A Study of the Supramolecular Approach in Controlling Diblock Copolymer Nanopatterning and Nanoporosity on Surfaces

Protein-Polymer Interaction Characteristics Unique to Nanoscale Interfaces: A Perspective on Recent Insights

Protein interactions at polymer interfaces represent a complex but ubiquitous phenomenon that demands an entirely different focus of investigation than what has been attempted before. With the advancement of nanoscience and nanotechnology, the nature of polymer materials interfacing proteins has evolved to exhibit greater chemical intricacy and smaller physical dimensions. Existing knowledge built from studying the interaction of macroscopic, chemically alike surfaces with an ensemble of protein molecules cannot be simply carried over to nanoscale protein-polymer interactions. In this Perspective, novel protein interaction phenomena driven by the presence of nanoscale polymer interfaces are discussed. Being able to discern discrete protein interaction events via simple visualization was crucial to attaining the much needed, direct experimental evidence of protein-polymer interactions at the single biomolecule level. Spatial and temporal tracking of particular proteins at specific polymer interfaces was made possible by resolving individual proteins simultaneously with those polymer nanodomains responsible for the protein interactions. Therefore, such single biomolecule level approaches taken to examine protein-polymer interaction mark a big departure from the mainstream approaches of collecting indirectly observed, ensemble-averaged protein signals on chemically simple substrates. Spearheading research efforts so far has led to inspiring initial discoveries of protein interaction mechanisms and kinetics that are entirely unique to nanoscale polymer systems. They include protein self-assembly/packing characteristics, protein-polymer interaction mechanisms/kinetics, and various protein functionalities on polymer nanoconstructs. The promising beginning and future of nanoscale protein-polymer research endeavors are presented in this article.

Monolayer Arrays of Nanoparticles on Block Copolymer Brush Films

Two-dimensional arrays of nanoparticles (NPs) have widespread applications in optical coatings, plasmonic sensors, and nanocomposites. Current bottom-up approaches that use homogeneous NP templates, such as silane self-assembled monolayers or homopolymers, are typically plagued by NP aggregation, whereas patterned block copolymer (BCP) films require specific compositions for specific NP distributions. Here, we show, using polystyrene- b-poly(4-vinylpyridine) (PS- b-P4VP) and gold NPs (AuNPs) of various sizes, that a nanothin PS- b-P4VP brushlike coating (comprised of a P4VP wetting layer and a PS overlayer), which is adsorbed onto flat substrates during their immersion in very dilute PS- b-P4VP tetrahydrofuran solutions, provides an excellent template for obtaining dense and well-dispersed AuNPs with little aggregation. These non-close-packed arrays have similar characteristics regardless of immersion time in solution (about 10-120 s studied), solution concentration below a critical value (0.1 and 0.05 mg/mL studied), and AuNP diameter (10-90 nm studied). Very dilute BCP solutions are necessary to avoid deposition, during substrate withdrawal, of additional material onto the adsorbed BCP layer, which typically leads to patterned surfaces. The PS brush coverage depends on immersion time (adsorption kinetics), but full coverage does not inhibit AuNP adsorption, which is attributed to PS molecular rearrangement during exposure to the aqueous AuNP colloidal solution. The simplicity, versatility and robustness of the method will enable applications in materials science requiring dense, unaggregated NP arrays.

Block Copolymer Brush Layer-Templated Gold Nanoparticles on Nanofibers for Surface-Enhanced Raman Scattering Optophysiology

A nanothin block copolymer (BCP) brush-layer film adsorbed on glass nanofibers is shown to address the long-standing challenge of forming a template for the deposition of dense and well-dispersed nanoparticles on highly curved surfaces, allowing the development of an improved nanosensor for neurotransmitters. We employed a polystyrene- block-poly(4-vinylpyridine) BCP and plasmonic gold nanoparticles (AuNPs) of 52 nm in diameter for the fabrication of the nanosensor on pulled fibers with diameters down to 200 nm. The method is simple, using only solution processes and a plasma cleaning step. The templating of the AuNPs on the nanofiber surprisingly gave rise to more than 1 order of magnitude improvement in the surface-enhanced Raman scattering (SERS) performance for 4-mercaptobenzoic acid compared to the same AuNPs aggregated on identical fibers without the use of a template. We hypothesize that a wavelength-scale lens formed by the nanofiber contributes to enhancing the SERS performance to the extent that it can melt the glass nanofiber under moderate laser power. We then show the capability of this nanosensor to detect the corelease of the neurotransmitters dopamine and glutamate from living mouse brain dopaminergic neurons with a sensitivity 1 order of magnitude greater than with aggregated AuNPs. The simplicity of fabrication and the far superior performance of the BCP-templated nanofiber demonstrates the potential of this method to efficiently pattern nanoparticles on highly curved surfaces and its application as molecular nanosensors for cell physiology.

Coordinated Responsive Arrays of Surface-Linked Polymer Islands-CORALs

The concept of co-ordinated responsive arrays of surface-linked islands (polymer CORALs) is introduced. This study targets a responsive system capable of revealing or covering the substrate surface in response to environmental changes in a reversible way. A convenient method of fabrication of polymer CORALs is proposed. It is based on microphase separation that occurs in thin films of supramolecular assemblies of block copolymers with reactive blocks. Such blocks form nanometer-size domains that may serve as anchors for surface-linked polymer islands. Two characteristics of the islands are critically important for the switching function: high grafting density within the islands and small lateral separation that allows interactions between polymer chains grafted to the neighboring islands. This combination permits complete coverage of the substrate surface upon exposure to a good solvent (relaxed state). In a weak solvent, the chains collapse within the islands, thus revealing the substrate (compact state). The morphology of the CORALs in both states and some details of the switching process were studied with atomic force microscopy, grazing incidence small-angle scattering, and coarse-grained molecular dynamic simulations.

Reversible pH Stimulus-Response Material Based on Amphiphilic Block Polymer Self-Assembly and Its Electrochemical Application

Stimulus-responsive microporous solid thin films were successfully fabricated by simple molecular self-assembly via an amphiphilic block polymer, polystryene–b–polyacrylic acid (PS–b–PAA). The solid thin films exhibit different surface morphologies in response to external stimuli, such as environments with different pH values in aqueous solutions. The experiments have successfully applied atomic force microscope (AFM) technology to observe in-situ surface morphological changes. There is a reversible evolution of the microstructures in buffer solutions over a pH range of 2.4–9.2. These observations have been explained by positing that there is no conventional PAA swelling but that the PAA chains in the micropores stretch and contract with changes in the pH of the solution environment. The hydrophobicity of the solid thin film surface was transformed into super-hydrophilicity, as captured by optical contact angle measurements. The stimulus-responsive dynamics of pore sizes was described by a two-stage mechanism. A promising electrochemical application of this film is suggested via combination with an electrochemical impedance technique. This study is aimed at strategies for the functionalization of stimulus-responsive microporous solid thin films with reversible tunable surface morphologies, and exploring new smart materials with switch-on/switch-off behavior.

Fundamentals of nanoscale polymer-protein interactions and potential contributions to solid-state nanobioarrays

Protein adsorption onto polymer surfaces is a very complex, ubiquitous, and integrated process, impacting essential areas of food processing and packaging, health devices, diagnostic tools, and medical products. The nature of protein-surface interactions is becoming much more complicated with continuous efforts toward miniaturization, especially for the development of highly compact protein detection and diagnostic devices. A large body of literature reports on protein adsorption from the perspective of ensemble-averaged behavior on macroscopic, chemically homogeneous, polymeric surfaces. However, protein-surface interactions governing the nanoscale size regime may not be effectively inferred from their macroscopic and microscopic characteristics. Recently, research efforts have been made to produce periodically arranged, nanoscopic protein patterns on diblock copolymer surfaces solely through self-assembly. Intriguing protein adsorption phenomena are directly probed on the individual biomolecule level for a fundamental understanding of protein adsorption on nanoscale surfaces exhibiting varying degrees of chemical heterogeneity. Insight gained from protein assembly on diblock copolymers can be effectively used to control the surface density, conformation, orientation, and biofunctionality of prebound proteins in highly miniaturized applications, now approaching the nanoscale. This feature article will highlight recent experimental and theoretical advances made on these fronts while focusing on single-biomolecule-level investigations of protein adsorption behavior combined with surface chemical heterogeneity on the length scale commensurate with a single protein. This article will also address advantages and challenges of the self-assembly-driven patterning technology used to produce protein nanoarrays and its implications for ultrahigh density, functional, and quantifiable protein detection in a highly miniaturized format.

Nanoporous morphology control of polyethylene membranes by block copolymer blends

A desirable combination of size-selective molecular permeation and robustness development for nanoporous membranes could be achieved via pore geometry control by a blending technique.

Hexagonally ordered nanoparticles templated using a block copolymer film through Coulombic interactions

We present a novel and simple method for forming hexagonal gold nanoparticle arrays that uses Coulombic interactions between negatively charged gold nanoparticles on positively charged vertically oriented poly(4-vinylpyridine) cylinders formed in a spin cast polystyrene-b-poly(4-vinylpyridine) block copolymer film. Exposure of the block copolymer film to dibromobutane vapor quaternizes and crosslinks the poly(4-vinylpyridine) domains which allows for the templated deposition of gold nanoparticles into a self-assembled hexagonal array through electrostatic interactions. These systems can form the basis for sensors or next generation nanoparticle based electronics.

Morphology, Thickness, and Composition Evolution in Supramolecular Block Copolymer Films over a Wide Range of Dip-Coating Rates

Dip-coating, an important industrial technique, has been underexploited for preparing block copolymer (BC) thin films, such that the knowledge regarding their general characteristics is limited. Here, we present an overview of the crucial factors that determine how BC film morphology evolves as a function of dip-coating rate (withdrawal speed) over a wide range, illustrated using THF solutions of a polystyrene-b-poly(4-vinyl pyridine) (PS-P4VP) diblock copolymer mixed with two small molecules, naphthol and naphthoic acid, which are hydrogen-bonders with P4VP. Key factors in determining the film morphology are the systematic variation in film thickness and, for supramolecular BCs, in film composition with dip-coating rate. The former shows a general V-shaped dependence, related to the so-called capillarity and draining regimes identified previously for dip-coated sol-gel films. The relative small molecule content in the films studied is shown to increase in the capillarity regime from low to that of the dip-coating solution and thereafter to remain constant. Together, these changes, in addition to solvent and other effects, determine the film morphology and its evolution with dip-coating rate.

An emerging pore-making strategy: confined swelling-induced pore generation in block copolymer materials

Block copolymers (BCPs) composed of two or more thermodynamically incompatible homopolymers self-assemble into periodic microdomains. Exposing self-assembled BCPs with solvents selective to one block causes a swelling of the domains composed of this block. Strong swelling in the confinement imposed by the matrix of the other glassy block leads to well-defined porous structures via morphology reconstruction. This confined swelling-induced pore-making process has emerged recently as a new strategy to produce porous materials due to synergic advantages that include extreme simplicity, high pore regularity, involvement of no chemical reactions, no weight loss, reversibility of the pore forming process, etc. The mechanism, kinetics, morphology, and governing parameters of the confined swelling-induced pore-making process in BCP thin films are discussed, and the main applications of nanoporous thin films in the fields of template synthesis, surface patterning, and guidance for the areal arrangements of nanomaterials and biomolecules are summarized. Recent, promising results of extending this mechanism to produce BCP nanofibers or nanotubes and bulk materials with well-defined porosity, which makes this strategy also attractive to researchers outside the nanocommunity, are also presented.

Self-assembled structures of hydrogen-bonded poly(vinyl chloride-g-4-vinyl pyridine) graft copolymers

An amphiphilic graft copolymer of poly(vinyl chloride-graft-4-vinyl pyridine), i.e., PVC-g-P4VP was synthesized via atom transfer radical polymerization (ATRP) and modified by introducing 1,5-dihydroxynaphthalene (DHN) as a hydrogen bonding donor to form hydrogen-bonded macromolecules. The PVC-g-P4VP graft copolymer prepared from a selective solvent for PVC, i.e., tetrahydrofuran (THF) exhibited a well-organized micellar morphology consisting of a P4VP core and a PVC corona. However, the graft copolymer prepared from a good solvent for both domains, i.e., N, N-dimethylformamide (DMF) showed a less-organized, random microphase-separated morphology. Upon the introduction of DHN, a more spherical morphology and a more-organized 'bicontinuous-like' morphology was observed in THF and DMF, respectively. This results from strong segregation between the two domains due to the enhanced interfacial energy by DHN. When DHN was removed by methanol, nanoporous films with versatile pore sizes and shapes were generated.

Size-selective diffusion in nanoporous but flexible membranes for glucose sensors

A series of nanoporous membranes prepared from polyethylene-block-polystyrene were applied for size-selective diffusion of glucose and albumin molecules. Millimeter-sized test cells for characterization of such molecular diffusions were designed assuming an implantable glucose sensor. The prepared nanoporous membrane exhibits excellent flexibility and toughness compared to conventional nanoporous membranes of brittle alumina. Pore size of the membranes could be controlled from 5 to 30 nm by varying preparation conditions. All of these nanoporous membranes prepared in this study let glucose pass through, indicating a continuous pore connection through the entire thickness of the membrane in a few tens of micrometers. In contrast, membranes prepared under optimum conditions could perfectly block albumin permeation. This means that these vital molecules having different sizes can be selectively diffused through the nanoporous membranes. Such a successful combination of size selectivity of molecular diffusion in nanoscale and superior mechanical properties in macroscale is also beneficial for other devices requesting down-sized manufacture.

Surface chemical functionalization of cylindrical nanopores derived from a polystyrene-poly(methylmethacrylate) diblock copolymer via amidation

This paper demonstrated covalent functionalization of surface -COOH groups on cylindrical nanopores derived from a polystyrene-poly(methylmethacrylate) diblock copolymer (PS- b-PMMA) via amidation mediated by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). The surface functionalization led to conversion of the surface charge of the nanopores and also to the shrinkage of effective pore radius, as verified using cyclic voltammetry for PS- b-PMMA-derived nanoporous films immobilized on gold substrates. For native PS- b-PMMA-derived nanoporous films, the redox current of anionic Fe(CN) 6 (3-) decreased with increasing solution pH due to the deprotonation of the surface -COOH groups, whereas those of cationic Ru(NH 3) 6 3+ and uncharged 1,1'-ferrocenedimethanol (Fc(CH 2OH) 2) were similar regardless of pH. In contrast, upon EDC-mediated amidation of the nanopore surface with ethylenediamine, the redox current of Ru(NH 3) 6 3+ decreased with decreasing pH and those of Fe(CN) 6 (3-) and Fc(CH 2OH) 2 were independent of pH. The decrease in redox current of Ru(NH 3) 6 3+ at acidic pH was consistent with the presence of -NH 2 groups on the nanopore surface as a result of the covalent immobilization of ethylenediamine. Furthermore, the redox current of Fc(CH 2OH) 2 decreased upon amidation of the nanopores with tetraethyleneglycol monoamine ((PEO) 4NH 2), reflecting the shrinkage of the effective pore radius. The control of the surface charge and effective radius of the nanopores via EDC-mediated amidation will provide a simple means for controlling the selectivity of molecular mass transport through PS- b-PMMA-derived nanopores.

Electrochemical characterization of nanoporous films fabricated from a polystyrene-poly(methylmethacrylate) diblock copolymer: monitoring the removal of the PMMA domains and exploring the functional groups on the nanopore surface

Cyclic voltammetry (CV) was used to assess fabrication of a nanoporous film from a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA) and also to explore the surface functional groups on the resulting nanopores. Polymer films containing vertically aligned cylindrical nanoscale pores (ca. 10 nm in pore radius, 20-30 nm in film thickness) were prepared on gold substrates by removing the cylindrical PMMA domains from PS-b-PMMA films via UV irradiation and subsequent acetic acid treatment. CV measurements provided a simple means for monitoring the extent of the removal of the PMMA domains and for assessing the formation of a recessed nanodisk-array electrode (RNE) structure. The resulting RNEs exhibited a decrease in redox current of anionic Fe(CN)6(3-) with increasing solution pH from 4.6 to 6.3 and a negligible change in CV of uncharged 1,1'-ferrocenedimethanol. The decrease in redox current of Fe(CN)6(3-) at the higher pH was due to electrostatic repulsion between Fe(CN)6(3-) and the electrical double layer formed in the neighborhood of the negatively charged nanopore surface. Indeed, the reduction of effective pore radius measured from CVs of Fe(CN)6(3-) was correlated to the change in the thickness of the electrical double layer. The pH range that showed the decrease in redox current of Fe(CN)6(3-) was consistent with the presence of -COOH groups on the nanopore surface, although they were not detected using Fourier transform infrared spectra of etched PS-b-PMMA films.