Investigation of Electrophoretic Exclusion Method for the Concentration and Differentiation of Proteins

Filtering and continuously separating microplastics from water using electric field gradients formed electrochemically in the absence of buffer

Here we use experiments and finite element simulations to investigate the electrokinetics within straight microchannels that contain a bipolar electrode and an unbuffered electrolyte solution. Our findings indicate that in the presence of a sufficiently high electric field, water electrolysis proceeds at the bipolar electrode and leads to variations in both solution conductivity and ionic current density along the length of the microchannel. The significance of this finding is twofold. First, the results indicate that both solution conductivity and ionic current density variations significantly contribute to yield sharp electric field gradients near the bipolar electrode poles. The key point is that ionic current density variations constitute a fundamentally new mechanism for forming electric field gradients in solution. Second, we show that the electric field gradients that form near the bipolar electrode poles in unbuffered solution are useful for continuously separating microplastics from water in a bifurcated microchannel. This result expands the potential scope of membrane-free separations using bipolar electrodes.

Water electrolysis at a bipolar electrode in the absence of buffer forms electric field gradients in a fundamentally new way. These electric field gradients are useful for continuously separating microplastics from water.

Electrophoretic exclusion microscale sample preparation for cryo-EM structural determination of proteins

Transmission electron microscopy (TEM) of biological samples has a long history and has provided many important insights into fundamental processes and diseases. While great strides have been made in EM data collection and data processing, sample preparation is still performed using decades-old techniques. Those sample preparation methods rely on extensive macroscale purification and concentration to achieve homogeneity suitable for high-resolution analyses. Noting that relatively few bioparticles are needed to generate high-quality protein structures, this work uses microfluidics that can accurately and precisely manipulate and deliver bioparticles to grids for imaging. The use of microfluidics enables isolation, purification, and concentration of specific target proteins at these small scales and does so in a relatively short period of time (minutes). These capabilities enable imaging of more dilute solutions and obtaining pure protein images from mixtures. In this system, spatially isolated, purified, and concentrated proteins are transferred directly onto electron microscopy grids for imaging. The processing enables imaging of more dilute solutions, as low as 5 × 10^-6 g/ml, with small total amounts of protein (<400 pg, 900 amol). These levels may be achieved with mixtures and, as proof-of-principle, imaging of one protein from a mixture of two proteins is demonstrated.

Simulation and experiment of asymmetric electrode placement for electrophoretic exclusion in a microdevice

Electrophoretic exclusion (EE) is a counterflow gradient technique that exploits hydrodynamic flow and electrophoretic forces to exclude, enrich, and separate analytes. Resolution for this technique has been theoretically examined and the smallest difference in electrophoretic mobilities that can be completely separated is estimated to be 10^-13  cm² /Vs. Traditional and mesoscale systems have been used, whereas microfluidics offers a greater range of geometries and configurations towards approaching this theoretical limit. To begin to understand the impact of seemingly subtle changes to the entrance flow and the electric field configurations, three closely related microfluidic interfaces were modeled, fabricated, and tested. These interfaces consisted of systematically varying placement of an asymmetric electrode relative to a channel entrance: leading electrode placed outside the channel entrance, leading electrode aligned with the channel, and leading electrode placed within the channel. A charged fluorescent dye is used as a sensitive and accurate probe for the model and to test the concentration variation at these interfaces. Models and experiments focused on visualizing the concentration profile in areas of high temporal dynamics, thus providing a severe test of the models. Experimental data and simulation results showed strong qualitative agreement. The complexity of the electric and flow fields about this interface and the agreement between models and testing suggests the theoretical assessment capabilities can be used to faithfully design novel and more efficient interfaces.

Development of the resolution theory for electrophoretic exclusion

Electrophoretic exclusion, a technique that differentiates species in bulk solution near a channel entrance, has been demonstrated on benchtop and microdevice designs. In these systems, separation occurs when the electrophoretic velocity of one species is greater than the opposing hydrodynamic flow, while the velocity of the other species is less than that flow. Although exclusion has been demonstrated in multiple systems for a range of analytes, a theoretical assessment of resolution has not been addressed. To compare the results of these calculations to traditional techniques, the performance is expressed in terms of smallest difference in electrophoretic mobilities that can be completely separated (R = 1.5). The calculations indicate that closest resolvable species (Δμmin ) differ by approximately 10(-13) m(2) /Vs and peak capacity (nc ) is 1000. Published experimental data were compared to these calculated results.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2010-2012)

CE 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 updates published in 2009 and 2011 and covers material published through to June 2012. 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 inline extraction methods that have been used for electrophoresis.

Quantitative assessment of flow and electric fields for electrophoretic focusing at a converging channel entrance with interfacial electrode

The electric field and flow field gradients near an electrified converging channel are amenable to separating and focusing specific classes of electrokinetic material, but the detailed local electric field and flow dynamics in this region have not been thoroughly investigated. Finite elemental analysis was used to develop a model of a buffer reservoir connected to a smaller channel to simulate the electrophoretic and flow velocities (which correspond directly to the respective electric and flow fields) at a converging entrance. A detailed PTV (Particle Tracking Velocimetry) study using charged fluorescent microspheres was performed to assess the model validity both in the absence and presence of an applied electric field. The predicted flow velocity gradient from the model agreed with the PTV data when no electric field was present. Once the additional forces that act on the large particles required for tracing (dielectrophoresis) were included, the model accurately described the velocity of the charged particles in electric fields.

Using electrophoretic exclusion to manipulate small molecules and particles on a microdevice

Electrophoretic exclusion, a novel separations technique that differentiates species in bulk solution using the opposing forces of electrophoretic velocity and hydrodynamic flow, has been adapted to a microscale device. Proof-of-principle experiments indicate that the device was able to exclude small particles (1 μm polystyrene microspheres) and fluorescent dye molecules (rhodamine 123) from the entrance of a channel. Additionally, differentiation of the rhodamine 123 and polystyrene spheres was demonstrated. The current studies focus on the direct observation of the electrophoretic exclusion behavior on a microchip.

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.