A 30 nm Nanopore Electrode: Facile Fabrication and Direct Insights into the Intrinsic Feature of Single Nanoparticle Collisions

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.

Monitoring molecule translocation through plasmonic nanopores based on surface enhanced Raman scattering

To study the translocation behavior of molecules through nanopores, the gold plasmonic nanopores (GPNs) structures with high Raman activity are fabricated. The process of molecule translocation (via potential, concentration and pH) is studied by Surface Enhanced Raman Scattering (SERS). The electrostatic effect is found critical for the translocation direction and speed of model molecule. What's more, the characteristic blinking signal of rhodamine 6G (R6G) molecules translocating through the nanopores is observed with millisecond temporal resolution.

Solid-state nanochannels based on electro-optical dual signals for detection of analytes

The sensitive detection of analytes of different sizes is crucial significance for environmental protection, food safety and medical diagnostics. The confined space of nanochannels provides a location closest to the molecular reaction behaviors in real systems, thereby opening new opportunities for the precise detection of analytes. However, due to the susceptibility to external interference on the confined space of nanochannels, the high sensitivity nature of the current signals through the nanochannels is more troubling for the detection reliability. Combining highly sensitive optical signals with the sensitive current signals of solid-state nanochannels establishes a nanochannel detection platform based on electro-optical dual signals, potentially offering more sensitive, specific, and accuracy detection of analytes. This review summarizes the last five years of applications of solid-state nanochannels based on electro-optical dual signals in analytes detection. Firstly, the detection principles of solid-state nanochannels and the construction strategies of nanochannel electro-optical sensing platforms are discussed. Subsequently, the review comprehensively outlines the applications involving nanochannels with electrical signals combined with fluorescence signals, electrical signals combined with surface-enhanced Raman spectroscopy signals, and electrical signals combined with other optical signals in analyte detection. Additionally, the perspectives and difficulties of nanochannels are investigated on the basis of electro-optical dual signals.

Electrochemical Monitoring of Real-Time Vesicle Dynamics Induced by Tau in a Confined Nanopipette

The microtubule-associated protein tau participates in neurotransmission regulation via its interaction with synaptic vesicles (SVs). The precise nature and mechanics of tau's engagement with SVs, especially regarding alterations in vesicle dynamics, remain a matter of discussion. We report an electrochemical method using a synapse-mimicking nanopipette to monitor vesicle dynamics induced by tau. A model vesicle of ~30 nm is confined within a lipid-modified nanopipette orifice with a comparable diameter to mimic the synaptic lipid environment. Both tau and phosphorylated tau (p-tau) present two-state dynamic behavior in this biomimetic system, showing typical ionic current oscillation, induced by lipid-tau interaction. The results indicate that p-tau has a stronger affinity to the lipid vesicles in the confined environment, blocking the vesicle movement to a higher degree. Taken together, this method bridges a gap for sensing synaptic vesicle dynamics in a confined lipid environment, mimicking vesicle movement near the synaptic membrane. These findings contribute to understanding how different types of tau protein regulate synaptic vesicle motility and to underlying its functional and pathological behaviours in disease.

In situ self-referenced intracellular two-electrode system for enhanced accuracy in single-cell analysis

Since the emergence of single-cell electroanalysis, the two-electrode system has become the predominant electrochemical system for real-time behavioral analysis of single-cell and multicellular populations. However, due to the transmembrane placement of the two electrodes, cellular activities can be interrupted by the transmembrane potentials, and the test results are susceptible to influences from factors such as intracellular solution, membrane, and bulk solution. These limitations impede the advancement of single-cell analysis. Here, we propose a highly miniaturized and integrated in situ self-referenced intracellular two-electrode system (IS-SRITES), wherein both the working and reference electrodes are positioned inside the cell. Additionally, we demonstrated the stability (0.28 mV/h) of the solid-contact in situ Ag/AgCl reference electrode and the ability of the system to conduct standard electrochemical testing in a wide pH range (pH 6.0-8.0). Cell experiments confirmed the non-destructive performance of the electrode system towards cells and its capacity for real-time monitoring of intra- and extracellular pH values. Moreover, through equivalent circuits, finite element simulations, and drug delivery experiments, we illustrated that the IS-SRITES can yield more accurate test results and exhibit enhanced resistance to interference from the extracellular environment. Our proposed system holds the potential to enable the precise detection of intracellular substances and optimize the existing model of the electrode system for intracellular signal detection, thereby spearheading advancements in single-cell analysis.

Single entity collision for inorganic water pollutants measurements: Insights and prospects

In the context of aquatic environmental issues, dynamic analysis of nano-sized inorganic water pollutants has been one of the key topics concerning their seriously amplified threat to natural ecosystems and life health. Its ultimate challenge is to reach a single-entity level of identification especially towards substantial amount of inorganic pollutants formed as natural or manufactured nanoparticles (NPs), which enter the water environments along with the potential release of constituents or other contaminating species that may have coprecipitated or adsorbed on the particles' surface. Here, we introduced a 'nano-impacts' approach-single entity collision electrochemistry (SECE) promising for in-situ characterization and quantification of nano-sized inorganic pollutants at single-entity level based on confinement-controlled electrochemistry. In comparison with ensemble analytical tools, advantages and features of SECE point at understanding 'individual' specific fate and effect under its free-motion condition, contributing to obtain more precise information for 'ensemble' nano-sized pollutants on assessing their mixture exposure and toxicity in the environment. This review gives a unique insight about the single-entity collision measurements of various inorganic water pollutants based on recent trends and directions of state-of-the-art single entity electrochemistry, the prospects for exploring nano-impacts in the field of inorganic water pollutants measurements were also put forward.

Single-Nanoparticle-Level Understanding of Oxidase-like Activity of Au Nanoparticles on Polymer Nanobrush-Based Proton Reservoirs

Enzyme-mimicking nanoparticles play a key role in important catalytic processes, from biosensing to energy conversion. Therefore, understanding and tuning their performance is crucial for making further progress in biological applications. We developed an efficient and sensitive electrochemical method for the real-time monitoring of the glucose oxidase (GOD)-like activity of single nanoparticle through collision events. Using brush-like sulfonate (-SO3-)-doped polyaniline (PANI) decorated on TiO2 nanotube arrays (TiNTs-SPANI) as the electrode, we fabricated a proton reservoir with excellent response and high proton-storage capacity for evaluating the oxidase-like activity of individual Au nanoparticles (AuNPs) via instantaneous collision processes. Using glucose electrocatalysis as a model reaction system, the GOD-like activity of individual AuNPs could be directly monitored via electrochemical tests through the nanoparticle collision-induced proton generation. Furthermore, based on the perturbation of the electrical double layer of SPANI induced by proton injection, we investigated the relationship between the measured GOD-like activities of the plasmonic nanoparticles (NPs) and the localized surface plasmon resonance (LSPR) as well as the environment temperature. This work introduces an efficient platform for understanding and characterizing the catalytic activities of nanozymes at the single-nanoparticle level.

Stochastic Sensing of Dynamic Interactions and Chemical Reactions with Nanopores/Nanochannels

Nanopore sensing technology is an emerging analysis method with the advantages of simple operation, high sensitivity, fast output and being label free, and it is widely used in protein analysis, gene sequencing, biomarker detection, and other fields. The confined space of the nanopore provides a place for dynamic interactions and chemical reactions between substances. The use of nanopore sensing technology to track these processes in real time is helpful to understand the interaction/reaction mechanism at the single-molecule level. According to nanopore materials, we summarize the development of biological nanopores and solid-state nanopores/nanochannels in the stochastic sensing of dynamic interactions and chemical reactions. The goal of this paper is to stimulate the interest of researchers and promote the development of this field.

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.

Monitoring Bipolar Electrochemistry and Hydrogen Evolution Reaction of a Single Gold Microparticle under Sub-Micropipette Confinement

Herein, an approach to track the process of autorepeating bipolar reactions and hydrogen evolution reaction (HER) on a micro gold bipolar electrode (BPE) is established. Once blocking the channel of the sub-micropipette tip, the formed gold microparticle is polarized into the wireless BPE, which induces the dissolution of the gold at the anode and the HER at the cathode. The current response shows a periodic behavior with three regions: the bubble generation region (I), the bubble rupture/generation region (II), and the channel opening region (III). After a stable low baseline current of region I, a series of positive spike signals caused by single H2 nanobubbles rupture/generation are recorded standing for the beginning of region II. Meanwhile, the dissolution of the gold blocking at the orifice will create a new channel, increasing the baseline current for region III, where the synthesis of gold occurs again, resulting in another periodic response. Finite element simulations are applied to unveil the mechanism thermodynamically. In addition, the integral charge of the H2 nanobubbles in region II corresponds to the consumption of the anode gold. It simultaneously monitors autorepeating bipolar reactions of a single gold microparticle and HER of a single H2 nanobubble electrochemically, which reveals an insightful physicochemical mechanism in nanoscale confinement and makes the glass nanopore an ideal candidate to further reveal the heterogeneity of catalytic capability at the single particle level.

Bipolar Electrochemistry - A Powerful Tool for Micro/Nano-Electrochemistry

The understanding of areas for "classical" electrochemistry (including catalysis, electrolysis and sensing) and bio-electrochemistry at the micro/nanoscale are focus on the continued performance facilitations or the exploration of new features. In the recent 20 years, a different mode for driving electrochemistry has been proposed, which is called as bipolar electrochemistry (BPE). BPE has garnered attention owing to the interesting properties: (i) its wireless nature facilitates electrochemical sensing and high throughput analysis; (ii) the gradient potential distribution on the electrodes surface is a useful tool for preparing gradient surfaces and materials. These permit BPE to be used for modification and analytical applications on a micro/nanoscale surface. This review aims to introduce the principle and classification of BPE and BPE at micro/nanoscale; sort out its applications in electrocatalysis, electrosynthesis, electrophoresis, power supply and so on; explain the confined BPE and summarize its analytical application for single entities (single cells, single particles and single molecules), and discuss finally the important direction of micro/nanoscale BPE.

In Situ Characterization of Oxygen Evolution Electrocatalysis of Silver Salt Oxide on a Wireless Nanopore Electrode

Silver salt oxide shows superior oxidation ability for the applications of superconductivity, sterilization, and catalysis. However, due to the easy decomposition, the catalytic properties of silver salt oxide are difficult to characterize by conventional methods. Herein, we used a closed-type wireless nanopore electrode (CWNE) to in situ and real-time monitor the electrocatalytic performance of Ag₇NO₁₁ in the oxygen evolution reaction. The real-time current recording revealed that the deposited Ag₇NO₁₁ on the CWNE tip greatly enhanced the oxidative capacity of the electrode, resulting in water splitting. The statistical event analysis reveals the periodic O₂ bubble formation and dissolution at the Ag₇NO₁₁ interface, which ensures the characterization of the oxygen evolution electrocatalytic process at the nanoscale. The calculated k_cat and Markov chain modeling suggest the anisotropy of Ag₇NO₁₁ at a low voltage may lead to multiple catalytic rates. Therefore, our results demonstrate the powerful capability of CWNE in direct and in situ characterization of gas-liquid-solid catalytic reactions for unstable catalysts.

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.

Enhanced Electrochemistry of Single Plasmonic Nanoparticles

The structure-function relationship of plasmon enhanced electrochemistry (PEEC) is of great importance for the design of efficient PEEC catalyst, but is rarely investigated at single nanoparticle level for the lack of efficient nanoscale methodology. Herein, we report the utilization of nanoparticle impact electrochemistry to allow single nanoparticle PEEC, where the effect of incident light on the plasmonic Ag/Au nanoparticles for accelerating Co-MOFNs catalyzed hydrogen evolution reaction (HER) is systematically explored. It is found that the plasmon excited hot carrier injection can lower the reaction activation energy, resulting in a much promoted reaction probability and the integral charge generated from individual collisions. Besides, a plasmonic nanoparticle filtering method is established to effectively distinguish different plasmonic nanoparticles. This work provides a unique view in understanding the intrinsic physicochemical properties for PEEC at the nano-confined domains.

Nanopipettes for the Electrochemical Study of Enhanced Enzymatic Activity in a Femtoliter Space

The chemical reaction in a confined space is known to be accelerated due to a high collision probability; however, the study of this confinement effect in a supersmall space down to femtoliter (fL) is seldom reported. Here, an adjustable volume [from picoliter (pL) to fL] of the aqueous phase is retrained at the tip of a nanopipette by an organic solvent so that the confinement effect on the specific activity of glucose oxidase is investigated. The activity is determined by the amount of hydrogen peroxide generated from the reaction between the oxidase and glucose using a nanoelectrode inside the nanopipette. As compared with the activity in bulk solution (82 U/mg), the activity increases up to 7500 U/mg in a 105 fL space. The 2 orders of magnitude increase in the enzymatic activity is the highest amplification in the volume-confined enzyme reaction as reported. A near-exponential drop in the activity is observed with the increase in the space volume, revealing the dominant enhancement in the confined space at the fL level for the first time. The established electrochemical nanopipettes should not only provide a strategy for the study of the enzymatic activity in supersmall confined space but also help understand the confinement effect of enzyme-catalyzed reactions.

Nanopipette-Based Nanosensor for Label-Free Electrochemical Monitoring of Cell Membrane Rupture under H2O2 Treatment

H₂O₂ is an essential signaling molecule in living cells that can cause direct damage to lipids, proteins, and DNA, resulting in cell membrane rupture. However, current studies mostly focus on probe-based sensing of intracellular H₂O₂, and these methods usually require sophisticated probe synthesis and instruments. In particular, local H₂O₂ treatment induces cell membrane rupture, but the level of cell membrane destruction is unknown because the mechanical properties of the cell membrane are difficult to accurately determine. Therefore, highly sensitive and label-free methods are required to measure and reflect mechanical changes in the cell membrane. Here, using an ultrasmall quartz nanopipette with a tip diameter less than 90 nm as a nanosensor, label-free and noninvasive electrochemical single-cell measurement is achieved for real-time monitoring of cell membrane rupture under H₂O₂ treatment. By spatially controlling the nanopipette tip to precisely approach a specific location on the membrane of a single living cell, stable cyclic membrane oscillations are observed under a constant direct current voltage. Specifically, upon nanopipette advancement, the mechanical status of the cell membrane can be sensibly displayed by continuous current versus time traces. The electrical signals are collected and processed, ultimately revealing the mechanical properties of the cell membrane and the degree of cell apoptosis. This nanopipette-based nanosensor paves the way for developing a facile, label-free, and noninvasive strategy to assay the mechanical properties of the cell membrane during external stimulation at the single-cell level.

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.

Confined Synthesis of Silver Wire at the Nanopipette-Liquid/Liquid Interface

Herein, silver wire is synthesized electrochemically within a nanopipette using the nanopipette-liquid/liquid interface. The i-t curve characterizes the growth state of the silver wire. The higher rate of current increase indicates the faster electron transfer and the faster growth of the silver wire; conversely, when the current does not increase significantly with time, i.e., the rate of increase of the current is small, the growth rate of the silver wire is slow. The main driving force for the growth of silver into a linear structure is the theoretical current differential between the water and oil, caused by the concentration difference between the silver nitrate and ferrocene. The growth of the silver wire is also influenced by the shape of the nanopipette. If the diameter of the pipet increases quickly, silver wire tends to produce multibranched structures, while a smaller diameter makes it easier to obtain silver wire with fewer branches due to the confinement effect. This method is also applicable to the synthesis of gold within a nanopipette. The combination of nanopipette and metallic material using a liquid-liquid interface results in a broader application of nanopipettes for nanopore sensors, nanopore electrodes, bipolar electrodes, etc.

Probing Multidimensional Structural Information of Single Molecules Transporting through a Sub-10 nm Conical Plasmonic Nanopore by SERS

Probing the orientation and oxygenation state of single molecules (SMs) is of great importance for understanding the advanced structure of individual molecules. Here, we manipulate molecules transporting through the hot spot of a sub-10 nm conical gold nanopore and acquire the multidimensional structural information of the SMs by surface enhanced Raman scattering (SERS) detection. The sub-10 nm size and conical shape of the plasmonic nanopore guarantee its high detection sensitivity. SERS spectra show a high correlation with the orientations of small-sized single rhodamine 6G (R6G) during transport. Meanwhile, SERS spectra of a single hemoglobin (Hb) reveal both the vertical/parallel orientations of the porphyrin ring and oxygenated/deoxygenated states of Hb. The present study provides a new strategy for bridging the primary sequence and the advanced structure of SMs.

Bipolar (Bio)electroanalysis

This contribution reviews a selection of the most recent studies on the use of bipolar electrochemistry in the framework of analytical chemistry. Despite the fact that the concept is not new, with several important studies dating back to the middle of the last century, completely novel and very original approaches have emerged over the last decade. This current revival illustrates that scientists still (re)discover some exciting virtues of this approach, which are useful in many different areas, especially for tackling analytical challenges in an unconventional way. In several cases, this "wireless" electrochemistry strategy enables carrying out measurements that are simply not possible with classic electrochemical approaches. This review will hopefully stimulate new ideas and trigger scientists to integrate some aspects of bipolar electrochemistry in their work in order to drive the topic into yet unexplored and eventually completely unexpected directions.

Dynamic Behavior of Charged Particles at the Nanopipette Orifice

Understanding the dynamic behavior of charged particles driven by flow and electric field in nanochannels/pores is highly important for both fundamental study and practical applications. While a great breakthrough has been made in understanding the translocation dynamics of charged particles within the nanochannels/pores, studies on the dynamics of particles at the orifice of nanochannels/pores are scarcely reported. Here, we study particle motion at a smaller-sized orifice of a nanopipette by combining experimentally observed current transients with simulated force conditions. The theoretical force analysis reveals that dielectrophoretic force plays an equally important role as electrophoretic force and electroosmotic force, although it has often been neglected in understanding the particle translocation dynamics within the nanopipette. Under the combined action of these forces, it thus becomes difficult for particles to physically collide with the orifice of the nanopipette, resulting in a relatively low decrease in the current transients, which coincides with experimental results. We then regulate the dynamic behavior by altering experimental conditions (i.e., bias potential, nanopipette surface charge, and particle size), and the results further validate the presence and influence of forces being considered. This study improves the understanding of the relationship between particle properties and observed current transients, providing more possibilities for accurate single-particle analysis and single-entity regulation.

Towards explicit regulating-ion-transport: nanochannels with only function-elements at outer-surface

Function elements (FE) are vital components of nanochannel-systems for artificially regulating ion transport. Conventionally, the FE at inner wall (FE_IW) of nanochannel⁻systems are of concern owing to their recognized effect on the compression of ionic passageways. However, their properties are inexplicit or generally presumed from the properties of the FE at outer surface (FE_OS), which will bring potential errors. Here, we show that the FE_OS independently regulate ion transport in a nanochannel⁻system without FE_IW. The numerical simulations, assigned the measured parameters of FE_OS to the Poisson and Nernst-Planck (PNP) equations, are well fitted with the experiments, indicating the generally explicit regulating-ion-transport accomplished by FE_OS without FE_IW. Meanwhile, the FE_OS fulfill the key features of the pervious nanochannel systems on regulating-ion-transport in osmotic energy conversion devices and biosensors, and show advantages to (1) promote power density through concentrating FE at outer surface, bringing increase of ionic selectivity but no obvious change in internal resistance; (2) accommodate probes or targets with size beyond the diameter of nanochannels. Nanochannel-systems with only FE_OS of explicit properties provide a quantitative platform for studying substrate transport phenomena through nanoconfined space, including nanopores, nanochannels, nanopipettes, porous membranes and two-dimensional channels.

Function elements are key components for nanochannel systems for artificial regulation of ion transport. Here, the authors investigate the independent role of function elements at the outer surface of nanochannel systems, without function elements at inner walls, in promoting osmotic energy conversion and biochemical sensing.

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.

Construction of a Smart Nanofluidic Sensor through a Redox Reaction Strategy for High-Performance Carbon Monoxide Sensing

Carbon monoxide (CO), an important gas signaling molecule, demonstrated various physiological and pathological functions by regulating the ion flux of biological channels. Herein, inspired by the CO-regulated K⁺ channel in vivo, we propose a smart CO-responsive nanosensor through the redox reaction strategy. Such nanosensor demonstrated an outstanding CO specificity and selectivity with high ion rectification (∼9) as well as excellent stability and recyclability. Therefore, these results will provide a new direction for the design of nanochannel-based sensors for future practical and biological applications.

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

The continuing interest in nanoscale research has spurred the development of nanosensors for liquid phase measurements. These include nanopore-based sensors typically employed for detecting nanoscale objects, such as nanoparticles, vesicles and biomolecules, and electrochemical nanosensors suitable for identification and quantitative analysis of redox active molecules. In this Perspective, we discuss conductive nanopipettes (CNP) that can combine the advantages of single entity sensitivity of nanopore detection with high selectivity and capacity for quantitative analysis offered by electrochemical sensors. Additionally, the small physical size and needle-like shape of a CNP enables its use as a tip in the scanning electrochemical microscope (SECM), thus, facilitating precise positioning and localized measurements in biological systems.

Electrowetting-Mediated Transport to Produce Electrochemical Transistor Action in Nanopore Electrode Arrays

Understanding water behavior in confined volumes is important in applications ranging from water purification to healthcare devices. Especially relevant are wetting and dewetting phenomena which can be switched by external stimuli, such as light and electric fields. Here, these behaviors are exploited for electrochemical processing by voltage-directed ion transport in nanochannels contained within nanopore arrays in which each nanopore presents three electrodes. The top and middle electrodes (TE and ME) are in direct contact with the nanopore volume, but the bottom electrode (BE) is buried beneath a 70 nm silicon nitride (SiN_x ) insulating layer. Electrochemical transistor operation is realized when small, defect-mediated channels are opened in the SiN_x . These defect channels exhibit voltage-driven wetting that mediates the mass transport of redox species to/from the BE. When BE is held at a potential maintaining the defect channels in the wetted state, setting the potential of ME at either positive or negative overpotential results in strong electrochemical rectification with rectification factors up to 440. By directing the voltage-induced electrowetting of defect channels, these three-electrode nanopore structures can achieve precise gating and ion/molecule separation, and, as such, may be useful for applications such as water purification and drug delivery.

Ionic Strength Gated Redox Current Rectification by Ferrocene Grafted in Silica Nanochannels

In this work, a silica nanochannel membrane (SNM) consisting of a high density of vertically aligned channels on an indium tin oxide (ITO) electrode surface was used as a stencil to confine the grafting of ferrocene (Fc) via electrochemical reduction of diazonium cations. The grafting selectively occurs on the underlying ITO surface (namely, the bottom of silica nanochannels) instead of on the nanochannel walls. Thus, the obtained electrode, designated as Fc@SNM/ITO, is composed of an array of silica nanochannels with redox activity on the bottom. Immobilized Fc molecules are able to rectify the electron transfer between the underlying ITO electrode and soluble redox species. In other words, they can promote the electron transfer in one direction while suppressing that in the opposite direction. Anodic and cathodic redox rectification was observed for Fe(CN)₆^4- and IrCl₆^2-, respectively, with the magnitude of current rectification directly proportional to the concentration of redox species. Moreover, because of strong permselectivity of silica nanochannels arising from their ultrasmall size (2-3 nm in diameter) and negatively charged surface (due to deprotonation of surface silanol groups), the access of Fe(CN)₆^4- and IrCl₆^2- to nanochannels strongly depends on the ionic strength of electrolyte solution. A higher ionic strength favors the access of anionic redox species, thus leading to a larger redox current rectification. The redox current rectification phenomenon not only proves the permselective nature of silica nanochannels but also holds potential for the development of electrochemical current rectification-based sensors.

Hierarchical Control of Plasmonic Nanochemistry in Microreactor

A microreactor that can confine chemical reactions exclusively in tiny vessels with the volume of ∼0.015 μm³ is introduced. Aluminum inversed hollow nanocone arrays (IHNAs) are fabricated by a simple and efficient colloidal lithography method. Ag and Au nanoparticles (NPs), as well as polypyrrole, grow exclusively in the conic cavities under light illumination. The photocatalytic effect arising from the plasmonic enhanced electric fields (E-fields) of IHNAs boosts the reactions and is in charge of the submicrometer site-selectivity. By partially inhibiting light to IHNAs, various hierarchical patterns at the macro-, micro-, and sub-microscale are obtained, inspiring a facile patterning technique by varying the light source. In addition, the Al IHNA films are transferred to flexible and curved substrates with unchanged performances, showing high flexibility for wide applications. Microreactors based on the IHNAs will contribute to the control of chemical reactions at different dimensions and offer great potentials in developing novel nanofabrication techniques.

Wireless nanopore electrodes for analysis of single entities

Measurements of a single entity underpin knowledge of the heterogeneity and stochastics in the behavior of molecules, nanoparticles, and cells. Electrochemistry provides a direct and fast method to analyze single entities as it probes electron/charge-transfer processes. However, a highly reproducible electrochemical-sensing nanointerface is often hard to fabricate because of a lack of control of the fabrication processes at the nanoscale. In comparison with conventional micro/nanoelectrodes with a metal wire inside, we present a general and easily implemented protocol that describes how to fabricate and use a wireless nanopore electrode (WNE). Nanoscale metal deposition occurs at the tip of the nanopipette, providing an electroactive sensing interface. The WNEs utilize a dynamic ionic flow instead of a metal wire to sense the interfacial redox process. WNEs provide a highly controllable interface with a 30- to 200-nm diameter. This protocol presents the construction and characterization of two types of WNEs-the open-type WNE and closed-type WNE-which can be used to achieve reproducible electrochemical measurements of single entities. Combined with the related signal amplification mechanisms, we also describe how WNEs can be used to detect single redox molecules/ions, analyze the metabolism of single cells, and discriminate single nanoparticles in a mixture. This protocol is broadly applicable to studies of living cells, nanomaterials, and sensors at the single-entity level. The total time required to complete the protocol is ~10-18 h. Each WNE costs ~$1-$3.

Reversing current rectification to improve DNA-sensing sensitivity in conical nanopores

Herein, we report the ultrasensitive DNA detection through designing an elegant nanopore biosensor as the first case to realize the reversal of current rectification direction for sensing. Attributed to the unique asymmetric structure, the glass conical nanopore exhibits the sensitive response to the surface charge, which can be facilely monitored by ion current rectification curves. In our design, an enzymatic cleavage reaction was employed to alter the surface charge of the nanopore for DNA sensing. The measured ion current rectification was strongly responsive to DNA concentrations, even reaching to the reversed status from the negative ratio (-6.5) to the positive ratio (+16.1). The detectable concentration for DNA was as low as 0.1 fM. This is an ultrasensitive and label-free DNA sensing approach, based on the rectification direction-reversed amplification in a single glass conical nanopore.

Discrimination of Protein Amino Acid or Its Protonated State at Single-Residue Resolution by Graphene Nanopores

The function of a protein is determined by the composition of amino acids and is essential to proteomics. However, protein sequencing remains challenging due to the protein's irregular charge state and its high-order structure. Here, a proof of principle study on the capability of protein sequencing by graphene nanopores integrated with atomic force microscopy is performed using molecular dynamics simulations. It is found that nanopores can discriminate a protein sequence and even its protonation state at single-residue resolution. Both the pulling forces and current blockades induced by the permeation of protein residues are found to be highly correlated with the type of amino acids, which makes the residues identifiable. It is also found that aside from the dimension, both the conformation and charge state of the residue can significantly influence the force and current signal during its permeation through the nanopore. In particular, due to the electro-osmotic flow effect, the blockade current for the double-protonated histidine is slightly smaller than that for single-protonated histidine, which makes it possible for discrimination of different protonation states of amino acids. The results reported here present a novel protein sequencing scheme using graphene nanopores combined with nanomanipulation technology.

Short-chain oligonucleotide detection by glass nanopore using targeting-induced DNA tetrahedron deformation as signal amplifier

Glass capillary nanopore has been developed as a promising sensing platform for bioassay with single-molecule resolution. Although the diameter of glass capillary nanopore can be easily tuned, direct event-readouts of small biomacromolecules, like short-chain oligonucleotide fragments (within ∼20 nucleotides) remain great challenge, which limited by the configuration of the conical-shaped nanopore and the instrumental temporal resolution. Here, we exploit a smart strategy for glass nanopore detection of short-chain oligonucleotides by using relatively big-sized tetrahedral DNA nanostructures as a signal amplifier, which can amplify the signals and retard the translocation speed meanwhile. The tetrahedral DNA nanostructure with a hairpin loop sequence in one edge, undergoes a shape transformation upon the complementary combination of the target oligonucleotides, in which the presence of short-chain target oligonucleotide can be readout due to obvious variation in amplitude of ion current pulse that caused by volume change of the DNA tetrahedral. Therefore, this strategy is promising for extending glass nanopore sensing platform for sensitive detection of short-chain oligonucleotides.