Micropatterning of Polymer Thin Films with pH-Sensitive and Cross-linkable Hydrogen-Bonded Polyelectrolyte Multilayers

Polyelectrolyte multilayers of poly(acrylic acid) (PAA) and polyacrylamide (PAAm) were prepared via hydrogen-bonding interactions. These multilayers as assembled were stable at low pH but dissolved quickly in neutral pH water. We developed methods for stabilizing these multilayers to high pH through cross-linking by heating or UV-irradiation. Thermal treatment of the multilayers, which resulted in a partial imidization between carboxylic acid and amide groups, gave the multilayer good stability at high pH. In addition, we introduced photoreactive groups in the multilayer, which rendered the film insoluble after UV irradiation. Using these selective stabilization approaches, we have succeeded in micropatterning these films by ink-jet printing and photolithography to create subtractive patterns

Mechanism of the pH-Induced Discontinuous Swelling/Deswelling Transitions of Poly(allylamine hydrochloride)-Containing Polyelectrolyte Multilayer Films

The mechanism of the discontinuous swelling/deswelling transitions exhibited by polyelectrolyte multilayers containing poly(allylamine hydrochloride) (PAH) was examined by FT-IR spectroscopy, in-situ atomic force microscopy (AFM), and in-situ ellipsometry. Assembly pH was found to play a critical role in determining the postassembly pH-dependent swelling behavior of multilayers containing PAH. Multilayer films assembled at pH < 8.5 were found to exhibit pH-independent swelling behavior over the pH range of 2.0−10.5, whereas dramatic discontinuous swelling transitions were observed when the assembly pH was greater than 8.5. FT-IR spectroscopy was used to demonstrate unequivocally that the pH-triggered, discontinuous swelling/deswelling transitions of PAH/sulfonated polystyrene (SPS) multilayers assembled at high pH (>8.5) are driven by changes in the degree of ionization of free amine groups of PAH that are established during multilayer assembly. The pH trigger points of these swelling/deswelling transitions are determined by the elimination/reestablishment of hydrophobically associated PAH chain segments. Such hydrophobic interactions are responsible for shifting the pKa of the free amine groups of PAH to unusually low values (ca. pH 4.0). The kinetics of deswelling were found to be strongly influenced by the type of polyanion assembled with PAH

Heterostructured Magnetic Nanotubes^†

Heterostructured magnetic tubes with submicrometer dimensions were assembled by the layer-by-layer deposition of polyelectrolytes and nanoparticles in the pores of track-etched polycarbonate membranes. Multilayers composed of poly(allylamine hydrochloride) and poly(styrene sulfonate) assembled at high pH (pH > 9.0) were first assembled into the pores of track-etched polycarbonate membranes, and then multilayers of magnetite (Fe3O4) nanoparticles and PAH were deposited. Transmission electron microscopy (TEM) confirmed the formation of multilayer nanotubes with an inner shell of magnetite nanoparticles. These tubes exhibited superparamagnetic characteristics at room temperature (300 K) as determined by a SQUID magnetometer. The surface of the magnetic nanotubes could be further functionalized by adsorbing poly(ethylene oxide)-b-poly(methacrylic acid) block copolymers. The separation and release behavior of low molecular weight anionic molecules (i.e., ibuprofen, rose bengal, and acid red 8) by/from the multilayer nanotubes were studied because these tubes could potentially be used as separation or targeted delivery vehicles. The magnetic tubes could be successfully used to separate (or remove) a high concentration of dye molecules (i.e., rose bengal) from solution by activating the nanotubes in acidic solution. The release of the anionic molecules in physiologically relevant buffer solution showed that whereas bulky molecules (e.g., rose bengal) release slowly, small molecules (i.e., ibuprofen) release rapidly from the multilayers. The combination of the template method and layer-by-layer deposition of polyelectrolytes and nanoparticles provides a versatile means to create functional nanotubes with heterostructures that can be used for separation as well as targeted delivery

All-Nanoparticle Thin-Film Coatings

All-nanoparticle thin-film coatings that exhibit antireflection, antifogging (superhydrophilicity), and self-cleaning properties have been prepared via layer-by-layer deposition of TiO2 and SiO2 nanoparticles. The porosity and chemical composition of the coatings were determined using a simple method that is based on ellipsometry and does not require any assumptions about the refractive indices of the constituent nanoparticles. The presence of nanopores in the TiO2/SiO2 nanoparticle coatings results in superhydrophilicity as well as antireflection properties. The superhydrophilicity of contaminated coatings could also be readily recovered and retained after ultraviolet irradiation

Antibacterial Properties of Ag Nanoparticle Loaded Multilayers and Formation of Magnetically Directed Antibacterial Microparticles

Antibacterial coatings based on hydrogen-bonded multilayers containing in situ synthesized Ag nanoparticles were created on planar surfaces and on magnetic colloidal particles. We report the antibacterial properties of these coatings, determined using a disk-diffusion (Kirby-Bauer) test, as a function of the film thickness and the concentration of Ag nanoparticles in the hydrogen-bonded multilayers. The zone of inhibition (ZoI) determined by the disk-diffusion test increases as the thickness of the multilayer film is increased. Results obtained for the values of the ZoI as a function of film thickness can be described adequately with a simple diffusion model (i.e., the square of the zone of inhibition (ZoI) depended linearly on the logarithm of the thickness of the silver-loaded films). This observation suggests that, in order to incrementally increase the ZoI, an exponentially increasing amount of Ag is required within the multilayers. In general, there was no statistically significant correlation between the zone of inhibition and the number of Ag loading and reduction cycles. The duration of sustained release of antibacterial Ag ions from these coatings, however, could be prolonged by increasing the total supply of zerovalent silver in the films via multiple loading and reduction cycles. These results indicate that the release of silver is controlled by an oxidation mechanism at the surface of the nanoparticles and that repeated loading and reduction of silver leads preferentially to growth of the existing silver nanoparticles in the film as opposed to nucleation of new Ag nanoparticles. We also show that magnetic microspheres coated with silver nanoparticle loaded hydrogen-bonded multilayer thin films can be used to deliver antibacterial agents to specific locations. The minimum inhibitory concentration (MIC) of nanocomposite coated microspheres was determined by the agar dilution technique:  antibacterial magnetic microspheres with higher concentrations of Ag nanoparticles exhibited lower MIC values

All-Nanoparticle Thin-Film Coatings

All-nanoparticle thin-film coatings that exhibit antireflection, antifogging (superhydrophilicity), and self-cleaning properties have been prepared via layer-by-layer deposition of TiO2 and SiO2 nanoparticles. The porosity and chemical composition of the coatings were determined using a simple method that is based on ellipsometry and does not require any assumptions about the refractive indices of the constituent nanoparticles. The presence of nanopores in the TiO2/SiO2 nanoparticle coatings results in superhydrophilicity as well as antireflection properties. The superhydrophilicity of contaminated coatings could also be readily recovered and retained after ultraviolet irradiation

pH-Responsive Reversibly Swellable Nanotube Arrays

We demonstrate a technique for synthesizing substrate-bound arrays of submicrometer-sized reversibly swellable tubes by using porous templates. The sacrificial template approach allows straightforward control over the tube length, diameter, and lateral arrangement of the resultant surface-bound nanotubes. We also explored methods for varying the tube opening structure by altering the pore shape at the surface of the template. A specific PEM system composed of poly(allylamine hydrochloride) and poly(acrylic acid) was chosen as the building block for the nanotube arrays because of its ability to undergo pH-triggered swelling−deswelling transitions. The activation of this transition results in dramatic changes in the length and diameter of the nanotubes as characterized in situ via confocal laser scanning microscopy (CLSM). The pH-driven reversible swelling−deswelling and nanoporosity behavior observed with planar films and nanotubes of this PEM system is a direct consequence of the breaking and reforming of ionic cross-links

Determining the Young's Modulus of Polyelectrolyte Multilayer Films via Stress-Induced Mechanical Buckling Instabilities

Determining the Young's Modulus of Polyelectrolyte Multilayer Films via Stress-Induced Mechanical Buckling Instabilitie

Amine-Rich Polyelectrolyte Multilayer Nanoreactors for in Situ Gold Nanoparticle Synthesis

In situ synthesis of inorganic clusters in polyelectrolyte multilayers (PEMs) has typically relied on free carboxylic acid groups as the binding sites, limiting the scheme to the loading of cationic precursor reagents. The use of amine groups for similar purposes has not been demonstrated because of the challenge of incorporating free amine groups that are not paired with the oppositely charged groups residing on the polyanion of the PEM. In this paper, we use specific PEM assembly conditions to produce ultrathin conformal films of PAH/PAA (poly(allylamine hydrochloride)/poly(acrylic acid)) and PAH/PSS (poly(styrene sulfonate)) that upon suitable postassembly treatment undergo substantial molecular rearrangements that generate free amine groups in the films. These PEMs are capable of binding anionic precursors including complexes widely used for the synthesis of gold nanoparticles. On the basis of our understanding of the gold binding mechanisms, we demonstrate systematic control over the size and spatial distribution of gold nanoparticles in the films by changing the PEM assembly and postassembly treatment conditions

Creating Effective Refractive Index Gradients within Polyelectrolyte Multilayer Films: Molecularly Assembled Rugate Filters

We demonstrate the use of aqueous-based layer-by-layer (LbL) processing and in situ nanoreactor chemistry for creating polyelectrolyte multilayer (PEM) nanocomposites in which the concentration of a nanoparticle species is modulated throughout the depth of the film in a complex way. Using the in situ growth of silver nanoparticles to selectively increase the refractive index of precisely defined regions of the PEM film, we have produced a digitized rugate filter, a special type of dielectric mirror possessing many thin layers that approximate a continuous, periodic refractive index profile. The design of the digital rugate from an initial smooth index profile was aided by a computer, and construction of the device was accomplished entirely by automated means. Poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) bilayer regions within the film supported subsequent nanoreactor chemistry and created the high refractive index regions of the film, while PAH and poly(styrenesulfonate) (PSS) bilayer regions remained unmodified and served as the low index regions. Spectroscopic results obtained from the assembled device confirm the presence of a reflection band that grows in amplitude with increasing silver incorporation, reaching a peak reflectance of 75% after five silver loading and exchange cycles. The rugate structure suppresses reflection sidebands, features common to Bragg reflectors and simpler dielectric mirror designs. Cross-sectional TEM reveals nanoscale control of the size and location of silver-containing domains within the PEM film. The ability of this method to control the placement of nanoparticles within PEMs enables the inexpensive, easy production of conformable optical coatings with tunable properties. In addition, these nanocomposite films may have applications in a number of other fields where precise control of effective gradients in physical properties throughout thin films has been shown to affect the overall performance in a desirable way