3D electrochemical and ion current imaging using scanning electrochemical-scanning ion conductance microscopy

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.

A review of innovative electrochemical strategies for bioactive molecule detection and cell imaging: Current advances and challenges

Cellular heterogeneity poses a major challenge for tumor theranostics, requiring high-resolution intercellular bioanalysis strategies. Over the past decades, the advantages of electrochemical analysis, such as high sensitivity, good spatio-temporal resolution, and ease of use, have made it the preferred method to uncover cellular differences. To inspire more creative research, herein, we highlight seminal works in electrochemical techniques for biomolecule analysis and bioimaging. Specifically, micro/nano-electrode-based electrochemical techniques enable real-time quantitative analysis of electroactive substances relevant to life processes in the micro-nanostructure of cells and tissues. Nanopore-based technique plays a vital role in biosensing by utilizing nanoscale pores to achieve high-precision detection and analysis of biomolecules with exceptional sensitivity and single-molecule resolution. Electrochemiluminescence (ECL) technology is utilized for real-time monitoring of the behavior and features of individual cancer cells, enabling observation of their dynamic processes due to its capability of providing high-resolution and highly sensitive bioimaging of cells. Particularly, scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) which are widely used in real-time observation of cell surface biological processes and three-dimensional imaging of micro-nano structures, such as metabolic activity, ion channel activity, and cell morphology are introduced in this review. Furthermore, the expansion of the scope of cellular electrochemistry research by innovative functionalized electrodes and electrochemical imaging models and strategies to address future challenges and potential applications is also discussed in this review.

Correlative Analysis of Ion-Concentration Profile and Surface Nanoscale Topography Changes Using Operando Scanning Ion Conductance Microscopy

Although various spectroscopic methods have been developed to capture ion-concentration profile changes, it is still difficult to visualize the ion-concentration profile and surface topographical changes simultaneously during the charging/discharging of lithium-ion batteries (LIBs). To tackle this issue, we have developed an operando scanning ion conductance microscopy (SICM) method that can directly visualize an ion-concentration profile and surface topography using a SICM nanopipette while controlling the sample potential or current with a potentiostat for characterizing the polarization state during charging/discharging. Using operando SICM on the negative electrode (anode) of LIBs, we have characterized ion-concentration profile changes and the reversible volume changes related to the phase transition during cyclic voltammetry (CV) and charge/discharge of the graphite anode. Operando SICM is a versatile technique that is likely to be of major value for evaluating the correlation between the electrolyte concentration profile and nanoscale surface topography changes.

Topography and Permeability Analyses of Vasculature-on-a-Chip Using Scanning Probe Microscopies

Microphysiological systems (MPS) or organs-on-chips (OoC) can emulate the physiological functions of organs in vitro and are effective tools for determining human drug responses in preclinical studies. However, the analysis of MPS has relied heavily on optical tools, resulting in difficulties in real-time and high spatial resolution imaging of the target cell functions. In this study, the role of scanning probe microscopy (SPM) as an analytical tool for MPS is evaluated. An access hole is made in a typical MPS system with stacked microchannels to insert SPM probes into the system. For the first study, a simple vascular model composed of only endothelial cells is prepared for SPM analysis. Changes in permeability and local chemical flux are quantitatively evaluated during the construction of the vascular system. The morphological changes in the endothelial cells after flow stimulation are imaged at the single-cell level for topographical analysis. Finally, the possibility of adapting the permeability and topographical analysis using SPM for the intestinal vascular system is further evaluated. It is believed that this study will pave the way for an in situ permeability assay and structural analysis of MPS using SPM.

Scanning Ion Conductance Microscopy

Scanning ion conductance microscopy (SICM) has emerged as a versatile tool for studies of interfaces in biology and materials science with notable utility in biophysical and electrochemical measurements. The heart of the SICM is a nanometer-scale electrolyte filled glass pipette that serves as a scanning probe. In the initial conception, manipulations of ion currents through the tip of the pipette and appropriate positioning hardware provided a route to recording micro- and nanoscopic mapping of the topography of surfaces. Subsequent advances in instrumentation, probe design, and methods significantly increased opportunities for SICM beyond recording topography. Hybridization of SICM with coincident characterization techniques such as optical microscopy and faradaic electrodes have brought SICM to the forefront as a tool for nanoscale chemical measurement for a wide range of applications. Modern approaches to SICM realize an important tool in analytical, bioanalytical, biophysical, and materials measurements, where significant opportunities remain for further exploration. In this review, we chronicle the development of SICM from the perspective of both the development of instrumentation and methods and the breadth of measurements performed.

Nanoscale Visualization of Morphological Alteration of Live-Cell Membranes by the Interaction with Oligoarginine Cell-Penetrating Peptides

The interactions between the cell membrane and biomolecules remain poorly understood. For example, arginine-rich cell-penetrating peptides (CPPs), including octaarginines (R8), are internalized by interactions with cell membranes. However, during the internalization process, the exact membrane dynamics introduced by these CPPs are still unknown. Here, we visualize arginine-rich CPPs and cell-membrane interaction-induced morphological changes using a system that combines scanning ion-conductance microscopy and spinning-disk confocal microscopy, using fluorescently labeled R8. This system allows time-dependent, nanoscale visualization of structural dynamics in live-cell membranes. Various types of membrane remodeling caused by arginine-rich CPPs are thus observed. The induction of membrane ruffling and the cup closure are observed as a process of endocytic uptake of the peptide. Alternatively suggested is the concave structural formation accompanied by direct peptide translocation through cell membranes. Studies using R8 without fluorescent labeling also demonstrate a non-negligible effect of the fluorescent moiety on membrane structural alteration.

Scanning ion conductance microscopy of isolated metaphase chromosomes in a liquid environment

Scanning probe microscopy (SPM) uses a probing tip which scans over a sample surface for obtaining information on the sample surface characteristics. Among various types of SPM, atomic force microscopy (AFM) has been widely applied to imaging of biological samples including chromosomes. Scanning ion conductance microscopy (SICM) has been also introduced for visualizing the surface structure of biological samples because it can obtain "contact-free" topographic images in liquid conditions by detecting ion current flow through a pipette opening. However, we recently noticed that the consistent imaging of chromosomes is difficult by SICM. In this paper, the behaviors of the ion current on the sample surfaces were precisely investigated for obtaining SICM images of isolated muntjac metaphase chromosomes more consistently than at present. The present study revealed that application of positive potential to the pipette electrode was acceptable for obtaining the topographic image of chromosomes, while application of negative potential failed in imaging. The approach curves were then studied for analyzing the relationship between the ion current and the tip sample distance when the pipette is approaching chromosomes. The current-voltage (I-V) curve further provided us the accurate interpretation of the ion current behavior during chromosome imaging. These data were further compared with those for SICM imaging of HeLa cells. Our findings indicated that chromosomes are electrically charged and the net charge is strongly negative in normal Dulbecco's phosphate buffered saline. We finally showed that the ion concentration of the bath electrolyte is important for imaging chromosomes by SICM.

A High Accuracy Ion Conductance Imaging Method Based on the Approach Curve Spectrum

Scanning ion conductance microscopy (SICM), as an emerging non-contact in situ topography measurement tool with nano resolution, has been increasingly used in recent years in biomedicine, electrochemistry and materials science. In the conventional measurement method of SICM, the sample topography is constructed according to the position of the probe at the feedback threshold of the ion current. Nevertheless, for different structures, a constant threshold cannot maintain a constant probe-sample distance. This phenomenon makes the measurement topography inconsistent with the real sample surface. In order to solve this problem and improve the measurement accuracy of SICM, a new ion conductance imaging method based on the approach curve spectrum is proposed in this work. In the new method, the local feature around the measurement point is firstly evaluated according to the change rate of ion current. Secondly, based on the local feature, the corresponding approach curve is searched from the prior approach curve spectrum to accurately evaluate the distance between the probe and the sample. Finally, the sample topography is constructed by the probe position subtracting the probe-sample distance. In this paper, we verify the feasibility of the new imaging method by combining finite element theory and experiments. To examine the measurement accuracy, the standard strip silicon and cylindrical polydimethylsiloxane (PDMS) samples are tested, and the improved imaging method can obtain the topography closer to the real samples and reduce the volumetric measurement error by 5.4%. The implementation of the new imaging method will further promote SICM application in related research fields.

Analysing single live cells by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) offers single live cell activities along its topography toward cellular physiology and pathology.

Biological imaging with scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is a powerful and versatile technique for visualizing the local electrochemical activity of a surface as an ultramicroelectrode tip is moved towards or over a sample of interest using precise positioning systems. In comparison with other scanning probe techniques, SECM not only enables topographical surface mapping but also gathers chemical information with high spatial resolution. Considerable progress has been made in the analysis of biological samples, including living cells and immobilized biomacromolecules such as enzymes, antibodies and DNA fragments. Moreover, combinations of SECM with comple-mentary analytical tools broadened its applicability and facilitated multi-functional analysis with extended life science capabilities. The aim of this review is to present a brief topical overview on recent applications of biological SECM, with particular emphasis on important technical improvements of this surface imaging technique, recommended applications and future trends.

Fabrication and Use of Nanopipettes in Chemical Analysis

This review summarizes progress in the fabrication, modification, characterization, and applications of nanopipettes since 2010. A brief history of nanopipettes is introduced, and the details of fabrication, modification, and characterization of nanopipettes are provided. Applications of nanopipettes in chemical analysis are the focus in several cases, including recent progress in imaging; in the study of single molecules, single nanoparticles, and single cells; in fundamental investigations of charge transfer (ion and electron) reactions at liquid/liquid interfaces; and as hyphenated techniques combined with other methods to study the mechanisms of complicated electrochemical reactions and to conduct bioanalysis.