Real-time and in-situ intracellular ATP assay with polyimidazolium brush-modified nanopipette

Nanosensors Monitor Intracellular GSH Depletion: GSH Triggers Cu(II) for Tumor Imaging and Inhibition

Glutathione (GSH) is the primary antioxidant in cells, and GSH consumption will break the redox balance in cells. Based on this, a method that uses high concentrations of GSH in the tumor microenvironment to trigger the redox reaction of Cu(II) to generate copper nanoprobes with fluorescence and tumor growth inhibition properties is proposed. The nanoprobe mainly exists in the form of Cu(I) and catalyzes the decomposition of hydrogen peroxide into hydroxyl radicals. At the same time, a simple and controllable carbon micro-nano electrode is used to construct a single-cell sensing platform, which enable the detection of glutathione content in single living cells after Cu(II) treatment, providing an excellent example for detecting single-cell biomolecules.

PNA-Functionalized, Silica Nanowires-Filled Glass Microtube for Ultrasensitive and Label-Free Detection of miRNA-21

MicroRNAs (miRNAs) are endogenous and noncoding single-stranded RNA molecules with a length of approximately 18-25 nucleotides, which play an undeniable role in early cancer screening. Therefore, it is very important to develop an ultrasensitive and highly specific method for detecting miRNAs. Here, we present a bottom-up assembly approach for modifying glass microtubes with silica nanowires (SiNWs) and develop a label-free sensing platform for miRNA-21 detection. The three-dimensional (3D) networks formed by SiNWs make them abundant and highly accessible sites for binding with peptide nucleic acid (PNA). As a receptor, PNA has no phosphate groups and exhibits an overall electrically neutral state, resulting in a relatively small repulsion between PNA and RNA, which can improve the hybridization efficiency. The SiNWs-filled glass microtube (SiNWs@GMT) sensor enables ultrasensitive, label-free detection of miRNA-21 with a detection limit as low as 1 aM at a detection range of 1 aM-100 nM. Noteworthy, the sensor can still detect miRNA-21 in the range of 102-108 fM in complex solutions containing 1000-fold homologous interference of miRNAs. The high anti-interference performance of the sensor enables it to specifically recognize target miRNA-21 in the presence of other miRNAs and distinguish 1-, 3-mismatch nucleotide sequences. Significantly, the sensor platform is able to detect miRNA-21 in the lysate of breast cancer cell lines (e.g., MCF-7 cells and MDA-MB-231 cells), indicating that it has good potential in the screening of early breast cancers.

In situ monitoring of cytoplasmic dopamine levels by noble metals decorated carbon fiber tips

Dopamine (DA), a catecholamine neurotransmitter, is crucial in brain signal transmission. Monitoring cytoplasmic DA levels can reflect changes in metabolic factors and provide valuable information for researching the mechanisms involved in neurodegenerative diseases. However, the in-situ detection of intracellular DA is constrained by its low contents in small-sized single cells. In this work, we report that noble metal (Au, Pt)-modified carbon fiber micro-nanoelectrodes are capable of real-time detection of DA in single cells with excellent sensitivity, selectivity, and anti-contamination capabilities. Notably, noble metals can be modified on the electrode surface through electrochemical deposition to enhance the conductivity of the electrode and the oxidation current of DA by 50 %. The nanosensors can work stably and continuously in rat adrenal pheochromocytoma cells (PC12) to monitor changes in DA levels upon K+ stimulation. The functionalized carbon fibers based nanosensors will provide excellent prospects for DA analysis in the brains of living animals.

Recent advances of nanopore technique in single cell analysis

Single cells and their dynamic behavior are closely related to biological research. Monitoring their dynamic behavior is of great significance for disease prevention. How to achieve rapid and non-destructive monitoring of single cells is a major issue that needs to be solved urgently. As an emerging technology, nanopores have been proven to enable non-destructive and label-free detection of single cells. The structural properties of nanopores enable a high degree of sensitivity and accuracy during analysis. In this article, we summarize and classify the different types of solid-state nanopores that can be used for single-cell detection and illustrate their specific applications depending on the size of the analyte. In addition, their research progress in material transport and microenvironment monitoring is also highlighted. Finally, a brief summary of existing research challenges and future trends in nanopore single-cell analysis is tentatively provided.

Iontronic Photoelectrochemical Biorecognition Probing

Herein, the first iontronic photoelectrochemical (PEC) biorecognition probing is devised by rational engineering of a dual-functional bioconjugate, i.e., a light-sensitive intercalated structural DNA, as a smart gating module confined within a nanotip, which could respond to both the incident light and biotargets of interest. Light stimulation of the bioconjugate could intensify the negative charge at the nano-orifice to sustain enhanced ionic current. The presence of proteins (e.g., acetylcholinesterase, AChE) or nucleic acids (e.g., microRNA (miR)-10b) could lead to bioconjugate release with altered ionic signaling. The practical applicability of the methodology is confirmed by AChE detection in human serum and miR-10b detection in single cells.

θ-Nanopipette for Single-Cell Resistive-Pulse Profiling of DNA Repair Proteins Accompanied by Drug Evaluation

Single-cell analysis of the DNA repair protein is important but remains unachieved. Exploration of nanopipettte technologies in single-cell electroanalysis has recently seen rapid growth, while the θ-nanopipette represents an emerging technological frontier with its potential largely veiled. Here a θ-nanopipette is first applied for single-cell resistive-pulse sensing (RPS) of the important DNA repair protein O6-alkylguanine DNA alkyltransferase (hAGT). The removal of alkyl mutations by hAGT could restore the damaged aptamer linking with a structural DNA carrier, allowing the selective binding of the aptamer to thrombin with precisely matched size to produce distinct RPS signals when passing through the orifice. Kinetic analysis of hAGT repair was studied. Meanwhile, the device shows the simultaneous on-demand infusion of inhibitors to inactivate the hAGT activity, indicative of its potential in drug screening for enhanced chemotherapy. This work provides a new paradigm for θ-nanopipette-based single-cell RPS of a DNA repair protein accompanied by drug evaluation.

A multi-stimuli-responsive nanochannel inspired by biological disulfide bond

Disulfide bonds exist widely in channel protein and play an essential role in matter exchange and signal transduction (e.g., rhodopsin, canonical transient receptor potential 5 (TRPC5)). The research on disulfide bond in nanochannel is significant for the cognition of their biological functions. However, the fragility of biological channel limits the in-situ study and practical application. Herein, an innovative biologically-inspired artificial nanochannel based on disulfide bond (NCDS) with excellent durability, adjustable surface property is proposed. The constructed NCDS has a multi-response to UV-light, thiol (e.g., cysteine (Cys)) or pH stimulation, and can obtain reversibility after regulation by hydrogen peroxide (H₂O₂) or H⁺. The biomimetic NCDS shows great potential in biosensor and intelligent response design. This study also shines new light to channel protein based on disulfide bond that despite the nanochannel has specificity, it will be modulated by the change of nature environment, such as UV-light and chemical microenvironment (e.g., redox state and pH), which might be the reason of some disease.

Solid-State Nanopore/Nanochannel Sensing of Single Entities

Solid-state nanopores/nanochannels, with their high stability, tunable geometry, and controllable surface chemistry, have recently become an important tool for constructing biosensors. Compared with traditional biosensors, biosensors constructed with solid-state nanopores/nanochannels exhibit significant advantages of high sensitivity, high specificity, and high spatiotemporal resolution in the detection single entities (such as single molecules, single particles, and single cells) due to their unique nanoconfined space-induced target enrichment effect. Generally, the solid-state nanopore/nanochannel modification method is the inner wall modification, and the detection principles are the resistive pulse method and the steady-state ion current method. During the detection process, solid-state nanopore/nanochannel is easily blocked by single entities, and interfering substances easily enter the solid-state nanopore/nanochannel to generate interference signals, resulting in inaccurate measurement results. In addition, the problem of low flux in the detection process of solid-state nanopore/nanochannel, these defects limit the application of solid-state nanopore/nanochannel. In this review, we introduce the preparation and functionalization of solid-state nanopore/nanochannel, the research progress in the field of single entities sensing, and the novel sensing strategies on solving the above problems in solid-state nanopore/nanochannel single-entity sensing. At the same time, the challenges and prospects of solid-state nanopore/nanochannel for single-entity electrochemical sensing are also discussed.

Recent Advances in the Development and Characterization of Electrochemical and Electrical Biosensors for Small Molecule Neurotransmitters

Neurotransmitters act as chemical messengers, determining human physiological and psychological function, and abnormal levels of neurotransmitters are related to conditions such as Parkinson's and Alzheimer's disease. Biologically and clinically relevant concentrations of neurotransmitters are usually very low (nM), so electrochemical and electronic sensors for neurotransmitter detection play an important role in achieving sensitive and selective detection. Additionally, these sensors have the distinct advantage to potentially be wireless, miniaturized, and multichannel, providing remarkable opportunities for implantable, long-term sensing capabilities unachievable by spectroscopic or chromatographic detection methods. In this article, we will focus on advances in the development and characterization of electrochemical and electronic sensors for neurotransmitters during the last five years, identifying how the field is progressing as well as critical knowledge gaps for sensor researchers.

Neuromorphic functions with a polyelectrolyte-confined fluidic memristor

Reproducing ion channel-based neural functions with artificial fluidic systems has long been an aspirational goal for both neuromorphic computing and biomedical applications. In this study, neuromorphic functions were successfully accomplished with a polyelectrolyte-confined fluidic memristor (PFM), in which confined polyelectrolyte-ion interactions contributed to hysteretic ion transport, resulting in ion memory effects. Various electric pulse patterns were emulated by PFM with ultralow energy consumption. The fluidic property of PFM enabled the mimicking of chemical-regulated electric pulses. More importantly, chemical-electric signal transduction was implemented with a single PFM. With its structural similarity to ion channels, PFM is versatile and easily interfaces with biological systems, paving a way to building neuromorphic devices with advanced functions by introducing rich chemical designs.

Organic Molecular Probe Enabled Ionic Current Rectification toward Subcellular Detection of Glutathione with High Selectivity, Sensitivity, and Recyclability

Single-cell interrogation with the solid-state nanoprobes enables understanding of the linkage between cellular behavior and heterogeneity. Herein, inspired by the charge property of the organic molecular probe (OMP), a generic ionic current rectification (ICR) single-cell methodology is established, exemplified by subcellular detection of glutathione (GSH) with high selectivity, sensitivity, and recyclability. The as-developed nanosensor can transduce the subcellular OMP-GSH interaction via a sensitive ionic response, which stems from the superior specificity of OMP and its essential charge property. In addition, the nanosensor exhibits good reversibility, since the subsequent tandem reaction after the recognition can well recover the sensing surface. Given the diverse structures and tailorable charge properties of OMP, this work underpins a new and general method of OMP-based ICR single-cell analysis.

Functional nucleic acid engineered double-barreled nanopores for measuring sodium to potassium ratio at single-cell level

The use of double-barreled nanopipette (θ-nanopipette) to electrically sample, manipulate, or detect biomaterials has recently seen strong growth in single-cell studies, driven by the potential of the nanodevices and applications that they may enable. Considering the pivotal roles of Na/K ratio (R_Na/K) at cellular level, herein we describe an engineered θ-nanopipette for measuring single-cell R_Na/K. The two independently addressable nanopores, located within one nanotip, allow respective customization of functional nucleic acids but simultaneous deciphering of Na and K levels inside a single cell of a non-Faradic manner. Two ionic current rectification signals, corresponding to the Na- and K-specific smart DNA responses, could be easily used to derive the R_Na/K. The applicability of this nanotool is validated by practical probing intracellular R_Na/K during the drug-induced primary stage of apoptotic volume decrease. Especially, the R_Na/K has been shown by our nanotool to be different in cell lines with different metastatic potential. This work is expected to contribute to futuristic study of single-cell R_Na/K in various physiological and pathological processes.

Enhanced Sensing Performance of a Microchannel-Based Electrochemiluminescence Biosensor for Adenosine Triphosphate via a dsDNA Superstructure Amplification Strategy

The sensing performance of a microchannel-based electrochemiluminescence (ECL) biosensor is related to the change ratio of charge density on the surface of microchannels caused by a target recognition reaction. In this study, adenosine triphosphate (ATP) served as a model target. The dsDNA superstructures containing a capture probe (CP, containing an ATP aptamer sequence) and alternating units of ssDNA probes of P1 and P2, CP/(P1/P2)_n, were grafted onto the inner wall of microchannels first. The CP in dsDNA superstructures captured ATP molecules, causing the release of dsDNA fragments containing alternating units of P1 and P2, (P1/P2)_n. The target recognition reaction significantly changed the charge density of microchannels, which altered the ECL intensity of the (1,10-phenanthroline)ruthenium(II)/tripropylamine system in the reporting interface. The ECL intensity of the constructed system had a linear relationship with the logarithm of ATP concentration ranging from 1 fM to 100 pM with a detection limit of 0.32 fM (S/N = 3). The biosensor was successfully applied to detect ATP in rat brains.

Versatile Construction of Biomimetic Nanosensors for Electrochemical Monitoring of Intracellular Glutathione

The current strategies for nanoelectrode functionalization usually involve sophisticated modification procedures, uncontrollable and unstable modifier assembly, as well as a limited variety of modifiers. To address this issue, we propose a versatile strategy for large-scale synthesis of biomimetic molecular catalysts (BMCs) modified nanowires (NWs) to construct functionalized electrochemical nanosensors. This design protocol employs an easy, controllable and stable assembly of diverse BMCs-poly(3,4-ethylenedioxythiophene) (PEDOT) composites on conductive NWs. The intrinsic catalytic activity of BMCs combined with outstanding electron transfer ability of conductive polymer enables the nanosensors to sensitively and selectively detect various biomolecules. Further application of sulfonated cobalt phthalocyanine functionalized nanosensors achieves real-time electrochemical monitoring of intracellular glutathione levels and its redox homeostasis in single living cells for the first time.

A highly selective ATP-responsive biomimetic nanochannel based on smart copolymer

ATP-sensitive potassium (K_ATP) channels couple intracellular metabolism to the electrical activity by regulating K⁺ flux across the plasma membrane, thus playing an important role in both normal and pathophysiology. To understand the mechanism of ATP regulating biological ion channels, developing an ATP-responsive artificial nanochannel is an appealing but challenging topic because K_ATP channel is a heteromultimer of two subunits (potassium channel subunit (Kir6.x) and sulfonylurea receptor (SUR)) and exhibit dynamic functions with adjustability and reversibility. Inspired by the structure of K_ATP channels, we designed a smart copolymer modified nanochannel that may address the challenge. In the tricomponent poly(N-isopropylacrylamide) (PNIPAAm, PNI)-based copolymer system, phenylthiourea was used to bind the phosphate units of nucleotides and phenylboronic acid was introduced to combine the pentose ring of the nucleoside unit. Besides, a -COOH group with electron-withdrawing property was added into the phenylthiourea units, which may promote the hydrogen-bond-donating ability of thiourea. Specially, the smart copolymer not only provided static binding sites for recognition but also translated the recognition of ATP into their dynamic conformational transitions by changing the hydrogen-bonding environments surrounding PNIPAAm chains, thus achieving the gating function of nanochannel, which resembled the integration and coordination of Kir6.x and SUR units in active K_ATP. The ATP-regulated ion channel exhibited excellent stability and reversibility. This study is the first example showing how to learn from nature to assemble the ATP-responsive artificial nanochannel and demonstrate the possible mechanism of ATP gating.

Discerning Tyrosine Phosphorylation from Multiple Phosphorylations Using a Nanofluidic Logic Platform

Discerning tyrosine phosphorylation (pTyr) catalyzed by Tyr kinase is central to the revelation of oncogenic mechanisms and the development of targeted anticancer drugs. Despite some techniques, this goal remains challenging, especially when faced with the interference of multiple phosphorylation events, including serine (pSer) and threonine phosphorylation (pThr). We describe here a functional polymer-modified artificial ion nanochannel, which enables the sensitive and selective recognition of phosphotyrosine (pY) peptide by the distinct ionic current change. Such a recognition effect allows for the nanochannel to work in a complex protein digest condition. Further, the implementation of nanofluidic logic functions with the addition of Ca²⁺ dramatically improves the selectivity of the nanochannel to pY peptide and thus can discern pTyr by the Tyr kinase from pSer by the Ser/Thr kinase through simultaneously monitoring multisite phosphorylation at the same or different peptide substrates in one-pot. This logic sensing platform displays the potential in differentiating Tyr kinase and Ser/Thr kinase and assessing multi-kinase activities in multi-targeted drug design.

Highly efficient zinc finger peptide detection with ZIF-8-modified micropipets

Zeolitic imidazolate framework-8 (ZIF-8)-modified micropipets can be an effective sensing platform for zinc finger peptides, the limit of detection of which reaches 10-2 μg ml-1. A series of techniques for detecting biomolecules are expected to emerge because of its simplicity, low cost, and universality by modifying other functional materials into the micropipets.

Photocontrolled Nanopipette Biosensor for ATP Gradient Electroanalysis of Single Living Cells

Emerging nanopipette tools have demonstrated substantial potential for advanced single-cell analysis, which plays vital roles from fundamental cellular biology to biomedical diagnostics. Highly recyclable nanopipettes with easy and quick regeneration are of special interest for precise and multiple measurements. However, existing recycle strategies are generally plagued by operational complexity and limited efficiency. Light, acting in a noncontact way, should be the ideal external stimulus to address this issue. Herein, we present the photocontrolled nanopipette capable of probing cellular adenosine triphosphate (ATP) gradient at single-cell level with good sensitivity, selectivity, and reversibility, which stems from the use of ATP-specific azobenzene (Azo)-incorporated DNA aptamer strands (AIDAS) and thereby the sensible transduction of variable nanopore size by the ionic currents passing through the aperture. Photoisomerized conformational change of the AIDAS by alternative UV/vis light stimulation ensures its noninvasive regeneration and repeated detection. Inducement and inhibition of the cellular ATP could also be probed by this nanosensor.

Twin Nanopipettes for Real-Time Electrochemical Monitoring of Cytoplasmic Microviscosity at a Single-Cell Level

Cytoplasmic microviscosity (CPMV) plays essential roles in governing the diffusion-mediated cellular processes and has been recognized as a reliable indicator of the cellular response of many diseases and malfunctions. Current CPMV studies are exclusively established by probe-assisted optical methods, which nevertheless necessitate the complicated synthesis and delivery of optical probes into cells and thus the issues of biocompatibility and bio-orthogonality. Using twin nanopipettes integrated with a patch-clamp system, a practical electrochemical single-cell measurement is presented, which is capable of real-time and long-term CPMV detection without cell disruption. Specifically, upon the operation of the twin nanopipettes, the cellular CPMV status, which is correlated to cytoplasmic ionic mobility, could be sensibly transduced via the ionic current passing through the nanosystem. The average CPMV value of HeLa cells was detected as ca. 86 cP. Notably, the correlation between chemotherapy and CPMV alterations makes this approach possible for the real-time and long-term assessment of the evolution of external stimuli, as exemplified by the two natural products taxol and colchicine. Integrated with the patch-clamp setup, this study features the first use of twin nanopipettes for electrochemical CPMV monitoring of single living cells, and it is expected to inspire more interest in the exploitation of dual- and multiple nanopipettes for advanced single-cell analysis.

Target-Triggered Assembly in a Nanopipette for Electrochemical Single-Cell Analysis

Engineered nanopipette tools have recently emerged as a powerful approach for electrochemical nanosensing, which has major implications in both fundamental biological research and biomedical applications. Herein, we describe a generic method of target-triggered assembly of aptamers in a nanopipette for nanosensing, which is exemplified by sensitive and rapid electrochemical single-cell analysis of adenosine triphosphate (ATP), a ubiquitous energy source in life and important signaling molecules in many physiological processes. Specifically, a layer of thiolated aptamers is immobilized onto a Au-coated interior wall of a nanopipette tip. With backfilled pairing aptamers, the engineered nanopipette is then used for probing intracellular ATP via the ATP-dependent linkage of the split aptamers. Due to the higher surface charge density from the aptamer assembly, the nanosensor would exhibit an enhanced rectification signal. Besides, this ATP-responsive nanopipette tool possesses excellent selectivity and stability as well as high recyclability. This work provides a practical single-cell nanosensor capable of intracellular ATP analysis. More generally, integrated with other split recognition elements, the proposed mechanism could serve as a viable basis for addressing many other important biological species.