NANOGAP-ENABLED STUDY OF ELECTRODE REACTIONS BY SCANNING ELECTROCHEMICAL MICROSCOPY

NANOGAP-ENABLED STUDY OF ELECTRODE REACTIONS BY SCANNING ELECTROCHEMICAL MICROSCOPY Nikoloz Nioradze, PhD University of Pittsburgh, 2014 The nanogap quasi-steady-state voltammetry, developed in my work, presents the way to monitor and study rapid electron transfer reactions on macroscopic substrates of scanning electrochemical microscopy (SECM). It combines the cyclic voltammetry and SECM and monitors substrate reaction as a tip current. The resulting plot of iT versus ES features the retraceable sigmoidal shape of a quasi-steady state voltammogram although a transient peak-shape voltammogram is obtained simultaneously at the macroscopic substrate. This simplifies measurement and analysis of a quasi-steady-state voltammogram and gives information about thermodynamic as well as kinetic parameters of the reaction taking place at the interface. No charging current at the amperometric tip, high and adjustable mass transport under the tip and high spatial resolution are all advantages of quasi-steady-state voltammetry. I also introduced generalized theory for nanoscale iT-ES voltammetry of substrate reactions with arbitrary reversibility and mechanism under comprehensive experimental conditions including any substrate potential and both SECM modes (feedback and substrate generation tip collection, SG/TC). I nanofabricated submicrometer size highly reliable Pt SECM tips and found the way of protection of these tiny electrodes from the damage caused either by electrostatic discharge or electrochemical etching. Subsequent application of quasi-steady-state voltammetry and reliable nanofabricated SECM probes enabled sensitive detection of adsorption of organic impurities from air and ultrapure water to the HOPG surface as evidenced by redox reaction of ferrocenylmethyl)trimethyl ammonium (FcTMA+). Study revealed that hydrophobic contaminant layer slows down the access of hydrophilic aqueous redox species to the underlying HOPG surface, thereby yielding a lower standard rate constant, k0. Moreover, this barrier effects stronger to a more charged form (FcTMA2+) of a redox couple so that the electron-transfer reaction of the more hydrophilic form is slower to yield a lower k0 value

DEVELOPMENT OF NANOFABRICATED PROBES FOR SCANNING ELECTROCHEMICAL MICROSCOPY

ABSTRACTNikoloz Nioradze, MSUniversity of Pittsburgh 2009The work describes development of novel Pt disk electrodes with nanometer diameters using focused ion beam (FIB) milling. The nanofabricated electrodes are applied as probes of scanning electrochemical microscopy (SECM) for studying of electrochemical processes on redox reactive substrates. Nanotips are fabricated by pulling a platinum wire sealed in a glass capillary using a laser pipette puller. An inlaid platinum disk eventually is exposed by milling the tip of a pulled capillary by FIB. Electrodes are as small as 300-500 nm in diameter with a ratio of insulating glass sheath and Pt wire (RG) as small as 3. Moreover, FIB milling gives a flat tip surface. The small RG and smooth surface are helpful for approaching a probe close to a substrate in SECM feedback experiments. Nanometer-sized electrodes can be readily positioned within 100 nm from a polished platinum surface. After placing the tip close to the Pt disk, a potential of the Pt substrate is swept to drive a redox reaction, which is monitored as a current response at the nanometer-sized electrode. Presented method of combination of SECM and substrate cyclic voltammetry (CV) will allow for accurate extraction of electrochemical kinetic parameters of a redox reaction. Overall, it can be concluded that tips fabricated by presented method can be successfully exploited for SECM experiments. Described SECM-substrate CV approach will be useful for monitoring of electrochemical behavior of different conductive substrates

Nanoscale Mechanism of Molecular Transport through the Nuclear Pore Complex As Studied by Scanning Electrochemical Microscopy

The nuclear pore complex (NPC) is the proteinaceous nanopore that solely mediates molecular transport across the nuclear envelope between the nucleus and cytoplasm of a eukaryotic cell. Small molecules (<40 kDa) diffuse through the large pore of this multiprotein complex. A passively impermeable macromolecule tagged with a signal peptide is chaperoned through the nanopore by nuclear transport receptors (e.g., importins) owing to their interactions with barrier-forming proteins. Presently, this bimodal transport mechanism is not well understood and is described by controversial models. Herein, we report on a dynamic and spatially resolved mechanism for NPC-mediated molecular transport through nanoscale central and peripheral routes with distinct permeabilities. Specifically, we develop a nanogap-based approach of scanning electrochemical microscopy to precisely measure the extremely high permeability of the nuclear envelope to a small probe molecule, (ferrocenylmethyl)­trimethylammonium. Effective medium theories indicate that the passive permeability of 5.9 × 10^–2 cm/s corresponds to the free diffusion of the probe molecule through ∼22 nanopores with a radius of 24 nm and a length of 35 nm. Peripheral routes are blocked by wheat germ agglutinin to yield 2-fold lower permeability for 17 nm-radius central routes. This lectin is also used in fluorescence assays to find that importins facilitate the transport of signal-tagged albumin mainly through the 7 nm-thick peripheral route rather than through the sufficiently large central route. We propose that this spatial selectivity is regulated by the conformational changes in barrier-forming proteins that transiently and locally expand the impermeably thin peripheral route while blocking the central route

Stabilizing Nanometer Scale Tip-to-Substrate Gaps in Scanning Electrochemical Microscopy Using an Isothermal Chamber for Thermal Drift Suppression

The control of a nanometer-wide gap between tip and substrate is critical for nanoscale applications of scanning electrochemical microscopy (SECM). Here, we demonstrate that the stability of the nanogap in ambient conditions is significantly compromised by the thermal expansion and contraction of components of an SECM stage upon a temperature change and can be dramatically improved by suppressing the thermal drift in a newly developed isothermal chamber. Air temperature in the chamber changes only at ∼0.2 mK/min to remarkably and reproducibly slow down the drift of tip–substrate distance to ∼0.4 nm/min in contrast to 5–150 nm/min without the chamber. Eventually, the stability of the nanogap in the chamber is limited by its fluctuation with a standard deviation of ±0.9 nm, which is mainly ascribed to the instability of a piezoelectric positioner. The subnanometer scale drift and fluctuation are measured by forming a ∼20 nm-wide gap under the 12 nm-radius nanopipet tip based on ion transfer at the liquid/liquid interface. The isothermal chamber is useful for SECM and, potentially, for other scanning probe microscopes, where thermal-drift errors in vertical and lateral probe positioning are unavoidable by the feedback-control of the probe–substrate distance

Nanometer Scale Scanning Electrochemical Microscopy Instrumentation

We report the crucial components required to perform scanning electrochemical microscopy (SECM) with nanometer-scale resolution. The construction and modification of the software and hardware instrumentation for nanoscale SECM are explicitly explained including (1) the LabVIEW code that synchronizes the SECM tip movement with the electrochemical response, (2) the construction of an isothermal chamber to stabilize the nanometer scale gap between the tip and substrate, (3) the modification of a commercial bipotentiostat to avoid electrochemical tip damage during SECM experiments, and (4) the construction of an SECM stage to avoid artifacts in SECM images. These findings enabled us to successfully build a nanoscale SECM, which can be utilized to map the electrocatalytic activity of individual nanoparticles in a typical ensemble sample and study the structure/reactivity relationship of single nanostructures

In Situ Detection of the Adsorbed Fe(II) Intermediate and the Mechanism of Magnetite Electrodeposition by Scanning Electrochemical Microscopy

Electrodeposition is an important approach that can produce functional compound materials by assembling multiple species at the electrode surface. However, a fundamental understanding of the electrodeposition mechanism has been limited by its complexity and is often gained only through ex situ studies of deposited materials. Here we report on the application of scanning electrochemical microscopy (SECM) to enable the in situ, real-time, and quantitative study of electrodeposition and electrodissolution. Specifically, we electrodeposit magnetite (Fe₃O₄) from an alkaline solution of Fe­(III)–triethanolamine as a robust route that can prepare this magnetic and electrocatalytic compound on various conductive substrates. The powerful combination of SECM with cyclic voltammetry (CV) at a gold substrate reveals that the electrodeposition of magnetite requires the preceding adsorption of Fe­(II)–triethanolamine on the substrate surface and, subsequently, is mediated through the highly complicated EC_adsC_mag mechanism, where both chemical steps occur at the substrate surface rather than in the homogeneous solution. SECM-based CV is obtained under high mass-transport conditions and analyzed by the finite element method to kinetically resolve all steps of the EC_adsC_mag mechanism and quantitatively determine relevant reaction parameters. By contrast, the adsorbed Fe­(II) intermediate is unresolvable from co-deposited magnetite in situ by other electrochemical techniques and is undetectable ex situ because of the facile air oxidation of the Fe­(II) intermediate. Significantly, SECM-based CV will be useful for the in situ characterization of various electrodeposited compounds to complement their ex situ characterization

Origins of Nanoscale Damage to Glass-Sealed Platinum Electrodes with Submicrometer and Nanometer Size

Glass-sealed Pt electrodes with submicrometer and nanometer size have been successfully developed and applied for nanoscale electrochemical measurements such as scanning electrochemical microscopy (SECM). These small electrodes, however, are difficult to work with because they often lose a current response or give a low SECM feedback in current–distance curves. Here we report that these problems can be due to the nanometer-scale damage that is readily and unknowingly made to the small tips in air by electrostatic discharge or in electrolyte solution by electrochemical etching. The damaged Pt electrodes are recessed and contaminated with removed electrode materials to lower their current responses. The recession and contamination of damaged Pt electrodes are demonstrated by scanning electron microscopy and X-ray energy dispersive spectroscopy. The recessed geometry is noticeable also by SECM but is not obvious from a cyclic voltammogram. Characterization of a damaged Pt electrode with recessed geometry only by cyclic voltammetry may underestimate electrode size from a lower limiting current owing to an invalid assumption of inlaid disk geometry. Significantly, electrostatic damage can be avoided by grounding a Pt electrode and nearby objects, most importantly, an operator as a source of electrostatic charge. Electrochemical damage can be avoided by maintaining potentiostatic control of a Pt electrode without internally disconnecting the electrode from a potentiostat between voltammetric measurements. Damage-free Pt electrodes with submicrometer and nanometer sizes are pivotal for reliable and quantitative nanoelectrochemical measurements

Organic Contamination of Highly Oriented Pyrolytic Graphite As Studied by Scanning Electrochemical Microscopy

Highly oriented pyrolytic graphite (HOPG) is an important electrode material as a structural model of graphitic nanocarbons such as graphene and carbon nanotubes. Here, we apply scanning electrochemical microscopy (SECM) to demonstrate quantitatively that the electroactivity of the HOPG basal surface can be significantly lowered by the adsorption of adventitious organic impurities from both ultrapure water and ambient air. An SECM approach curve of (ferrocenylmethyl)­trimethylammonium (FcTMA⁺) shows the higher electrochemical reactivity of the HOPG surface as the aqueous concentration of organic impurities, i.e., total organic carbon (TOC), is decreased from ∼20 to ∼1 ppb. SECM-based nanogap voltammetry in ∼1 ppb-TOC water yields unprecedentedly high standard electron-transfer rate constants, <i>k</i>⁰, of ≥17 and ≥13 cm/s for the oxidation and reduction of the FcTMA^2+/+ couple, respectively, at the respective tip–HOPG distances of 36 and 45 nm. Anomalously, <i>k</i>⁰ values and nanogap widths are different between the oxidation and reduction of the same redox couple at the same tip position, which is ascribed to the presence of an airborne contaminant layer on the HOPG surface in the noncontaminating water. This hydrophobic layer is more permeable to FcTMA⁺ with less charge than its oxidized form so that the oxidation of FcTMA⁺ at the HOPG surface results in the higher tip current and, subsequently, apparently narrower gap and higher <i>k</i>⁰. Mechanistically, we propose that HOPG adsorbs organic impurities mainly from ambient air and then additionally from ∼20 ppb-TOC water. The latter tightens a monolayer of airborne contaminants to yield lower permeability