Materials.
2 M KCl and 50 mM FeCl 3 solutions were prepared using deionized (DI) Milli-Q water (resistivity, ∼18 MΩ • cm at 25 °C). The FeCl 3 solution was used to prepare Ag/AgCl electrodes [19] . KCl and FeCl 3 were purchased from Funakoshi Co. (Tokyo, Japan) and Sigma-Aldrich (MS, USA), respectively. A flexible syringe needle made of plastic and fused silica (World Precision Instruments, Microfil, MF34G-5) was used to fill nanopipettes with DI water or KCl solution.
Sub-10 nm Nanopipette Fabrication.
All sub-10 nm nanopipettes used in this study were fabricated from quartz capillaries with an inner diameter of 0.3 mm and an outer diameter of 1.0 mm (Q100-30-1.5, Sutter Instrument, CA, USA). The capillaries were cut into 75 mm length with a custom-made pipe cutter. Nanopipettes were then fabricated with a laser puller (P-2000, Sutter Instrument, CA, USA). The details of pulling parameters used for the laser puller are provided in Supporting Information, Table S1.
2 M KCl and 50 mM FeCl 3 solutions were prepared using deionized (DI) Milli-Q water (resistivity, ∼18 MΩ • cm at 25 °C). The FeCl 3 solution was used to prepare Ag/AgCl electrodes [19] . KCl and FeCl 3 were purchased from Funakoshi Co. (Tokyo, Japan) and Sigma-Aldrich (MS, USA), respectively. A flexible syringe needle made of plastic and fused silica (World Precision Instruments, Microfil, MF34G-5) was used to fill nanopipettes with DI water or KCl solution.
Sub-10 nm Nanopipette Fabrication.
All sub-10 nm nanopipettes used in this study were fabricated from quartz capillaries with an inner diameter of 0.3 mm and an outer diameter of 1.0 mm (Q100-30-1.5, Sutter Instrument, CA, USA). The capillaries were cut into 75 mm length with a custom-made pipe cutter. Nanopipettes were then fabricated with a laser puller (P-2000, Sutter Instrument, CA, USA). The details of pulling parameters used for the laser puller are provided in Supporting Information, Table S1.