(320d) Rectification, Hysterisis and Oscillations In Nanoscale Electrokinetics

Our group has been studying several nonlinear ion current phenomena in ion-selective nanopores, nanoslots and nanoporous membranes under an applied DC field. We are able to design nanofluidic components that exhibit rectifying  and excitable/oscillatory ion-current dynamics of biological membranes with protein ion channels. We are designing devices with such nonlinear current/voltage features to improve the performance of biosensors and mass spectrometry.We have shown that the ion current rectification phenomenon first observed by Z. Siwy (Nature Nanotechnology, 2008) when a low voltage is applied across a conic pore is due to internal and external depletion/enrichment driven by asymmetric conductivity jumps at the two ends. A cross-section averaged theory quantitatively captures the rectification factor and is favorably compared to simulations and experimental measurements. Beyond a critical voltage related to the Donnan potential of the pore, the rectification is due to asymmetric external depletion at the two ends (G. Yossifon and H-C Chang, Phys Rev Lett, 2009) and is opposite in direction as low-voltage rectification (L-J Chen and L-J Guo, Chem Soc Rev, 2010). For symmetric electrolyte, the concentration profile in the electroneutral depletion zone is governed by the diffusion Laplace equation and the classical solution for a disk sink provides an estimate of the large-voltage rectification factor. We report the first experimental data on this rectification reversal phenomenon at a critical voltage that is consistent with our theory. Generalization to other nanofluidic diodes with different spatial asymmetries will also be discussed. We have also experimentally and theoretically examined the use of external depletion to concentrate analytes, as was first reported by J. Han’s group (Anal Chem, 2005). It is found that the concentration only occurs for minority coion analytes. The concentration slug develops at a particular location near the boundary of the depletion zone due to a balance of electrophoretic flux and convective flux. Analyte stacking is possible for analytes of different mobilities but vortices produced by pressure-driven back flow, to ensure flow continuity, can destroy the concentrated slugs. With proper channel/membrane design, based on our theoretical analysis, we are able to produce negative differential resistance, excitability, bistability and hysteresis. Upon coupling this nonlinear ion circuit with a linear electronic capacitor, the hybrid ion-electron nonlinear circuit can sustain relaxation type oscillations in the voltage and current. This oscillatory nanofluidic clock is used to concentrate and release specific analytes periodically for an on-chip biosensor and for a continuous spray emitter for mass spectrometry.

      An exciting new direction is to use feedback control to sustain the local pH, ionic strength and electric field to produce non-equilibrium chemical kinetics in nanoscale electrokinetics.  Two companion papers on water-breaking reactions for pH actuation and DNA hybridization detection will be presented by L.-J. Chen and Z. Slouka.