Integrated Preconcentration SDS−PAGE of Proteins in Microchips Using Photopatterned Cross-Linked Polyacrylamide Gels

Microscale Interfacial Polymerization on a Chip

Forming hydrogels with precise geometries is challenging and mostly done using photopolymerization, which involves toxic chemicals, rinsing steps, solvents, and bulky optical equipment. Here, we introduce a new method for in situ formation of hydrogels with a well‐defined geometry in a sealed microfluidic chip by interfacial polymerization. The geometry of the hydrogel is programmed by microfluidic design using capillary pinning structures and bringing into contact solutions containing hydrogel precursors from vicinal channels. The characteristics of the hydrogel (mesh size, molecular weight cut‐off) can be readily adjusted. This method is compatible with capillary‐driven microfluidics, fast, uses small volumes of reagents and samples, and does not require specific laboratory equipment. Our approach creates opportunities for filtration, hydrogel functionalization, and hydrogel‐based assays, as exemplified by a rapid, compact competitive immunoassay that does not require a rinsing step.

Direct Measurement of Initial Rate of Enzyme Reaction by Electrokinetic Filtration Using a Hydrogel-plugged Capillary Device

A novel electrokinetic filtration device using a plugged hydrogel was developed to directly measure the initial rate of enzyme reactions. In the proposed method, the enzyme reaction proceeded only for a short time when the substrate was passed through a thin layer of enzyme trapped by the hydrogel without any lag times for mixing and detection. In experimental conditions, alkaline phosphatase (enzyme) was filtrated at a cathodic-side interface of the plugged hydrogel by molecular sieving effect, providing the thin enzyme zone whose thickness was approximately 100 μm. When 4-methylumberiferyl phosphate (substrate) was electrokinetically introduced into the device after trapping the enzyme, 4-methylumberiferone (product) was generated by the enzyme reaction for only 1.26 s as the substrate passed through the trapped enzyme zone. As a result, the initial rate of the enzyme reaction could be directly calculated to 31.0 μM/s by simply dividing the concentration of the product by the tunable reaction time. Compared to the initial rate obtained by mixing the enzyme and substrate solutions, the value of the maximum velocity of the enzyme reaction was 30-fold larger than that in the mixing method due to the preconcentration of the enzyme by trapping. The Michaelis-Menten constant in the proposed method was 2.7-fold larger than that in the mixing method, suggesting the variation of changes in the equilibrium of complex formation under the experimental conditions.

Origami paper-based sample preconcentration using sequentially driven ion concentration polarization

Ion concentration polarization (ICP) is one of the preconcentration techniques which can acquire a high preconcentration factor. Still, the main hurdles of ICP are its instability and low efficiency under physiological conditions with high ionic strength and abundant biomolecules. Here, we suggested a sequentially driven ICP process, which enhanced the electrokinetic force required for preconcentration, enabling enrichment of highly ionic raw samples without increasing the electric field. We acquired a 13-fold preconcentration factor (PF) in human serum using a paper-based origami structure consisting of multiple layers for three-dimensional sequential ICP (3D seq-ICP). Moreover, we demonstrated a paper-based enzyme-linked immunosorbent assay (ELISA) by 3D seq-ICP using tau protein, showing a 6-fold increase in ELISA signals.

Direct whole blood analysis by the antigen-antibody chemically-delayed dissociation from nanosensors arrays

A wide range of fields, starting from basic research in life sciences and up to medical applications, are highly interested in the investigation and detection of biomarkers in all their forms, including proteins. However, direct analytical detection of specific protein biomarkers from a physiological biosample is still extremely challenging due to the abundant variety and amount of its components. In this work, we apply the chemically-controlled antigen-dissociation detection approach on silicon nanowires-based field-effect transistor arrays, by creating a suitable 'chemical environment' which enabled the clear-cut splitting of the dissociation regime window into two sub-regimes, thus allowing the complete washing of the nonspecifically adsorbed salts and biomolecules, while significantly delaying the dissociation of specific surface-bounded antigen-antibody pairs. This was accomplished by the addition of the water-miscible organic reagent ethylene glycol, which radically alters the properties of the aqueous solvent, by means of dramatically reducing its interactions with the particular protein antigen, and thus allowing for the increase in the antigen-antibody interaction strength. This in turn, deeply reduces the solubility of the surface-bound protein molecules and increases their interaction with the specific receptor antibody units, which brings to a substantial delay in the antibody-antigen dissociation behavior. This phenomenon allows the clear-cut splitting of the dissociation regime window and the quantitative and accurate analysis of proteins in physiological samples. We demonstrated the direct and quantitative detection of protein biomarkers, down to concentrations in the fM range, from unprocessed whole blood minuscule samples of only a few microliters. This work is the first demonstration on the chemically-controlled dissociation kinetics of antibody-antigen pairs by the use of water-miscible organic solvent mixtures, and its application in the direct ultrasensitive detection of protein biomarkers from whole blood samples.

Electrokinetic stacking of particle zones in confined channels enabling their UV absorbance detection on microchips

In this article, we report a simple approach to stacking micro- and nanoparticle zones by electrokinetically migrating them through moderately confined channels of uniform cross-section. Experiments show the reported pre-concentration process to initiate at the tail end of the zone following its electrokinetic injection, with the stacked region migrating faster than the rest of the sample band. This effect causes the particles traveling in front to merge into the stacked region making it grow both in size and concentration. Because the stacked zone also gradually loses particles from its trailing edge, it eventually disintegrates upon running out of particles at its front end. Nevertheless, enhancements in peak height by over 100-fold were recorded using the reported approach for polystyrene beads with diameters comparable to the channel depth. This enhancement however, exhibited a temporal variation as the particle band migrated through the analysis column reaching a maximum value that depended on the particle diameter, particle concentration, channel depth, electric field strength, electroosmotic mobility, etc. Interestingly, the peak area recorded by the detector remained relatively constant during this particle migration period allowing reliable sample quantitation. Moreover, upon incubating antibody-coated particles against an antigen sample, the peak area for the particle zone was seen to scale linearly with the antigen concentration establishing the utility of the reported focusing phenomenon for chemical/biochemical analysis. The noted stacking technique was further applied to enabling UV absorbance detection of particle zones on microchips which then allowed us to determine the colloidal content in actual natural water samples. .

Quantifying the pH shift induced by selective anodic electrochemical reactions in the ion concentration polarization phenomenon

Recently, the ion concentration polarization (ICP) phenomenon has been actively utilized for low abundance biomolecular preconcentration applications. Since ICP significantly rearranges the ion distribution near a permselective membrane, its detailed investigation should be conducted for developing efficient platforms. In particular, proton transport through the membrane critically affects the pH of sample solutions so that continuous monitoring or batch measurement of pH is the priority task to be carried out. Moreover, electrochemical reactions have been overlooked, even though an overpotential is applied to preconcentrate a sample under physiological conditions, and the electrodes are in direct contact with the sample biomolecules. In this work, we experimentally visualized and directly measured how the electrochemical reaction dominated the preconcentration efficiency using two types of electrode configurations; large exposed electrode area (LEEA) and small exposed electrode area (SEEA). Interestingly, significant pH variation was confirmed only in the case of SEEA. As a result, the BSA preconcentration was impeded within a short period in the case of SEEA, but loss-free preconcentration was achieved in the case of LEEA. Therefore, one should pay careful attention to the electrode design of electrokinetic operation, especially when pH-sensitive biomolecules are involved.

Numerical investigation of the current transition regimes in nanochannels

The concentration polarization phenomena and its effects represent one of the main challenges for the optimal operation of many nanofluidic systems. A numerical investigation of the different electric current transition regimes observed during the concentration polarization phenomena in nanochannels is performed. This included a 2D-axisymmetric simulation of the nanofluidic system (reservoir-nanochannel-reservoir). From these simulations, a novel mechanism is discovered that explains that different current transition regimes. This driving mechanism involves the applied electric field penetration while the convective flow mechanism is found to be negligible. This differs with the classical statement that the mixing process with less depleted areas initiated by an electrokinetic vortex instability starts the overlimiting regime. Additionally, the numerical approach allows us to identify new characteristics of the linear-limiting transition such as source-like and saddle-like points of the electric field streamlines. The three voltage-current regimes (linear, limiting and overlimiting) are explained by observing and quantifying changes in electric field, potential, ion concentration and ion concentration gradients within the system.

Nanoporous Hydrogels for the Observation of Anthrax Exotoxin Translocation Dynamics

The ability to observe lethal anthrax exotoxins translocating through size-constricting nanopores in vitro, combined with detailed sequence and structural data, has aided in elucidated mechanisms of exotoxin cell entry and toxicity. However, due to limited observations of anthrax exotoxins translocating through protective antigen nanopores in vitro and the instability of protective antigen-functionalized suspended lipid bilayers, questions remain regarding the native mechanisms of cell entry. Nanoporous hydrogel membranes offer a robust tool for studying protein translocation with ensemble measurements that complement conventional single-molecule translocation measurements. Here, we utilize nanoporous hydrogel membranes to assess the translocation of full-length anthrax lethal and edema factors through nanopores similar in diameter to protective antigen translocons. We find that, relative to globular serum and other proteins that do not translocate natively through nanopores, anthrax exotoxins demonstrate significantly reduced barriers to pore entry. Computed free-energy barriers to the unfolding of proteins and the dissociation of macromolecular complexes are generally found to coincide with translocation. Finally, a nanopore-blocking strategy is developed that utilizes nonspecific synthetic peptide constructs and effectively prevents LF translocation within the nanoporous hydrogel.

In situ photo-patterning of pressure-resistant hydrogel membranes with controlled permeabilities in PEGDA microfluidic channels

We report the fabrication of highly permeable membranes in poly(ethylene glycol) diacrylate (PEGDA) channels, for investigating ultra- or micro-filtration, at the microfluidic scale. More precisely, we used a maskless UV projection setup to photo-pattern PEG-based hydrogel membranes on a large scale (mm-cm), and with a spatial resolution of a few microns. We show that these membranes can withstand trans-membrane pressure drops of up to 7 bar without any leakage, thanks to the strong anchoring of the hydrogel to the channel walls. We also report in situ measurements of the Darcy permeability of these membranes, as a function of the deposited energy during photo-polymerization, and their formulation composition. We show that the use of PEG chains as porogens, as proposed in [Lee et al., Biomacromolecules, 2010, 11, 3316], significantly increases the porosity of the hydrogels, up to Darcy permeabilities of about 1.5 × 10-16 m2, while maintaining the strong mechanical stability of the membranes. We finally illustrate the opportunities offered by this technique, by investigating frontal filtration of colloidal dispersions in a straight microfluidic channel.

Expression of Aspergillus niger N5-5 in E. coli and purification and identification of products

Due to the feature of high hydrolysis, tannase is widely used in food, beverage, brewing and other fields. However, high cost in producing natural tannase makes it difficult to apply tannase to industry in a large-scale. Microbial expression systems can be used for preparing numerous amount of enzyme at low cost, so in this paper Aspergillus niger N5-5 was expressed using E. coli system. Specific primers were designed based on the Aspergillus niger N5-5 sequence N3 (GenBank, No.: KP677552), and tannase gene tan was promoted to carry 6 His tag and enzyme cutting site which contains NdeI/HindIII using PCR amplification. Then, tannase gene tan was connected to expression vector by NdeI/HindIII enzyme cutting. In this way, recombinant expression vector tan-pET43.1a was formed. Then, the expression vector pET43.1a by NdeI/HindIII enzyme cutting was transformed into E. coli BL21 (DE3) to induce expression of Aspergillus niger N5-5. When the induced fungi were disrupted by the ultrasonic wave, the crude enzyme was extracted and purified by using the IMAC, and then the activity of the crude enzyme and pure enzyme was determined. According to the results of determination of the tannase activity, the tannase activity of the crude enzyme was greatly improved after the crude enzyme was purified, and the specific activity of the pure enzyme was about 8 times of that of the crude enzyme. The results of SDS-PAGE of the pure enzyme showed that the molecular mass of the pure enzyme was about 65 kDa/64–65 kDa, which was consistent with the expected result (64.2 kDa), It can be concluded that the crude enzyme solution was purified successfully. The results of pure enzyme’s protein identification by Western Blotting showed that clear protein bands pro-3 were observed. Molecular mass of clear protein bands pro-3 was about 65 kDa, which was in line with the expected results (64.2 kDa). It can be seen that the aforementioned expression protein could be specifically combined with His tag. It proved expression protein to be a recombinant fusion protein with 6 His tag.

Nanofluidic device for continuous multiparameter quality assurance of biologics

Process analytical technology (PAT) is critical for the manufacture of high-quality biologics as it enables continuous, real-time and on-line/at-line monitoring during biomanufacturing processes. The conventional analytical tools currently used have many restrictions to realizing the PAT of current and future biomanufacturing. Here we describe a nanofluidic device for the continuous monitoring of biologics' purity and bioactivity with high sensitivity, resolution and speed. Periodic and angled nanofilter arrays served as the molecular sieve structures to conduct a continuous size-based analysis of biologics. A multiparameter quality monitoring of three separate commercial biologic samples within 50 minutes has been demonstrated, with 20 µl of sample consumption, inclusive of dead volume in the reservoirs. Additionally, a proof-of-concept prototype system, which integrates an on-line sample-preparation system and the nanofluidic device, was demonstrated for at-line monitoring. Thus, the system is ideal for on-site monitoring, and the real-time quality assurance of biologics throughout the biomanufacturing processes.

Solid supports for extraction and preconcentration of proteins and peptides in microfluidic devices: A review

Determination of proteins and peptides is among the main challenges of today's bioanalytical chemistry. The application of microchip technology in this field is an exhaustively developed concept that aims to create integrated and fully automated analytical devices able to quantify or detect one or several proteins from a complex matrix. Selective extraction and preconcentration of targeted proteins and peptides especially from biological fluids is of the highest importance for a successful realization of these microsystems. Incorporation of solid structures or supports is a convenient solution employed to face these demands. This review presents a critical view on the latest achievements in sample processing techniques for protein determination using solid supports in microfluidics. The study covers the period from 2006 to 2015 and focuses mainly on the strategies based on microbeads, monolithic materials and membranes. Less common approaches are also briefly discussed. The reviewed literature suggests future trends which are discussed in the concluding remarks.

Antigen-Dissociation from Antibody-Modified Nanotransistor Sensor Arrays as a Direct Biomarker Detection Method in Unprocessed Biosamples

The detection of biomolecules is critical for a wide spectrum of applications in life sciences and medical diagnosis. Nonetheless, biosamples are highly complex solutions, which contain an enormous variety of biomolecules, cells, and chemical species. Consequently, the intrinsic chemical complexity of biosamples results in a significant analytical background noise and poses an immense challenge to any analytical measurement, especially when applied without prior efficient separation and purification steps. Here, we demonstrate the application of antigen-dissociation regime, from antibody-modified Si-nanowire sensors, as a simple and effective direct sensing mechanism of biomarkers of interest in complex biosamples, such as serum and untreated blood, which does not require ex situ time-consuming biosample manipulation steps, such as centrifugation, filtering, preconcentration, and desalting, thus overcoming the detrimental Debye screening limitation of nanowire-based biosensors. We found that two key parameters control the capability to perform quantitative biomarkers analysis in biosamples: (i) the affinity strength (k_off rate) of the antibody-antigen recognition pair, which dictates the time length of the high-affinity slow dissociation subregime, and (ii) the "flow rate" applied during the solution exchange dissociation step, which controls the time width of the low-affinity fast-dissociation subregime. Undoubtedly, this is the simplest and most convenient approach for the SiNW FET-based detection of antigens in complex untreated biosamples. The lack of ex situ biosample manipulation time-consuming processes enhances the portability of the sensing platform and reduces to minimum the required volume of tested sample, as it allows the direct detection of untreated biosamples (5-10 μL blood or serum), while readily reducing the detection cycle duration to less than 5 min, factors of great importance in near-future point-of-care medical applications. We believe this is the first ever reported demonstration on the real-time, direct label-free sensing of biomarkers from untreated blood samples, using SiNW-based FET devices, while not compromising the ultrasensitive sensing capabilities inherent to these devices.

A Reversibly Sealed, Easy Access, Modular (SEAM) Microfluidic Architecture to Establish In Vitro Tissue Interfaces

Microfluidic barrier tissue models have emerged as advanced in vitro tools to explore interactions with external stimuli such as drug candidates, pathogens, or toxins. However, the procedures required to establish and maintain these systems can be challenging to implement for end users, particularly those without significant in-house engineering expertise. Here we present a module-based approach that provides an easy-to-use workflow to establish, maintain, and analyze microscale tissue constructs. Our approach begins with a removable culture insert that is magnetically coupled, decoupled, and transferred between standalone, prefabricated microfluidic modules for simplified cell seeding, culture, and downstream analysis. The modular approach allows several options for perfusion including standard syringe pumps or integration with a self-contained gravity-fed module for simple cell maintenance. As proof of concept, we establish a culture of primary human microvascular endothelial cells (HMVEC) and report combined surface protein imaging and gene expression after controlled apical stimulation with the bacterial endotoxin lipopolysaccharide (LPS). We also demonstrate the feasibility of incorporating hydrated biomaterial interfaces into the microfluidic architecture by integrating an ultra-thin (< 1 μm), self-assembled hyaluronic acid/peptide amphiphile culture membrane with brain-specific Young’s modulus (~ 1kPa). To highlight the importance of including biomimetic interfaces into microscale models we report multi-tiered readouts from primary rat cortical cells cultured on the self-assembled membrane and compare a panel of mRNA targets with primary brain tissue signatures. We anticipate that the modular approach and simplified operational workflows presented here will enable a wide range of research groups to incorporate microfluidic barrier tissue models into their work.

Microfluidic chip-capillary electrophoresis device for the determination of urinary metabolites and proteins

Microfluidic chip-CE (MC-CE) devices have caught recent attention for diagnostic applications in urine. This is due to the successes reported in handling real urine samples by integrating microfluidic chips (MC) with analyte enrichment and sample cleanup to CE with high separation efficiency and sensitive analyte detection. Here, we review the determination of urinary metabolites and proteins by MC-CE devices within the past 7 years. The application scope for MC-CE integrated devices was found to exceed the use of either technique alone, showing comparable performance to laser-induced fluorescence detection using less sensitive UV detectors, offering the flexibility to handle difficult urine samples with on-chip dilution and online standard addition and delivering enhanced performance as compared with commercial microfluidic chip electrophoresis chips.

Preconcentration of diluted mixed-species samples following separation and collection in a micro-nanofluidic device

A microfluidic device consisting of a nanoscale Nafion membrane and a polydimethylsiloxane microchannel is proposed for the preconcentration of diluted multi-mixed species samples then following separation and collection. When an electric field is applied across the microchip, an accumulation of the mixed-species sample occurs at the junction between the microchannel and the membrane by means of ion concentration polarization effect. A separation of the sample then takes place due to the difference in the electrophoretic mobilities of the sample components. Finally, the component of interest is guided to a collection reservoir by manipulating the external potential configuration and is trapped in place by means of a magnetically actuated valve. The preconcentration performance of the proposed device is evaluated in both straight and convergent microchannels using a fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) sample. It is shown that a preconcentration factor of 40 times can be achieved using a straight microchannel. By contrast, the preconcentration factor increases to 50 times when using a convergent channel. The practical feasibility of the proposed device is demonstrated by performing the preconcentration, separation, and collection of a mixed FITC-BSA and Tetramethylrhodamine sample.

Photopatterning of poly(N-isopropylacrylamide) membranes for a high level of enrichment and cleanup of nucleic acids in microfluidic chips

An ideal nanoporous poly(N-isopropylacrylamide) membrane has been fabricated in glass microchannels by means of spatially controlled photopatterning technology for a high level of enrichment and cleanup of nucleic acids.

Spatiotemporal pH dynamics in concentration polarization near ion-selective membranes

We present a detailed analysis of the transient pH dynamics for a weak, buffered electrolyte subject to voltage-driven transport through an ion-selective membrane. We show that pH fronts emanate from the concentration polarization zone next to the membrane and that these propagating fronts change the pH in the system several units from its equilibrium value. The analysis is based on a 1D model using the unsteady Poisson-Nernst-Planck equations with nonequilibrium chemistry and without assumptions of electroneutrality or asymptotically thin electric double layers. Nonequilibrium chemical effects, especially for water splitting, are shown to be important for the dynamical and spatiotemporal evolution of the pH fronts. Nonetheless, the model also shows that at steady state the assumption of chemical equilibrium can still lead to good approximations of the global pH distribution. Moreover, our model shows that the transport of the hydronium ion in the extended space charge region is governed by a balance between electromigration and water self-ionization. On the basis of this observation, we present a simple model showing that the net flux of the hydronium ion is proportional to the length of the extended space charge region and the water self-ionization rate. To demonstrate these effects in practice, we have adopted the experiment of Mai et al. (Mai, J.; Miller, H.; Hatch, A. V. Spatiotemporal Mapping of Concentration Polarization Induced pH Changes at Nanoconstrictions. ACS Nano 2012, 6, 10206) as a model problem, and by including the full chemistry and transport, we show that the present model can capture the experimentally observed pH fronts. Our model can, among other things, be used to predict and engineer pH dynamics, which can be essential to the performance of membrane-based systems for biochemical separation and analysis.

Laminated microfluidic system for small sample protein analysis

We describe a technology based on lamination that allows for the production of highly integrated 3D devices suitable for performing a wide variety of microfluidic assays. This approach uses a suite of microfluidic coupons ("microfloupons") that are intended to be stacked as needed to produce an assay of interest. Microfloupons may be manufactured in paper, plastic, gels, or other materials, in advance, by different manufacturers, then assembled by the assay designer as needed. To demonstrate this approach, we designed, assembled, and characterized a microfloupon device that performs sodium-dodecyl-sulfate polyacrylamide gel electrophoresis on a small sample of protein. This device allowed for the manipulation and transport of small amounts of protein sample, tight injection into a thin polyacrylamide gel, electrophoretic separation of the proteins into bands, and subsequent removal of the gel from the device for imaging and further analysis. The microfloupons are rugged enough to handle and can be easily aligned and laminated, allowing for a variety of different assays to be designed and configured by selecting appropriate microfloupons. This approach provides a convenient way to perform assays that have multiple steps, relieving the need to design highly sophisticated devices that incorporate all functions in a single unit, while still achieving the benefits of small sample size, automation, and high speed operation.

Microfluidic devices with permeable polymer barriers for capture and transport of biomolecules and cells

We report a method for fabricating permeable polymer microstructure barriers in polydimethylsiloxane (PDMS) microfluidic devices and the use of the devices to capture and transport DNA and cells. The polymer microstructure in a desired location in a fluidic channel is formed in situ by the polymerization of acrylamide and polyethylene diacrylate cross-linker (PEG-DA) monomer in a solution which is trapped in the location using a pair of PDMS valves. The porous polymer microstructure provides a mechanical barrier to convective fluid flow in the channel or between two microfluidic chambers while it still conducts ions or small charged species under an electric field, allowing for the rapid capture and transport of biomolecules and cells by electrophoresis. We have demonstrated the application of the devices for the rapid capture and efficient release of bacteriophage λ genomic DNA, solution exchange and for the transport and capture of HeLa cells. Our devices will enable the multi-step processing of biomolecules and cells or individual cells within a single microfluidic chamber.

Integration of nanoporous membranes into microfluidic devices: electrokinetic bio-sample pre-concentration

The integration of nanoporous membranes into microfluidic devices allows a wide range of analytical and biochemical applications such as stable concentration gradient generation, sample pre-concentration, and ion and biomolecule filtration in a controllable manner. However, further applications of nanoporous membranes in microfluidic devices require rapid and controllable fabrication methods of various nanoporous precursor materials; currently, few such methods exist. Here, we describe simple and robust methods that can be used for microfabricating four different precursor materials as leakage-tight membranes in a microfluidic channel network. The methods consist of a common integration process and individual solidification processes such as solvent evaporation, UV-curing, and temperature treatment. We demonstrate that the fabricated membranes can be used for electrokinetic, nanofluidic pre-concentration of bio-samples such as proteins, cells, and microspheres on either the anodic or cathodic side of the membranes. In addition, we not only characterize the physicochemical properties of the membranes such as conductance of membrane-integrated microchannels, relative permselectivity, and pre-concentration ability, but also compare fabrication availability, membrane robustness, surface charge density tunability and biocompatibility with buffer solutions. The methods are versatile for many nanoporous precursor materials and easy to control the location and dimension of the membranes. Hence, the methods developed and the characterized properties of the membranes tested in this work could be widely employed for further applications of nanoporous membranes in microfluidic systems.

Novel moving reaction boundary-induced stacking and separation of human hemoglobins in slab polyacrylamide gel electrophoresis

We developed a novel polyacrylamide gel electrophoresis (PAGE) method to stack and separate human hemoglobins (Hbs) based on the concept of moving reaction boundary (MRB). This differs from the classic isotachophoresis (ITP)-based stacking PAGE in the aspect of buffer composition, including the electrode buffer (pH 8.62 Tris-Gly), sample buffer (pH 6.78 Tris-Gly), and separation buffer (pH 8.52 Tris-Gly). In the MRB-PAGE system, a transient MRB was formed between alkaline electrode buffer and acidic sample buffer, being designed to move toward the anode. Hbs carried partial positive charges in the sample buffer due to its pH below pI values of Hbs, resulting in electromigrating to the cathode. Hbs would carry negative charges quickly when migrated into the alkaline electrode buffer and be transported to the anode until meeting the sample buffer again. Thus, Hbs were stacked within a MRB until the transient MRB reached the separation buffer and then separated by zone electrophoresis with molecular sieve effect of the gel. The experimental results demonstrated that there were three clear and sharp protein zones of Hbs (HbA1c, HbA0, and HbA2) in MRB-PAGE, in contrast to only one protein zone (HbA0) in ITP-PAGE for large-volume loading (≥15 μl), indicating high stacking efficiency, separation resolution, and good sensitivity of MRB-PAGE. In addition, MRB-PAGE was performed in a conventional slab PAGE device, requiring no special device. Thus, it could be widely used in separation and analysis of diluted protein in a standard laboratory.

Chip-based immunoassays

Microfluidic immunoassay techniques offer advantages in speed, automation, and portability over -bench-top gold standard counterparts. In particular, on-chip immunosubtraction is a rapid homogeneous immunoassay used for reporting both protein native mobility and binding specificity. Immunosubtraction is performed by removing antibody-bound target proteins from electrophoretic detection via a size-based exclusion filter, while unbound nontarget proteins are able to pass through the filter for downstream detection. Immunosubtraction is achieved on-chip by fabrication of discrete patterned polyacrylamide (PA) gel regions. Additionally, PA gel regions are used to define on-chip sample preparation regions for protein enrichment, fluorescent labeling, and antibody-target binding prior to immunosubtraction. Here we describe the immunosubtraction device fabrication technique as well as the electrophoretic assay protocol for determining target protein mobility and binding specificity within complex biological samples including cerebrospinal fluid.

Spatiotemporal mapping of concentration polarization induced pH changes at nanoconstrictions

Under an applied electric field, concentration polarization (CP) arises from ion permselectivity of most nanoporous materials and biological ion channels. We present novel methods to quantitatively assess CP-induced spatiotemporal changes of pH that may significantly impact transport dynamics, device functionality, and physicochemical properties of molecular analytes in devices with nanofluidic constrictions. We measured pH fluctuations of >1.5 pH units and changes extending over 100's of micrometers from nanoconstrictions. The degree of change depends on key system parameters including buffer composition, surface charge, and strength of electric field. The results highlight the importance of neglected contributions of pH changes, and the approach can aid characterization and manipulation of mass transport in nanofluidic systems.

Continuous-flow biomolecule concentration and detection in a slanted nanofilter array

We demonstrate continuous-flow biomolecule concentration and detection in a microfabricated slanted sieving structure, which we term a herringbone nanofilter array (HNA). The HNA structure consists of periodically-patterned deep and shallow nanoslits meeting at right angles. In addition to concentration, we can discriminate different sized analytes by mixing a fluorescent probe with the sample and measuring the extent of the concentrating effect. Using this principle, we interrogate biomolecular interactions, including protein-DNA binding, protein-protein interaction and antibody-antigen binding. The final example demonstrates a novel method to perform a homogeneous immunoassay for detecting a disease marker, human C-reactive protein (CRP), using fluorescent-labeled antibodies at clinically relevant concentrations. The signal amplification potential and continuous flow operation provide a significant advantage over other microfluidic batch separation techniques for the easy integration of this device into a common point-of-care diagnostic platform.

[In situ photopolymerization of polyacrylamide-based preconcentrator on a microfluidic chip for capillary electrophoresis]

Microchip electrophoresis is widely used for microfluidics and has been studied extensively over the past decade. Translation of capillary electrophoresis methods from traditional capillary systems to a microchip platform provides rapid separation and easy quantitation of sample components. However, most microfluidic systems suffer from critical scaling problems. One promising solution to this problem is online sample preconcentration of all analytes in a sample reservoir before the separation channel. Herein, the following three techniques for online preconcentration during microchip electrophoresis are proposed: (1) in situ fabrication of an ionic polyacrylamide-based preconcentrator on a simple poly(methyl methacrylate) microfluidic chip for perm-selective preconcentration and capillary electrophoretic separation of anionic compounds, (2) simultaneous concentration enrichment and electrophoretic separation of weak acids on a microchip using an in situ photopolymerized carboxylate-type polyacrylamide gels as the perm-selective preconcentrator, and (3) microchip electrophoresis of oligosaccharides using lectin-immobilized preconcentrator gels fabricated by in situ photopolymerization. These techniques are expected to be powerful tools for clinical and pharmaceutical studies with on-line preconcentration during microchip electrophoresis.

Si nanowires forest-based on-chip biomolecular filtering, separation and preconcentration devices: nanowires do it all

The development of efficient biomolecular separation and purification techniques is of critical importance in modern genomics, proteomics, and biosensing areas, primarily due to the fact that most biosamples are mixtures of high diversity and complexity. Most of existent techniques lack the capability to rapidly and selectively separate and concentrate specific target proteins from a complex biosample, and are difficult to integrate with lab-on-a-chip sensing devices. Here, we demonstrate the development of an on-chip all-SiNW filtering, selective separation, desalting, and preconcentration platform for the direct analysis of whole blood and other complex biosamples. The separation of required protein analytes from raw biosamples is first performed using a antibody-modified roughness-controlled SiNWs (silicon nanowires) forest of ultralarge binding surface area, followed by the release of target proteins in a controlled liquid media, and their subsequent detection by supersensitive SiNW-based FETs arrays fabricated on the same chip platform. Importantly, this is the first demonstration of an all-NWs device for the whole direct analysis of blood samples on a single chip, able to selectively collect and separate specific low abundant proteins, while easily removing unwanted blood components (proteins, cells) and achieving desalting effects, without the requirement of time-consuming centrifugation steps, the use of desalting or affinity columns. Futhermore, we have demonstrated the use of our nanowire forest-based separation device, integrated in a single platform with downstream SiNW-based sensors arrays, for the real-time ultrasensitive detection of protein biomarkers directly from blood samples. The whole ultrasensitive protein label-free analysis process can be practically performed in less than 10 min.

Enhancement in the sensitivity of microfluidic enzyme-linked immunosorbent assays through analyte preconcentration

In this Article, we describe a microfluidic enzyme-linked immunosorbent assay (ELISA) method whose sensitivity can be substantially enhanced through preconcentration of the target analyte around a semipermeable membrane. The reported preconcentration has been accomplished in our current work via electrokinetic means allowing a significant increase in the amount of captured analyte relative to nonspecific binding in the trapping/detection zone. Upon introduction of an enzyme substrate into this region, the rate of generation of the ELISA reaction product (resorufin) was observed to increase by over a factor of 200 for the sample and 2 for the corresponding blank compared to similar assays without analyte trapping. Interestingly, in spite of nonuniformities in the amount of captured analyte along the surface of our analysis channel, the measured fluorescence signal in the preconcentration zone increased linearly with time over an enzyme reaction period of 30 min and at a rate that was proportional to the analyte concentration in the bulk sample. In our current study, the reported technique has been shown to reduce the smallest detectable concentration of the tumor marker CA 19-9 and Blue Tongue Viral antibody by over 2 orders of magnitude compared to immunoassays without analyte preconcentration. When compared to microwell based ELISAs, the reported microfluidic approach not only yielded a similar improvement in the smallest detectable analyte concentration but also reduced the sample consumption in the assay by a factor of 20 (5 μL versus 100 μL).

Microfluidic digital isoelectric fractionation for rapid multidimensional glycoprotein analysis

Here we present an integrated microfluidic device for rapid and automated isolation and quantification of glycoprotein biomarkers directly from biological samples on a multidimensional analysis platform. In the first dimension, digital isoelectric fractionation (dIEF) uses discrete pH-specific membranes to separate proteins and their isoforms into precise bins in a highly flexible spatial arrangement on-chip. dIEF provides high sample preconcentration factors followed by immediate high-fidelity transfer of fractions for downstream analysis. We successfully fractionate isoforms of two potential glycoprotein cancer markers, fetuin and prostate-specific antigen (PSA), with 10 min run time, and results are compared qualitatively and quantitatively to conventional slab gel IEF. In the second dimension, functionalized monolithic columns are used to capture and detect targeted analytes from each fraction. We demonstrate rapid two-dimensional fractionation, immunocapture, and detection of C-reactive protein (CRP) spiked in human serum. This rapid, flexible, and automated approach is well-suited for glycoprotein biomarker research and verification studies and represents a practical avenue for glycoprotein isoform-based diagnostic testing.

Continuous microfluidic DNA and protein trapping and concentration by balancing transverse electrokinetic forces

Sample pre-concentration can be a critical element to improve sensitivity of integrated microchip assays. In this work a converging Y-inlet microfluidic channel with integrated coplanar electrodes was used to investigate transverse DNA and protein migration under uniform direct current (DC) electric fields to assess the ability to concentrate a sample prior to other enzymatic modifications or capillary electrophoretic separations. Employing a pressure-driven flow to perfuse the microchannel, negatively charged samples diluted in low and high ionic strength buffers were co-infused with a receiving buffer of the same ionic strength into a main daughter channel. Experimental results demonstrated that, depending of the buffer selection, different DNA migration and accumulation dynamics were seen. Charged analytes could traverse the channel width and accumulate at the positive bias electrode in a low electroosmotic mobility, high electrophoretic mobility, high ionic strength buffer or migrated towards an equilibrium position within the channel in a high electroosmotic mobility, high electrophoretic mobility, low ionic strength buffer. The various migration behaviours are the result of a balance between the electrophoretic force and a drag force induced by a recirculating electroosmotic flow generated across the channel width due to the bounding walls. Under continuous flow conditions, DNA samples were concentrated several-fold by balancing these transverse electrokinetic forces. The electrokinetic trapping technique presented here is a simple technique which could be expanded to concentrate or separate other analytes as a preconditioning step for downstream processes.

Protein separation by capillary gel electrophoresis: a review

Capillary gel electrophoresis (CGE) has been used for protein separation for more than two decades. Due to the technology advancement, current CGE methods are becoming more and more robust and reliable for protein analysis, and some of the methods have been routinely used for the analysis of protein-based pharmaceuticals and quality controls. In light of this progress, we survey 147 papers related to CGE separations of proteins and present an overview of this technology. We first introduce briefly the early development of CGE. We then review the methodology, in which we specifically describe the matrices, coatings, and detection strategies used in CGE. CGE using microfabricated channels and incorporation of CGE with two-dimensional protein separations are also discussed in this section. We finally present a few representative applications of CGE for separating proteins in real-world samples.