High speed nanofluidic protein accumulator

Nylon Monofilament Mold Three-dimensional Microfluidic Chips for Size-exclusion Microchip Electrophoresis: Application to Specific Online Preconcentration of Proteins

We present a lithography-free procedure for fabricating intrinsically three-dimensional microchannels within PDMS elastomers using nylon monofilament molds. We embedded nylon monofilaments in an uncured PDMS composite to fabricate straight channels of desired length, for use as molds to form the microchannels. Next, we fabricated two layer devices consisting of dialysis membranes, which preconcentrate specific proteins in accordance with molecular weight, in between two layers of PDMS substrates with embedded microchannels. Because of the membrane isolation, analyte exchange between two fluidic layers can be precisely controlled by an applied voltage. More importantly, given that only small molecules pass through the dialysis membrane, the integrated membrane is suitable for molecular sieving or size exclusion for a concentrator prior to microchip electrophoresis. Researchers can use our microchip design for online purification and preconcentration of proteins in the presence of excess reagent immediately after fluorescent labeling. This method's technical advantage is that three-dimensional microstructures, such as microchannels that have a circular cross-section, are readily attainable and can be fabricated in a straightforward manner without using specialized equipment. Our method is a low-cost, environmentally sustainable procedure for fabricating microfluidic devices, and will render microfluidic processes more accessible and easy to implement.

A compact microfluidic chip with integrated impedance biosensor for protein preconcentration and detection

In this study, a low-cost, compact biochip is designed and fabricated for protein detection. Nanofractures formed by self-assembled gold nanoparticles at junction gaps are applied for ion enrichment and depletion to create a trapping zone when electroosmotic flow occurs in microchannels. An impedance measurement module is implemented based on the lock-in amplifier technique to measure the impedance change during antibody growth on the gold electrodes which is caused by trapped proteins in the detection region. The impedance measurement results confirm the presence of trapped proteins. Distinguishable impedance profiles, measured at frequencies in the range of 10-100 kHz, for the detection area taken before and after the presence of proteins validate the performance of the proposed system.

Recent advances in protein analysis by capillary and microchip electrophoresis

This review article describes the significant recent advances in the analysis of proteins by capillary and microchip electrophoresis during the period from mid-2014 to early 2017. This review highlights the progressions, new methodologies, innovative instrumental modifications, and challenges for efficient protein analysis in human specimens, animal tissues, and plant samples. The protein analysis fields covered in this review include analysis of native, reduced, and denatured proteins in addition to Western blotting, protein therapeutics and proteomics.

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.

Sample pre-concentration with high enrichment factors at a fixed location in paper-based microfluidic devices

The lack of sensitivity is a major problem among microfluidic paper-based analytical devices (μPADs) for early disease detection and diagnosis. Accordingly, the present study presents a method for improving the enrichment factor of low-concentration biomarkers by using shallow paper-based channels realized through a double-sided wax-printing process. In addition, the enrichment factor is further enhanced by exploiting the ion concentration polarization (ICP) effect on the cathodic side of the nanoporous membrane, in which a stationary sample plug is obtained. The occurrence of ICP on the shallow-channel μPAD is confirmed by measuring the current-voltage response as the external voltage is increased from 0 to 210 V (or the field strength from 0 to 1.05 × 10(4) V m(-1)) over 600 s. In addition, to the best of our knowledge, the electroosmotic flow (EOF) speed on the μPAD fabricated with a wax-channel is measured for the first time using a current monitoring method. The experimental results show that for a fluorescein sample, the concentration factor is increased from 130-fold in a conventional full-thickness paper channel to 944-fold in the proposed shallow channel. Furthermore, for a fluorescein isothiocyanate-labeled bovine serum albumin (FITC-BSA) sample, the proposed shallow-channel μPAD achieves an 835-fold improvement in the concentration factor. The concentration technique presented here provides a novel strategy for enhancing the detection sensitivity of μPAD applications.

A simple electrokinetic protein preconcentrator utilizing nano-interstices

This work proposes a simple method for creating nanofluidic channels for protein preconcentration through self-assembled gold nanoparticles (AuNPs) using the exclusion-enrichment effect. A depletion force is elicited in nano-interstices among self-assembled AuNPs due to the overlap of electrical double layers (EDLs); therefore, proteins quickly accumulate. The experimental results show that the generation of depletion forces is correlated with the size of the AuNPs. The self-assembled monolayer of AuNPs (13 nm in diameter) can successfully preconcentrate proteins through effective EDL overlapping. This approach provides a new process to produce nanochannels that does not require high-voltage or time-consuming fabrication.

Sample preconcentration utilizing nanofractures generated by junction gap breakdown assisted by self-assembled monolayer of gold nanoparticles

The preconcentration of proteins with low concentrations can be used to increase the sensitivity and accuracy of detection. A nonlinear electrokinetic flow is induced in a nanofluidic channel due to the overlap of electrical double layers, resulting in the fast accumulation of proteins, referred to as the exclusion-enrichment effect. The proposed chip for protein preconcentration was fabricated using simple standard soft lithography with a polydimethylsiloxane replica. This study extends our previous paper, in which gold nanoparticles were manually deposited onto the surface of a protein preconcentrator. In the present work, nanofractures were formed by utilizing the self-assembly of gold-nanoparticle-assisted electric breakdown. This reliable method for nanofracture formation, involving self-assembled monolayers of nanoparticles at the junction gap between microchannels, also decreases the required electric breakdown voltage. The experimental results reveal that a high concentration factor of 1.5×10(4) for a protein sample with an extremely low concentration of 1 nM was achieved in 30 min by using the proposed chip, which is faster than our previously proposed chip at the same conditions. Moreover, an immunoassay of bovine serum albumin (BSA) and anti-BSA was carried out to demonstrate the applicability of the proposed chip.

Protein preconcentration using nanofractures generated by nanoparticle-assisted electric breakdown at junction gaps

Sample preconcentration is an important step that increases the accuracy of subsequent detection, especially for samples with extremely low concentrations. Due to the overlapping of electrical double layers in the nanofluidic channel, the concentration polarization effect can be generated by applying an electric field. Therefore, a nonlinear electrokinetic flow is induced, which results in the fast accumulation of proteins in front of the induced ionic depletion zone, the so-called exclusion-enrichment effect. Nanofractures were created in this work to preconcentrate proteins via the exclusion-enrichment effect. The protein sample was driven by electroosmotic flow and accumulated at a specific location. The preconcentration chip for proteins was fabricated using simple standard soft lithography with a polydimethylsiloxane replica. Nanofractures were formed by utilizing nanoparticle-assisted electric breakdown. The proposed method for nanofracture formation that utilizes nanoparticle deposition at the junction gap between microchannels greatly decreases the required electric breakdown voltage. The experimental results indicate that a protein sample with an extremely low concentration of 1 nM was concentrated to 1.5×10⁴-fold in 60 min using the proposed chip.

Multiscale Multiphysics and Multidomain Models I: Basic Theory

This work extends our earlier two-domain formulation of a differential geometry based multiscale paradigm into a multidomain theory, which endows us the ability to simultaneously accommodate multiphysical descriptions of aqueous chemical, physical and biological systems, such as fuel cells, solar cells, nanofluidics, ion channels, viruses, RNA polymerases, molecular motors and large macromolecular complexes. The essential idea is to make use of the differential geometry theory of surfaces as a natural means to geometrically separate the macroscopic domain of solvent from the microscopic domain of solute, and dynamically couple continuum and discrete descriptions. Our main strategy is to construct energy functionals to put on an equal footing of multiphysics, including polar (i.e., electrostatic) solvation, nonpolar solvation, chemical potential, quantum mechanics, fluid mechanics, molecular mechanics, coarse grained dynamics and elastic dynamics. The variational principle is applied to the energy functionals to derive desirable governing equations, such as multidomain Laplace-Beltrami (LB) equations for macromolecular morphologies, multidomain Poisson-Boltzmann (PB) equation or Poisson equation for electrostatic potential, generalized Nernst-Planck (NP) equations for the dynamics of charged solvent species, generalized Navier-Stokes (NS) equation for fluid dynamics, generalized Newton's equations for molecular dynamics (MD) or coarse-grained dynamics and equation of motion for elastic dynamics. Unlike the classical PB equation, our PB equation is an integral-differential equation due to solvent-solute interactions. To illustrate the proposed formalism, we have explicitly constructed three models, a multidomain solvation model, a multidomain charge transport model and a multidomain chemo-electro-fluid-MD-elastic model. Each solute domain is equipped with distinct surface tension, pressure, dielectric function, and charge density distribution. In addition to long-range Coulombic interactions, various non-electrostatic solvent-solute interactions are considered in the present modeling. We demonstrate the consistency between the non-equilibrium charge transport model and the equilibrium solvation model by showing the systematical reduction of the former to the latter at equilibrium. This paper also offers a brief review of the field.

Accelerating microfluidic immunoassays on filter membranes by applying vacuum

This paper describes a vacuum-accelerated microfluidic immunoassay (we abbreviate it as VAMI) by sandwiching a filter membrane between a two-layer chip. A direct assay of IgG demonstrated that VAMI could simultaneously achieve higher sensitivity and require less time compared with conventional microfluidic immunoassays. We further applied VAMI to carry out a 3-step competitive assay (including antigen immobilization, competitive reaction and 2(nd) antibody reaction) for detecting the illegal food additive Sudan Red. A total assay time of 15 min with a limit of detection (LOD) of 1 ng ml(-1) is achieved.

Flow injection/sequential injection analysis systems: potential use as tools for rapid liver diseases biomarker study

Flow injection/sequential injection analysis (FIA/SIA) systems are suitable for carrying out automatic wet chemical/biochemical reactions with reduced volume and time consumption. Various parts of the system such as pump, valve, and reactor may be built or adapted from available materials. Therefore the systems can be at lower cost as compared to other instrumentation-based analysis systems. Their applications for determination of biomarkers for liver diseases have been demonstrated in various formats of operation but only a few and limited types of biomarkers have been used as model analytes. This paper summarizes these applications for different types of reactions as a guide for using flow-based systems in more biomarker and/or multibiomarker studies.

Application of a biotin functionalized QD assay for determining available binding sites on electrospun nanofiber membrane

BACKGROUND

The quantification of surface groups attached to non-woven fibers is an important step in developing nanofiber biosensing detection technologies. A method utilizing biotin functionalized quantum dots (QDs) 655 for quantitative analysis of available biotin binding sites within avidin immobilized on electrospun nanofiber membranes was developed.

RESULTS

A method for quantifying nanofiber bound avidin using biotin functionalized QDs is presented. Avidin was covalently bound to electrospun fibrous polyvinyl chloride (PVC 1.8% COOH w/w containing 10% w/w carbon black) membranes using primary amine reactive EDC-Sulfo NHS linkage chemistry. After a 12 h exposure of the avidin coated membranes to the biotin-QD complex, fluorescence intensity was measured and the total amount of attached QDs was determined from a standard curve of QD in solution (total fluorescence vs. femtomole of QD 655). Additionally, fluorescence confocal microscopy verified the labeling of avidin coated nanofibers with QDs. The developed method was tested against 2.4, 5.2, 7.3 and 13.7 mg spray weights of electrospun nanofiber mats. Of the spray weight samples tested, maximum fluorescence was measured for a weight of 7.3 mg, not at the highest weight of 13.7 mg. The data of total fluorescence from QDs bound to immobilized avidin on increasing weights of nanofiber membrane was best fit with a second order polynomial equation (R(2) = .9973) while the standard curve of total fluorescence vs. femtomole QDs in solution had a linear response (R(2) = .999).

CONCLUSION

A QD assay was developed in this study that provides a direct method for quantifying ligand attachment sites of avidin covalently bound to surfaces. The strong fluorescence signal that is a fundamental characteristic of QDs allows for the measurement of small changes in the amount of these particles in solution or attached to surfaces.

Ion-permeable membrane for on-chip preconcentration and separation of cancer marker proteins

Cancer marker proteins have been electrophoretically concentrated and then separated in a microfluidic device. On-chip preconcentration was achieved using an ion-permeable membrane, consisting of acrylamide, N,N'-methylene-bisacrylamide and 2-(acrylamido)-2-methylpropanesulfonate. This negatively charged membrane was photopolymerized in the microdevice near the injection intersection. Anionic proteins were excluded from the porous membrane based on both size and charge, which concentrated target components in the injection intersection prior to separation by microchip capillary electrophoresis (μ-CE). Bovine serum albumin was used in the initial characterization of the system and showed a 40-fold enrichment in the μ-CE peak with 4 min of preconcentration. Adjustment of buffer pH enabled baseline resolution of two cancer biomarkers, α-fetoprotein (AFP) and heat shock protein 90 (HSP90), while fine control over preconcentration time limited peak broadening. Our optimized preconcentration and μ-CE approach was applied to AFP and HSP90, where enrichment factors of >10-fold were achieved with just 1 min of preconcentration. Overall, the process was simple and rapid, providing a useful tool for improving detection in microscale systems.

Microfluidic chips for point-of-care immunodiagnostics

We might be at the turning point where research in microfluidics undertaken in academia and industrial research laboratories, and substantially sponsored by public grants, may provide a range of portable and networked diagnostic devices. In this Progress Report, an overview on microfluidic devices that may become the next generation of point-of-care (POC) diagnostics is provided. First, we describe gaps and opportunities in medical diagnostics and how microfluidics can address these gaps using the example of immunodiagnostics. Next, we conceptualize how different technologies are converging into working microfluidic POC diagnostics devices. Technologies are explained from the perspective of sample interaction with components of a device. Specifically, we detail materials, surface treatment, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the light of various biosensing concepts. Finally, we discuss the integration of components into accurate and reliable devices.

Radial sample preconcentration

In this paper we present a radial sample preconcentration strategy enabled by axisymmetric concentration polarization in a microfluidic chamber on a uniform nanoporous film. Sample analytes are focused into the centre, creating a concentrated plug that is injected vertically into the microfluidic analysis layer. No balancing pressure driven flows or tangential fields are required, and the process has essentially zero footprint on the analysis layer. An electrokinetic loading scheme enables repeat loading/concentration cycles, and a finned radial chamber geometry dampens instabilities and accommodates larger volumes. Modelling results indicate over 1800-fold concentration increases are possible in 10 s, for high mobility buffers and high applied field strength. At moderate field strength and buffer mobility, experiments demonstrate a 168-fold increase in concentration of FITC-BSA protein in 36 s.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2008-2010)

Capillary electrophoresis has been alive for over two decades now; yet, its sensitivity is still regarded as being inferior to that of more traditional methods of separation such as HPLC. As such, it is unsurprising that overcoming this issue still generates much scientific interest. This review continues to update this series of reviews, first published in Electrophoresis in 2007, with an update published in 2009 and covers material published through to June 2010. It includes developments in the fields of stacking, covering all methods from field-amplified sample stacking and large volume sample stacking, through to ITP, dynamic pH junction and sweeping. Attention is also given to on-line or in-line extraction methods that have been used for electrophoresis.

Large scale lithography-free nano channel array on polystyrene

This paper reports a new fabrication method of lithography-free nanochannel array. It is based on the cracking process on the surface of a polystyrene (PS) Petri-dish, one type of thermoplastic that is composed of uni-axial macromolecular chains. Under proper conditions, parallel nanochannels with equal interspaces are obtained. Control over the channel depth from 20 nm to 200 nm is achieved, with the channel length reaching tens of millimetres. The PDMS replication based on PS nanochannel array has been successfully carried out. In combination with the microstructure, both an ion enrichment device and a current rectification device are fabricated, and their quantified characters manifested the applicability of the channel array structure in nanofluidics.

Sub-10-nm nanochannels by self-sealing and self-limiting atomic layer deposition

We report on a novel fabrication method of a nanochannel ionic field effect transistor (IFET) structure with sub-10-nm dimensions. A self-sealing and self-limiting atomic layer deposition (ALD) facilitates the fabrication of lateral type nanochannels smaller than the e-beam or optical lithographic limits. Using highly conformal ALD film structures, including TiO(2), TiO(2)/TiN, and Al(2)O(3)/Ru, we have fabricated lateral sub-10-nm nanochannels with good control over channel diameter. Nanochannels surrounded by core/shell (high-k dielectric/metal) layers give rise to all-around-gating IFETs, an important functional element in an electrofluidic-based circuit system.

Nanofluidic technology for biomolecule applications: a critical review

In this review, we present nanofluidic phenomena, particularly as they relate to applications involving analysis of biomolecules within nanofabricated devices. The relevant length scales and physical phenomena that govern biomolecule transport and manipulation within nanofabricated nanofluidic devices are reviewed, the advantages of nanofabricated devices are presented, and relevant applications are cited. Characteristic length scales include the Debye length, the Van der Waals radius, the action distance of hydrogen bonding, the slip length, and macromolecular dimensions. On the basis of the characteristic lengths and related nanofluidic phenomena, a nanofluidic toolbox will be assembled. Nanofluidic phenomena that affect biomolecule behavior within such devices can include ion depletion and enrichment, modified velocity and mobility, permselectivity, steric hindrance, entropy, adsorption, and hydrodynamic interaction. The complex interactions and coupled physics of such phenomena allow for many applications, including biomolecule separation, concentration, reaction/hybridization, sequencing (in the case of DNA) and detection. Examples of devices for such applications will be presented, followed by a discussion of near-term challenges and future thoughts for the field.

Irreversible, direct bonding of nanoporous polymer membranes to PDMS or glass microdevices

A method for integrating porous polymer membranes such as polycarbonate, polyethersulfone and polyethylene terephthalate to microfluidic devices is described. The use of 3-aminopropyltriethoxysilane as a chemical crosslinking agent was extended to integrate membranes with PDMS and glass microfluidic channels. A strong, irreversible bond between the membranes and microfluidic structure was achieved. The bonding strength in the APTES treated devices was significantly greater than in devices fabricated using either a PDMS "glue" or two-part epoxy bonding method. Evaluation of a filtering microdevice and the pore structure via SEM indicates the APTES conjugation does not significantly alter the membrane transport function and pore morphology.

Study on the kinetics of homogeneous enzyme reactions in a micro/nanofluidics device

In this paper, a micro/nanofluidic preconcentration device integrated with an electrochemical detector has been used to study the enrichment of enzymes and homogeneous enzyme reaction kinetics. The enzymes are first concentrated in front of a nanochannel via an exclusion-enrichment effect (EEE) mechanism of the nanochannel integrated in a microfluidics device. If a substrate is electrokinetically transported to the concentrated enzymes, homogeneous enzymatic reaction occurs. The enzymatic reaction product can penetrate through the nanochannel to be detected electrochemically. In this device, the enriched enzymes can be well retained and repeatedly used, thus, the enzymatic reaction occurs in a continuous-flow mode. For demonstration, Glucose oxidase (GOx) was chosen as the model enzyme to study the influence of enzyme concentration on its reaction kinetics. The different concentration of GOx in front of the nanochannel was simply achieved by using different enrichment time. When substrate glucose was introduced electrokinetically, a rapid electrochemical steady-state response could be obtained. It was found that the electrochemical response to a constant glucose concentration increased with the increase of enzyme enrichment time, which is expected for homogeneous enzymatic reactions. Under proper conditions, the electrochemical responds linearly to the glucose concentration ranging from 0 to 15 mM, and the Michaelis constants (K(m)) are relatively low, which indicates a more efficient complex formation between enzyme and substrate. These results suggest that the present micro/nanofluidics device is promising for the study of enzymatic reaction kinetics and other bioassays such as cell assays, drug discovery, and clinical diagnosis.

Immunoassay on free-standing electrospun membranes

For the purpose of immunoassay, electrospun membranes can be thought as the threadlike self-assembling of nano/microbeads. Nonwoven membranes of electrospun poly(epsilon-caprolactone) (PCL) fibers display excellent tenacity, flexibility and suitable surface energy. These PCL membranes exhibit easy handling in air, fast spreading, and wetting in aqueous solution, and rapid adsorption of protein molecules by hydrophobic interaction. After a fold-and-press process, the membrane porosity was reduced from approximately 75% to less than 10%, whereas the thickness increased from 5.3 to 280 microm. The resulting fluorescence signal from adsorbed protein increased>120x. With anti-HSA and HSA-FITC as an immunoassay model, a linear detection range from 500 ng/mL down to 1 ng/mL is obtained, with a detection of limit (LOD) of approximately 0.08 ng/mL. By comparison, conventional nitrocellulose and a 24.3 microm PCL fiber electrospun membrane displayed a much higher LOD of approximately 100 ng/mL. Immunoassay on free-standing electrospun membrane successfully combines the low-cost and simplicity of conventional membrane immunoassay, with the fast reaction speed and high sensitivity characteristic of magnetic nano/microbeads bioassays.