Chapter 2 – Overview of the Thesis
2.3 Superparamagnetic and fluorescent thermo-responsive Core-Shell-Corona hybrid
Corona hybrid Nanogels with a protective Silica Shell
Colloidal hybrid nanostructures can be used as components for multifunctional materials in future technologies, in which the most interesting functionalities of different material classes are combined in a single entity.7,8
Particular attention was paid to the combination of the properties of inorganic NPs with stimuli-responsive polymers in hybrid inorganic/inorganic/organic core-shell-corona par-ticles. So, an easy and completely reproducible strategy for the preparation and character-ization of the solution behavior and functional properties of superparamagnetic and/or fluorescent, thermo-responsive inorganic/organic hybrid nanogels with a γ-Fe2O3/SiO2, CdSe(ZnS)/SiO2 and γ-Fe2O3/CdSe(ZnS)/SiO2 intermediate core-shell structure and a thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) corona was developed.
Figure 2-8. Schematic drawing of the synthesis of NP/SiO2/PNIPAAm core-shell-corona hybrid nanogels and representative TEM images of CdSe(ZnS)-NPs, CdSe(ZnS)/SiO2-NPs and CdSe(ZnS)/SiO2/PNIPAAm nanogels.
These well-defined and nearly monodisperse multifunctional nanogels were prepared via two consecutive encapsulation processes of superparamagnetic and/or fluorescent semi-conductor nanocrystals with a silica layer, further covered with a crosslinked and respon-sive polymer corona via a “grafting from/grafting onto” polymerization process. The
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thetic strategy towards monodisperse thermo-sensitive hybrid materials is schematically illustrated in Figure 2-8.
The main focus was to investigate the challenging experimental preparation conditions in detail and then, to transfer this encapsulation strategy to other hydrophobic nanoparticles and provide a platform technology for such core-shell-corona hybrids. With a precise adjustment of the conditions it was possible to achieve a reliable encapsulation and to either entrap several or single particles. The thickness of the SiO2 shell can be controlled and the composition of the NPs within the silica particle can be manipulated according to the needs of a given application. The total diameter can be reliably varied between 20 nm to 60 nm for both types of NPs (CdSe(ZnS) and γ-Fe2O3) upon increasing the concentra-tion of tetraethyl orthosilicate (TEOS) from 0.05 to 1 mmol (Figure 2-9). The intermedi-ate silica shell was not only chosen as a synthetic intermediintermedi-ate and for an improvement of the biocompatibility, but also to impart the final nanocomposite particles with a beneficial barrier layer that also protects the nanocrystals against harmful chemicals degrading the functionality of the core nanocrystals.
Figure 2-9. (A) Dependence of the particle radius and product homogeneity of CdSe(ZnS)/SiO2 core-shell particles as a function of the TEOS concentration and constant NP concentration. (B) TEM images of CdSe(ZnS)/SiO2 core shell particles prepared using different TEOS amounts.
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The hydrodynamic radius distributions of all nanogels showed a consistent increase throughout the various steps and the obtained distribution functions were remarkably nar-row pointing towards monodisperse and well-defined core-shell-corona particles (Figure 2-10).
Figure 2-10. Intensity-weighted hydrodynamic radii distribution (DLS) of (A) γ-Fe2O3, γ-Fe2O3/SiO2 and γ-Fe2O3/SiO2/PNIPAAm and (B) CdSe(ZnS)/SiO2 and CdSe(ZnS)/SiO2/PNIPAAm hybrid nanoparticles at 10% cross-linking density and RT.
To confirm the structures of the materials, TEM and cryo-TEM measurements of the fluo-rescent CdSe(ZnS)/SiO2/PNIPAAm hybrids were performed and are shown in Figure 2-11. All resulting hybrid core-shell-corona particle were very uniform in size and shape and exhibited a narrow size distribution (Figure 2-11 A). The radius of the core-shell-corona materials obtained via TEM was 110 ± 15 nm for both types of hybrid materials.
This radius was lower than the dimensions determined by DLS, which measures the fully extended particle in solution, whereas the TEM values corresponded to sizes in dried state. Therefore, also cryo-TEM images were recorded. The micrographs of CdSe(ZnS)/SiO2/PNIPAAm in Figure 2-11 B showed a fuzzy corona, highlighted by the encircled areas (Figure 2-11 B) and the greyscale analysis (Figure 2-11 C).
Nevertheless, the imaging data in combination with DLS data confirm the successful formation of the hybrid core-shell-corona nanogels and a successful immobilization of a PNIPAAm gel-like corona on the activated core-shell particles.
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Figure 2-11. (A) TEM image and (B) cryo-TEM images of CdSe(ZnS)/SiO2/PNIPAAm nanogels.
(C) Greyscale analysis of the cross section indicated in micrograph B.
The hybrid core-shell-corona particles retained full functionality of the superparamagnet-ic and fluorescent core materials, whsuperparamagnet-ich was investigated using a vibrating sample mag-netometer and a fluorescence spectroscopy (Figure 2-12 A/B). Further, they combined it with the barrier properties and ease of chemical functionalization of the silica shell.
The thermo-responsive character was investigated by DLS (Figure 2-12 C). The samples exhibited a thermo-responsive volume phase-transition nearly at 33-34 °C, originating from the PNIPAAm corona around the core-shell particles. With an increase in tempera-ture the dimensions of the hybrids decreased strongly and the shrinking and swelling pro-cess of the CdSe(ZnS)/SiO2/PNIPAAm particles was fully reproducible as depicted in the inset of Figure 2-12 C. This behavior is comparable to spherical PNIPAAm mi-cro/nanogels. But herein, the overall property profile was amplified by the presence of functional components inside the core.
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Figure 2-12. (A) Magnetic hysteresis curves of γ-Fe2O3 nanoparticles, γ-Fe2O3/SiO2 core-shell particles and γ-Fe2O3/SiO2/PNIPAAm hybrid material with a 10% crosslinking density at RT. (B) Fluorescence spectra for CdSe(ZnS), CdSe(ZnS)/SiO2 and CdSe(ZnS)/SiO2/PNIPAAm. (C) Dependence of the z-average hydro-dynamic radius of CdSe(ZnS)/SiO2/PNIPAAm nanogel particles upon temperature with a crosslinking density of 10%. The inset depicts the changes in the z-average hydrodynamic radius for various temperature cycles below and above the volume transition temperature, respectively.
The reported well-defined size-controlled core-shell particles have high potential as flexi-ble carrier materials in a lot of applications and further, the multifunctional hybrid core-shell-corona nanogels could be applied to possible applications in cell and tissue imaging, delivery of acid-sensitive imaging probes and clinical diagnosis.
The aim of further studies is to use this new knowledge about the synthesis of core-shell-corona particles and their stimuli-responsive properties to create hybrid core-shell-core-shell-corona Janus particles with two different polymers immobilized to the opposite sides of the core-shell particles on the basis of our particles.
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