Scanning electrochemical microscopy of living cells. 5. Imaging of fields of normal and metastatic human breast cells

Temperature Effect on the Electrochemical Current Response during Scanning Electrochemical Microscopy of Living Cells

Scanning electrochemical microscopy (SECM) is being used increasingly to monitor electrochemical processes at the interface of living cells and electrodes. This allows the detection and quantification of biomarkers that further the understanding of various diseases. Rapid SECM experiments are often carried out without monitoring the analyte solution temperature or are performed at room temperature. The reported research demonstrates that temperature control is crucial during SECM imaging of living cells to obtain reliable data. In this study, a SECM-integrated thermostatic ring on the sample stage enabled imaging of living biological cells in a constant height mode at various temperatures. Two-dimensional line scans were conducted while scanning single Adenocarcinoma Cervical cancer (HeLa) cells. Numerical modeling was carried out to evaluate the effect of the temperature on the electrochemical current response of living cells to compare the apparent heterogeneous rate constant (k0), representing cellular reaction kinetics. This study reveals that even slight temperature variations of approximately 2 °C affect the reaction kinetics of single living cells, altering the measured current during SECM.

In situ monitoring of the effect of Cu2+ on the membrane permeability of a single living cell with a dual-electrode tip of a scanning electrochemical microscope

Here, an Au-Cu dual-electrode tip was designed to monitor the effect of Cu²⁺ on the membrane permeability of a single living cell in situ using scanning electrochemical microscopy. The probe approach curves (PACs) were obtained using potassium ferricyanide as a redox mediator. Meanwhile, according to the simulation, theoretical PACs could be acquired. Thus, the cell membrane permeability coefficient (P_m) values were obtained by overlapping the experimental PACs with the theoretical values. Cu²⁺ was directly generated by electrolyzing the Cu electrode of the dual-electrode tip to investigate its effect on the cell membrane permeability in situ. This work has potential value to improve the understanding of the mechanism of acute heavy metal damage on the cell membrane and will also help clarify the role of heavy metal ions in physiological or pathological processes.

Electrochemical monitoring of reactive oxygen/nitrogen species and redox balance in living cells

Levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in cells and cell redox balance are of great interest in live cells as they are correlated to several pathological and physiological conditions of living cells. ROS and RNS detection is limited due to their spatially restricted abundance: they are usually located in sub-cellular areas (e.g., in specific organelles) at low concentration. In this work, we will review and highlight the electrochemical approach to this bio-analytical issue. Combining electrochemical methods and miniaturization strategies, specific, highly sensitive, time, and spatially resolved measurements of cellular oxidative stress and redox balance analysis are possible. Graphical abstract In this work, we highlight and review the use of electrochemistry for the highly spatial and temporal resolved detection of ROS/RNS levels and of redox balance in living cells. These levels are central in several pathological and physiological conditions and the electrochemical approach is a vibrant bio-analytical trend in this field.

Correlating Live Cell Viability with Membrane Permeability Disruption Induced by Trivalent Chromium

Cr(III) is often regarded as a trace essential micronutrient that can be found in many dietary supplements due to its participation in blood glucose regulation. However, increased levels of exposure have been linked to adverse health effects in living organisms. Herein, scanning electrochemical microscopy (SECM) was used to detect variation in membrane permeability of single cells (T24) resulting from exposure to a trivalent Cr-salt, CrCl₃. By employing electrochemical mediators, ferrocenemethanol (FcMeOH) and ferrocenecarboxylic acid (FcCOO^-), initially semipermeable and impermeable, respectively, complementary information was obtained. Three-dimensional COMSOL finite element analysis simulations were successfully used to quantify the permeability coefficients of each mediator by matching experimental and simulated results. Depending on the concentration of Cr(III) administered, three regions of membrane response were detected. Following exposure to low concentrations (up to 500 μM Cr(III)), their permeability coefficients were comparable to that of control cells, 80 μm/s for FcMeOH and 0 μm/s for FcCOO^-. This was confirmed for both mediators. As the incubation concentrations were increased, the ability of FcMeOH to permeate the membrane decreased to a minimum of 17 μm/s at 7500 μM Cr(III), while FcCOO^- remained impermeable. At the highest examined concentrations, both mediators were found to demonstrate increased membrane permeability. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability studies were also conducted on Cr(III)-treated T24 cells to correlate the SECM findings with the toxicity effects of the metal. The viability experiments revealed a similar concentration-dependent trend to the SECM cell membrane permeability study.

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.

Electrochemical detection of single cancer and healthy cell collisions on a microelectrode

The electrochemical detection of single cancer cells and healthy cells is reported. Detection was achieved by monitoring the consumption of a single cell's contents upon its collisions with a microelectrode in the presence of surfactant. The electrochemical response between acute lymphoblastic lymphoma T-cells and healthy thymocytes differed by two orders of magnitude.

Scanning Electrochemical Microscopy (SECM): Fundamentals and Applications in Life Sciences

In recent years, scanning electrochemical microscopy (SECM) has made significant contributions to the life sciences. Innovative developments focusing on high-resolution imaging, developing novel operation modes, and combining SECM with complementary optical or scanning probe techniques renders SECM an attractive analytical approach. This chapter gives an introduction to the essential instrumentation and operation principles of SECM for studying biologically-relevant systems. Particular emphasis is given to applications aimed at imaging the activity of biochemical constituents such as enzymes, antibodies, and DNA, which play a pivotal role in biomedical diagnostics. Furthermore, the unique advantages of SECM and combined techniques for studying live cells is highlighted by discussion of selected examples.

One-dimensional nanoprobes for single-cell studies

Owing to variation of individual cells within a population, single-cell studies are of great interest to researchers. Recent developments in nanofabrication technology have made this area increasingly attractive as one-dimensional (1D) nanoscale probes can be manufactured with increasing accuracy. Here, we provide an overview and description of the major designs that have been reported to date. For more details of what applications could be realized and how, based on the probe shapes and designs, we summarize the most recently reported performances of 1D single-cell probes with their advantages and limitations. Minimally invasive probes are required for long-term experiments on single cells. Carbon nanotubes with their unique properties and structure are excellent candidates for multitask robotic intracellular probes. Carbon nanotube-tipped cellular endoscopes are less invasive compared with pipettes or cantilever tips. Advances in nanofabrication techniques have made it possible to produce more consistent nanoscale cellular probes that can capture a variety of information from optical, electrical and chemical signals. In addition, these tools can transfer tiny amounts of fluids and molecular materials in a highly localized fashion for the purpose of analyzing or stimulating a variety of responses at the level of individual cells and even cellular organelles. We conclude with a critical analysis of the current state of the field as well as the major obstacles for further probe development of minimally invasive probes and their widespread use in cell biology.

Patterning bacterial communities on epithelial cells

Micropatterning of bacteria using aqueous two phase system (ATPS) enables the localized culture and formation of physically separated bacterial communities on human epithelial cell sheets. This method was used to compare the effects of Escherichia coli strain MG1655 and an isogenic invasive counterpart that expresses the invasin (inv) gene from Yersinia pseudotuberculosis on the underlying epithelial cell layer. Large portions of the cell layer beneath the invasive strain were killed or detached while the non-invasive E. coli had no apparent effect on the epithelial cell layer over a 24 h observation period. In addition, simultaneous testing of the localized effects of three different bacterial species; E. coli MG1655, Shigella boydii KACC 10792 and Pseudomonas sp DSM 50906 on an epithelial cell layer is also demonstrated. The paper further shows the ability to use a bacterial predator, Bdellovibriobacteriovorus HD 100, to selectively remove the E. coli, S. boydii and P. sp communities from this bacteria-patterned epithelial cell layer. Importantly, predation and removal of the P. Sp was critical for maintaining viability of the underlying epithelial cells. Although this paper focuses on a few specific cell types, the technique should be broadly applicable to understand a variety of bacteria-epithelial cell interactions.

Recent advances in high resolution scanning electrochemical microscopy of living cells--a review

This review discusses advances in the field of high resolution scanning electrochemical microscopy (HR-SECM) and scanning ion conductance microscopy (SICM) to study living cells. Relevant references from the advent of this technique in the late 1980s to most recent contributions in 2012 are presented with special discussion on high resolution images. A clear progress especially within the last 5 years can be seen in the field of HR-SECM. Furthermore, we also concentrate on the intrinsic properties of SECM imaging techniques e.g. different modes of image acquisition, their advantages and disadvantages in imaging living cells and strategies for further enhancement of image resolution, etc. Some of the recent advances of SECM in nanoimaging have also been discussed which may have potential applications in high resolution imaging of cellular processes.

A new application of scanning electrochemical microscopy for the label-free interrogation of antibody-antigen interactions: Part 2

Within this paper we describe the use of scanning electrochemical microscopy (SECM) to fabricate a dotted array of biotinylated polyethyleneimine which was then used to immobilise first neutravidin and then a biotinylated antibody towards a relevant antigen of interest (PSA, NTx, ciprofloxacin). These antigens were selected both for their clinical relevance but also since they display a broad range of molecular weights, to determine whether the size of the antigen used effects the sensitivity of this approach. The SECM was then used to image the binding of both complementary and non-complementary antigens in a label-free assay. Imaging of the arrays before and following exposure to various concentrations of antigen in buffer showed clear evidence for specific binding of the complementary antigens to the antibody functionalised dots. Non-specific binding was also quantified by control experiments with other antigens. This demonstrated non-specific binding across the whole of the substrate, thereby confirming that specific binding does occur between the antibody and antigen of interest at the surface of the dots. The binding of ciprofloxacin was investigated both in simple buffer solution and in a more complex media, bovine milk.

Imaging enzymes at work: metabolic mapping by enzyme histochemistry

For the understanding of functions of proteins in biological and pathological processes, reporter molecules such as fluorescent proteins have become indispensable tools for visualizing the location of these proteins in intact animals, tissues, and cells. For enzymes, imaging their activity also provides information on their function or functions, which does not necessarily correlate with their location. Metabolic mapping enables imaging of activity of enzymes. The enzyme under study forms a reaction product that is fluorescent or colored by conversion of either a fluorogenic or chromogenic substrate or a fluorescent substrate with different spectral characteristics. Most chromogenic staining methods were developed in the latter half of the twentieth century but still find new applications in modern cell biology and pathology. Fluorescence methods have rapidly evolved during the last decade. This review critically evaluates the methods that are available at present for metabolic mapping in living animals, unfixed cryostat sections of tissues, and living cells, and refers to protocols of the methods of choice.

Glucose and lactate biosensors for scanning electrochemical microscopy imaging of single live cells

We have developed glucose and lactate ultramicroelectrode (UME) biosensors based on glucose oxidase and lactate oxidase (with enzymes immobilized onto Pt UMEs by either electropolymerization or casting) for scanning electrochemical microscopy (SECM) and have determined their sensitivity to glucose and lactate, respectively. The results of our evaluations reveal different advantages for sensors constructed by each method: improved sensitivity and shorter manufacturing time for hand-casting, and increased reproducibility for electropolymerization. We have acquired amperometric approach curves (ACs) for each type of manufactured biosensor UME, and these ACs can be used as a means of positioning the UME above a substrate at a known distance. We have used the glucose biosensor UMEs to record profiles of glucose uptake above individual fibroblasts. Likewise, we have employed the lactate biosensor UMEs for recording the lactate production above single cancer cells with the SECM. We also show that oxygen respiration profiles for single cancer cells do not mimic cell topography, but are rather more convoluted, with a higher respiration activity observed at the points where the cell touches the Petri dish. These UME biosensors, along with the application of others already described in the literature, could prove to be powerful tools for mapping metabolic analytes, such as glucose, lactate, and oxygen, in single cancer cells.

Nanoelectrochemistry of mammalian cells

There is a significant current interest in development of new techniques for direct characterization of the intracellular redox state and high-resolution imaging of living cells. We used nanometer-sized amperometric probes in combination with the scanning electrochemical microscope (SECM) to carry out spatially resolved electrochemical experiments in cultured human breast cells. With the tip radius approximately 1,000 times smaller than that of a cell, an electrochemical probe can penetrate a cell and travel inside it without apparent damage to the membrane. The data demonstrate the possibility of measuring the rate of transmembrane charge transport and membrane potential and probing redox properties at the subcellular level. The same experimental setup was used for nanoscale electrochemical imaging of the cell surface.

Scanning electrochemical microscopy

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

Advances in the application of scanning electrochemical microscopy to bioanalytical systems

Scanning electrochemical microscopy (SECM) is a powerful surface characterisation technique that allows for the electrochemical profiling of surfaces with sub micrometer resolution. While SECM has been most widely used to electrochemically study and profile non-biological surfaces and processes, the technique has in recent years, been increasingly used for the study of biological systems - and this is the focus of this review. An overview of SECM and how the technique may be applied to the study of biological systems will first be given. SECM and its application to the study of cells, enzymes and DNA will each be considered in detail. The review will conclude with a discussion of future directions and scope for further developments and applications.

Review: Recent applications of scanning electrochemical microscopy to the study of charge transfer kinetics

Scanning electrochemical microscopy (SECM) has been proven to be a valuable technique for the quantitative investigation and surface analysis of a wide range of processes that occur at interfaces. In particular, there is a great deal of interest in studying the kinetics of charge transfer characteristics at the solid/liquid and liquid/liquid interface. This overview outlines recent advances and applications of SECM to the investigation of charge transfer reactions at the solid/liquid interface and liquid/liquid interface.

Feedback effects in combined fast-scan cyclic voltammetry-scanning electrochemical microscopy

Fast-scan cyclic voltammetry at scan rates between 5 and 1000 V s(-1) was performed at the tip of a scanning electrochemical microscope immersed in a solution of redox mediator. The effect of conducting and insulating substrates on the voltammetric signal was investigated as a function of scan rate and tip-substrate distance. It was found that diffusional interactions between the tip and the substrate are greatest at lower scan rates and on the reverse sweep of the voltammogram. At the fastest scan rates used, the tip could be brought to with 1 microm of the substrate without appreciable perturbation of the voltammogram. By selecting scan rates and tip-substrate distances such that feedback effects were negligible, it was possible to image the diffusion layer of a 10 microm Pt substrate electrode. With the tip placed 1 microm above a biological cell, tip-substrate diffusional interactions were greatly diminished at a scan rate of 100 V s(-1) and absent at a scan rate of 1000 V s(-1). These results suggest conditions can be selected that allow chemical imaging of substrates without the feedback interactions typically encountered in scanning electrochemical microscopy.

Scanning electrochemical microscopy for direct imaging of reaction rates

Not only in electrochemistry but also in biology and in membrane transport, localized processes at solid-liquid or liquid-liquid interfaces play an important role at defect sites, pores, or individual cells, but are difficult to characterize by integral investigation. Scanning electrochemical microscopy is suitable for such investigations. After two decades of development, this method is based on a solid theoretical foundation and a large number of demonstrated applications. It offers the possibility of directly imaging heterogeneous reaction rates and locally modifying substrates by electrochemically generated reagents. The applications range from classical electrochemical problems, such as the investigation of localized corrosion and electrocatalytic reactions in fuel cells, sensor surfaces, biochips, and microstructured analysis systems, to mass transport through synthetic membranes, skin and tissue, as well as intercellular communication processes. Moreover, processes can be studied that occur at liquid surfaces and liquid-liquid interfaces.

Imaging and detection of morphological changes of single cells before and after secretion using scanning electrochemical microscopy

Images of Human umbilical vein endothelial cells (HUVECs) have been obtained and the regulation of cell morphology changes after nitric oxide release has been recorded and discerned quantitatively for the first time using scanning electrochemical microscopy.

Regulation and characterization of the polarity of cells embedded in a reconstructed basement matrix using a three-dimensional micro-culture system

Three cell lines, that is, the human breast cancer cell line (MCF-7) and the human mammary epithelial cell line (S-1) and its malignant form (T4-2) were embedded in a reconstituted basement membrane (Matrigel) that had 20-nL pyramid-shaped silicon microstructures. The proliferative behavior of the MCF-7 cells was dependent on the surrounding conditions (2-D, collagen gel, or Matrigel), whereas the respiratory activity of a single cell (F(c)) was almost identical under different culture conditions. The F(c) value changed with cellular polarity. The F(c) value for the S-1 cells was observed to decrease slightly, whereas that of the T4-2 cells increased 2 days after cultivation in the microstructures within the Matrigel. However, when the T4-2 cells were cultured in the presence of tyrphostin AG 1478 (T4-2 tyr) to inhibit epidermal growth factor (EGF) signaling, the F(c) value decreased slightly and remained almost constant for an additional 1 week; this was similar to the behavior of the S-1 cells. Further, fluorescence images showed that the T4-2 tyr cells formed polar structures that were similar to those formed by the S-1 cells whereas the T4-2 cells did not form such structures. These results indicate that cellular polarity can be assessed by measuring cellular respiratory activity.

Scanning electrochemical microscopy in the 21st century

The fundamentals of and recent advances in scanning electrochemical microscopy (SECM) are described. The focus is on applications of this method to studies of systems and processes of active current interest ranging from nanoelectrochemistry to electron transfer reactions and electrocatalysis to biological imaging.