Characterization of Nanopipet-Supported ITIES Tips for Scanning Electrochemical Microscopy of Single Solid-State Nanopores

Abstract

Nanoscale scanning electrochemical microscopy (SECM) is a powerful scanning probe technique that enables high-resolution imaging of chemical processes at single nanometer-sized objects. However, it has been a challenging task to quantitatively understand nanoscale SECM images, which requires to accurately characterize the size and geometry of nanoelectrode tips. Herein, we address this challenge through transmission electron microscopy (TEM) of quartz nanopipets for SECM imaging of single solid-state nanopores by using nanopipet-supported interfaces between two immiscible electrolyte solutions (ITIES) as tips. We take advantage of the high resolution of TEM to demonstrate that laser-pulled quartz nanopipets reproducibly yield not only an extremely small tip diameter of ~30 nm, but also a substantial tip roughness of ~5 nm. The size and roughness of a nanopipet can be reliably determined by optimizing the intensity of the electron beam not to melt or deform the quartz nanotip without metal coating. Electrochemically, the nanoscale ITIES supported by a rough nanotip gives higher amperometric responses to tetrabutylammonium than expected for a 30 nm-diameter disk tip. The finite element simulation of sphere-cap ITIES tips accounts for the high current responses and also reveals that the SECM images of 100 nm-diameter Si3N4 nanopores are enlarged along the direction of the tip scan. Nevertheless, spatial resolution is not significantly compromised by a sphere-cap tip, which can be scanned in closer proximity to the substrate. This finding augments the utility of a protruded tip, which can be fabricated and miniaturized more readily to facilitate nanoscale SECM imaging.

Nanobubble-controlled nanofluidic transport

“…Between ionic current recording and EM imaging, nanopipette tips were grid-mounted and cryogenically frozen to preserve the as-measured nanofluidic systems ( fig. S2) (40)(41)(42). Because the EM signal is dependent on mass density, nanobubbles in micrographs appear lighter than liquid and solid materials.…”

Advances and Perspectives in Chemical Imaging in Cellular Environments Using Electrochemical Methods

“…Shigeru and coworkers [55] used a 30 nm diameter probe (silanized quartz nanopipette) filled with 1,2-dichloroethane (DCE) to produce a nanoscale ITIES, and used this to image a nanoporous Si 3 N 4 membrane. The ITIES protruded from the tip of a nanopipette, in a sphere-cap geometry [16], which did not significantly compromise spatial resolution, in part due to the fact that the tip could be scanned closer to the substrate.…”

“…As the variety of probes used in SEPM platforms has increased, understanding the exact geometry has become increasingly important for quantitation of the current response [70]. Recently, comprehensive tip characterization of SICM probes has been explored using transmission electron microscopy (TEM) and ion conductance measurements, taking into account the effects of surface chemistry on the tip current response [55,70,71].…”

Advances and Perspectives in Chemical Imaging in Cellular Environments Using Electrochemical Methods

“…An interesting development in nanoscale SECM is the use of a nanopipette-supported interface between two immiscible electrolyte solutions (ITIES). Shigeru and coworkers used a 30 nm diameter probe (silanized quartz nanopipette) filled with DCE to produce a nanoscale ITIES, and used this to image a nanoporous Si3N4 membrane [44]. The ITIES protruded from the tip of a nanopipette, in a sphere-cap geometry [16], which did not significantly compromise spatial resolution, in part due to the fact that the tip could be scanned closer to the substrate.…”

“…As the variety of probes used in SEPM platforms has increased, understanding the exact geometry has become increasingly important for quantitation of the current response [58]. Recently, comprehensive tip characterization of SICM probes has been explored using transmission electron microscopy (TEM) and ion conductance measurements, taking into account the effects of surface chemistry on the tip current response [44][58] [59].…”