Veronika Kozlovskaya

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 LbL shells for living cell surface engineering

We report on the design of cytocompatible synthetic shells from highly permeable, hydrogen-bonded multilayers for cell surface engineering with preservation of long-term cell functioning. In contrast to traditional polyelectrolyte layer-by-layer (LbL) systems, shells suggested here are based on hydrogen bonding allowing gentle cell encapsulation using non-toxic, non-ionic and biocompatible components such as poly(N-vinylpyrrolidone) (PVPON) and tannic acid (TA) which were earlier exploited on abiotic surfaces but never assembled on cell surfaces. Here, we demonstrate that these LbL shells with higher diffusion facilitate outstanding cell survivability reaching 79% in contrast to only 20% viability level achieved with ionically paired coatings. We suggest that the drastic increase in cell viability and preservation of cell functioning after coating with synthetic shell stems from the minimal exposure of the cells to toxic polycations and high shell permeability.

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.

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.

Fluorescence correlation spectroscopy studies of diffusion of a weak polyelectrolyte in aqueous solutions

We apply fluorescent correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The technique brings single molecule sensitivity and molecular specificity to dynamic measurements of polyelectrolyte solutions. Translational diffusion of Alexa-labeled poly(methacrylic acid), PMAA*, chains was studied in very dilute, 10(-4) mg/ml, solutions as a function of solution pH and ionic strength. The observed changes in diffusion coefficients were consistent with about twofold expansion of PMAA* coils when pH was changed from 5 to 8, and with chain contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The dependence of the hydrodynamic size of PMAA* chains on the counterion type followed the sequence: Li(+)>Na(+) approximately equal to Cs(+)>K(+). The dependence of translational diffusion on polyacid concentration was weak at the low concentration limit, but chain motions were significantly slower at higher polymer concentrations when PMAA chains overlapped. Finally, measurements of dynamics of PMAA* chains in salt-free solutions showed that self-diffusion of PMAA* chains significantly slowed down when PMAA concentration was increased, probably reflecting the sensitivity of PMAA* translational motions to the onset of interchain domain formation. These results illustrate the utility of the FCS technique for studying hydrodynamic sizes of polyelectrolyte coils in response to variation in solution pH or concentration of salt and polyelectrolytes. They also suggest that FCS will be a promising technique for selective observation of the dynamics of polyelectrolyte components in complex polymer mixtures.

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.

Tuning swelling pH and permeability of hydrogel multilayer capsules

We report on tuning swelling pH transitions of hydrogel hollow capsules that were derived from hydrogen-bonded multilayers via chemical cross-linking. The capsules were either of a single component - a weak poly(carboxylic acid), such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), or poly(ethacrylic acid) (PEAA) - or contained two hydrogen-bonding polymers, such as in poly(carboxylic acid)/poly(N-vinylpyrrolidone-co-NH2) (PVPON-co-NH2) or in poly(carboxylic acid)/poly(N-vinylcaprolactam-co-NH2) (PVCL-co-NH2) systems. By varying the acidity of weak polyelectrolytes, the capsule swelling can be tuned over a wide pH range from 5 to 10. We show that differently from one-component capsules, the swelling amplitude of two-component capsules is limited by the number of cross-links provided by amino-containing units of a PVCL-co-NH2 copolymer. For two-component capsules with the same degree of cross-linking, permeability at the minimum swelling pH was decreased for PMAA-neutral copolymer capsules as compared to those of PAA-neutral copolymer. We also demonstrate that swelling pH of one-component capsules in the acidic region can be modulated via reversible association with a polycation. The fine control over the swelling pH transitions and permeability of hydrogel capsules enables their use in biomedical applications.