We report on the application of mixed polymer brushes to the reversible in situ switching colloidal systems (suspensions of responsive 200 nm in diameter particles in individual solvents and immiscible liquids). We used mixed copolymer brushes to fabricate responsive nanoparticles and employed the particles to prepare responsive colloidal systems, which demonstrated drastic transformation/switching of material properties upon external stimuli. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change of solvent and pH. We show that this behavior can be used for a reversible formation of particle aggregates, stabilization and switching between w/o and o/w emulsions, and regulation of the particle transport between immiscible liquids across the interface. We demonstrate an example of the application of the responsive colloids for the fabrication of ultrahydrophobic coatings with textured surfaces from aqueous dispersion, and no surfactant application, using the switching properties of the responsive particles.
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.
Stabilized with a polyelectrolyte complex, superparamagnetic wires were prepared from Fe3O4 nanoparticles and used to fabricate structures of a complex architecture on solid substrates by manipulating the wires in an external magnetic field.
The ability to vary, adjust, and control hydrophobic interactions is crucial in manipulating interactions between biological objects and the surface of synthetic materials in aqueous environment. To this end a grafted polymer layer (multi‐component mixed polymer brush) is synthesized that is capable of reversibly exposing nanometer‐sized hydrophobic fragments at its hydrophilic surface and of tuning, turning on, and turning off the hydrophobic interactions. The reversible switching occurs in response to changes in the environment and alters the strength and range of attractive interactions between the layer and hydrophobic or amphiphilic probes in water. The grafted layer retains its overall hydrophilicity, while local hydrophobic forces enable the grafted layer to sense and attract the hydrophobic domains of protein molecules dissolved in the aqueous environment. The hydrophobic interactions between the material and a hydrophobic probe are investigated using atomic force microscopy measurements and a long‐range attractive and contact‐adhesive interaction between the material and the probe is observed, which is controlled by environmental conditions. Switching of the layer exterior is also confirmed via protein adsorption measurements.
The objective of this research was the fabrication of superhydrophobic surfaces by deposition (casting) of the aggregates of nanoparticles from water-born solutions with no surfactant application. This approach to superhydrophobic surfaces is important for technologies which avoid organic solvents, surfactants, and applications of complex methods (e.g., lithography, microprinting, micromolding, etc.) for the fabrication of textured functional surfaces. The casting of textured coatings from water-born dispersions is a non-toxic and environmentally friendly method that can be conducted without expensive equipment and tools.The goal of this research was achieved by the fabrication of aqueous dispersions of hybrid responsive nanoparticles. The hybrid particles consisted of a silica core with a grafted mixed block copolymer brush of poly(styrene-block-4-vinylpyridine) P(S-b-4VP) constituting the responsive particle shell. The responsive shell was used to tune and stabilize the secondary aggregates of the particles of the appropriate size and morphology in an aqueous environment. The suspension of the particles formed a textured hydrophilic coating on various substrates upon casting and evaporation of water. Heating the coating above the glass transition temperature of polystyrene (PS) resulted in production of the superhydrophobic material.Superhydrophobic surfaces have received much attention due to their important applications ranging from self-cleaning materials to microfluidic devices. A superhydrophobic surface has a very high advancing water contact angle, typically of 150°or higher, and a very low contact angle hysteresis, that is a very low sliding angle, typically below 15°. [1,2] The wettability of a solid surface is governed by the combination of two factors: the surface chemical composition and the surface texture. [1,[3][4][5][6][7] The surface roughness amplifies the hydrophobic behavior of hydrophobic surfaces due to two possible mechanisms: the Wenzel regime [6] (homogeneous wetting of a rough surface when the wetting liquid (water) penetrates surface cavities) and the Cassie regime [7] (heterogeneous wetting of a rough surface when air is trapped beneath a droplet of water and water escapes from cavities, crevices, and grooves occupied by trapped air). In the Wenzel regime, the water contact angle and the contact angle hysteresis increase with surface roughness resulting in a high roll-off angle. This surface is not a superhydrophobic surface because of the high wetting hysteresis. A low wetting hysteresis is indeed obtained in the Cassie regime for a "composite" surface with trapped air in the surface grooves. The transition between the Wenzel regime and the Cassie regime depends on the intrinsic contact angle, as measured on the reference smooth surface of the same chemical composition as for the rough sample, and the surface texture. McCarthy and Oner [1,2] demonstrated experimentally that the Cassie regime can be approached for characteristic dimensions of surface features of the rough substrate i...
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°.
A novel procedure to architecture nanoelectrode arrays with enhanced electrochemical properties was developed. Magneto-assisted formation of conducting nanowires upon self-assembling of Au-shell/CoFe2O4-magnetic-core nanoparticles (18 ± 3 nm diameter) was demonstrated on a Au electrode surface by application of an external magnetic field. The nanowires were visualized by atomic force microscopy showing similar diameters (40 nm) and a length increase from 0.57 to 1.53 μm when the time intervals allowed for the self-assembling process ranged from 15 to 120 min. The conducting nanowires caused an increase of the electrode surface area yielding an electrochemical response to a diffusional redox probe (ferrocenemonocarboxylic acid) enhanced by ∼6.5-fold after 120 min. The enhancement factor for the electrochemical process was controlled by the time intervals allowed for the nanoelectrode array formation. The primary electrochemical reaction of the electron relay was coupled with the bioelectrocatalytic oxidation of glucose in the presence of soluble glucose oxidase resulting in the amplification of the biocatalytic cascade controlled by the growth of the nanostructured assembly on the electrode surface. The studied nanoelectrode array was suggested as a general platform for electrochemical biosensors with the enhanced current outputs controlled by the structure of the self-assembled nanowires.
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