TAILORING THE MICRO- AND MESO-POROSITY OF SPHERICAL SILICA PARTICLES USING NANO/MICROBUBBLES AS TEMPLATES

Applications such as hydrogen storage, separation, catalysis, delivery of poorly soluble drugs all demand internally micro- or meso-porous inorganic materials, with specific requirements for pore size and available surface area, which can be produced reliably, easily and cheaply. Therefore there is a great need to improve existing methods for production of porous materials.

The proposal aims to investigate, by experiment, entirely novel micro- and meso-porous silica particles using nano/microbubbles as templating material. Recently published developments on the stability and long life of nano/microbubbles in aqueous and organic solvents have paved the way for their application in various fields and the proposed research intends to use stable nano/microbubbles to tune the internal porosity/architecture of an inorganic material. The work aims to identify the main parameters influencing the nano/microbubble size and relate it to the resulting internal structure as well as those influencing the silica particle size and uniformity. An efficient method (ultrasound sonicator and cavitation venturi tube) will be used to generate the nano/microbubbles and their size and stability will be validated allowing their use as templating material within the silica droplets to tailor the internal structure of spherical silica particles. Improved production of silica droplets containing nano/microbubbles using membrane emulsification will be a significant leap toward reducing surfactant templating methods and slow batch operation to grow silica particles.
The aim is to facilitate the development of an eco-friendly process (that does not rely on templating surfactants) for the production of highly uniform porous spherical silica particles.

Although silica will be used as a case study material, the process has the potential to be applied to tailor the internal architecture of both inorganic and polymeric nanostructures. Such nanostructures have great potential for applications in drug delivery, energy (e.g. hydrogen) storage as well as catalyst supports.

There is no academic or industrial activity taking place within Europe and the UK when it comes on to the use of micro- nanobubbles for tuning the internal architecture (pore size, pore connectivity, pore volume, and surface area) of inorganic or organic materials. As such, there is an urgent need for research on this subject so as to enable the UK to keep up with the emerging scientific field of fine bubble technology from then Far East, and so that the UK can benefit from the vast potential of this novel technology.
Application driven technological demands require vastly superior nanoscale control of structure relative to the current state of the art. Currently, many porous inorganic materials are synthesised using expensive templating surfactants. Their primary role during wall formation is to promote the pore structure so that swelling liquids can be introduced to enlarge the pores even further.

The proposed project aims to radically change the current expensive templating method with the one which has never been previously explored i.e. through the use of nano/microbubbles in order to fine tune the porosity of silica.

The principal ultimate aim of the project (and its follow-on work) is to ultimately provide better control, greener and cheaper processes for the production of the porous silica particles. Bearing in mind numerous applications of porous silica ranging from catalytic supports, photonic crystals, drug and gene delivery, photodynamic therapy, biomedical imaging, biotechnology, separation agent, inorganic ion exchange resin I can freely say that the potential impact on the society will be immense.

One significant example is drug delivery. Having in mind that 40% of marketed drugs and 70% of oral drugs in development are only weakly soluble in aqueous media, which limits the amount of drug available for absorption by the body and for those applications, silica technology is being considered to offer a means of drug delivery (no rejection by immune system). Silica is currently being tested for this application, generated by batch production followed by crushing which gives low yield and produces significant amount of chemical waste. Therefore any improvement to the process which will increase the reliability and yield and reduce the production costs will have significant implications for patients by reducing the costs of the drug. A particular good example is Antavir (used in the treatment of HIV) where silica has been successfully used to increase the bioavailability of the drug. Reducing drug costs would have even greater impact in the poorer countries, where cheaper overall solutions are needed.
Another significant example is potential use of silica particles for reduction of pressure during hydrogen storage helping to achieve the reduction in global emission of greenhouse gas: in the absence of adsorption media a storage pressure of approximately 700 bar is required, but in the presence of adsorption media this can be reduced to less than 70 bar. However, the 'extra space' taken up by the adsorption media has to be offset by media that has a very high internal surface area, leading to high hydrogen storage capacity. The ability to successfully and simply template this is using 'air' would have a major impact. The proposed research will investigate the scientific foundations on which this can be delivered.