Confined Nanopipette Sensing: From Single Molecules, Single Nanoparticles, to Single Cells

Electric Field-Controlled Synthesis and Characterisation of Single Metal-Organic-Framework (MOF) Nanoparticles

Achieving control over the size distribution of metal-organic-framework (MOF) nanoparticles is key to biomedical applications and seeding techniques. Electrochemical control over the nanoparticle synthesis of the MOF, HKUST-1, is achieved using a nanopipette injection method to locally mix Cu²⁺ salt precursor and benzene-1,3,5-tricarboxylate (BTC^3- ) ligand reagents, to form MOF nanocrystals, and collect and characterise them on a TEM grid. In situ analysis of the size and translocation frequency of HKUST-1 nanoparticles is demonstrated, using the nanopipette to detect resistive pulses as nanoparticles form. Complementary modelling of mass transport in the electric field, enables particle size to be estimated and explains the feasibility of particular reaction conditions, including inhibitory effects of excess BTC^3- . These new methods should be applicable to a variety of MOFs, and scaling up synthesis possible via arrays of nanoscale reaction centres, for example using nanopore membranes.

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.

Target-Driven Nanozyme Growth in TiO2 Nanochannels for Improving Selectivity in Electrochemical Biosensing

Nanozymes have been used in colorimetric and electrochemical sensing because of their low cost and high stability. However, the wide applications of nanozymes in sensing devices are largely limited due to their poor selectivity. In this study, unlike traditional methods using prepared nanozymes for target detection, we designed a target-driven nanozyme growth strategy in TiO₂ nanochannels to detect analytes. Using telomerase as an example, the established recognition event was used to expand the photocatalytic activity of TiO₂ to visible-light region, thus triggering Prussian blue nanoparticle (PBNP) growth in visible light. Benefiting from the peroxidase (POD)-like activity of PBNPs, the uncharged 3,5,3',5'-tetramethylbenzidine (TMB) is oxidized to positively charged oxTMB, which induces significant ionic transport changes in nanochannels, and thus in turn provides information about telomerase activity. Such a nanozyme-triggered sensing system exhibited excellent performance in telomerase detection in urine specimens from patients with bladder cancer. This innovative target-driven signal generation strategy might provide a new method for applying nanozymes in developing sensitive, rapid, and accurate biological sensing systems.

Toward Nanopore Electrospray Mass Spectrometry: Nanopore Effects in the Analysis of Bacteria

The shape and structure analyses capability of nanopore is powerful and complementary to mass spectrometry analysis. It is extremely attractive but challenging to integrate these two techniques. The feasibility of combining nanopore electrospray with mass spectrometry was explored in this study. A nanopore effect was observed during the nano-electrospray of single bacterium, through which the shape and dimension of a single bacterium could be obtained. Molecular information on these bacteria was then acquired by analyzing these bacteria deposited on the counter electrode through laser spray ionization mass spectrometry experiments. Proof-of-concept experiments were carried out for four types of bacteria. Results show that the combination of nanopore results with mass spectrum data could effectively improve the identification accuracy of these bacteria from 72.5% to 100%. Although initial experiments were demonstrated in this work, results showed that it is feasible and promising to integrate nanopore technology with mass spectrometry for large biomolecule studies in the near future.

Deconvolution of electroosmotic flow in hysteresis ion transport through single asymmetric nanopipettes

Unveiling the contributions of electroosmotic flow (EOF) in the electrokinetic transport through structurally-defined nanoscale pores and channels is challenging but fundamentally significant because of the broad relevance of charge transport in energy conversion, desalination and analyte mixing, micro and nano-fluidics, single entity analysis, capillary electrophoresis etc. This report establishes a universal method to diagnose and deconvolute EOF in the nanoscale transport processes through current-potential measurements and analysis without simulation. By solving Poisson, Nernst-Planck (PNP) with and without Navier-Stokes (NS) equations, the impacts of EOF on the time-dependent ion transport through asymmetric nanopores are unequivocally revealed. A sigmoidal shape in the I-V curves indicate the EOF impacts which further deviate from the well-known non-linear rectified transport features. Two conductance signatures, an absolute change in conductance and a 'normalized' one relative to ion migration, are proposed as EOF impact (factor). The EOF impacts can be directly elucidated from current-potential experimental results from the two analytical parameters without simulation. The EOF impact is found more significant in intermediate ionic strength, and potential and pore size dependent. The less-intuitive ionic strength and size dependence is explained by the combined effects of electrostatic screening and non-homogeneous charge distribution/transport at nanoscale interface. The time-dependent conductivity and optical imaging experiments using single nanopipettes validate the proposed method which is applicable to other channel type nanodevices and membranes. The generalizable approach eliminates the need of simulation/fitting of specific experiments and offers previously inaccessible insights into the nanoscale EOF impacts under various experimental conditions for the improvement of separation, energy conversions, high spatial and temporal control in single entity sensing/manipulation, and other related applications.

A Gold Nanoparticle Bouquet held on plasma membrane: An ultrasensitive dark-field imaging approach for Cancer Cell Analysis

Rational: p53 is suppressing tumor protein correlated with the cell cycle factors and apoptosis. Here, a gold nanoparticle bouquet is designed for an ultrasensitive dark-field imaging approach for cancer cell analysis.

Methods: AuNP60/APBA is functionalized by a gold nanoparticle bouquet-plasmonic 60 nm gold nanoparticles. And consistent APBA can be held on the plasma membrane. After 13 nm gold nanoparticles are functionalized with mannose (AuNP13/MN), the AuNP60/APBA gold nanoparticles are captured. The absorption spectrum of aggregation gold nanoparticles (AuNPs) shifts to near-infrared (NIR) region which can be observed under dark-field microscopy (DFM) and is treated the subsequent with photothermal therapy.

Results: The results that MCF-7 cells were successfully destroyed under the near-infrared (NIR) irradiation and the intracellular WTp53 increased while the MTp53 decreased. These results indicated that p53 is the key molecule in the apoptosis signaling pathway. Photothermal therapy can stimulate the MTp53 in the cell signal conductive pathway.

Conclusion: This work offers a new method for intracellular p53 analysis and a potential targeted cancer treatment.

Rapid 3-dimensional shape determination of globular proteins by mobility capillary electrophoresis and native mass spectrometry

Established high-throughput proteomics methods provide limited information on the stereostructures of proteins. Traditional technologies for protein structure determination typically require laborious steps and cannot be performed in a high-throughput fashion. Here, we report a new medium throughput method by combining mobility capillary electrophoresis (MCE) and native mass spectrometry (MS) for the 3-dimensional (3D) shape determination of globular proteins in the liquid phase, which provides both the geometric structure and molecular mass information of proteins. A theory was established to correlate the ion hydrodynamic radius and charge state distribution in the native mass spectrum with protein geometrical parameters, through which a low-resolution structure (shape) of the protein could be determined. Our test data of 11 different globular proteins showed that this approach allows us to determine the shapes of individual proteins, protein complexes and proteins in a mixture, and to monitor protein conformational changes. Besides providing complementary protein structure information and having mixture analysis capability, this MCE and native MS based method is fast in speed and low in sample consumption, making it potentially applicable in top–down proteomics and structural biology for intact globular protein or protein complex analysis.

Using native mass spectrometry and mobility capillary electrophoresis, the ellipsoid dimensions of globular proteins or protein complexes could be measured efficiently.

DNA nanotechnology assisted nanopore-based analysis

Nanopore technology is a promising label-free detection method. However, challenges exist for its further application in sequencing, clinical diagnostics and ultra-sensitive single molecule detection. The development of DNA nanotechnology nonetheless provides possible solutions to current obstacles hindering nanopore sensing technologies. In this review, we summarize recent relevant research contributing to efforts for developing nanopore methods associated with DNA nanotechnology. For example, DNA carriers can capture specific targets at pre-designed sites and escort them from nanopores at suitable speeds, thereby greatly enhancing capability and resolution for the detection of specific target molecules. In addition, DNA origami structures can be constructed to fulfill various design specifications and one-pot assembly reactions, thus serving as functional nanopores. Moreover, based on DNA strand displacement, nanopores can also be utilized to characterize the outputs of DNA computing and to develop programmable smart diagnostic nanodevices. In summary, DNA assembly-based nanopore research can pave the way for the realization of impactful biological detection and diagnostic platforms via single-biomolecule analysis.

Fabrication of a Biomimetic Nanochannel Logic Platform and Its Applications in the Intelligent Detection of miRNA Related to Liver Cancer

Nanochannel-based analytical techniques have great potential applications for nucleic acid sequencing and high sensitivity detection of biological molecules. However, the sensitivity of conventional solid-state nanochannel sensors is hampered by a lack of effective signal amplification strategies, which has limited its utility in the field of analytical chemistry. Here we selected a solid-state nanochannnel modified with polyethylenimine and Zr⁴⁺ in combination with graphene oxide as the sensing platform. The high-performance sensor is based upon the change of the surface charge of the nanochannel, which is resulted from DNA cascade signal amplification in solution. The target miRNA (miR-122) can be indirectly quantitated with a detection limit of 97.2 aM with an excellent selectivity. Depending on the nucleic acid's hybridization and configuration transform, the designed nanochannel sensing systems can realize the intelligent detection of multiple liver cancer-related miRNA (miR-122 and miR Let-7a) integrating with cascaded INHIBIT-OR logic gate to provide theoretical guidance and technical support for clinical diagnosis and therapeutic evaluation of liver cancer.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

Individually Addressable Multi-nanopores for Single-Molecule Targeted Operations

The fine-tuning of molecular transport is a ubiquitous problem of single-molecule methods. The latter is evident even in powerful single-molecule techniques such as nanopore sensing, where the quest for resolving more detailed biomolecular features is often limited by insufficient control of the dynamics of individual molecules within the detection volume of the nanopore. In this work, we introduce and characterize a reconfigurable multi-nanopore architecture that enables additional channels to manipulate the dynamics of DNA molecules in a nanopore. We show that the fabrication process of this device, consisting of four adjacent, individually addressable nanopores located at the tip of a quartz nanopipette, is fast and highly reproducible. By individually tuning the electric field across each nanopore, these devices can operate in several unique cooperative detection modes that allow moving, sensing, and trapping of DNA molecules with high efficiency and increased temporal resolution.

Low-Noise Nanopore Enables In-Situ and Label-Free Tracking of a Trigger-Induced DNA Molecular Machine at the Single-Molecular Level

Solid-state nanopores have shown special high potential in a label-free molecular assay, structure identification, and target-index at the single-molecular level, even though frustrating electrical baseline noise is still one of the major factors that limit the spatial resolution and signaling reliability of solid-state nanopores, especially in small target detection. Here we develop a significant and easy-operating noise-reduction approach via mixing organic solvents with high dielectric constants into a traditional aqueous electrolyte. The strategy is generally effective for pores made of different materials, such as the most commonly used conical glass (CGN) or SiN_x. While the mechanism should be multisourced, MD simulations suggest the noise reduction may partially arise from the even ionic distribution caused by the addition of higher dielectric species. Among all solvents experimentally tested, the two with the highest dielectric constants, formamide and methylformamide, exhibit the best noise reduction effect for target detection of CGN. The power spectral density at the low-frequency limit is reduced by nearly 3 orders with the addition of 20% formamide. Our work qualifies the reliability of solid-state nanopores into much subtler scales of detection, such as dsDNAs under 100 bp. As a practical example, bare CGN is innovatively employed to perform in-situ tracking of trigger-responsive DNA machine forming oligomers.

Review-Recent Advances in Nanosensors Built with Pre-Pulled Glass Nanopipettes and Their Applications in Chemical and Biological Sensing

Nanosensors built with pre-pulled glass nanopipettes, including bare or chemically modified nanopipettes and fully or partially filled solid nanoelectrodes, have found applications in chemical and biological sensing via resistive-pulse, current rectification, and electrochemical sensing. These nanosensors are easily fabricated and provide advantages through their needle-like geometry with nanometer-sized tips, making them highly sensitive and suitable for local measurements in extremely small samples. The variety in the geometry and layout have extended sensing capabilities. In this review, we will outline the fundamentals in fabrication, modification, and characterization of those pre-pulled glass nanopipette based nanosensors and highlight the most recent progress in their development and applications in real-time monitoring of biological processes, chemical ion sensing, and single entity analysis.

Specific Recognition of Uranyl Ion Employing a Functionalized Nanochannel Platform for Dealing with Radioactive Contamination

Radioactive contamination is a highly concerning global environmental issue along with the development of the nuclear industry. On account of sophisticated operations and high cost of instrument detection methods, numerous efforts have been focused on rapid and simple detection of pollution elements and uranium is the most common one. It is an enormous challenge to push the limit of determination as low as possible while carrying out ultrasensitive detection. Here, we report an intelligent platform based on functionalized solid nanochannels to monitor ultratrace uranyl ions. The platform has a detection limit of 1 fM, which is far below the value that traditional instrumental methods can reach. What is more, the system also exhibits uranyl removal property. The mesenchymal stem cells cultivated in media containing uranyl can achieve excellent viability in the presence of the membranes. This work provides a new choice for handling global radioactive contamination of water.

Single-cell ATP detection and content analyses in electrostimulus-induced apoptosis using functionalized glass nanopipettes

A glass nanopipette functionalized with ATP-responsive gold nanoparticle assemblies was developed for ATP detection in single-cells and used for analysing the content change of ATP during electrostimulus (ES)-induced apoptosis. The variation of ATP content in single normal cells and cancer cells during apoptosis was detected by the method.

The fabrication of a gold nanoelectrode–nanopore nanopipette for dopamine enrichment and multimode detection

It is important to further improve the electrophysiology and electrochemistry techniques of neurotransmitter detection.

Label-free single-molecule identification of telomere G-quadruplexes with a solid-state nanopore sensor

Telomere sequences can spontaneously form G-quadruplexes (G4) in the presence of some cations. In view of their relevance to genetic processes and potential as therapeutic-targets, hitherto, a wealth of conventional techniques have been reported for interrogation of G4 conformation diversity and corresponding folding kinetics, most of which are limited in precision and sensitivity. This work introduces a label-free solid-state nanopore (SSN) approach for the determination of inter-, intra- and tandem molecular G4 with distinct base permutation in various cation buffers with a tailored aperture and meanwhile captures the single-molecule G4 folding process. SSN translocation properties elucidated that both inter- and intramolecular G4 generated higher current blockage with longer duration than flexible homopolymer nucleotide, and intramolecular G4 are structurally more stable with higher event frequency and longer blockage time than intermolecular ones; base arrangement played weak role in translocation behaviors; the same sequences with one, two and three G4 skeletons displayed an increase in current blockage and a gradual extension in dwell time with the increase of molecule size recorded in the same nanopore. We observed the conformation change of single-molecule G4 which indicated the existence of folding/unfolding equilibration in nanopore, and real-time test suggested a gradual formation of G4 with time. Our results could provide a continued and improved understanding of the underlying relevance of structural stability and G4 folding process by utilizing SSN platform which exhibits strategic potential advances over the other methods with high spatial and temporal resolution.

Nanopore detection of single-molecule G-quadruplexes.

Glucose level determination in single cells in their satiety and starvation states using an enzymatic functional glass nanopore

Enzymatic functional glass nanopipettes containing glucose oxidase (GOx) and cytochrome c (Cyt c) were developed for detecting glucose in single cells.

Nanopore biphasic-pulse biosensor

Nanopores as artificial biomimetic nanodevices are of great importance for their applications in biosensing, nanomedicine and bioelectronics. However, it remains a challenge to detect small biomolecules especially small-sized proteins with high sensitivity and selectivity. In the article, we report a simple and efficient method for small-sized protein detection by constructing biphasic-pulse nanopore biosensor. Unlike the traditional resistive pulse sensing, the biphasic-pulse event can provide unique and abundant fingerprint information. Although the nanopore biphasic-pulse electrical signal is originated from both the molecular exclusion electrical resistance and the surface-charged effect of confined molecule, its frequency and amplitude of the waveform can be adjusted by pH, applied potential and salt concentration. Based on the frequency of the biphasic pulse, nanomolar concentration of proteins could be specifically detected and the limit of detection is 1.2 nM. In addition, the biphasic-pulse nanopore shows well discrimination in similar-sized protein detection and its signal generation is highly reproducible. The nanopore biphasic-pulse biosensor should have broad applications as a new generation of powerful single-molecule device.

Single molecule sensing of amyloid-β aggregation by confined glass nanopores

We have developed a glass nanopore based single molecule tool to investigate the dynamic oligomerization and aggregation process of Aβ1-42 peptides. The intrinsic differences in the molecular size and surface charge of amyloid aggregated states could be distinguished through single molecule induced characteristic current fluctuation. More importantly, our results reveal that the neurotoxic Aβ1-42 oligomer tends to adsorb onto the solid surface of nanopores, which may explain its instability and highly neurotoxic features.

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.

Single-cell identification by microfluidic-based in situ extracting and online mass spectrometric analysis of phospholipids expression

This work describes a microfluidic system for in situ extraction of a single-cell and its phosphatidylcholine analysis through mass spectrometry. This approach uncovered cellular heterogeneity among seemingly identical cells and provided a new platform for identification and classification of cells.

Ultrasensitive Detection of Dopamine with Carbon Nanopipets

Carbon fiber micro- and nanoelectrodes have been extensively used to measure dopamine and other neurotransmitters in biological systems. Although the radii of some reported probes were ≪1 μm, the lengths of the exposed carbon were typically on the micrometer scale, thus limiting the spatial resolution of electroanalytical measurements. Recent attempts to determine neurotransmitters in single cells and vesicles have provided additional impetus for decreasing the probe dimensions. Here, we report two types of dopamine sensors based on carbon nanopipets (CNP) prepared by chemical vapor deposition of carbon into prepulled quartz capillaries. These include 10-200 nm radius CNPs with a cavity near the orifice and CNPs with an open path in the middle, in which the volume of sampled solution can be controlled by the applied pressure. Because of the relatively large surface area of carbon exposed to solution inside the pipet, both types of sensors yielded well-shaped voltammograms of dopamine down to ca. 1 nM concentrations, and the unprecedented voltammetric response to 100 pM dopamine was obtained with open CNPs. TEM tomography and numerical simulations were used to model CNP responses. The effect of dopamine adsorption on the CNP detection limit is discussed along with the possibilities of measuring other physiologically important analytes (e.g., serotonin) and eliminating anionic and electrochemically irreversible interferences (e.g., ascorbic acid).

Surface-Charge Effects on Voltammetry in Carbon Nanocavities

Ion transport controlled by electrostatic interactions is an important phenomenon in biological and artificial membranes, channels, and nanopores. Here, we employ carbon-coated nanopipets (CNPs) for studying permselective electrochemistry in a conductive nanopore. A significant accumulation (up to 2000-fold) of cationic redox species and anion depletion inside a CNP by diffuse-layer and surface-charge effects in a solution of low ionic strength were observed as well as the shift of the voltammetric midpeak potential. Finite-element simulations of electrostatic effects on CNP voltammograms show permselective ion transport in a single conducting nanopore and semiquantitatively explain our experimental data. The reported results are potentially useful for improving sensitivity and selectivity of CNP sensors for ionic analytes.

One-Dimensional Assemblies of a DNA Tetrahedron: Manipulations on the Structural Conformation and Single-Molecule Behaviors

DNA nanotechnology can construct various nanostructures with diverse functionalities. However, conformation fluctuations due to the structural flexibility of duplex DNA compromise the efficiency to realize the functionality and reactivity of DNA nanostructures. To understand and control the structural deviation from the design represents a major challenge as well as an opportunity for DNA nanotechnology. In the present work, two series of one-dimensional assemblies of DNA tetrahedrons (DTHs) were fabricated and applied to demonstrate the manipulations of conformation dynamics of a one-dimensional DTH assembly by simple variation on linkage styles at single-molecule resolution. A stepwise strategy allows both nanoassembly with a high fidelity in the number and sequence of DTH units to be assembled with a minimum number of linkage sequences. The characterization for these nanostructures with atomic force microscope (AFM) and a solid-state nanopore technique indicates the difference in conformation dynamics and bending stiffness between two analogous nanoassemblies both in the immobilized state on the surface and free state in solution. This work showed the power of fine-tuning the dynamic conformation of the nanostructures and could see the applications in single-molecule biosensing and functionalization of DNA nanostructures.

Exploration of solid-state nanopores in characterizing reaction mixtures generated from a catalytic DNA assembly circuit

Recent advances have proven that using solid-state nanopores is a promising single molecular technique to enrich the DNA assembly signaling library. Other than using them for distinguishing structures, here we innovatively adapt solid-state nanopores for use in analyzing assembly mixtures, which is usually a tougher task for either traditional characterization techniques or nanopores themselves. A trigger induced DNA step polymerization (SP-CHA), producing three-way-DNA concatemers, is designed as a model. Through counting and integrating the translocation-induced current block when each concatemer passes through a glass conical glass nanopore, we propose an electrophoresis-gel like, but homogeneous, quantitative method that can comprehensively profile the "base-pair distribution" of SP-CHA concatemer mixtures. Due to the higher sensitivity, a number of super long concatemers that were previously difficult to detect via gel electrophoresis are also revealed. These ultra-concatemers, longer than 2 kbp, could provide a much enhanced signal-to-noise ratio for nanopores and are thus believed to be more accurate indicators for the existence of a trigger, which may be of benefit for further applications, such as molecular machines or biosensors.

Biomimetic Nanochannel-Ionchannel Hybrid for Ultrasensitive and Label-Free Detection of MicroRNA in Cells

A biomimetic nanochannel-ionchannel hybrid coupled with electrochemical detector was developed for label-free and ultrasensitive detection of microRNA (miRNA) in cells. Probe single stranded DNA (ssDNA) was first immobilized on the outer surface of the nanochannel-ionchannel hybrid membrane, which can hybridize with the target miRNA in cells. Due to the unique mass transfer property of the hybrid, the DNA-miRNA hybridization kinetics can be sensitively monitored in real-time using the electrochemical technique. More importantly, due to the super small size of the ionchannels, the DNA probe immobilization and hybridization process can be carried out on the outer surface of the ionchannel side, which can effectively avoid the blockage and damage of channels and thus considerably enhance the reproducibility and accuracy of the method. Using this strategy, the miRNA ranging from 0.1 fM to 0.1 μM can be facilely detected with a low detection limit of 15.4 aM, which is much lower than most reported work. The present strategy provides a sensitive and label-free miRNA detection platform, which will be of great significance in biomedical research and clinical diagnosis.

A simple strategy for the fabrication of gold-modified single nanopores and its application for miRNA sensing

A simple strategy was developed for nanopore modification, and was used for miRNA sensing with good sensitivity and selectivity.

Redox Cycling in Individually Encapsulated Attoliter-Volume Nanopores

Redox cycling electrochemistry in arrays of individually encapsulated attoliter-volume ( V ∼ 10 aL) nanopores is investigated and reported here. These nanopore electrode array (NEA) structures exhibit distinctive electrochemical behaviors not observed in open NEAs, which allow free diffusion of redox couples between the nanopore interior and bulk solution. Confined nanopore environments, generated by sealing NEAs with a layer of poly(dimethylsiloxane), are characterized by enhanced currents-up to 250-fold compared with open NEAs-owing to effective trapping of the redox couple inside the nanopores and to enhanced mass transport effects. In addition, electrochemical rectification ( ca. 1.5-6.3) was observed and is attributed to ion migration. Finite-element simulations were performed to characterize the concentration and electric potential gradients associated with the disk electrode, aqueous medium, and ring electrode inside the nanopores, and the results are consistent with experimental observations. The additional signal enhancement and redox-cycling-based rectification behaviors produced in these self-confined attoliter-volume nanopores are potentially useful in devising ultrasensitive sensors and molecular-based iontronic devices.