Nanoelectrochemical quantification of single-cell metabolism

An Insulin-Modified pH-Responsive Nanopipette Based on Ion Current Rectification

The properties of nanopipettes largely rely on the materials introduced onto their inner walls, which allow for a vast extension of their sensing capabilities. The challenge of simultaneously enhancing the sensitivity and selectivity of nanopipettes for pH sensing remains, hindering their practical applications. Herein, we report insulin-modified nanopipettes with excellent pH response performances, which were prepared by introducing insulin onto their inner walls via a two-step reaction involving silanization and amidation. The pH response intensity based on ion current rectification was significantly enhanced by approximately 4.29 times when utilizing insulin-modified nanopipettes compared with bare ones, demonstrating a linear response within the pH range of 2.50 to 7.80. In addition, insulin-modified nanopipettes featured good reversibility and selectivity. The modification processes were monitored using the I-V curves, and the relevant mechanisms were discussed. The effects of solution pH and insulin concentration on the modification results were investigated to achieve optimal insulin introduction. This study showed that the pH response behavior of nanopipettes can be greatly improved by introducing versatile molecules onto the inner walls, thereby contributing to the development and utilization of pH-responsive nanopipettes.

Non-Mass Spectrometric Targeted Single-Cell Metabolomics

Metabolic assays serve as pivotal tools in biomedical research, offering keen insights into cellular physiological and pathological states. While mass spectrometry (MS)-based metabolomics remains the gold standard for comprehensive, multiplexed analyses of cellular metabolites, innovative technologies are now emerging for the targeted, quantitative scrutiny of metabolites and metabolic pathways at the single-cell level. In this review, we elucidate an array of these advanced methodologies, spanning synthetic and surface chemistry techniques, imaging-based methods, and electrochemical approaches. We summarize the rationale, design principles, and practical applications for each method, and underscore the synergistic benefits of integrating single-cell metabolomics (scMet) with other single-cell omics technologies. Concluding, we identify prevailing challenges in the targeted scMet arena and offer a forward-looking commentary on future avenues and opportunities in this rapidly evolving field.

A troubleshooting guide for laser pulling platinum nanoelectrodes

While there are numerous publications on laser-assisted fabrication and characterization of Pt nanoelectrodes, the exact replication of those procedures is not as straightforward as following a single recipe across laboratories. Often, the working procedures vary by day, by laser puller, or by person. Only a handful of nanoelectrode fabrication papers record their parameters, and even fewer offer troubleshooting advice. Here, we provide a step-by-step guide for laser-assisted Pt nanoelectrode fabrication using low-cost equipment including a laser puller, voltammetry, and simple microscope images captured via cell phone. We also offer solutions for common failures experienced throughout the process to guide beginners as they troubleshoot their own fabrication procedures.

Triple-Barrel Ultramicroelectrodes for Multipurpose, Submilliliter Electroanalysis

Here, we have developed and applied a triple-barrel microelectrode. This device incorporates a platinum disk working electrode, a platinum disk counter electrode, and a low-leakage Ag/AgCl reference electrode into a small probe. We demonstrate that the incorporated low-leakage reference electrode shows similar voltammetry, potentiometry, and drift when compared to a commercial reference electrode in bulk solution. We also demonstrate the versatility of such a small three-channel system via voltammetry in nanoliter droplets and through electroanalysis of captured aerosols. Finally, we demonstrate the probe's potential utility in single-cell electroanalysis by making measurements within salmon eggs.

On the mechanism of the bipolar reference electrode

Commercial silver/silver chloride (Ag/AgCl) reference electrodes are some of the most commonly used reference electrodes, but they suffer from a number of issues due to their porous frits. Such issues include difficulty miniaturizing, silver and chloride ion leakage, charge screening effects at low ionic strength, frit drying if left unattended in air, and incompatibility with organic solvents. To solve these issues, we recently designed a reference electrode that is leakless in principle by replacing the porous frit with a sealed, conductive wire, where the ends of the wire are exposed to the reference electrode solution and the working electrode solution. We hypothesized that the reference electrode operated like a closed, bipolar electrochemical cell, and we termed the name bipolar reference electrode (BPRE). Here, we provide evidence that the BPRE can either act as a reference electrode by operating through an ion transfer mechanism via leakage through the imperfect seal, or it can act as a highly stable quasi-reference electrode through a bipolar electron transfer mechanism (BPQRE). Finally, we demonstrate the effectiveness of the BPRE in other types of common electrochemical studies, including chronoamperometry, linear sweep voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy.

Trends in single-impact electrochemistry for bacteria analysis

Single-impact electrochemistry for the analysis of bacteria is a powerful technique for biosensing applications at the single-cell scale. The sensitivity of this electro-analytical method has been widely demonstrated based on chronoamperometric measurements at an ultramicroelectrode polarized at the appropriate potential of redox species in solution. Furthermore, the most recent studies display a continuous improvement in the ability of this sensitive electrochemical method to identify different bacterial strains with better selectivity. To achieve this, several strategies, such as the presence of a redox mediator, have been investigated for detecting and identifying the bacterial cell through its own electrochemical behavior. Both the blocking electrochemical impacts method and electrochemical collisions of single bacteria with a redox mediator are reported in this review and discussed through relevant examples. An original sensing strategy for virulence factors originating from pathogenic bacteria is also presented, based on a recent proof of concept dealing with redox liposome single-impact electrochemistry. The limitations, applications, perspectives, and challenges of single-impact electrochemistry for bacteria analysis are briefly discussed, based on the most significant published data.

Oxidation of Cysteine by Electrogenerated Hexacyanoferrate(III) in Microliter Droplets

Chemical reactivity in droplets is often assumed to mimic reactivity in bulk, continuous water. Here, we study the catalytic oxidation of cysteine by electrogenerated hexacyanoferrate(III) in microliter droplets. These droplets are adsorbed onto glassy carbon macroelectrodes and placed into an immiscible 1,2-dichloroethane phase. We combined cyclic voltammetry, optical microscopy, and finite element simulations to quantify the apparent bimolecular rate constant, k_c,app, in microdroplets and bulk water. Statistical analyses reveal that the apparent bimolecular rate constant (k_c,app) values formicrodroplets are larger than those in the continuous phase. Reactant adsorption to the droplet boundary has previously been implicated as the cause of such rate accelerations. Finite element modeling of this system suggests that molecular adsorption to the liquid|liquid interface cannot alone account for our observations, implicating kinetics of the bimolecular reaction either at the boundary or throughout the microliter volume. Our results indicate that cysteine oxidation by electrogenerated hexacyanoferrate(III) can be accelerated within a microenvironment, which may have profound implications on understanding biological processes within a cell.

Investigating the cytotoxic redox mechanism of PFOS within Hep G2 by hyperspectral-assisted scanning electrochemical microscopy

Perfluorooctane sulfonate (PFOS) is one of the most lethal per- and poly-fluoroalkyl substances (PFAS). Generally, exposure effects are studied through case-controlled studies, cohort studies, or cell assays. Unfortunately, most studies involving two-dimensional cell cultures require cell lysis or fixation. For in vitro studies, fluorescence microscopy has been useful, but methods to simultaneously discern phototoxic effects during an experiment are limited. Here, we use hepatocarcinoma (Hep G2) cells to examine the redox mechanism of PFOS cytotoxicity in vitro, while using hyperspectral-assisted scanning electrochemical microscopy (SECM) to differentiate between PFOS and redox mediator induced stress. Specifically, we correlate an increase in the electrochemical response of ferrocenemethanol oxidation with an increase in intracellular reactive oxygen species. Corresponding hyperspectral images of redox indicative-fluorophores implicate superoxide in the cytotoxic redox mechanism.

Nanotechnology Tools Enabling Biological Discovery

Over the years, the engineering aspect of nanotechnology has been significantly exploited. Medical intervention strategies have been developed by leveraging existing molecular biology knowledge and combining it with nanotechnology tools to improve outcomes. However, little attention has been paid to harnessing the strengths of nanotechnology as a biological discovery tool. Fundamental understanding of controlling dynamic biological processes at the subcellular level is key to developing personalized therapeutic and diagnostic interventions. Single-cell analyses using intravital microscopy, expansion microscopy, and microfluidic-based platforms have been helping to better understand cell heterogeneity in healthy and diseased cells, a major challenge in oncology. Also, single-cell analysis has revealed critical signaling pathways and biological intracellular components with key biological functions. The physical manipulation enabled by nanotools can allow real-time monitoring of biological changes at a single-cell level by sampling intracellular fluid from the same cell. The formation of intercellular highways by nanotube-like structures has important clinical implications such as metastasis development. The integration of nanomaterials into optical and molecular imaging techniques has rendered valuable morphological, structural, and biological information. Nanoscale imaging unravels mechanisms of temporality by enabling the visualization of nanoscale dynamics never observed or measured between individual cells with standard biological techniques. The exceptional sensitivity of nanozymes, artificial enzymes, make them perfect components of the next-generation mobile diagnostics devices. Here, we highlight these impactful cancer-related biological discoveries enabled by nanotechnology and producing a paradigm shift in cancer research and oncology.

Versatile Construction of Biomimetic Nanosensors for Electrochemical Monitoring of Intracellular Glutathione

The current strategies for nanoelectrode functionalization usually involve sophisticated modification procedures, uncontrollable and unstable modifier assembly, as well as a limited variety of modifiers. To address this issue, we propose a versatile strategy for large-scale synthesis of biomimetic molecular catalysts (BMCs) modified nanowires (NWs) to construct functionalized electrochemical nanosensors. This design protocol employs an easy, controllable and stable assembly of diverse BMCs-poly(3,4-ethylenedioxythiophene) (PEDOT) composites on conductive NWs. The intrinsic catalytic activity of BMCs combined with outstanding electron transfer ability of conductive polymer enables the nanosensors to sensitively and selectively detect various biomolecules. Further application of sulfonated cobalt phthalocyanine functionalized nanosensors achieves real-time electrochemical monitoring of intracellular glutathione levels and its redox homeostasis in single living cells for the first time.

Versatile potentiometric metabolite sensing without dioxygen interference

The field of electrochemical biosensors has been dominated by amperometric and voltammetric sensors; however, these are limited greatly in their signal dependence on electrode size. Open circuit potentiometric sensors are emerging as an alternative due to their signal insensitivity to electrode size. Here, we present a second-generation biosensor that uses a modified chitosan hydrogel to entrap a dehydrogenase or other oxidoreductase enzyme of interest. The chitosan is modified with a desired electron mediator such that in the presence of the analyte, the enzyme will oxidize or reduce the mediator, thus altering the measured interfacial potential. Using the above design, we demonstrate a swift screening method for appropriate enzyme-mediator pairs based on open circuit potentiometry, as well as the efficacy of the biosensor design using two dehydrogenase enzymes (FADGDH and ADH) and peroxidase. Using 1,2-naphthoquinone as the mediator for FADGDH, dynamic ranges from 0.1 to 50 mM glucose are achieved. We additionally demonstrate the ease of fabrication and modification, a lifetime of ≥28 days, insensitivity to interferents, miniaturization to the microscale, and sensor efficacy in the presence of the enzyme's natural cofactor. These results forge a foundation for the generalized use of potentiometric biosensors for a wide variety of analytes within biologically-relevant systems where oxygen can be an interferent.

A subcellular cookie cutter for spatial genomics in human tissue

Intracellular heterogeneity contributes significantly to cellular physiology and, in a number of debilitating diseases, cellular pathophysiology. This is greatly influenced by distinct organelle populations and to understand the aetiology of disease, it is important to have tools able to isolate and differentially analyse organelles from precise location within tissues. Here, we report the development of a subcellular biopsy technology that facilitates the isolation of organelles, such as mitochondria, from human tissue. We compared the subcellular biopsy technology to laser capture microdissection (LCM) that is the state-of-the-art technique for the isolation of cells from their surrounding tissues. We demonstrate an operational limit of  >20 µm for LCM and then, for the first time in human tissue, show that subcellular biopsy can be used to isolate mitochondria beyond this limit.

Vesicular release dynamics are altered by the interaction between the chemical cargo and vesicle membrane lipids

The release of the cargo from soft vesicles, an essential process for chemical delivery, is mediated by multiple factors. Among them, the regulation by the interaction between the chemical cargo species and the vesicular membrane, widely existing in all vesicles, has not been investigated to date. Yet, these interactions hold the potential to complicate the release process. We used liposomes loaded with different monoamines, dopamine (DA) and serotonin (5-HT), to simulate vesicular release and to monitor the dynamics of chemical release from isolated vesicles during vesicle impact electrochemical cytometry (VIEC). The release of DA from liposomes presents a longer release time compared to 5-HT. Modelling the release time showed that DA filled vesicles had a higher percentage of events where the time for the peak fall was better fit to a double exponential (DblExp) decay function, suggesting multiple kinetic steps in the release. By fitting to a desorption-release model, where the transmitters adsorbed to the vesicle membrane, the dissociation rates of DA and 5-HT from the liposome membrane were estimated. DA has a lower desorption rate constant, which leads to slower DA release than that observed for 5-HT, whereas there is little difference in pore size. The alteration of vesicular release dynamics due to the interaction between the chemical cargo and vesicle membrane lipids provides an important mechanism to regulate vesicular release in chemical and physiological processes. It is highly possible that this introduces a fundamental chemical regulation difference between transmitters during exocytosis.

Leakless, Bipolar Reference Electrodes: Fabrication, Performance, and Miniaturization

Reference electrodes must maintain a well-defined potential for long periods of time to be useful. The silver/silver chloride (Ag/AgCl) reference electrode is arguably the most widely used reference electrode, but it leaks silver and chloride ions into the sample solution through the porous frit over time. Further, the porous frit makes miniaturization to the micro- and nanoscale challenging. Here, we present an alternative, where the traditional Ag/AgCl reference electrode porous frit is replaced by a conductive wire, preventing ion leakage and allowing miniaturization to the microscale. Charge balance is maintained through a closed bipolar electrochemical mechanism, where faradaic processes occur on each end of the sealed wire. Using the above design, we demonstrate the efficacy of the leakless, bipolar reference electrode (BPRE) and miniaturize it to the microscale (μ-leakless BPRE). Importantly, we demonstrate that leakless and μ-leakless BPREs behave the same as commercial reference electrodes during potentiometric measurements and leakless BPREs perform similarly during voltammetric measurements on ultramicroelectrodes. We demonstrate that the drift during voltammetry using a leakless BPRE on a macroelectrode is slightly more appreciable compared to the drift seen with a commercial reference electrode. We detail design principles for the use of leakless BPREs in nonaqueous solvents and in sealing other conductive materials (e.g., gold and carbon). Using mass spectrometry, we show that the maximum leakage of methylene blue is 0.36 fmol/s, at least 2 orders of magnitude smaller than that of commercial reference electrodes. Finally, we demonstrate the efficacy of using leakless BPREs in potentiometric glucose sensing.