Fabio Baldessari

Electrokinetics in nanochannels: Part I. Electric double layer overlap and channel-to-well equilibrium

In this paper a new model is described for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. Electrolyte concentration in the nanochannel is predicted self-consistently via equilibrium between ionic solution in the wells and within the nanochannel. Differently than published models that require detailed iterative numerical solutions of coupled differential equations, the framework presented here is self-consistent and predictions are obtained solving a simple one-dimensional integral. The derivation clearly shows that the electric potential field depends on three new parameters: the ratio of ion density in the channel to ion density in the wells; the ratio of free-charge density to bulk ion density within the channel; and a modified Debye-Hückel thickness, which is the relevant scale for shielding of surface net charge. For completeness, three wall-surface boundary conditions are analyzed: specified zeta-potential; specified surface net charge density; and charge regulation. Predictions of experimentally observable quantities based on the model proposed here, such as depth-averaged electroosmotic flow and net ionic current, are significantly different than results from previous overlapped electric double layer models. In this first paper of a series of two, predictions are presented where channel depth is varied at constant well concentration. Results show that under conditions of electric double layer overlap, electroosmosis contributes only a small fraction of the net ionic current, and that most of the measurable current is due to ionic conduction in conditions of increased counterion density in the nanochannel. In the second of this two-paper series, predictions are presented where well-concentration is varied and the channel depth is held constant, and the model described here is employed to study the dependence of ion mobility on ionic strength, and compare predictions to measurements of ionic current as a function of channel depth and ion density.

Lymphocyte Electrotaxis In Vitro and In Vivo

Electric fields are generated in vivo in a variety of physiologic and pathologic settings, including penetrating injury to epithelial barriers. An applied electric field with strength within the physiologic range can induce directional cell migration (i.e., electrotaxis) of epithelial cells, endothelial cells, fibroblasts, and neutrophils suggesting a potential role in cell positioning during wound healing. In the present study, we investigated the ability of lymphocytes to respond to applied direct current (DC) electric fields. Using a modified Transwell assay and a simple microfluidic device, we show that human PBLs migrate toward the cathode in physiologically relevant DC electric fields. Additionally, electrical stimulation activates intracellular kinase signaling pathways shared with chemotactic stimuli. Finally, video microscopic tracing of GFP-tagged immunocytes in the skin of mouse ears reveals that motile cutaneous T cells actively migrate toward the cathode of an applied DC electric field. Lymphocyte positioning within tissues can thus be manipulated by externally applied electric fields, and may be influenced by endogenous electrical potential gradients as well.

Electrophoresis in nanochannels: brief review and speculation

The relevant physical phenomena that dominate electrophoretic transport of ions and macromolecules within long, thin nanochannels are reviewed, and a few papers relevant to the discussion are cited. Sample ion transport through nanochannels is largely a function of their interaction with electric double layer. For small ions, this coupling includes the net effect of the external applied field, the internal field of the double layer, and the non-uniform velocity of the liquid. Adsorption/desorption kinetics and the effects of surface roughness may also be important in nanochannel electrophoresis. For macromolecules, the resulting motion is more complex as there is further coupling via steric interactions and perhaps polarization effects. These complex interactions and coupled physics represent a valuable opportunity for novel electrophoretic and chromatographic separations.

Electrokinetics in nanochannels Part II. Mobility dependence on ion density and ionic current measurements

In the first of this two-paper series, a new model was developed for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. The model takes into account the dependence of ion mobility on local ion densities and pH. This model is used here to study and demonstrate the effect of ion density and pH on ionic current measurements. A comparison is shown of predictions based on each of three boundary conditions, as studied in Part I. The model developed in Part I is validated by comparing simulations with measurements of ionic current as a function of sodium borate concentration. Results show that predictions based on extended Debye-Hückel theory for ion mobility significantly improve the accuracy of simulations, but that these do not predict exact scaling behavior. A simple bulk conductivity measurement used as input parameter for the simulations, in place of the predicted bulk conductivity (K(0)), guarantees agreement with data in the thin EDL region. Results also indicate that the charge regulation boundary condition, complemented with an adequate bulk electrolyte model, provides better agreement with experimental trends than the specified zeta potential or specified surface net charge boundary conditions. Further, it is shown that currents due to advection (by electroosmotic flow) are in all cases studied less than 25% of the total current in the system.

Electrophoresis in Nanochannels

Micro- and nanofabrication technology enables the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically-controlled microchip systems with no moving parts. We are studying and leveraging the unique separation modalities offered by nanoscale electrokinetic channels. We report analytical, numerical, and experimental investigations of nanochannel electrophoretic transport and separation dynamics of neutral and charged analytes. Our study includes continuum-theory-based analytical and numerical studies of nanofluidic electrophoretic separation dynamics, as well as experimental validation of these models. We have used 40, 100, and 1,560 nm deep channels etched in fused silica to independently measure mobility and valence of small ions. We also use these devices to separate 10 to 100 base pair DNA in the absence of a gel separation matrix. The effective free-solution mobilities of the ds-DNA oligonucleotides measured in 1560 nm deep channel are consistent with reported literature values, while smaller values of the mobility were measured for 4o nm deep channels for the same charge-species. The goal of our work is to explore and exploit electrokinetic flow regimes with extreme scales of length and charge density.Copyright