Functional Nanochannels for Sensing Tyrosine Phosphorylation

Stimuli-Responsive Ion Transport Regulation in Nanochannels by Adhesion-Induced Functionalization of Macroscopic Outer Surface

Responsive regulation of ion transport through nanochannels is crucial in the design of smart nanofluidic devices for sequencing, sensing, and water-energy nexus. Functionalization of the inner wall of the nanochannel enhances interaction with ions and fluid but restricts versatile chemical approaches and accurate characterizations of fluidic interfaces. Herein, we reveal a responsive regulating mechanism of ion transport through nanochannels by polydopamine (PDA)-induced functionalization on the macroscopic outer surface of nanochannels. Responsive molecules were codeposited with PDA on the outer surface of nanochannels and formed a valve of nanometer thickness to manually manipulate ion transport by changing its gap spacing, surface charge, and wettability under external stimulus. The response ratio can be up to 100-fold by maximizing the proportion of responsive molecules on the outer surface. Laminating the codepositions of different responsive molecules with PDA on the channel's outer surface produces multiple responses. A nearly universal adhesion of PDA with responsive molecules on the open outer surface induces nanochannels responsive to different external stimuli with variable response ratios and arbitrary combinations. The results challenge the primary role of functionalization on the nanoconfined interface of nanofluidics and open opportunities for developing new-style nanofluidic devices through the functionalization of macroscopic interface.

Harnessing Cooperative Multivalency in Thioguanine for Surface-Enhanced Raman Scattering (SERS)-Based Differentiation of Polyfunctional Analytes Differing by a Single Functional Group

Small-molecule receptors are increasingly employed to probe various functional groups for (bio)chemical analysis. However, differentiation of polyfunctional analogs sharing multiple functional groups remains challenging for conventional mono- and bidentate receptors because their insufficient number of binding sites limits interactions with the least reactive yet property-determining functional group. Herein, we introduce 6-thioguanine (TG) as a supramolecular receptor for unique tridentate receptor-analyte complexation, achieving ≥97 % identification accuracy among 16 polyfunctional analogs across three classes: glycerol derivatives, disubstituted propane, and vicinal diols. Crucially, we demonstrate distinct spectral changes induced by the tridentate interaction between TG's three anchoring points and all the analyte's functional groups, even the least reactive ones. Notably, hydrogen bond (H-bond) networks formed in the TG-analyte complexes demonstrate additive effects in binding strength originating from good bond linearity, cooperativity, and resonance, thus strengthening complexation events and amplifying the differences in spectral changes induced among analytes. It also enhances spectral consistency by selectively forming a sole configuration that is stronger than the respective analyte-analyte interaction. Finally, we achieve 95.4 % accuracy for multiplex identification of a mixture consisting of multiple polyfunctional analogs. We envisage that extension to other multidentate non-covalent interactions enables the development of interference-free small molecule-based sensors for various (bio)chemical analysis applications.

Injectable Thermo-Responsive Peptide Hydrogels and Its Enzyme Triggered Dynamic Self-Assembly

Endogenous stimuli-responsive injectable hydrogels hold significant promise for practical applications due to their spatio-temporal controllable drug delivery. Herein, we report a facile strategy to construct a series of in situ formation polypeptide hydrogels with thermal responsiveness and enzyme-triggered dynamic self-assembly. The thermo-responsive hydrogels are from the diblock random copolymer mPEG-b-P(Glu-co-Tyr). The L-glutamic acid (Glu) segments with different γ-alkyl groups, including methyl, ethyl, and n-butyl, offer specific secondary structure, facilitating the formation of hydrogel. The L-tyrosine (Tyr) residues not only provide hydrogen-bond interactions and thus adjust the sol-gel transition temperatures, but also endow polypeptide enzyme-responsive properties. The PTyr segments could be phosphorylated, and the phosphotyrosine copolymers were amphiphilies, which could readily self-assemble into spherical aggregates and transform into sheet-like structures upon dephosphorylation by alkaline phosphatase (ALP). P(MGlu-co-Tyr/P) and P(MGlu-co-Tyr) copolymers showed good compatibility with both MC3T3-E1 and Hela cells, with cell viability above 80% at concentrations up to 1000 μg/mL. The prepared injectable polypeptide hydrogel and its enzyme-triggered self-assemblies show particular potential for biomedical applications.

DNA-Peptide Interaction-Modulated Charge Reversal in Biomimetic Nanochannels for Simple and Efficient Detection of Histone Deacetylases

Protein acetylation, a fundamental post-translational modification, plays a critical role in the regulation of gene expression and cellular processes. Monitoring histone deacetylases (HDACs) is important for understanding epigenetic dynamics and advancing the early diagnosis of malignancies. Here, we leverage the dynamic characteristics of DNA-peptide interactions in biomimetic nanochannels to develop a HDAC detection method. In specific, the catalysis of peptide deacetylation by HDACs triggers alterations in the charge states of the nanochannel surface to accommodate DNA molecules. Then, the interaction between DNA and peptides shifts the nanochannel surface charge from positive to negative, leading to a reversal of the ion current rectification (ICR). By calculation of the ICR ratio, quantitative detection of HDACs can be efficiently achieved using the nanochannel-based method in an enzyme-free and label-free manner. Our experimental results demonstrate that HDACs can be detected by using this method within a concentration range of 0.5-500 nM. The innate simplicity and efficiency of this strategy may render it a valuable tool for advancing both fundamental research and clinical applications in the realm of epigenetics and personalized medicine.

Graphene Oxide-Mediated Regulation of Volume Exclusion and Wettability in Biomimetic Phosphorylation-Responsive Ionic Gates

Replicating phosphorylation-responsive ionic gates via artificial fluidic systems is essential for biomolecular detection and cellular communication research. However, current approaches to governing the gates primarily rely on volume exclusion or surface charge modulation. To overcome this limitation and enhance ion transport controllability, we introduce graphene oxide (GO) into nanochannel systems, simultaneously regulating the volume exclusion and wettability. Moreover, inspired by (cAMP)-dependent protein kinase A (PKA)-regulated L-type Ca2+ channels, we employ peptides for phosphorylation which preserves them as nanoadhesives to coat nanochannels with GO. The coating boosts steric hindrance and diminishes wettability, creating a substantial ion conduction barrier, which represents a significant advancement in achieving precise ion transport regulation in abiotic nanochannels. Leveraging the mechanism, we also fabricated a sensitive biosensor for PKA activity detection and inhibition exploration. The combined regulation of volume exclusion and wettability offers an appealing strategy for controlled nanofluidic manipulation with promising biomedical applications in diagnosis and drug discovery.

Solid-state nanochannels for bio-marker analysis

Bio-markers, such as ions, small molecules, nucleic acids, peptides, proteins and cells, participate in the construction of living organisms and play important roles in biological processes. It is of great significance to accurately detect these bio-markers for studying their basic functions, the development of molecular diagnosis and to better understand life processes. Solid-state nanochannel-based sensing systems have been demonstrated for the detection of bio-markers, due to their rapid, label-free and high-throughput screening, with high sensitivity and specificity. Generally, studies on solid-state nanochannels have focused on probes on the inner-wall (PIW), ignoring probes on the outer-surface (POS). As a result, the direct detection of cells is difficult to realize by these inner-wall focused nanochannels. Moreover, the sensitivity for detecting ions, small molecules, nucleic acids, peptides and proteins requires further improvement. Recent research has focused on artificial solid-state nanochannels with POS, which have demonstrated the ability to independently regulate ion transport. This design not only contributes to the in situ detection of large analytes, such as cells, but also provides promising opportunities for ultra-high sensitivity detection with a clear mechanism. In this tutorial review, we present an overview of the detection principle used for solid-state nanochannels, inner-wall focused nanochannels and outer-surface focused nanochannels. Furthermore, we discuss the remaining challenges faced by current nanochannel technologies and provide insights into their prospects.

A highly sensitive nanochannel device for the detection of SUMO1 peptides

SUMOylation is an important and highly dynamic post-translational modification (PTM) process of protein, and its disequilibrium may cause various diseases, such as cancers and neurodegenerative disorders. SUMO proteins must be accurately detected to understand disease states and develop effective drugs. Reliable antibodies against SUMO2/3 are commercially available; however, efficient detectors are yet to be developed for SUMO1, which has only 50% homology with SUMO2 and SUMO3. Here, using phage display technology, we identified two cyclic peptide (CP) sequences that could specifically bind to the terminal dodecapeptide sequence of SUMO1. Then we combined the CPs and polyethylene terephthalate conical nanochannel films to fabricate a nanochannel device highly sensitive towards the SUMO1 terminal peptide and protein; sensitivity was achieved by ensuring marked variations in both transmembrane ionic current and Faraday current. The satisfactory SUMO1-sensing ability of this device makes it a promising tool for the time-point monitoring of the SENP1 enzyme-catalyzed de-SUMOylation reaction and cellular imaging. This study not only solves the challenge of SUMO1 precise recognition that could promote SUMO1 proteomics analysis, but also demonstrates the good potential of the nanochannel device in monitoring of enzymes and discovery of effective drugs.

Nanofluidic Device for Detection of Lysine Methylpeptides and Sensing of Lysine Methylation

Protein methylation is the smallest possible yet vitally important post-translational modification (PTM). This small and chemically inert addition in proteins makes the analysis of methylation more challenging, thus calling for an efficient tool for the sake of recognition and detection. Herein, we present a nanofluidic electric sensing device based on a functionalized nanochannel that was constructed by introducing monotriazole-containing p-sulfonatocalix[4]arene (TSC) into a single asymmetric polymeric nanochannel via click chemistry. The device can selectively detect lysine methylpeptides with subpicomole sensitivity, distinguish between different lysine methylation states, and monitor the lysine methylation process by methyltransferase at the peptide level in real time. The introduced TSC molecule, with its confined asymmetric configuration, presents the remarkable ability to selectively bind to lysine methylpeptides, which, coupled with the release of the complexed Cu ions, allows the device to transform the molecular-level recognition to the discernible change in ionic current of the nanofluidic electric device, thus enabling detection. This work could serve as a stepping stone to the development of a new methyltransferase assay and the chemical that specifically targets lysine methylation in PTM proteomics.

Accurate detection of reactive oxygen species by tuning an elastic motif (GPGGA)4 in nanopores

We have developed a reactive oxygen species (ROS) sensor based on nanopores modified with GGGCEG(GPGGA)₄CEG. The formation of an intramolecular disulfide bond oxidized by ROS leads to conformation changes in GGGCEG(GPGGA)₄CEG, which then induces an obvious change in the size of the nanopores and a corresponding ionic current change. This work allows the accurate and dynamic monitoring of ROS through the combination of (GPGGA)₄ and nanopores.

Super-Assembled Multi-Level Asymmetric Mesochannels for Coupled Accelerated Dual-Ion Selective Transport

Asymmetric nanofluidic devices hold great potential in energy conversion applications. However, most of the existing asymmetric nanofluidic devices remain a single-level asymmetric structure and a single-ion selective layer, which results in weak ion selectivity and limited energy conversion efficiency. Herein, a multi-level asymmetric mesoporous carbon/anodized aluminum/mesoporous silica (MC/AAO/MS) nanofluidic device with abundant and ordered mesochannels is constructed from super-assembly strategy. The resultant MC/AAO/MS exhibits diode-like ion transport and outstanding ion storage-release performance. Importantly, MC/AAO/MS couples the MC and MS dual-ion selective layers, which ensures a high ionic conductance and evidently enhances the cation selectivity. Thereby, the MC/AAO/MS demonstrates ascendant salinity gradient energy conversion performance. The power density and conversion efficiency can reach up to 5.37 W m^-2 and 32.79%, respectively. Noteworthy, a good energy conversion performance of 63 mW m^-2 can still be achieved upon high working area, outperforming 300% of the performance of MC/AAO and MS/AAO single-level asymmetric nanochannels. Theoretical calculation further verifies that the multi-level asymmetric structure and dual-ion selective transport are the reason for the enhanced cation selectivity and energy conversion efficiency. This work opens a new avenue for constructing multi-level asymmetric structured nanofluidic devices for various applications.

Recent advance in solid state nanopores modification and characterization

Nanopore, due to its advantages of modifiable, controllability and sensitivity, has made a splash in recent years in the fields of biomolecular sequencing, small molecule detection, salt differential power generation, and biomimetic ion channels, etc. In these applications, the role of chemical or biological modification is indispensable. Compared with small molecules, the modification of polymers is more difficult and the methods are more diverse. Choosing appropriate modification method directly determines the success or not of the research, therefore, it is necessary to summarize the polymer modification methods toward nanopores. In addition, it is also important to provide clear and convincing evidence that the nanopore modification is successful, the corresponding characterization methods are also indispensable. Therefore, this review will summarize the methods of polymer modification of nanopores and efficient characterization methods. And we hope that this review will provide some reference value for like-minded researchers.

Specific extraction of nucleic acids employing pillar[6]arene-functionalized nanochannel platforms

The rapid extraction of high-purity nucleic acids from complex biological samples using conventional methods is complicated. Therefore, in this study, glycine-pillar[6]arene (Gly-P6)-functionalized tapered nanochannels were constructed using 32-mer single-stranded E. coli DNA (ssDNA) as a model sequence, which can selectively transport ssDNA by multiple noncovalent forces (transport flux of 2.65 nM cm^-2 h^-1) under the interference of amino acids and other substances. In view of these prospective results, the selective transport of nucleic acids with nanochannels could be applied in the design of nucleic acid enrichment and separation systems in the future.

Bioinspired Hydrogen Peroxide-Activated Nanochannels and Their Applications in Cancer Cell Analysis

Bioinspired nanochannels that manipulate ion transport have shown great potential for understanding complex physiological processes. Herein, inspired by the gating function of the biological ion channels, we designed and constructed artificial hydrogen peroxide (H₂O₂)-activated nanochannels by decorating the inner pore surface with 4-(phenoxymethyl) benzeneboronic acid pinacol ester (PBAE). Benefiting from the specific hydrolysis reaction between H₂O₂ and PBAE in the confined nanochannels, the functionalized artificial nanochannels exhibited a highly selective and sensitive response toward H₂O₂. The system could switch between open/closed states in the presence/absence of H₂O₂ by the ionic current test. Meanwhile, comsol simulations were carried out to evidence the mechanism of hydrogen peroxide triggered regulation of ion transport by the nanochannels. It was found that the surface charge density of the nanochannels changed along with the addition of H₂O₂. Furthermore, based on the sensing strategy, the PBAE-functionalized nanochannel membrane was applied in the detection of H₂O₂ in the tumor microenvironment, which achieved highly selective distinguishing of cancerous cells from normal cells. This work provides a versatile method to construct bioinspired nanochannel-based platforms for detecting small reactive molecules and offers prospects for the application of disease diagnosis and prognosis.

Pushing the limits of nanopore transport performance by polymer functionalization

Inspired by the design and performance of biological pores, polymer functionalization of nanopores has emerged as an evolving field to advance transport performance within the last few years. This feature article outlines developments in nanopore functionalization and the resulting transport performance including gating based on electrostatic interaction, wettability and ligand binding, gradual transport controlled by polymerization as well as functionalization-based asymmetric nanopore and nanoporous material design going towards the transport direction. Pushing the limits of nanopore transport performance and thus reducing the performance gap between biological and technological pores is strongly related to advances in polymerization chemistry and their translation into nanopore functionalization. Thereby, the effect of the spatial confinement has to be considered for polymer functionalization as well as for transport regulation, and mechanistic understanding is strongly increased by combining experiment and theory. A full mechanistic understanding together with highly precise nanopore structure design and polymer functionalization is not only expected to improve existing application of nanoporous materials but also opens the door to new technologies. The latter might include out of equilibrium devices, ionic circuits, or machine learning based sensors.

Rational design of a self-assembled surfactant film in nanopipettes: combined fluorescence and electrochemical sensing

Herein, a generalizable method based on the formation of a self-assembled surfactant film was reported to build a nanopipette system. Using this nanopipette, it was found that arginine metabolism shows an age-related difference in Alzheimer's disease.

Peptide Assembled in a Nano-confined Space as a Molecular Rectifier for the Availability of Ionic Current Modulation

Bioinspired nanochannels have emerged as a powerful tool for bioengineering and biomedical research due to their robust mechanical and controllable chemical properties. Inspired by inward-rectifier potassium (K⁺) channels, herein, the charged peptide assembly has been introduced into a nano-confined space for the modulation of ion current rectification (ICR). Peptide-responsive reaction-triggered sequence changes can contribute to polarity conversion of the surface charge; therefore, ICR reversal (ICRR) is generated. Compared with other responsive elements, natural charged peptides show the merit of controllable charge polarity. By electrochemically monitoring the ICRR as an output signal, one can utilize the peptide assembly-mediated ICRR to construct an ionic sensory platform. In addition, a logic gate has been established to demonstrate the availability of an ionic sensory platform for inhibitor screening. As peptide nanoassemblies may also have various structures and functions due to their diverse properties, the ionic modulation system can provide alternatives for the assay of peptide-associated biotargets with biomedical applications.

Ferrofluids transport in bioinspired nanochannels: Application to electrochemical biosensing with magnetic-controlled detection

Controllable transport of ions, molecules or fluids in bioinspired nanochannels is crucial to study biointeraction occurred in confined space and also develop biosensing platforms or devices. Herein, ferrofluids transport in biofunctionalized nanochannels was investigated and a novel electrochemical biosensing platform with the characteristic of label-free, high sensitivity and rapid response was constructed. The hydrophilic ferrofluids can flux swiftly through the antibody-immobilized nanochannels with the assistance of a permanent magnet. It was initially found that the presence of ferrofluids would depress the redox current of the electrochemical probe [Fe(CN)₆]^3-. The mechanism of the depressing effect was ascribed to the constrained diffusion of [Fe(CN)₆]^3- which lowered the concentration of it at the electrode surface and the weak adsorption of the ferrofluids which increased the charge transfer resistance of the interface. Therefore, redox current of the probe was applied to indicate the amount of the ferrofluids fluxing through the bioinspired nanochannels. The steric hindrance of the bioinspired nanochannels changed with the amount of the corresponding target being incubated, resulting in quantitative variation of the redox current. In this way, electrochemical biosensing platform based on ferrofluids transport was constructed. Using carbohydrate antigen 153 (CA153) as a model target, a low detection limit of 0.0013 U·mL^-1 was acquired. This magnetic-controlled bioelectrochemical platform was expected to be expanded to other applications such as genetic testing, drug analysis, and molecular identification.

In situ peptide self-assembly on ionic nanochannel for dynamic monitoring of MMPs in extracellular matrix

The extracellular matrix (ECM) of tumor mediates malignant transformation and distant metastasis with extracellular proteinases, especially the matrix metalloproteinases (MMPs). However, there is no assay method to trace the dynamic content of MMPs in ECM. In this work, we have proposed a strategy by assembling peptide scaffold on ionic nanochannels to monitor the target proteinases. The short peptide unit is designed to induce self-assembly with good stability, biocompatibility and programmability, while ion nanochannels can provide electrochemical response upon the MMP activities. Taking MMP-2 as an example, the peptide unit includes two regions, one for self-assembly and one for bio-recognition, so the assembly region (KLVFF) can self-assemble to nanofiber networks. In the meantime, since the reactive region (PLGVR) has MMP-2 recognition site, the peptide assembly on nanochannel can thus be used for the detection of active MMP-2 in tumor microenvironment, with a wide linear detection range (10 fg/mL-10 ng/mL) and 6.6 fg/mL limit of detection. Moreover, the availability of the established ECM mimic is able to distinguish active MMP-2 from latent proMMP-2 in tumor samples. By designing different peptide units for self-assembly on the ionic nanochannel, the assay platform can be promisingly used for other proteinases in ECM, so this work may provide a useful approach to trace the dynamic content of the MMPs in tumor microenvironment (TEM).

Bio-inspired Track-Etched Polymeric Nanochannels: Steady-State Biosensors for Detection of Analytes

Bio-inspired polymeric nanochannel (also referred as nanopore)-based biosensors have attracted considerable attention on account of their controllable channel size and shape, multi-functional surface chemistry, unique ionic transport properties, and good robustness for applications. There are already very informative reviews on the latest developments in solid-state artificial nanochannel-based biosensors, however, which concentrated on the resistive-pulse sensing-based sensors for practical applications. The steady-state sensing-based nanochannel biosensors, in principle, have significant advantages over their counterparts in term of high sensitivity, fast response, target analytes with no size limit, and extensive suitable range. Furthermore, among the diverse materials, nanochannels based on polymeric materials perform outstandingly, due to flexible fabrication and wide application. This compressive Review summarizes the recent advances in bio-inspired polymeric nanochannels as sensing platforms for detection of important analytes in living organisms, to meet the high demand for high-performance biosensors for analysis of target analytes, and the potential for development of smart sensing devices. In the future, research efforts can be focused on transport mechanisms in the field of steady-state or resistive-pulse nanochannel-based sensors and on developing precisely size-controlled, robust, miniature and reusable, multi-functional, and high-throughput biosensors for practical applications. Future efforts should aim at a deeper understanding of the principles at the molecular level and incorporating these diverse pore architectures into homogeneous and defect-free multi-channel membrane systems. With the rapid advancement of nanoscience and biotechnology, we believe that many more achievements in nanochannel-based biosensors could be achieved in the near future, serving people in a better way.

Aptamer-Functionalized Nanochannels for One-Step Detection of SARS-CoV-2 in Samples from COVID-19 Patients

With the outbreak of COVID-19, which is fast transmitting and highly contagious, the development of rapid, highly specific, and sensitive detection kits has become a research hotspot. The existing assay methods for SARS-CoV-2 are mainly based on enzymatic reactions, which require expensive reagents, hindering popular use, especially in resource-constrained areas. Herein, we propose an aptamer-based method for the assay of SARS-CoV-2 via binding of the spike protein using functionalized biomimetic nanochannels. To get the analogous effect of human ACE2, a receptor for the spike protein, the aptamer to bind to the spike S1 protein has been first screened by a SELEX technique and then immobilized on the previously prepared nanochannels. In the presence of SARS-CoV-2, the changes in steric hindrance and charge density on the surface of the nanochannels will affect the ion transport, along with a rapid electrochemical response. Our method has been successfully applied to detect the viral particles in clinical pharyngeal swab specimens in one step without sample treatment. We expect this rapid, reagent-free, and sensitive assay method to be developed as a useful tool for diagnosing COVID-19.

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.

A novel aggregation-induced enhanced emission aromatic molecule: 2-aminophenylboronic acid dimer

Aggregation-induced enhanced emission (AIEE) molecules have significant applications in optoelectronics, biomedical probes and chemical sensors, and large amounts of AIEE molecules have been reported since the concept of AIEE was proposed. Most aromatic AIEE molecules have complex structures consisting of multiple aromatic rings and/or polycyclic skeletons. In this study, we find that 2-aminophenylboronic acid (2-APBA) with a simple structure is highly emissive in the solid state. Further studies reveal that 2-APBA exists in a dimeric form, and the 2-APBA dimer is a novel AIEE molecule. The underlying AIEE mechanism is that the 2-APBA dimeric units aggregate through intermolecular interactions to produce highly ordered molecular packing without the presence of π–π stacking interactions that would lead to aggregation-caused quenching. Furthermore, the 2-APBA dimer aggregates could reversibly transform into its non-fluorescent monomer form driven by new kinds of dynamic covalent B–N and B–O bonds, illustrating its good potential in molecular recognition, nanogating, chemo/bio-sensing and controlled drug release.

The 2-APBA dimer tending to aggregate into a highly ordered structure is discovered to be AIEE active. Through alternate treatment with CO₂ and N₂, 2-APBA can switch between monomer and dimer aggregates driven by dynamic covalent B–N and B–O bonds.

Revealing Ionic Signal Enhancement with Probe Grafting Density on the Outer Surface of Nanochannels

Probe-modified nanopores/nanochannels are one of the most advanced sensors because the probes interact strongly with ions and targets in nanoconfinement and create a sensitive and selective ionic signal. Recently, ionic signals have been demonstrated to be sensitive to the probe-target interaction on the outer surface of nanopores/nanochannels, which can offer more open space for target recognition and signal conversion than nanoconfined cavities. To enhance the ionic signal, we investigated the effect of grafting density, a critical parameter of the sensing interface, of the probe on the outer surface of nanochannels on the change rate of the ionic signal before and after target recognition (β). Electroneutral peptide nucleic acids and negatively charged DNA are selected as probes and targets, respectively. The experimental results showed that when adding the same number of targets, the β value increased with the probe grafting density on the outer surface. A theoretical model with clearly defined physical properties of each probe and target has been established. Numerical simulations suggest that the decrease of the background current and the aggregation of targets at the mouth of nanochannels with increasing probe grafting density contribute to this enhancement. This work reveals the signal mechanism of probe-target recognition on the outer surface of nanochannels and suggests a general approach to the nanochannel/nanopore design leading to sensitivity improvement on the basis of relatively good selectivity.

Biomimetic calcium-inactivated ion/molecular channel

A phosphopeptide-modified nanochannel was prepared based on a conical polymeric nanopore. It shows a reversible Ca2+-induced inactivation effect toward the ion flow and molecular transport, resulting from Ca2+ binding-caused surface charge neutralization and hydrophilicity reduction, and Ca2+ removal by the competitive binding.

Peptide Nucleic Acid-Functionalized Nanochannel Biosensor for the Highly Sensitive Detection of Tumor Exosomal MicroRNA

Compared with free miRNAs in blood, miRNAs in exosomes have higher abundance and stability. Therefore, miRNAs in exosomes can be regarded as an ideal tumor marker for early cancer diagnosis. Here, a peptide nucleic acid (PNA)-functionalized nanochannel biosensor for the ultrasensitive and specific detection of tumor exosomal miRNAs is proposed. After PNA was covalently bound to the inner surface of the nanochannels, the detection of tumor exosomal miRNAs was achieved by the charge changes on the surface of nanochannels before and after hybridization (PNA-miRNA). Due to the neutral characteristics of PNA, the efficiency of PNA-miRNA hybridization was improved by significantly reducing the background signal. This biosensor could not only specifically distinguish target miRNA-10b from single-base mismatched miRNA but also achieve a detection limit as low as 75 aM. Moreover, the biosensor was further used to detect exosomal miRNA-10b derived from pancreatic cancer cells and normal pancreatic cells. The results indicate that this biosensor could effectively distinguish pancreatic cancer tumor-derived exosomes from the normal control group, and the detection results show good consistency with those of the quantitative reverse-transcription polymerase chain reaction method. In addition, the biosensor was used to detect exosomal miRNA-10b in clinical plasma samples, and it was found that the content of exosomal miRNA-10b in cancer patients was generally higher than that of healthy individuals, proving that the method is expected to be applied for the early diagnosis of cancer.

Exploring the contribution of charged species at the outer surface to the ion current signal of nanopores: a theoretical study

Nanopores attached to charged species realize the artificial regulation of ion transport by the electrostatic effect in nanoconfines, produce a sensitive ion current signal and play a critical role in nanopore-based analyses. However, until now, the contribution of the charged species at the outer surface, an inherent component of nanopores, to the ion current signal has not yet been fully investigated. Here, we theoretically investigate the contribution of the charged species at the outer surface to the ion current signal of a conical nanopore. The results indicate that when the electrostatic effect at the tip of the conical nanopore is strengthened, the contribution from the charged species at the outer surface to the ionic current signal becomes stronger or even predominant compared with that of the inner walls. This effect can be further enhanced using nanopore arrays with small openings and low pore density in a low concentration electrolyte. This work focuses on the working mechanism of nanopores with a high-efficient signal conversion and promotes the performance of nanopores with a regional distribution of charged probes and targets.

Mass spectrometry-based chemical mapping and profiling toward molecular understanding of diseases in precision medicine

Precision medicine has been strongly promoted in recent years. It is used in clinical management for classifying diseases at the molecular level and for selecting the most appropriate drugs or treatments to maximize efficacy and minimize adverse effects. In precision medicine, an in-depth molecular understanding of diseases is of great importance. Therefore, in the last few years, much attention has been given to translating data generated at the molecular level into clinically relevant information. However, current developments in this field lack orderly implementation. For example, high-quality chemical research is not well integrated into clinical practice, especially in the early phase, leading to a lack of understanding in the clinic of the chemistry underlying diseases. In recent years, mass spectrometry (MS) has enabled significant innovations and advances in chemical research. As reported, this technique has shown promise in chemical mapping and profiling for answering "what", "where", "how many" and "whose" chemicals underlie the clinical phenotypes, which are assessed by biochemical profiling, MS imaging, molecular targeting and probing, biomarker grading disease classification, etc. These features can potentially enhance the precision of disease diagnosis, monitoring and treatment and thus further transform medicine. For instance, comprehensive MS-based biochemical profiling of ovarian tumors was performed, and the results revealed a number of molecular insights into the pathways and processes that drive ovarian cancer biology and the ways that these pathways are altered in correspondence with clinical phenotypes. Another study demonstrated that quantitative biomarker mapping can be predictive of responses to immunotherapy and of survival in the supposedly homogeneous group of breast cancer patients, allowing for stratification of patients. In this context, our article attempts to provide an overview of MS-based chemical mapping and profiling, and a perspective on their clinical utility to improve the molecular understanding of diseases for advancing precision medicine.

Electrochemical Trans-Channel Assay for Efficient Evaluation of Tumor Cell Invasiveness

Efficiently assessing the invasive capability of tumor cells is critical both for the research and treatment of cancer. Here, we report a novel method called the electrochemical trans-channel assay for efficient evaluation of tumor cell invasiveness. A bioinspired extracellular matrix degradation model (EDM) has been first fabricated on a porous anodic alumina (PAA) membrane to construct the electrochemical apparatus. Upon contacting the invasive tumor cells, invasive capability can be sensitively evaluated by the degree of EDM impairment, which is recorded by the electrochemical trans-channel ionic currents in a label-free manner. Compared to the most commonly used trans-well migration method, this assay can be accomplished in an efficient way that is significantly faster (20 min) and more convenient. Besides, quantitation can also be realized for monitoring the invasion process, which cannot be achieved by other currently used methods. Our proposed electrochemical trans-channel assay method has shown a synergistic effect for the evaluation of tumor cell invasiveness, providing a promising method for clinical assessment or prognostic applications of tumor metastasis.

Advances in epitope molecularly imprinted polymers for protein detection: a review

Epitope molecularly imprinted polymers (EMIPs) are novel imprinted materials using short characteristic peptides as templates rather than entire proteins. To be specific, the amino acid sequence of the template peptide is the same as an exposed N- or C-terminus of a target protein, or its amino acid composition and sequence replicate a similar conformational arrangement as the same amino acid residues on the surface of the target protein. EMIPs have a good application prospect in protein research. Herein, we focus on classification of epitope imprinting techniques, methods of epitope immobilization on matrix materials including boronate affinity immobilization, covalent bonding immobilization, physical adsorption immobilization and metal ion chelation immobilization, and application of EMIPs in peptides, proteins, target imaging and target therapy fields. Finally, the main problems and future development are summarized.

Two-Dimensional Metalloporphyrinic Framework Nanosheet-Based Dual-Mechanism-Driven Ratiometric Electrochemiluminescent Biosensing of Protein Kinase Activity

A dual-mechanism-driven ratiometric electrochemiluminescent (ECL) biosensor was developed for the ultrasensitive detection of protein kinase activity, which was based on a competitive catalytic reaction and resonance energy transfer (RET) by assembling gold nanoparticles (GNPs) on two-dimensional (2D) porphyrinic metal-organic framework (MOF) nanosheets. In this work, an ECL catalytic reaction competing for dissolved O₂ proceeded between 2D copper-based zinc porphyrinic MOF (Cu-TCPP(Zn)) nanosheets and luminol. Meanwhile, the cathodic ECL of singlet oxygen (¹O₂), derived from the electrocatalytic reaction of 2D Cu-TCPP(Zn), would be reduced by the assembled GNPs due to RET, while the anodic emission of luminol could be enhanced by GNPs with excellent electrocatalytic activity. With the detection of protein kinase A (PKA) as an example, this dual-mechanism-driven ECL biosensor exhibited a broad linear range (0.005-5.0 U mL^-1) and a sensitive detection limit (0.0037 U mL^-1). Compared with the traditional single-mechanism-driven sensing strategies, the developed dual-mechanism-driven ratiometric ECL biosensor may provide an effective method for the design of green and ultrasensitive ECL sensors.

Interfacial Super-Assembly of Nanofluidic Heterochannels from Layered Graphene and Alumina Oxide Arrays for Label-Free Histamine-Specific Detection

Nanofluidic devices with well-defined channels have shown great potential for biosensing, separation and, energy conversion. Recently, two-dimensional (2D) materials have been widely used for constructing novel nanofluidic devices owing to their high specific surface, abundant surface charge, and low cost. However, 2D-based nanofluidic devices for highly sensitive biosensing have drawn little attention. Herein, we developed a 2D material-based nanofluidic heterochannel with an asymmetric T-mode nanochannel structure and surface charge polarization distribution. This heterochannel was composed of layered graphene oxide modified with Nα, Nα-bis(carboxymethyl)-l-lysine (containing metal-nitrilotriacetic chelates, NTA) and an oxide array (NTA-GO/AAO), which can achieve remarkable selectivity, specificity, and label-free detection of the neurotransmitter histamine based on a metal ion displacement mechanism. A detection limit of 1 nM can be obtained using the NTA-GO/AAO heterochannel. This study provides a simple and label-free platform for developing a 2D-based nanofluidic heterochannel for specific molecular detection.