Supramolecular hybrid structures and gels from host-guest interactions between alpha-cyclodextrin and PEGylated organosilica nanoparticles

Polypseudorotaxanes are polymer chains threaded by molecular rings that are free to unthread; these "pearl-necklace" can self-assemble further, leading to higher-order supramolecular structures with interesting functionalities. In this work, the complexation between alpha-cyclodextrin (alpha-CD), a cyclic oligosaccharide of glucopyranose units, and poly(ethylene glycol) (PEG) grafted to silica nanoparticles was studied. The threading of alpha-CD onto the polymeric chains leads to their aggregation into bundles, followed by either the precipitation of the inclusion complex or the formation of a gel phase, in which silica nanoparticles are incorporated. The kinetics of threading, followed by turbidimetry, revealed a dependence of the rate of complexation on the following parameters: the concentration of alpha-CD, temperature, PEG length (750, 4000, and 5000 g mol(-1)), whether the polymer is grafted or free in solution, and the density of grafting. Complexation is slower, and temperature has a higher impact on PEG grafted on silica nanoparticles compared to PEG free in solution. Thermodynamic parameters extracted from the transition-state theory showed that inclusion complex formation is favored with grafted PEG compared to free PEG and establishes a ratio of complexation of five to six ethylene oxide units per cyclodextrin. The complexation yields, determined by gravimetry, revealed that much higher yields are obtained with longer chains and higher grafting density

Formation of supramolecular gels from host-guest interactions between PEGylated chitosan and alpha-Cyclodextrin

Chitosan-based hydrogels are prepared via the formation of polypseudorotaxanes (PPR), by selectively threading alpha-cyclodextrin (alpha-CD) macrocycles onto polymeric chains, which, through the formation of microcrystalline domains, act as junction points for the network. Specifically, host-guest inclusion complexes are formed between alpha-CD and PEGylated chitosan (PEG-Ch), resulting in the formation of supramolecular gels. PEG-grafted chitosan is obtained with a reaction yield of 79.8%, a high degree of grafting (50.9% GW) and water solubility (approximate to 16 mg mL(-1)), as assessed by turbidimetry. A range of compositions for mixtures of PEG-Ch solutions (0.2-0.8% w/w) and alpha-CD solutions (2-12% w/w, or 0.04-0.2% mol) are studied. Regardless of PEG content, gels are not formed at low alpha-CD concentrations (<4%). Dynamic rheology measurements reveal stiff gels (G' above 15k) and a narrow linear viscoelastic region, reflecting their brittleness. The highest elastic modulus is obtained for a hydrogel composition of 0.4% PEG-Ch and 6% alpha-CD. Steady-state measurements, cycling between low and high shear rates, confirm the thixotropic nature of the gels, demonstrating their capacity to fully recover their mechanical properties after being exposed to high stress, making them good candidates to use as in-situ gel-forming materials for drug delivery to topical or parenteral sites

Supramolecular hybrid structures and gels from host-guest interactions between alpha-cyclodextrin and PEGylated organosilica nanoparticles

Polypseudorotaxanes are polymer chains threaded by molecular rings that are free to unthread; these "pearl-necklace" can self-assemble further, leading to higher-order supramolecular structures with interesting functionalities. In this work, the complexation between alpha-cyclodextrin (alpha-CD), a cyclic oligosaccharide of glucopyranose units, and poly(ethylene glycol) (PEG) grafted to silica nanoparticles was studied. The threading of alpha-CD onto the polymeric chains leads to their aggregation into bundles, followed by either the precipitation of the inclusion complex or the formation of a gel phase, in which silica nanoparticles are incorporated. The kinetics of threading, followed by turbidimetry, revealed a dependence of the rate of complexation on the following parameters: the concentration of alpha-CD, temperature, PEG length (750, 4000, and 5000 g mol(-1)), whether the polymer is grafted or free in solution, and the density of grafting. Complexation is slower, and temperature has a higher impact on PEG grafted on silica nanoparticles compared to PEG free in solution. Thermodynamic parameters extracted from the transition-state theory showed that inclusion complex formation is favored with grafted PEG compared to free PEG and establishes a ratio of complexation of five to six ethylene oxide units per cyclodextrin. The complexation yields, determined by gravimetry, revealed that much higher yields are obtained with longer chains and higher grafting density