Ion transport in surface modified cylindrical silicon-on-insulator nanopore with field effect modulation

Solid-state nanopore research, used in the field of biomolecule detection and separation, has developed rapidly during the last decade. An electric field generated from the nanopore membrane to the aperture surface by a bias voltage can be used to electrostatically control the transport of charges. This results in ionic current rectification that can be used for applications such as biomolecule filtration and DNA sequencing.

In this doctoral research, a voltage bias was applied on the device silicon layer of Silicon-on-Insulator (SOI) cylindrical single nanopore to analyze how the perpendicular gate electrical field affected the ionic current through the pore. The nanopore was fabricated using electron beam lithography (EBL) and reactive ion etching (RIE) which are standard CMOS processes and can be integrated into any electronic circuit with massive production. The long cylindrical pore shape provides a larger surface area inside the aperture compared to other nanopores whose surface charge is of vital importance to ion transport.

Ionic transport through the nanopore was characterized by measuring the ionic conductance of the nanopore in aqueous hydrochloric acid and potassium chloride solutions under field effect modulation. The nanopores were separately coated with negatively charged thermal silicon oxide and positively charged aluminum oxide using Atomic Layer Deposition. Both layers worked as electrical insulation layers preventing leakage current once the substrate bias was applied. Different surface charges also provided different counterion-coion configurations. The transverse conductance of the nanopore at low electrolyte concentrations (<10-4 M) changed with voltage bias when the Debye length was comparable to the dimensions of the nanopore.

Ionic transport through nanopores coated with polyelectrolyte (PE) brushes were also investigated in ionic solutions with various pH values using Electrochemical Impedance spectroscopy (EIS). The pH sensitive poly[2–(dimethylamino) ethyl methacrylate] (PDMAEMA) PE brushes were integrated on the inner walls as well as the surface of the thermal oxidized SOI cylindrical nanopore using surface-initiated atom transfer radical polymerization (SI-ATRP). An equivalent circuit model was developed to extract conductive and resistive values of the nanopore in ionic solutions. The ionic conductance of PE coated nanopore was effectively rectified by varying the pH and gate bias.