Morphological control of grafted polymer films via attraction to small nanoparticle inclusions

Colloidal adsorption in planar polymeric brushes

The design of nano-functionalised membranes or channels, able to effectively adsorb pollutants in aqueous solutions, is a topic that is gaining a great deal of attention in the materials science community. With this work we explore, through a combination of scaling theories and molecular dynamics simulations, the adsorption of spherical non-deformable colloidal nanoparticles within planar polymeric brushes. Our strategy is twofold: first, we generalise the Alexander-de Gennes theory for planar homopolymeric brushes to the case of diblock copolymer brushes, then we map the adsorbing homopolymeric brushes onto a diblock copolymer system, where the adsorbed colloids and all interacting monomers are considered monomers in bad solvent and we apply the generalised scaling theory to this effective diblock copolymer. This allows the prediction of the average conformation of the grafted substrate, i.e. its average height, as a function of the amount of loaded particles, as well as the introduction of a continuous mapping between a homopolymeric brush, the fraction of loaded particles and the average height of the adsorbing substrate.

Effects of Sequence Composition, Patterning and Hydrodynamics on the Conformation and Dynamics of Intrinsically Disordered Proteins

Intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs) perform diverse functions in cellular organization, transport and signaling. Unlike the well-defined structures of the classical natively folded proteins, IDPs and IDRs dynamically span large conformational and structural ensembles. This dynamic disorder impedes the study of the relationship between the amino acid sequences of the IDPs and their spatial structures and dynamics, with different experimental techniques often offering seemingly contradictory results. Although experimental and theoretical evidence indicates that some IDP properties can be understood based on their average biophysical properties and amino acid composition, other aspects of IDP function are dictated by the specifics of the amino acid sequence. We investigate the effects of several key variables on the dimensions and the dynamics of IDPs using coarse-grained polymer models. We focus on the sequence "patchiness" informed by the sequence and biophysical properties of different classes of IDPs-and in particular FG nucleoporins of the nuclear pore complex (NPC). We show that the sequence composition and patterning are well reflected in the global conformational variables such as the radius of gyration and hydrodynamic radius, while the end-to-end distance and dynamics are highly sequence-specific. We find that in good solvent conditions highly heterogeneous sequences of IDPs can be well mapped onto averaged minimal polymer models for the purpose of prediction of the IDPs dimensions and dynamic relaxation times. The coarse-grained simulations are in a good agreement with the results of atomistic MD. We discuss the implications of these results for the interpretation of the recent experimental measurements, and for the further applications of mesoscopic models of FG nucleoporins and IDPs more broadly.

Morphology of Polymer Brushes in the Presence of Attractive Nanoparticles: Effects of Temperature

We study the role of temperature on the structure of pure polymer brushes and their mixture with attractive nanoparticles in flat and cylindrical geometries. It has previously been established that the addition of such nanoparticles causes the polymer brush to collapse and the intensity of the collapse depends on the attraction strength, the nanoparticle diameter, and the grafting density. In this work, we carry out molecular dynamics simulation under good solvent conditions to show how the collapse transition is affected by the temperature, for both plane grafted and inside-cylinder grafted brushes. We first examine the pure brush morphology and verify that the brush height is insensitive to temperature changes in both planar and cylindrical geometries, as expected for a polymer brush in a good solvent. On the other hand, for both system geometries, the brush structure in the presence of attractive nanoparticles is quite responsive to temperature changes. Generally speaking, for a given nanoparticle concentration, increasing the temperature causes the brush height to increase. A brush which contracts when nanoparticles are added eventually swells beyond its pure brush height as the system temperature is increased. The combination of two easily controlled external parameters, namely, concentration of nanoparticles in solution and temperature, allows for sensitive and reversible adjustment of the polymer brush height, a feature which could be exploited in designing smart polymer devices.

Physics of the Nuclear Pore Complex: Theory, Modeling and Experiment

The hallmark of eukaryotic cells is the nucleus that contains the genome, enclosed by a physical barrier known as the nuclear envelope (NE). On the one hand, this compartmentalization endows the eukaryotic cells with high regulatory complexity and flexibility. On the other hand, it poses a tremendous logistic and energetic problem of transporting millions of molecules per second across the nuclear envelope, to facilitate their biological function in all compartments of the cell. Therefore, eukaryotes have evolved a molecular "nanomachine" known as the Nuclear Pore Complex (NPC). Embedded in the nuclear envelope, NPCs control and regulate all the bi-directional transport between the cell nucleus and the cytoplasm. NPCs combine high molecular specificity of transport with high throughput and speed, and are highly robust with respect to molecular noise and structural perturbations. Remarkably, the functional mechanisms of NPC transport are highly conserved among eukaryotes, from yeast to humans, despite significant differences in the molecular components among various species. The NPC is the largest macromolecular complex in the cell. Yet, despite its significant complexity, it has become clear that its principles of operation can be largely understood based on fundamental physical concepts, as have emerged from a combination of experimental methods of molecular cell biology, biophysics, nanoscience and theoretical and computational modeling. Indeed, many aspects of NPC function can be recapitulated in artificial mimics with a drastically reduced complexity compared to biological pores. We review the current physical understanding of the NPC architecture and function, with the focus on the critical analysis of experimental studies in cells and artificial NPC mimics through the lens of theoretical and computational models. We also discuss the connections between the emerging concepts of NPC operation and other areas of biophysics and bionanotechnology.

Effects of cosolvent partitioning on conformational transitions and tethered chain flexibility in spherical polymer brushes

In this work, based on the framework of preferential adsorption concept and analytical self-consistent field (SCF) theory, a model is proposed to investigate the reentrant transition experimentally observed from the thermoresponsive spherical brush in a series of aqueous alcohol solutions. The interaction between monomers is incorporated into the model. Conformational transitions of the spherical brush are quantitatively correlated to the physical parameters, including the number of adsorbed cosolvents which facilitates the solvent quality, the number of cosolvent bridges which drives the brush collapse, as well as their partition coefficients between the brush and the bulk solution. An analytical formula for the number of Kuhn segments per tethered chain is obtained based on the analytical SCF theory, which elucidates the flexibility of tethered chains in the intricate system of multicomponents involving the conformational transitions. Under the experimental conditions associated with the cosolvent-brush interaction, the variation of the monomer chemical potential with the monomer concentration indicates that the monomer distribution of the spherical brush remains continuous. The analysis based on the SFC theory also reveals that the distribution of adsorbed cosolvents is a positive parabola while the distribution of cosolvent bridges appears to be an exponential decay function, implying that the intervening space between tethered chains, rather than the number of adsorbed cosolvents, plays a crucial role in forming the cosolvent bridge. We demonstrate that the model formulated for the reentrant transition under weaker cosolvent-brush interactions provides guidelines for the one under stronger nanoparticle-brush interactions.

Nanoelectrochemical Study of Molecular Transport through the Nuclear Pore Complex

The nuclear pore complex (NPC) is the proteinaceous nanopore that solely mediates the transport of both small molecules and macromolecules between the nucleus and cytoplasm of a eukaryotic cell to regulate gene expression. In this personal account, we introduce recent progress in our nanoelectrochemical study of molecular transport through the NPC. Our work represents the importance of chemistry in understanding and controlling of NPC-mediated molecular transport to enable the efficient and safe delivery of genetic therapeutics into the nucleus, thereby fundamentally contributing to human health. Specifically, we employ nanoscale scanning electrochemical microscopy to test our hypothesis that the nanopore of the NPC is divided by transport barriers concentrically into peripheral and central routes to efficiently mediate the bimodal traffic of protein transport and RNA export, respectively, through cooperative hydrophobic and electrostatic interactions.

Sorption Characteristics of Polymer Brushes in Equilibrium with Solvent Vapors

While polymer brushes in contact with liquids have been researched intensively, the characteristics of brushes in equilibrium with vapors have been largely unexplored, despite their relevance for many applications, including sensors and smart adhesives. Here, we use molecular dynamics simulations to show that solvent and polymer density distributions for brushes exposed to vapors are qualitatively different from those of brushes exposed to liquids. Polymer density profiles for vapor-solvated brushes decay more sharply than for liquid-solvated brushes. Moreover, adsorption layers of enhanced solvent density are formed at the brush–vapor interface. Interestingly and despite all of these effects, we find that solvent sorption in the brush is described rather well with a simple mean-field Flory–Huggins model that incorporates an entropic penalty for stretching of the brush polymers, provided that parameters such as the polymer–solvent interaction parameter, grafting density, and relative vapor pressure are varied individually.

A unified understanding of the cononsolvency of polymers in binary solvent mixtures

The standard random phase approximation (RPA) model is applied to investigate the cononsolvency of polymers in mixtures of two good solvents. It is shown that in the RPA framework, the two types of cononsolvency behaviors reported in previous theoretical studies can be unified under the same concept of mean-field density correlations. The two types of cononsolvency are distinguished by the solvent composition at which maximum immiscibility is predicted to occur. The maximum immiscibility occurs with the cosolvent being the minor solvent if the driving mechanism is the preferential solvation of polymers. For the cononsolvency driven by the preferential mixing of solvents, the maximum immiscibility is predicted at a symmetric solvent composition. An interplay of the two driving forces gives rise to a reentrant behavior in which the cononsolvency of the two types switches from one to the other, through a "conventional" region where the overall solvent quality varies monotonically with the solvent composition. The RPA model developed in this work provides a unified analytical framework for understanding the conformational and solubility transition of polymers in multi-solvent mixtures. Such findings highlight the complex role played by the solvents in polymer solutions, a problem of fundamental and practical interest in diverse applications of materials science.

The Role of Cohesiveness in the Permeability of the Spatial Assemblies of FG Nucleoporins

Nuclear pore complexes (NPCs) conduct selective, bidirectional transport across the nuclear envelope. The NPC passageway is lined by intrinsically disordered proteins that contain hydrophobic phenylalanine-glycine (FG) motifs, known as FG nucleoporins (FG nups), that play the key role in the NPC transport mechanism. Cohesive interactions among the FG nups, which arise from the combination of hydrophobic, electrostatic, and other forces, have been hypothesized to control the morphology of the assemblies of FG nups in the NPC, as well as their permeability with respect to the transport proteins. However, the role of FG nup cohesiveness is still vigorously debated. Using coarse-grained polymer theory and numerical simulations, we study the effects of cohesiveness on the selective permeability of in vitro FG nup assemblies in different geometries that have served as proxies for the morphological and transport properties of the NPC. We show that in high-density FG nup assemblies, increase in cohesiveness leads to the decrease in their permeability, in accordance with the accepted view. On the other hand, the permeability of low-density assemblies is a nonmonotonic function of the cohesiveness, and a moderate increase in cohesiveness can enhance permeability. The density- and cohesiveness-dependent effects on permeability are explained by considering the free-energy cost associated with penetrating the FG nup assemblies. We discuss the implications of these findings for the organization and function of the NPC.

Controlling the Surface Properties of Binary Polymer Brush-Coated Colloids via Targeted Nanoparticles

This paper explores a novel mechanism for controlling the surface properties of polymer-coated colloids using targeted ("sticky") nanoparticles which attract monomers of certain polymer species. In our study, colloids are coated by two types of tethered polymer chains having different chemical properties. Attraction of nanoparticles to the monomers of one polymer type causes these polymer chains to contract toward the grafting surface, rendering the other type more exposed to the environment. Thus, the effective surface properties of the colloid are dominated by the intended polymer type. We use coarse-grained molecular dynamics (CGMD) simulation to demonstrate that introducing nanoparticles which interact preferentially with certain types of polymers makes it possible to switch between different surface properties of the colloid. This mechanism can in principle be exploited in drug delivery systems and self-assembly applications.

Statistical description of co-nonsolvency suppression at high pressures

We present an application of Flory-type theory of a flexible polymer chain dissolved in a binary mixture of solvents to theoretical description of co-nonsolvency. We show that our theoretical predictions are in good quantitative agreement with the recently published MD simulation results for the conformational behavior of a Lennard-Jones flexible chain in a binary mixture of the Lennard-Jones fluids. We show that our theory is able to describe co-nonsolvency suppression through pressure enhancement to extremely high values recently discovered in experiments and reproduced by full atomistic MD simulations. By analysing the co-solvent concentration in the internal polymer volume at different pressure values, we speculate that this phenomenon is caused by the suppression of the co-solvent preferential solvation of the polymer backbone at the rather high pressure imposed. We show that when the co-solvent-induced coil-globule transition takes place, the entropy and enthalpy contributions to the solvation free energy abruptly decrease, while the solvation free energy remains continuous.

The spatial arrangement of a single nanoparticle in a thin polymer film and its effect on the nanoparticle diffusion

The spatial arrangement of nanoparticles (NPs) within thin polymer films may influence their properties such as the glass transition temperature. Questions regarding what may affect the spatial arrangement of NPs, however, still remain unanswered at a molecular level. In this work, we perform molecular dynamics simulations for a free-standing thin polymer film with a single NP. We find from simulations that depending on the NP size and the inter-particle interaction between the NP and polymers, one may control the spatial arrangement of the NP. When the interaction between the NP and polymers is sufficiently attractive (repulsive), the NP is likely to be placed at the center (at the surface) of the thin film in equilibrium. Interestingly, for a moderate interaction between the NP and polymers, the first-order transition occurs in the spatial arrangement of the NP as one increases the NP size: a small NP prefers the surface of the polymer film whereas a large NP prefers the center. Such a first-order transition is corroborated by calculating the free energy of the NP as a function of the position and can be understood in terms of a sixth-order Landau free energy. More interestingly, the diffusion of the NP also changes drastically due to the first-order transition in the spatial arrangement. The NP diffusion is enhanced drastically (more than expected in bulk polymer melts) as the NP is shifted to the polymer film surface.

Conformational collapse of spherical poly(N-isopropylacrylamide) brushes under the constraint of bound micelles

In this paper, we investigate the micelle (charge)-constrained collapse of a spherical poly(N-isopropylacrylamide) (PNIPAM) brush.

How to operate a nuclear pore complex by Kap-centric control

Nuclear pore complexes (NPCs) mediate molecular transport between the nucleus and cytoplasm in eukaryotic cells. Tethered within each NPC lie numerous intrinsically disordered proteins known as FG nucleoporins (FG Nups) that are central to this process. Over two decades of investigation has converged on a view that a barrier mechanism consisting of FG Nups rejects non-specific macromolecules while promoting the speed and selectivity of karyopherin (Kaps) receptors (and their cargoes). Yet, the number of NPCs in the cell is exceedingly small compared to the number of Kaps, so that in fact there is a high likelihood the pores are always populated by Kaps. Here, we contemplate a view where Kaps actively participate in regulating the selectivity and speed of transport through NPCs. This so-called "Kap-centric" control of the NPC accounts for Kaps as essential barrier reinforcements that play a prerequisite role in facilitating fast transport kinetics. Importantly, Kap-centric control reconciles both mechanistic and kinetic requirements of the NPC, and in so doing potentially resolves incoherent aspects of FG-centric models. On this basis, we surmise that Kaps prime the NPC for nucleocytoplasmic transport by fine-tuning the NPC microenvironment according to the functional needs of the cell.

The Multiple Faces of Disordered Nucleoporins

An evolutionary advantage of intrinsically disordered proteins (IDPs) is their ability to bind a variety of folded proteins-a paradigm that is central to the nucleocytoplasmic transport mechanism, in which nuclear transport receptors mediate the translocation of various cargo through the nuclear pore complex by binding disordered phenylalanine-glycine-rich nucleoporins (FG-Nups). FG-Nups are highly dynamic, which poses a substantial problem when trying to determine precisely their function using common experimental approaches. FG-Nups have been studied under a variety of conditions, ranging from those that constitute single-molecule measurements to physiological concentrations at which they can form supramolecular structures. In this review, I describe the physicochemical properties of FG-Nups and compare them to those of other disordered systems, including well-studied IDPs. From this comparison, it is apparent that FG-Nups not only share some properties with IDPs in general but also possess unique characteristics that might be key to their central role in the nucleocytoplasmic transport machinery.

Simple biophysics underpins collective conformations of the intrinsically disordered proteins of the Nuclear Pore Complex

Nuclear Pore Complexes (NPCs) are key cellular transporter that control nucleocytoplasmic transport in eukaryotic cells, but its transport mechanism is still not understood. The centerpiece of NPC transport is the assembly of intrinsically disordered polypeptides, known as FG nucleoporins, lining its passageway. Their conformations and collective dynamics during transport are difficult to assess in vivo. In vitro investigations provide partially conflicting results, lending support to different models of transport, which invoke various conformational transitions of the FG nucleoporins induced by the cargo-carrying transport proteins. We show that the spatial organization of FG nucleoporin assemblies with the transport proteins can be understood within a first principles biophysical model with a minimal number of key physical variables, such as the average protein interaction strengths and spatial densities. These results address some of the outstanding controversies and suggest how molecularly divergent NPCs in different species can perform essentially the same function.

DOI:http://dx.doi.org/10.7554/eLife.10785.001

Animal, plant and fungal cells contain a structure called the nucleus, inside which the genetic material of the cell is stored. For the cell to work properly, certain proteins and other molecules need to be able to enter and exit the nucleus. This transport is carried out by pore-like molecular “devices” known as Nuclear Pore Complexes, whose architecture and mode of operation are unique among cellular transporters.

Nuclear Pore Complexes are charged with a daunting task of deciding which of the hundreds of molecules it conducts per second should go through and which should not. Small molecules can pass freely through Nuclear Pore Complexes. However, larger molecules can only pass through the pore efficiently if they are bound to specialized transport proteins that interact with the proteins – called FG nucleoporins – that line the pore. A unique feature of the FG nucleoporins is that, unlike typical proteins, they do not have a defined three-dimensional structure. Instead, they form a soft and pliable lining inside the Nuclear Pore Complex passageway.

Exactly how interacting with transport proteins affects the structure and spatial arrangements of the FG nucleoporins in a way that allows them to control transport is not well understood. This is in part because existing experimental techniques are unable to study the structures of the FG nucleoporins in enough detail to track how they change during transport. The complexity and the diversity of the FG nucleoporins also make them difficult to model in detail.

Vovk, Gu et al. have developed a theoretical model that is based on just three basic physical properties of the FG nucleoporins – their flexibility, their ability to interact with each other, and their binding with the transport proteins. Future work can refine the model by incorporating further molecular details about the interactions between FG nucleoporins and transport proteins.

The predictions made by this simple model agree well with experimental results in a wide range of situations – from single molecules to complex spatial assemblies. They also explain why some of the experimental results appear to contradict each other and suggest how several outstanding controversies in the field can be reconciled. Because the model invokes only fundamental physical principles of FG nucleoporin assemblies, it shows that some of their general properties do not depend on the exact conditions. In particular, this might shed light on why Nuclear Pore Complexes in different organisms perform essentially the same function, although the details of their molecular structure may differ. This also suggests how the FG nucleoporins can be manipulated to build artificial devices based on the same principles.

DOI:http://dx.doi.org/10.7554/eLife.10785.002

Polymer-brush lubrication: a review of recent theoretical advances

This review compiles recent theoretical advances to describe compressive and shear forces of polymer-brush bilayers, which consist of two opposing brushes in contact. Such model systems for polymer-brush lubrication are frequently used as a benchmark to gain insight into biological problems, e.g., synovial joint lubrication. Based on scaling theory, I derive conformational and collective properties of polymer-brush bilayers in equilibrium and out-of-equilibrium situations, such as shear forces in the linear and nonlinear response regimes of stationary shear and under non-stationary shear. Furthermore, I discuss the influence of macromolecular inclusions and electrostatic interactions on polymer-brush lubrication. Comparisons to alternative analytical approaches, experiments and numerical results are performed. Special emphasis is given to methods for simulating polymer-brush bilayers using molecular dynamics simulations.

A physical model describing the interaction of nuclear transport receptors with FG nucleoporin domain assemblies

The permeability barrier of nuclear pore complexes (NPCs) controls bulk nucleocytoplasmic exchange. It consists of nucleoporin domains rich in phenylalanine-glycine motifs (FG domains). As a bottom-up nanoscale model for the permeability barrier, we have used planar films produced with three different end-grafted FG domains, and quantitatively analyzed the binding of two different nuclear transport receptors (NTRs), NTF2 and Importin β, together with the concomitant film thickness changes. NTR binding caused only moderate changes in film thickness; the binding isotherms showed negative cooperativity and could all be mapped onto a single master curve. This universal NTR binding behavior - a key element for the transport selectivity of the NPC - was quantitatively reproduced by a physical model that treats FG domains as regular, flexible polymers, and NTRs as spherical colloids with a homogeneous surface, ignoring the detailed arrangement of interaction sites along FG domains and on the NTR surface.

Deciphering the Structure and Function of Nuclear Pores Using Single-Molecule Fluorescence Approaches

Due to its central role in macromolecular trafficking and nucleocytoplasmic information transfer, the nuclear pore complex (NPC) has been studied in great detail using a wide spectrum of methods. Consequently, many aspects of its architecture, general function, and role in the life cycle of a cell are well understood. Over the last decade, fluorescence microscopy methods have enabled the real-time visualization of single molecules interacting with and transiting through the NPC, allowing novel questions to be examined with nanometer precision. While initial single-molecule studies focused primarily on import pathways using permeabilized cells, it has recently proven feasible to investigate the export of mRNAs in living cells. Single-molecule assays can address questions that are difficult or impossible to answer by other means, yet the complexity of nucleocytoplasmic transport requires that interpretation be based on a firm genetic, biochemical, and structural foundation. Moreover, conceptually simple single-molecule experiments remain technically challenging, particularly with regard to signal intensity, signal-to-noise ratio, and the analysis of noise, stochasticity, and precision. We discuss nuclear transport issues recently addressed by single-molecule microscopy, evaluate the limits of existing assays and data, and identify open questions for future studies. We expect that single-molecule fluorescence approaches will continue to be applied to outstanding nucleocytoplasmic transport questions, and that the approaches developed for NPC studies are extendable to additional complex systems and pathways within cells.

Promiscuous binding of Karyopherinβ1 modulates FG nucleoporin barrier function and expedites NTF2 transport kinetics

The transport channel of nuclear pore complexes (NPCs) contains a high density of intrinsically disordered proteins that are rich in phenylalanine-glycine (FG)-repeat motifs (FG Nups). The FG Nups interact promiscuously with various nuclear transport receptors (NTRs), such as karyopherins (Kaps), that mediate the trafficking of nucleocytoplasmic cargoes while also generating a selectively permeable barrier against other macromolecules. Although the binding of NTRs to FG Nups increases molecular crowding in the NPC transport channel, it is unclear how this impacts FG Nup barrier function or the movement of other molecules, such as the Ran importer NTF2. Here, we use surface plasmon resonance to evaluate FG Nup conformation, binding equilibria, and interaction kinetics associated with the multivalent binding of NTF2 and karyopherinβ1 (Kapβ1) to Nsp1p molecular brushes. NTF2 and Kapβ1 show different long- and short-lived binding characteristics that emerge from varying degrees of molecular retention and FG repeat binding avidity within the Nsp1p brush. Physiological concentrations of NTF2 produce a collapse of Nsp1p brushes, whereas Kapβ1 binding generates brush extension. However, the presence of prebound Kapβ1 inhibits Nsp1p brush collapse during NTF2 binding, which is dominated by weak, short-lived interactions that derive from steric hindrance and diminished avidity with Nsp1p. This suggests that binding promiscuity confers kinetic advantages to NTF2 by expediting its facilitated diffusion and reinforces the proposal that Kapβ1 contributes to the integral barrier function of the NPC.

A Polymer-Brush-Based Nanovalve Controlled by Nanoparticle Additives: Design Principles

Polymer-grafted surfaces and channels are increasingly used for the design of responsive materials and sensors due to robust performance and ease of use. Various strategies for the control of the nanoscale morphologies of such materials and devices are being tested. Entropic repulsion between the polymer chains in a grafted brush of sufficient density causes the chains to extend in the direction perpendicular to the grafting surface in comparison to the position of unattached polymers. When nanoparticles having attractive interactions with the polymers are introduced into the solvent, these nanoparticles tend to infiltrate into the brush and reduce its extension. Under certain conditions, a sharp reduction in brush height extension can occur over a narrow range of nanoparticle concentrations in solution. We describe a way of controlling transport through polymer-functionalized nanochannels with nanoparticle additives, relying on the physics of nanoparticles and polymer brushes under confinement, and we suggest a blueprint for the creation of a tunable nanovalve. The nanovalve is modeled as a cylinder with a polymer brush grafted on its inside surface. Brush properties such as the chain length and the grafting density are chosen so that the brush chains extend into the center of the cylinder in the absence of nanoparticles, occluding the flux of analyte molecules through the pore. When nanoparticles that are attracted to the polymers are introduced into solution, they infiltrate into the brush and partially collapse it against the cylindrical grafting surface, opening space in the center of the cylinder through which analyte molecules can flow. The operation of such a nanovalve is analyzed via self-consistent field theory calculations in the strong-stretching approximation. Self-consistent field analysis is supported by Langevin dynamics simulations of the underlying coarse-grained model of the polymer-nanoparticle system.

Effects of surface affinity on the ordering dynamics of self-assembled monolayers of chain molecules: Transition from a parallel to a perpendicular structure

The effects of surface interactions on the ordering dynamics of self-assembled monolayers (SAM) of chain molecules were studied using molecular dynamics simulations. When the strength of surface-chain interactions was equal to or less than that of chain-chain interactions, domains of chain molecules adsorbed perpendicular to the surface ("upright" chains) formed on the surface. Although chain molecules adsorbed parallel to the surface ("lying" chains) were initially observed on the surface, they did not develop into two-dimensionally aligned structures. In contrast, when the strength of surface-chain interactions was at least twice that of chain-chain interactions, the proportion of upright chain molecules was initially small, and the reorientation of lying chains was observed shortly afterwards. In this case, the reorientation from lying to upright configuration developed slowly from the domain boundaries of two-dimensionally aligned structures late in the calculation period. Although the orientation processes of chain molecules on surfaces were strongly influenced by the strength of surface-chain interactions, the total adsorption rate on the surface was not. We also analyzed the maximum area of domains formed by lying chains. The development of two-dimensionally aligned domains required strong surface-chain interactions to prevent the spontaneous formation of nuclei of upright domains.

Ion permeability of the nuclear pore complex and ion-induced macromolecular permeation as studied by scanning electrochemical and fluorescence microscopy

Efficient delivery of therapeutic macromolecules and nanomaterials into the nucleus is imperative for gene therapy and nanomedicine. Nucleocytoplasmic molecular transport, however, is tightly regulated by the nuclear pore complex (NPC) with the hydrophobic transport barriers based on phenylalanine and glycine repeats. Herein, we apply scanning electrochemical microscopy (SECM) to quantitatively study the permeability of the NPCs to small probe ions with a wide range of hydrophobicity as a measure of their hydrophobic interactions with the transport barriers. Amperometric detection of the redox-inactive probe ions is enabled by using the ion-selective SECM tips based on the micropipet- or nanopipet-supported interfaces between two immiscible electrolyte solutions. The remarkably high ion permeability of the NPCs is successfully measured by SECM and theoretically analyzed. This analysis demonstrates that the ion permeability of the NPCs is determined by the dimensions and density of the nanopores without a significant effect of the transport barriers on the transported ions. Importantly, the weak ion-barrier interactions become significant at sufficiently high concentrations of extremely hydrophobic ions, i.e., tetraphenylarsonium and perfluorobutylsulfonate, to permeabilize the NPCs to naturally impermeable macromolecules. Dependence of ion-induced permeabilization of the NPC on the pathway and mode of macromolecular transport is studied by using fluorescence microscopy to obtain deeper insights into the gating mechanism of the NPC as the basis of a new transport model.

Morphology of polymer brushes infiltrated by attractive nanoinclusions of various sizes

Addition of nanoparticles can control the morphologies of grafted polymer layers that are important in a variety of natural and artificial systems. We study the morphologies of grafted polymer layers interacting attractively with nanoparticle inclusions, as a function of particle size and the interaction strength, using self-consistent field theory and Langevin dynamics simulations. We find that the addition of nanoparticles causes distinctive changes in the layer morphology. For sufficiently strong interaction/binding, increasing the concentration of nanoparticles causes a compression of the polymer layer into a compact, low height state, followed by a subsequent rebound and swelling at sufficiently high concentrations. For nanoparticles of small size, the compression of the layer is sharp and occurs over a narrow range of nanoparticle concentrations. The transition region widens as the nanoparticle size increases. The transition is initiated via a dense layer of tightly bound monomers and nanoparticles near the grafting surface, with a low density region above it. For nanoparticles much larger than the characteristic graft spacing in the brush, the behavior is reversed: the nanoparticles penetrate only the dilute region near the top of the polymer layer without causing the layer to collapse.