Low-cost and convenient fabrication of polymer micro/nanopores with the needle punching process and their applications in nanofluidic sensing

Solid-state micro/nanopores play an important role in the sensing field because of their high stability and controllable size. Aiming at problems of complex processes and high costs in pore manufacturing, we propose a convenient and low-cost micro/nanopore fabrication technique based on the needle punching method. The thin film is pierced by controlling the feed of a microscale tungsten needle, and the size variations of the micropore are monitored by the current feedback system. Based on the positive correlation between the micropore size and the current threshold, the size-controllable preparation of micropores is achieved. The preparation of nanopores is realized by the combination of needle punching and chemical etching. First, a conical defect is prepared on the film with the tungsten needle. Then, nanopores are obtained by unilateral chemical etching of the film. Using the prepared conical micropores, resistive-pulse detection of nanoparticles is performed. Significant ionic current rectification is also obtained with our conical nanopores. It is proved that the properties of micro/nanopores prepared by our method are comparable to those prepared by the track-etching method. The simple and controllable fabrication process proposed here will advance the development of low-cost micro/nanopore sensors.

Hydrophobic Gating and Spatial Confinement in Hierarchically Organized Block Copolymer-Nanopore Electrode Arrays for Electrochemical Biosensing of 4-Ethyl Phenol

Hydrophobic gating in biological transport proteins is regulated by stimulus-specific switching between filled and empty nanocavities, endowing them with selective mass transport capabilities. Inspired by these, solid-state nanochannels have been integrated into functional materials for a broad range of applications, such as energy conversion, filtration, and nanoelectronics, and here we extend these to electrochemical biosensors coupled to mass transport control elements. Specifically, we report hierarchically organized structures with block copolymers on tyrosinase-modified two-electrode nanopore electrode arrays (BCP@NEAs) as stimulus-controlled electrochemical biosensors for alkylphenols. A polystyrene-b-poly(4-vinyl)pyridine (PS-b-P4VP) membrane placed atop the NEA endows the system with potential-responsive gating properties, where water transport is spatially and temporarily gated through hydrophobic P4VP nanochannels by the application of appropriate potentials. The reversibility of hydrophobic voltage-gating makes it possible to capture and confine analyte species in the attoliter-volume vestibule of cylindrical nanopore electrodes, enabling redox cycling and yielding enhanced currents with amplification factors >100× when operated in a generator-collector mode. The enzyme-coupled sensing capabilities are demonstrated using nonelectroactive 4-ethyl phenol, exploiting the tyrosinase-catalyzed turnover into reversibly redox-active quinones, then using the quinone-catechol redox reaction to achieve ultrasensitive cycling currents in confined BCP@NEA sensors giving a limit-of-detection of ∼120 nM. The mass transport controlled sensing platform described here is relevant to the development of enzyme-coupled multiplex biosensors for sensitive and selective detection of biomarkers and metabolites in next-generation point-of-care devices.

Using Metadynamics To Explore the Free Energy of Dewetting in Biologically Relevant Nanopores

Water confined within hydrophobic spaces can undergo cooperative dewetting transitions due to slight changes in water density and pressure that push water toward the vapor phase. Many transmembrane protein ion channels contain nanoscale hydrophobic pores that could undergo dewetting transitions, sometimes blocking the flow of ions without physical blockages. Standard molecular dynamics simulations have been extensively applied to study the behavior of water in nanoscale pores, but the large free energy barriers of dewetting often prevent direct sampling of both wet and dry states and quantitative studies of the hydration thermodynamics of biologically relevant pores. Here, we describe a metadynamics protocol that uses the number of waters within the pore as the collective variable to drive many reversible transitions between relevant hydration states and calculate well-converged free energy profiles of pore hydration. By creating model nanopore systems and changing their radius and morphology and including various cosolvents, we quantify how these pore properties and cosolvents affect the dewetting transition. The results reveal that the dewetting free energy of nanoscale pores is determined by two key thermodynamic parameters, namely, the effective surface tension coefficients of water-air and water-pore interfaces. Importantly, while the effect of salt can be fully captured in the water activity dependence, amphipathic cosolvents such as alcohols modify both dry and wet states of the pore and dramatically shift the wet-dry equilibrium. The metadynamics approach could be applied to studies of dewetting transitions within nanoscale pores of proteins and provide new insights into why different pore properties evolved in biological systems.

Rectified and Salt Concentration Dependent Wetting of Hydrophobic Nanopores

Nanopores lined with hydrophobic groups function as switches for water and all dissolved species, such that transport is allowed only when applying a sufficiently high transmembrane pressure difference or voltage. Here we show a hydrophobic nanopore system whose wetting and ability to transport water and ions is rectified and can be controlled with salt concentration. The nanopore we study contains a junction between a hydrophobic zone and a positively charged hydrophilic zone. The nanopore is closed for transport at low salt concentrations and exhibits finite current only when the concentration reaches a threshold value that is dependent on the pore opening diameter, voltage polarity and magnitude, and type of electrolyte. The smallest nanopore studied here had a 4 nm diameter and did not open for transport in any concentration of KCl or KI examined. A 12 nm nanopore was closed for all KCl solutions but conducted current in KI at concentrations above 100 mM for negative voltages and opened for both voltage polarities at 500 mM KI. Nanopores with a hydrophobic/hydrophilic junction can thus function as diodes, such that one can identify a range of salt concentrations where the pores transport water and ions for only one voltage polarity. Molecular dynamics simulations together with continuum models provided a multiscale explanation of the observed phenomena and linked the salt concentration dependence of wetting with an electrowetting model. Results presented are crucial for designing next-generation chemical and ionic separation devices as well as understanding fundamental properties of hydrophobic interfaces under nanoconfinement.

Potential-induced wetting and dewetting in pH-responsive block copolymer membranes for mass transport control

Wetting and dewetting behavior in channel-confined hydrophobic volumes is used in biological membranes to effect selective ion/molecular transport. Artificial biomimetic hydrophobic nanopores have been devised utilizing wetting and dewetting, however, tunable mass transport control utilizing multiple transport modes is required for applications such as controllable release/transport, water separation/purification and energy conversion. Here, we investigate the potential-induced wetting and dewetting behavior in a pH-responsive membrane composed of a polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) when fabricated as a hierarchically-organized sandwich structure on a nanopore electrode array (NEA), i.e. BCP@NEA. At pH < pK_a(P4VP) (pK_a ∼ 4.8), the BCP acts as an anion-exchange membrane due to the hydrophilic, protonated P4VP cylindrical nanodomains, but at pH > pK_a(P4VP), the P4VP domains exhibit charge-neutral, hydrophobic and collapsed structures, blocking mass transport via the hydrophobic membrane. However, when originally prepared in a dewetted condition, mass transport in the BCP membrane may be switched on if sufficiently negative potentials are applied to the BCP@NEA architecture. When the hydrophobic BCP membrane is introduced on top of 2-electrode-embedded nanopore arrays, electrolyte solution in the nanopores is introduced, then isolated, by exploiting the potential-induced wetting and dewetting transitions in the BCP membrane. The potential-induced wetting/dewetting transition and the effect on cyclic voltammetry in the BCP@NEA structures is characterized as a function of the potential, pH and ionic strength. In addition, chronoamperometry and redox cycling experiments are used to further characterize the potential response. The multi-modal mass transport system proposed in this work will be useful for ultrasensitive sensing and single-molecule studies, which require long-time monitoring to explore reaction dynamics as well as molecular heterogeneity in nanoconfined volumes.

Gating of Hydrophobic Nanopores with Large Anions

Understanding ion transport in nanoporous materials is critical to a wide variety of energy and environmental technologies, ranging from ion-selective membranes, drug delivery, and biosensing, to ion batteries and supercapacitors. While nanoscale transport is often described by continuum models that rely on a point charge description for ions and a homogeneous dielectric medium for the solvent, here, we show that transport of aqueous solutions at a hydrophobic interface can be highly dependent on the size and hydration strength of the solvated ions. Specifically, measurements of ion current through single silicon nitride nanopores that contain a hydrophobic-hydrophilic junction show that transport properties are dependent not only on applied voltage but also on the type of anion. We find that in Cl^--containing solutions the nanopores only conducted ionic current above a negative voltage threshold. On the other hand, introduction of large polarizable anions, such as Br^- and I^-, facilitated the pore wetting, making the pore conductive at all examined voltages. Molecular dynamics simulations revealed that the large anions, Br^- and I^-, have a weaker solvation shell compared to that of Cl^- and consequently were prone to migrate from the aqueous solution to the hydrophobic surface, leading to the anion accumulation responsible for pore wetting. The results are essential for designing nanoporous systems that are selective to ions of the same charge, for realization of ion-induced wetting in hydrophobic pores, as well as for a fundamental understanding on the role of ion hydration shell on the properties of solid/liquid interfaces.

Information Dynamics of a Nonlinear Stochastic Nanopore System

Nanopores have become a subject of interest in the scientific community due to their potential uses in nanometer-scale laboratory and research applications, including infectious disease diagnostics and DNA sequencing. Additionally, they display behavioral similarity to molecular and cellular scale physiological processes. Recent advances in information theory have made it possible to probe the information dynamics of nonlinear stochastic dynamical systems, such as autonomously fluctuating nanopore systems, which has enhanced our understanding of the physical systems they model. We present the results of local (LER) and specific entropy rate (SER) computations from a simulation study of an autonomously fluctuating nanopore system. We learn that both metrics show increases that correspond to fluctuations in the nanopore current, indicating fundamental changes in information generation surrounding these fluctuations.

Microstructural and associated chemical changes during the composting of a high temperature biochar: Mechanisms for nitrate, phosphate and other nutrient retention and release

Recent studies have demonstrated the importance of the nutrient status of biochar and soils prior to its inclusion in particular agricultural systems. Pre-treatment of nutrient-reactive biochar, where nutrients are loaded into pores and onto surfaces, gives improved yield outcomes compared to untreated biochar. In this study we have used a wide selection of spectroscopic and microscopic techniques to investigate the mechanisms of nutrient retention in a high temperature wood biochar, which had negative effects on Chenopodium quinoa above ground biomass yield when applied to the system without prior nutrient loading, but positive effects when applied after composting. We have compared non-composted biochar (BC) with composted biochar (BCC) to elucidate the differences which may have led to these results. The results of our investigation provide evidence for a complex series of reactions during composting, where dissolved nutrients are first taken up into biochar pores along a concentration gradient and through capillary action, followed by surface sorption and retention processes which block biochar pores and result in deposition of a nutrient-rich organomineral (plaque) layer. The lack of such pretreatment in the BC samples would render it reactive towards nutrients in a soil-fertilizer system, making it a competitor for, rather than provider of, nutrients for plant growth.

Impact of geometry changes in the channel pore by the gating movements on the channel's conductance

Kv 1.2 are voltage-dependent potassium channels of great biological importance. Despite the existence of many reports considering structure - function relations of the Kv 1.2 channel's quantitative domains, some details of the voltage gating remain ambiguous, or even unknown. One of the examples is the range of the S4-S6 domains motions involved in channel activation and gating. Another important question is to what extent the channel geometry influences the observable channel conductance at different voltages, and what mechanism stands behind. Does the narrowing of the pore reduce the conductance by ohmic resistance growth? The answer is surprisingly negative. But it can be explained in an alternative way by considering the fluctuations. To address these problems, we formulate geometric models that mimic the generic features of voltage sensor movement and trigger the movement of the other domains involved in gating. We carry out a complete simulation of S4-S6 domains translations and tilts. The obtained pore profiles allow to estimate the (ohmic) conductance dependency on the voltage. From a family of analysed models, we choose the one most concurring with the experimental data. The results allow to suggest the most probable scenario of S4-S6 domains movement during channel activation by membrane depolarization.

Electrostatic-Charge- and Electric-Field-Induced Smart Gating for Water Transportation

Regulating and controlling the transport of water across nanochannels is of great importance in both fundamental research and practical applications because it is difficult to externally control water flow through nanochannels as in biological channels. To date, only a few hydrophobic nanochannels controlling the transport of water have been reported, all of which use exotic hydrophobic molecules. However, the effect of electrostatic charges, which plays an indispensable role in membrane proteins and dominates the energetics of water permeation across aquaporins, has not gained enough attention to control water transport through a solid-state nanochannel/nanopore. Here, we report electrostatic-charge-induced water gating of a single ion track-etched sub-10 nm channel. This system can directly realize the gating transition between an open, conductive state and a closed, nonconductive state by regulating the surface charge density through a process that involves alternating capillary evaporation and capillary condensation. Compared to the introduction of exotic hydrophobic molecules, water gating controlled by electrostatic charges is simple, convenient, and effective. Such a system anticipates potential applications including desalination, controllable valves, and drug delivery systems.

Biomimetic Solid-State Nanochannels: From Fundamental Research to Practical Applications

In recent years, solid-state smart nanopores/nanochannels for intelligent control of the transportation of ions and molecules as organisms have been extensively studied, because they hold great potential applications in molecular sieves, nanofluidics, energy conversion, and biosensors. To keep up with the fast development of this field, it is necessary to summarize the construction, characterization, and application of biomimetic smart nanopores/nanochannels. These can be classified into four sections: the fabrication of solid-state nanopores/nanochannels, the functionalization methods and materials, the mechanism explanation about the ion rectification, and the practical applications. A brief conclusion and outlook for the biomimetic nanochannels is provided, highlighting those that could be developed and integrated into devices for use in tackling current and the future problems including resources, energy, environment, and health.