The Kv channel blocker 4-aminopyridine enhances Ag+ uptake: a scanning electrochemical microscopy study of single living cells

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.

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

Herein, we feature our recent efforts toward the development and application of nanoelectrochemistry at liquid/liquid interfaces, which are also known as interfaces between two immiscible electrolyte solutions (ITIES). Nanopipets, nanopores, and nanoemulsions are developed to create the nanoscale ITIES for the quantitative electrochemical measurement of ion transfer, electron transfer, and molecular transport across the interface. The nanoscale ITIES serves as an electrochemical nanosensor to enable the selective detection of various ions and molecules as well as high-resolution chemical imaging based on scanning electrochemical microscopy. The powerful nanoelectroanalytical methods will be useful for biological and material applications as illustrated by in situ studies of solid-state nanopores, nuclear pore complexes, living bacteria, and advanced nanoemulsions. These studies provide unprecedented insights into the chemical reactivity of important biological and material systems even at the single nanostructure level.

Mechanistic Assessment of Metabolic Interaction between Single Oral Commensal Cells by Scanning Electrochemical Microscopy

The human oral microbiome heavily influences the status of oral and systemic diseases through different microbial compositions and complex signaling between microbes. Recent evidence suggests that investigation of interactions between oral microbes can be utilized to understand how stable communities are maintained and how they may preserve health. Herein, we investigate two highly abundant species in the human supragingival plaque, Streptococcus mitis and Corynebacterium matruchotii, to elucidate their real-time chemical communication in commensal harmony. Specifically, we apply nanoscale scanning electrochemical microscopy (SECM) using a submicropipet-supported interface between two immiscible electrolyte solutions as an SECM probe not only to image the permeability of S. mitis and C. matruchotii membranes to tetraethylammonium (TEA⁺) probe ions but also to real-time visualize the metabolic interaction between two microbes via lactate production/consumption at a single-cell level. The metabolic relationship between two strains is quantitatively assessed by determining (1) the passive permeability of both bacterial membranes of 2.4 × 10^-4 cm/s to the free diffusion of TEA⁺, (2) 0.5 mM of the lactate concentration produced by a single S. mitis strain at a rate of 2.7 × 10^-4 cm/s, and (3) a lactate oxidation rate ≥5.0 × 10⁶ s^-1 by an individual C. matruchotii strain. Significantly, this study, for the first time, describes a mechanism of in situ metabolic interaction between oral commensals at the single-cell level through quantitative analysis, which supports the observed in vivo spatial arrangements of these microbes.

Can Ag+ Permeate through a Potassium Ion Channel? A Bottom-Up Approach by Infrared Spectroscopy of the Ag+ Complex with the Partial Peptide of a Selectivity Filter

Silver and silver ions have a long history of antimicrobial activity and medical applications. Nevertheless, the activity of Ag⁺ against bacteria, how it enters a cell, has not yet been established. The K⁺ channel, a membrane protein, is a possible route. The addition of a channel inhibitor (4-aminopyridine) to modulate the Ag⁺ uptake could support this view. However, the inhibitor enhances the uptake of Ag⁺, the opposite result. We have applied cold ion trap infrared laser spectroscopy to complexes of Ag⁺ and Ac-Tyr-NHMe (a model for GYG) which is a portion of the selectivity filter in the K⁺ channel to consider the question of permeation. With support from quantum chemical calculations, we have determined the stable conformations of the complex. The conformations strongly suggest that Ag⁺ would not readily permeate the K⁺ channel. The mechanism of the unexpected enhancement by the inhibitor is discussed.

Effect of metal cations on antimicrobial activity and compartmentalization of silver in Shewanella oneidensis MR-1 upon exposure to silver ions

Silver is an antimicrobial agent that is used extensively in consumer products, such as fabrics and humidifiers. Silver ion (Ag⁺) uptake in bacteria represents a crucial phase of antimicrobial activity. However, the uptake mechanism of Ag⁺ in bacteria remains largely unknown. The genus Shewanella drives many geochemical processes of nutrients and pollutants in soils. In the present study, Ag⁺ uptake by Shewanella oneidensis MR-1 was first investigated in a laboratory in defined anaerobic, oligotrophic, and inorganic media with or without cations (potassium ions [K⁺], magnesium ions [Mg²⁺], and zinc ions [Zn²⁺]). Our results revealed variations in antimicrobial activity of Ag⁺ in the presence of Mg²⁺ and Zn²⁺. First, Mg²⁺ significantly decreased antimicrobial activity of Ag⁺ in S. oneidensis MR-1 by inhibiting cellular Ag⁺ uptake when compared with K⁺. The results were consistent with that of Co²⁺ (Mg²⁺ channel blocker) decreased Ag⁺ uptake by S. oneidensis MR-1. Moreover, Mg²⁺ promoted riboflavin secretion and facilitated the formation of metallic Ag nanoparticles on bacterial surfaces, which was beneficial for extracellular electron transfer and consequently reduced antibacterial activity of Ag⁺. Second, Zn²⁺ increased the antimicrobial activity of Ag⁺ in S. oneidensis MR-1, although the effect on Ag⁺ uptake was minimal. A synergistic interaction between Zn²⁺ and Ag⁺ led to an increase in dead cells and decreased ferrihydrite reduction capacity. The findings suggest that Mg²⁺ could reduce the environmental risk of Ag⁺ to soil bacteria, while Zn²⁺ should be of particular concern due to its synergistic antimicrobial effect on bacteria.

Biotic Process Dominated the Uptake and Transformation of Ag+ by Shewanella oneidensis MR-1

Silver ions (Ag⁺) directly emitted from industrial sources or released from manufactured Ag nanoparticles (AgNPs) in biosolid-amended soils have raised concern about the risk to ecosystems. However, our knowledge of Ag⁺ toxicity, internalization, and transformation mechanisms to bacteria is still insufficient. Here, we combine the advanced technologies of hyperspectral imaging (HSI) and single-particle inductively coupled plasma mass spectrometry to visualize the potential formed AgNPs inside the bacteria and evaluate the contributions of biological and non-biological processes in the uptake and transformation of Ag⁺ by Shewanella oneidensis MR-1. The results showed a dose-dependent toxicity of Ag⁺ to S. oneidensis MR-1 in the ferrihydrite bioreduction process, which was primarily induced by the actively internalized Ag. Moreover, both HSI and cross-section high-resolution transmission electron microscopy results confirmed that Ag inside the bacteria existed in the form of particulate. The Ag mass distribution in and around live and inactivated cells demonstrated that the uptake and transformation of Ag⁺ by S. oneidensis MR-1 were mainly via biological process. The bioaccumulation of Ag⁺ may be lethal to bacteria. A better understanding of the uptake and transformation of Ag⁺ in bacteria is central to predict and monitor the key factors that control Ag partitioning dynamics at the biointerface, which is critical to develop practical risk assessment and mitigation strategies.

Scanning electrochemical microscopy and its potential for studying biofilms and antimicrobial coatings

Biofilms are known to be well-organized microbial communities embedded in an extracellular polymeric matrix, which supplies bacterial protection against external stressors. Biofilms are widespread and diverse, and despite the considerable large number of publications and efforts reported regarding composition, structure and cell-to-cell communication within biofilms in the last decades, the mechanisms of biofilm formation, the interaction and communication between bacteria are still not fully understood. This knowledge is required to understand why biofilms form and how we can combat them or how we can take advantage of these sessile communities, e.g. in biofuel cells. Therefore, in situ and real-time monitoring of nutrients, metabolites and quorum sensing molecules is of high importance, which may help to fill that knowledge gap. This review focuses on the potential of scanning electrochemical microscopy (SECM) as a versatile method for in situ studies providing temporal and lateral resolution in order to elucidate cell-to-cell communication, microbial metabolism and antimicrobial impact, e.g. of antimicrobial coatings through the study of electrochemical active molecules. Given the complexity and diversity of biofilms, challenges and limitations will be also discussed.

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.

Mapping Cd²⁺-induced membrane permeability changes of single live cells by means of scanning electrochemical microscopy

Scanning Electrochemical Microscopy (SECM) is a powerful, non-invasive, analytical methodology that can be used to investigate live cell membrane permeability. Depth scan SECM imaging allowed for the generation of 2D current maps of live cells relative to electrode position in the x-z or y-z plane. Depending on resolution, one depth scan image can contain hundreds of probe approach curves (PACs). Individual PACs were obtained by simply extracting vertical cross-sections from the 2D image. These experimental PACs were overlaid onto theoretically generated PACs simulated at specific geometry conditions. Simulations were carried out using 3D models in COMSOL Multiphysics to determine the cell membrane permeability coefficients at different locations on the surface of the cells. Common in literature, theoretical PACs are generated using a 2D axially symmetric geometry. This saves on both compute time and memory utilization. However, due to symmetry limitations of the model, only one experimental PAC right above the cell can be matched with simulated PAC data. Full 3D models in this article were developed for the SECM system of live cells, allowing all experimental PACs over the entire cell to become usable. Cd(2+)-induced membrane permeability changes of single human bladder (T24) cells were investigated at several positions above the cell, displaced from the central axis. The experimental T24 cells under study were incubated with Cd(2+) in varying concentrations. It is experimentally observed that 50 and 100 μM Cd(2+) caused a decrease in membrane permeability, which was uniform across all locations over the cell regardless of Cd(2+) concentration. The Cd(2+) was found to have detrimental effects on the cell, with cells shrinking in size and volume, and the membrane permeability decreasing. A mapping technique for the analysis of the cell membrane permeability under the Cd(2+) stress is realized by the methodology presented.

Dielectric-dependent electron transfer behaviour of cobalt hexacyanides in a solid solution of sodium chloride

Here we emphasise the importance of the dielectric environment on the electron transfer behavior in interfacial electrochemical systems. Through doping cobalt hexacyanide (Co(CN)63-) into single microcrystals of sodium chloride (NaCl), for the first time, we obtained the direct electrochemical behavior of Co(CN)63- which is hardly ever obtained in either aqueous or conventional nonaqueous solutions. DFT calculations elucidate that, as the Co(CN)63- anions occupy the lattice units of NaCl65- in the NaCl microcrystal, the redox energy barrier of Co(CN)63-/4- is decreased dramatically due to the low dielectric constant of NaCl. Meanwhile, the low-spin Co(CN)64- anions are stabilized in the lattices of the NaCl microcrystal. The results also show that the NaCl microcrystal is a potential solvent for solid-state electrochemistry at ambient temperature.

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.

On-chip metal/polypyrrole quasi-reference electrodes for robust ISFET operation

To operate an ion-sensitive field-effect transistor (ISFETs) it is necessary to set the electrolyte potential using a reference electrode. Conventional reference electrodes are bulky, fragile, and too big for applications where the electrolyte volume is small. Several researchers have proposed tackling this issue using a solid-state planar micro-reference electrode or a reference field-effect transistor. However, these approaches are limited by poor robustness, high cost, or complex integration with other microfabrication processes. Here we report a simple method to create robust on-chip quasi-reference electrodes by electrodepositing polypyrrole on micro-patterned metal leads. The electrodes were fabricated through the polymerization of pyrrole on patterned metals with a cyclic voltammetry process. Open circuit potential measurements were performed to characterize the polypyrrole electrode performance, demonstrating good stability (±1 mV), low drift (∼1 mV h(-1)), and reduced pH response (5 mV per pH). In addition, the polypyrrole deposition was repeated in microelectrodes made of different metals to test compatibility with standard complementary metal-oxide-semiconductor (CMOS) processes. Our results suggest that nickel, a metal commonly used in semiconductor foundries for silicide formation, is a good candidate to form the polypyrrole quasi-reference electrodes. Finally, the polypyrrole microelectrodes were used to operate foundry fabricated ISFETs. These experiments demonstrated that transistors biased with polypyrrole electrodes have pH sensitivity and resolution comparable to ones that are biased with standard reference electrodes. Therefore, the simple fabrication, high compatibility, and robust electrical performance make polypyrrole an ideal choice for the fabrication of outstanding microreference electrodes that enable robust and sensitive operation of multiple ISFET sensors on a chip.

Ion permeability of the nuclear pore complex and ion-induced macromolecular permeation as studied by scanning electrochemical and fluorescence microscopy

Efficient delivery of therapeutic macromolecules and nanomaterials into the nucleus is imperative for gene therapy and nanomedicine. Nucleocytoplasmic molecular transport, however, is tightly regulated by the nuclear pore complex (NPC) with the hydrophobic transport barriers based on phenylalanine and glycine repeats. Herein, we apply scanning electrochemical microscopy (SECM) to quantitatively study the permeability of the NPCs to small probe ions with a wide range of hydrophobicity as a measure of their hydrophobic interactions with the transport barriers. Amperometric detection of the redox-inactive probe ions is enabled by using the ion-selective SECM tips based on the micropipet- or nanopipet-supported interfaces between two immiscible electrolyte solutions. The remarkably high ion permeability of the NPCs is successfully measured by SECM and theoretically analyzed. This analysis demonstrates that the ion permeability of the NPCs is determined by the dimensions and density of the nanopores without a significant effect of the transport barriers on the transported ions. Importantly, the weak ion-barrier interactions become significant at sufficiently high concentrations of extremely hydrophobic ions, i.e., tetraphenylarsonium and perfluorobutylsulfonate, to permeabilize the NPCs to naturally impermeable macromolecules. Dependence of ion-induced permeabilization of the NPC on the pathway and mode of macromolecular transport is studied by using fluorescence microscopy to obtain deeper insights into the gating mechanism of the NPC as the basis of a new transport model.

Towards single-cell analysis for pharmacokinetics

Traditional ‘macroscopic’ pharmacokinetics (PK) investigates the fate of drugs or toxicants administered externally to living organisms, described by the extent and rate of absorption, distribution, metabolism and excretion. However, how a single cell affects a specific pharmaceutical after administration still remains a largely untouched area, primarily due to the technical restrictions imposed by minute amounts of chemicals involved. With the fast development of high-temporal and spatial-resolution detection techniques and single-cell handling techniques, it becomes possible to pursue single-cell PK. This review summarizes useful methodological and experimental techniques to investigate PK at the level of the single cell, including the microfluidics-based single-cell manipulation and the MS and electrochemical methods for single-cell analysis.

Real-time mapping of a hydrogen peroxide concentration profile across a polymicrobial bacterial biofilm using scanning electrochemical microscopy

Quantitative detection of hydrogen peroxide in solution above a Streptococcus gordonii (Sg) bacterial biofilm was studied in real time by scanning electrochemical microscopy (SECM). The concentration of hydrogen peroxide was determined to be 0.7 mM to 1.6 mM in the presence of 10 mM glucose over a period of 2 to 8 h. The hydrogen peroxide production measured was higher near the biofilm surface in comparison to Sg grown planktonically. Differential hydrogen peroxide production was observed both by fluorometric as well as by SECM measurements. The interaction between two different species in a bacterial biofilm of Sg and Aggregatibacter actinomycetemcomitans (Aa) in terms of hydrogen peroxide production was also studied by SECM. One-directional y-scan SECM measurements showed the unique spatial mapping of hydrogen peroxide concentration across a mixed species biofilm and revealed that hydrogen peroxide concentration varies greatly dependent upon local species composition.

Ion-selective permeability of an ultrathin nanoporous silicon membrane as probed by scanning electrochemical microscopy using micropipet-supported ITIES tips

We report on the application of scanning electrochemical microscopy (SECM) to the measurement of the ion-selective permeability of porous nanocrystalline silicon membrane as a new type of nanoporous material with potential applications in analytical, biomedical, and biotechnology device development. The reliable measurement of high permeability in the molecularly thin nanoporous membrane to various ions is important for greater understanding of its structure-permeability relationship and also for its successful applications. In this work, this challenging measurement is enabled by introducing two novel features into amperometric SECM tips based on the micropipet-supported interface between two immiscible electrolyte solutions (ITIES) to reveal the important ion-transport properties of the ultrathin nanopore membrane. The tip of a conventional heat-pulled micropipet is milled using the focused ion beam (FIB) technique to be smoother, better aligned, and subsequently, approach closer to the membrane surface, which allows for more precise and accurate permeability measurement. The high membrane permeability to small monovalent ions is determined using FIB-milled micropipet tips to establish a theoretical formula for the membrane permeability that is controlled by free ion diffusion across water-filled nanopores. Moreover, the ITIES tips are rendered selective for larger polyions with biomedical importance, i.e., polyanionic pentasaccharide Arixtra and polycationic peptide protamine, to yield the membrane permeability that is lower than the corresponding diffusion-limited permeability. The hindered transport of the respective polyions is unequivocally ascribed to electrostatic and steric repulsions from the wall of the nanopores, i.e., the charge and size effects.

Ctr1 transports silver into mammalian cells

Silver is a non-essential, toxic metal. The use of silver as an antimicrobial agent in many applications and its presence as a contaminant in foods and air can lead to accumulation in tissues. Despite its widespread use, the systems involved in the uptake of silver into mammalian cells are presently unknown. Previous studies have shown that copper uptake at the plasma membrane by copper transporter 1 (Ctr1) is inhibited by an excess of silver, suggesting that Ctr1 may function in importing silver into cells. In this study we examined directly the role of Ctr1 in the accumulation of silver in mammalian cells using over-expression experiments and mouse embryonic fibroblast cells lacking Ctr1. COS-7 cells transfected to express a human Ctr1-green fluorescent protein (hCtr1-GFP) fusion protein hyper-accumulated silver when incubated in medium supplemented with low micromolar concentrations (2.5-10 micromol/L) of AgNO(3). An hCtr1-GFPM150L,M154L variant deficient for copper transport failed to stimulate accumulation of silver. Mouse embryonic fibroblast cells lacking Ctr1 showed approximately a 50% reduction in silver content when incubated in silver-supplemented medium compared to a wild-type isogenic cell line. Collectively, these data demonstrate that Ctr1 transports both copper and silver and suggest that Ctr1 is an important transport protein in the accumulation of silver in mammalian cells.

Scanning electrochemical microscopy in neuroscience

This article reviews recent work involving the application of scanning electrochemical microscopy (SECM) to the study of individual cultured living cells, with an emphasis on topographical and functional imaging of neuronal and secretory cells of the nervous and endocrine system. The basic principles of biological SECM and associated negative amperometric-feedback and generator/collector-mode SECM imaging are discussed, and successful use of the methodology for screening soft and fragile membranous objects is outlined. The drawbacks of the constant-height mode of probe movement and the benefits of the constant-distance mode of SECM operation are described. Finally, representative examples of constant-height and constant-distance mode SECM on a variety of live cells are highlighted to demonstrate the current status of single-cell SECM in general and of SECM in neuroscience in particular.

Silver ions induce oxidative stress and intracellular zinc release in human skin fibroblasts

Silver compounds used as topical antimicrobial agents are known to exert toxic effects on skin cells. The aim of this study was to investigate whether the toxicity of silver ions, in analogy to other transition metal ions, depends on pro-oxidant effects. We treated human skin fibroblasts with concentrations of AgNO(3) not affecting cell proliferation, mitochondrial activity, or cell viability and found that Ag(+) strongly increases the production of reactive oxygen species, including superoxide anion radicals. These effects correspond to a strong decrease in intracellular reduced glutathione and to an increased susceptibility to H(2)O(2)-induced cell death. In addition, AgNO(3) down-regulates the expression of antioxidant genes such as the transcription factor Nrf2 and its target gene glutamate-cysteine ligase catalytic subunit. Furthermore Ag(+) induces a transient intracellular zinc release and increases the mRNA and protein expression of the zinc-binding protein metallothionein by activating the metal-responsive transcription factor 1, as verified by RNA interference. In conclusion, we show for the first time that Ag(+) induces oxidative stress and affects intracellular zinc homeostasis in human skin fibroblasts. The understanding of the mechanism involved in silver toxicity might contribute to new strategies for managing the therapy of skin infections.