The extension of microfluidic devices to include three-dimensional fluidic networks allows complex fluidic and chemical manipulations but requires innovative methods to interface fluidic layers. Externally controllable interconnects, employing nuclear track-etched polycarbonate membranes containing nanometer-diameter capillaries, are described that produce hybrid three-dimensional fluidic architectures. Controllable nanofluidic transfer is achieved by controlling applied bias, polarity, and density of the immobile nanopore surface charge and the impedance of the nanocapillary array relative to the microfluidic channels. Analyte transport between vertically separated microchannels has three stable transfer levels, corresponding to zero, reverse, and forward bias. The transfer can even depend on the properties of the analyte being transferred such as the molecular size, illustrating the flexible character of the analyte transfer. In a specific analysis implementation, nanochannel array gating is applied to capillary electrophoresis separations, allowing selected separated components to be isolated for further manipulation, thereby opening the way for preparative separations at attomole analyte mass levels.

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Molecular transport through nanoporous nuclear-track-etched membranes was investigated with fluorescent probes by manipulating applied electric field polarity, pore size, membrane surface functionality, pH, and the ionic strength. Three forces contribute to analyte transport through membranes: ion migration, electroosmosis, and diffusion. Diffusion dominates under field-free conditions with surface hydrophobicity controlling solvent access to the nanochannels and hence the magnitude of transport by diffusion. In low ionic strength solutions (µ ∼ 10 mM), electroosmosis dominates transport when the membranes are biased, and the charge state of the surface determines the direction of flow. At high ionic strength (µ ∼ 1 M), ion migration dominates in hydrophobic membranes, and diffusion is controlling in hydrophilic membranes. The magnitude and polarity of the interior surface charge is controlled by surface functionality and displays the largest impact on molecular transport. The analyte can migrate in opposite directions under the same applied electric field by modifying either membrane surface charge or solution ionic strength. Transport can be fine-tuned by adjusting pH under low ionic strength conditions in either type of membrane. Increasing the surface charge density, σs, enhances the mobile counterion concentration, increasing the electroosmotically driven flux. Comparisons of behavior under different conditions are understood by reference to the product, κa, of the inverse Debye length, κ, and the pore diameter, a.

An electrokinetic injection technique is described which uses a nuclear track-etched nanocapillary array to inject sample plugs from one layer of a microfluidic device into another vertically separated layer for electrophoretic separations. Gated injection protocols for analyte separations, reported here, establish nanocapillary array interconnects as a route to multilevel microfluidic analytical designs. The hybrid nanofluidic/microfluidic gated injection protocol allows sample preparation and separation to be implemented in separate horizontal planes, thereby achieving multilayer integration. Repeated injections and separations of FITC-labeled arginine and tryptophan, using 200-nm pore-diameter capillary array injectors in place of traditional cross injectors are used to demonstrate gated injection with a bias configuration that uses relay switching of a single high-voltage source. Injection times as rapid as 0.3 s along with separation reproducibilities as low as 1% for FITC-labeled arginine exemplify the capability for fast, serial separations and analyses. Impedance analysis of the micro-/nanofluidic network is used to gain further insight into the mechanism by which this actively controlled nanofluidic-interconnect injection method works. Gated sample introduction via a nanocapillary array interconnect allows the injection and separation protocols to be optimized independently, thus realizing the versatility needed for real-world implementation of rapid, serial microchip analyses.

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