Formation and structure of polyelectrolyte and nanoparticle multilayers: effect of particle characteristics

Investigating the effect of polymeric approaches on circulation time and physical properties of nanobubbles

PURPOSE

A challenge in the field of nanobubbles, including lipobubbles and polymeric nanobubbles, is identification of formulation approaches to enhance circulation time or "bubble life" in the specific organ to allow for organ visualization. The aim of this study was to investigate the potential of two specific preparation approaches, polymeric surface modification to lipobubbles and a one-step approach for the preparation of ionotropically originated polymeric hydrogel nanobubbles for the production of biocompatible, biodegradable, and sufficiently echogenic ('flexible') bubbles, preferably within the nanometer range, that possess an enhanced in vivo lifetime compared to an unmodified lipobubble to allow visualization of the lymph node vasculature.

METHODS

In the first approach, formed liposomes (basic and polymerically enhanced) were sequentially layered with appropriate cationic and anionic polyelectrolytes followed by transformation into polymer-coated nanobubbles. In addition, a one-step approach was employed for the fabrication of ionotropically originated polymeric hydrogel bubbles.

RESULTS

Bubble lifetime was marginally enhanced by self-deposition of polyelectrolytes onto the normal lipobubble, however, not significantly (P = 0.0634). In general, formulations possessing a higher ratio of anionic:cationic coats and highly anionic overall surface charge (-20.62 mV to -17.54 mV) possessed an enhanced lifetime. The improvement in bubble lifetime was significant when a purely polymeric polyionic hydrogel bubble shell was instituted compared to a normal unmodified lipobubble (P = 0.004). There was enhanced persistence of these systems compared to lipobubbles, attributed to the highly flexible, interconnected hydrogel shell which minimized gas leakage. The prolonged contrast signal may also be attributed to a degree of polymeric deposition/endothelial attachment.

CONCLUSIONS

This study identified the relevance of polymeric modifications to nanobubbles for an improved circulating lifetime, which would be essential for application of these systems in passive drug or gene targeting via the enhanced permeability and retention effect.

Engineering nanoassemblies of polysaccharides

Polysaccharides offer a wealth of biochemical and biomechanical functionality that can be used to develop new biomaterials. In mammalian tissues, polysaccharides often exhibit a hierarchy of structure, which includes assembly at the nanometer length scale. Furthermore, their biochemical function is determined by their nanoscale organization. These biological nanostructures provide the inspiration for developing techniques to tune the assembly of polysaccharides at the nanoscale. These new polysaccharide nanostructures are being used for the stabilization and delivery of drugs, proteins, and genes, the engineering of cells and tissues, and as new platforms on which to study biochemistry. In biological systems polysaccharide nanostructures are assembled via bottom-up processes. Many biologically derived polysaccharides behave as polyelectrolytes, and their polyelectrolyte nature can be used to tune their bottom-up assembly. New techniques designed to tune the structure and composition of polysaccharides at the nanoscale are enabling researchers to study in detail the emergent biological properties that arise from the nanoassembly of these important biological macromolecules.

Characteristics of model polyelectrolyte multilayer films containing laponite clay nanoparticles

Polyelectrolyte films structure formed by the "layer-by-layer" (LbL) technique can be enriched by addition of charged nanoparticles like carbon nanotubes and silver or hydroxyapatite nanoparticles, which can improve properties of the polyelectrolyte films or modify their functionality. In our paper we examined the formation and properties of model polyelectrolyte multilayers containing a synthetic layered silicate, Laponite. The Laponite nanoparticles were incorporated into multilayer films, which were formed from weak, branched polycation PEI and strong polyanion PSS. Since charge of PEI is pH-dependent, we build up multilayer films in two deposition conditions: pH = 6 when PEI was strongly charged and pH = 10.5 when charge density of PEI was low. Thicknesses of the films constructed with various numbers of Laponite layers were measured by single wavelength ellipsometry. We also determined the differences in permeability for selected electroactive molecules using cyclic voltamperometry. Properties of the films containing clay nanoparticles were compared with model polyelectrolyte multilayer films PEI/PSS formed at the same conditions. We found that Laponite nanoparticles strongly influenced PEI/PSS multilayer film properties. Replacement of PSS by Laponite eliminated the oscillations of the film thickness in the case when PEI was weakly charged. PSS layer adsorbed on top of PEI/Laponite bilayers increased the thickness of multilayer films and improved their barrier properties so synergistic effects between these properties for polyelectrolytes and Laponite nanoparticles could be observed.

Targeted functionalization of nanoparticle thin films via capillary condensation

Capillary condensation, an often undesired natural phenomenon in nanoporous materials, was used advantageously as a universal functionalization strategy in nanoparticle thin films assembled layer-by-layer. Judicious choice of nanoparticle (and therefore pore) size allowed targeted capillary condensation of chemical vapors of both hydrophilic and hydrophobic molecules across film thickness. Heterostructured thin films with modulated refractive index profiles produced in this manner exhibited broadband antireflection properties with an average reflectance over the visible region of the spectrum of only 0.4%. Capillary condensation was also used to modify surface chemistry and surface energy. Photosensitive capillary-condensates were UV-cross-linked in situ. Undesired adventitious condensation of humidity could be avoided by condensation of hydrophobic materials such as poly(dimethyl siloxane).

Polymer/colloid surface micromachining: micropatterning of hybrid multilayers

Fabrication of multicomponent patterned films comprising polymer/nanoparticle multilayers using conventional lithography and bottom-up layer-by-layer nanofabrication techniques is described. The work is motivated by the potential to extend polymer surface micromachining capabilities toward construction of integrated systems by connecting discrete domains of active materials containing functional nanoparticles. Modified surfaces illustrate tunability of the physical (thickness, roughness, 3D structures) and chemical (inorganic/organic material combinations) properties of the nanocomposite micropatterns. Intriguing nanoscale phenomena were observed for the structures when the order of material deposition was changed; the final multilayer thickness and surface roughness and mechanical integrity of the patterns were found to be interdependent and related to the roughness of layers deposited earlier in the process.

Hydrothermal treatment of nanoparticle thin films for enhanced mechanical durability

The mechanical durability of nanoporous all-nanoparticle and polymer-nanoparticle layer-by-layer (LbL) films (80-150 nm thick) on both glass and polycarbonate substrates has been greatly enhanced by hydrothermal treatment (124-134 degrees C). Polymer-nanoparticle films were more durable than all-nanoparticle films after hydrothermal treatment. The optical properties of nanoporous antireflection (AR) films were exploited in an abrasion test (25-100 kPa normal stress) to quantify the extent of abrasive wear observed qualitatively by scanning electron microscopy (SEM). Marginal damage was observed under optimal reinforcement conditions. Untreated films not only delaminated from the surface completely but also damaged their underlying glass and polycarbonate substrates during testing. The nature of the substrate was found to play an important role in determining abrasion resistance, regardless of the level of particle fusion in the film. The low-temperature process enables in situ mechanical reinforcement of otherwise delicate nanoparticle assemblies on plastic substrates. Tribochemical wear was found to planarize the nanoscale surface texture of these films, similar to what is observed in chemo-mechanical polishing (CMP). This finding is useful for anyone trying to make robust superhydrophobic or superhydrophilic coatings. To our knowledge, this is the first report on hydrothermal reinforcement of LbL films.

Structural color in porous, superhydrophilic, and self-cleaning SiO2/TiO2 Bragg stacks

Thin-film Bragg stacks exhibiting structural color have been fabricated by a layer-by-layer (LbL) deposition process involving the sequential adsorption of nanoparticles and polymers. High- and low-refractive-index regions of quarter-wave stacks were generated by calcining LbL-assembled multilayers containing TiO(2) and SiO(2) nanoparticles, respectively. The physical attributes of each region were characterized by a recently developed ellipsometric method. The structural color characteristics of the resultant nanoporous Bragg stacks could be precisely tuned in the visible region by varying the number of stacks and the thickness of the high- and low-refractive-index stacks. These Bragg stacks also exhibited potentially useful superhydrophilicity and self-cleaning properties.

Structure and mechanism of the deposition of multilayers of polyelectrolytes and nanoparticles

MD simulation of the layer-by-layer assembly of polyelectrolytes (PEs) and nanoparticles (NPs) revealed that the assembly process is electrostatically driven with alternating charge reversal and an overcompensation mechanism. Layers were observed to grow in the lateral direction as well as in a direction normal to the surface. Weakly adsorbed PE molecules were observed to desorb from the flat and NP surfaces. Those molecules are attracted by suspended NPs in solution. PE molecules do not only pull NPs toward the surface but bridge NPs both in solution and on the surface, forming agglomerates and islands. The first double layer differs in structure from the second double layer as a result of strong adsorption of the PE molecules to the rigid surface.

Nanoporosity-driven superhydrophilicity: a means to create multifunctional antifogging coatings

Multifunctional nanoporous thin films have been fabricated from layer-by-layer assembled silica nanoparticles and a polycation. The resultant multilayer films were found to exhibit both antifogging and antireflection properties. The antifogging properties are a direct result of the development of superhydrophilic wetting characteristics (water droplet contact angle <5 degrees within 0.5 s or less). The nearly instantaneous sheetlike wetting promoted by the superhydrophilic multilayer prevents light scattering water droplets from forming on a surface. The low refractive index of the multilayer film (as low as 1.22) resulting from the presence of nanopores was found to impart excellent antireflection properties. Glass slides coated on both sides with a nanoporous multilayer film exhibited transmission levels as high as 99.8%. Stable superhydrophilic wetting characteristics were obtained only after a critical number of bilayers were deposited onto a surface. The assembly conditions (solution pH and nanoparticle concentration), as well as the choice of nanoparticle size, were found to strongly influence film properties. It is suggested that the superhydrophilic behavior is driven by the rapid infiltration of water into a 3D nanoporous network created under specific assembly conditions.