Tunneling Ultramicroelectrode: Nanoelectrodes and Nanoparticle Collisions

Abstract

We describe the fabrication of a nanometer-size electrode based on an insulating TiO2 film and a metal nanoparticle (NP). The TiO2 film is deposited on the conducting Pt surface of an ultramicroelectrode (UME) to block electron transfer (ET) to solution species. The film thickness is, however, thin enough to enable tunneling to Pt NPs; thus, the subsequent contact of metal NP to the TiO2 film restores the ET to solution species solely on the NP surface via facile electron tunneling. Consequently, the composite of UME/metal oxide film/NP offers nm-scale active area. The TiO2 film is electrochemically deposited on the Pt UME (Pt UME/TiO2), monitoring the cyclic voltammetry (CV) of ferrocenemethanol until the oxidation wave just disappears. A single Pt NP is captured in a collision experiment by observing the current increase upon contact of the Pt NP with the Pt UME/TiO2 by means of Pt NP-mediated electrochemical reduction of Fe(CN)6(3-). The resultant Pt UME/TiO2/Pt NP (or tunneling UME, T-UME) showed long-term stability and robustness with well-defined electrochemical response, suggesting applicability as a novel nm-size electrode for CV and steady-state measurements such as those with scanning electrochemical microscopy (SECM). Here, we employed the T-UME to measure SECM approach curves and showed remarkable approach capability for a nm-size SECM probe.

Addressing Colloidal Stability for Unambiguous Electroanalysis of Single Nanoparticle Impacts

“…We broadly define NIE as any method based on the electrochemical detection of discrete NPs in solution as they collide with the surface of an electrode. [1][2][3][4][5][6][7] The ability to probe individual microscopic particles in solution with UMEs has inspired fundamental studies of single NP electrochemistry and NP/electrode interactions, [8][9][10][11][12][13][14][15][16][17] diffusive and electro/magnetophoretic particle transport, [18][19][20][21][22] and the development of electrochemical bioassays with single-molecule sensitivity. [23][24] Colloidal stability [25][26] is of critical importance for the accurate interpretation of NP/electrode impacts.…”

Boosting water oxidation layer-by-layer

“…6, although the Fermi levels of the electrode and IrO 2 NPs equilibrate, those of the O 2 /H 2 O redox couple in solution do not. The latter arises as the kinetics of the rate of discharging the IrO 2 NPs, by electron transfer to the electrode surface facilitated by tunneling of electrons between IrO 2 NPs in the film due to the overlapping high densities of states of the metallic oxides, 52 are far superior to the kinetics of charging the IrO 2 NPs, via the OER at the surface of each IrO 2 NP. Thus, a constant thermodynamic driving force is present that drives electron transfer efficiently to the electrode surface.…”

Focused-Ion-Beam-Milled Carbon Nanoelectrodes for Scanning Electrochemical Microscopy

“…Moreover, an approach curve at a gold-coated silicon wafer shows a higher positive feedback response as the concentration of a redox mediator is lowered, thereby revealing the limited conductivity of the thin gold film under high mass-transport conditions across carbon–gold nanogaps. Importantly, we enabled reliable and quantitative nanoelectrochemical measurements by protecting carbon nanotips from nanoscale electrostatic damage (30, 31), unlike the recent study of pyrolytic carbon nanoelectrodes (17). …”