Electrokinetic control of fluid transport in gold-coated nanocapillary array membranes in hybrid nanofluidic-microfluidic devices.

The introduction of metallic elements into microfluidic devices that support electrokinetic transport creates several fundamental issues relative to the high conductivity of the metal, which can act as a current shunt, causing profound effects on the transport process. Here we examine the use of Au-coated nanocapillary array membranes (Au NCAMs) as electrically addressable fluid control elements in multi-layer microfluidic architectures. Three alternative methods for fluid injection across Au NCAMs are presented: electrokinetic injection across NCAMs with Au coated on one side (asymmetric NCAM), electrokinetic injection across NCAMs with an embedded Au layer (symmetric NCAM), and field-free electroosmotic flow (EOF) pumping across either type of Au NCAM. Injection efficiency across asymmetric NCAMs depends on the orientation of the asymmetric membrane relative to the driving potential. Efficient injections are enabled when the Au coating is on the receiving side of the membrane, however, some distortion of the injected volume element is observed, especially with large injection potentials. These results for asymmetric membranes agree qualitatively with two-dimensional numerical simulations of injections across a single slit pore, which suggest that the direction-selective transport behavior is related to electrophoretic transport of the anionic fluorescein probe. Reproducible, high quality injections are also achieved in symmetric Au NCAMs having an embedded gold nanoband region within the nanopores. Nanoband Au NCAMs are excellent candidates for a range of applications, including high efficiency electrochemical sensing, electrochemically catalyzed conversion or pretreatment and label free sensing utilizing extraordinary optical transmission. EOF pumping could be an alternative to electrokinetic injections in some applications, however, this approach is only useful for relatively large pore sizes (>400 nm) and presents considerably worse sample spreading via Taylor dispersion.