Evaluation of microchip material and surface treatment options for IEF of allergenic milk proteins on microchips

Modular microfluidic system for on-chip extraction, preconcentration and detection of the cytokine biomarker IL-6 in biofluid

The cytokine interleukin 6 (IL-6) is involved in the pathogenesis of different inflammatory diseases, including cancer, and its monitoring could help diagnosis, prognosis of relapse-free survival and recurrence. Here, we report an innovative microfluidic approach that uses the fluidization of magnetic beads to specifically extract, preconcentrate and fluorescently detect IL-6 directly on-chip. We assess how the physical properties of the beads can be tuned to improve assay performance by enhancing mass transport, reduce non-specific binding and multiply the detection signal threefold by transitioning between packed and fluidization states. With the integration of a full ELISA protocol in a single microfluidic chamber, we show a twofold reduction in LOD compared to conventional methods along with a large dynamic range (10 pg/mL to 2 ng/mL). We additionally demonstrate its application to IL-6 detection in undiluted serum samples.

Array of multifunctional polymers for localized immobilization of biomolecules on microarray substrates

The performance of microarray assays results from the optimization of several parameters: in particular, the physical-chemical properties of the surface play a pivotal role in determining the robustness of the technology. Usually, microarray substrates are entirely modified with coatings able to bind, covalently or not, bioprobes. Here we present a new, fully automated approach for the immobilization of biomolecules, based on the deposition of pL amounts of water solutions of DMA based copolymers on an uncoated surface, followed by the deposition, on the same spot, of the probe. Starting from a common precursor, polymers with different characteristics and functionalities are obtained by post-polymerization modification and by combining different monomers during the synthesis. This strategy, allows to functionalize and tailor the surface properties of discrete areas of the same array with different chemistries, that coexist on a single substrate. As a consequence, probes with different functionalities are bound simultaneously to neutral, positively, negatively charged, hydrophobic, hydrophilic polymers, in micrometer-sized spots. The proposed polymer array, applicable to both DNA or protein, offers advantages in terms of time and costs reduction, since pretreatment and coating steps are totally avoided, and the requested polymer amount is extremely low. Moreover, it provides a strategy perfectly suitable for miniaturization applicable to integrated biosensors or Lab-on-a-chip devices.

Surface isoelectric focusing (sIEF) with carrier ampholyte pH gradient

Isoelectric focusing (IEF) is a powerful tool for amphoteric protein separations because of high sensitivity, bio-compatibility, and reduced complexity compared to chromatography or mechanical separation techniques. IEF miniaturization is attractive because it enables rapid analysis, easier adaptation to point of care applications, and smaller sample demands. However, existing small-scale IEF tools have not yet been able to analyze single protein spots from array libraries, which are ubiquitous in many pharmaceutical discovery and screening protocols. Thus, we introduce an in situ, novel, miniaturized protein analysis approach that we have termed surface isoelectric focusing (sIEF). Low volume printed sIEF gels can be run at length scales of ∼300 μm, utilize ∼0.9 ng of protein with voltages below 10 V. Further, the sIEF device platform is so simple that it can be integrated with protein library arrays to reduce cost; devices demonstrate reusability above 50 uses. An acrylamide monomer solution containing broad-range carrier ampholytes was microprinted with a Nano eNabler^TM between micropatterned gold electrodes spaced 300 μm apart on a glass slide. The acrylamide gel was polymerized in situ followed by protein loading via printed diffusional exchange. A pH gradient formed via carrier ampholyte stacking when electrodes were energized; the gradient was verified using ratiometric pH-sensitive FITC/TRITC dyes. Green fluorescent protein (GFP) and R-phycoerythrin (R-PE) were utilized both as pI markers and to test sIEF performance as a function of electric field strength and ampholyte concentration. Factors hampering sIEF included cathodic drift and pH gradient compression, but were reduced by co-printing non-ionic Synperonic® F-108 surfactant to reduce protein-gel interactions. sIEF gels achieved protein separations in <10 min yielding bands < 50 μm wide with peak capacities of ∼8 and minimum pI differences from 0.12 to 0.14. This new sIEF technique demonstrated comparable focusing at ∼100 times smaller dimensions than any previous IEF. Further, sample volumes required were reduced four orders of magnitude from 20 μL for slab gel IEF to 0.002 μL for sIEF. In summary, sIEF advantages include smaller volumes, reduced power consumption, and microchip surface accessibility to focused bands along with equivalent separation resolutions to prior IEF tools. These attributes position this new technology for rapid, in situ protein library analysis in clinical and pharmaceutical settings.

Dyneon THV, a fluorinated thermoplastic as a novel material for microchip capillary electrophoresis

In this work, we have investigated Dyneon THV, a fluorinated material, as a new material to afford electrokinetic separations in microfluidic devices. To overcome protein adsorption, two poly(ethylene oxide) (PEO)-based coatings have been investigated: Pluronic F127 and PEO stearate 40. The best results were obtained with the PEO stearate 40 coating which allowed decreasing the surface contact angle from 91 ± 3 to 76°± 3. With this surface treatment, a 66% reduction of the electroosmotic mobility at pH 8.0 and a marked suppression of protein adsorption were observed compared to a native Dyneon THV microchip. Finally, a separation of fluorescently labeled proteins (bovine serum albumin and trypsin inhibitor), well-known for their strong tendency to adsorb on hydrophobic surfaces, was successfully achieved in an HEPES buffer with a PEO stearate 40 treated microchip by capillary zone electrophoresis. Furthermore, we demonstrated the possibility to perform non-aqueous capillary electrophoresis analysis of hydrophobic dyes using various solvents in untreated microchips. The overall results demonstrated not only the suitability of the Dyneon THV microchip for electrokinetic separations, but also its versatility allowing different separation modes to be implemented with the same microchip material.

Neutral polymers as coatings for high resolution electrophoretic separation of Aβ peptides on glass microchips

This study reports a comparison of the performances of two neutral polymers, poly ethylene-oxide (PEO) and poly(dimethylacrylamide-co-allyl glycidyl ether) (EpDMA), in glass microchips to achieve zone electrophoresis separation of several truncated forms of beta amyloid (Aβ) peptides, sharing very similar structures. The peptides were derivatized by FluoProbes 488 NHS to allow their fluorescence detection. Two protocols based either on PEO or EpDMA led to good pH stabilities in addition to a significant reduction of the electroosmotic flow. These two polymer coatings allowed repeatable analyses and high resolution for the simultaneous analysis of three Aβ peptides, Aβ 1-38, Aβ 1-40 and Aβ 1-42, considered as potential biomarkers of Alzheimer's disease. A recovery study showed that EpDMA was superior in reducing the adsorption of the Aβ peptides on the coated inner wall. Finally, the separation method relying on the EpDMA coated microchips was validated as linear using a calibration curve and the LOD was estimated to be close to 200 nM. Despite very short migration distances, different N-terminal or C-terminal truncated Aβ peptides, corresponding to promising biomarker combinations for the future diagnostic, were fully resolved. The method was successfully applied to detect these peptides in spiked cerebrospinal fluid and has provided a first achievement towards the development of a microsystem that would integrate preconcentration and separation steps.

Development of an SDS-gel electrophoresis method on SU-8 microchips for protein separation with LIF detection: Application to the analysis of whey proteins

This work describes the development of an SDS-gel electrophoresis method for the analysis of major whey proteins (α-lactalbumin, β-lactoglobulin, and BSA) carried out in SU-8 microchips. The method uses a low-viscosity solution of dextran as a sieving polymer. A commercial coating agent (EOTrol LN) was added to the separation buffer to control the EOF of the chips. The potential of this coating agent to prevent protein adsorption on the walls of the SU-8 channels was also evaluated. Additionally, the fluorescence background of the SU-8 material was studied to improve the sensitivity of the method. By selecting an excitation wavelength of 532 nm at which the background fluorescence remains low and by replacing the mercury arc lamp by a laser in the detection system, an LOD in the nanomolar range was achieved for proteins derivatized with the fluorogenic reagent Chromeo P540. Finally, the method was applied to the analysis of milk samples, demonstrating the potential of SU-8 microchips for the analysis of proteins in complex food samples.

Determination of binding parameters between lysozyme and its aptamer by frontal analysis continuous microchip electrophoresis (FACMCE)

An original and simple methodology based on microchip electrophoresis (MCE) in a continuous frontal analysis mode (named frontal analysis continuous microchip electrophoresis, FACMCE) was developed for the simultaneous determination of the binding parameters, i.e. ligand-site dissociation constant (k(d)) and number of binding sites on the substrate (n). This simultaneous determination was exemplified with the interaction between an aptamer and its target. The selected target is a strongly basic protein, lysozyme, as its quantification is of great interest due to its antimicrobial and allergenic properties. A glass microdevice equipped with a fluorescence detection system was coated with hydroxypropylcellulose, reducing the electroosmotic flow and adsorption onto the channel walls. This microdevice allowed the continuous electrokinetic injection of a mixture of fluorescently labelled aptamer and non-labelled lysozyme. By determining the concentration of the free fluorescently labelled aptamer thanks to its corresponding plateau height, mathematical linearization methods allowed to determine a k(d) value of 48.4±8.0 nM, consistent with reported results (31 nM), while the average number of binding sites n on lysozyme, never determined before, was 0.16±0.03. These results seem to indicate that the buffer nature and the SELEX process should influence the number and affinity of the binding sites. In parallel it has been shown that the binding between lysozyme and its aptamer presents two sites of different binding affinities.

Micropillar array chips toward new immunodiagnosis

In this paper, we demonstrate the possibility to use a micropillar array to perform molecular immunodiagnosis. A polydimethylsiloxane (PDMS) microdevice consisting of a rectangular array of micropillars (45 µm in height, 100 × 100 µm square cross section) was used to replace microchannels or gels (polyacrylamide or agarose) to perform electrokinetic separation. This microarray was used to mimic highly diluted gel and to maintain electrolyte within the pillar zone by capillary effect. The electrolyte composition (glycerol and agarose content) was investigated in order to improve protein separation by isoelectric focusing (IEF). The influence of glycerol on focusing time and on the different evaporative contributions was further evaluated. In order to perform an immunodiagnostic of milk allergy, different surface treatments were optimized to prevent milk allergen adsorption on PDMS surface. Poly(dimethylacrylamide)-co-allyl glycidyl ether (PDMA-AGE) as well as gelatin led to a satisfactory signal to noise ratio. Finally the possibility to perform protein mixture separation using this micropillar array chip followed by immunoblotting was demonstrated by using the serum from an allergic individual, confirming the great potential of this analytical platform in the field of immunodiagnosis.