Ion Gating in Nanopore Electrode Arrays with Hierarchically Organized pH-Responsive Block Copolymer Membranes

Azo-PMA nanopores of sub-20 nm length for unimolecular resolution of nucleic acids and proteins

Owing to the facile fabrication and surface modification, the cost-effective polymer nanopores are widely employed in unimolecular determination of biomacromolecules and selective sensing of small molecules, nanoparticles and biomarkers. However, the documented polymer nanochannels are generally microscale in length with low spatial resolution. We herein synthesized azobenzene side-chain polymer (Azo-PMA) and spin-coated on silicon nitride membrane to obtain a polymer film of nanoscale thickness for further nanopore generation via controlled dielectric breakdown (CDB) approach. The Azo-PMA nanopores demonstrate good ions transporting activities, pH tolerance and stability in high concentration of electrolyte with low ionic current noise. In addition, the azobenzene-containing polymer nanopores exhibit photo-response upon UV/Vis. light irradiation. The Azo-PMA nanopore devices are utilized for linear and quadruple nucleic acids discrimination, sensing of proteins with distinct shapes and sizes, as well as the single amino acid resolution with good capture rate and sensitivity. We established an unimolecular sensing platform using polymer nanopores for nucleic acids and proteins detection with good spatial resolution, which will be an addition for the nanopore-carrier material exploration and applications in potential genomics and proteomics with high spatiotemporal resolution and low cost.

Single-molecule resolution of macromolecules with nanopore devices

Nanopore-based electrical detection technology holds single-molecule resolution and combines the advantages of high sensitivity, high throughput, rapid analysis, and label-free detection. It is widely applied in the determination of organic and biological macromolecules, small molecules, and nanomaterials, as well as in nucleic acid and protein sequencing. There are a wide variety of organic polymers and biopolymers, and their chemical structures, and conformation in solution directly affect their ensemble properties. Currently, there is limited approach available for the analysis of single-molecule conformation and self-assembled topologies of polymers, dendrimers and biopolymers. Nanopore single-molecule platform offers unique advantages over other sensing technologies, particularly in molecular size differentiation of macromolecules and complex conformation analysis. In this review, the classification of nanopore devices, including solid-state nanopores (SSNs), biological nanopores, and hybrid nanopores is introduced. The recent developments and applications of nanopore devices are summarized, with a focus on the applications of nanopore platform in the resolution of the structures of synthetic polymer, including dendritic, star-shaped, block copolymers, as well as biopolymers, including polysaccharides, nucleic acids and proteins. The future prospects of nanopore sensing technique are ultimately discussed.

Electrochemical Reactions Affected by Electric Double Layer Overlap in Conducting Nanopores

When a potential is applied to an electrode immersed in an electrolyte solution, ions with opposite charges accumulate around the electrode, forming an electrical double layer (EDL). Unlike flat electrodes, nanoporous electrodes with pore sizes comparable to the EDL thickness experience overlapping EDLs, altering the electrochemically effective surface area. Although previous research has primarily examined the ion charging dynamics and EDL formation in nanoporous electrodes, the impact of EDL overlap on Faraday reactions remains underexplored. In this study, we examined the influence of EDL overlap on electrochemical reactions within nanoporous electrodes using chronoamperometry and DC and AC voltammetry. We used the electrolyte concentration, measurement duration, overpotential, and electrode material as variables to determine the relationship between the extent of EDL overlap and the electrochemical reaction. The electrolyte concentration-dependent electrochemical reaction due to the EDL overlap was more pronounced for electrodes with faster potential changes, shorter measurement times, lower overpotentials, and slower catalytic activity. This is a unique nanoporous electrochemical phenomenon that is not observed on flat electrodes. These findings provide insight into the utilization of nanoporous electrodes in catalytic and sensor applications.

Alcohol-gating femtosecond laser-induced micro/nano-structured membranes with reversible switching wettability and breathability

A reversible liquid gating membrane with the ability to regulate gas/liquid transport is critical for many fields, such as biological applications, multiphase separation, and sewerage treatment. Numerous membranes can respond to external stimuli and dynamically control gas/liquid fluid transport; however, simultaneously achieving regulated gas/liquid transport membranes through simple manufacturing remains a challenge. In this work, we investigated an alcohol-regulation gating membrane via femtosecond laser one-step processing, allowing in situ dynamically controllable gas/liquid transfer. More specifically, the porous membrane, processed by laser, exhibits excellent superhydrophobicity (WCA ∼ 153.4°) and breathability (water-vapor evaporation rates ∼118.3 mg (cm2 h)-1), enabling gas to penetrate but not water. In contrast, it allows the passage of water while preventing the permeation of gas subsequent to the introduction of alcohol. Furthermore, the porous membrane still possesses superbly consistent performance after being placed in air for 90 days or over 100 dropping-drying ethanol cycles test, indicating outstanding durability and reversibility. Significantly, the porous membrane has broad potential applications in medical dressings, providing a new strategy to fabricate next-generation bandages.

pH-responsive membranes: Mechanisms, fabrications, and applications

Understanding the mechanisms of pH-responsiveness allows researchers to design and fabricate membranes with specific functionalities for various applications. The pH-responsive membranes (PRMs) are particular categories of membranes that have an amazing aptitude to change their properties such as permeability, selectivity and surface charge in response to changes in pH levels. This review provides a brief introduction to mechanisms of pH-responsiveness in polymers and categorizes the applied polymers and functional groups. After that, different techniques for fabricating pH-responsive membranes such as grafting, the blending of pH-responsive polymers/microgels/nanomaterials, novel polymers and graphene-layered PRMs are discussed. The application of PRMs in different processes such as filtration membranes, reverse osmosis, drug delivery, gas separation, pervaporation and self-cleaning/antifouling properties with perspective to the challenges and future progress are reviewed. Lastly, the development and limitations of PRM fabrications and applications are compared to provide inclusive information for the advancement of next-generation PRMs with improved separation and filtration performance.

Layer-by-Layer Nanofluidic Membranes for Promoting Blue Energy Conversion

Access to and use of energy resources are now crucial components of modern human existence thanks to the exponential growth of technology. Traditional energy sources provide significant challenges, such as pollution, scarcity, and excessive prices. As a result, there is more need than ever before to replace depleting resources with brand-new, reliable, and environmentally friendly ones. With the aid of reverse electrodialysis, the salinity gradient between rivers and seawater as a clean supply with easy and infinite availability is a viable choice for energy generation. The development of nanofluidic-based reverse electrodialysis (NRED) as a novel high-efficiency technology is attributable to the progress of nanoscience. However, understanding the predominant mechanisms of this process at the nanoscale is necessary to develop and disseminate this technology. One viable option to gain insight into these systems while saving expenses is to employ simulation tools. In this study, we looked at how a layer-by-layer (LBL) soft layer influences ion transport and energy production in charged nanochannels. We solved the steady-state Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations for three different types of nanochannels with a trumpet geometry, where the narrow part is covered with a built-up LbL soft layer and the rest is a hard wall with a surface charge density of σ = -10, 0, or +10 mC/m2. The findings show that in type (I) nanochannels, at NPEL/NA = 100 mol/m3 and pH = 7, the maximum power output rises 675-fold as the concentration ratio rises from 10 to 1000. The results of this study can aid in a better understanding of energy harvesting processes using nanofluidic-based reverse electrodialysis in order to identify optimal conditions for the design of an intelligent route with great controllability and minimal pollution.

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.

Enhanced Ionic Current Rectification through Innovative Integration of Polyelectrolyte Bilayers and Charged-Wall Smart Nanochannels

The tools utilized by humans continue to shrink and speed up. Lab-on-a-chip (LOC) is one of the most recent techniques for decreasing the size of chemical systems. Today, LOCs have made substantial strides in developing nanomaterial fabrication techniques. Controlling and regulating the fluid and ion mobility in these systems is crucial. Layer-by-layer (LBL) soft layers are one of the most effective strategies for controlling fluid flow in channels. In light of the present constraints for developing these systems and the high expense of experimental investigations, it is vital to employ modeling to minimize costs and comprehend their underlying ideas and operations. In this study, we examined the influence of the LBL soft layer's presence in the charged nanochannels on the ion transport parameters. To examine the effect of the coating length of the LBL soft layer, we first examined three lengths of coating: one with a length greater than half (type (I)), one with a length equal to half (type (II)), and one with a length less than half (type (III)) of the nanochannel length. Then, by solving Poisson-Nernst-Planck and Navier-Stokes equations, we determined the influences of pH, soft layer charge density (N_PEL/N_A), bulk concentration (C₀), and hard surface charge density (σ) on the ionic current rectification (R_f) and selectivity (S) of the nanochannel. The maximum rectification of 30.65 was achieved using a nanochannel of type (III) and σ = +10 mC/m². The current results demonstrate a promising hybrid architecture consisting of an LBL soft layer and a smart charged nanochannel for enhanced rectification.

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.

Actively Controllable Solid-Phase Microextraction in a Hierarchically Organized Block Copolymer-Nanopore Electrode Array Sensor for Charge-Selective Detection of Bacterial Metabolites

Pseudomonas aeruginosa produces a number of phenazine metabolites, including pyocyanin (PYO), phenazine-1-carboxamide (PCN), and phenazine-1-carboxylic acid (PCA). Among these, PYO has been most widely studied as a biomarker of P. aeruginosa infection. However, despite its broad-spectrum antibiotic properties and its role as a precursor in the biosynthetic route leading to other secondary phenazines, PCA has attracted less attention, partially due to its relatively low concentration and interference from other highly abundant phenazines. This challenge is addressed here by constructing a hierarchically organized nanostructure consisting of a pH-responsive block copolymer (BCP) membrane with nanopore electrode arrays (NEAs) filled with gold nanoparticles (AuNPs) to separate and detect PCA in bacterial environments. The BCP@NEA strategy is designed such that adjusting the pH of the bacterial medium to 4.5, which is above the pK_a of PCA but below the pK_a of PYO and PCN, ensures that PCA is negatively charged and can be selectively transported across the BCP membrane. At pH 4.5, only PCA is transported into the AuNP-filled NEAs, while PYO and PCN are blocked. Structural characterization illustrates the rigorous spatial segregation of the AuNPs in the NEA nanopore volume, allowing PCA secreted from P. aeruginosa to be quantitatively determined as a function of incubation time using square-wave voltammetry and surface-enhanced Raman spectroscopy. The strategy proposed in this study can be extended by changing the nature of the hydrophilic block and subsequently applied to detect other redox-active metabolites at a low concentration in complex biological samples and, thus, help understand metabolism in microbial communities.

Molecular weight and dispersity affect chain conformation and pH-response in weak polyelectrolyte brushes

The pH-dependence of the conformation of annealed polyelectrolyte brushes can be tuned by varying the molecular weight distribution, as characterized via weight-average molecular weight and dispersity.