Leo Petrossian

Electromigration current rectification in a cylindrical nanopore due to asymmetric concentration polarization

We present experimental measurements of electromigration current through a single cylindrical nanopore. A single cylindrical nanopore with 175 nm diameter was fabricated in silicon in series with two micropores with 2 and 100 microm diameters. Thick electrical double layers (EDLs) (kappa a approximately 1) exhibit current rectification due to asymmetric concentration polarization while thinner EDLs show nearly symmetric conductance. After the electric field is turned off, electrical current is measured and observed due to redistribution of ions in the concentration polarization layer.

Field effect modulation of ionic conductance of cylindrical silicon-on-insulator nanopore array

Results demonstrating the field effect modulation of ionic transport through an array of cylindrical nanopores fabricated in silicon-on-insulator substrates are presented. Pronounced modulation of the conductance is observed at low electrolyte concentrations when the electric double layers within the nanopores are overlapping. A numerical model based on Brownian dynamics reproduces the measured data.

Fabrication of Cylindrical Nanopores and Nanopore Arrays in Silicon-On-Insulator Substrates

Electron-beam lithography and reactive ion etching were used to process silicon-on-insulator substrates for the fabrication of single cylindrical high-aspect-ratio solid-state nanopores and high-packing-density nanopore arrays. Minimum pore diameters of 40 nm were readily achieved with a high yield. The electrolyte concentration dependence of ion transport through single nanopores was measured for pores with diameters ranging from 40 to 140 nm. Measured single-nanopore conductances in high salt concentrations were compared to a simple model using a cylindrical resistance path and bulk solution conductivity. Electrochemical impedance spectroscopy was used to study the ac response of the device.

Demonstration of Coulter counting through a cylindrical solid state nanopore

The technique known as a Coulter counting, or resistive-pulse sensing, can be used to measure the size of a nanoscale analyte as it passes through a fluidic constriction separating two reservoirs. We have developed a fabrication procedure capable of reproducibly manufacturing cylindrical nanopores with diameters as small as 20 nm using a silicon-on-insulator substrate and electron beam lithography. The ionic conductance of these nanopores was measured across six orders of magnitude in electrolyte concentration. Polystyrene nanoparticles were then passed through the cylindrical pores while monitoring the current that flowed due to a constant bias voltage. Current pulses due to the passage of individual nanoparticles of various dimensions through a nanopore were observed and compared to theory.

Electromigration of Charged Polystyrene Beads Through Silicon Nanopores Filled With Low Ionic Strength Solutions

In this work we present preliminary results demonstrating the influence of electrical double layer overlap on the electromigration of polystyrene beads (PSB) through an array of 25 cylindrical nanopores. Each of the cylindrical nanopores of the array used in this study is 360nm long with a diameter of 90nm. We observe frequent Coulter events for solutions of higher ionic strength and absence of Coulter events at low ionic strength solution. At higher ionic strengths, the electric double layers in the nanopore are thin and ion transport through the nanopore follows the bulk behavior of the ionic solution. For solutions of lower ionic strength, the electric double layers are comparable to the nanopore dimensions and start to overlap, suggesting surface charge interaction with the polystyrene beads that pass through the nanopore. The work continues towards detailed statistical analysis of the characteristic events observed for different concentrations.Copyright

Integrated platform for ion channel sensing

Conductance measurement of single ion channels and related stochastic signals is a promising technique for the development of a functional biosensor. We present results showing that silicon substrates can be used as a low noise, universal platform for recording the electrical activity of single ion channels inserted into bilayer membranes. Bilayers were suspended on polytetrafluoroethylene coated, 150mum apertures etched into silicon substrates and then oxidized with common silicon processing techniques. The overall noise of the system was reduced with a 75mum thick SU-8 layer and integrated silver/silver chloride electrodes. Phase sensitive detection of single OmpF porin ion channels using the silicon device and a lock-in amplifier were made and demonstrate the systems capability as a sensor in high noise environments

Ion conductance of cylindrical solid state nanopores used in coulter counting experiments

Nanoscale apertures that provide a fluidic path between two reservoirs can be used for numerous applications. These applications include patch-clamp type measurements, Coulter counting and molecular studies. 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. Measurements of the conductance of the pores as a function of KCl concentration reveal the presence of a surface conduction mechanism that dominates the conductance of the nanopore and leads to a deviation of the concentration dependence of the conductance from the case of bulk solution. From current traces recorded, the passage of individual particles through the pore can be concluded.

Silicon-Based Ion Channel Platforms

We demonstrate that silicon substrates can be used as a universal platform for recording the electrical activity of ion channels inserted into suspended bilayers. The bilayers span narrow openings etched into silicon substrates using standard microelectronics processing techniques. Reversible Ag/AgCl electrodes are integrated around the circumference of the opening and provide long-term stable measurements of the ion channel currents. To demonstrate the utility of the silicon platform we have measured the electrical activity of OmpF porin ion channel proteins inserted into a lipid bilayer formed using the Montal — Mueller method. Systematic measurements of the lipid giga-seal characteristics are presented, including ac conductance measurements and statistical analysis in order to resolve the conductance of individual ion-channels.

Ion channel conductance measurements on a silicon-based platform

Conductance measurements of the transmembrane porin protein OmpF as a function of pH and bath concentration have been made with both a microfabricated silicon substrate device and a commercially available polystyrene aperture. Ion transport through the channel was simulated in atomic detail: the measured current was compared with theoretically calculated current, using a Brownian Dynamics kernel coupled to the Poisson equation by a P3M force field. The explicit protein structure and fixed charge distribution in the protein are calculated using the molecular dynamics code, GROMACS. Reasonable agreement is obtained in the simulated versus measured conductance over the range of experimental concentrations studied.