Functional materials for Pickering emulsions

This doctoral thesis highlights the most interesting experimental results achieved between 2015 and 2018 by the author, during his stay in the laboratories of the Department of Chemical Sciences of the University of Padova. The fil rouge of this thesis work is Pickering Emulsions (PE); emulsions where solid particles alone are used as stabilizers instead of the typical molecular or macromolecular surfactants that are employed to produce widely used emulsions in pharmaceutics, drug delivery, cosmetics, food industry to name a few. Solid micro/nanoparticles accumulate at the interface between two immiscible liquids (typically denoted as oil and water phase) and stabilize droplets against coalescence. A great advantage of a PE is that it is relatively stable once made and that many solid particles can be endowed with useful characteristics: conductivity, responsiveness, porosity, catalysis and so on. The chemical modification of silica nanoparticles (SiNP) and their use as PE stabilizers is the first theme treated in this thesis work. SiNP are functionalized with hydrophobic molecular structures for tuning their wettability, and with photo-active moieties to impart photocatalytic properties. In addition, silica-based PE are used to confine special ingredients needed for the colorimetric detection of acetone whose presence is associated to triacetone triperoxide, a deadly explosive used by terrorists in recent attacks, and for the development of a testing kit in the form of a pen. Another interesting PE stabilizer is nanocrystalline cellulose (NCC). An extensive sperimentation was carried out to learn how to handle and to chemically functionalize NCC. This led to the development of robust protocols that allowed to install on cellulose nanocrystals a pH-sensitive dye and magnetic nanoparticles that were used to develop, as a proof of principle, a solid pH sensor for urea detection and a colorimetric/magnetic, doubly-responsive system. The modified NCC materials hold the potential as PE stabilizers whose study is underway. The last part of the thesis reports the study, in collaboration with prof. S. Gross of the Chemical Sciences Department in Padova and prof. E. Hensen of the University of Eindhoven, for the production of ZnS nanoparticles through a controlled nucleation and crystallization under continuous flow conditions at room temperature, in water and without the use of any stabilizing ligand. The colloids display an average size of 5 nm and an impressively high specific surface area of 287 m2/g. Nanostructured ZnS is well known to be a direct wide-bandgap semiconductor and, for its tunable photophysical and electrochemical properties, is used for a broad range of applications ranging from catalysis and photocatalysis to nonlinear optics, optoelectronic devices and optical bioimaging. The possibility to prepare stable nanoparticles without the need of special ligand stabilization open the interesting perspective to use those as-prepared particles directly from the continuous flow reactor to stabilize functional PE. Work in this direction is currently underway. Furthermore, a toolbox that was always available during this thesis work was flow chemistry, an ensemble of techniques for the manipulation of fluids on the micrometer scale. Such manipulation is carried out inside microchannels, confined environments whose geometries can be exploited to optimize unit operations – such as mixing and heat transfer – of profound interest for chemists. A flow chemistry approach was used to prepare ZnS nanoparticles and also to develop a selective bromination protocol of tetraphenyl porphyrins. This latter study, along with a study on the application of functionalized NCC as a flame retardant have been a unique opportunity to face interesting problems, although they were not related to the Pickering emulsions general theme of this thesis work.