Harman, Clarissa Lorraine Grace; (2023) Systematic Design of Magnetophoretic Pickering Emulsions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Pickering emulsions, stabilised by organic or inorganic particles, offer long-term dispersibility of liquid droplets and resistance to coalescence. The versatility of stabilising particles, their external stimulus-response, and their ability to encapsulate and release cargo with high internal payload capacity make them attractive in applications ranging from catalysis to the cosmetic and food industry, and of enormous promise for pharmaceutical and clinical use. Magnetically responsive Pickering emulsions show excellent capabilities for biomedical applications, due to their controlled remote movement and/or destabilisation with an applied magnetic field. Despite this promise, more proof-of- concept research is needed to enable these emulsions to become responsive to an external stimulus. To this end, the work herein encompasses all aspects of the development of magnetically responsive Pickering emulsions, namely (I) the optimisation of suitable magnetic nanoparticles building blocks, (II) their formulation into stable emulsions, and (III) an investigation of the resulting magnetophoretic behaviour. Initial work investigated the preparation and formulation of silica-stabilised Pickering emulsions by evaluating different oils and homogenisation techniques. Silica nanoparticles (SiO2 NPs, 50 nm) were prepared via the Stöber method, were used in formulations with different oils, with isopropyl myristate identified as the most compatible. Emulsification preparation routes were examined with ultrasonication and rotor-stator homogenisation, in order to prepare emulsions that were oil-in-water (o/w) droplets must be homogenous and between 10 to 50 µm in size. Pickering emulsions prepared using ultrasonic homogenisation were small, with a mean droplet diameter of 4 µm. Those prepared using homogenisation had a mean droplet diameter of 10 µm with a shorter preparation time. This work illustrated that small changes in preparation parameters could result in PEs with varying properties. Magnetic nanocomposites were prepared of superparamagnetic iron oxide nanoparticles (SPIONs) coated with silica, which protected surface oxidation and provided improved colloidal and surface properties. The reproducible synthesis of monodisperse, single-core SPION coated with a silica shell of controlled thickness was investigated using design of experiments (DoE). Herein, a 3^3 full factorial design was utilised to study the experimental parameters of the reverse microemulsion route, studying the influence that reactant concentration of tetraethyl orthosilicate (TEOS) and ammonium hydroxide (NH4OH), and the number of fractionated additions of TEOS had on the silica shell. This investigation facilitated a reproducible and controlled approach for the high-yield synthesis of SPION@SiO2 NPs with uniform silica shell thickness. Application of a multiple linear regression analysis established a relationship between the applied experimental variables and the resulting silica shell thickness. These experimental variables were similarly found to dictate the monodispersity of the SPION@SiO2 NPs formed. While the synthesis of NPs can be a complicated and demanding process to understand, by implementing DoE the intricate reaction process can be studied and modelled. Following the preparation of SPION@SiO2 NPs (with a size of 50 nm), they were used to generate magnetically responsive Pickering emulsions, whose magnetic response was investigated at the macroscopic and microscopic scales. For macroscopic studies, the bulk emulsion was exposed to a bar magnet with magnetic field strength (B) of 245 mT. Microscopic studies investigated the magnetophoretic response of the droplets travelling towards the magnet source. Different parameters affecting magnetophoresis including magnetic field strength, the temperature of the system, and droplet size were explored; increasing each of these parameters was observed to increase the velocity of the droplets' movement towards the magnet. A 3^2 central composite design (CCD) was used to establish a relationship between the formulation of the Pickering emulsion and the magnetophoretic response, evaluating the concentration of SPION@SiO2 NPs dispersed in water. Regression analysis found that the total distance travelled, the mean square displacement, and velocity were dependent on the interaction term between the mass of SPION@SiO2 and the water fraction. This body of work provides insights into the production and use of particles to form magnetic Pickering emulsions, whose magnetic response has been mapped for external forces and internal formulation control. The unique contribution of this work sets to develop an experimental model for the design of nanoparticles, by utilising a statistical approach it was possible to determine how experimental factors in fluence the synthesis of magnetic nanoparticles. A similar statistical approach was implemented to understand how the magnetic response of emulsions could be fine tuned with respect to magnetic strength and temperature. This work is the first of its kind and intends to provide a platform for the fundamental understanding and better design for this family of materials.

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