Understanding metal film deposition conditions for coating sub-micron Pickering emulsions: Towards localised cytotoxic drug delivery

Chemotherapy is one of the main methods to treat cancer. However, chemotherapy is usually accompanied by serious side effects, for example cytotoxic drugs used in chemotherapy indiscriminatingly kill healthy cells as well as tumour cells. Thus, targeted delivery of such cytotoxic cancer drugs has been developed with the aim of decreasing the side effects that patients suffer from. The current targeted delivery platforms of cytotoxic drugs include liposomes, micelles, polymersomes, nanoparticles (NPs), dendrimers etc. Especially, the delivery carriers with core-shell structure such as liposomes, polymersomes, polymeric microcapsules are utilised to isolate drugs from environments and avoid the leaching of drugs. However, these delivery carriers with organic shell cannot significantly stop the leaching of drugs due to the fast diffusion of drugs with small molecular weight through their organic material-based shells. Therefore, these carriers with organic shell are limited by their short drug retention time. As a result, developing core-shell drug carriers with impermeable shells that allow for high drug dosages to be delivered locally without drug leaching elsewhere in the body is still a significant challenge in overcoming the limitations of chemotherapy.
Metal-shell microcapsules have been developed as a new targeted delivery strategy of cytotoxic drugs because they show prospect in tackling this particular challenge. Indeed, as compared to the conventional microcapsules with organic material shells, the close-packed crystalline structure of metallic shells imparts them with low permeability to small molecules. Therefore, the metal-shell microcapsules can retain small molecules for months, which potentially provides a solution to stop undesired leaching of cytotoxic drug during chemotherapy. Nevertheless, the targeted delivery efficiency of current metal shell microcapsules should be improved to meet the application requirements in the clinic. Generally, the conventional delivery carriers of drugs confront various challenges while delivering drugs within the body such as fast renal clearance of NPs (diameter < 5nm), nonspecific accumulation of carriers (diameter ranging from 50 - 100 nm) within livers, splenic filtration for carriers (diameter > 200nm). These problems decline the drug dosages delivered into tumours. Enforced permeability and retention effect (EPR effect) suggests mutated tumour cells show larger gaps between endothelial cells than healthy cells, which allows carriers loading drugs with diameter of ~100 nm to pass through endothelial cells and accumulate within tumours. Indeed, EPR effect provides a solution for efficiently targeting the drug delivery to tumours. Therefore, metal shell capsules with diameter of ~100 nm are most suitable but have not yet been systematically produced as targeted delivery platform of cytotoxic drugs.
In developing such small metal shell capsules, one needs to take into account various factors that may affect the encapsulation and delivery efficiency of metal shell capsules. The impermeability of metal shell capsules is not only resulted from the crystalline structure, but also relies on the growth of a homogeneous and dense metal film on the capsule surface. So far, dense metal films with thickness of 50 - 100 nm have been coated on surface of microcapsules (mostly of such microcapsules show diameter ranging from 1 and 10 m). The metal films are grown at capsule surface through concentrating the metal reduction process at the oil/water interface in the presence of catalysts. However, the volume fraction of the core phase is significantly reduced as developing capsules with diameter of ~100 nm, which makes the production of thin impermeable films most important. Nevertheless, it is challenging to create very thin and impermeable metal films onto droplet surfaces since the growth of metal films onto an oil-water interface is not yet fully understood.
Additionally, catalysts play a key role for preparing small capsules with thin metallic film. Taking gold shell capsules as an example, platinum nanoparticles are applied for stabilising oil-water interface of emulsions (templates of preparing metal-shell capsules) as well as catalysing reduction of gold film onto droplet surface. The growth of gold shell at oil-water interface may be affected by properties of platinum NPs including size, stability and adsorption behaviour of platinum NPs at the oil-water interface, which has not been fully studied. Polyvinylpyrrolidone (PVP) is used to provide stability of platinum NPs. However, excess PVP in platinum NPs suspension potentially affect the adsorption of platinum NPs at oil-water interface. It is required to find the balance between preparation of stable platinum NPs and minimised negative influence of PVP on adsorption of platinum NPs at oil-water interface and even the growth of metallic film.
In this work, platinum NPs were synthesised in the presence of limited PVP (less than 0.0134 wt%). The preparation of platinum NPs stabilised by PVP was optimised through modifying the PVP and reducing agent concentrations. These platinum NPs were then used to manufacture Pickering emulsions, the emulsion droplets stabilised by solid NPs, through adsorbing onto oil-water interface. Additionally, the size of Pickering emulsions has been optimised since they act as templates of preparing metal-shell capsules. The growth mechanism of the gold film on droplet surface was investigated through monitoring the growth process of the gold film on a 2D model surface in electroless plating method. Finally, gold shell nanocapsules were prepared by coating gold shell onto the surface of Pickering emulsions.
In detail, platinum NPs were synthesised through reducing platinum salt using sodium borohydride in the presence of limited PVP. The successful reduction of platinum salt to platinum NPs has been confirmed by a UV-vis test. The effect of reducing agent and PVP concentrations on properties of platinum NPs was investigated through dynamic light scattering (DLS) and transmission electron microscopy (TEM). A possible synthesis mechanism of platinum NPs in limited PVP was discussed. The formation of platinum NPs experiences four stages including 1) reduction of platinum salt, 2) first coalescence of platinum NPs, 3) metastable stage and 4) second coalescence of platinum NPs. Generally, more PVP and larger sodium borohydride concentration are beneficial for the formation of more stable platinum NPs. More sodium borohydride leads to a longer metastable stage. Therefore, PVP got enough time to diffuse to platinum NPs at metastable stage and hinder the coalescence of platinum NPs at second coalescence stage. Therefore, there are less clusters of platinum NPs. However, low reducing agent concentration results in short metastable stage and thus brings more clusters of platinum NPs even at high PVP concentration. The optimised PVP concentration and reducing agent concentration of synthesising platinum NPs were 0.0134 wt% and 22.4 mM, respectively. The platinum NPs obtained at different conditions were used to prepare Pickering emulsions and subsequently grow a gold film on the droplet surfaces. Optical microscopy and laser diffraction (LD) particle size analysis showed that the stability and droplet size were closely related to size of synthesised platinum NPs, which fits with theoretical calculations of droplet size. Scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) confirmed that the thickness and properties of the gold film growing on the microcapsules were affected by the properties of prepared platinum NPs. Small platinum NPs resulted in a thin and homogeneous gold film on the droplet surfaces.
Then, the 2D solid model surface was used to study the growth mechanism of the gold film in the presence of platinum NPs. The density of platinum NPs was successfully controlled by 1) adding excess PVP into the platinum NPs suspension before conducting NPs adsorption and 2) modifying the adsorption time of platinum NPs. The growth of gold film on a 2D model surface was monitored using TEM and energy dispersive X-ray spectroscopy (EDX). The influence of different gold salt concentrations and platinum NPs densities on properties of the gold film including size distribution, density, surface coverage, interparticle distance and diameter of gold films was investigated. Large gold patches with wide size distributions formed as a result of more gold salt when other parameters were constant. Additionally, more platinum NPs adsorbed at the 2D surface resulted in the formation of larger gold patches with more monodisperse size distribution. Therefore, the possible reasons affecting the growth of gold film include Ostwald ripening and coalescence of gold NPs, which are related to the platinum salt concentrations and platinum NPs density.
Finally, gold shell nanocapsules were successfully manufactured through electroless plating coating a gold film onto emulsion droplets with a diameter of ~100 nm. As the template of preparing gold shell nanocapsules, the synthesis of Pickering emulsions was optimised. Specifically, the influences of preparation parameters including sonication time, electrolyte concentrations, platinum NPs concentrations, oil volume fractions and extra PVP concentrations on the size of Pickering emulsions were investigated using DLS, DL, optical microscopy, cryo-TEM. Oil volume fractions, platinum NPs and extra PVP concentrations play important roles of controlling droplet size distribution. Based on the understanding of the gold film growing on a 2D solid model surface, the gold films with different thickness and structure were grown on the surfaces of Pickering emulsions. The addition of excess PVP potentially led to the formation of a heterogeneous gold film on the droplet surface, which shows an incomplete encapsulation of the oil core. Our understanding about the optimisation preparation of gold film nanocapsules is expected to develop a targeted delivery platform of cytotoxic drugs stopping the leaching of cytotoxic drugs and providing the long retention of drugs.