Control of shape and material composition of solid-state nanopores.

Solid-state nanopores fabricated by a high-intensity electron beam in ceramic membranes can be fine-tuned on three-dimensional geometry and composition by choice of materials and beam sculpting conditions. For similar beam conditions, 8 nm diameter nanopores fabricated in membranes containing SiO(2) show large depletion areas (70 nm in radius) with small sidewall angles (55 degrees ), whereas those made in SiN membranes show small depletion areas (40 nm) with larger sidewall angles (75 degrees ). Three-dimensional electron tomograms of nanopores fabricated in a SiO(2)/SiN/SiO(2) membrane show a biconical shape with symmetric top and bottom and indicate a mixing of SiN and SiO(2) layers up to 30 nm from the edge of nanopore, with Si-rich particles throughout the membrane. Electron-energy-loss spectroscopy (EELS) reveals that the oxygen/nitrogen ratio near the pore depends on the beam sculpting conditions.

Salt dependence of ion transport and DNA translocation through solid-state nanopores.

We report experimental measurements of the salt dependence of ion transport and DNA translocation through solid-state nanopores. The ionic conductance shows a three-order-of-magnitude decrease with decreasing salt concentrations from 1 M to 1 muM, strongly deviating from bulk linear behavior. The data are described by a model that accounts for a salt-dependent surface charge of the pore. Subsequently, we measure translocation of 16.5-mum-long dsDNA for 50 mM to 1 M salt concentrations. DNA translocation is shown to result in either a decrease ([KCl] > 0.4 M) or increase of the ionic current ([KCl] < 0.4 M). The data are described by a model where current decreases result from the partial blocking of the pore and current increases are attributed to motion of the counterions that screen the charge of the DNA backbone. We demonstrate that the two competing effects cancel at a KCl concentration of 370 +/- 40 mM.

Fabrication and characterization of nanopore-based electrodes with radii down to 2 nm.

We report on the fabrication and characterization of gold nanoelectrodes with carefully controlled nanometer dimensions in a matrix of insulating silicon nitride. A focused electron beam was employed to drill nanopores in a thin silicon nitride membrane. The size and shape of the nanopores were studied with high-resolution transmission electron microscopy and electron-energy-loss two-dimensional maps. The pores were subsequently filled with gold, yielding conically shaped nanoelectrodes. The nanoelectrodes were examined by atomic and electrostatic force microscopy. Their applicability in electrochemistry was demonstrated by steady-state cyclic voltammetry. Pores with a radii down to 0.4 nm and electrodes with radii down to 2 nm are demonstrated.

Nanopore tomography of a laser focus.

We demonstrate that the ionic current through a solid-state nanopore can be used to measure at single nanometer resolution the three-dimensional intensity profile of a laser directly in the focus of a microscope objective. We find a linear dependence of the ionic current on the incident laser power since the laser-induced heat increases the temperature locally in the solution. Our data show a temperature increase of up to 20 K in the center of the focus for a laser wavelength of 1064 nm. Measurements of the two-dimensional temperature profiles at different positions along the optical axis allow us to reconstruct the three-dimensional temperature profile of the laser focus, similar to tomography. Our new technique does not rely on the help of any optical elements and allows quantitative measurement of optical intensity or temperature distributions in aqueous environments with nanometer resolution.

High rectifying efficiencies of microtubule motility on kinesin-coated gold nanostructures.

We demonstrate highly efficient rectification of microtubule motility on gold nanofabricated structures. First, we present a novel nanofabrication process for the creation of gold tracks for microtubule motility recessed in silicon oxide. This approach is particularly useful because it enables the use of the well-understood PEG-silane chemistry on SiO2 for the blocking of kinesin, whereas the gold tracks allow possible electrical control. We demonstrate excellent confinement of microtubule motility to the gold nanostructures and that microtubules move on the gold with speeds comparable to that on glass. Second, we present designs of three advanced rectifier geometries. We analyze the microtubule pathways through the geometries, and we demonstrate highly efficient rectification with up to 92% efficiency. As a result, we find that up to 97% of the microtubules move unidirectionally.

Lithographically fabricated nanopore-based electrodes for electrochemistry.

We report a new technique for fabricating electrodes for electrochemical applications with lateral dimensions in the range 15-200 nm and a reproducible, well-defined geometry. This technique allows determining the electrode size by electron microscopy prior to electrochemical measurements and without contamination of the metal electrode. We measured the diffusion-limited current with stepped-current voltammetry and showed that its dependence on electrode size can be quantitatively understood if the known geometry of the electrodes is explicitly taken into account.