Mikhail Motornov

Chemical Gating with Nanostructured Responsive Polymer Brushes: Mixed Brush versus Homopolymer Brush

In this report, we describe a novel approach to create an electrochemical gating system using mixed polymer brushes grafted to an electrode surface, and we explore the switchable properties of these mixed polymer brushes. The morphological transitions in the mixed polymer brushes associated with the electrode surface result in the opening, closing, or precise tuning of their permeability for ion transport through the channels formed in the nanostructured thin film in response to an external stimulus (pH change). The gating mechanism was studied by atomic force microscopy, ellipsometry, contact angle measurements, force-distance measurements, and electrochemical impedance spectroscopy. In comparison to a homopolymer brush system, the mixed brush demonstrates much broader variation of ion transport through the thin film. We suggest that this approach could find important applications in electrochemical sensors and devices with tunable/switchable access to the electrode surface.

Molecular-engineered stimuli-responsive thin polymer film: a platform for the development of integrated multifunctional intelligent materials

The area of stimuli-responsive thin polymer films (brushes, layer-by-layer multilayered structures, networks, and hybrid systems with inorganic particles) is discussed in this article with the major focus on the properties and potential applications for sensors, smart coatings, miniaturized devices, and hierarchically assembled multifunctional systems. We suggest directions for further expansion of this area using all aspects of the mechanism of response (transduction, dynamics, selectivity, sensitivity, and amplifications) and combinations of different thin film structural designs.

Field-Directed Self-Assembly with Locking Nanoparticles

A reversible locking mechanism is established for the generation of anisotropic nanostructures by a magnetic field pulse in liquid matrices by balancing the thermal energy, short-range attractive and long-range repulsive forces, and dipole-dipole interactions using a specially tailored polymer shell of nanoparticles. The locking mechanism is used to precisely regulate the dimensions of self-assembled magnetic nanoparticle chains and to generate and disintegrate three-dimensional (3D) nanostructured materials in solvents and polymers.

Adapting low-adhesive thin films from mixed polymer brushes.

The concept of the responsive/adaptive mixed polymer brushes was applied to the development of the thin film coatings possessing low adhesive properties that were evaluated with AFM probes in different media. Mixed brushes composed of polydimethylsiloxane (PDMS) and polyethyleneoxide (PEO) revealed a selective layered segregation in air and water. Immersion of the sample into an aqueous environment drove PEO chains to the brush-water interface while upon drying the surface undergoing reconstruction and was occupied with PDMS. Low interfacial energies of PDMS in air and PEO in water provided low-adhesive properties of the PDMS-PEO brushes to the probes in both media due to the spontaneous and rapid reconstruction of the mixed brush.

Single Nanoparticle Plasmonic Devices by the “Grafting to” Method

Hierarchically organized single-nanoparticle structures synthesized in this work consisted of a 200 nm silica core and a pH-responsive poly(2-vinylpyridine) shell decorated with 15 nm gold nanoparticles. pH changes in the range of 3-6 back and forth results in a swelling-shrinking polymer brush shell and, thus, in the tuning distance between noble nanoparticles. A change in the interparticle distance is accompanied by a very pronounced shift in the maximum wavelength of the surface plasmon absorption peak. The dispersion of the resulting composite nanoparticles reversibly changed color from red to purple-blue as the pH changed from 2.5 to 6. Such hierarchically assembled nanostructures can be used as free-standing single-particle sensors in various miniaturized analytical systems.

Electrochemical Nanotransistor from Mixed‐Polymer Brushes

Functional interfaces with switchable properties controlled by external physical signals (light, electrical potential, magnetic field) or chemical inputs (presence or absence of chemicals, variation of their concentrations) have received considerable attention in the last two decades. Their applications for controlled surface wettability, thin-films permeability, catalysis, and particularly for assembling modified electrodes, used in switchable sensors, fuel cells, and memory units, resulted in novel materials, devices, and systems. Recent integration of signal-responsive materials, functionalized switchable interfaces and biocomputing systems processing biochemical information resulted in the novel concept of biochemically controlled ‘‘smart’’ materials with built-in Boolean logic. Stimuli-responsive thin polymer films and particularly polyelectrolyte brushes bound at interfaces allowed efficient transduction of biochemically processed inputs into electronic output signals using the polymer thin-film transition between shrunken and swollen states changing the electrochemical activity. In most of the studied systems, including switchable electrode surfaces, membranes and nanostructured assemblies, the changes of their interfacial properties were controlled by altering the bulk solution pH value introduced into the systems or generated by in situ biochemical processes. Very few of these systems operated upon local pH changes generated at the interfaces without alteration of the solution properties. Various applications of the switchable interfaces for sizeand charge-selective separation, microfluidic devices, and in the future implantable biomedical devices require their functioning without changing their physiological environment, thus using only local signals resulting in the interfacial changes. Many applications are in fact limited by the nature of the external signal that can interfere with other functions of the device. Electrical potential, magnetic field, or nearinfrared and visible light are the most-demanded external signals to tune and switch the properties of stimuli-responsive materials. Only a few examples of changes in stimuli-responsive materials upon application of electrical potential are reported. Polyelectrolyte brushes with electroactive counterions were recently used as an effective platform for surfaces with electrochemically switchable wetting properties. The electroactuation of microcantilevers coated on one side with cationic polyelectrolyte brushes represent an exciting example of a miniaturized electromechanical device based on stimuliresponsive polymer brushes, where the response is triggered by an electrical potential applied to the functionalized electrode. This Communication demonstrates the possibility to reversibly switch the electrochemical activity of a polymerbrush-functionalized electrode between OFF/ON states by applying electrical signals which alternate only local, interfacial pH values. The fabricated device mimics the performance of switching electronic devices such as transistors. It operates in aqueous solutions and could be used in buffered biological environments. This device is only 25 nm thick in Z-direction and could be as small as the size of the grafted polymer coil (ca. 10 nm) in the XY-plain. The indium tin oxide (ITO) single-side coated conducting glass-electrode surface was modified with a mixed-polymer brush composed of two different polyelectrolytes poly(2-vinyl pyridine) (P2VP, molecular weight 56000 g mol ) and polyacrylic acid (PAA, molecular weight 100500 gmol ) covalently bound to the primary silane layer, Scheme 1. The thickness, composition, and structure of the P2VP/PAA mixed-polymer brush were studied by atomic force microscopy (AFM), ellipsometry, and contact angle measurements, revealing the P2VP/PAA ratio of ca. 1:3 (w/w) and the polymer film thickness of 24 2 nm in dry condition. The polymers can be protonated or deprotonated depending on the pH value of the electrolyte solution resulting in thin films with different charges, thus providing access to the electrode for ionic species charged opposite to the polymer and insulating the surface against the species carrying the same electrical charge. At the isoelectric point, when the charges on one of the polymers are compensated by the charges on another and the polymers form a polyelectrolyte complex, the neutral polymer film creates a hydrophobic barrier for the penetration of all ionic species to the electrode conducting support. The pH-switchable electrochemical activity of the P2VP/PAA-mixed-brush-modified electrode for selective reactions of anionic and cationic redox species ([Fe(CN)6] 4 and [Ru(NH3)6] 3þ, respectively) at different pH values was studied in details and reported elsewhere. Applying cyclic voltammetry to follow the transition of the

Stimuli-responsive hydrogel hollow capsules by material efficient and robust cross-linking-precipitation synthesis revisited.

Monodisperse stimuli-responsive hydrogel capsules were synthesized in the 100-nm-diameter to 10-μm-diameter range from poly(4-vinylpyridine) (P4VP) and poly(ethyleneimine) (PEI) through a simple, efficient two-step cross-linking-precipitation template method under conditions of a good solvent. In this method, the core-shell particles were obtained by the deposition (heterocoagulation mechanism) of the cross-linked P4VP, PEI, or their mixtures on the surfaces of the template colloidal silica particles in nitromethane (for PEI) or a nitromethane-acetone mixture (for P4VP and P4VP-PEI mixtures) in the presence of cross-linker 1,4-diiodobutane. The cross-linked polymeric shell swollen in a good solvent stabilized the core-shell colloids. This mechanism provided the conditions for the synthesis of core-shell colloids in a submicrometer range of dimensions with an easily adjusted shell thickness (wall of the capsules) ranging from a few to hundreds of nanometers. The chemical composition of the shell was tuned by varying the ratio of co-cross-linked shell-forming polymers (P4VP and PEI). In the second step, the hollow capsules were obtained by etching the silica core in HF solutions. In this step, the colloidal stability of the hollow capsules was provided by ionized P4VP and PEI cross-linked shells. The hollow capsules demonstrate that the pH- and ionic-strength-triggered swelling and shrinking result in size-selective uptake and release properties. Cross-linked via quaternized functional groups, P4VP capsules undergo swelling and shrinking transitions at a physiologically relevant pH of around 6. The 200-nm-diameter hollow capsule with 25-nm-thick walls demonstrated a factor of 2 greater capacity to accommodate cargo molecules than the core-shell particles of the same dimensions because of an internal compartment and a combination of radial and a circumferential swelling modes in the capsules.