Person:

Li, Jiali

Voltage biased solid-state nanopores are used to detect and characterize individual single stranded DNA molecules of fixed micrometer length by operating a nanopore detector at pH values greater than ∼11.6. The distribution of observed molecular event durations and blockade currents shows that a significant fraction of the events obey a rule of constant event charge deficit (ecd) indicating that they correspond to molecules translocating through the nanopore in a distribution of folded and unfolded configurations. A surprisingly large component is unfolded. The result is an important milestone in developing solid-state nanopores for single molecule sequencing applications.

We report the design of an “ion sculpting” instrument that enables the controlled fabrication of nanometer-sized structures in solid-state materials. The instrument employs a beam of kilo-electron-volt argon ions that impinge on a solid-state membrane containing prefabricated structures such as holes, slits, or cavities whose properties are to be modified. By controlling both the ion beam parameters and sample temperature, the instrument can be adjusted to either deliver or remove material from these articulations, for example opening or closing holes of various shapes. The instrument is unique in its use of feedback control for the crafting of structures that define a hole through which a component of the incident ion beam is permitted to pass and be monitored. Electrostaticion optics refocus ions transmitted unimpeded through the hole, onto a detector capable of registering single ions. The transmission rate is a direct, real-time measure of the transmitting area that is used as a feedback signal to trigger the termination of the ion irradiation process precisely when a desired dimension is obtained. The ions thus serve the dual role of modifying and measuring the size of the nanoscale structures. The sensitivity of the ion beam sculpting apparatus to atomic-scale material rearrangement at the perimeter of a hole also enables the study of ion beam induced material transport at solid-state surfaces. The utility of the instrument as a fabrication tool has been demonstrated by the fabrication of nanopores used for recent single-molecule biophysics studies.

A nanometre scale pore in a solid state membrane provides a new way to electronically probe the structure of single linear polymers, including those of biological interest in their native environments. Previous work with biological protein pores wide enough to pass and sense single stranded DNA molecules demonstrates the power of the nanopore approach, but many future tasks and applications call for a robust solid-state pore whose nanometre scale dimensions and properties may be selected, as one selects the lenses of a microscope. Here we demonstrate a solid-state nanopore microscope capable of observing individual molecules of double stranded DNA and their folding behaviour. We discuss extensions of the nanopore microscope concept to alternative probing mechanisms and applications including the study of molecular structure and sequencing.