Hybrid three-dimensional nanofluidic/microfluidic devices using molecular gates

Crossed microfluidic channels are fabricated in poly(dimethylsiloxane) (PDMS) monoliths in planes above and below a 6-10 mum thick nanoporous polycarbonate nuclear track-etched (PCTE) membrane to form a three-dimensional hybrid microfluidic and nanofluidic system. The use of commercially available nanoporous membranes allows quick and economical fabrication of nanochannel architectures to provide fluidic communication between microfluidic layers. More importantly, these nanoporous membranes add functionality to the system as gateable interconnects. These nanofluidic interconnects enable control of net fluid flow based on a number of different physical characteristics of the sample stream, the microfluidic channels and the nanochannels, leading to hybrid fluidic architectures of considerable versatility. Because the nanofluidic membrane can have surfaces with excess charge of either polarity, the net flow direction inside the microdevices is principally controlled by two factors: the magnitude of the electrical and physical flow impedance of the nanoporous membrane relative to that of the microchannels and the surface chemical functionalities which determine the polarity of the excess charge in the nanochannels. The nanochannel impedance may be manipulated by varying membrane pore size. Flow control is investigated by monitoring electrokinetic transport of both neutral and negatively charged fluorescent probes, by means of laser-induced fluorescence and fluorescence microscopy, while varying solution and nanochannel properties. When the pore size of the PCTE membrane is small, the impedance is large and the polarity of the nanochannel surface charge determines the overall direction of the net electroosmotic flow. When the combined impedance of the upper and lower microchannels exceeds 30 times the impedance of the nanochannel membrane, the direction of the flow is based on the negative surface charge of the PDMS microchannels. (C) 2003 Elsevier Science B.V. All rights reserved.