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.

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.

Fluorescent nanoparticles stabilized by poly(ethylene glycol) containing shell for pH-triggered tunable aggregation in aqueous environment.

Fluorescent silica nanoparticles decorated with a responsive shell, a mixed polymer brush, were synthesized. Specifically, a poly(2-vinylpyridine), P2VP, and poly(ethylene glycol), PEG, binary polymer brush was synthesized on silica nanoparticles via the "grafting to" technique. The selection of the components (PEG and P2VP) for the responsive brush shell was motivated by potential biomedical applications. Poly(glycidyl methacrylate), PGMA, labeled with Rhodamine B, RhB, was used to form a reactive and fluorescent shell on the nanoparticle surface. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change in pH. At lower pH, aqueous dispersions of the particles are stable, since PEG and P2VP are water-soluble, extended and contribute to the steric and electrostatic mechanisms of colloidal stability. An increase of pH to 6 causes a slow aggregation as a consequence of the hydrophobic attraction between the collapsed and almost nonprotonated P2VP macromolecules. The aggregation was well controlled and occurred within 90-120 min of the pH change. The aggregation was fully reversible after the decrease in pH. The pH variation did not quench the fluorescence of the colloidal suspensions. The pH-tunable aggregation of the fluorescent nanoparticles could find diverse applications for labeling and contrasting of cells and tissues when the size of the label and the intensity of the optical signals can be tuned by and related to the pH of the host environment.

Reversible "closing" of an electrode interface functionalized with a polymer brush by an electrochemical signal.

The poly(4-vinyl pyridine) (P4VP)-brush-modified indium tin oxide (ITO) electrode was used to switch reversibly the interfacial activity by the electrochemical signal. The application of an external potential (-0.85 V vs Ag|AgCl|KCl, 3M) that electrochemically reduced O(2) resulted in the concomitant consumption of hydrogen ions at the electrode interface, thus yielding a higher pH value and triggering the restructuring of the P4VP brush on the electrode surface. The initial swollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)(6)](4-) redox species, but the electrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The produced hydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting its redox process at the electrode surface. The interface's return to the electrochemically active state was achieved by disconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of the electrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggered inhibition ("closing") of the electrode interface. The application of this approach to different interfacial systems will allow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlled activity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating as electroswitchable drug releasers).

Switchable selectivity for gating ion transport with mixed polyelectrolyte brushes: approaching 'smart' drug delivery systems.

A pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) and poly(2-vinylpyridine) (P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used for selective gating transport of anions and cations across the thin film. An ITO glass electrode was modified with the polymer brush and used to study the switchable permeability of the mixed brush triggered by changes in pH of the aqueous environment in the presence of two soluble redox probes: [Fe(CN)(6)](4-) and [Ru(NH(3))(6)](3+). The responsive behavior of the brush was also investigated using the in situ ellipsometric measurements of the brush swelling, examination of the brush morphology with atomic force microscopy (AFM), and contact angle measurements of the brush samples extracted from aqueous solutions at different pH values. The mixed brush demonstrated a bipolar permselective behavior. At pH<3 the positively charged P2VP chains enabled the electrochemical process for the negatively charged redox probe, [Fe(CN)(6)](4-), while the redox process for the positively charged redox probe was effectively inhibited. On the contrary, at pH>6 a reversible redox process for the positively charged redox probe, [Ru(NH(3))(6)](3+), was observed, while the redox process for the negatively charged redox species, [Fe(CN)(6)](4-), was fully inhibited. Stepwise changing the pH value and recording cyclic voltammograms for the intermediate states of the polymer brush allowed electrochemical observation of the brush transition from the positively charged state, permeable for the negatively charged species, to the negatively charged state, permeable for the positively charged species. The data of ellipsometric, AFM and contact angle measurements are in accord with the electrochemical study. The discovered properties of the brush could be used for the development of 'smart' sensors and drug delivery systems, for example, a smart drug delivery capsule which could release negatively charged molecules of drugs in acidic conditions, while positively charged molecules of drugs will be released in neutral conditions.

Responsive colloidal systems: reversible aggregation and fabrication of superhydrophobic surfaces.

We report on a method of fabricating stimuli-responsive core-shell nanoparticles using block copolymers covalently bound to a silica nanoparticle surface. We used the "grafting to" approach to graft amphiphilic block copolymer brushes of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) and poly(styrene-b-4-vinylpyridine) onto silica nanoparticles with two different diameters: colloidal silica 200 nm in diameter and fumed silica 15 nm in diameter. We used the pH-responsive properties of the grafted brush to regulate the interactions between the particles, and between the particles and their environment. We show that this behavior can be applied for a reversible formation of particle aggregates, and can be used to tune and stabilize the secondary aggregates of particles of the appropriate size and morphology in an aqueous environment. The suspensions of the particles form a textured hydrophilic coating on various substrates upon casting and the evaporation of water. Heating above the polymer's glass transition temperature or treatment in acidic water result in back and forth switching between superhydrophobic and hydrophilic surfaces, respectively.

Stimuli-responsive hydrogel membranes coupled with biocatalytic processes.

A nanostructured signal-responsive thin hydrogel membrane was coupled with enzyme-based systems to yield "smart" multisignal-responsive hybrid systems with built-in "logic". The enzyme systems transduce biochemical input signals into structural changes of the membrane, thus resulting in the amplification of the biochemical signals and their transformation into the gated transport of molecules through the membrane. Coupling of the biocatalytic systems with a stimuli-responsive membrane is a promising approach for the development of materials that can regulate transport and release of chemicals/drugs by receiving and processing the biochemical information via biochemical reactions.

Chemical and structural changes in a pH-responsive mixed polyelectrolyte brush studied by infrared ellipsometry.

This work provides direct chemical and structural insight into pH-dependent changes of an ultrathin (d=12 nm) mixed polyelectrolyte brush. In-situ infrared spectroscopic ellipsometry was used for the first time to study the gradual pH-responsive behavior of the brush, constituted of weak anionic and cationic polyelectrolytes, poly(acrylic acid) (PAA) and poly(2-vinylpyridine) (P2VP), respectively. The pH-dependent infrared fingerprints in the mid-infrared spectral range were analyzed as a function of chemical and structural changes in the mixed brush caused by pH changes. Thereby, the IR spectra were directly correlated to different chemical states of the brush, giving previously not accessible new information on the ionization of the thin film. In contrast to other techniques (e.g., classical attenuated total reflection IR spectroscopy) we used almost plane Si-substrates for the IR ellipsometric approach with application of a single reflection mode. The optical path through Si is of minimal length, which makes a large spectral range accessible. For the most pronounced bands of the carboxyl group at 1718 cm(-1) and the carboxylate ion at 1565 cm(-1), the band amplitudes were correlated with the degree of ionization of the carboxylic groups. Interpretation of the pH-dependent changes in the spectral signature reveals gradual changes of the chemical structures of the mixed brush between three distinct switchable states: strongly ionized PAA at pH 10, strongly ionized P2VP at pH 2, and mainly nonionized functional groups in a "dry" PAA-P2VP polyelectrolyte complex in the range from pH=4 to pH=7. At intermediate pH, the IR spectra confirm the previously made hypothesis of the formation of a polyelectrolyte complex between P2VP and PAA in the mixed brush. From IR spectra it is also concluded that the polyelectrolyte complex is formed as a result of a small fraction of ionized functional groups.

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.

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.

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.

"Chemical transformers" from nanoparticle ensembles operated with logic.

The pH-responsive nanoparticles were coupled with information-processing enzyme-based systems to yield "smart" signal-responsive hybrid systems with built-in Boolean logic. The enzyme systems performed AND/OR logic operations, transducing biochemical input signals into reversible structural changes (signal-directed self-assembly) of the nanoparticle assemblies, thus resulting in the processing and amplification of the biochemical signals. The hybrid system mimics biological systems in effective processing of complex biochemical information, resulting in reversible changes of the self-assembled structures of the nanoparticles. The bioinspired approach to the nanostructured morphing materials could be used in future self-assembled molecular robotic systems.

Nonwettable thin films from hybrid polymer brushes can be hydrophilic.

Hybrid brushes composed of two liquid polymers, poly(dimethylsiloxane) (PDMS) and a highly branched ethoxylated polyethylenimine (EPEI), were synthesized on Si wafers by the "grafting to" method and by applying a combinatorial approach (fabrication of gradient brushes). The combinatorial approach revealed a strong effect of "layer assisted tethering", which allowed us to synthesize hybrid brushes twice as thick as the reference homopolymer brushes. The hybrid brushes are stable thin films that can rapidly and reversibly switch between hydrophilic and hydrophobic states in water and air, respectively. The switching in water affects a rapid release of amino functional groups which can be used to regulate adhesion and reactivity of the material. The switching in air rapidly returns the brush to a hydrophobic state. The hybrid brush is hydrophilic because of two mechanisms: (1) exposure of EPEI chains to the brush-water interface under water, and (2) retention of some fraction of water via swollen EPEI chains (the EPEI chains swell by 2-3 times), which are conserved by a PDMS cap in air. The hybrid brush is wettable under water, and at the same time, the brush is nonwettable in air because water droplets are trapped in a metastable state when the water contact angle is above 90 degrees .