We report the unexpected radial migration of DNA molecules in capillary electrophoresis (CE) with applied Poiseuille flow. Such movement can contribute to anomalous migration times, peak dispersion, and size and shape selectivity in CE. When Poiseuille flow is applied from the cathode to the anode, DNA molecules move toward the center of the capillary, forming a narrow, highly concentrated zone. Conversely, when the flow is applied from the anode to the cathode, DNA molecules move toward the walls, leaving a DNA-depleted zone around the axis. We showed that the deformation and orientation of DNA molecules under Poiseuille flow was responsible for the radial migration. By analyzing the forces acting on the deformed and oriented DNA molecules, we derived an expression for the radial lift force, which explained our results very well under different conditions with Poiseuille flow only, electrophoresis only, and the combination of Poiseuille flow and electrophoresis. Factors governing the direction and velocity of radial migration were elucidated. Potential applications of this phenomenon include an alternative to sheath flow in flow cytometry, improving precision and reliability of single-molecule detection, reduction of wall adsorption, and size separation with a mechanism akin to field-flow fractionation. On the negative side, nonuniform electroosmotic flow along the capillary or microfluidic channel is common in CE, and radial migration of certain analytes cannot be neglected.
Under specific experimental conditions, the electrokinetic separation of certain microorganisms can produce peaks of very high apparent efficiencies (approximately 10(6)-10(10) theoretical plates/m). This is unusual in that no deliberate focusing mechanism was employed. To investigate this process further, the separation was monitored in real time using a charge-coupled device (CCD) imaging system. At least two different processes seem to be operative when these narrow peaks are observed. The initial field-induced association of cells appears to require a dilute polymer solution, electroosmotic flow (preferably countercurrent to the direction of cell electrophoresis), and a direct current electric field. Three possible models are presented that may explain aspects of the observed behavior. The balance between dispersive forces and intercellular adhesive forces also affects the observed bandwidths. Understanding and controlling the dynamic and aggregation of cells in microfluidic processes is essential, since it can be beneficial for some experiments and detrimental to others.
Well-aligned ZnO/ZnSe core/shell nanowire arrays with type-II energy alignment are synthesized via a two-step chemical vapor deposition method. Morphology and structure studies reveal a transition layer of wurtzite ZnSe between the wurtzite ZnO core and the cubic ZnSe shell. Type-II interfacial transitions are observed in the spectral region from visible to near infrared in transmission and photoluminescence. More significantly, for the first time, the interfacial transition is shown to extend the photoresponse of the prototype photovoltaic device based on the coaxial nanowire array to a threshold much below the bandgap of either component (3.3 and 2.7 eV, respectively) at 1.6 eV, with an external quantum efficiency of $4% at 1.9 eV and 9.5% at 3 eV. These results represent a major advance towards the realization of all-inorganic type-II heterojunction photovoltaic devices in an optimal device architecture.
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