Andrij Pich

Microgels by Precipitation Polymerization: Synthesis, Characterization, and Functionalization

This chapter reviews recent work on the synthesis of aqueous microgel particles by precipitation polymerization. Precipitation polymerization allows flexible control over important physicochemical properties of aqueous microgels, such as size distribution, surface charge, chemical composition, and microstructure. The microgel systems discussed in this review are mainly based on poly(N-isopropyl acrylamide) and poly(N-vinylcaprolactam) due to their ability to react to external stimuli such as the pH or temperature of the surrounding medium. We discuss synthetic routes to obtain microgels based on homo- or copolymers as well as colloids with complex core–shell morphology. The functionalization of microgels is of crucial importance from the application point of view. Different routes for incorporation of functional groups, synthetic polymers, proteins, or nanoparticles in microgel structures are discussed.

Synthesis of novel tantalum oxide sub-micrometer hollow spheres with tailored shell thickness.

Sub-micrometer-sized hollow tantalum oxide (Ta2O5) spheres with tunable shell thickness and void size have been fabricated exploiting beta-diketone-functionalized polystyrene (PS) beads as sacrificial templates in a sol-gel process. First, a controlled precipitation of Ta2O5 nanoparticles was carried out on the template surface by hydrolyzing tantalum ethoxide (Ta(OEt)5) at room temperature, and subsequently, the polymer core was removed either via chemical treatment with toluene or calcination at 650 degrees C. The thickness of the tantala shell precipitated on the PS core during the coating process was tuned between 100 and 142 nm by varying the concentration of tantala precursor in the reaction media. The obtained Ta2O5-coated PS particles and hollow microspheres were characterized by scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. Due to the unique optical and dielectric properties, these nanostructured materials are envisaged to be used in applications such as novel building blocks for the fabrication of advanced materials, surface coatings, catalysts, and drug delivery systems.

Theranostic USPIO-Loaded Microbubbles for Mediating and Monitoring Blood-Brain Barrier Permeation

Efficient and safe drug delivery across the blood-brain barrier (BBB) remains to be one of the major challenges of biomedical and (nano-) pharmaceutical research. Here, we show that poly(butyl cyanoacrylate)-based microbubbles (MB), carrying ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles within their shell, can be used to mediate and monitor BBB permeation. Upon exposure to transcranial ultrasound pulses, USPIO-MB are destroyed, resulting in acoustic forces inducing vessel permeability. At the same time, USPIO are released from the MB shell, they extravasate across the permeabilized BBB and they accumulate in extravascular brain tissue, thereby providing non-invasive R2*-based magnetic resonance imaging information on the extent of BBB opening. Quantitative changes in R2* relaxometry were in good agreement with 2D and 3D microscopy results on the extravascular deposition of the macromolecular model drug FITC-dextran into the brain. Such theranostic materials and methods are considered to be useful for mediating and monitoring drug delivery across the BBB, and for enabling safe and efficient treatment of CNS disorders.

All-Silica Colloidosomes with a Particle-Bilayer Shell

We report on the preparation of all-silica colloidosomes with adjustable size, shell structure, mechanical strength, and permeability. Our approach is based on the coassembly at the water/oil interface of silica nanoparticles and a silica precursor polymer-hyperbranched polyethoxysiloxane-which acts as a binder for particles as well as an additional interfacial component. Remarkably, the shell of colloidosomes can be fine-tuned from a particle monolayer up to a bilayer bound with a sandwiched thin silica film. This method presents a facile approach toward multiscale production of microcapsules which have a high potential in encapsulation technology and in smart coating formulations.

Formation of catalytically active gold–polymer microgel hybrids via a controlled in situ reductive process

A newly developed N-vinylcaprolactam/acetoacetoxyethyl methacrylate/acrylic acid based microgel displays in situ reductive reactivity towards HAuCl4, forming hybrid polymer–gold nanostructures at ambient temperature without additional reducing agents. The colloidal gold nanostructure is selectively formed in the core of the microgel and the composite structure is used as a noble metal catalyst, the activity of which can be tuned depending on the size of the formed core. The hybrid particles can easily be isolated after catalysis via centrifugation and re-used with retention of the catalytic activity.

Design of multicomponent microgels by selective deposition of nanomaterials.

In the present paper a method for the targeted deposition of different nanomaterials on aqueous microgels is described. In the first stage poly(3,4-ethylenedioxythiophene) (PEDOT) nanorods are introduced into the microgel structure by in situ oxidative polymerization. In the second stage hydrogen tetrachloroaurate is used to transform PEDOT chains to an oxidized state in the microgel structure, leading to the fixation of chloroaurate anions on the surface of the PEDOT nanorods. The reduction of chloroaurate ions induces the formation of gold nanoparticles (AuNPs) predominantly located on the PEDOT surface. Obtained microgel/PEDOT/AuNP hybrid particles with different nanoparticle loadings exhibit superior colloidal stability and temperature sensitivity. The microgel/PEDOT/AuNP hybrid microgels exhibit extraordinary catalytic activity in aqueous media.

Temperature sensitive hybrid microgels loaded with ZnO nanoparticles

We report on a generic approach for the preparation of ZnO loaded temperature sensitive hybrid microgels with structural hierarchy. The synthetic process involves controlled hydrolysis of Zn(CH3COO)2·2H2O salt in the presence of poly(N-vinylcaprolactam-co-acetoacetoxyethylmethacrylate) (PVCL-AAEM) microgel templates in 2-propanol. Transmission electron microscopy (TEM) and electron mapping image (EMI) analysis have been employed to prove the presence of ZnO nanoparticles inside the polymeric templates. Dynamic light scattering (DLS) measurements reveal that hybrid microgels display a temperature sensitivity similar to the pure template particles, even at a high loading of ZnO nanoparticles. Changes in size, morphology and physical properties of hybrid microgels as a function of the loading amount of ZnO nanoparticles have been discussed. We demonstrate that these nanostructured materials can effectively be used as transparent UV-shielding materials. In addition, these submicrometre-size hybrid microgels could be applied in the fields of optoelectronic devices, UV-detectors, and photocatalysts.

The special behaviours of responsive core–shell nanogels

Responsive core–shell nanogels that consist of polymers with different sensitivities to an environmental stimulus are soft matter systems with very unique properties. The coupling of the networks leads to a mutual influence of swelling and thus affects the internal structure and dynamics. We highlight the recent progress in theoretical modelling that provides important information on the inhomogeneous internal structures of core–shell nanogels and also discuss interesting discrepancies between theoretical models and experimental data. In addition we describe amphoteric core–shell nanogels that reveal further specific properties, which are of utmost relevance for applications but very challenging for theoretical models. Finally we discuss unexpected dynamic properties of core–shell nanogels with shell-restricted swelling, which indicate confinement effects.

Biocompatible Hybrid Nanogels

Various types of nanoobjects such as metal, semiconductor, and polymer particles, various types of hydrogels and polymer micelles have been incorporated into living cells. [1] Many of these experiments were directed to cell imaging or drug delivery whereas others were intended to introduce optical sensors into the cells.[2]

Monitoring the internal structure of poly(N-vinylcaprolactam) microgels with variable cross-link concentration.

The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.