help to distinguish between bubbles resulting from a phase transition and/or APR product formation, but is also important from a heat and mass transport perspective.

Chip-Based Multicapillary Column with Maximal Interconnectivity to Combine Maximum Efficiency and Maximum Loadability.

On the basis of our previous work on the design of pillar array columns for liquid chromatography, we report on a new pillar array design for high-efficiency, high volumetric loadability gas chromatography columns. The proposed pillar array configuration leads to a column design which can either be considered as a packed bed with perfectly ordered and uniform flow paths or as multicapillary columns (8 parallel tracks) with a maximal interconnectivity between the flow paths to avoid the so-called polydispersity effect (dispersion arising from the inevitable differences in migration velocity between parallel flow paths). Despite our relative inexperience with column coating, and most probably (not supported by data) suffering from the same problem of stationary phase pooling in the right-angled corners of the flow-through channels as other chip-based GC devices, the efficiencies obtained in a L = 70 cm long and 75 µm deep and 6.195 mm wide chip for, respectively, quasi-unretained and retained components (k = 7) went up to N = 60 000 and 12 500 under isothermal conditions using H2 as carrier gas and a downstream restriction. Under programmed temperature conditions (Ti = 80 °C, Tf = 175 °C at 30 °C/min, and a H2 flow of 0.4 mL/min), a peak capacity of 170 was obtained in 3.6 min. For retained compounds, the optimal flow rate is found to be on the order of 0.4 mL/min, achieved at an operating pressure of 2.3 bar. Intrinsically, the column combines the efficiency of a 75 µm capillary with the volumetric loadability of a 240 µm capillary.

In-line sample concentration by evaporation through porous hollow fibers and micromachined membranes embedded in microfluidic devices.

Two types of microfluidic systems, a porous hollow fiber and a thin supported membrane with an array of micromachined holes, are investigated for concentrating mass-limited analyte samples. Water evaporation is driven by the partial pressure difference across the hydrophobic membrane, induced by dry sweeping gas on the permeate side. An analytical model permitting clarification of the contribution of design and process parameters on acquisition of concentrated solution and prediction of achievable concentration factors is presented. Concentrating an exemplary solution utilizing the two systems has been studied at different experimental conditions to validate the model. The results show that the hollow fiber gives controllable concentration factors of more than 10. For the micromachined membrane concentrator concentration factors of 6-8 were achieved, at much lower flow rates than predicted by the model. Because of the asymptotic dependence of concentration factor on flow rate, accurate control of the liquid feed is extremely critical in the flow rate range where high concentration factors are obtained, and the smallest variations in liquid flow rate may easily lead to supersaturation and deposition of solutes in the pores. This changes membrane porosity in an unpredictable way and limits the maximum attainable concentration factor.

A zero-gap silicon membrane with defined pore size and porosity for alkaline electrolysis.

Porous separators are a key component in alkaline water electrolyzers and are significant sources of overpotential. In this paper, porous silicon separators were fabricated by etching precise arrays of cylindrical pores into silicon substrates through lithography. Chemical stability of the silicon-based separators is ensured through the deposition of a silicon nitride layer. Platinum or nickel were vapor-deposited directly on the faces of the separator to complete a zero-gap configuration. Separator porosity (ε) was varied by changing the pore diameter and the pore spacing. These well-controlled porous silicon zero gap electrodes (PSi-ZGEs) were used to study the trade-off between separator resistance and gas-crossover at different porosities. It was found that separator resistances comparable to commercially used Zirfon UTP 500 were achieved at much lower ε. Gas crossover remained within the explosive limits for ε ≤ 0.15%. The PSi-ZGEs achieved stable performance at 100 mA cm-2 for 24 hours without significant surface damage in the alkaline electrolyte. In the broad perspective, the current work can pave the path for the development of ionomer-free separators for alkaline water electrolysis which rely on the separator geometry to limit gas-crossover.

Disposable attenuated total reflection-infrared crystals from silicon wafer: a versatile approach to surface infrared spectroscopy.

Attenuated total reflection-infrared (ATR-IR) spectroscopy is increasingly used to characterize solids and liquids as well as (catalytic) chemical conversion. Here we demonstrate that a piece of silicon wafer cut by a dicing machine or cleaved manually can be used as disposable internal reflection element (IRE) without the need for polishing and laborious edge preparation. Technical aspects, fundamental differences, and pros and cons of these novel disposable IREs and commercial IREs are discussed. The use of a crystal (the Si wafer) in a disposable manner enables simultaneous preparation and analysis of substrates and application of ATR spectroscopy in high temperature processes that may lead to irreversible interaction between the crystal and the substrate. As representative application examples, the disposable IREs were used to study high temperature thermal decomposition and chemical changes of polyvinyl alcohol (PVA) in a titania (TiO(2)) matrix and assemblies of 65-450 nm thick polystyrene (PS) films.

Glucose level determination with a multi-enzymatic cascade reaction in a functionalized glass chip.

In this work we show the functionalization of the interior of microfluidic glass chips with poly(2-hydroxyethyl methacrylate) polymer brushes as anchors for co-immobilization of the enzymes glucose-oxidase and horseradish peroxidase. The formation of the brush layer and subsequent immobilization of these enzymes have been characterized on flat surfaces by atomic force microscopy and Fourier transform infrared spectroscopy, and studied inside glass chips by field emission scanning microscopy. Enzyme-functionalized glass chips have been applied for performing a multi-enzymatic cascade reaction for the fast (20 s) determination of glucose in human blood samples and the result is in excellent agreement with values obtained from the conventional hospital laboratory. The limit of detection of this bi-enzymatic method is 60 µM. With the advantages of high selectivity and reproducibility, this functionalization method can be used for improving the efficiency of glucose sensors.

Attenuated total reflection-infrared nanofluidic chip with 71 nL detection volume for in situ spectroscopic analysis of chemical reaction intermediates.

We present a micromachined silicon attenuated total reflection-infrared (ATR-IR) crystal with integrated nanofluidic glass channels which enables infrared spectroscopic studies with only 71 nL sample volume. Because of the short path length through silicon, the system allows IR spectroscopy down to 1200 cm(-1), which covers the typical fingerprint region of most organic compounds. To demonstrate proof-of-principle, the chip was used to study a Knoevenagel condensation reaction between malononitrile and p-anisaldehyde catalyzed by different concentrations of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in solvent acetonitrile. By in situ measurement, it was demonstrated for the first time that at certain concentrations of DBU, reaction intermediates become stabilized, an effect that slows down or even stops the reaction. This is thought to be caused by increased ionic character of the solvent, in which protonated DBU stabilizes the intermediates. This clearly demonstrates that infrared mechanistic studies of chemical reactions are feasible in volumes as little as 71 nL.

On the pathway of photoexcited electrons: probing photon-to-electron and photon-to-phonon conversions in silicon by ATR-IR.

Photoexcitation and charge carrier thermalization inside semiconductor photocatalysts are two important steps in solar fuel production. Here, photoexcitation and charge carrier thermalization in a silicon wafer are for the first time probed by a novel, yet simple and user-friendly Attenuated Total Reflectance Infrared spectroscopy (ATR-IR) system.

A supramolecular sensing platform for phosphate anions and an anthrax biomarker in a microfluidic device.

A supramolecular platform based on self-assembled monolayers (SAMs) has been implemented in a microfluidic device. The system has been applied for the sensing of two different analyte types: biologically relevant phosphate anions and aromatic carboxylic acids, which are important for anthrax detection. A Eu(III)-EDTA complex was bound to ß-cyclodextrin monolayers via orthogonal supramolecular host-guest interactions. The self-assembly of the Eu(III)-EDTA conjugate and naphthalene ß-diketone as an antenna resulted in the formation of a highly luminescent lanthanide complex on the microchannel surface. Detection of different phosphate anions and aromatic carboxylic acids was demonstrated by monitoring the decrease in red emission following displacement of the antenna by the analyte. Among these analytes, adenosine triphosphate (ATP) and pyrophosphate, as well as dipicolinic acid (DPA) which is a biomarker for anthrax, showed a strong response. Parallel fabrication of five sensing SAMs in a single multichannel chip was performed, as a first demonstration of phosphate and carboxylic acid screening in a multiplexed format that allows a general detection platform for both analyte systems in a single test run with µM and nM detection sensitivity for ATP and DPA, respectively.

Room-temperature intermediate layer bonding for microfluidic devices.

In this work a novel room-temperature bonding technique based on chemically activated Fluorinated Ethylene Propylene (FEP) sheet as an intermediate between chemically activated substrates is presented. Surfaces of silicon and glass substrates are chemically modified with APTES bearing amine terminal groups, while FEP sheet surfaces are treated to form carboxyl groups and subsequently activated by means of EDC-NHS chemistry. The activation procedures of silicon, glass and FEP sheet are characterized by contact angle measurements and XPS. Robust bonds are created at room-temperature by simply pressing two amine-terminated substrates together with activated FEP sheet in between. Average tensile strengths of 5.9 MPa and 5.2 MPa are achieved for silicon-silicon and glass-glass bonds, respectively, and the average fluidic pressure that can be operated is 10.2 bar. Moreover, it is demonstrated that FEP-bonded microfluidic chips can handle mild organic solvents at elevated pressures without leakage problems. This versatile room-temperature intermediate layer bonding technique has a high potential for bonding, packaging, and assembly of various (bio-) chemical microfluidic systems and MEMS devices.

Electrokinetic sorting and collection of fractions for preparative capillary electrophoresis on a chip.

A microfabricated device capable of selecting and collecting multiple components from a mixture separated by capillary electrophoresis (CE) is described. This collection is automated and can be easily controlled by a set of rules defined by an operator, enabling fast and consistent operation. The device consists of an electrokinetically steered fluidic network that can be divided into three sections: a CE part, a fractions distribution region and a set of storage channels. Sample fractions leave the CE channel and are detected in the interfacial region by fluorescence intensity measurements. If an upcoming peak is detected, separation is withheld and the potentials are reconfigured to force the fraction into one of the collection channels, where they become available for further processing or analysis. The sequence of separation and collection is repeated until all the bands of interest are captured. A mixture of three fluorescent dyes (Rhodamine 6G, Rhodamine B and Fluorescein) was used to demonstrate the principle. The components were repeatedly separated by means of CE and pooled in their respective storage channels. In comparison to previous developments, the system presented in this paper offers automatic collection of all fractions in a single run. Furthermore, it is possible to run the system in a repetitive mode for accumulative pooling if more fractionated sample is required.

Synchronized, continuous-flow zone electrophoresis.

A new method for performing continuous electrophoretic separation of complex mixtures in microscale devices is proposed. Unlike in free-flow electrophoresis devices, no mechanical pumping is required--both fluid transport and separation are driven electrokinetically. This gives the method great potential for on-a-chip integration in multistep analytical systems. The method enables us to collect fractionated sample and tenfold purification is possible. The model of the operation is presented and a detailed description of the optimal conditions for performing purification is given. The chip devices with 10-microm-deep separation chamber of 1.5 mm x 4 mm in size were fabricated in glass. A standard microchip electrophoresis setup was used. Continuous separation of rhodamine B, rhodamine 6G, and fluorescein was accomplished. Purification was demonstrated on a mixture containing rhodamine B and fluorescein, and the recovery of both fractions was achieved. The results show the feasibility of the method.

Bubble-free operation of a microfluidic free-flow electrophoresis chip with integrated Pt electrodes.

In order to ensure a stable and efficient separation in microfluidic free-flow electrophoresis (FFE) devices, various methods and chips have been presented until now. A major concern hereby is the generation of gas bubbles caused by electrolysis and the resulting disturbances in the position of the separated analyte lanes. Instable lane positions would lead to a decreased resolution in sample collection over time which certainly would be problematic when incorporating a stationary detector system. In contrast to our previous publications, in which we implemented laborious semipermeable membranes to keep bubbles outside the separation region, here we describe an electrochemical approach to suppress the electrolysis of water molecules and therefore bubble formation. This approach allowed a simpler and additionally a closed chip device with integrated platinum electrodes. With the use of this chip, the successful separation of three fluorescent compounds was demonstrated. Quinhydrone, which is a complex of hydroquinone and p-benzoquinone, was added only to the local flow streams along the electrodes, preventing mixing with the separation media and sample. The electrical current was generated via the oxidization and reduction of hydroquinone and p-benzoquinone up to a certain limit of the electrical current without gas formation. The separation stability was investigated for the chip with and without quinhydrone, and the results clearly indicated the improvement. In contrast to the device operating without quinhydrone, a 2.5-fold increase in resolution was achieved. Furthermore, separation was demonstrated within tens of milliseconds. This chemical approach with its high miniaturization possibilities offers an interesting alternative, in particular for low-current miniaturized FFE systems, in which large and open electrode reservoirs are not tolerable.

Forced splitting of fractions in CE.

In order to increase the electrophoretic separation between fractions of analytes on a microfluidic chip, without the need for a longer separation channel, we propose and demonstrate a preparative electrokinetic procedure by which overlapping or closely spaced fractions are automatically split. The method involves an extra T-junction at the end of a separation channel and detector-triggered reconfiguration of voltages at channel outlets. Forced splitting of a separated four-component mixture is demonstrated, and possible sources of errors leading to contamination of split fractions are also discussed in detail and illustrated both by computational fluid dynamics and experiments. The splitting method can be applied in preparative CE-on-a-chip systems, for which it greatly simplifies downstream fraction manipulation and helps in reducing cross-contamination between collected fractions.

Three-Dimensional Fractal Geometry for Gas Permeation in Microchannels.

The novel concept of a microfluidic chip with an integrated three-dimensional fractal geometry with nanopores, acting as a gas transport membrane, is presented. The method of engineering the 3D fractal structure is based on a combination of anisotropic etching of silicon and corner lithography. The permeation of oxygen and carbon dioxide through the fractal membrane is measured and validated theoretically. The results show high permeation flux due to low resistance to mass transfer because of the hierarchical branched structure of the fractals, and the high number of the apertures. This approach offers an advantage of high surface to volume ratio and pores in the range of nanometers. The obtained results show that the gas permeation through the nanonozzles in the form of fractal geometry is remarkably enhanced in comparison to the commonly-used polydimethylsiloxane (PDMS) dense membrane. The developed chip is envisioned as an interesting alternative for gas-liquid contactors that require harsh conditions, such as microreactors or microdevices, for energy applications.

Toward jet injection by continuous-wave laser cavitation.

This is a study motivated by the need to develop a needle-free device for eliminating major global healthcare problems caused by needles. The generation of liquid jets by means of a continuous-wave laser, focused into a light absorbing solution, was studied with the aim of developing a portable and affordable jet injector. We designed and fabricated glass microfluidic devices, which consist of a chamber where thermocavitation is created and a tapered channel. The growth of a vapor bubble displaces and expels the liquid through the channel as a fast traveling jet. Different parameters were varied with the purpose of increasing the jet velocity. The velocity increases with smaller channel diameters and taper ratios, whereas larger chambers significantly reduce the jet speed. It was found that the initial position of the liquid-air meniscus interface and its dynamics contribute to increased jet velocities. A maximum velocity of 94±3 m/s for a channel diameter of D=120 µm, taper ratio n=0.25, and chamber length E=200 µm was achieved. Finally, agarose gel-based skin phantoms were used to demonstrate the potential of our devices to penetrate the skin. The maximum penetration depth achieved was ∼1 mm, which is sufficient to penetrate the stratum corneum and for most medical applications. A meta-analysis shows that larger injection volumes will be required as a next step to medical relevance for laser-induced jet injection techniques in general.

An All-Glass Microfluidic Network with Integrated Amorphous Silicon Photosensors for on-Chip Monitoring of Enzymatic Biochemical Assay.

A lab-on-chip system, integrating an all-glass microfluidics and on-chip optical detection, was developed and tested. The microfluidic network is etched in a glass substrate, which is then sealed with a glass cover by direct bonding. Thin film amorphous silicon photosensors have been fabricated on the sealed microfluidic substrate preventing the contamination of the micro-channels. The microfluidic network is then made accessible by opening inlets and outlets just prior to the use, ensuring the sterility of the device. The entire fabrication process relies on conventional photolithographic microfabrication techniques and is suitable for low-cost mass production of the device. The lab-on-chip system has been tested by implementing a chemiluminescent biochemical reaction. The inner channel walls of the microfluidic network are chemically functionalized with a layer of polymer brushes and horseradish peroxidase is immobilized into the coated channel. The results demonstrate the successful on-chip detection of hydrogen peroxide down to 18 µM by using luminol and 4-iodophenol as enhancer agent.

Free-flow zone electrophoresis and isoelectric focusing using a microfabricated glass device with ion permeable membranes.

This paper describes a microfabricated free-flow electrophoresis device with integrated ion permeable membranes. In order to obtain continuous lanes of separated components an electrical field is applied perpendicular to the sample flow direction. This sample stream is sandwiched between two sheath flow streams, by hydrodynamic focusing. The separation chamber has two open side beds with inserted electrodes to allow ventilation of gas generated during electrolysis. To hydrodynamically isolate the separation compartment from the side electrodes, a photo-polymerizable monomer solution is exposed to UV light through a slit mask for in situ membrane formation. These so-called salt-bridges resist the pressure driven fluid, but allow ion transport to enable electrical connection. In earlier devices the same was achieved by using open side channel arrays. However, only a small fraction of the applied voltage was effectively utilized across the separation chamber during free-flow electrophoresis and free-flow isoelectric focusing. Furthermore, the spreading of the carrier ampholytes into the side channels resulted in a very restricted pH gradient inside the separation chamber. The chip presented here allows at least 10 times more efficient use of the applied potential and a nearly linear pH gradient from pH 3 to 10 during free-flow isoelectric focusing could be established. Furthermore, the application of hydrodynamic focusing in combination with free-flow electrophoresis can be used for guiding the separated components to specific chip outlets. As a demonstration, several standard fluorescent markers were separated and focused by free-flow zone electrophoresis and by free-flow isoelectric focusing employing a transversal voltage of up to 150 V across the separation chamber.

Continuous-wave laser generated jets for needle free applications.

We designed and built a microfluidic device for the generation of liquid jets produced by thermocavitation. A continuous wave (CW) laser was focused inside a micro-chamber filled with a light-absorbing solution to create a rapidly expanding vapor bubble. The chamber is connected to a micro-channel which focuses and ejects the liquid jet through the exit. The bubble growth and the jet velocity were measured as a function of the devices geometry (channel diameter D and chamber width A). The fastest jets were those for relatively large chamber size with respect to the channel diameter. Elongated and focused jets up to 29 m/s for a channel diameter of [Formula: see text] and chamber size of [Formula: see text] were obtained. The proposed CW laser-based device is potentially a compact option for a practical and commercially feasible needle-free injector.

Microfluidic high-resolution free-flow isoelectric focusing.

A microfluidic free-flow isoelectric focusing glass chip for separation of proteins is described. Free-flow isoelectric focusing is demonstrated with a set of fluorescent standards covering a wide range of isoelectric points from pH 3 to 10 as well as the protein HSA. With respect to an earlier developed device, an improved microfluidic FFE chip was developed. The improvements included the usage of multiple sheath flows and the introduction of preseparated ampholytes. Preseparated ampholytes are commonly used in large-scale conventional free-flow isoelectric focusing instruments but have not been used in micromachined devices yet. Furthermore, the channel depth was further decreased. These adaptations led to a higher separation resolution and peak capacity, which were not achieved with previously published free-flow isoelectric focusing chips. An almost linear pH gradient ranging from pH 2.5 to 11.5 between 1.2 and 2 mm wide was generated. Seven isoelectric focusing markers were successfully and clearly separated within a residence time of 2.5 s and an electrical field of 20 V mm-1. Experiments with pI markers proved that the device is fully capable of separating analytes with a minimum difference in isoelectric point of Delta(pI) = 0.4. Furthermore, the results indicate that even a better resolution can be achieved. The theoretical minimum difference in isoelectric point is Delta(pI) = 0.23 resulting in a peak capacity of 29 peaks within 1.8 mm. This is an 8-fold increase in peak capacity to previously published results. The focusing of pI markers led to an increase in concentration by factor 20 and higher. Further improvement in terms of resolution seems possible, for which we envisage that the influence of electroosmotic flow has to be further reduced. The performance of the microfluidic free-flow isoelectric focusing device will enable new applications, as this device might be used in clinical analysis where often low sample volumes are available and fast separation times are essential.