Fabrication of Layer-by-Layer Assembled Biomimetic Nanochannels for Highly Sensitive Acetylcholine Sensing

Electricity-Wettability Controlled Fast Transmission of Dopamine in Nanochannels

Achieving fast transmembrane transmission of molecules in organisms is a challenging problem. Inspired by the transport of Dopmine (DA) in organisms, the DA transporter (DAT) binds to DA in a way that has a ring recognition (the recognition group is the tryptophan group). Herein, D-Tryptophan-pillar[5]arene (D-Trp-P5) functionalized conical nanochannel is constructed to achieve fast transmission of DA. The D-Trp-P5 functionalized nanochannel enables specific wettability recognition of DA molecules and has great cycle stability. With the controlling of voltage to wettability, the transport flux of DA is up to 499.73 nmol cm^-2 h^-1 at -6 V, 16.88 times higher than that under positive voltages. In response to these results, a high-throughput DA transport device based on controlled electricity-wettability is provided.

Guest Binding with Sulfated Cyclodextrins: Does the Size of Cavity Matter?

Sulfated cyclodextrins have recently emerged as potential candidates for producing host-induced guest aggregation with properties better than p-sulfonatocalixarenes that have previously shown numerous applications involving the phenomena of host-induced guest aggregation. In the class of sulfated cyclodextrins (SCD), sulfated β-cyclodextrin (β-SCD) remains the most extensively investigated host molecule. Although it is assumed that the host-induced guest aggregation is predominantly an outcome of interaction of the guest molecule with the charges on the exterior of SCD cavity, it has not been deciphered whether the variation in the cavity size will make a difference in the efficiency of host-induced guest-aggregation process. In this investigation, we present a systematic study of host-induced guest aggregation of a cationic molecular rotor dye, Thioflavin T (ThT) with three different sulfated cyclodextrin molecules, α-SCD, β-SCD and γ-SCD, which differ in their cavity size, using steady-state emission, ground-state absorption and time-resolved emission measurements. The obtained photophysical properties of ThT, upon interaction with different SCD molecules, indicate that the binding strength of ThT with different SCD molecules correlate with the cavity size of the host molecule, giving rise to the strongest complexation of ThT with the largest host molecule (γ-SCD). The binding affinity of ThT towards different host molecules has been supported by molecular docking calculations. The results obtained are further supported with the temperature and ionic strength dependent studies performed on the host-guest complex. Our results indicate that for host-induced guest aggregation, involving oppositely charged molecules, the size of the cavity also plays a crucial role beside the charge density on the exterior of host cavity.

Nanopore-Based Detection of Trace Concentrations of Multivalent Ions When Impurity Ions Are Present

The feasibility of detecting a trace concentration of multivalent ions based on the ionic current rectification (ICR) of a nanopore when impurity ions might present is assessed. Adopting a bullet-shaped nanopore surface modified with tannic acid as an example, the detection of trace concentrations of Cu²⁺ (target ion) when Fe³⁺ (impurity) is present with K⁺ as background ions under various conditions is simulated. In particular, the influence of the reaction order of the association of target ions and tannic acid on the nanopore performance is examined. We show that the lower the background concentration the better the detection performance. For the examined background concentrations of 1, 10, 100, and 1000 mM, the optimal detection ranges are [0.5, 1000 μM] and [1, 1000 nM] for Cu²⁺ and Fe³⁺, respectively. The detection limits, 0.5 μM for Cu²⁺ and 1 nM for Fe³⁺, are lower than those that can be obtained from conventional instruments, suggesting the potential of applying the present nanopore-based approach. In addition, we also consider the presence of multiple ions, which can occur, for example, in detecting Cu²⁺ (target ion) when Fe³⁺ (impurity) might present or vice versa with K⁺ as background ions. The competitive adsorption of these three kinds of ions can yield complicated ICR behaviors.

Highly sensitive acetylcholine biosensing via chemical amplification of enzymatic processes in nanochannels

Acetylcholinesterase-modified nanochannels are proposed as reliable and reproducible nanofluidic sensors for highly sensitive detection of acetylcholine. The operation mechanism relies on the use of weak polyelectrolytes as "chemical amplifiers" that adjust/reconfigure the nanochannel surface charge in the presence of local pH changes induced by the enzymatic reaction. Experimental results show that the presence of acetylcholine can be transduced into measurable ionic signals with a low limit of detection.

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.

Nanosensing of Acetylcholine Molecules: Influence of the Association Mechanism

A bullet-shaped nanopore surface modified by two polyelectrolyte (PE) layers, an inner polyethyleneimine (PEI) layer and an outer p-sulfonatocalix[4]-arene (SCX4) layer, is applied to sense trace levels of acetylcholine (Ach) molecules. We show that the higher the order of the association reaction of Ach with SCX4, the smaller the difference between the ionic current when Ach is present and that when it is absent, and so is the difference in the space charge density. In addition, the larger the binding constant K of that reaction, the lower the detection limit but narrower the detection range. Choosing pH 7 is most appropriate because if the pH is low, the concentration polarization of H⁺ is significant, and as it gets high, both PE layers become uncharged. At pH 7 and K = 2 × 10⁷ L/mol, the detection limit of the nanopore ranges from 1 to 10 nM, which is orders of magnitude lower than that of the other approaches.

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.

Detecting Pesticide Dodine by Displacement of Fluorescent Acridine from Cucurbit[10]uril Macrocycle

According to a simple guest-replacement fluorescence turn-on mechanism, we constructed a fluorescent probe system based on cucurbit[10]uril (Q[10]) and protonated acridine (AD) to detect the pesticide dodine (DD). Formation of a homoternary inclusion complex AD₂@Q[10] in both aqueous solution and solid state was studied by means of ¹H NMR spectroscopy and X-ray crystallography. Although AD can emit strong fluorescence in aqueous solution, the homoternary inclusion complex AD₂@Q[10] does not exhibit any fluorescence. Upon the addition of the pesticide DD into the aqueous solution of AD₂@Q[10], the AD molecules in the Q[10] cavity are displaced by the pesticide DD, and strong fluorescence recovers. The fluorescent probe system based on Q[10] and AD provided a wide determination of DD from 0 to 4.0 × 10^-5 mol·L^-1 with a low limit of detection of 1.827 × 10^-6 mol·L^-1. The guest-replacement fluorescence turn-on mechanism is also confirmed by ¹H NMR spectroscopy. Further, the fluorescent probe can directly detect DD residues in real agricultural products, and obvious fluorescence signal was observed under UV irradiation.

Ion track etching of polycarbonate membranes monitored by in situ small angle X-ray scattering

In situ small angle X-ray scattering (SAXS) measurements of ion track etching in polycarbonate foils are used to directly monitor the selective dissolution of ion tracks with high precision, including the early stages of etching. Detailed information about the track etching kinetics and size, shape, and size distribution of an ensemble of nanopores is obtained. Time resolved measurements as a function of temperature and etchant concentration show that the pore radius increases almost linearly with time for all conditions and the etching process can be described by an Arrhenius law. The radial etching shows a power law dependency on the etchant concentration. An increase in the etch rate with increasing temperature or concentration of the etchant reduces the penetration of the etchant into the polymer but does not affect the pore size distribution. The in situ measurements provide an estimate for the track etch rate, which is found to be approximately three orders of magnitude higher than the radial etch rate. The measurement methodology enables new experiments studying membrane fabrication and performance in liquid environments.

Stimulus-Responsive Supramolecular Host-Guest Assembly of a Cationic Pyrene Derivative with Sulfated β-Cyclodextrin

In general, aggregation-prone organic molecules are prevented from self-aggregation in the presence of macrocyclic hosts like β-cyclodextrin because of their preference for the formation of inclusion complex with guest molecules. On the contrary, sulfate-laced β-cyclodextrin has been recently reported to induce the aggregation of some of the non-aggregation-prone organic dyes, which have been subsequently utilized for biosensing applications. In the present contribution, we report the interaction of a cationic organic probe molecule, 1-pyrene methyl amine (PMA), which belongs to one of the most useful families of organic fluorescent probes, that is, pyrene, with a sulfated β-cyclodextrin derivative (SCD). Interaction of a cationic probe with a β-cyclodextrin derivative was studied using a variety of photophysical methods such as ground-state absorption, steady-state emission, and time-resolved emission techniques. Detailed photophysical investigations have revealed that SCD induces the ground-state association of PMA molecules. This SCD-induced aggregation of PMA molecules has been attributed to the charge neutralization of the cationic probe by negatively charged sulfate groups, which subsequently lead to their association because of the close proximity on the rims of cyclodextrin. This monomer-dimer equilibrium of the PMA-SCD system is found to be extremely responsive to external chemical stimuli like temperature, pH, ionic strength of the medium, and organic solvent (dimethyl sulfoxide), which projects them as potential platforms for various sensing applications including bioanalytes. The supramolecular assembly has been demonstrated to sense arginine.

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.

Ultrasensitive Detection of MicroRNAs with Morpholino-Functionalized Nanochannel Biosensor

Here, we demonstrate a phosphorodiamidate morpholino oligos (PMO)-functionalized nanochannel biosensor for label-free detection of microRNAs (miRNAs) with ultrasensitivity and high sequence specificity. PMO, as a capture probe, was covalently anchored on the nanochannel surface. Because of the neutral character and high sequence-specific affinity of PMO, hybridization efficiency between PMO and miRNAs was enhanced, thus largely decreasing background signals and highly improving the detection specificity and sensitivity. The miRNAs detection was realized through observing the change of surface charge density when PMO/miRNAs hybridization occurred. Not only could the developed biosensor specifically discriminate complementary miRNAs (Let-7b) from noncomplementary miRNAs (miR-21) and one-base mismatched miRNAs (Let-7c), but also it could detect target miRNAs in serum samples. In addition, this nanochannel-based biosensor attained a reliable limit of detection down to 1 fM in PBS and 10 fM in serum sample, respectively. It is expected that such a new method will benefit miRNA detection in clinical diagnosis.

Bioinspired integrated nanosystems based on solid-state nanopores: "iontronic" transduction of biological, chemical and physical stimuli

The ability of living systems to respond to stimuli and process information has encouraged scientists to develop integrated nanosystems displaying similar functions and capabilities. In this regard, biological pores have been a source of inspiration due to their exquisite control over the transport of ions within cells, a feature that ultimately plays a major role in multiple physiological processes, e.g. transduction of physical stimuli into nervous signals. Developing abiotic nanopores, which respond to certain chemical, biological or physical inputs producing "iontronic" signals, is now a reality thanks to the combination of "soft" surface science with nanofabrication techniques. The interplay between the functional richness of predesigned molecular components and the remarkable physical characteristics of nanopores plays a critical role in the rational integration of molecular functions into nanopore environments, permitting us to envisage nanopore-based biomimetic integrated nanosystems that respond to a variety of external stimuli such as pH, redox potential, molecule concentration, temperature, or light. Transduction of these stimuli into a predefined "iontronic" response can be amplified by exploiting nanoconfinement and physico-chemical effects such as charge distribution, steric constraints, equilibria displacement, or local changes in ionic concentration, to name but a few examples. While in past decades the focus has been mostly on their fundamental aspects and the in-depth study of their interesting transport properties, for several years now nanopore research has started to shift towards specific practical applications. This work is dedicated to bringing together the latest developments in the use of nanopores as "iontronic" transducing elements. Our aim is to show the wide potential of abiotic nanopores in sensing and signal transduction and also to promote the potential of this technology among doctoral students, postdocs, and researchers. We believe that even a casual reader of this perspective will not fail to be impressed by the wealth of opportunities that solid-state nanopores can offer to the transduction of biological, physical and chemical stimuli.

Selective molecular recognition on calixarene-functionalized 3D surfaces

Calixarene based various 3D surface materials with unique signal amplification in molecular recognition are presented, including quantum dots (QDs), metal nanoparticles (NPs), nanotubes, and mesoporous silica.

Fluoride responsive single nanochannel: click fabrication and highly selective sensing in aqueous solution

A F^– responsive nanochannel based on hydrogen-bonding interactions was designed to accomplish highly selective sensing in aqueous solution.

Fluoride is important to explore because it plays important roles in nature and biological processes. To accomplish selective F^– sensing in aqueous solution, a nanochannel modified with 1,3-dipropargylaza-p-tert-butyl calix[4]crown (C4CE) was designed on the basis of hydrogen-bonding interactions. The nanodevice not only exhibits high selectivity for F^–, even in serum, but also shows the useful property of recyclability.

A bio-inspired, sensitive, and selective ionic gate driven by silver (I) ions

By grafting specific response DNA on the interior surface of ion track-etched conical nanochannels, a highly sensitive and selective ionic gate that can be driven by silver (I) ions is demonstrated. The switches between the OFF-state and the ON-state are mainly dependent on silver (I) ions and cysteine. Such a biomimetic nanodevice shows potential for application in sensing, pharmaceuticals, and sterilization.

Gating of responsive multiple nanochannels by ultra-low concentration of saccharides

We report a saccharide recognition system by modifying responsive copolymers on the solid-based multiple nanochannels.

PEI/Zr⁴⁺-coated nanopore for selective and sensitive detection of ATP in combination with single-walled carbon nanotubes

By virtue of a biomimetic nanopore and single-walled carbon nanotubes, a new sensor for adenosine triphosphate (ATP) detection is designed. As compared to the routine approach, the present scenario does not entail the surface modification of nanopore with analyte-specific probes. The underlying mechanism relies on a symmetric nanopore sequentially modified with polyethyleneimine (PEI) and Zr(4+) that can quantitate the concentration of ATP-bound aptamer, while other free aptamers are removed by single-walled carbon nanotubes (SWNTs). The detection limit of the nanopore sensor is 27.46 nM, and the linear range is from 50 nM to 400 nM. The biosensor with an excellent selectivity against guanosine triphosphate (GTP), uridine triphosphate (UTP), and cytosine triphosphate (CTP) can be applied in the real samples such as Hela cell.

Supramolecular chemistry of p-sulfonatocalix[n]arenes and its biological applications

CONSPECTUS: Developments in macrocyclic chemistry have led to supramolecular chemistry, a field that has attracted increasing attention among researchers in various disciplines. Notably, the discoveries of new types of macrocyclic hosts have served as important milestones in the field. Researchers have explored the supramolecular chemistry of several classical macrocyclic hosts, including crown ethers, cyclodextrins, calixarenes, and cucurbiturils. Calixarenes represent a third generation of supramolecular hosts after cyclodextrins and crown ethers. Easily modified, these macrocycles show great potential as simple scaffolds to build podand-like receptors. However, the inclusion properties of the cavities of unmodified calixarenes are not as good as those of other common macrocycles. Calixarenes require extensive chemical modifications to achieve efficient endo-complexation. p-Sulfonatocalix[n]arenes (SCnAs, n = 4-8) are a family of water-soluble calixarene derivatives that in aqueous media bind to guest molecules in their cavities. Their cavities are three-dimensional and π-electron-rich with multiple sulfonate groups, which endow them with fascinating affinities and selectivities, especially toward organic cations. They also can serve as scaffolds for functional, responsive host-guest systems. Moreover, SCnAs are biocompatible, which makes them potentially useful for diverse life sciences and pharmaceutical applications. In this Account, we summarize recent work on the recognition and assembly properties unique to SCnAs and their potential biological applications, by our group and by other laboratories. Initially examining simple host-guest systems, we describe the development of a series of functional host-guest pairs based on the molecular recognition between SCnAs and guest molecules. Such pairs can be used for fluorescent sensing systems, enzymatic activity assays, and pesticide detoxification. Although most macrocyclic hosts prevent self-aggregation of guest molecules, SCnAs can induce self-aggregation. Researchers have exploited calixarene-induced aggregation to construct supramolecular binary vesicles. These vesicles respond to internal and external stimuli, including temperature changes, redox reactions, additives, and enzymatic reactions. Such structures could be used as drug delivery vehicles. Although several biological applications of SCnAs have been reported, this field is still in its infancy. Continued exploration of the supramolecular chemistry of SCnAs will not only improve the existing biological functions but also open new avenues for the use of SCnAs in the fields of biology, biotechnology, and pharmaceutical research. In addition, we expect that other interdisciplinary research efforts will accelerate developments in the supramolecular chemistry of SCnAs.

Extended-nanofluidics: fundamental technologies, unique liquid properties, and application in chemical and bio analysis methods and devices

Engineering using liquids confined in channels 10-1000 nm in dimension, or "extended-nanofluidics," is the next target of microfluidic science. Liquid properties at this scale were unrevealed until recently because of the lack of fundamental technologies for investigating these ultrasmall spaces. In this article, the fundamental technologies are reviewed, and the emerging science and technology in the extended-nanospace are discussed.

Biomimetic nanopore for sensitive and selective detection of Hg(ii) in conjunction with single-walled carbon nanotubes

Through SWNTs, duplex DNA derived from folding of single-stranded DNA can be quantitated with Zr⁴⁺–PEI coated cone-shaped nanopore. With Hg²⁺ detection, sensitivity and selectivity based on this paradigm is guaranteed without probe immobilization.