| Nanoscale fabrication techniques are able to produce sophisticated nanofluidic devices that have tremendous potential as analytical instruments. Ion transport properties in these devices are easily manipulated by proper selection of the channel dimensions, channel geometry, and applied potential, and consequently, phenomena such as ion current rectification, surface charge, double-layer overlap, and entropy lead to ion enrichment, depletion, and separation. By fabricating devices in plane, we are able to create fluidic circuits with any two-dimensional architecture that have a variety of nano- and microscale elements coupled in series, parallel, or both. Moreover, in-plane devices enable simultaneous electrical and optical characterization to better understand transport phenomena.;In this work, we developed in-plane fabrication techniques to produce nanofluidic devices with asymmetric channels, e.g., funnels, and studied ion transport behavior through them. These nanoscale funnels were cast in high-modulus poly(dimethylsiloxane) (h-PDMS) on SU-8 masters formed by electron beam lithography. We also shaped the SU-8 masters by electron beam induced etching with water as a precursor gas to create nanochannels with three-dimensional topography. The tip shape, taper angle, and length of individual funnels and their relative orientation within a nanofluidic circuit were systematically examined by conductivity and fluorescence measurements. Longer nanofunnels with smaller taper angles rectified ion current to a greater extent than shorter funnels with larger taper angles. Current rectification systematically increased when one, two, three, and four funnels were stacked in series. To mimic a logic circuit and demonstrate an AND function, we designed a device with two of these stacked funnels as parallel inputs and a single straight nanochannel as an output. Similarly, a circuit with transistor-like behavior was formed from three funnels connected to a common microchannel. In both circuit designs, the observed ion transport, enrichment, and depletion were controlled by the relative orientation of the funnels and applied potentials. Our work shows that complex nanofluidic architectures can be designed and fabricated to exploit electrokinetic processes at the nanoscale. |
