The Analytical and Surface Chemistry Program supports Prof. Stephen Jacobson at Indiana University to produce tunable filters for transporting, trapping, concentrating, and reacting particles and molecules by combining nanoscale conduits with AC electrokinetics. The nanoporous elements provide a physical barrier and the applied AC field enhances selectivity. High field strengths and field gradients generated in the vicinity of the nanopore structures give rise to both electrophoretic and dielectrophoretic effects, enabling fractionation of particles based on size, charge, and polarizability. Having the trapping elements (e.g., nanopores) integrated with microfluidic structures permits isolation of single nanopores, improves the mass transfer to the nanopores, and allows the pores to be addressed in an array format. Dr. Jacobson and his students study how transport, trapping, and separation are influenced by the shape and amplitude of the applied waveform; dimensions, geometry, and density of the nanoscale conduit; surface properties of the conduit; composition of the surrounding medium; and particle shape and composition.
This project fits into the broader arena of liquid phase separations, which are central to analytical sciences. Having a fundamental understanding of what happens when nanostructured materials are combined with AC electrokinetics provides a framework from which more challenging separation problems can be undertaken. The research provides a unique opportunity for underrepresented groups and undergraduate students to work with state-of-the-art separation techniques and provides a stimulating research environment for both graduate and undergraduate students.
Nanofluidic devices are able to sense, separate, and sort individual molecules with unprecedented precision because of the unique transport properties these nanoscale conduits exhibit. With nanofabrication techniques, we are able to create fluidic circuits with a wide range of well-defined geometries and dimensions. Nanoscale dimensions coupled with symmetric or asymmetric channel geometries impact the transport of ions by phenomena such as surface charge, entropic forces, and ion current rectification, which are either insignificant or absent in larger microchannels. Moreover, resistive-pulse sensing on these devices provides a real time, label-free approach to characterize single molecules and particles at biologically relevant concentrations. During this project, our research group studied the fundamental transport properties of ions and particles in several types of nanofluidic devices. In our initial work, we evaluated the impact of the surface charge in nanochannels on ion enrichment, ion depletion, and sample stacking. We also observed ion current rectification in large conical nanopores, e.g., tip diameters greater than 100 nm. To eliminate ion current rectification, we modified the pore surface with triethylene glycol, which minimized the surface charge in the pores. To enhance the degree of rectification, we fabricated nanofluidic devices with one, two, three, and four nanofunnels in series. With each funnel added in series, the ion current rectification increased. To control ion transport precisely, we developed nanofluidic circuits with two inputs (asymmetric funnels) and one output (straight nanochannel). Current measurements on these devices showed behavior similar to an AND logic circuit. We applied what we have learned from these ion current measurements to sense hepatitis B virus capsids by resistive-pulse sensing, which measures a change in ion current as a particle passes through a pore of comparable dimensions. We were able to discriminate easily between particle sizes by their differences in current pulse amplitude and transit time distributions. Moreover, we evaluated particle transport through nanochannel devices with two pores in series. Current measurements showed two current pulses for each particle passing through the two pores in series. From the time between pulses, we are able to calculate physical parameters for the particles, e.g., electrophoretic mobilities.
