Eugenia Kharlampieva

Responsive microcapsule reactors based on hydrogen-bonded tannic acid layer-by-layer assemblies

We explore responsive properties of hollow multilayer shells of tannic acid assembled with a range of neutral polymers, poly(N-vinylpyrrolidone) (PVPON), poly(N-vinylcaprolactam) (PVCL) or poly(N-isopropylacrylamide) (PNIPAM). We found that properties of the nanoscale shells fabricated through hydrogen-bonded layer-by-layer (LbL) assembly can be tuned changing the interaction strength of a neutral polymer with tannic acid, and by a change in counterpart hydrophobicity. Unlike most hydrogen-bonded LbL films with two polymer components, the produced tannic acid-based multilayer shells are extremely stable in the wide pH range from 2 to 10. We demonstrate that gold nanoparticles can be grown within tannic acid-containing shell walls under mild environmental conditions paving the way for further modification of the capsule walls through thiol-based surface chemistry. Moreover, these shells show reversible pH-triggered changes in surface charge and permeability towards FITC-labeled polysaccharide molecules. The permeability of these LbL containers can be controlled by changing pH providing an opportunity for loading and release of a functional cargo under mild conditions.

Hydrogen‐Bonded Layer‐by‐Layer Polymer Films

New developments in the area of hydrogen‐bonded layer‐by‐layer assembly composed of weak polyelectrolytes are reviewed, with emphasis on self‐assembly in an aqueous environment. Advances in fundamental understanding of polymer layering at surfaces are addressed. The effects of molecular weight of polymers, ionic strength, pH, and temperature on growth and post‐self‐assembly response of hydrogen‐bonded films are summarized and contrasted with trends known for electrostatically assembled films. Deposition of hydrogen‐bonded films onto particulate substrates and properties of produced capsules are discussed. Strategies to stabilize hydrogen‐bonded multilayers at neutral and basic pH through crosslinking and response properties of produced ultrathin hydrogel films deposited onto flat substrates or comprising the wall of capsules are also described. The potential of hydrogen‐bonding self‐assembly in surface modification and functionalization, in construction of responsive functional containers and membranes, or as solid‐state matrices demonstrating superior ion conductivity make these materials promising for future biomedical and device applications.

Spin-Assisted Layer-by-Layer Assembly: Variation of Stratification as Studied with Neutron Reflectivity †

We apply neutron reflectivity to probe the internal structure of spin-assisted layer-by-layer (SA-LbL) films composed of electrostatically assembled polyelectrolytes. We find that the level of stratification and the degree of layer intermixing can be controlled by varying the type and concentration of salt during SA-LbL assembly. We observe well-defined layer structure in SA-LbL films when deposited from salt-free solutions. These films feature 2-nm-thick bilayers, which are approximately 3-fold thicker than those in conventional LbL films assembled under similar conditions. The addition of a 10 mM phosphate buffer promotes progressive layer interdiffusion with increasing distance from the substrate. However, adding 0.1 M NaCl to the phosphate buffer solution restores the layer stratification. We also find that SA-LbL films obtained from buffer solutions are more stratified as compared to the highly intermixed layers seen in conventional LbL films from buffer. Our results yield new insights into the mechanism of SA-LbL assembly and the final microstructure in comparison with traditional LbL assemblies.

Multilayer-derived, ultrathin, stimuli-responsive hydrogels

In addition to a well-known capacity of the layer-by-layer (LbL) technique to create multilayers with strongly bound polymer chains, the technique also provides a unique opportunity to fabricate highly swollen, hydrogel-like films and capsules. Layered, ultrathin hydrogel-like membranes can be fabricated using electrostatically assembled or hydrogen-bonded multilayers as template matrices, or using a click chemistry approach. This review describes recent progress in the area, comparing various approaches used for the fabrication of these surface-mediated, LbL-templated structures, and discusses future applications of ultrathin hydrogel materials.

Anisotropic Micro‐ and Nano‐Capsules

In this work, we introduce anisotropically shaped, ultrathin micro- and nano-capsules fabricated by layer-by-layer approach. The original cubic and tetrahedral shapes of the template particles were replicated to produce hollow capsules with well-defined edges. Introducing tannic acid as a component of LbL shells resulted in enhanced chemical stability of these hollow polymer structures under a wide pH range due to high pK(a) value. Computational studies demonstrated increased mechanical stability of the anisotropic capsules under osmotic pressure variation due to sharp edges and vertices acting as a reinforcing frame in contrast to spherical microcapsules that undergo random buckling.

pH-responsive photoluminescent LbL hydrogels with confined quantum dots

We report on responsive photoluminescent hybrid materials with quantum dots immobilized in organized manner fabricated by spin-assisted layer-by-layer assembly (SA LbL). The strongly interacting polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) serve for confining CdTe nanoparticles stabilized by thioglycolic acid, while a poly(methacrylic acid) (PMAA) hydrogel matrix presents an elastomeric network with pH-responsive properties. Quantum dot layers encapsulated in PSS-PAH bilayers are confined inside this hybrid hydrogel matrix. The system undergoes reversible changes in photoluminescent intensity in response to pH variations. Photoluminescent intensity of the hybrid matrix is suppressed in excess negative charge at high pH, but excess positive charge at low pH results in significant photoluminescence increase. Such hybrid quantum dot-containing hydrogel-LbL assemblies provide a way for a novel design of materials with precisely controlled structure and pH-triggered optical properties which might be developed into or pH- or chemical sensors.

Bimetallic Nanostructures as Active Raman Markers: Gold‐Nanoparticle Assembly on 1D and 2D Silver Nanostructure Surfaces

It is demonstrated that bimetallic silver-gold anisotropic nanostructures can be easily assembled from various nanoparticle building blocks with well-defined geometries by means of electrostatic interactions. One-dimensional (1D) silver nanowires, two-dimensional (2D) silver nanoplates, and spherical gold nanoparticles are used as representative building blocks for bottom-up assembly. The gold nanoparticles are electrostatically bound onto the 1D silver nanowires and the 2D silver nanoplates to give bimetallic nanostructures. The unique feature of the resulting nanostructures is the particle-to-particle interaction that subjects absorbed analytes to an enhanced electromagnetic field with strong polarization dependence. The Raman activity of the bimetallic nanostructures is compared with that of the individual nanoparticle blocks by using rhodamine 6G solution as the model analyte. The Raman intensity of the best-performing silver-gold nanostructure is comparable with the dense array of silver nanowires and silver nanoplates that were prepared by means of the Langmuir-Blodgett technique. An optimized design of a single-nanostructure substrate for surface-enhanced Raman spectroscopy (SERS), based on a wet-assembly technique proposed here, can serve as a compact and low-cost alternative to fabricated nanoparticle arrays.

Hydrogen-bonded polymer multilayers probed by neutron reflectivity.

We present a neutron reflectivity study of the internal structure of multilayers made of a weak polyelectrolyte and a neutral component where interactions between adjacent layers are controlled by hydrogen-bonding. We found the degree of interpenetration of polymer layers expressed as the interlayer roughness to be strongly correlated with the strength of intermolecular interactions between the adjacent layers. In addition, polymer layers become more diffuse with a distance from the substrate. Our results demonstrate that hydrogen-bonded films exhibit a close correlation between their structure and properties, which is essential for various applications.

Replication of anisotropic dispersed particulates and complex continuous templates

Anisotropic nano-, micro- and mesoscale natural and synthetic structures possess a unique combination of physical properties due to a complex balance of steric factors and intermolecular interactions at multiple length scales. Utilizing such structures as templates for conformal replication allows reproduction of their unique shapes and properties into different synthetic materials. This review is devoted to the recent progress made on anisotropic microstructures suitable as sacrificial templates as well as techniques currently used for their precise replication into various materials. We present an overview of synthetic strategies used for the replication of both dispersed particulates and continuous templates with a number of recent examples of anisotropic organic and inorganic replicas presented. Strategies for generating adequate robust replicas and their expected assembling behavior are also briefly discussed.

Secondary structure of silaffin at interfaces and titania formation

We report on the secondary structure of the recombinant silaffin protein, rSilC, at liquid–solid and air–solid interfaces with polyelectrolyte layer-by-layer (LbL) films serving as templates to mediate protein adsorption. By exploiting in situ ATR-FTIR spectroscopy directly we revealed that the molecular layer of rSilC adsorbed on the LbL surface exhibits a random coil conformation in a hydrated state. In contrast, the partial transition into β-sheet state is observed when the protein is deposited by spin casting with fast water removal. Both forms of rSilC surface layers are capable of mineralization of titania nanostructures at ambient conditions. We suggest that the careful tailoring of the silaffin secondary structure both at interfaces and in solution with particular amino acid sequences capable of intra- and inter-molecular transformations is essential for directing the “bio-titania” mineralization resulting in nanoparticles to large microstructures.