Asymmetric nanoscale conduits, such as conical track-etch pores, rectify ion current due to surface charge effects. To date, most data concerning this phenomenon have been obtained for small nanopores with diameters comparable to the electrical double layer thickness. Here, we systematically evaluate rectification for nanopores in poly(ethylene terephthalate) membranes with tip diameters of 10, 35, 85, and 380 nm. Current-voltage behavior is determined for buffer concentrations from 1 mM to 1 M and pHs 3.4 and 6.7. In general, ion current rectification increases with decreasing tip diameter, with decreasing ionic strength, and at higher pH. Surface charge contributes to increased pore conductivities compared to bulk buffer conductivities, though double layer overlap is not necessary for rectification to occur. Interestingly, the 35 nm pore exhibits a maximum rectification ratio for the 0.01 M buffer at pH 6.7, and the 380 nm pores exhibit nearly diodelike current-voltage curves when initially etched and strong rectification after the ion current has stabilized.

We report characterization of Hepatitis B virus (HBV) capsids by resistive-pulse sensing through single track-etched conical nanopores formed in poly(ethylene terephthalate) membranes. The pores were ~40 nm in diameter at the tip, and the pore surface was covalently modified with triethylene glycol to reduce surface charge density, minimize adsorption of the virus capsids, and suppress electroosmotic flow in the pore. The HBV capsids were assembled in vitro from Cp149, the assembly domain of HBV capsid protein. Assembled T=3 (90 Cp149 dimer) and T=4 (120 dimer) capsids are 31 and 36 nm in diameter, respectively, and were easily discriminated by monitoring the change in current as capsids passed through an electrically biased pore. The ratio of the number of T=3 to T=4 capsids transiting a pore did not reflect actual concentrations, but favored transport of smaller T=3 capsids. These results combined with longer transit times for the T=4 capsids indicated that the capsids must overcome an entropic barrier to enter a pore.Nanopores and nanochannels exhibit unique transport properties1 and have a number of potential applications.2 Of particular interest is developing label-free, nondestructive techniques for rapid sensing, characterization, and sorting of particles with nanometer dimensions. The resistive-pulse technique3 measures changes in ion current resulting from transit of particles through an electrically biased nanopore filled with electrolyte. As sensing elements, protein pores,4 e.g., α-hemolysin, exhibit highly reproducible pore composition and dimensions, but lack robustness when suspended in lipid bilayers. Alternatively, microand nanofabrication techniques are used to fabricate solid-state and synthetic nanopores5 , 6 with a wide range of well-defined geometries and dimensions. Forming these pores parallel or perpendicular to the substrate surface permits straightforward integration with other device features. Solid-state and synthetic nanopores exhibit ion depletion/concentration,7 -9 ion permittivity,10 enhanced channel conductance,11 ion current rectification,12 , 13 and pressure-induced salt flux rectification.14 , 15 The ability to control pore dimensions over a range of length scales permits analysis of a variety of samples, including DNA,16 -18 proteins,19 viruses,20 immune complexes,21 nanoparticles,22 and small molecules,23 and similarly designed pores may be used to sequence DNA.24 In some cases, the molecule of interest, e.g., DNA, must overcome an entropic barrier to enter nanoscale slits25 and pores. 26 Related to this work is the characterization of viruses with track-etched pores20 and immune complexes with femtosecond laser-machined pores.21 In both examples, the studied protein complexes are ~100-150 nm in diameter. The reassembly process is inherently of interest, and this system offers a unique opportunity to characterize capsid transport, capsid properties, and nanopore properties. The T=3 and T=4 capsids are similar in diameter, 31 and 36 nm, respectively, and have identic...

We report fabrication and characterization of nanochannel devices with two nanopores in series for resistive-pulse sensing of hepatitis B virus (HBV) capsids. The nanochannel and two pores are patterned by electron beam lithography between two microchannels and etched by reactive ion etching. The two nanopores are 50-nm wide, 50-nm deep, and 40-nm long and are spaced 2.0-μm apart. The nanochannel that brackets the two pores is 20x wider (1 μm) to reduce the electrical resistance adjacent to the two pores and to ensure the current returns to its baseline value between resistive-pulse events. Average pulse amplitudes differ by <2% between the two pores and demonstrate the fabrication technique is able to produce pores with nearly identical geometries. Because the two nanopores in series sense single particles at two discrete locations, particle properties, e.g., electrophoretic mobility, are determined from the pore-to-pore transit time.

We report fabrication of nanofluidic channels with asymmetric features (e.g., funnels) that were cast in high modulus poly(dimethylsiloxane) and had well-defined geometries and dimensions. Masters used to cast the funnels were written in the negative tone resist SU-8 by electron beam lithography. Replicated funnels had taper angles of 5, 10, and 20 degrees and were 80 nm wide at the tip, 1 microm wide at the base, and 120 nm deep. The planar format permitted easy coupling of the funnels to microfluidic channels and simultaneous electrical and optical characterization of ion transport. All three designs rectified ion current, and the 5 degrees funnel exhibited the highest rectification ratio. Fluorescence measurements at the funnel base showed that an anionic probe was enriched and depleted in the high and low conductance states, respectively.

We report integrated nanopore/microchannel devices in which single nanopores are isolated between two microfluidic channels. The devices were formed by sandwiching track-etched conical nanopores in a poly(ethylene terephthalate) membrane between two poly(dimethylsiloxane) microchannels. Integration of the nanopores into microfluidic devices improves mass transport to the nanopore and allows easy coupling of applied potentials. Electrical and optical characterization of these individual nanopores suggests double layer overlap is not required to form an ion depletion region adjacent to the nanopore in the microchannel; rather, excess surface charge in the nanopore contributes to the formation of this ion depletion region. We used fluorescent probes to optically map the ion depletion region and the stacking of fluorescein near the nanopore/microchannel junction, and current measurements confirmed formation of the ion depletion region.