Characterizing the catecholamine content of single mammalian vesicles by collision-adsorption events at an electrode

Spike by spike frequency analysis of amperometry traces provides statistical validation of observations in the time domain

Amperometry is a commonly used electrochemical method for studying the process of exocytosis in real-time. Given the high precision of recording that amperometry procedures offer, the volume of data generated can span over several hundreds of megabytes to a few gigabytes and therefore necessitates systematic and reproducible methods for analysis. Though the spike characteristics of amperometry traces in the time domain hold information about the dynamics of exocytosis, these biochemical signals are, more often than not, characterized by time-varying signal properties. Such signals with time-variant properties may occur at different frequencies and therefore analyzing them in the frequency domain may provide statistical validation for observations already established in the time domain. This necessitates the use of time-variant, frequency-selective signal processing methods as well, which can adeptly quantify the dominant or mean frequencies in the signal. The Fast Fourier Transform (FFT) is a well-established computational tool that is commonly used to find the frequency components of a signal buried in noise. In this work, we outline a method for spike-based frequency analysis of amperometry traces using FFT that also provides statistical validation of observations on spike characteristics in the time domain. We demonstrate the method by utilizing simulated signals and by subsequently testing it on diverse amperometry datasets generated from different experiments with various chemical stimulations. To our knowledge, this is the first fully automated open-source tool available dedicated to the analysis of spikes extracted from amperometry signals in the frequency domain.

Computer-Assisted Processing of Current Step Signals in Single Blocking Impact Electrochemistry

Current step signals related to single-entity collisions in blocking impact electrochemistry were analyzed by computer-assisted processing for estimating the size distributions of various particles. In this work, three different types of entities were studied by single blocking impact electrochemistry: polystyrene nanospheres (350 nm diameter) and microspheres (1 μm diameter), phospholipid liposomes (300 nm diameter) and two different strains of Gram-negative bacillus bacteria (Escherichia coli and Shewanella oneidensis). The size estimations of these different entities from the current step signal analysis were compared and discussed according to the shape and size of each entity. From the magnitude of the current step transient, the size distribution of each entity was calculated by a new computer program assisting in the detection and analysis of single impact events in chronoamperometry measurements. The data processing showed that the size distributions obtained from the electrochemical data agreed with the dynamic light scattering and atomic force microscopy data for nanospheres and liposomes. In contrast, the size estimation calculated from the electrochemical data was underestimated for microspheres and bacteria. We demonstrated that our computer program was efficient for detecting and analyzing the collision events in single blocking impact electrochemistry for various entities from spherical hard nanoparticles to micrometer-sized rod-shaped living bacteria.

Analyzing Fusion Pore Dynamics and Counting the Number of Acetylcholine Molecules Released by Exocytosis

Acetylcholine (ACh) is a critical neurotransmitter influencing various neurophysiological functions. Despite its significance, quantitative methods with adequate spatiotemporal resolution for recording a single exocytotic ACh efflux are lacking. In this study, we introduce an ultrafast amperometric ACh biosensor that enables 50 kHz electrochemical recording of spontaneous single exocytosis events at axon terminals of differentiated cholinergic human SH-SY5Y neuroblastoma cells with sub-millisecond temporal resolution. Characterization of the recorded amperometric traces revealed seven distinct current spike types, each displaying variations in shape, time scale, and ACh quantities released. This finding suggests that exocytotic release is governed by complex fusion pore dynamics in these cells. The absolute number of ACh molecules released during exocytosis was quantified by calibrating the sensor through the electroanalysis of liposomes preloaded with varying ACh concentrations. Notably, the largest quantal release involving approximately 8000 ACh molecules likely represents full exocytosis, while a smaller release of 5000 ACh molecules may indicate partial exocytosis. Following a local administration of bafilomycin A1, a V-ATPase inhibitor, the cholinergic cells exhibited both a larger quantity of ACh released and a higher frequency of exocytosis events. Therefore, this ACh sensor provides a means to monitor minute amounts of ACh and investigate regulatory release mechanisms at the single-cell level, which is vital for understanding healthy brain function and pathologies and optimizing drug treatment for disorders.

Monitoring the electroactive cargo of extracellular vesicles can differentiate various cancer cell lines

Extracellular vesicles (EVs) are pivotal in cell-to-cell communication due to the array of cargo contained within these vesicles. EVs are considered important biomarkers for identification of disease, however most measurement approaches have focused on monitoring specific surface macromolecular targets. Our study focuses on exploring the electroactive component present within cargo from EVs obtained from various cancer and non-cancer cell lines using a disk carbon fiber microelectrode. Variations in the presence of oxidizable components were observed when the total cargo from EVs were measured, with the highest current detected in EVs from MCF7 cells. There were differences observed in the types of oxidizable species present within EVs from MCF7 and A549 cells. Single entity measurements showed clear spikes due to the detection of oxidizable cargo within EVs from MCF7 and A549 cells. These studies highlight the promise of monitoring EVs through the presence of varying electroactive components within the cargo and can drive a wave of new strategies towards specific detection of EVs for diagnosis and prognosis of various diseases.

Impact electrochemistry for biosensing: advances and future directions

Impact electrochemistry allows for the investigation of the properties of single entities, ranging from nanoparticles (NPs) to soft bio-particles. It has introduced a novel dimension in the field of biological analysis, enhancing researchers' ability to comprehend biological heterogeneity and offering a new avenue for developing novel diagnostic devices for quantifying biological analytes. This review aims to summarize the recent advancements in impact electrochemistry-based biosensing over the past two to three years and provide insights into the future directions of this field.

Amperometry and Electron Microscopy show Stress Granules Induce Homotypic Fusion of Catecholamine Vesicles

An overreactive stress granule (SG) pathway and long-lived, stable SGs formation are thought to participate in the progress of neurodegenerative diseases (NDs). To understand if and how SGs contribute to disorders of neurotransmitter release in NDs, we examined the interaction between extracellular isolated SGs and vesicles. Amperometry shows that the vesicular content increases and dynamics of vesicle opening slow down after vesicles are treated with SGs, suggesting larger vesicles are formed. Data from transmission electron microscopy (TEM) clearly shows that a portion of large dense-core vesicles (LDCVs) with double/multiple cores appear, thus confirming that SGs induce homotypic fusion between LDCVs. This might be a protective step to help cells to survive following high oxidative stress. A hypothetical mechanism is proposed whereby enriched mRNA or protein in the shell of SGs is likely to bind intrinsically disordered protein (IDP) regions of vesicle associated membrane protein (VAMP) driving a disrupted membrane between two closely buddled vesicles to fuse with each other to form double-core vesicles. Our results show that SGs induce homotypic fusion of LDCVs, providing better understanding of how SGs intervene in pathological processes and opening a new direction to investigations of SGs involved neurodegenerative disease.

Reconstruction of Vesicle Assemblies with DNA Nanorulers for Resolving Heterogeneity of Vesicles in Live Cells

Nanoscale vesicles such as synaptic vesicles play a pivotal role in efficient interneuronal communications in vivo. However, the coexistence of single vesicle and vesicle clusters in living cells increases the heterogeneity of vesicle populations, which largely complicates the quantitative analysis of the vesicles. The high spatiotemporal monitoring of vesicle assemblies is currently incompletely resolved. Here, this work uses synthetic vesicles and DNA nanorulers to reconstruct in vitro the vesicle assemblies that mimic vesicle clusters in living cells. DNA nanorulers program the lateral distance of vesicle assemblies from 3 to 10 nm. This work uses the carbon fiber nanoelectrode (CFNE) to amperometric monitor artificial vesicle assemblies with sub-10 nm interspaces, and obtain a larger proportion of complex events. This work resolves the heterogeneity of individual vesicle release kinetics in PC12 cells with the temporal resolution down to ≈0.1 ms. This work further analyzes the aggregation state of intracellular vesicles and the exocytosis of living cells with electrochemical vesicle cytometry. The results indicate that the exocytosis of vesicle clusters is critically dependent on the size of clusters. This technology has the potential as a tool to shed light on the heterogeneity analysis of vesicle populations.

Probing conformational kinetics of catalase with and without magnetic field by single-entity collision electrochemistry

The conformational motions of enzymes are crucial for their catalytic activities, but these fluctuations are usually spontaneous and unsynchronized and thus difficult to obtain from ensemble-averaged measurements. Here, we employ label-free single-entity electrochemical measurements to monitor in real time the fluctuating enzymatic behavior of single catalase molecules toward the degradation of hydrogen peroxide. By probing the electrochemical signals of single catalase molecules at a carbon nanoelectrode, we were able to observe three distinct current traces that could be attributed to conformational changes on the sub-millisecond timescale. Whereas, nearly uniform single long peaks were observed for single catalase molecules under a moderate magnetic field due to the restricted conformational changes of catalase. By combining high-resolution current signals with a multiphysics simulation model, we studied the catalytic kinetics of catalase with and without a magnetic field, and further estimated the maximum catalytic rate and conformational transition rate. This work introduces a new complementary approach to existing single-molecule enzymology, giving further insight into the enzymatic reaction mechanism.

Observing Discrete Blocking Events at a Polarized Micro- or Submicro-Liquid/Liquid Interface

Single-entity collisional electrochemistry (SECE), a subfield of single-entity electrochemistry, enables directly characterizing entities and particles in the electrolyte solution at the single-entity resolution. Blockade SECE at the traditional solid ultramicroelectrode (UME)/electrolyte interface suffers from a limitation: only redox-inactive particles can be studied. The wide application of the classical Coulter counter is restricted by the rapid translocation of entities through the orifice, which results in a remarkable proportion of undetected signals. In response, the blocking effect of single charged conductive or insulating nanoparticles (NPs) at low concentrations for ion transfer (IT) at a miniaturized polarized liquid/liquid interface was successfully observed. Since the particles are adsorbed at the liquid/liquid interface, our method also solves the problem of the Coulter counter having a too-fast orifice translocation rate. The decreasing quantal staircase/step current transients are from landings (controlled by electromigration) of either conductive or insulating NPs onto the interface. This interfacial NP assembly shields the IT flux. The size of each NP can be calculated by the step height. The particle size measured by dynamic light scattering (DLS) is used for comparison with that calculated from electrochemical blocking events, which is in fairly good agreement. In short, the blocking effect of IT by single entities at micro- or submicro-liquid/liquid interface has been proven experimentally and is of great reference in single-entity detection.

Sensitive quantification of mercury ions in real water systems based on an aggregation-collision electrochemical detection

Nanoparticle impact electrochemistry (NIE) is an emerging electroanalytical technique that has been utilized to the sensitive detection of a wide range of biological species. So far, the NIE based trace ion detection is largely unexplored due to the lack of effective signal amplification strategies. We herein develop an NIE-based electrochemical sensing platform that utilizes T-Hg2+-T coordination induced AgNP aggregation to detect Hg2+ in aqueous solution. The proposed aggregation-collision strategy enables highly sensitive and selective detection. A dual-mode analysis based on the change in impact frequency and oxidative charge of the anodic oxidation of the AgNPs in NIE allows for more accurate self-validated quantification. Furthermore, the current NIE-based sensor demonstrates reliable analysis of Hg2+ of real water samples, showing great potential for practical environmental monitoring and point-of-care testing (POCT) applications.

Amphetamine-induced reverse transport of dopamine does not require cytosolic Ca2

Amphetamines (AMPHs) are substrates of the dopamine transporter (DAT) and reverse the direction of dopamine (DA) transport. This has been suggested to depend on activation of Ca2+-dependent pathways, but the mechanism underlying reverse transport via endogenously expressed DAT is still unclear. Here, to enable concurrent visualization by live imaging of extracellular DA dynamics and cytosolic Ca2+ levels, we employ the fluorescent Ca2+ sensor jRGECO1a expressed in cultured dopaminergic neurons together with the fluorescent DA sensor GRABDA1H expressed in cocultured "sniffer" cells. In the presence of the Na+-channel blocker tetrodotoxin to prevent exocytotic DA release, AMPH induced in the cultured neurons a profound dose-dependent efflux of DA that was blocked both by inhibition of DAT with cocaine and by inhibition of the vesicular monoamine transporter-2 with Ro-4-1284 or reserpine. However, the AMPH-induced DA efflux was not accompanied by an increase in cytosolic Ca2+ and was unaffected by blockade of voltage-gated calcium channels or chelation of cytosolic Ca2+. The independence of cytosolic Ca2+ was further supported by activation of N-methyl-D-aspartate-type ionotropic glutamate receptors leading to a marked increase in cytosolic Ca2+ without affecting AMPH-induced DA efflux. Curiously, AMPH elicited spontaneous Ca2+ spikes upon blockade of the D2 receptor, suggesting that AMPH can regulate intracellular Ca2+ in an autoreceptor-dependent manner regardless of the apparent independence of Ca2+ for AMPH-induced efflux. We conclude that AMPH-induced DA efflux in dopaminergic neurons does not require cytosolic Ca2+ but is strictly dependent on the concerted action of AMPH on both vesicular monoamine transporter-2 and DAT.

Enhanced Single-Particle Collision Electrochemistry at Polysulfide-Functionalized Microelectrodes for SARS-CoV-2 Detection

Single-particle collision electrochemistry (SPCE) has shown great promise in biosensing applications due to its high sensitivity, high flux, and fast response. However, a low effective collision frequency and a large number of interfering substances in complex matrices limit its broad application in clinical samples. Herein, a novel and universal SPCE biosensor was proposed to realize sensitive detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) based on the collision and oxidation of single silver nanoparticles (Ag NPs) on polysulfide-functionalized gold ultramicroelectrodes (Ps-Au UMEs). Taking advantage of the strong interaction of the Ag-S bond, collision and oxidation of Ag NPs on the Ps-Au UME surface could be greatly promoted to generate enhanced Faraday currents. Compared with bare Au UMEs, the collision frequency of Ps-Au UMEs was increased by 15-fold, which vastly improved the detection sensitivity and practicability of SPCE in biosensing. By combining magnetic separation, liposome encapsulation release, and DNAzyme-assisted signal amplification, the SPCE biosensor provided a dynamic range of 5 orders of magnitude for spike proteins with a detection limit of 6.78 fg/mL and a detection limit of 21 TCID50/mL for SARS-CoV-2. Furthermore, SARS-CoV-2 detection in nasopharyngeal swab samples of infected patients was successfully conducted, indicating the potential of the SPCE biosensor for use in clinically relevant diagnosis.

Single Exosome Amperometric Measurements Reveal Encapsulation of Chemical Messengers for Intercellular Communication

In multicellular organisms, cells typically communicate by sending and receiving chemical signals. Chemical messengers involved in the exocytosis of neuroendocrine cells or neurons are generally assumed to only originate from the fusing of intracellular large dense core vesicles (LDCVs) or synaptic vesicles with the cellular membrane following stimulation. Accumulated evidence suggests that exosomes─one of the main extracellular vesicles (EVs)─carrying cell-dependent DNA, mRNA, proteins, etc., play an essential role in cellular communication. Due to experimental limitations, it has been difficult to monitor the real-time release of individual exosomes; this restricts a comprehensive understanding of the basic molecular mechanisms and the functions of exosomes. In this work, we introduce amperometry with microelectrodes to capture the dynamic release of single exosomes from a single living cell, distinguish them from other EVs, and differentiate the molecules inside exosomes and those secreted from LDCVs. We show that, similar to many LDCVs and synaptic vesicles, exosomes released by neuroendocrine cells also contain catecholamine transmitters. This finding reveals a different mode of chemical communication via exosome-encapsulated chemical messengers and a potential interconnection between the two release pathways, changing the canonical view of exocytosis of neuroendocrine cells and possibly neurons. This defines a new mechanism for chemical communication at the fundamental level and opens new avenues in the research of the molecular biology of exosomes in the neuroendocrine and central nervous systems.

The Microelectrode Insulator Influences Water Nanodroplet Collisions

Studying chemical reactions in very small (attoliter to picoliter) volumes is important in understanding how chemistry proceeds at all relevant scales. Stochastic electrochemistry is a powerful tool to study the dynamics of single nanodroplets, one at a time. Perhaps the most conceptually simple experiment is that of the current blockade, where the collision of an insulating particle is observed electrochemically as a stepwise decrease in current. Here, we demonstrate that nanodroplet collisions on microelectrodes are not as simple as water droplets adsorbing to the electrode to block current and that the environment immediately around the microelectrode (glass insulator) plays a pivotal role in the electrochemical collision response. We use correlated opto-electrochemical measurements to understand a variety of electrochemical responses when water nanodroplets collide with a microelectrode during the heterogeneous oxidation of decamethylferrocene in oil. The amperometric current reports not only on current blockades but also on nanodroplet coalescence events and preferential wetting to the glass around the microelectrode. Treating the glass with dichlorodimethylsilane creates a hydrophobic environment around the working electrode, and the simple current blockade response expected from the absorption of insolating nanoparticles is observed. These results highlight the importance of the environment around the working electrode for nanodroplet collision studies.

Hofmeister Series: From Aqueous Solution of Biomolecules to Single Cells and Nanovesicles

Hofmeister effects play a critical role in numerous physicochemical and biological phenomena, including the solubility and/or accumulation of proteins, the activities of enzymes, ion transport in biochannels, the structure of lipid bilayers, and the dynamics of vesicle opening and exocytosis. This minireview focuses on how ionic specificity affects the physicochemical properties of biomolecules to regulate cellular exocytosis, vesicular content, and nanovesicle opening. We summarize recent progress in further understanding Hofmeister effects on biomacromolecules and their applications in biological systems. These important steps have increased our understanding of the Hofmeister effects on cellular exocytosis, vesicular content, and nanovesicle opening. Increasing evidence is firmly establishing that the ions along the Hofmeister series play an important role in living organisms that has often been ignored.

The dynamic nature of exocytosis from large secretory vesicles. A view from electrochemistry and imaging

In this brief review, we discuss the factors that modulate the quantum size and the kinetics of exocytosis. We also discuss the determinants which motivate the type of exocytosis from the so-called kiss-and-run to full fusion and along the intermediate mode of partial release. Kiss-and-run release comprises the transient opening of a nanometer (approx. 2 nm diameter) fusion pore between vesicle and plasma membrane allowing a small amount of release. Partial release comprises a larger more extended opening of the pore to allow a larger fraction of released vesicle content and is what is observed as normal full release in most electrochemical measurements. Partial release appears to be dominant in dense core vesicles and perhaps synaptic vesicles. The concept of partial release leads to the fraction released as a plastic component of exocytosis. Partial vesicular distension and the kinetics of exocytosis can be modulated by second messengers, physiological modulators, and drugs. This concept adds a novel point of regulation for the exocytotic process.

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.

Homogeneous Electrochemical Immunoassay Using an Aggregation-Collision Strategy for Alpha-Fetoprotein Detection

Homogeneous immunoassays represent an attractive alternative to traditional heterogeneous assays due to their simplicity and high efficiency. Homogeneous electrochemical assays, however, are not commonly accessed due to the requirement of electrode immobilization of the recognition elements. Herein, we demonstrate a new homogeneous electrochemical immunoassay based on the aggregation-collision strategy for the quantification of tumor protein biomarker alpha-fetoprotein (AFP). The detection principle relies on the aggregation of AgNPs induced by the molecular biorecognition between AFP and AgNPs-anti-AFP probes, which leads to an increased AgNP size and decreased AgNP concentration, allowing an accurate self-validated dual-mode immunoassay by performing nanoimpact electrochemistry (NIE) of the oxidation of AgNPs. The intrinsic one-by-one analytical capability of NIE as well as the participation of all of the atoms of the AgNPs in signal transduction greatly elevates the detection sensitivity. Accordingly, the current sensor enables a limit of detection (LOD) of 5 pg/mL for AFP analysis with high specificity and efficiency. More importantly, reliable detection of AFP in diluted human sera of hepatocellular carcinoma (HCC) patients is successfully achieved, indicating that the NIE-based homogeneous immunoassay shows great potential in HCC liquid biopsy.

Calcium-dependent subquantal peptide release from single docked lawn-resident vesicles of pituitary lactotrophs

Regulated exocytosis consists of the fusion between vesicles and the plasma membranes, leading to the formation of a narrow fusion pore through which secretions exit the vesicle lumen into the extracellular space. An increase in the cytosolic concentration of free Ca²⁺ ([Ca²⁺]_i) is considered the stimulus of this process. However, whether this mechanism can be preserved in a simplified system of membrane lawns with docked secretory vesicles, devoid of cellular components, is poorly understood. Here, we studied peptide discharge from individual secretory vesicles docked at the plasma membrane, prepared from primary endocrine pituitary cells (the lactotrophs), releasing hormone prolactin. To label secretory vesicles, we transfected lactotrophs to express the fluorescent atrial natriuretic peptide (ANP.emd), previously shown to be expressed in and released from prolactin-containing vesicles. We used stimulating solutions containing different [Ca²⁺] to evoke vesicle peptide discharge, which appeared similar in membrane lawns and in intact stimulated lactotrophs. All vesicles examined discharged peptides in a subquantal manner, either exhibiting a unitary or sequential time course. In the membrane lawns, the unitary vesicle peptide discharge was predominant and slightly slower than that recorded in intact cells, but with a shorter delay with respect to the stimulation onset. This study revealed directly that Ca²⁺ triggers peptide discharge from docked single vesicles in the membrane lawns with a half-maximal response of ∼8 µM [Ca²⁺], consistent with previous whole-cell patch-clamp studies in endocrine cells where the rapid component of exocytosis, interpreted to represent docked vesicles, was fully activated at <10 µM [Ca²⁺]. Interestingly, the sequential subquantal peptide vesicle discharge indicates that fluctuations between constricted and dilated fusion pore states are preserved in membrane lawns and that fusion pore regulation appears to be an autonomously controlled process.

Clinically Applicable Homogeneous Assay for Serological Diagnosis of Alpha-Fetoprotein by Impact Electrochemistry

Tumor protein quantification with high specificity, sensitivity, and efficiency is of great significance to enable early diagnosis and effective treatment. The existing methods for protein analysis usually suffer from high cost, time-consuming operation, and insufficient sensitivity, making them not clinically friendly. In this work, a label-free homogeneous sensor based on the nano-impact electroanalytic (NIE) technique was proposed for the detection of tumor protein marker alpha-fetoprotein (AFP). The detection principle is based on the recovery of current of single PtNP catalyzed hydrazine oxidation due to the release of the pre-adsorbed passivating aptamers on PtNPs from the competition of the stronger binding between the specific interaction of the AFP aptamer and AFP. The intrinsic one-by-one analytical ability of NIE allows highly sensitive detection, which can be further improved by reducing the reaction/incubation volume. Meanwhile, the current sensor avoids a laborious labeling procedure as well as the separation and washing steps due to the in situ characteristic of NIE. Accordingly, the current sensor enables efficient, highly sensitive, and specific AFP analysis. More importantly, the reliable detection of AFP in diluted real sera from hepatocellular carcinoma (HCC) patients is successfully achieved, indicating that the impact electrochemistry-based sensing platform has great potential to be applied in point-of-care devices for HCC liquid biopsy.

Rapid Screening of Bimetallic Electrocatalysts Using Single Nanoparticle Collision Electrochemistry

The pace of nanomaterial discovery for high-performance electrocatalysts could be accelerated by the development of efficient screening methods. However, conventional electrochemical characterization via drop-casting is inherently inaccurate and time-consuming, as such ensemble measurements are serially performed through nanocatalyst synthesis, morphological characterization, and performance testing. Herein, we propose a rapid electrochemical screening method for bimetallic electrocatalysts that combines nanoparticle (NP) preparation and performance testing at the single NP level, thus avoiding any inhomogeneous averaging contribution. We employed single NP collision electrochemistry to realize in situ electrodeposition of a precisely tunable Pt shell onto individual parent NPs, followed by instantaneous electrocatalytic measurement of the newborn bimetallic core-shell NPs. We demonstrated the utility of this approach by screening bimetallic Au-Pt NPs and Ag-Pt NPs, thereby exhibiting promising electrocatalytic activity at optimal atomic ratios for methanol oxidation and oxygen reduction reactions, respectively. This work provides a new insight for the rapid screening of other bimetallic electrocatalysts.

Pore-Opening Dynamics of Single Nanometer Biovesicles at an Electrified Interface

Release from nanobiovesicles via a pore generated by membrane electroporation at an electrified interface can be monitored by vesicle impact electrochemical cytometry (VIEC) and provides rich information about the various vesicular content transfer processes, including content homeostasis, intraphase content transfer, or the transient fusion of vesicles. These processes are primarily influenced by the vesicular pore-opening dynamics at the electrified interface which has not been disclosed at the single nanobiovesicle level yet. In this work, after simultaneously measuring the size and release dynamics of individual vesicles, we employed a moving mesh-finite element simulation algorithm to reconstruct the accurate pore-opening dynamics of individual vesicles with different sizes during VIEC. We investigated the expansion times and maximal pore sizes as two characteristics of different vesicles. The pore expansion times between nanobiovesicles and pure lipid liposomes were compared, and that of the nanobiovesicles is much longer than that for the liposomes, 2.1 ms vs 0.18 ms, respectively, which reflects the membrane proteins limiting the electroporation process. For the vesicles with different sizes, a positive relationship of pore size (R_p,max) with the vesicle size (R_ves) and also their ratio (R_p,max/R_ves) versus the vesicle sizes is observed. The mechanism of the pore size determination is discussed and related to the membrane proteins and the vesicle size. This work accurately describes the dynamic pore-opening process of individual vesicles which discloses the heterogeneity in electroporation of different sized vesicles. This should allow us to examine the more complicated vesicular content transfer process between intravesicular compartments.

Chemical Imaging and Analysis of Single Nerve Cells by Secondary Ion Mass Spectrometry Imaging and Cellular Electrochemistry

A nerve cell is a unit of neuronal communication in the nervous system and is a heterogeneous molecular structure, which is highly mediated to accommodate cellular functions. Understanding the complex regulatory mechanisms of neural communication at the single cell level requires analytical techniques with high sensitivity, specificity, and spatial resolution. Challenging technologies for chemical imaging and analysis of nerve cells will be described in this review. Secondary ion mass spectrometry (SIMS) allows for non-targeted and targeted molecular imaging of nerve cells and synapses at subcellular resolution. Cellular electrochemistry is well-suited for quantifying the amount of reactive chemicals released from living nerve cells. These techniques will also be discussed regarding multimodal imaging approaches that have recently been shown to be advantageous for the understanding of structural and functional relationships in the nervous system. This review aims to provide an insight into the strengths, limitations, and potentials of these technologies for synaptic and neuronal analyses.

Simultaneous Counting of Molecules in the Halo and Dense-Core of Nanovesicles by Regulating Dynamics of Vesicle Opening

We report the discovery that in the presence of chaotropic anions (SCN- ) the opening of nanometer biological vesicles at an electrified interface often becomes a two-step process (around 30 % doublet peaks). We have then used this to independently count molecules in each subvesicular compartment, the halo and protein dense-core, and the fraction of catecholamine binding to the dense-core is 68 %. Moreover, we differentiated two distinct populations of large dense-core vesicles (LDCVs) and quantified their content, which might correspond to immature (43 %) and mature (30 %) LDCVs, to reveal differences in their biogenesis. We speculate this is caused by an increase in the electrostatic attraction between protonated catecholamine and the negatively charged dense-core following adsorption of SCN- .

Visualization of Partial Exocytotic Content Release and Chemical Transport into Nanovesicles in Cells

For decades, "all-or-none" and "kiss-and-run" were thought to be the only major exocytotic release modes in cell-to-cell communication, while the significance of partial release has not yet been widely recognized and accepted owing to the lack of direct evidence for exocytotic partial release. Correlative imaging with transmission electron microscopy and NanoSIMS imaging and a dual stable isotope labeling approach was used to study the cargo status of vesicles before and after exocytosis; demonstrating a measurable loss of transmitter in individual vesicles following stimulation due to partial release. Model secretory cells were incubated with ¹³C-labeled l-3,4-dihydroxyphenylalanine, resulting in the loading of ¹³C-labeled dopamine into their vesicles. A second label, di-N-desethylamiodarone, having the stable isotope ¹²⁷I, was introduced during stimulation. A significant drop in the level of ¹³C-labeled dopamine and a reduction in vesicle size, with an increasing level of ¹²⁷I^-, was observed in vesicles of stimulated cells. Colocalization of ¹³C and ¹²⁷I^- in several vesicles was observed after stimulation. Thus, chemical visualization shows transient opening of vesicles to the exterior of the cell without full release the dopamine cargo. We present a direct calculation for the fraction of neurotransmitter release from combined imaging data. The average vesicular release is 60% of the total catecholamine. An important observation is that extracellular molecules can be introduced to cells during the partial exocytotic release process. This nonendocytic transport process appears to be a general route of entry that might be exploited pharmacologically.

Dynamic Visualization and Quantification of Single Vesicle Opening and Content by Coupling Vesicle Impact Electrochemical Cytometry with Confocal Microscopy

In this work, we introduce a novel method for visualization and quantitative measurement of the vesicle opening process by correlation of vesicle impact electrochemical cytometry (VIEC) with confocal microscopy. We have used a fluorophore conjugated to lipids to label the vesicle membrane and manipulate the membrane properties, which appears to make the membrane more susceptible to electroporation. The neurotransmitters inside the vesicles were visualized by use of a fluorescence false neurotransmitter 511 (FFN 511) through accumulation inside the vesicle via the neuronal vesicular monoamine transporter 2 (VMAT 2). Optical and electrochemical measurements of single vesicle electroporation were carried out using an in-house, disk-shaped, gold-modified ITO (Au/ITO) microelectrode device (5 nm thick, 33 μm diameter), which simultaneously acted as an electrode surface for VIEC and an optically transparent surface for confocal microscopy. As a result, the processes of adsorption, electroporation, and opening of single vesicles followed by neurotransmitter release on the Au/ITO surface have been simultaneously visualized and measured. Three opening patterns of single isolated vesicles were frequently observed. Comparing the vesicle opening patterns with their corresponding VIEC spikes, we propose that the behavior of the vesicular membrane on the electrode surface, including the adsorption time, residence time before vesicle opening, and the retention time after vesicle opening, are closely related to the vesicle content and size. Large vesicles with high content tend to adsorb to the electrode faster with higher frequency, followed by a shorter residence time before releasing their content, and their membrane remains on the electrode surface longer compared to the small vesicles with low content. With this approach, we start to unravel the vesicle opening process and to examine the fundamentals of exocytosis, supporting the proposed mechanism of partial or subquantal release in exocytosis.

Detection of Bacterial Rhamnolipid Toxin by Redox Liposome Single Impact Electrochemistry

The detection of Rhamnolipid virulence factor produced by Pseudomonas aeruginosa involved in nosocomial infections is reported by using the redox liposome single impact electrochemistry. Redox liposomes based on 1,2-dimyristoyl-sn-glycero-3-phosphocholine as a pure phospholipid and potassium ferrocyanide as an encapsulated redox content are designed for using the interaction of the target toxin with the lipid membrane as a sensing strategy. The electrochemical sensing principle is based on the weakening of the liposomes lipid membrane upon interaction with Rhamnolipid toxin which leads upon impact at an ultramicroelectrode to the breakdown of the liposomes and the release/electrolysis of its encapsulated redox probe. We present as a proof of concept the sensitive and fast sensing of a submicromolar concentration of Rhamnolipid which is detected after less than 30 minutes of incubation with the liposomes, by the appearing of current spikes in the chronoamperometry measurement.

Quantifying Intracellular Single Vesicular Catecholamine Concentration with Open Carbon Nanopipettes to Unveil the Effect of L-DOPA on Vesicular Structure

Understanding the regulatory mechanisms of exocytosis is essential for uncovering the pathologies of neuronal disorders and developing related pharmaceuticals. In this work intracellular vesicle impact electrochemical cytometry (IVIEC) measurements with different‐sized (50–500 nm radius) open carbon nanopipettes (CNPs) were performed to quantify the vesicular content and release kinetics of specific vesicle populations grouped by orifice sizes. Intracellular vesicles with radius below 100 nm were captured and narrowed between 50 and 100 nm. On the basis of this, single vesicular catecholamine concentrations in the intracellular environment were quantified as 0.23–1.1 M. Our results with L‐3,4‐dihydroxyphenylalanine (L‐DOPA)‐exposure indicate that L‐DOPA regulates exocytosis by increasing the dense core size and vesicular content while catecholamine concentrations did not show obvious alterations. These were all achieved simultaneously and relatively noninvasively with open CNPs.

Vesicle Impact Electrochemical Cytometry to Determine Carbon Nanotube-Induced Fusion of Intracellular Vesicles

Carbon nanotube (CNT)-modified electrodes are used to obtain new measurements of vesicle content via amperometry. We have investigated the interaction between CNTs and isolated adrenal chromaffin vesicles (as a model) by vesicle impact electrochemical cytometry. Our data show that the presence of CNTs not only significantly increased the vesicular catecholamine number from 2,250,000 ± 112,766 molecules on a bare electrode to 3,880,000 ± 686,573 molecules on CNT/carbon fiber electrodes but also caused an enhancement in the maximum intensity of the current, which implies the existence of strong interactions between vesicle biolayers and CNTs and an altered electroporation process. We suggest that CNTs might perturb and destabilize the membrane structure of intracellular vesicles and cause the aggregation or fusion of vesicles into new vesicles with larger size and higher content. Our findings are consistent with previous computational and experimental results and support the hypothesis that CNTs as a mediator can rearrange the phospholipid bilayer membrane and trigger homotypic fusion of intracellular vesicles.

Intestinal microbiota modulates adrenomedullary response through Nod1 sensing in chromaffin cells

The intestinal microbiota closely interacts with the neuroendocrine system and exerts profound effects on host physiology. Here, we report that nucleotide-binding oligomerization domain 1 (Nod1) ligand derived from intestinal bacteria modulates catecholamine storage and secretion in mouse adrenal chromaffin cells. The cytosolic peptidoglycan receptor Nod1 is involved in chromogranin A (Chga) retention in dense core granules (DCGs) in chromaffin cells. Mechanistically, upon recognizing its ligand, Nod1 localizes to DCGs, and recruits Rab2a, which is critical for Chga and epinephrine retention in DCGs. Depletion of Nod1 ligand or deficiency of Nod1 leads to a profound defect in epinephrine storage in chromaffin cells and subsequently less secretion upon stimulation. The intestine-adrenal medulla cross talk bridged by Nod1 ligand modulates adrenal medullary responses during the immobilization-induced stress response in mice. Thus, our study uncovers a mechanism by which intestinal microbes modulate epinephrine secretion in response to stress, which may provide further understanding of the gut-brain axis.

Artificial Synapse: Spatiotemporal Heterogeneities in Dopamine Electrochemistry at a Carbon Fiber Ultramicroelectrode

An artificial synapse is developed that mimics ultramicroelectrode (UME) amperometric detection of single cell exocytosis. It comprises the nanopipette of a scanning ion conductance microscope (SICM), which delivers rapid pulses of neurotransmitter (dopamine) locally and on demand at >1000 defined locations of a carbon fiber (CF) UME in each experiment. Analysis of the resulting UME current-space-time data reveals spatiotemporal heterogeneous electrode activity on the nanoscale and submillisecond time scale for dopamine electrooxidation at typical UME detection potentials. Through complementary surface charge mapping and finite element method (FEM) simulations, these previously unseen variations in electrochemical activity are related to heterogeneities in the surface chemistry of the CF UME.

Dopamine in Parkinson's disease

Parkinson's disease is a neurodegenerative disease caused by the death of neurons, ie, cells critical to the production of dopamine, an important neurotransmitter in the brain. Here, we present a brief review of the dopamine synthetic pathway, binding to the dopamine receptors, and subsequent action. The production of dopamine (a monoamine neurotransmitter) occurs in the ventral tegmental area (VTA) of the substantia nigra, specifically in the hypothalamic nucleus and midbrain. Compared to other monoamines, dopamine is widely distributed in the olfactory bulb, midbrain substantia nigra, hypothalamus, VTA, retina, and the periaqueductal gray area. Dopamine receptors are large G-protein coupled receptor family members, of which there are five subtypes including D1, D2, D3, D4, and D5. These subtypes are further divided into two subclasses: D1-like family receptors (types 1 and 5) and D2-like family receptors (types 2, 3, and 4). Four different pathways and functions of the dopaminergic system are presented in this review. In the oxidation of dopamine, 5,6-indolequinone, dopamine-o-quinone, and aminochrome are formed. It is difficult to separate the roles of 5,6-indolequinone and dopamine-o-quinone in the degenerative process of Parkinson's diseases due to their instability. The role of aminochrome in Parkinson's disease is to form and stabilize the neurotoxic protofibrils of alpha-synuclein, mitochondrial dysfunction, oxidative stress, and the degradation of protein by lysosomal systems and proteasomes. The neurotoxic effects of aminochrome can be inhibited by preventing the polymerization of 5,6-indolequinone, dopamine-o-quinone, and aminochrome into neuromelanin, by reducing aminochrome catalysis by DT-diaphorase, and by preventing dopamine oxidative deamination catalyzed by monoamine oxidase. In addition to these, the conversion of dopamine in the neuromelanin (NM) shows both protective and toxic roles. Therefore, the aims of this review were to discuss and explain the role of dopamine and explore its physiology and specificity in Parkinson's disease, as well as its role in other physiological functions.

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.

Electrochemical Measurements Reveal Reactive Oxygen Species in Stress Granules*

Stress granules (SGs) are membrane-less organelles that assemble in the cytoplasm to organize cellular contents and promote rapid adaptation during stress. To understand how SGs contribute to physiological functions, we used electrochemical measurements to detect electroactive species in SGs. With amperometry, we discovered that reactive oxygen species (ROS) are encapsulated inside arsenite-induced SGs, and H2 O2 is the main species. The release kinetics of H2 O2 from single SGs and the number of H2 O2 molecules were quantified. The discovery that SGs contain ROS implicates them as communicators of the cellular stresses rather than a simple endpoint. This may explain how SGs regulate cellular metabolism and stress responses. This may also help better understand their cytoprotective functions in pathological conditions associated with SGs such as neurodegenerative diseases (NDs), cancers and viral infections.

Rapid and Accurate Data Processing for Silver Nanoparticle Oxidation in Nano-Impact Electrochemistry

In recent years, nano-impact electrochemistry (NIE) has attracted widespread attention as a new electroanalytical approach for the analysis and characterization of single nanoparticles in solution. The accurate analysis of the large volume of the experimental data is of great significance in improving the reliability of this method. Unfortunately, the commonly used data analysis approaches, mainly based on manual processing, are often time-consuming and subjective. Herein, we propose a spike detection algorithm for automatically processing the data from the direct oxidation of sliver nanoparticles (AgNPs) in NIE experiments, including baseline extraction, spike identification and spike area integration. The resulting size distribution of AgNPs is found to agree very well with that from transmission electron microscopy (TEM), showing that the current algorithm is promising for automated analysis of NIE data with high efficiency and accuracy.

Faradaic Counter for Liposomes Loaded with Potassium, Sodium Ions, or Protonated Dopamine

Collisional electrochemistry between single particles and a biomimetic polarized micro-liquid/liquid interface has emerged as a novel and powerful analytical method for measurements of single particles. Using this platform, rapid detection of liposomes at the single particle level is reported herein. Individual potassium, sodium, or protonated dopamine-encapsulated (pristine or protein-decorated) liposomes collide and fuse with the polarized micro-liquid/liquid interface accompanying the release of ions, which are recorded as spike-like current transients of stochastic nature. The sizing and concentration of the liposomes can be readily estimated by quantifying the amount of encapsulated ions in individual liposomes via integrating each current spike versus time and the spike frequency, respectively. We call this type of nanosensing technology "Faradaic counter". The estimated liposome size distribution by this method is in line with the dynamic light scattering (DLS) measurements, implying that the quantized current spikes are indeed caused by the collisions of individual liposomes. The reported electrochemical sensing technology may become a viable alternative to DLS and other commercial nanoparticle analysis systems, for example, nanoparticle tracking analysis.

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.

Membrane Tension Modifies Redox Loading and Release in Single Liposome Electroanalysis

Here, we present a study of how liposomes are loaded and release their contents during their electrochemical detection. We loaded 200 nm liposomes with a redox mediator, ferrocyanide, and used amperometry to detect their collision on a carbon-fiber microelectrode (CFE). We found that we could control the favorability of their electroporation process and the amount of ferrocyanide released by modifying the osmolarity of the buffer in which the liposomes were suspended. Interestingly, we observed that the quantity of the released ferrocyanide varied significantly with buffer osmolarity in a nonmonotonic fashion. Using stimulated Raman scattering (SRS), we confirmed that this behavior was partly explained by fluctuations in the intravesicular redox concentration in response to osmotic pressure. To our surprise, the redox concentration obtained from SRS was much greater than that obtained from amperometry, implying that liposomes may release only a fraction of their contents during electroporation. Consistent with this hypothesis, we observed barrages of electrochemical signals that far exceeded the frequency predicted by Poisson statistics, suggesting that single liposomes can collide with the CFE and electroporate multiple times. With this study, we have resolved some outstanding questions surrounding electrochemical detection of liposomes while extending observations from giant unilamellar vesicles to 200 nm liposomes with high temporal resolution and sensitivity.

Recent Progress in Quantitatively Monitoring Vesicular Neurotransmitter Release and Storage With Micro/Nanoelectrodes

Exocytosis is one of the essential steps for chemical signal transmission between neurons. In this process, vesicles dock and fuse with the plasma membrane and release the stored neurotransmitters through fusion pores into the extracellular space, and all of these steps are governed with various molecules, such as proteins, ions, and even lipids. Quantitatively monitoring vesicular neurotransmitter release in exocytosis and initial neurotransmitter storage in individual vesicles is significant for the study of chemical signal transmission of the central nervous system (CNS) and neurological diseases. Electrochemistry with micro/nanoelectrodes exhibits great spatial-temporal resolution and high sensitivity. It can be used to examine the exocytotic kinetics from the aspect of neurotransmitters and quantify the neurotransmitter storage in individual vesicles. In this review, we first introduce the recent advances of single-cell amperometry (SCA) and the nanoscale interface between two immiscible electrolyte solutions (nanoITIES), which can monitor the quantity and release the kinetics of electrochemically and non-electrochemically active neurotransmitters, respectively. Then, the development and application of the vesicle impact electrochemical cytometry (VIEC) and intracellular vesicle impact electrochemical cytometry (IVIEC) and their combination with other advanced techniques can further explain the mechanism of neurotransmitter storage in vesicles before exocytosis. It has been proved that these electrochemical techniques have great potential in the field of neuroscience.