Chemically irreversible redox mediator for SECM kinetics investigations: determination of the absolute tip-sample distance

Exploring the Living Cell: Applications and Advances of Scanning Electrochemical Microscopy

A living cell is a complex network of molecular, biochemical and physiological processes. Cellular activities, such as ion transport, metabolic processes, and cell-cell interactions can be determined electrochemically by detecting the electrons or ions exchanged in these processes. Electrochemical methods often are noninvasive, and they can enable the real-time monitoring of cellular processes. Scanning electrochemical microscopy (SECM) is an advanced scanning probe electroanalysis technique that can map the surface topography and local reactivity of a substrate with high precision at the micro- or nanoscale. By measuring electrochemical signals, such as redox reactions, ion fluxes, and pH changes, SECM can provide valuable insights into cellular activity. As a result of its compatibility with liquid medium measurements and its nondestructive nature, SECM has gained popularity in living cell research. This review aims to furnish an overview of SECM, elucidating its principles, applications, and its potential to contribute significantly to advancements in cell biology, electroporation, and biosensors. As a multidisciplinary tool, SECM is distinguished by its ability to unravel the intricacies of living cells and offers promising avenues for breakthroughs in our understanding of cellular complexity.

Quantitative Feedback Referencing for Improved Kinetic Fitting of Scanning Electrochemical Microscopy Measurements

Scanning electrochemical microscopy (SECM) has matured as a technique for studying local electrochemical processes. The feedback mode is most commonly used for extracting quantitative kinetic information. However, approaching individual regions of interest, as is commonly done, does not take full advantage of the spatial resolution that SECM has to offer. Moreover, fitting of experimental approach curves remains highly subjective due to the manner of estimating the tip-to-substrate distance. We address these issues using negative or positive feedback currents as a reference to calculate the tip-to-substrate distance directly for quantitative kinetic fitting of approach curves and line profiles. The method was first evaluated by fitting simulated data and then tested experimentally by resolving negative feedback and intermediate kinetics behavior in a spatially controlled fashion using (i) a flat, binary substrate composed of Au and SiO₂ segments and (ii) a dual-mediator system for live-cell measurements. The methodology developed herein, named quantitative feedback referencing (QFR), improves fitting accuracy, removes fitting subjectivity, and avoids substrate-microelectrode contact.

Pt/Polypyrrole Quasi-References Revisited: Robustness and Application in Electrochemical Energy Storage Research

Choosing reference electrodes for nonaqueous electrochemical measurements, especially in energy storage research, is challenging due to lengthy experiments (>1 day), the lack of alternatives to the commonly used Ag/Ag⁺ reference electrode (RE), the introduction of junction potentials, and the possibility of sample contamination. Often, quasi-reference electrodes (QREs) such as Ag wires and Li metal strips are used. However, small changes in electrolyte composition can cause large potential drifts, and their surfaces may be reactive to the solution. Here, we propose an alternative QRE based on polypyrrole electrodeposited on Pt wire (PPyQRE) encased in a glass tube with the open end sealed with commercial frits. While freestanding PPyQRE wires have been reported in the literature, simple encasing of the PPyQRE overcomes the above-mentioned drawbacks of QREs while providing a reliable reference potential that is closer to the performance of an RE. During cyclic voltammetric and bulk electrolysis testing of a redox mediator in solution, the encased PPyQRE exhibited stable reference potentials over multiple charge/discharge cycles with minimal drift (∼5 mV) after ∼2.25 days of operation. We also tested the reliability of our reference during the testing of multilayer graphene Li-ion anodes, which often involve cycling samples at highly reducing potentials (<-3 V vs Fc/Fc⁺) over long durations (>1 day). In the same testing conditions, the Ag/Ag⁺ electrode led to observable Ag deposits on the graphene and large potential drifts (∼50 mV), while the PPyQRE exhibited no measurable drift and revealed changes in voltammetric features that were obscured by reference drift when using Ag/Ag⁺. Minor reference drifts of ∼30 mV over long usage of the PPyQRE (∼2 months) can be addressed by calibration with a ferrocene couple at the end of experiments. These results highlight the advantages of using an encased PPyQRE as a simple and practical reference electrode for electrochemical measurements in the field of nonaqueous energy storage research.

Theoretical and experimental verification of imaging resolution factors in scanning electrochemical microscopy

The imaging resolution of scanning electrochemical microscopy (SECM) depends strongly on the tip electrode size and the tip-substrate distance. Herein, etched glass encapsulation was applied to fabricate a gold disk electrode, and the size of the tip electrode was accurately determined from the steady-state limiting current. Referring to the theoretical research carried out by our predecessors, the formula for the imaging resolution was derived, followed by the imaging of gold spots and cells with the prepared microelectrodes of different sizes and with different tip-substrate distances. A depth scan was performed to generate 2D current maps of the gold spot relative to the position of the microelectrode in the x-z plane. Probe approach curves and horizontal sweeps were obtained from one depth scan image by simply extracting vertical and horizontal cross-sectional lines, and further characterized by comparison with simulated curves through modeling of the experimental system. The experimental results were basically consistent with the theory, revealing that the highest imaging resolution can be obtained with the smallest tip electrode when d/a = 1, and when the size of the tip electrode is fixed the smallest tip-substrate distance can give the highest imaging resolution.

Scanning electrochemical microscopy with conducting polymer probes: Validation and applications

Scanning electrochemical microscopy (SECM) allows spatially and temporally resolved measurements of a broad range of reactive surfaces and specimens, typically using electrochemically active metal probes. While conducting polymers (CPs) present several analytical properties of interest due to their chemical versatility, potentially enabling the measurement of ionic fluxes as well as redox processes, they have not been widely used as probe materials for SECM. CPs can be modified and fine-tuned to improve experimental parameters and they can be easily prepared by electrodeposition. In this paper, we show a new type of CP probe for SECM that retains the spatial resolution of conventional metal probes and introduces the possibility to exploit a wide range of ionic and redox systems. Poly-3,4-ethylenedioxythiophene (PEDOT) was electrochemically deposited on flat and recessed Pt microdisks to generate CP SECM probes. To demonstrate their usefulness, an insulating substrate with conducting features was imaged. Well-defined SECM feedback images were observed for both the CP well-probe and the Pt probe, proving the efficiency of the new electrode to image redox reactions. Additionally, an organosulfur compound was used as mediator taking advantage of the electrocatalytic effect PEDOT has on the molecule's kinetics. Finally, these probes were also used in a mediator-less fashion, taking advantage of the ion flux required to electrochemically oxidize the PEDOT deposit. We investigated the impact of anion size and concentration on current-distance relationships for SECM probe positioning. CP probes pose exciting prospects for the imaging and measurement of combined redox and ionic processes in energy materials.

Photo-electrochemical properties of quantum rods studied by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is used for studying the intrinsic photo-electrochemical properties of CdSe/CdS quantum rods.