Ion Conductance of Cylindrical Solid State Nanopores Used in Coulter Counting Experiments
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Ion Conductance of Cylindrical Solid State Nanopores Used in Coulter Counting Experiments Leo Petrossian1, Seth J Wilk1, Punarvasu Joshi1, Michael Goryll1, Jonathan D Posner2, Stephen M Goodnick1, and Trevor J Thornton1 1 Electrical Engineering, Arizona State University, Tempe, AZ, 85287 2 Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ, 85287 Nanoscale apertures that provide a fluidic path between two reservoirs can be used for numerous applications. These applications include patch-clamp type measurements [1], Coulter counting [2] and molecular studies [3]. Many of the aforementioned applications benefit from a reduction in aperture dimensions [4]. In the case of Coulter counting, analytes with dimensions similar to the diameter of the cylindrical aperture pass through this aperture. The particle passing through the aperture is reducing the volume, resulting in increased impedance, which is observed as a reduction in the ionic current while applying a constant bias voltage [2]. For an aperture of known geometry, the size of a passing analyte may be measured by observing the induced resistance increase. In addition, the duration of the event may be used to examine physical characteristics of the analyte by decoupling the analyte independent factors from the factors that depend on the analytes’ velocities. Minimizing the pressure and concentration differences leaves the electrophoretic mobility as the only unknown which may be used to identify specific analytes. For Coulter counting of nanometer-sized analytes, we have developed a process capable of reproducibly fabricating cylindrical apertures in a silicon-on-insulator substrate with diameters less than 30 nm. The fabrication process utilizes electron beam lithography for the lithographic definition of the apertures enabling accurate control of final device dimensions. Silicon-on-insulator substrates are used, featuring a top silicon thickness of 320 nm and a buried oxide (BOX) thickness of 1 μm. On the back side of the substrate, an area is photolithographically patterned that is subsequently etched using a KOH etch to form a recess. The recess etch stops on the buried oxide. The buried oxide is then finally etched using buffered oxide etch via the nanometer aperture, allowing a precise control of the oxide removal, resulting in an aperture size of about 2 μm in diameter. Fig. 1 displays a cross-sectional scanning electron micrograph of the nanometer-sized aperture prepared via reactive ion etching on top of a void in the BOX layer. In this case, no back side recess etch was performed. In order to be able to accurately determine the size of the analyte, the dependence of the conductance of the nanopore on its diameter has to be known precisely. Therefore, the conductance of the pores as a function of KCl concentration has been measured for pores of different diameter. The conductance vs. KCl concentration is shown in Fig. 2. With decreasing KCl concentration and thus a decreasing conductivity of the solution, a linear (ohmi
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