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.

Glucose micro-biosensor for scanning electrochemical microscopy characterization of cellular metabolism in hypoxic microenvironments

Mapping of the metabolic activity of tumor tissues represents a fundamental approach to better identify the tumor type, elucidate metastatic mechanisms and support the development of targeted cancer therapies. The spatially resolved quantification of Warburg effect key metabolites, such as glucose and lactate, is essential. Miniaturized electrochemical biosensors scanned over cancer cells and tumor tissue to visualize the metabolic characteristics of a tumor is attractive but very challenging due to the limited oxygen availability in the hypoxic environments of tumors that impedes the reliable applicability of glucose oxidase-based glucose micro-biosensors. Herein, the development and application of a new glucose micro-biosensor is presented that can be reliably operated under hypoxic conditions. The micro-biosensor is fabricated in a one-step synthesis by entrapping during the electrochemically driven growth of a polymeric matrix on a platinum microelectrode glucose oxidase and a catalytically active Prussian blue type aggregate and mediator. The as-obtained functionalization improves significantly the sensitivity of the developed micro-biosensor for glucose detection under hypoxic conditions compared to normoxic conditions. By using the micro-biosensor as non-invasive sensing probe in Scanning Electrochemical Microscopy (SECM), the glucose uptake by a breast metastatic adenocarcinoma cell line, with an epithelial morphology, is measured.

Scanning electrochemical microscopy based irreversible destruction of living cells

In this research, scanning electrochemical microscopy combined with electrochemical impedance spectroscopy has been applied to irreversible electroporation of active yeast cells by causing cell death. This finding is important for the development of irreversible electroporation technique, which could be suitable for the curing of cancerous tissues, because during this research cell death has been achieved using relatively low ultramicro-electrode (UME) voltage, precisely of 2.0 V vs Ag/AgCl,Cl^-sat. It was determined that the irreversibly electroporated area of immobilized yeast cells was located directly below the UME and was of approximately 20 times larger width than the diameter of the UME, leaving undamaged cells out of this area. The ability of SECM to move the UME with high accuracy in x, y, and z directions and the ability to use electrodes of various diameters as well as the fact that the diameter of the electroporated area depends on the diameter of the UME and on the distance between the UME and the surface, what offers the possibility to establish targeted electroporation systems for selective treatment of tissues.

Scanning electrochemical microscope as a tool for the electroporation of living yeast cells

In this study, a scanning electrochemical microscope (SECM) was for the first time adapted to perform the electroporation process of living yeast cells. We have demonstrated that relatively low voltage pulses of 1-2 V vs. Ag/AglCl,Cl^-_sat applied to gold-based ultramicroelectrode (Au-UME) are performing reversible electroporation of yeast cells immobilized on fluorine-doped tin oxide (FTO)/glass surface. SECM and electrochemical impedance spectroscopy (EIS) were used for the determination of quantitative electrochemical characteristics before and after the electroporation. The electrochemical impedance spectroscopy (EIS) illustrated significant electrochemical changes of electroporated yeast cells, while SECM feedback mode surface vertical scan current-distance curves showed that the diameter of the area affected by the electrical pulse is about 25 times larger than the diameter of the Au-UME used for the electroporation process. The results presented in this research open up a possibility to develop a targeted electroporation system which will affect only the selected area of tissue or some other cell-covered surface. Such model is promising for the selective treatment of selected cells in tissues and/or other sensitive biological systems while selecting the location and size of electroporated areas.

A Microfluidic Dielectric Spectroscopy System for Characterization of Biological Cells in Physiological Media

Dielectric spectroscopy (DS) is a promising cell screening method that can be used for diagnostic and drug discovery purposes. The primary challenge of using DS in physiological buffers is the electrode polarization (EP) that overwhelms the impedance signal within a large frequency range. These effects further amplify with the miniaturization of the measurement electrodes. In this study, we present a microfluidic system and the associated equivalent circuit models for real-time measurements of cell membrane capacitance and cytoplasm resistance in physiological buffers with 10 s increments. The current device captures several hundreds of biological cells in individual microwells through gravitational settling and measures the system's impedance using microelectrodes covered with dendritic gold nanostructures. Using PC-3 cells (a highly metastatic prostate cancer cell line) suspended in cell growth media (CGM), we demonstrate stable measurements of cell membrane capacitance and cytoplasm resistance in the device for over 15 min. We also describe a consistent application of the equivalent circuit model, starting from the reference measurements used to determine the system parameters. The circuit model is tested using devices with varying dimensions, and the obtained cell parameters between different devices are nearly identical. Further analyses of the impedance data have shown that accurate cell membrane capacitance and cytoplasm resistance can be extracted using a limited number of measurements in the 5 MHz to 10 MHz range. This will potentially reduce the timescale required for real-time DS measurements below 1 s. Overall, the new microfluidic device can be used for the dielectric characterization of biological cells in physiological buffers for various cell screening applications.

Evaluating Young's Modulus of Single Yeast Cells Based on Compression Using an Atomic Force Microscope with a Flat Tip

In this research, atomic force microscopy (AFM) with a flat tip cantilever is utilized to measure Young's modulus of a whole yeast cell (Saccharomyces cerevisiae BY4741). The results acquired from AFM are similar to those obtained using a microfluidic chip compression system. The mechanical properties of single yeast cells are important parameters which can be examined using AFM. Conventional studies apply AFM with a sharp cantilever tip to indent the cell and measure the force-indentation curve, from which Young's modulus can be calculated. However, sharp tips introduce problems because the shape variation can lead to a different result and cannot represent the stiffness of the whole cell. It can lead to a lack of broader meaning when evaluating Young's modulus of yeast cells. In this report, we confirm the differences in results obtained when measuring the compression of a poly(dimethylsiloxane) bead using a commercial sharp tip versus a unique flat tip. The flat tip effectively avoids tip-derived errors, so we use this method to compress whole yeast cells and generate a force–deformation curve. We believe our proposed method is effective for evaluating Young's modulus of whole yeast cells.