| Popis: |
Encapsulation and controlled release of active ingredients in formulated products can provide benefits to a number of industries such as cosmetics, pharmaceuticals, agrochemicals, home and personal care products and fragrances. The main roles of encapsulation are to isolate an active material from its surroundings so as to act as a form of protection from exterior environments and to provide a mechanism for controlled release. One of the prominent challenges of encapsulating small compounds, such as those found in drugs, vitamins, fragrance and flavour oils, arises from the fact that the permeation of such species through most microcapsule membranes is fast because current capsule shell materials (polymeric/lipid based membranes/ particulate) have large diffusion coefficients. Although this can be compensated for in some ways, the unavoidable active ingredient leaching resulting from common processes occurring during the application/storage (i.e. lifetime) of the product such as dilution or interaction with other formulation ingredients (surfactants, polymers, biological species) is a significant challenge. Such process often leads to wastage of the valuable active and to side effects. In this work, a method for preventing the loss of encapsulated actives whereby an impermeable metallic film is deposited directly onto an emulsion droplet, hence allowing 100 % of the emulsion core (potentially corresponding to 100 % of the active material depending on conditions) has been developed. A metallic nanoparticle emulsifier, stabilised by a polymer, is used as a catalyst for the electroless deposition of a secondary metal film at the emulsion droplet interface, thereby forming an impermeable shell. In this case a platinum-catalysed gold film electroless deposition mechanism is used as a model system to demonstrate the feasibility of the process. Polyvinylpyrrolidone (PVP) stabilised platinum nanoparticles (Pt-PVP NPs) were specifically synthesised to reduce the quantity of excess polymer in the resulting nanoparticle dispersion, in order to reduce competition for adsorption at the oil-water interface for the Pt nanoparticles and hence to maximise the nanoparticle packing at the interface. Successful adsorption of the Pt nanoparticles at the oil-water interface was confirmed by electron microscopy techniques and energy dispersive x-ray (EDX) spectroscopy, whereby a densely packed nanoparticle array was observed at curved interfaces. Interfacial rheology experiments also showed the build up of a nanoparticle film over time at a planar oil-water interface, which as a result exhibited increasing elastic properties. Cryo-transmission electron microscopy (cryo-TEM) illustrated the importance of reducing the amount of excess PVP in the nanoparticle dispersion as the polymer was found to competitively adsorb at the interface. This observation was supported by interfacial tension measurements of the nanoparticles in oil, whereby dispersions on the nanoparticles took significantly longer to reach equilibrium than those with added polymer. The optimised Pt-PVP NP dispersions were used to stabilise hexadecane emulsions and key parameters in the emulsification process were investigated to demonstrate control over the resulting emulsion. Emulsification with excess PVP in the nanoparticle dispersion further confirmed that the PVP was competitively adsorbing and reduced the density of catalytic nanoparticles at the oil-water interface. For a given nanoparticle concentration stable emulsions could be produced with oil volume fractions between 5% and 13 %. The influence of electrolyte concentration was investigated and the significance of screening interactions between the charged oil-water interface and the charged nanoparticle surface for efficient nanoparticle adsorption was highlighted. Finally, the Pt-PVP NP stabilised emulsions were used as scaffolds for the electroless deposition of a secondary gold film, thereby encapsulating the oil core. Retention of the core was demonstrated over 35 days in an ethanol:water (4:1) environment at 40 °C and was compared to the rapid release of oil from uncoated polymeric shells. The effect of the polymeric stabiliser in the gold plating solution was investigated, with longer chained polymers appearing to provide better stabilisation of the microcapsules during the electroless deposition process. The shell thickness and therefore density of the microcapsules was controlled by altering the concentration of gold salt in the plating solution. Preliminary ultrasound and IR irradiation experiments were also carried out to demonstrate the possibility of remotely triggering the metallic film fracture and associated release of the capsule payload. These initial tests show promise and offer potential for applications such as controlled drug delivery. |
