Recent innovations in protein separation on microchips by electrophoretic methods: An update

A rapid, simple and sensitive microfluidic chip electrophoresis mass spectrometry method for monitoring amino acids in cell culture media

The development and optimization of cell culture media for biotech applications is a fundamental step of process development. The composition of cell culture media requires an ideal blend of amino acids, vitamins, nucleosides, lipids, carbohydrates, trace elements and other components. The ability to monitor these constituents is required to ensure that cells receive sufficient nutrients to facilitate growth, viability and productivity. Analysis of cell culture media is challenging due to the range and diversity of compounds contained in this matrix and normally requires time consuming methods. A rapid, simple and sensitive microfluidic chip CE-MS method is described to monitor amino acids in chemically defined cell culture media from a Chinese hamster ovary cell line cultured over a period of 10 days. The described platform enabled the separation of 16 amino acids in less than 2 minutes and without the requirement for extensive sample preparation. The analytical parameters evaluated were precision, linearity, limit of detection and limit of quantification. The majority of essential amino acids were present in cell culture growth in high concentrations compared to non-essential amino acids. Over the course of the 10 days cell culture the concentration of certain amino acids declined by up to 100%. Microfluidic chip based CE-MS methods can be used effectively to obtain the consumption rates of amino acids in cell culture media during cell growth and to perform at-line monitoring and screening of cell culture status.

Comparison of separation modes for microchip electrophoresis of proteins

Separation of a set of model proteins was tested on a microchip electrophoresis analytical platform capable of sample injection by two different electrokinetic mechanisms. A range of separation modes-microchip capillary zone electrophoresis, microchip micellar electrokinetic chromatography, and nanoparticle-based sieving-was tested on glass and polydimethylsiloxane/glass microchips and with silica-nanoparticle colloidal arrays. The model proteins calmodulin (18 kiloDalton), bovine serum albumin (66 kDa), and concanavalin (106 kDa) were labeled with Alexa Fluor 647 for laser-induced fluorescence detection. The best separation and resolution were obtained in a silica-nanoparticle colloidal array chip.

Analysis of proteins and peptides by electromigration methods in microchips

This review presents the developments and applications of microchip electromigration methods in the separation and analysis of peptides and proteins in the period 2011-mid-2016. The developments in sample preparation and preconcentration, microchannel material, and surface treatment are described. Separations by various microchip electromigration methods (zone electrophoresis in free and sieving media, affinity electrophoresis, isotachophoresis, isoelectric focusing, electrokinetic chromatography, and electrochromatography) are demonstrated. Advances in detection methods are reported and novel applications in the areas of proteomics and peptidomics, quality control of peptide and protein pharmaceuticals, analysis of proteins and peptides in biomatrices, and determination of physicochemical parameters are shown.

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.

Surface Modification of Poly(dimethylsiloxane) Using Ionic Complementary Peptides to Minimize Nonspecific Protein Adsorption

Poly(dimethylsiloxane) (PDMS) has become a widely used material for microfluidic and biological applications. However, PDMS has unacceptably high levels of nonspecific protein adsorption, which significantly lowers the performance of PDMS-based microfluidic chips. Most existing methods to reduce protein fouling of PDMS are to make the surface more hydrophilic by surface oxidization, polymer grafting, and physisorbed coatings. These methods suffer from the relatively short-term stability, the multistep complex treatment procedure, or the insufficient adsorption reduction. Herein, we developed a novel and facile modification method based on self-assembled peptides with well-tailored amino acid composition and sequence, which can also interact strongly with the PDMS surface in the same way as proteins, for suppressing the nonspecific protein fouling and improving the biocompatibility of PDMS-based microfluidic chips. We first demonstrated that an ionic complementary peptide, EAR16-II with a sequence of [(Ala-Glu-Ala-Glu-Ala-Arg-Ala-Arg)2], can readily self-assemble into an amphipathic film predominantly composed of tightly packed β-sheets on the native hydrophobic and plasma-oxidized hydrophilic PDMS surfaces upon low concentrations of carbohydrates. The self-assembled EAR16-II amphipathic film exposed its hydrophobic side to the solution and thus rendered the PDMS surface hydrophobic with water contact angles (WCAs) of around 110.0°. However, the self-assembled EAR16-II amphipathic film exhibited excellent protein-repelling and blood compatibility properties comparable to or better than those obtained with previously reported methods. A schematic model has been proposed to explain the interactions of EAR16-II with the PDMS surface and the antifouling capability of EAR16-II coatings at a molecular level. The current work will pave the way to the development of novel coating materials to address the nonspecific protein adsorption on PDMS, thereby broadening the potential uses of PDMS-based microfluidic chips in complex biological analysis.

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.

Introduction to glass microstructuring techniques

In this chapter an overview of manufacturing methods, leading to the fabrication of microstructures in glass substrates, is presented. Glass is a material of excellent optical properties, a very good electric insulator, biocompatible and chemically stable. In addition to its intrinsic qualities, glass can be processed with the use of manufacturing methods originating from the microelectronic industry. In this text two complete manufacturing protocols are described, each composed of standard microfabrication steps; namely, the deposition of masking layers, photolithographic patterning and pattern transfer via wet or dry etching. As a result, a set of building blocks is provided, allowing the manufacture of various microfluidic components that are frequently used in the domain of micro-total analysis system technology.

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.

Sample preconcentration in open microchannels combined with MALDI-MS

In this work, a method for preconcentrating samples in 1 cm long, 50-150 μm wide open microchannels is presented. Platinum electrodes were positioned at the channel ends, voltage was applied, and charged analyte was preconcentrated at the oppositely charged side during continuous supply of sample. The preconcentration was initially studied in a closed system, where an influence on the analyte position from a pH gradient, generated by water electrolysis, was observed. In the open channel, the analyte distribution after preconcentration was evaluated using MALDI-MS with the channel as MALDI target. MALDI matrix was applied with an airbrush or by electrospray matrix deposition and by using the latter technique higher degrees of crystallization in the channels were obtained. After preconcentrating a 1 nM cytochrome c solution for 5 min, corresponding to a supplied amount of 1.25 fmol, a signal on the cathodic channel end could be detected. When a solution of cytochrome c trypsin digest was supplied, the peptides were preconcentrated at different positions along the channel depending on their charge.

Dual-channel bipolar electrode focusing: simultaneous separation and enrichment of both anions and cations

In this paper we show that a microelectrochemical cell comprising two parallel microchannels spanned by a single bipolar electrode can be used to simultaneously enrich and separate both anions and cations within a single microchannel. This is possible because reduction and oxidation of water at the cathodic and anodic poles of the bipolar electrode, respectively, lead to ion depletion zones. Specifically, TrisH(+) is neutralized by OH(-) at the cathodic pole, while acetate buffer is neutralized by H(+) at the anodic pole. This action creates a local electric field gradient having both positive and negative components, and hence positive and negative ions follow their respective field gradients leading to separation. In the presence of an opposing counter-flow (pressure driven flow in this case), enrichment also occurs. In addition to separation and enrichment in a single channel, it is also possible to simultaneously enrich cations in one microchannel and anions in the other. Enrichment is achieved by controlling experimental parameters, including the type of buffer and the direction and magnitude of the opposing counter-flow.

Microfluidic systems for diagnostic applications: a review

Because of intensive developments in recent years, the microfluidic system has become a powerful tool for biological analysis. Entire analytic protocols including sample pretreatment, sample/reagent manipulation, separation, reaction, and detection can be integrated into a single chip platform. A lot of demonstrations on the diagnostic applications related to genes, proteins, and cells have been reported because of their advantages associated with miniaturization, automation, sensitivity, and specificity. The aim of this article is to review recent developments in microfluidic systems for diagnostic applications. Based on the categories of various fluid-manipulating mechanisms and biological detection approaches, in-depth discussion of the microfluidic-based diagnostic systems is provided. Moreover, a brief discussion on materials and manufacturing techniques will be included. The current excellent integration of microfluidic systems and diagnostic applications suggests a solid foundation for the development of practical point-of-care devices.

Microfluidic carbon-blackened polydimethylsiloxane device with reduced ultra violet background fluorescence for simultaneous two-color ultra violet/visible-laser induced fluorescence detection in single cell analysis

In single cell analysis (SCA), individual cell-specific properties and inhomogeneous cellular responses are being investigated that is not subjected to ensemble-averaging or heterogeneous cell population effects. For proteomic single cell analysis, ultra-sensitive and reproducible separation and detection techniques are essential. Microfluidic devices combined with UV laser induced fluorescence (UV-LIF) detection have been proposed to fulfill these requirements. Here, we report on a novel microfluidic chip fabrication procedure that combines straightforward production of polydimethylsiloxane (PDMS) chips with a reduced UV fluorescence background (83%-reduction) by using PDMS droplets with carbon black pigments (CBP) as additives. The CBP-droplet is placed at the point of detection, whereas the rest of the chip remains transparent, ensuring full optical control of the chip. We systematically studied the relation of the UV background fluorescence at CBP to PDMS ratios (varying from 1:10 to 1:1000) for different UV laser powers. Using a CBP/PDMS ratio of 1:20, detection of a 100 nM tryptophan solution (S/N = 3.5) was possible, providing a theoretical limit of detection of 86 nM (with S/N = 3). Via simultaneous two color UV/VIS-LIF detection, we were able to demonstrate the electrophoretic separation of an analyte mixture of 500 nM tryptophan (UV) and 5 nM fluorescein (VIS) within 30 s. As an application, two color LIF detection was also used for the electrophoretic separation of the protein content from a GFP-labeled single Spodoptera frugiperda (Sf9) insect cell. Thereby just one single peak could be measured in the visible spectral range that could be correlated with one single peak among others in the ultraviolet spectra. This indicates an identification of the labeled protein γ-PKC and envisions a further feasible identification of more than one single protein in the future.

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.

Toward point-of-care microchip profiling of proteins

Profiling of protein biomarkers is powerful for the analysis of complex proteomes altered during the progression of diseases. Lab-on-a-chip technologies can potentially provide the throughput and efficiency required for point-of-care and clinical applications. While initial studies utilized 1D microchip separation techniques, researchers have recently developed novel 2D microchip separation platforms with the ability to profile thousands of proteins more effectively. Despite advancements in lab-on-a-chip technologies, very few reports have demonstrated a point-of-care microchip-based profiling of proteins. In this review, recent progress in 1D and 2D microchip profiling of protein mixtures of a biological sample with potential point-of-care applications are discussed. A selection of recent microchip immunoassay-based techniques is also highlighted.