The double life of conductive nanopipette: a nanopore and an electrochemical nanosensor

A single-microbe living bioelectronic sensor for intracellular amperometric analysis

Subcellularly amperometric analysis in situ is crucial for understanding intracellular redox biochemistry and subcellular heterogeneity. Unfortunately, the ultra-small size and complex microenvironment inside the cell pose a great challenge to achieve this goal. To address the challenge, a minimized living microbial sensor has been fabricated in this work for amperometric analysis. Here, by fabricating the dimidiate microelectrode as the working electrode, while fitting a living electroactive bacterium (EAB) as the transducer, outward extracellular electron transfer (EET) of the sensory EAB is correlated with the concentration of lactic acid, which is electrochemically recorded and thus displays an electrical signal output for detection. In specific, the S. oneidensis modified dimidiate microelectrode (S.O.@GNE-NPE) acts as an integrated electroanalytical device to generate the electrical signal in situ. The established microcircuit provides unprecedented precision and sensitivity, contributing to subcellular amperometric measurement. The microbial sensor shows a linear response in the concentration range of 0-60 mM, with a limit of detection (LOD) at 0.3 mM. The microsensor also demonstrates good selectivity against interferences. Additionally, intracellular analysis of lactic acid provides direct evidence of enhanced lactic metabolism in cancer cells as a result of "Warburg Effect". This work shows an example of nano-, bio- and electric technologies that have been integrated on the EAB-modified dimidiate microelectrode, and achieves intracellular biosensing application through such integration. It may give a new strategy on the combination of micro/nanotechnologies with sensory EAB for the necessary development of bioelectronic devices.

Nanopipette dynamic microscopy unveils nano coffee ring

Liquid-phase electron microscopy (LP-EM) imaging has revolutionized our understanding of nanosynthesis and assembly. However, the current closed geometry limits its application for open systems. The ubiquitous physical process of the coffee-ring phenomenon that underpins materials and engineering science remains elusive at the nanoscale due to the lack of experimental tools. We introduce a quartz nanopipette liquid cell with a tunable dimension that requires only standard microscopes. Depending on the imaging condition, the open geometry of the nanopipette allows the imaging of evaporation-induced pattern formation, but it can also function as an ordinary closed-geometry liquid cell where evaporation is negligible despite the nano opening. The nano coffee-ring phenomenon was observed by tracking individual nanoparticles in an evaporating nanodroplet created from a thin liquid film by interfacial instability. Nanoflows drive the assembly and disruption of a ring pattern with the absence of particle-particle correlations. With surface effects, nanoflows override thermal fluctuations at tens of nanometers, in which nanoparticles displayed a "drunken man trajectory" and performed work at a value much smaller than kBT.

An Electrochemical Perspective on Reaction Acceleration in Microdroplets

Analytical techniques operating at the nanoscale introduce confinement as a tool at our disposal. This review delves into the phenomenon of accelerated reactivity within micro- and nanodroplets. A decade of accelerated reactivity observations was succeeded by several years of fundamental studies aimed at mechanistic enlightenment. Herein, we provide a brief historical context for rate enhancement in and around micro- and nanodroplets and summarize the mechanisms that have been proposed to contribute to such extraordinary reactivity. We highlight recent electrochemical reports that make use of restricted mass transfer to enhance electrochemical reactions and/or quantitatively measure reaction rates within droplet-confined electrochemical cells. A comprehensive approach to nanodroplet reactivity is paramount to understanding how nature takes advantage of these systems to provide life on Earth and, in turn, how to harness the full potential of such systems.

High-Resolution and Dynamic Visualization of Intracellular Redox Potential Using a Metal-Organic Framework-Functionalized Nanopotentiometer

Redox potential plays a key role in regulating intracellular signaling pathways, with its quantitative analysis in individual cells benefiting our understanding of the underlying mechanism in the pathophysiological events. Here, a metal organic framework (MOF)-functionalized SERS nanopotentiometer has been developed for the dynamic monitoring of intracellular redox potential. The approach is based on the encapsulation of zirconium-based MOF (Uio-66-F4) on a surface of gold-silver nanorods (Au-Ag NRs) that is modified with the newly synthesized redox-sensitive probe ortho-mercaptohydroquinone (HQ). Thanks to size exclusion of MOF as the chemical protector, the nanopotentiometer can be adapted to long-term use and possess high anti-interference ability toward nonredox species. Combining the superior fingerprint identification of SERS with the electrochemical activity of the quinone/hydroquinone, the nanopotentiometer shows a reversible redox responsivity and can quantify redox potential with a relatively wide range of -250-100 mV. Furthermore, the nanopotentiometer allows for dynamic visualization of intracellular redox potential changes induced by drugs' stimulation in a high-resolution manner. The developed approach would be promising for offering new insights into the correlation between redox potential and tumor proliferation-involved processes such as oxidative stress and hypoxia.

Multiphase Chemistry under Nanoconfinement: An Electrochemical Perspective

Most relevant systems of interest to modern chemists rarely consist of a single phase. Real-world problems that require a rigorous understanding of chemical reactivity in multiple phases include the development of wearable and implantable biosensors, efficient fuel cells, single cell metabolic characterization techniques, and solar energy conversion devices. Within all of these systems, confinement effects at the nanoscale influence the chemical reaction coordinate. Thus, a fundamental understanding of the nanoconfinement effects of chemistry in multiphase environments is paramount. Electrochemistry is inherently a multiphase measurement tool reporting on a charged species traversing a phase boundary. Over the past 50 years, electrochemistry has witnessed astounding growth. Subpicoampere current measurements are routine, as is the study of single molecules and nanoparticles. This Perspective focuses on three nanoelectrochemical techniques to study multiphase chemistry under nanoconfinement: stochastic collision electrochemistry, single nanodroplet electrochemistry, and nanopore electrochemistry.

Thermoelectric Nanofluidics Probing Thermal Heterogeneity inside Single Cells

Accurate temperature measurement in one living cell is of great significance for understanding biological functions and regulation. Here, a nanopipet electric thermometer (NET) is established for real-time intracellular temperature measurement. Based on the temperature-controlled ion migration, the temperature change in solution results in altered ion mobilities and ion distributions, which can be converted to the thermoelectric responses of NET in a galvanostatic configuration. The exponential relationship between the voltage and the temperature promises highly sensitive thermoelectric responses up to 11.1 mV K-1, which is over an order of magnitude higher than previous thermoelectric thermometry. Moreover, the NET exhibits superior thermal resolution of 25 mK and spatiotemporal resolution of 100 nm and 0.9 ms as well as excellent stability and reproducibility. Benefiting from these unique features, both thermal fluctuations in steady-state cells and heat generation and dissipation upon drug administration can be successfully monitored, which are hardly achieved by current methods. By using NET, thermal heterogeneities of single cancer cells during immunotherapy were reported first in this work, in which the increased intracellular temperature was demonstrated to be associated with the survival benefit and resistance of cancer cells in immunotherapy. This work not only provides a reliable method for microscopic temperature monitoring but also gains new insights to elucidate the mechanism of immune evasion and therapeutic resistance.

Molecular Electrocatalytic Processes in Carbon Nanopipettes

Conductive nanopipettes have been recognized as powerful multifunctional platforms for electrochemical sensing applications in confined spaces. However, the electron-transfer processes of many biological analytes (i.e., enzymes or proteins) are slow and coupled with chemical reactions, which have not been well elucidated in conductive nanopipettes. In this Letter, both experimental and simulation methods are used to study electron-transfer processes coupled to chemical reactions (EC mechanism) in carbon nanopipettes (CNPs). It is demonstrated that the electroactive species can serve as redox mediator to help oxidize and reduce the nonelectroactive analytes of interest in the solution and produce noticeable catalytic current signals. Besides, glutathione was directly measured by using ferrocenemethanol as the redox mediator in the CNPs. The elucidated EC processes in CNPs would offer a new opportunity to measure nonelectroactive analytes in biological fields.

The Modification and Applications of Nanopipettes in Electrochemical Analysis

Nanopipette, which is fabricated by glasses and possesses a nanoscale pore in the tip, has been proven to be immensely useful in electrochemical analysis. Numerous nanopipette-based sensors have emerged with improved sensitivity, selectivity, ease of use, and miniaturization. In this minireview, we provide an overview of the recent developments of nanopipette-based electrochemical sensors based on different types of nanopipettes, including single-nanopipettes, self-referenced nanopipettes, dual-nanopipettes, and double-barrel nanopipettes. Several important modification materials for nanopipette functionalization are highlighted, such as conductive materials, macromolecular materials, and functional molecules. These materials can improve the sensing performance and targeting specificities of nanopipettes. We also discuss examples of related applications and the future development of nanopipette-based strategies.

Simultaneous observation of the spatial and temporal dynamics of single enzymatic catalysis using a solid-state nanopore

We developed a bipolar SiNx nanopore for the observation of single-molecule heterogeneous enzymatic dynamics. Single glucose oxidase was immobilized inside the nanopore and its electrocatalytic behaviour was real-time monitored via continuous recording of ionic flux amplification. The temporal heterogeneity in enzymatic properties and its spatial dynamic orientations were observed simultaneously, and these two properties were found to be closely correlated. We anticipate that this method offers new perspectives on the correlation of protein structure and function at the single-molecule level.

Current oscillations from bipolar nanopores for statistical monitoring of hydrogen evolution on a confined electrochemical catalyst

Taking advantage of bipolar electrochemistry and a glass nanopipette, continuous single bubbles can be controlled which are generated and detached from a nanometer-sized area of confined electrochemical catalysts. The observed current oscillations offer opportunities to rapidly collect data for the statistical analysis of single-bubble generation on and departure from the catalysts.

Mass Transport and Electron Transfer at the Electrochemical-Confined Interface

Single entity measurements based on the stochastic collision electrochemistry provide a promising and versatile means to study single molecules, single particles, single droplets, etc. Conceptually, mass transport and electron transfer are the two main processes at the electrochemically confined interface that underpin the most transient electrochemical responses resulting from the stochastic and discrete behaviors of single entities at the microscopic scale. This perspective demonstrates how to achieve controllable stochastic collision electrochemistry by effectively altering the two processes. Future challenges and opportunities for stochastic collision electrochemistry are also highlighted.

Multivalent Ion-Modulated Electron Transfer Processes in Carbon Nanopipettes

Conductive nanopipettes with both an electroactive interface and a pipet geometry have been recognized as powerful multifunctional probes in various electrochemical sensing and imaging applications. As confined inside the nanopipette, the excess surface charges at the solid/solution interface would then play a dominant role in the resulting charge transport processes. Herein, the effects of a multivalent ion on the resulting electron transfer (ET) processes in the carbon nanopipettes are investigated with both experimental and simulation methods. The multivalent cations (i.e., Ca²⁺, Mg²⁺, Co²⁺, and Ni²⁺) are shown to strongly adsorb at the negatively charged carbon surface and attract more Fe(CN)₆^4- ions inside the cavity, as indicated by the increasing ET current responses. In addition to elucidating the fundamental charge transport processes in conductive nanopipettes to afford better usage as electrochemical probes, these results could also help in the development of new sensing methods for measuring the non-electroactive ions in biological or environmental systems.

Electrodeposition of Metal Nanoparticles inside Carbon Nanopipettes for Sensing Applications

Conductive nanopipettes offer promising confined spaces to enable advanced electrochemical sensing applications in small spaces. Herein, a series of metal-decorated carbon nanopipettes (CNPs) were developed, in which Au, Ag, and Pt are modified at the inner walls of CNPs by a simple electrodeposition method. The fabricated tips show good sensing performances for a variety of important analytes, such as glucose, hydrogen peroxide, and chloride and hydrogen ions in biological and catalytic systems. This simple and effective approach can be further extended to prepare other functionalized nanopipette electrodes toward more versatile and powerful measurements in electrochemical sensing and imaging applications.

Conductive polymer hydrogel-coated nanopipette sensor with tunable size

Nanopipette-based sensors are one of the most effective tools for detecting nanoparticles, bioparticles, and biomolecules. Quantitative analysis of nanoparticles with different shapes and electrical charges is achieved through measurement of the blockage currents that occur when particles pass through the nanopore. However, typical nanopipette sensors fabricated using a conventional needle-pulling method have a typical pore-diameter limitation of around 100 nm. Herein, we report a novel conductive hydrogel-composited nanopipette sensor with a tunable inner-pore diameter. This is made by electrodepositing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate onto the surface of a nanopipette with a prefabricated sacrificial copper layer. Because of the presence of copper ions, the conductive polymer can stably adhere to the tip of the nanopipette to form a nanopore; when nanoparticles pass through the conductive nanopore, more distinct blocking events are observed. The size of the nanopore can be changed simply by adjusting the electrodeposition time. In this way, suitable nanopores can be obtained for highly sensitive screening of a series of particles with diameters of the order of tens of nanometers.

Label-free imaging and biomarker analysis of exosomes with plasmonic scattering microscopy

Exosome analysis is a promising tool for clinical and biological research applications. However, detection and biomarker quantification of exosomes is technically challenging because they are small and highly heterogeneous. Here, we report an optical approach for imaging exosomes and quantifying their protein markers without labels using plasmonic scattering microscopy (PSM). PSM can provide improved spatial resolution and distortion-free image compared to conventional surface plasmon resonance (SPR) microscopy, with the signal-to-noise ratio similar to objective coupled surface plasmon resonance (SPR) microscopy, and millimeter-scale field of view as a prism-coupled SPR system, thus allowing exosome size distribution analysis with high throughput. In addition, PSM retains the high specificity and surface sensitivity of the SPR sensors and thus allows selection of exosomes from extracellular vesicles with antibody-modified sensor surfaces and in situ analyzing binding kinetics between antibody and the surface protein biomarkers on the captured exosomes. Finally, the PSM can be easily constructed on a popular prism-coupled SPR system with commercially available components. Thus, it may provide an economical and powerful tool for clinical exosome analysis and exploration of fundamental issues such as exosome biomarker binding properties.

Dielectrophoresis: An Approach to Increase Sensitivity, Reduce Response Time and to Suppress Nonspecific Binding in Biosensors?

The performance of receptor-based biosensors is often limited by either diffusion of the analyte causing unreasonable long assay times or a lack of specificity limiting the sensitivity due to the noise of nonspecific binding. Alternating current (AC) electrokinetics and its effect on biosensing is an increasing field of research dedicated to address this issue and can improve mass transfer of the analyte by electrothermal effects, electroosmosis, or dielectrophoresis (DEP). Accordingly, several works have shown improved sensitivity and lowered assay times by order of magnitude thanks to the improved mass transfer with these techniques. To realize high sensitivity in real samples with realistic sample matrix avoiding nonspecific binding is critical and the improved mass transfer should ideally be specific to the target analyte. In this paper we cover recent approaches to combine biosensors with DEP, which is the AC kinetic approach with the highest selectivity. We conclude that while associated with many challenges, for several applications the approach could be beneficial, especially if more work is dedicated to minimizing nonspecific bindings, for which DEP offers interesting perspectives.

A High Spatiotemporal Iontronic Single-Cell Viscometer

Ideal single-cell viscometer has remained unachieved, leaving a gap in current palette of single-cell nanotools. Information of single-cell viscosity could contribute to our knowledge of fundamental biological processes, e.g., mass diffusion, biochemical interaction, and cellular responses to many diseases and pathologies. Although advances have been made to this end, existing methods generally suffer from limitations, e.g., low spatiotemporal resolution. Here, we describe a high spatiotemporal iontronic single-cell viscometer that operates upon a patch clamp integrated with double-barreled nanopores separated by a septum of ca. 32 nm. The system enables reversible electroosmotic manipulation of the adjacent small fluid bridging two nanopores, the viscous alternation of which could be sensitively monitored by the ionic responses. In practical cellular studies, significantly, our findings reveal not only the less deviated medium viscosities than those of lysosomes and mitochondria but also the highest viscosities in the near-nuclear region than those of mitochondrion-dense and lysosome-dense regions. This work has provided an accessible single-cell viscometer and enriched the armory of single-cell nanotools.

Nanoconfined Electrochemical Collision and Catalysis of Single Enzyme inside Carbon Nanopipettes

Revealing the electrocatalytic features of single redox enzyme is significant to both fundamental biological processes and practical catalysis and sensing applications. Herein, we directly reveal the electrocatalytic current from a single enzyme inside the carbon nanopipettes via electrochemical collision strategies, based on the increased activity at nanoscale confinement. Besides the staircase current steps from surface blockage, discrete H₂O₂ oxidation and reduction current transients catalyzed by a single enzyme are also displayed and analyzed. The carbon nanopipette would increase the catalytic activities of enzymes and lead to a detectable current response, thus opening a new way to investigate the fundamental enzymatic mechanisms at the single enzyme level.

Chemistry solutions to facilitate nanopore detection and analysis

Characteristic ionic current modulations will be produced in a single molecule manner during the communication of individual molecules with a nanopore. Hence, the information regarding the length, composition, and structure of a molecule can be extracted from deciphering the electrical message. However, until now, achieving a satisfactory resolution for observation and quantification of a target analyte in a complex system remains a nontrivial task. In this review, we summarize the progress and especially the recent advance in utilizing chemistry solutions to facilitate nanopore detection and analysis. The discussed chemistry solutions are classified into several major categories, including covalent/non-covalent chemistry, redox chemistry, displacement chemistry, back titration chemistry, chelation chemistry, hydrolysis-chemistry, and click chemistry. Considering the significant success of using chemical reaction-assisted nanopore sensing strategies to improve sensor sensitivity & selectivity and to study various topics, other non-chemistry based methodologies can undoubtedly be employed by nanopore sensors to explore new applications in the interdisciplinary area of chemistry, biology, materials, and nanotechnology.

Pressure-Regulated Single-Entity Electrochemistry Inside Carbon Nanopipettes

Conductive nanopipettes have been widely used as a multifunctional platform for emerging sensing applications in small spaces, although the electrochemical processes involved are not well controlled and fully quantified. Herein, we use an external pressure to precisely control the solution volume and regulate the electrochemical signals in carbon nanopipettes. In addition to polarizing the redox concentration profile, the pressure is found to generate a convective flow to control the transport processes of redox molecules and nanoparticles as well, and their quantitative correlation is established by a numerical simulation. The elucidated pressure-regulated electrochemistry in conductive nanopipettes would reveal the fundamental charge transport processes at the nanoscale and promote better usage of conductive nanopipettes for delivery and sensing applications in single-cell analysis.

Detection of small-sized DNA fragments in a glassy nanopore by utilization of CRISPR-Cas12a as a converter system

The fabrication of nanopores with a matched pore size, and the existence of multiple interferents make the reproducible detection of small-sized molecules by means of solid-state nanopores still challenging. A useful method to solve these problems is based on the detection of large DNA nanostructures related to the existence of small-sized targets. In particular, a DNA tetrahedron with a well-defined 3D nanostructure is the ideal candidate for use as a signal transducer. Here, we demonstrate the detection of an L1-encoding gene of HPV18 as a test DNA target sequence in a reaction buffer solution, where long single-stranded DNA linking DNA tetrahedra onto the surface of the magnetic beads is cleaved by a target DNA-activated CRISPR-cas12 system. The DNA tetrahedra are subsequently released and can be detected by the current pulse in a glassy nanopore. This approach has several advantages: (1) one signal transducer can be used to detect different targets; (2) a glassy nanopore with a pore size much larger than the target DNA fragment can boost the tolerance of the contaminants and interferents which often degrade the performance of a nanopore sensor.

Quantification of the charge transport processes inside carbon nanopipettes

Conductive nanopipettes have been extensively used as powerful multifunctional probes for electrochemical and ion transport measurements, while the involved charge transfer processes have not been fully explored. In this paper, we use both experimental and simulation methods to de-convolute and quantify the respective electron transfer (ET) and ion transport (IT) contributions to the resulting current signals in carbon nanopipettes (CNPs). The results present that the current signals in CNPs are determined by ET in the case of low solution depth and long timescales, while IT becomes dominant at short timescales or high solution depth. In addition, the electrochemically and chemically irreversible ET processes in CNPs were also quantified. The elucidated and quantified charge transport processes inside CNPs will help control and optimize the IT and ET processes at the nanoscale, promoting better and broad usage of conductive nanopipettes in single-entity sensing and imaging applications.

The coupled electron transfer (ET) and ion transport (IT) processes in conductive nanopipettes, at both steady and transient states, are elucidated and quantified by experiments and simulation.

Bioinspired Solid-State Nanochannel Sensors: From Ionic Current Signals, Current, and Fluorescence Dual Signals to Faraday Current Signals

Inspired from bioprotein channels of living organisms, constructing "abiotic" analogues, solid-state nanochannels, to achieve "smart" sensing towards various targets, is highly seductive. When encountered with certain stimuli, dynamic switch of terminal modified probes in terms of surface charge, conformation, fluorescence property, electric potential as well as wettability can be monitored via transmembrane ionic current, fluorescence intensity, faraday current signals of nanochannels and so on. Herein, the modification methodologies of nanochannels and targets-detecting application are summarized in ions, small molecules, as well as biomolecules, and systematically reviewed are the nanochannel-based detection means including 1) by transmembrane current signals; 2) by the coordination of current- and fluorescence-dual signals; 3) by faraday current signals from nanochannel-based electrode. The coordination of current and fluorescence dual signals offers great benefits for synchronous temporal and spatial monitoring. Faraday signals enable the nanoelectrode to monitor both redox and non-redox components. Notably, by incorporation with confined effect of tip region of a needle-like nanopipette, glorious in-vivo monitoring is conferred on the nanopipette detector at high temporal-spatial resolution. In addition, some outlooks for future application in reliable practical samples analysis and leading research endeavors in the related fantastic fields are provided.

A Practical Electrochemical Nanotool for Facile Quantification of Amino Acids in Single Cell

Though significant advances are made in the arena of single-cell electroanalysis, quantification of intracellular amino acids of human cells remains unsolved. Exemplified by l-histidine (l-His), this issue is addressed by a practical electrochemical nanotool synergizing the highly accessible nanopipette with commercially available synthetic DNAzyme. The fabricated nanotools are screened before operation of a single-use manner, and the l-His-provoked cleavage of the DNA molecules can be sensibly transduced by the ionic current rectification response, the intrinsic property of nanopipette governed by its interior surface charges. Regional distribution of cytosolic l-His level in human cells is electrochemically quantified for the first time, and time-dependent drug treatment effects are further revealed. This work unveils the possibility of electrochemistry for quantification of cytosolic amino acids of a spatial- and time-based manner and ultimately enables a better understanding of amino acid-involved events in living cells.

Electrochemical Collision of Single Silver Nanoparticles in Carbon Nanopipettes

Exploring the electrochemical collision features at nanoelectrodes is highly desirable for revealing new physical insights and further expanding its applications at smaller spaces. Herein, we study the collision processes of single silver nanoparticles (AgNPs) inside carbon nanopipettes (CNPs). Results show that AgNPs undergo multiple collision and oxidation processes prior to fully oxidation after entering into the CNPs. Different from the disk electrodes, the produced Ag⁺ cannot immediately diffuse away from the cavity and will be reduced once switching to reductive potentials. More intriguingly, we observe discrete cathodic spikes from the Ag⁺ reduction, which are presumably due to the negatively charged carbon surface confined in the CNPs. The elucidated collision features in a CNP would enable its better usage for single entity measurements at confined spaces.

Nanoconfined Electrochemical Sensing of Single Silver Nanoparticles with a Wireless Nanopore Electrode

Single entity electrochemistry (SEE) has emerged as a promising method for precise measurement and fundamental understanding of the heterogeneity of single entities. Herein, we propose the dual responsive SEE sensing of the silver nanoparticles (AgNPs) collisions through a wireless nanopore electrode (WNE). Given the high temporal resolution and low background noise features, the Faradaic and capacitive currents provide the AgNPs' collision response. The electron transfer between the AgNPs and the electrode surface is identified under a bipolar electrochemical mechanism. Compared to the ultramicroelectrode, multistep oxidation of 30 nm AgNPs is observed due to the decreased interaction of the nanoparticles to the electrode. Moreover, the nanoconfinement of WNE plays a vital role in the repeated capturing of nanoparticles from the nontunneling region into the tunneling region until a complete oxidation. As a comparison, the collision of 5 nm AgNPs with higher interaction at the electrode surface shows great decrease in the multistep events. Thus, we propose a nanoconfined interaction based SEE method which could be used for simultaneously capturing the Faradaic and capacitive response. The nanoconfined interaction based SEE method holds great promise in the better understanding of heterogeneity of single particles.

In situ detection of hydroxyl radicals in mitochondrial oxidative stress with a nanopipette electrode

A nanopipette sensor was designed for the in situ detection of ˙OH around mitochondria with high selectivity and sensitivity.