Scanning electrochemical microscopy imaging with laser-pulled probes

Integration of Scanning Electrochemical Microscopy and Scanning Electrochemical Cell Microscopy in a Bifunctional Nanopipette toward Simultaneous Mapping of Activity and Selectivity in Electrocatalysis

Scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) were integrated in a single bifunctional probe for simultaneous mapping of the oxygen reduction current and the oxidation current of the produced H2O2. The dual probe is fabricated from a double-barrel θ capillary, comprising one open barrel filled with the electrolyte and another filled with pyrolytic carbon. Pt is deposited with a gas injection system (GIS) at the end of the carbon barrel. The probe integrates the advantages of both SECM and SECCM by forming an electrochemical droplet cell that embeds the Pt working electrode of the carbon barrel directly into the electrolyte meniscus formed upon sample contact from the electrolyte barrel. The versatility of the dual probe is demonstrated by mapping the oxygen reduction reaction (ORR) current and the H2O2 oxidation current of a Pt microstrip on a gold substrate. This allows simultaneous localized electrochemical measurements, highlighting the potential of the dual probe for broader applications in characterizing the electrocatalytic properties of materials.

Unraveling Mass and Electron Transfer Kinetics at Silica Nanochannel Membrane Modified Electrodes by Scanning Electrochemical Microscopy

An in-depth understanding of kinetic processes convoluting mass and charge transfer at nanoporous membrane modified electrodes is crucial for developing high-performance electrochemical sensors. In this work, we propose a theoretical model to unravel mass (k_m) and electron transfer rate (k_f) from the apparent electrochemical rate constant (k_app) at silica nanoporous membrane (SNM) modified indium tin oxide (ITO) electrodes (designated as SNM/ITO for simplicity). Using scanning electrochemical microscopy (SECM), the k_app of charged redox species was first determined at the SNM/ITO in the absence and presence of surfactant micelles inside SNM. On the basis of the theory, in the presence of micelles inside SNM, k_m equals zero for all charged probes (Ru(NH₃)₆²⁺, Ru(CN)₆^3-, and FcMeOH⁺), thus the SNM behaves as an insulating barrier and the overall electrode reactivity is dominated by the permeability of SNM. After excluding micelles from SNM, the k_m of Ru(CN)₆^3-/4- is strongly dependent on the KCl concentration in the solution, decreasing from 0.23/0.15 mm s^-1 to almost zero upon decreasing the KCl concentration from 1.0 to 0.01 M. In contrast, k_m increases from 1.33 to 2.4 mm s^-1 for Ru(NH₃)₆²⁺ and from 0.18 to 0.33 mm s^-1 for FcMeOH⁺, which are comparable to the electron transfer rate at the underlying ITO electrode surface (0.8 and 0.35 mm s^-1). In these cases, both mass and electron transfer processes are important in determining the overall redox activity of SNM/ITO electrodes. The methodology reported in this work can provide a quantitative way of unraveling these processes and their respective contributions.

Recent Advances in Scanning Electrochemical Microscopy for Biological Applications

Scanning electrochemical microscopy (SECM) is a chemical microscopy technique with high spatial resolution for imaging sample topography and mapping specific chemical species in liquid environments. With the development of smaller, more sensitive ultramicroelectrodes (UMEs) and more precise computer-controlled measurements, SECM has been widely used to study biological systems over the past three decades. Recent methodological breakthroughs have popularized SECM as a tool for investigating molecular-level chemical reactions. The most common applications include monitoring and analyzing the biological processes associated with enzymatic activity and DNA, and the physiological activity of living cells and other microorganisms. The present article first introduces the basic principles of SECM, followed by an updated review of the applications of SECM in biological studies on enzymes, DNA, proteins, and living cells. Particularly, the potential of SECM for investigating bacterial and biofilm activities is discussed.

Submicron probes for scanning electrochemical microscopy

To improve the spatial resolutions of scanning electrochemical microscopy (SECM) imaging, the laser-pulled submicron electrode fabrication method was explored in this work. Manual polishing of a laser-pulled Pt nanoelectrode exposed a Pt tip diameter of 250 nm with a ratio of the tip glass to exposed Pt disc (RG) of 30. This fabricated submicron probe was then utilized to study the electrochemical functionality of an independently addressable microband electrodes (IAME) sample using SECM. In the constant imaging mode of SECM, where the probe is scanned linearly across the sample at a fixed z position, SECM demonstrated higher resolution than that of the conventional micrometer electrodes when the feedback currents from the Pt and glass microbands were characterized. In addition, the depth scan imaging mode of SECM was also used to extract experimental horizontal line scans and probe approach curves for analysis. Three-dimensional (3D) simulations of the IAME–SECM probe experiments were explored for the first time to quantify the tip-to-sample distances, tilt angle of the sample (or electrode), and height of the Pt microbands. The experimentally characterized height was found to be similar to manufacturer specification (125 nm vs 110 nm). Furthermore, the more computationally demanding 3D simulation of the true IAME sample geometry (110 nm height of the Pt microbands) revealed minimal difference in feedback behaviours in comparison with the idealized flat geometry. The removal of this simulation complexity was proved to be sufficient for SECM analysis of the IAME sample by a 250 nm Pt probe, which greatly saves computation resources.