Real-Time Intracellular Measurements of ROS and RNS in Living Cells with Single Core-Shell Nanowire Electrodes

Application and outlook of electrochemical technology in single-cell analysis

Cellular heterogeneity, especially in some important diseased cells like tumor cells, acts as an invisible driver for disease development like cancer progression in the tumor ecosystem, contributing to differences in the macroscopic and microscopic detection of disease lesions like tumors. Traditional analysis techniques choose group information masked by the mean as the analysis sample, making it difficult to achieve precise diagnosis and target treatment, on which could be shed light via the single-cell level determination/bioanalysis. Hence, in this article we have reviewed the special characteristic differences among various kinds of typical single-cell bioanalysis strategies and electrochemical techniques, and then focused on the recent advance and special bio-applications of electrochemiluminescence and micro-nano electrochemical sensing mediated in single-cell bioimaging & bioanalysis. Especially, we have summarized the relevant research exploration of the possibility to establish the in-situ single-cell electrochemical methods to detect cell heterogeneity through determination of specific biomolecules and bioimaging of some important biological processes. Eventually, this review has explored some important advances of electrochemical single-cell detection techniques for the real-time cellular bioimaging and diagnostics of some disease lesions like tumors. It raises the possibility to provide the specific in-situ platform to exploit the versatile, sensitive, and high-resolution electrochemical single-cell analysis for the promising biomedical applications like rapid tracing of some disease lesions or in vivo bioimaging for precise cancer theranostics.

Electrochemical Single-Cell Protein Therapeutics Using a Double-Barrel Nanopipette

Single-cell protein therapeutics is expected to promote our in-depth understanding of how a specific protein with a therapeutic dosage treats the cell without population averaging. However, it has not yet been tackled by current single-cell nanotools. We address this challenge by the use of a double-barrel nanopipette, in which one lumen was used for electroosmotic cytosolic protein delivery and the other was customized for ionic evaluation of the consequence. Upon injection of protein DJ-1 through the delivery lumen, upregulation of the antioxidant protein could protect neural PC-12 cells against oxidative stress from phorbol myristate acetate exposure, as deduced by targeting of the cytosolic hydrogen peroxide by the detecting lumen. The nanotool developed in this study for single-cell protein therapeutics provides a perspective for future single-cell therapeutics involving different therapeutic modalities, such as peptides, enzymes and nucleic acids.

An Electrochemical Nanosensor for Monitoring the Dynamics of Intracellular H2 O2 Upon NADH Treatment

Reactive oxygen species (ROS)-based therapeutic strategies play an important role in cancer treatment. However, in situ, real-time and quantitative analysis of intracellular ROS in cancer treatment for drug screening is still a challenge. Herein we report one selective hydrogen peroxide (H₂ O₂ ) electrochemical nanosensor, which is prepared by electrodeposition of Prussian blue (PB) and polyethylenedioxythiophene (PEDOT) onto carbon fiber nanoelectrode. With the nanosensor, we find that the level of intracellular H₂ O₂ increases with NADH treatment and that increase is dose-dependent to the concentration of NADH. High-dose of NADH (above 10 mM) can induce cell death and intratumoral injection of NADH is validated for inhibiting tumor growth in mice. This study highlights the potential of electrochemical nanosensor for tracking and understanding the role of H₂ O₂ in screening new anticancer drug.

Nano- and Microsensors for In Vivo Real-Time Electrochemical Analysis: Present and Future Perspectives

Electrochemical nano- and microsensors have been a useful tool for measuring different analytes because of their small size, sensitivity, and favorable electrochemical properties. Using such sensors, it is possible to study physiological mechanisms at the cellular, tissue, and organ levels and determine the state of health and diseases. In this review, we highlight recent advances in the application of electrochemical sensors for measuring neurotransmitters, oxygen, ascorbate, drugs, pH values, and other analytes in vivo. The evolution of electrochemical sensors is discussed, with a particular focus on the development of significant fabrication schemes. Finally, we highlight the extensive applications of electrochemical sensors in medicine and biological science.

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.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

An Integrated Photoelectrochemical Nanotool for Intracellular Drug Delivery and Evaluation of Treatment Effect

With reduced background and high sensitivity, photoelectrochemistry (PEC) may be applied as an intracellular nanotool and open a new technological direction of single-cell study. Nevertheless, the present palette of single-cell tools lacks such a PEC-oriented solution. Here a dual-functional photocathodic single-cell nanotool capable of direct electroosmotic intracellular drug delivery and evaluation of oxidative stress is devised by engineering a target-specific organic molecule/NiO/Ni film at the tip of a nanopipette. Specifically, the organic molecule probe serves simultaneously as the biorecognition element and sensitizer to synergize with p-type NiO. Upon intracellular delivery at picoliter level, the oxidative stress effect will cause structural change of the organic probe, switching its optical absorption and altering the cathodic response. This work has revealed the potential of PEC single-cell nanotool and extended the boundary of current single-cell electroanalysis.

Large-Scale Synthesis of Functionalized Nanowires to Construct Nanoelectrodes for Intracellular Sensing

A strategy for one-pot and large-scale synthesis of functionalized core-shell nanowires (NWs) to high-efficiently construct single nanowire electrodes is proposed. Based on the polymerization reaction between 3,4-ethylenedioxythiophene (EDOT) and noble metal cations, manifold noble metal nanoparticles-polyEDOT (PEDOT) nanocomposites can be uniformly modified on the surface of any nonconductive NWs. This provides a facile and versatile approach to produce massive number of core-shell NWs with excellent conductivity, adjustable size, and well-designed properties. Nanoelectrodes manufactured with such core-shell NWs exhibit excellent electrochemical performance and mechanical stability as well as favorable antifouling properties, which are demonstrated by in situ intracellular monitoring of biological molecules (nitric oxide) and unraveling its relevant unclear signaling pathway inside single living cells.

Quantitative Nano-amperometric Measurement of Intravesicular Glutamate Content and its Sub-Quantal Release by Living Neurons

Quantitative measurements of intravesicular glutamate (Glu) and of transient exocytotic release contents directly from individual living neurons are highly desired for understanding the mechanisms (full or sub-quantal release?) of synaptic transmission and plasticity. However, this could not be achieved so far due to the lack of adequate experimental strategies relying on selective and sensitive Glu nanosensors. Herein, we introduce a novel electrochemical Glu nanobiosensor based on a single SiC nanowire that can selectively measure in real-time Glu fluxes released via exocytosis by large Glu vesicles (ca. 125 nm diameter) present in single hippocampal axonal varicosities as well as their intravesicular content before exocytosis. These measurements revealed a sub-quantal release mode in living hippocampal neurons, viz., only ca. one third to one half of intravesicular Glu molecules are released by individual vesicles during exocytotic events. Importantly, this fraction remained practically the same when hippocampal neurons were pretreated with L-Glu-precursor L-glutamine, while it significantly increased after zinc treatment, although in both cases the intravesicular contents were drastically affected.

Electrochemical Quantification of Neurotransmitters in Single Live Cell Vesicles Shows Exocytosis is Predominantly Partial

Exocytosis plays an essential role in the communication between cells in the nervous system. Understanding the regulation of neurotransmitter release during exocytosis and the amount of neurotransmitter content that is stored in vesicles is of importance, as it provides fundamental insights to understand how the brain works and how neurons elicit a certain behavior. In this minireview, we summarize recent progress in amperometric measurements for monitoring exocytosis in single cells and electrochemical cytometry measurements of vesicular neurotransmitter content in individual vesicles. Important steps have increased our understanding of the different mechanisms of exocytosis. Increasing evidence is firmly establishing that partial release is the primary mechanism of release in multiple cell types.

A Robust Au-C≡C Functionalized Surface: Toward Real-Time Mapping and Accurate Quantification of Fe2+ in the Brains of Live AD Mouse Models

Described here is that Au-C≡C bonds showed the highest stability under biological conditions, with abundant thiols, and the best electrochemical performance compared to Au-S and Au-Se bonds. The new finding was also confirmed by theorical calculations. Based on this finding, a specific molecule for recognition of Fe²⁺ was designed and synthesized, and used to create a selective and accurate electrochemical sensor for the quantification of Fe²⁺ . The present ratiometric strategy demonstrates high spatial resolution for real-time tracking of Fe²⁺ in a dynamic range of 0.2-120 μM. Finally, a microelectrode array with good biocompatibility was applied in imaging and biosensing of Fe²⁺ in the different regions of live mouse brains. Using this tool, it was discovered that the uptake of extracellular Fe²⁺ into the cortex and striatum was largely mediated by cyclic adenosine monophosphate (cAMP) through the CREB-related pathway in the brain of a mouse with Alzheimer's disease.

Sensitive and Selective Measurement of Hydroxyl Radicals at Subcellular Level with Tungsten Nanoelectrodes

Hydroxyl radical (^•OH) is an essential reactive oxygen species involved in critical cell functions. However, the mechanisms controlling its subcellular localization and intracellular level during health and disease remain poorly understood. This is due to the challenge of detecting ^•OH that are highly reactive and consequently short-lived (in vivo half-life of ∼10^-9 s). Herein, we present tungsten nanoelectrodes functionalized with stable 1-hexanethiol (HAT) for selective and sensitive detection of ^•OH at the subcellular level via the destruction of the self-assembled monolayer of HAT on the nanoelectrode tip. Taking advantage of the ultrasmall nanotip and the super mechanical toughness, the tungsten nanoelectrode could easily penetrate a single living cell without inducing any observable damage. Controlled by a high precision micromanipulator, the ^•OH level in RAW 264.7 murine macrophages under amyloid β (Aβ) induced oxidative stress were first investigated by the nanoelectrodes at the subcellular level. Moreover, the results revealed the cordycepin-mediated cytoprotection of macrophages through modulation of PI3K/Akt pathway activity and introduction of heme oxygenase-1 (HO-1). We believe that the developed nanoelectrochemical method has shown great capacities for the study of potential drugs for therapeutic intervention of Alzheimer's disease.

Electrochemistry at the Synapse

Electrochemical measurements of neurotransmitters provide insight into the dynamics of neurotransmission. In this review, we describe the development of electrochemical measurements of neurotransmitters and how they started with extrasynaptic measurements but now are pushing toward synaptic measurements. Traditionally, biosensors or fast-scan cyclic voltammetry have monitored extrasynaptic levels of neurotransmitters, such as dopamine, serotonin, adenosine, glutamate, and acetylcholine. Amperometry and electrochemical cytometry techniques have revealed mechanisms of exocytosis, suggesting partial release. Advances in nanoelectrodes now allow spatially resolved, electrochemical measurements in a synapse, which is only 20-100 nm wide. Synaptic measurements of dopamine and acetylcholine have been made. In this article, electrochemical measurements are also compared to optical imaging and mass spectrometry measurements, and while these other techniques provide enhanced spatial or chemical information, electrochemistry is best at monitoring real-time neurotransmission. Future challenges include combining electrochemistry with these other techniques in order to facilitate multisite and multianalyte monitoring.

Single-cell Analysis with Microfluidic Devices

A microfluidic device as a pivotal research tool in chemistry and life science is now widely recognized. Indeed, microfluidic techniques have made significant advancements in fundamental research, such as the inherent heterogeneity of single-cells studies in cell populations, which would be helpful in understanding cellular molecular mechanisms and clinical diagnosis of major diseases. Single-cell analyses on microdevices have shown great potential for precise fluid control, cell manipulation, and signal output with rapid and high throughput. Moreover, miniaturized devices also have open functions such as integrating with traditional detection methods, for example, optical, electrochemical or mass spectrometry for single-cell analysis. In this review, we summarized recent advances of single-cell analysis based on various microfluidic approaches from different dimensions, such as in vitro, ex vivo, and in vivo analysis of single cells.

Optimization of a microchip electrophoresis method with electrochemical detection for the determination of nitrite in macrophage cells as an indicator of nitric oxide production

Nitric oxide (NO) is involved in many biological functions, including blood pressure regulation, the immune response, and neurotransmission. However, excess production of NO can lead to the generation of reactive nitrogen species and nitrosative stress and has been linked to several neurodegenerative diseases and cardiovascular disorders. Because NO is short-lived and generally difficult to detect, its primary stable degradation product, nitrite, is frequently monitored in its place. In this paper, an improved method using microchip electrophoresis with electrochemical detection (ME-EC) was developed for the separation and detection of nitrite in cell lysates. A separation of nitrite from several electroactive cell constituents and interferences was optimized, and the effect of sample and buffer conductivity on peak efficiency was explored. It was found that the addition of 10 mM NaCl to the run buffer caused stacking of the nitrite peak and improved limits of detection. A platinum black working electrode was also evaluated for the detection of nitrite and other electroactive cellular species after electrophoretic separation. The use of a modified platinum working electrode resulted in 2.5-, 1.7-, and 7.2-fold signal enhancement for nitrite, ascorbic acid, and hydrogen peroxide, respectively, and increased the sensitivity of the method for nitrite twofold. The optimized ME-EC method was used to compare nitrite production by native and lipopolysaccharide-stimulated RAW 264.7 macrophage cells.