In this work we describe for the first time the integration of "smart" polymer brushes into single conical nanopores to obtain a new highly functional signal-responsive chemical nanodevice. The responsive brushes were constituted of zwitterionic monomers whose charge is regulated via pH changes in the environmental conditions. The pH-dependent chemical equilibrium of the monomer units provides a fine-tuning of the ionic transport though the nanopore by simply presetting the pH of the electrolyte solution. Our results demonstrate that this strategy enables a higher degree of control over the rectification properties when compared to the nanochannels modified with charged monolayer assemblies. We envision that these results will create completely new avenues to build-up "smart" nanodevices using responsive polymer brushes integrated into single conical nanopores.

Bottom-up surface processing with well-defined polymeric structures becomes increasingly important in many current technologies. Polymer brushes, that is, assemblies of macromolecules tethered at one end to a substrate, provide an exemplary system of materials capable of achieving such a goal. While the focus in the past decades has been mostly on their synthetic aspects and the in-depth study of their interesting properties, from several years now the core area of research has already started to shift towards specific practical applications. Ample functional versatility and relative ease of preparation are special strengths of polymer brushes, lending them a strong interdisciplinary character. To this end, this work is entirely dedicated to bringing together the latest research on applications of polymer brushes in multiple research fields. The aim of this review are twofold: first, to give a critical discussion of the current status of development of application-oriented research on polymer brushes, and second, to inform the reader as to what can be done with polymer brushes in multiple research fields. It is therefore hoped that the juxtaposition of perspectives from different disciplines in one place will stimulate and contribute to the ongoing process of cross-fertilization that is driving this fascinating and emerging area of polymer science. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 2012

Solid state nanochannels modified with supramolecular architectures are a new and interesting class of stimuli-responsive nanofluidic element. Their fundamental understanding requires describing the behavior of soft-materials in confined geometries and its responses to changes in solution conditions. Here, a nanochannel modified with a polyelectrolyte brush is studied with a molecular theory that incorporates the conformational behavior of the polymers, electrostatic, van der Waals, and repulsive interactions coupled with the ability of the polymer segments to regulate their charge through acid-base equilibrium. The theory predicts pH-dependent ionic conductivity in excellent agreement with experimental observations. The polymer chains undergo large conformational changes triggered by variations in the outer solution environment and the conductivity of the device is shown to be controlled by the charge state of the polymer. The degree of polymer charge is largely affected by charge regulation and nanoconfinement effects. The molecular calculations show that the apparent pK(a) inside the pore departs from that in solution when increasing the curvature of the nanochannel.

The creation of switchable and tunable nanodevices displaying transport properties similar to those observed in biological pores poses a major challenge in molecular nanotechnology. Here, we describe the construction of a fully "abiotic" nanodevice whose transport properties can be accurately controlled by manipulating the proton concentration in the surrounding environment. The ionic current switching characteristics displayed by the nanochannels resemble the typical behavior observed in many biological channels that fulfill key pH-dependent transport functions in living organisms, that is, the nanochannel can be switched from an "off" state to an "on" state in response to a pH drop. The construction of such a chemical nanoarchitecture required the integration of stable and ductile macromolecular building blocks constituted of pH-responsive poly(4-vinyl pyridine) brushes into solid state nanopores that could act as gate-keepers managing and constraining the flow of ionic species through the confined environment. In this context, we envision that the integration of environmental stimuli-responsive brushes into solid-state nanochannels would provide a plethora of new chemical alternatives for molecularly design robust signal-responsive "abiotic" devices mimicking the function of proton-gated ion channels commonly encountered in biological membranes.

Molecular design of ionic current rectifiers created on the basis of single conical nanopores is receiving increasing attention by the scientific community. Part of the appeal of this topic relies on the interest in sensors and fluidic nanoactuators based on the transport of ions and molecules through nanopore architectures that can readily be integrated into functional systems. The chemical modification of the pore walls controls not only the diameter of these nanoarchitectures but also their selectivity and transport properties. In order to confer selectivity to solid-state nanopores, it is necessary to develop and explore new methods for functionalizing the pore walls. Hence, the creation of functional nanopores capable of acting as selective ion channels or smart nanofluidic sensors depends critically on our ability to assemble and build up molecular architectures in a predictable manner within confined geometries with dimensions comparable to the size of the building blocks themselves. In this context, layer-by-layer deposition of polyelectrolytes offers a straightforward process for creating nanoscopic supramolecular assemblies displaying a wide variety of functional features. In this work, we describe for the first time the integration of layer-by-layer polyelectrolyte assemblies into single conical nanopores in order to study and explore the functional features arising from the creation of charged supramolecular assemblies within the constrained geometry of the nanofluidic device. To address this challenging topic, we used a combined experimental and theoretical approach to elucidate and quantify the electrostatic changes taking place inside the nanopore during the supramolecular assembly process. The multilayered films were built up through consecutive layer-by-layer adsorption of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on the pore surface. Our results show that the charge transport properties of single conical nanopores functionalized with PAH/PSS assemblies are highly dependent on the number of layers assembled on the pore wall. In contrast to what happens with PAH/PSS films deposited on planar surfaces (quantitative charge reversal), the surface charge of the pore walls decreases dramatically with the number of PAH/PSS layers assembled into the nanopore. This behavior was attributed to the nanoconfinement-induced structural reorganization of the polyelectrolyte layers, leading to the efficient formation of ion pairs and promoting a marked decrease in the net fixed charges on the nanopore walls. We consider that these results are of paramount relevance for the modification of nanopores, nanopipets, and nanoelectrodes using charged supramolecular assemblies, as well as of importance in "soft nanotechnology" provided that structural complexity, induced by nanoconfinement, can define the functional properties of self-assembled polymeric nanostructures.

Polymer brushes have recently emerged as an extremely versatile way to modify surface properties in a robust and controlled way. The introduction of responsive polymers and block copolymers in polymer‐brush systems has also opened up new routes to ‘smart' surfaces with switchable surface properties. Here, the use of polyelectrolyte brushes as a supramolecular platform for the immobilization of a wide range of species, leading to a tunable wettability of substrates, is presented.

We have studied the changes in physical and chemical properties of cationic poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) brushes after collapse driven by ion-pairing interactions in the presence of ClO 4anions. Results derived from the quartz crystal microbalance technique, atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle goniometry indicate that ion-paired collapsed polyelectrolyte brushes suffer a dramatic loss of water accompanied by conformational changes leading to markedly different mechanical properties. This scenario is completely different from polyelectrolyte brushes whose collapse is simply driven by pure Coulombic screening, for example, in the presence of Clanions. In addition, wetting measurements indicated that ion-pairing interactions can be used to switch surface characteristics from hydrophilic to hydrophobic in a reversible manner. The immediate implications of these experimental results are related to the promising use of polyelectrolyte brushes as biolubricants and the design of "smart" surfaces exhibiting ion-sensitive reversible changes in interfacial properties.

Single solid‐state nanopores modified with poly‐N‐isopropylacrylamide (NIPAM) brushes display thermally controlled gating properties. Below the lower critical solubility temperature (LCST) NIPAM brushes are swollen and, consequently, dramatically reduce the effective cross section of the nanopores (see image). Conversely, above the LCST the brushes dehydrate and suffer a transition into a collapsed state, which promotes the widening of the nanopore and enables a substantial flow of ions.