Although separation of polymers based on the combination of dielectrophoretic trapping and electrophoretic forces was proposed 15 years ago, experimental proof has not yet been reported. Here, we address this problem for long DNA fragments in a simple and easy-to-fabricate microfluidic device, in which the DNA is manipulated by electrophoresis and by electrodeless dielectrophoresis. By slowly increasing the strength of the dielectrophoretic traps in the course of the separation experiments, we are able to perform efficient and fast DNA separation according to length for two different DNA conformations: linear DNA (lambda (48.5-kbp) and T2 (164-kbp) DNA) and supercoiled covalently closed circular plasmid DNA (7 and 14 kbp). The underlying migration mechanism-thermally induced escape processes out of the dielectrophoretic traps in the direction of the electrophoretic force-is sensitive to different DNA fragments because of length-dependent DNA polarizabilities. This is analyzed in a second series of experiments, where the migration mechanism is exploited for the quantitative measurement of the DNA polarizabilities. This new and simple technique allows for the systematic characterization of the polarizability not only for DNA but also for other biomolecules like proteins. Furthermore, our results have direct implications to future biotechnological applications such as gene therapy and DNA vaccination.

Dielectrophoresis is a non-destructive, label-free method to manipulate and separate (bio-) particles and macromolecules. The mechanism is based on the movement of polarizable objects in an inhomogeneous electric field. Here, microfluidic devices are reviewed that generate those inhomogeneous electric fields with insulating posts or constrictions, an approach called electrodeless or insulator-based dielectrophoresis. Possible advantages compared to electrode-based designs are a less complex, monolithic fabrication process with low-cost polymeric substrates and no metal surface deterioration within the area of sample analysis. The electrodeless design has led to novel devices, implementing the functionality directly into the channel geometry and covering many areas of bioanalysis, like manipulation and separation of particles, cells, DNA, and proteins.

Control of surface properties in microfluidic systems is an indispensable prerequisite for successful bioanalytical applications. Poly(dimethylsiloxane) (PDMS) microfluidic devices are hampered from unwanted adsorption of biomolecules and lack of methods to control electroosmotic flow (EOF). In this paper, we propose different strategies to coat PDMS surfaces with poly(oxyethylene) (POE) molecules of varying chain lengths. The native PDMS surface is pretreated by exposure to UV irradiation or to an oxygen plasma, and the covalent linkage of POE-silanes as well as physical adsorption of a triblock-copolymer (F108) are studied. Contact angle measurements and atomic force microscopy (AFM) imaging revealed homogeneous attachment of POE-silanes and F108 to the PDMS surfaces. In the case of F108, different adsorption mechanisms to hydrophilic and hydrophobic PDMS are discussed. Determination of the electroosmotic mobilities of these coatings in PDMS microchannels prove their use for electrokinetic applications in which EOF reduction is inevitable and protein adsorption has to be suppressed.

Dielectrophoresis is a convenient tool for controlled manipulation of DNA with numerous applications, including DNA trapping, stretching, and separation. However, the mechanisms behind the dielectrophoretic properties of DNA are still under debate, and the role of conformation has not been addressed yet. Here, we quantify dielectrophoretic effects on DNA by determining its polarizability from microfluidic single molecule trapping experiments. We systematically study different DNA configurations (linear and supercoiled, 6-164 kbp) and demonstrate that the polarizability strongly depends on the specific conformation and size of the DNA molecules. The connection to its spatial extension is established by measuring diffusion coefficients and from that the radii of gyration; details about the spatial DNA structure are obtained from atomic force microscopy images. For linear and supercoiled DNA fragments, we found a power-law scaling for the polarizabilities and the diffusion coefficients. Our results imply a scaling of the polarizability with the radius of gyration, alpha approximately Rg0.9+/-0.1 and alpha approximately Rg1.6+/-0.2 for linear and supercoiled DNA, respectively. As an application, we demonstrate the separation of DNA topoisomers based on their dielectrophoretic properties, achieving baseline resolution within 210 s. Purified DNA samples of specific configuration may be of great importance for DNA nanoassembly or future DNA vaccines.