Selectivity enhancements in gel-based DNA-nanoparticle assays by membrane-induced isotachophoresis: thermodynamics versus kinetics

Nanoparticle-assisted detection of nucleic acids in a polymeric nanopore with a large pore size

Rapid and accurate detection of nucleic acids is of paramount importance in many fields, including medical diagnosis, gene therapy and virus identification. In this work, by taking advantage of two DNA hybridization probes, one of which was immobilized on the surface of gold nanoparticles, while the other was free in solution, detection of short length nucleic acids was successfully achieved using a large size (20 nm tip diameter) polyethylene terephythalate (PET) nanopore. The sensor was sensitive and selective: DNA samples with concentrations at as low as 0.5 nM could be detected within minutes and the number of mismatches can be discerned from the translocation frequency. Furthermore, the nanopore can be repeatedly used many times. Our developed large-size nanopore sensing platform offers the potential for fieldable/point-of-care diagnostic applications.

A multiplexed immuno-sensor for on-line and automated monitoring of tissue culture protein biomarkers

Frequent on-line and automated monitoring of multiple protein biomarkers level secreted in the culture media during tissue growth is essential for the successful development of Tissue Engineering and Regenerative Medicine (TERM) products. Here, we present a low-cost, rapid, reliable, and integrable anion-exchange membrane-(AEM) based multiplexed sensing platform for this application. Unlike the gold-standard manual ELISA test, incubation/wash steps are optimized for each target and precisely metered in microfluidic chips to enhance selectivity. Unlike optical detection and unreliable visual detection for the ELISA test, which require standardization for every usage, the AEM ion current signal also offers robustness, endowed by the pH and ionic strength control capability of the ion-selective membrane, such that a universal standard curve can be used to calibrate all runs. The electrical signal is enhanced by highly charged silica nanoparticle reporters, which also act as hydrodynamic shear amplifiers to enhance selectivity during wash. This AEM-based sensing platform is tested with vascular protein biomarkers, Endothelin-1 (ET-1), Angiogenin (ANG) and Placental Growth Factor (PlGF). The limit of detection and three-decade dynamic range are comparable to ELISA assay but with a significantly reduced assay time of 1 h vs 7 h, due to the elimination of calibration and blocking steps. Optimized protocol for each target renders the detection highly reliable with more than 98% confidence. The multiplexed detection capability of the platform is also demonstrated by simultaneous detection of ET-1, ANG and PlGF in 40 μl of the vascular endothelial cell culture supernatants using three-membrane AEM sensor and the performance is validated against ELISA.

Novel Homogeneous Anion Exchange Membranes for Reproducible and Sensitive Nucleic Acid Detection via Current-Voltage Characteristic Measurement

One-pot synthesis of novel hydrogel-based anion exchange membranes (AEMs), with only a single-phase monomer mixture, was used to eliminate surface heterogeneity and generate reproducible electroconvective microvortices in the over-limiting region of the current-voltage characteristic (CVC) curves. Diallyldimethylammonium chloride (DDA) was used as the main component to provide the cation charge groups, and 2-hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethyl acrylate (EGDMA) were used as the auxiliary structure monomers. The uniform membrane structure allowed reproducible and sensitive DNA detection and quantification, as probe-target surface complexes can gate the ion flux and produce large voltage shifts in the over-limiting region. Suppressed membrane curvature due to controlled swelling is a crucial part to avoid the reduction of depletion region for maintaining the influence of target gene hybridization. Fourier-transform infrared (FTIR) spectroscopy verified the synthesized membrane structure, with a residual vinyl group that allows easy carboxylation via additional photografting reaction. Consequently, a significantly higher DNA probe functionalization efficiency is obtained on the homogeneous AEMs, evidenced by the increasing nitrogen element content and bonding via X-ray photoelectron spectroscopy (XPS). The DDA content was optimized to provide a sufficient coulomb force between AEM and nucleic acid backbone to promote the specific binding efficiency but without high dimensional swelling which might change the surface geometry and restrict the voltage shifting for sensing in the over-limiting region, and the optimal DDA/HEMA ratio was found to be 4/10. The synthesized AEM sensor for recombinant 35S promoter sequence identification exhibited a reproducible calibration standard curve with dynamic range between 30 fM and 1 μM and high selectivity with only 0.01 V shift for 1 μM nontarget oligo.

Liquid biopsy technologies based on membrane microfluidics: High-yield purification and selective quantification of biomarkers in nanocarriers

Liquid biopsy, screening cancer non-invasively and frequently by detecting and quantifying molecular markers in physiological fluids, would significantly improve cancer survival rate but it remains a distant goal. The key obstacles presented by the highly heterogeneous samples are rapid/high-yield purification and precise/selective marker capture by their antibody and oligo probes. As irregular expressions of these molecular biomarkers are the key signals, quantifying only those from the cancer cells would greatly enhance the performance of the screening tests. The recent discovery that the biomarkers are carried by nanocarriers, such as exosomes, with cell-specific membrane proteins suggests that such selection may be possible, although a new suite of fractionation and quantification technologies would need to be developed. Although under-appreciated, membrane microfluidics has made considerable contributions to resolving these issues. We review the progress made so far, based on ion-selective, track-etched, and gel membranes and advanced electrophoretic and nano-filtration designs, in this perspective and suggest future directions.

Dielectrophoresis of proteins: experimental data and evolving theory

The ability to selectively move and trap proteins is core to their effective use as building blocks and for their characterization. Analytical and preparative strategies for proteins have been pursued and modeled for nearly a hundred years, with great advances and success. Core to all of these studies is the separation, isolation, purification, and concentration of pure homogeneous fractions of a specific protein in solution. Processes to accomplish this useful solution include biphasic equilibrium (chromatographies, extractions), mechanical, bulk property, chemical equilibria, and molecular recognition. Ultimately, the goal of all of these is to physically remove all non-like protein molecules-to the finest detail: all atoms in the full three-dimensional structure being identical down the chemical bond and bulk structure chirality. One strategy which has not been effectively pursued is exploiting the higher order subtle electrical properties of the protein-solvent system. The advent of microfluidic systems has enabled the use of very high electric fields and well-defined gradients such that extremely high resolution separations of protein mixtures are possible. These advances and recognition of these capabilities have caused a re-evaluation of the underlying theoretical models and they were found to be inadequate. New theoretical descriptions are being considered which align more closely to the total forces present and the subtlety of differences between similar proteins. These are focused on the interfacial area between the protein and hydrating solvent molecules, as opposed to the macroscale assumptions of homogeneous solutions and particles. This critical review examines all data which has been published that place proteins in electric field gradients which induce collection of those proteins, demonstrating a force greater than dispersive effects or countering forces. Evolving theoretical constructs are presented and discussed, and a general estimate of future capabilities using the higher order effects and the high fields and precise gradients of microfluidic systems is discussed. Graphical abstract.

Dynamics of driftless preconcentration using ion concentration polarization leveraged by convection and diffusion

Over the past several decades, separation and preconcentration methods of (bio)molecules have been actively developed for various biomedical and chemical processes such as disease diagnostics, point of care test and environmental monitoring. Among the great developments of the electrokinetic method in a micro/nanofluidic platform is the ion concentration polarization (ICP) phenomenon, in which a target molecule is accumulated near a permselective nanoporous membrane under an applied electric field. ICP method has been actively studied due to its easy implementation and high preconcentration/separation efficiency. However, the dynamic behavior of preconcentrated analytes has not yet been fully studied, especially driftless migration, where the applied electric field is orthogonal to the direction of the drift migration. Here, we demonstrate anomalous shapes of preconcentrated analytes (either plug or dumbbell shape) and the morphologies were analytically modeled by the leverage of convection and diffusion migration. This model was experimentally verified with various lengths of DNA and the limiting cases (convection-free environment in paper-based microfluidic device and extremely low diffusivity of red blood cells) were also shown to confirm the model. Thus, this study not only provides an insight into the fundamental electrokinetic dynamics of molecules in an ICP platform but also plays a guiding role for the design of a nanofluidic preconcentrator for a lab on a chip application.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2016-2018)

One of the most cited limitations of capillary and microchip electrophoresis is the poor sensitivity. This review continues to update this series of biannual reviews, first published in Electrophoresis in 2007, on developments in the field of online/in-line concentration methods in capillaries and microchips, covering the period July 2016-June 2018. It includes developments in the field of stacking, covering all methods from field-amplified sample stacking and large-volume sample stacking, through to isotachophoresis, dynamic pH junction, and sweeping. Attention is also given to online or in-line extraction methods that have been used for electrophoresis.

Simultaneous isolation and preconcentration of exosomes by ion concentration polarization

Exosomes carry microRNA biomarkers, occur in higher abundance in cancerous patients than in healthy ones, and because they are present in most biofluids, including blood and urine, these can be obtained noninvasively. Standard laboratory techniques to isolate exosomes are expensive, time consuming, provide poor purity, and recover on the order of 25% of the available exosomes. We present a new microfluidic technique to simultaneously isolate exosomes and preconcentrate them by electrophoresis using a high transverse local electric field generated by ion-depleting ion-selective membrane. We use pressure-driven flow to deliver an exosome sample to a microfluidic chip such that the transverse electric field forces them out of the cross flow and into an agarose gel which filters out unwanted cellular debris while the ion-selective membrane concentrates the exosomes through an enrichment effect. We efficiently isolated exosomes from 1× PBS buffer, cell culture media, and blood serum. Using flow rates from 150 to 200 μL/h and field strengths of 100 V/cm, we consistently captured between 60 and 80% of exosomes from buffer, cell culture media, and blood serum as confirmed by both fluorescence spectroscopy and nanoparticle tracking analysis. Our microfluidic chip maintained this recovery rate for more than 20 min with a concentration factor of 15 for 10 min of isolation.