Adenosine-Activated Nanochannels Inspired by G-Protein-Coupled Receptors

Nanochannel-based biosensor for ultrasensitive and label-free detection of thymidine kinase activity

Thymidine Kinase 1 (TK1) is a pivotal enzyme in fundamental biochemistry and molecular diagnosis, but recognition and molecule detection is a challenging task. Here, we constructed a DNA-integrated hybrid nanochannel sensor for TK1 activity and inhibition assay. Single-stranded DNA containing thymidine was used as a substrate to functionalize the nanochannels, restricting the ion current through channels. With kinase, the thymidine at the termini of the substrate DNA is phosphorylated, elevating surface charge density and mitigating the pore-obstruction effect by increasing transmembrane ion current. The kinase-induced distinctness can be accurately monitored by this hybrid nanodevice, which benefits from its high sensitivity to the change of surface charge. The excellent analytical performance in both kinase enzyme activity and inhibition analysis resulted in efficient and selective evaluation in human serum. Furthermore, compared to current approaches, it greatly simplifies and offers a direct method of analysis, making it a promising sensor technology for cancer management as well as the activities of multiple types of nucleic acid kinases.

A Visible-Light Regulated ATP Transport in Retinal-Modified Pillar[6]arene Layer-by-Layer Self-Assembled Sub-Nanochannel

Natural light-responsive rhodopsins play a critical role in visual conversion, signal transduction, energy transmission, etc., which has aroused extensive interest in the past decade. Inspired by these gorgeous works of living beings, scientists have constructed various biomimetic light-responsive nanochannels to mimic the behaviors of rhodopsins. However, it is still challenging to build stimuli-responsive sub-nanochannels only regulated by visible light as the rhodopsins are always at the sub-nanometer level and regulated by visible light. Pillar[6]arenes have an open cavity of 6.7 Å, which can selectively recognize small organic molecules. They can be connected to ions of ammonium or carboxylate groups on the rims. Therefore, we designed and synthesized the amino and carboxyl-derived side chains of pillar[6]arenes with opposite charges. The sub-nanochannels were constructed through the electrostatic interaction of layer-by-layer self-assembled amino and carboxyl-derived pillar[6]arenes. Then, the natural chromophore of the retinal with visible light-responsive performance was modified on the upper edge of the sub-nanochannel to realize the visible light switched on and off. Finally, we successfully constructed a visible light-responsive sub-nanochannel, providing a novel method for regulating the selective transport of energy-donating molecules of ATP.

Nanoionics from Biological to Artificial Systems: An Alternative Beyond Nanoelectronics

Ion transport under nanoconfined spaces is a ubiquitous phenomenon in nature and plays an important role in the energy conversion and signal transduction processes of both biological and artificial systems. Unlike the free diffusion in continuum media, anomalous behaviors of ions are often observed in nanostructured systems, which is governed by the complex interplay between various interfacial interactions. Conventionally, nanoionics mainly refers to the study of ion transport in solid-state nanosystems. In this review, to extent this concept is proposed and a new framework to understand the phenomena, mechanism, methodology, and application associated with ion transport at the nanoscale is put forward. Specifically, here nanoionics is summarized into three categories, i.e., biological, artificial, and hybrid, and discussed the characteristics of each system. Compared with nanoelectronics, nanoionics is an emerging research field with many theoretical and practical challenges. With this forward-looking perspective, it is hoped that nanoionics can attract increasing attention and find wide range of applications as nanoelectronics.

Biomimetic Nanochannels: From Fabrication Principles to Theoretical Insights

Biological nanochannels which can regulate ionic transport across cell membranes intelligently play a significant role in physiological functions. Inspired by these nanochannels, numerous artificial nanochannels have been developed during recent years. The exploration of smart solid-state nanochannels can lay a solid foundation, not only for fundamental studies of biological systems but also practical applications in various fields. The basic fabrication principles, functional materials, and diverse applications based on artificial nanochannels are summarized in this review. In addition, theoretical insights into transport mechanisms and structure-function relationships are discussed. Meanwhile, it is believed that improvements will be made via computer-guided strategy in designing more efficient devices with upgrading accuracy. Finally, some remaining challenges and perspectives for developments in both novel conceptions and technology of this inspiring research field are stated.

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.

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

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

Ultra-Sensitive and Selective Electrochemical Bio-Fluid Biopsy for Oral Cancer Screening

The early diagnosis of recurrence and metastasis is critically important for decreasing the morbidity and mortality associated with oral cancers. Although liquid biopsy methods hold great promise that provide a successive "time-slice" profile of primary and metastatic oral cancer, the development of non-invasive, rapid, simple, and cost-effective liquid biopsy techniques remains challenging. In this study, an ultrasensitive and selective electrochemical liquid biopsy is developed for oral cancer screening based on tracking trace amounts of cancer biomarker by functionalized asymmetric nano-channels. Detection via antigen-antibody reactions is assayed by evaluating changes in ionic current. Upon the recognition of cancer biomarker antigens in bio-fluids, the inner wall of nano-channel immobilized with the corresponding antibodies undergoes molecular conformation transformation and surface physicochemical changes, which significantly regulate the ion transport through the nano-channel and help achieve sensitivity with a detection limit of 10^-12 g mL^-1 . Furthermore, owing to the specificity of the monoclonal antibody for the antigen, the nano-channel exhibits high selectivity for the biomarker than for structurally similar biological molecules present in bio-fluids. The effectiveness of this technique is confirmed through the diagnosis of clinical cases of oral squamous cell carcinoma. This study presents a novel diagnostic tool for oral cancer detection in bio-fluids.

Sensing with Nanopores and Aptamers: A Way Forward

In the 90s, the development of a novel single molecule technique based on nanopore sensing emerged. Preliminary improvements were based on the molecular or biological engineering of protein nanopores along with the use of nanotechnologies developed in the context of microelectronics. Since the last decade, the convergence between those two worlds has allowed for biomimetic approaches. In this respect, the combination of nanopores with aptamers, single-stranded oligonucleotides specifically selected towards molecular or cellular targets from an in vitro method, gained a lot of interest with potential applications for the single molecule detection and recognition in various domains like health, environment or security. The recent developments performed by combining nanopores and aptamers are highlighted in this review and some perspectives are drawn.

SERS Detection of Nucleobases in Single Silver Plasmonic Nanopores

Conventional ion current-based nanopore techniques that identify single molecules are hampered by limitations of providing only the ionic current information. Here, we introduce a silver nanotriangle-based nanopore (diameter < 50 nm) system for detecting molecule translocation using surface-enhanced Raman scattering. Rhodamine 6G is used as a model molecule to study the effect of an electric field (-1 V) on the mass transport. The four DNA bases also show significantly different SERS signals when they are transported into the plasmonic nanopore. The observations suggest that in the electric field, analyte molecules are driven into the nanopipette through the hot spot of the silver nanopore. The plasmonic nanopore shows great potential as a highly sensitive SERS platform for detecting molecule transport and paves the way for single molecule probing.

Molecular Design of Solid-State Nanopores: Fundamental Concepts and Applications

Solid-state nanopores are fascinating objects that enable the development of specific and efficient chemical and biological sensors, as well as the investigation of the physicochemical principles ruling the behavior of biological channels. The great variety of biological nanopores that nature provides regulates not only the most critical processes in the human body, including neuronal communication and sensory perception, but also the most important bioenergetic process on earth: photosynthesis. This makes them an exhaustless source of inspiration toward the development of more efficient, selective, and sophisticated nanopore-based nanofluidic devices. The key point responsible for the vibrant and exciting advance of solid nanopore research in the last decade has been the simultaneous combination of advanced fabrication nanotechnologies to tailor the size, geometry, and application of novel and creative approaches to confer the nanopore surface specific functionalities and responsiveness. Here, the state of the art is described in the following critical areas: i) theory, ii) nanofabrication techniques, iii) (bio)chemical functionalization, iv) construction of nanofluidic actuators, v) nanopore (bio)sensors, and vi) commercial aspects. The plethora of potential applications once envisioned for solid-state nanochannels is progressively and quickly materializing into new technologies that hold promise to revolutionize the everyday life.

Biomolecule-Functionalized Solid-State Ion Nanochannels/Nanopores: Features and Techniques

Solid-state ion nanochannels/nanopores, the biomimetic products of biological ion channels, are promising materials in real-world applications due to their robust mechanical and controllable chemical properties. Functionalizations of solid-state ion nanochannels/nanopores by biomolecules pave a wide way for the introduction of varied properties from biomolecules to solid-state ion nanochannels/nanopores, making them smart in response to analytes or external stimuli and regulating the transport of ions/molecules. In this review, two features for nanochannels/nanopores functionalized by biomolecules are abstracted, i.e., specificity and signal amplification. Both of the two features are demonstrated from three kinds of nanochannels/nanopores: nucleic acid-functionalized nanochannels/nanopores, protein-functionalized nanochannels/nanopores, and small biomolecule-functionalized nanochannels/nanopores, respectively. Meanwhile, the fundamental mechanisms of these combinations between biomolecules and nanochannels/nanopores are explored, providing reasonable constructs for applications in sensing, transport, and energy conversion. And then, the techniques of functionalizations and the basic principle about biomolecules onto the solid-state ion nanochannels/nanopores are summarized. Finally, some views about the future developments of the biomolecule-functionalized nanochannels/nanopores are proposed.

Aptamer superstructure-based electrochemical biosensor for sensitive detection of ATP in rat brain with in vivo microdialysis

Highly sensitive and selective sensing of ATP in rat brain has attracted increasing interest from interdisciplinary fields of analytical chemistry and neuroscience owing to the importance of ATP in cellular metabolism and signal transduction. Herein, we demonstrated an electrochemical biosensor having an aptamer superstructure as a recognition element for the selective and sensitive detection of ATP in rat brain. Unlike the electrochemical aptamer-based sensors (aptasensors) built by assembling a simple DNA structure containing only one aptamer unit onto the electrode substrate, the aptasensor described here was developed by assembling an aptamer superstructure consisting of consecutive aptamer units in DNA strands onto the electrode substrate. Each aptamer unit in the superstructure was labelled with an electrochemical probe (i.e., methylene blue, MB) for signal readout. The aptamer superstructure was assembled onto the surface of a gold electrode to form the electrochemical aptasensor. In the presence of ATP, the strong electrochemical signals produced by multiple redox molecules labeled on the aptamer units clearly decreased because of the disassembling of the aptamer superstructure from the electrode surface due to strong interactions between ATP and the aptamer units. In this approach, the aptasensor was well responsive to the ATP concentration, and the current decrease was linearly related to the ATP concentration ranging from 0.1 nM to 1 mM. Moreover, the aptasensor has high selectivity and good regenerability. Due to these properties, the aptasensor with an aptamer superstructure can exhibit practical applications for ATP assay in rat brain combined with in vivo microdialysis.

Nanochannel-Ion Channel Hybrid Device for Ultrasensitive Monitoring of Biomolecular Recognition Events

We propose an in situ and label-free method for detection of biomolecular recognition events by use of a nanochannel-ion channel hybrid device integrated with an electrochemical detector. The aptamer is first immobilized on the outer surface of the nanochannel-ion channel hybrid. Its binding with target thrombin in solution considerably regulates the mass-transfer behavior of the device owing to the varied surface charge density and effective channel size. Via the electrochemical detector, the changed mass-transport property can be monitored in real time, which enables in situ and label-free detection of thrombin-aptamer recognition. The solution pH has a significant influence on detection sensitivity. Under optimal pH conditions, a detection limit as low as 0.22 fM thrombin can be achieved, which is much lower than most reported work. The present nanofluidic device provides a simple, ultrasensitive, and label-free platform for monitoring biomolecular recognition events, which would hold great potential in exploring the functions and reaction mechanisms of biomolecules in living systems.

Integrated Solid-State Nanopore Electrochemistry Array for Sensitive, Specific, and Label-Free Biodetection

Nanopore ionic current measurement is currently a prevailing readout and offers considerable opportunities for bioassays. Extending conventional electrochemistry to nanoscale space, albeit noteworthy, remains challenging. Here, we report a versatile electrochemistry array established on a nanofluidic platform by controllably depositing gold layers on the two outer sides of anodic aluminum oxide (AAO) nanopores, leading to form an electrochemical microdevice capable of performing amperometry in a label-free manner. Electroactive species ferricyanide ions passing through gold-decorated nanopores act as electrochemical indicator to generate electrolytic current signal. The electroactive species flux that dominates current signal response is closely related to the nanopore permeability. Such well-characteristic electrolytic current-species flux correlation lays a premise for quantitative electrochemical analysis. As a proof-of-concept demonstration, we preliminarily verify the analytical utility by detection of nucleic acid and protein at picomolar concentration levels. Universal surface modification and molecule assembly, specific target recognition and reliable signal output in nanopore enable direct electrochemical detection of biomolecules without the need of cumbersome probe labeling and signal amplification.

Dynamics of a Molecular Plug Docked onto a Solid-State Nanopore

Docking of a protein-DNA complex onto a nanopore can provide ample observation time, and has enabled collection of analytic applications of biological nanopores, including DNA sequencing. However, the application of the same principle to solid-state nanopores is tempered by poor understanding of the docking process. Here, we elucidate the behavior of individual protein-DNA complexes docked onto a solid-state nanopore by monitoring the nanopore ionic current. Repeat docking of monovalent streptavidin-DNA complexes is found to produce ionic current blockades that fluctuate between discrete levels. We elucidate the roles of the protein plug and the DNA tether in the docking process, finding the docking configurations to determine the multitude of the current blockade levels, whereas the frequency of the current level switching is determined by the interactions between the molecules and the solid-state membrane. Finally, we prove the feasibility of using the nanopore docking principle for single-molecule sensing using solid-state nanopores by detecting conformational changes of a tethered DNA molecule from a random coil to an i-motif state.

Bioinspired smart asymmetric nanochannel membranes

Bioinspired smart asymmetric nanochannel membranes (BSANM) have been explored extensively to achieve the delicate ionic transport functions comparable to those of living organisms. The abiotic system exhibits superior stability and robustness, allowing for promising applications in many fields. In view of the abundance of research concerning BSANM in the past decade, herein, we present a systematic overview of the development of the state-of-the-art BSANM system. The discussion is focused on the construction methodologies based on raw materials with diverse dimensions (i.e. 0D, 1D, 2D, and bulk). A generic strategy for the design and construction of the BSANM system is proposed first and put into context with recent developments from homogeneous to heterogeneous nanochannel membranes. Then, the basic properties of the BSANM are introduced including selectivity, gating, and rectification, which are associated with the particular chemical and physical structures. Moreover, we summarized the practical applications of BSANM in energy conversion, biochemical sensing and other areas. In the end, some personal opinions on the future development of the BSANM are briefly illustrated. This review covers most of the related literature reported since 2010 and is intended to build up a broad and deep knowledge base that can provide a solid information source for the scientific community.

A temperature, pH and sugar triple-stimuli-responsive nanofluidic diode

In this article, we have demonstrated for the first time a triple stimuli-responsive nanofluidic diode that can rectify ionic current under multiple external stimuli including temperature, pH, and sugar. This diode was fabricated by immobilizing poly[2-(dimethylamino)ethyl methacrylate]-co-[4-vinyl phenylboronic acid] (P(DMAEMA-co-VPBA)) onto the wall of a single glass conical nanopore channel via surface-initiator atom transfer radical polymerization (SI-ATRP). The copolymer brushes contain functional groups sensitive to pH, temperature and sugar that can induce charge and configuration change to affect the status of the pore wall. The experimental results confirmed that the P(DMAEMA-co-VPBA) brush modified nanochannel regulated the ionic current rectification successfully under three different external stimuli. This biomimetically inspired research simulates the complex biological multi-functions of ion channels and promotes the development of "smart" biomimetic nanochannel systems for actuating and sensing applications.

Bacteria-templated fabrication of a charge heterogeneous polymeric interface for highly specific bacterial recognition

Using bacteria-templated polymerization, a novel bacteria-imprinted polymer (BIP) was fabricated for bacterial recognition.