Stimuli-Responsive Polymer Brushes for Flow Control through Nanopores

Photo-responsive anti-fouling polyzwitterionic brushes: a mesoscopic simulation

The antifouling effects of a toothbrush-shaped photo-responsive polyzwitterionic membrane were studied via dissipative particle dynamics simulations in this work. The results reveal that the membrane modified by spiropyran methacrylate brushes displays photo-switchable and antifouling capability due to the photo-induced ring-opening reaction. Namely, surface morphology and hydrophilicity change in response to visible or UV light irradiation, which can be observed visually by protein adsorption and desorption. Further study indicates that: (1) brush-modification density can influence the structure and properties of the membrane. With low modification density, systems cannot establish an intact selective layer, which hinders the antifouling ability; as the modification density increases, the intact selective layer can be formed, which is conducive to the expression of photo-responsiveness and antifouling capability. (2) Factors of toothbrush-hair length and grafting ratio can influence the establishment of a light-responsive surface: as the grafting ratio and toothbrush-hair length increase, the light-responsive surface is gradually formed, meanwhile, the antifouling ability can be continuously reinforced under UV light irradiation. (3) As the brushes switch into a zwitterionic merocyanine state under UV exposure, the selective layer swelling becomes stronger than that with a hydrophobic spiropyran state under visible exposure. This is owing to the enhanced interaction between zwitterionic brushes and water, which is the root of the antifouling effect. The present work is expected to provide some guidelines for the design and development of novel antifouling membrane surfaces.

Hydrodynamic Effects on the Collapse Kinetics of Flexible Polyelectrolytes

Understanding the collapse kinetics of polyelectrolytes (PEs) is crucial for comprehending various biological and industrial phenomena. Despite occurring in an aqueous environment, previous computational studies have overlooked the influence of hydrodynamic interactions (HIs) facilitated by fluid motion. Here, we directly compute the Navier-Stokes equation to investigate the collapse kinetics of a highly charged flexible PE. Our findings reveal that HI accelerates PE collapse induced by hydrophobicity and multivalent salt. In the case of hydrophobicity, HI induces long-range collective motion of monomers, accelerating the coarsening of local clusters through either Brownian-coagulation-like or evaporation-condensation-like processes, depending on the strength of hydrophobicity with respect to electrostatic interaction. Regarding multivalent salt, HI does not affect the condensation dynamics of multivalent ions but facilitates quicker movement of local dipolar clusters along the PE, thereby expediting the collapse process. These results provide valuable insights into the underlying mechanisms of HI in PE collapse kinetics.

Synthesis and Characterization of Stimuli-Responsive Polymer Brushes in Nanofluidic Channels

Nanochannels with controllable gating behavior are attractive features in a wide range of nanofluidic applications including viral detection, particle sorting, and flow regulation. Here, we use selective sidewall functionalization of nanochannels with a polyelectrolyte brush to investigate the channel gating response to variations in solution pH and ionic strength. The conformational and structural changes of the interfacial brush layer within the channels are interrogated by specular and off-specular neutron reflectometry. Simultaneous fits of the specular and off-specular signals, using a dynamical theory model and a fitting optimization protocol, enable detailed characterization of the brush conformations and corresponding channel geometry under different solution conditions. Our results indicate a collapsed brush state under basic pH, equivalent to an open gate, and an expanded brush state representing a partially closed gate upon decreasing the pH and salt concentration. These findings open new possibilities in noninvasive in situ characterization of tunable nanofluidics and lab-on-chip devices with advanced designs and improved functionality.

Temperature-Responsive Nanoporous Membranes from Self-Assembly of Poly(N-isopropylacrylamide) Hairy Nanoparticles

Nanoporous membranes play a critical role in numerous separations on laboratory and industrial scales, ranging from water treatment to biotechnology. However, few strategies exist that allow for the preparation of mechanically robust nanoporous membranes whose separation properties can be easily tuned. Here, we introduce a new family of tunable nanoporous membranes based on nanoparticles decorated with temperature-responsive polymer brushes. We prepared mechanically robust membranes from hairy nanoparticles (HNPs) carrying PNIPAM polymer brushes. We assembled the HNPs into thin films through pressure-driven deposition of nanoparticle suspensions and measured the permeability and filtration cutoff of these membranes at different temperatures. The membrane pore diameter at room temperature varied between 10 and 30 nm depending on the polymer length. The water permeability of these membranes could be controlled by temperature, with the effective pore diameter increasing by a factor of 3-6 (up to 100 nm) when the temperature was increased to 60 °C. The size selectivity of these membranes in the filtration of nanoparticles could also be attenuated by temperature. Molecular dynamics computer simulations of a coarse-grained HNP model show that temperature-sensitive pores sizes are consistent with our experimental results and reveal the polymer configurations responsible for the observed filtration membrane permeability. We expect that these membranes will be useful for separations and in the preparation of responsive microfluidic devices.

Concave polymer brushes inwardly grafted in spherical cavities

The structure and scaling properties of inwardly curved polymer brushes, tethered under good solvent conditions to the inner surface of spherical shells such as membranes and vesicles, are studied by extensive molecular dynamics simulations and compared with earlier scaling and self-consistent field theory predictions for different molecular weights of the polymer chains N and grafting densities σ_g in the case of strong surface curvature, R^-1. We examine the variation of the critical radius R*(σ_g), separating the regimes of weak concave brushes and compressed brushes, predicted earlier by Manghi et al. [Eur. Phys. J. E 5, 519-530 (2001)], as well as various structural properties such as the radial monomer- and chain-end density profiles, orientation of bonds, and brush thickness. The impact of chain stiffness, κ, on concave brush conformations is briefly considered as well. Eventually, we present the radial profiles of the local pressure normal, P_N, and tangential, P_T, to the grafting surface, and the surface tension γ(σ_g), for soft and rigid brushes, and find a new scaling relationship P_N(R)∝σ_g ⁴, independent of the degree of chain stiffness.

Advances in design of polymer brush functionalized inorganic nanomaterials and their applications in biomedical arena

Grafting of polymer brush (assembly of polymer chains tethered to the substrate by one end) is emerging as one of the most viable approach to alter the surface of inorganic nanomaterials. Inorganic nanomaterials despite their intrinsic functional superiority, their applications remain restricted due to their incompatibility with organic or biological moieties vis-à-vis agglomeration issues. To overcome such a shortcoming, polymer brush modified surfaces of inorganic nanomaterials have lately proved to be of immense potential. For example, polymer brush-modified inorganic nanomaterials can act as efficient substrates/platforms in biomedical applications, ranging from drug-delivery to protein-array due to their integrated advantages such as amphiphilicity, stimuli responsiveness, enhanced biocompatibility, and so on. In this review, the current state of the art related to polymer brush-modified inorganic nanomaterials focusing, not only, on their synthetic strategies and applications in biomedical field but also the architectural influence of polymer brushes on the responsiveness properties of modified nanomaterials have comprehensively been discussed and its associated future perspective is also presented. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

A nanofluidic system based on cylindrical polymer brushes: how to control the size of nanodroplets

In molecular dynamics simulations we investigate the self-organized formation of droplets from a continuous flow of incoming nanoparticles. This transformation is facilitated by a cylindrical channel that is decorated with a polymer brush in a marginally poor solvent. We analyze droplet formation and propagation by means of simple scaling arguments which are tested in the simulations. Polymer brushes in marginally poor solvents serve as a pressure feedback system, exhibit a collapse transition under the moderate pressure of the incident flow, without the need for additional external stimuli, and finally close spontaneously after droplet passage. Our results qualitatively demonstrate the control of polymer brushes over continuous fluids and droplet formation, and its effectiveness as a means of fluid control can be used to design nanofluidic rectification devices that operate reliably under moderate pressure.

Pore Size Dependence of Permeability in Bicontinuous Nanoporous Media

In this work, we employ many-body dissipative particle dynamics (mDPD) simulations to investigate the fluid flow process through bicontinuous nanoporous media, which are representative models for a broad class of nanoporous materials. The mDPD formulation includes attractive and repulsive interactions describing accurately fluid-fluid and fluid-solid interactions. As a mesoscale simulation method, mDPD can bridge the length and time scale gap between continuum and atomistic simulations. The bicontinuous nanoporous models are constructed considering a defined morphology, the porosity level, and varying pore sizes in the range from 3.41 to 13.63 nm. All models have a 0.65 porosity level and the same topology. The models provide a stochastic description of the morphology and pore size distribution and allow for a direct investigation of the dependence of permeability on the average pore size. The stationary nanoporous models are filled with fluid particles, and flow is induced by the action of confining pistons. Simulation results, obtained by imposing different pressure differences on the surfaces of the nanoporous media, indicate a linear pressure drop within the nanoporous model. Regardless of the complexities and different scales of the porous media considered, the steady-state fluid flow through the nanoporous models is proportional to the pressure gradient applied, in agreement with Darcy's law. The calculated pore size dependence of permeability is well described by the Hagen-Poiseuille law, considering a single shape correction factor that accounts for the flow resistance due to the complex nanoporous morphology. This work highlights the effect of the average pore size of a complex stochastic bicontinuous nanoporous medium on fluid properties. The results indicate rather a relatively simple dependence of permeability on the average pore size. The novel method we employ to generate the stochastic bicontinuous nanoporous structure allows the control of different geometric features that can be explored in future studies.

Electrostatic Potential Analysis in Polyelectrolyte Brush-Grafted Microchannels Filled with Polyelectrolyte Dispersion

In this study, the model framework that includes almost all relevant parameters of interest has been developed to quantify the electrostatic potential and charge density occurring in microchannels grafted with polyelectrolyte brushes and simultaneously filled with polyelectrolyte dispersion. The brush layer is described by the Alexander-de Gennes model incorporated with the monomer distribution function accompanying the quadratic decay. Each ion concentration due to mobile charges in the bulk and fixed charges in the brush layer can be determined by multi-species ion balance. We solved 2-dimensional Poisson–Nernst–Planck equations adopted for simulating electric field with ion transport in the soft channel, by considering anionic polyelectrolyte of polyacrylic acid (PAA). Remarkable results were obtained regarding the brush height, ionization, electrostatic potential, and charge density profiles with conditions of brush, dispersion, and solution pH. The Donnan potential in the brush channel shows several times higher than the surface potential in the bare channel, whereas it becomes lower with increasing PAA concentration. Our framework is fruitful to provide comparative information regarding electrostatic interaction properties, serving as an important bridge between modeling and experiments, and is possible to couple with governing equations for flow field.

Rheological properties of polymer chains at a copper oxide surface: Impact of the chain length, surface coverage, and grafted polymer shape

We employ a recently derived semirealistic set of coarse-grained interactions to simulate polymer brushes of cis-1,4-polybutadiene grafted on a cuprous-oxide surface within the framework of dissipative particle dynamics. We consider two types of brushes, I and Y, that differ in the way they are connected to the surface. Our model explores the impact of free polymer chain length, grafting density of the brush, and imposed shear rate on the structural and dynamic properties of complex metal oxide polymer interfaces.

Liquid and Droplet Transport in Brush-Coated Cylindrical Nanochannels: Brush-Assisted Droplet Formation

We study, by coarse-grained molecular dynamics simulations, equilibrium and flow properties of a liquid in cylindrical nanochannels, coated with polymer brushes. The parameters of the interaction potential model confer a chemical incompatibility between brush monomers and liquid particles. First, we study cylindrical channels whose radii are larger than the brush height and a continuous column of liquid forms at the center of the channel. These results are contrasted to the limiting case in which the radius of the cylinder is comparable to the brush height. In this second case, the grafted polymers interact across the channel and "close" it. We observe a train of droplets as the stable liquid morphology. The droplet size is comparable to the cylinder radius. By applying a constant body force onto the liquid, we induce a Poiseuille-like flow and investigate the morphology and flow rate as a function of driving force. Upon increasing the driving force, we encounter a nonequilibrium transition from a closed channel with slowly moving droplets to a flowing liquid thread at the center. The switching between these two states is reversible.

Charge regulation mechanism in end-tethered weak polyampholytes

Weak polyampholytes, containing oppositely charged dissociable groups, are expected to be responsive to changes in ionic conditions. Here, we determine structural and thermodynamic properties, including the charged groups' degrees of dissociation, of end-tethered weak polyampholyte layers as a function of salt concentration, pH, and the solvent quality. For diblock weak polyampholytes grafted by their acidic blocks, we find that the acidic monomers increase their charge while the basic monomers decrease their charge with decreasing salt concentration for pH values less than the pKa value of both monomers and vice versa when the pH > pKa. This complex charge regulation occurs because the electrostatic attraction between oppositely charged blocks is stronger than the repulsion between monomers with the same charge in both good and poor solvents when the screening by salt ions is weak. This is evidenced by the retraction of the top block into the bottom layer. In the case of poor solvent conditions to the basic block (the top block), we find lateral segregation of basic monomers into micelles, forming a two-dimensional hexagonal pattern on the surface at intermediate and high pH values for monovalent salt concentrations from 0.01 to 0.1 M. When the solvent is poor to both blocks, we find lateral segregation of the grafted acidic block into lamellae with longitudinal undulations of low and high acidic monomer density. By exploiting weak block polyampholytes, our work expands the parameter space for creating responsive surfaces stable over a wide range of pH and salt concentration.

Real-Time Measurement of Polymer Brush Dynamics Using Silicon Photonic Microring Resonators: Analyte Partitioning and Interior Brush Kinetics

Polymer brushes are found in biomedical and industrial technologies, where they exhibit functionalities considerably dependent on polymer brush-solvent-analyte interactions. It remains a difficult challenge to quickly analyze solvent-swollen polymer brushes, both at the solvent-polymer brush interface and in the brush interior, as well as to monitor the kinetics of interaction of solvent-swollen brushes with key analytes. Here, we demonstrate the novel use of silicon photonic microring resonators to characterize in situ swollen polymer brush-analyte interactions. By monitoring resonant wavelength shifts, we find that brush-solvent-analyte interaction parameters can be extracted from a single set of data or from successive analyte introductions using a single brush-coated sensor. The partition coefficient of three industrially relevant plasticizers into hydrophobic and hydrophilic brushes was determined and found to be in agreement with known solubility trends. We found that the diffusion coefficient of the plasticizer into the brush decreases as brush thickness increases, supporting a model of a dense inner brush layer and diffuse outer layer. pK_a's of pH-sensitive brushes were determined on the microring resonator platform; upon increasing the dry brush thickness, the pK_a for poly(2-dimethylamino ethyl methacrylate) decreased from 8.5 to approach the bulk material pK_a of 7.3 and showed dependence on the presence and concentration of salt. These proof-of-concept experiments show how the surface-sensitive nature of the microring resonator detection platform provides valuable information about the interaction of the polymer brushes with the solvents and analytes, not easily accessed by other techniques.

Surface Modification of TFC-PA RO Membrane by Grafting Hydrophilic pH Switchable Poly(Acrylic Acid) Brushes

The grafting of pH-responsive poly(acrylic acid) (PAA) brushes was carried out on the surface of a commercial TFC-PA membrane using surface-initiated atom transfer radical polymerization (SI-ATRP). Poly(t-butyl acrylate) was polymerized through the SI-ATRP method followed by its acid hydrolysis to form PAA hydrophilic polymer brushes. Surface morphology, permeation flux, salt rejection, and pore sizes were investigated. The contact angle for water was reduced from 50° for a pristine membrane to 27° for the modified membrane due to a modification with the hydrophilic functional group and its brush on membrane surfaces. The flux rate also increased noticeably at lower pH values relative to higher pH for the modified membranes, while the flux remains stable in the case of pristine TFC-PA membranes. There is slight transition in the water flux rate that was also observed when going from pH values of 3 to 5. This was attributed to the pH-responsive conformational changes for the grafted PAA brushes. At these pH values, ionization of the COOH group takes place below and above pKa to influence the effective pore dimension of the modified membranes. At a lower pH value, the PAA brushes seem to permit tight structure conformation resulting in larger pore sizes and hence more flux. On the other hand, at higher pH values, PAA brushes appeared to be in extended conformation to induce smaller pore sizes and result in less flux. Further, pH values were observed to not significantly affect the NaCl salt rejection with values observed in between 98.8% and 95% and close to that of the pristine TFC-PA membranes. These experimental results are significant and have immediate implication for advances in polymer technology to design and modify the “switchable membrane surfaces” with controllable charge distribution and surface wettability, as well as regulation of water flux and salt.

Counterion-Tunable Thermosensitivity of Strong Polyelectrolyte Brushes

In this work, poly(sodium styrene sulfonate) brushes have been employed as a precursor to prepare thermosensitive strong polyelectrolyte brushes (SPBs) through a counterion exchange strategy. The substitution of hydrophilic Na⁺ counterions by hydrophobic tetraalkylphosphonium counterions leads to a thermoresponsivity of the SPBs. The thermosensitive properties including hydration, stiffness, and surface water wettability of the SPBs can be modulated by the type of the tetraalkylphosphonium counterions. Nevertheless, the wet thickness of the SPBs with tetraalkylphosphonium counterions does not exhibit an obvious temperature dependency due to the high steric barrier in the crowded environment of SPBs generated by the large tetraalkylphosphonium counterions. The mixtures of small Na⁺ counterions and large tetraalkylphosphonium counterions are employed to realize the thermosensitive wet thickness without sacrificing other thermoresponsive properties of the SPBs because the mixed counterions can bring both a certain hydrophobicity and some free space to the brushes. This work opens up the opportunities available for the use of counterions to tune the thermosensitivity of SPBs.

Modular Fabrication of Intelligent Material-Tissue Interfaces for Bioinspired and Biomimetic Devices

One of the goals of biomaterials science is to reverse engineer aspects of human and nonhuman physiology. Similar to the body's regulatory mechanisms, such devices must transduce changes in the physiological environment or the presence of an external stimulus into a detectable or therapeutic response. This review is a comprehensive evaluation and critical analysis of the design and fabrication of environmentally responsive cell-material constructs for bioinspired machinery and biomimetic devices. In a bottom-up analysis, we begin by reviewing fundamental principles that explain materials' responses to chemical gradients, biomarkers, electromagnetic fields, light, and temperature. Strategies for fabricating highly ordered assemblies of material components at the nano to macro-scales via directed assembly, lithography, 3D printing and 4D printing are also presented. We conclude with an account of contemporary material-tissue interfaces within bioinspired and biomimetic devices for peptide delivery, cancer theranostics, biomonitoring, neuroprosthetics, soft robotics, and biological machines.

Salt- and pH-induced swelling of a poly(acrylic acid) brush via quartz crystal microbalance w/dissipation (QCM-D)

We infer the swelling/de-swelling behavior of weakly ionizable poly(acrylic acid) (PAA) brushes of 2-39 kDa molar mass in the presence of KCl concentrations from 0.1-1000 mM, pH = 3, 7, and 9, and grafting densities σ = 0.12-2.15 chains per nm2 using a Quartz Crystal Microbalance with Dissipation (QCM-D), confirming and extending the work of Wu et al. to multiple chain lengths. At pH 7 and 9 (above the pKa ∼ 5), the brush initially swells at low KCl ionic strength (<10 mM) in the "osmotic brush" regime, and de-swells at higher salt concentrations, in the "salted brush" regime, and is relatively unaffected at pH 3, below the pKa, as expected. At pH 7, at low and moderate grafting densities, our results in the high-salt "salted brush" regime (Cs > 10 mM salt) agree with the predicted scaling H ∼ Nσ+1/3Cs-1/3 of brush height H, while in the low-salt "osmotic brush" regime (Cs < 10 mM salt), we find H ∼ Nσ+1/3Cs+0.28-0.38, whose dependence on Cs agrees with scaling theory for this regime, but the dependence on σ strongly disagrees with it. The predicted linearity in the degree of polymerization N is confirmed. The new results partially confirm scaling theory and clarify where improved theories and additional data are needed.

Influence of Chain Architecture on Nanopore Accessibility in Polyelectrolyte Block-Co-Oligomer Functionalized Mesopores

Functionalized ordered mesoporous silica materials are commonly investigated for applications such as drug release, sensing, and separation processes. Although, various homopolymer functionalized responsive mesopores are reported, little focus has been put on copolymers in mesopores. Mesoporous silica films are functionalized with responsive and orthogonally charged block-co-oligomers. Responsive 2-dimethylamino)ethyl methacrylate)-block-2-(methacryloyloxy)ethyl phosphate (DMAEMA-b-MEP) block-co-oligomers are introduced into mesoporous films using controlled photoiniferter initiated polymerization. This approach allows a very flexible charge composition design. The obtained block-co-oligomer functionalized mesopores show a complex gating behavior indicating a strong interplay between the different blocks emphasizing the strong influence of charge distribution inside mesopores on ionic pore accessibility. For example, in contrast to mesopores functionalized with zwitterionic polymers, DMAEMA-b-MEP block-co-oligomer functionalized mesopores, containing two oppositely charged blocks, do not show bipolar ion exclusion, demonstrating the influence of the chain architecture on mesopore accessibility. Furthermore, ligand binding-based selective gating is strongly influenced by this chain architecture as demonstrated by an expansion of pore accessibility states for block-co-oligomer functionalized mesopores as compared to the individual polyelectrolyte functionalization for calcium induced gating.

Hydrodynamic Properties of Polymers Screening the Electrokinetic Flow: Insights from a Computational Study

Understanding the hydrodynamic properties of polymeric coatings is crucial for the rational design of molecular transport involving polymeric surfaces and is relevant to drug delivery, sieving, molecular separations, etc. It has been found that the hydrodynamic radius of a polymer segment is an order of magnitude smaller than its physical size, but the origin of this effect does not seem to be well understood. Herein, we study the hydrodynamic properties of polymeric coatings by using molecular dynamics simulations, navigated by the continuous Navier-Stokes-Brinkman model. We confirm that the averaged hydrodynamic radius of a polymer bead is about one order of magnitude smaller than its physical radius, and, in addition, we show that it exhibits a strong dependence on the degree of polymerization. We relate this variation of the hydrodynamic radius to the structural properties and hydrodynamic shielding by surrounding polymer beads. This is done by separating the effects originating from near and far beads. For the near beads, shielding is mainly due to the two nearest beads (of the same polymer) and leads to about a 5-fold reduction in the hydrodynamic radius. Assuming the additivity of the hydrodynamic shielding by far beads, we suggest a simple model, which captures correctly the qualitative behaviour of the hydrodynamic radius with the degree of polymerization. The revealed shielding effects provide important insights relevant to the advanced modelling of hydrodynamic properties of polymeric coatings.

Dielectric Effects on Ion Transport in Polyelectrolyte Brushes

Surface-grafted polyelectrolytes provide a versatile way to create functionalized interfaces and nanochannels with externally controllable properties. Understanding the behavior of ions within the brush-like assemblies is crucial for the further development of these devices. We demonstrate that the ion transport through the brushes is governed by the interplay of electrostatic ion-polymer binding and steric effects, leading to a mobility that depends nonmonotonically on grafting density. However, the ion-polymer binding can be modulated by the dielectric properties of the substrate. As a result, surface polarization suppresses ion mobility near insulating interfaces and enhances it near conducting interfaces, even causing a shift from nonmonotonic to monotonic variation with grafting density.

Pressure responsive gating in nanochannels coated by semiflexible polymer brushes

We study by coarse-grained molecular-dynamics simulations the liquid flow in a slit channel with the inner walls coated by semiflexible polymer brushes. The distance between walls is close enough such that polymers grafted to opposing walls interact among each other and form bundles across the channel in poor solvent conditions. The solvent is simulated explicitly, including particles that fill the interior of the channel. The system is studied in equilibrium and under flow, by applying a constant body force on each particle of the system. A non-linear relation between external force and flow rate is observed, for a particular set of parameters. This non-linear response is linked to a morphological change of the polymer brushes. For large enough forces, the bundle structures formed across the channel break as the chains lean in the direction of the flow, and clear the middle of the channel. This morphological alteration of the polymer configurations translates in a sudden increase in the flow rate, acting as a pressure-responsive gate. The relation between flow and external force is investigated for various parameters, such as grafting density, quality of the solvent and polymer bending rigidity. We observe a non-monotonic dependence of the flow as a function of the polymer rigidity, and find an optimum value for the persistence length. We also find that the force threshold at which the morphological changes happen in the polymer brush, depends linearly on the grafting density. These findings can lead to new flow control techniques in micro and nano-fluidic devices.

A Temperature-, pH- and Voltage-Responsive Nanogate with a Remarkably High Factor of Change in Ion Currents due to ON/OFF Switching

In biological cells, nuclear pore complexes (NPCs) embedded in cell membranes are capable of controlling the flow of ions, for example, Na⁺ , K⁺ , and Ca²⁺ by responding to stimuli, for example, pH and voltage. Inspired by NPCs, researchers have been endeavoring to develop nanogates to achieve the control of ion transport, but the developed nanogates only have a low factor of change in ion currents due to ON/OFF switching. As such nanopores with high temperature and pH responsivities were developed in this work. According to the experimental results, at a voltage of 3 V, the change in ion currents due to pH change is up to a factor of 170, which is remarkably high compared to other nanogates reported. Quantum chemical (QC) calculation results show that a protonated cytosine molecule (C⁺ ) and an unprotonated cytosine molecule (C) form three pairs of hydrogen bonds and consequently a nucleobase pair, CC⁺ , leading to the binding of various strands, assembly of a strand net, and blockage of ion transport. The nanogate developed is capable of responding to temperature change. At a voltage of 3 V, the factor of change in ion currents in response to temperature variation is as high as 110. Further experiments were performed to investigate the influence of the NaCl concentrations and small opening diameters exerted on nanogate performance.

Flow and aggregation of rod-like proteins in slit and cylindrical pores coated with polymer brushes: an insight from dissipative particle dynamics

We use a meso-scale dissipative particle dynamics method to simulate the flow and aggregation of rod-like protein solutions through pores grafted with a solvent-sensitive polymer brush. The coated pores can control protein permeability and aggregation by a stretch-to-collapse conformational transition of the brush polymers in response to changes in the solvent quality. The protein solutions mimic aqueous glycoprotein solutions and proteins are represented as rod-like objects formed by coarse-grain beads. The model further employs two types of beads to represent the existence of cystein-like terminal groups in real glycoproteins and mimic the aggregation of real glycoproteins in aqueous solutions. We vary the solvent quality with respect to the brush chains and study the flow and aggregation of rod-like proteins in the slit and cylindrical pores as the brush polymers undergo the stretch-to-collapse transition. The results show that stretched brush chains close the pore, hamper proteins' flow and promote proteins' aggregation. The collapsed brush chains open the pores for proteins' flow and suppress their aggregation. Therefore, we observe more than a ten-fold reduction in the permeation rate of proteins in both pore geometries. Finally, due to pore confinement, larger proteins' aggregates are formed in the slit pore than in the cylindrical pore, while more pronounced orientation of proteins in the flow direction is seen in the cylindrical pore than in the slit pore.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Surface conductivity in electrokinetic systems with porous and charged interfaces: Analytical approximations and numerical results

We derive an approximate analytical representation of the conductivity for a 1D system with porous and charged layers grafted onto parallel plates. Our theory improves on prior work by developing approximate analytical expressions applicable over an arbitrary range of potentials, both large and small as compared to the thermal voltage (RTF). Further, we describe these results in a framework of simplifying nondimensional parameters, indicating the relative dominance of various physicochemical processes. We demonstrate the efficacy of our approximate expression with comparisons to numerical representations of the exact analytical conductivity. Finally, we utilize this conductivity expression, in concert with other components of the electrokinetic coupling matrix, to describe the streaming potential and electroviscous effect in systems with porous and charged layers.

Polyelectrolyte brushes: theory, modelling, synthesis and applications

Polyelectrolyte (PE) brushes are a special class of polymer brushes (PBs) containing charges. Polymer chains attain "brush"-like configuration when they are grafted or get localized at an interface (solid-fluid or liquid-fluid) with sufficiently close proximity between two-adjacent grafted polymer chains - such a proximity triggers a particular nature of interaction between the adjacent polymer molecules forcing them to stretch orthogonally to the grafting interface, instead of random-coil arrangement. In this review, we discuss the theory, synthesis, and applications of PE brushes. The theoretical discussion starts with the standard scaling concepts for polymer and PE brushes; following that, we shed light on the state of the art in continuum modelling approaches for polymer and PE brushes directed towards analysis beyond the scaling calculations. A special emphasis is laid in pinpointing the cases for which the PE electrostatic effects can be de-coupled from the PE entropic and excluded volume effects; such de-coupling is necessary to appropriately probe the complicated electrostatic effects arising from pH-dependent charging of the PE brushes and the use of these effects for driving liquid and ion transport at the interfaces covered with PE brushes. We also discuss the atomistic simulation approaches for polymer and PE brushes. Next we provide a detailed review of the existing approaches for the synthesis of polymer and PE brushes on interfaces, nanoparticles, and nanochannels, including mixed brushes and patterned brushes. Finally, we discuss some of the possible applications and future developments of polymer and PE brushes grafted on a variety of interfaces.

Effect of nanoporous structure and polymer brushes on the ionic conductivity of poly(methacrylic acid)/anode aluminum oxide hybrid membranes

Anode aluminum oxide (AAO) porous materials have been widely used in ionic translocation for many biological and chemical studies.

Electrostatics of soft charged interfaces with pH-dependent charge density: effect of consideration of appropriate hydrogen ion concentration distribution

Explicit consideration of hydrogen ion concentration for describing the electrostatics of grafted polyelectrolyte layers with pH-dependent charge density exhibits the necessity of considering a non-uniform depth dependent monomer distribution.

Stimuli-sensitive intrinsically disordered protein brushes

Grafting polymers onto surfaces at high density to yield polymer brush coatings is a widely employed strategy to reduce biofouling and interfacial friction. These brushes almost universally feature synthetic polymers, which are often heterogeneous and do not readily allow incorporation of chemical functionalities at precise sites along the constituent chains. To complement these synthetic systems, we introduce a biomimetic, recombinant intrinsically disordered protein that can assemble into an environment-sensitive brush. This macromolecule adopts an extended conformation and can be grafted to solid supports to form oriented protein brushes that swell and collapse dramatically with changes in solution pH and ionic strength. We illustrate the value of sequence specificity by using proteases with mutually orthogonal recognition sites to modulate brush height in situ to predictable values. This study demonstrates that stimuli-responsive brushes can be fabricated from proteins and introduces them as a new class of smart biomaterial building blocks.

Thermally switchable aligned nanopores by magnetic-field directed self-assembly of block copolymers

A scalable approach for developing large area polymer films, with stimuli responsive vertically aligned nanopores is reported. Magnetic fields are used to create highly aligned hexagonally packed block copolymer cylindrical microdomains with order parameters exceeding 0.95. Selective etch removal of material yields nanoporous films which demonstrate reversible pore closure on heating.

DNA-modified polymer pores allow pH- and voltage-gated control of channel flux

Biological channels embedded in cell membranes regulate ionic transport by responding to external stimuli such as pH, voltage, and molecular binding. Mimicking the gating properties of these biological structures would be instrumental in the preparation of smart membranes used in biosensing, drug delivery, and ionic circuit construction. Here we present a new concept for building synthetic nanopores that can simultaneously respond to pH and transmembrane potential changes. DNA oligomers containing protonatable A and C bases are attached at the narrow opening of an asymmetric nanopore. Lowering the pH to 5.5 causes the positively charged DNA molecules to bind to other strands with negative backbones, thereby creating an electrostatic mesh that closes the pore to unprecedentedly high resistances of several tens of gigaohms. At neutral pH values, voltage switching causes the isolated DNA strands to undergo nanomechanical movement, as seen by a reversible current modulation. We provide evidence that the pH-dependent reversible closing mechanism is robust and applicable for nanopores with opening diameters of up to 14 nm. The concept of creating an electrostatic mesh may also be applied to different organic polymers.

Physical modelling of the nuclear pore complex

An in-depth discussion of nuclear pore complexes illustrates the importance of macromolecular confinement in biological systems.

Physically interesting behaviour can arise when soft matter is confined to nanoscale dimensions. A highly relevant biological example of such a phenomenon is the Nuclear Pore Complex (NPC) found perforating the nuclear envelope of eukaryotic cells. In the central conduit of the NPC, of ∼30–60 nm diameter, a disordered network of proteins regulates all macromolecular transport between the nucleus and the cytoplasm. In spite of a wealth of experimental data, the selectivity barrier of the NPC has yet to be explained fully. Experimental and theoretical approaches are complicated by the disordered and heterogeneous nature of the NPC conduit. Modelling approaches have focused on the behaviour of the partially unfolded protein domains in the confined geometry of the NPC conduit, and have demonstrated that within the range of parameters thought relevant for the NPC, widely varying behaviour can be observed. In this review, we summarise recent efforts to physically model the NPC barrier and function. We illustrate how attempts to understand NPC barrier function have employed many different modelling techniques, each of which have contributed to our understanding of the NPC.

Single polymer gating of channels under a solvent gradient

We study the effect of a gradient of solvent quality on the coil-globule transition for a polymer in a narrow pore. A simple self-attracting, self-avoiding walk model of a polymer in solution shows that the variation in the strength of the interaction across the pore leads the system to go from one regime (good solvent) to the other (poor solvent) across the channel. This may be thought to be analogous to thermophoresis, where the polymer goes from the hot region to the cold region under the temperature gradient. The behavior of short chains is studied using exact enumeration while the behavior of long chains is studied using transfer matrix techniques. The distribution of the monomer density across the layer suggests that a gatelike effect can be created, with potential applications as a sensor.