Preparation of Silica Nanoparticles for Biomedical Applications: Bacterial Detection, Oral Drug Delivery and Colloidal Stability

The new branch of medicine, nanomedicine, implies the use of engineered devices and nanostructures in order to accelerate the knowledge of biomolecular-biological processes, to improve the efficiency of disease therapeutics and to develop novel diagnostic techniques. In the present work, three aspects related to the use of nanoparticles (NPs) in biomedical applications have been investigated. Overall, NPs can be applied for in vivo and in vitro bio-applications and the results achieved are strongly related with the stability of the material in suspension and in biological fluids. In particular, silica nanoparticles (SiNPs) have been synthesised and used for (I) the development of a probe for the fluorescent detection of bacteria (II) as carriers in oral drug delivery and (III) they have been used as proxy for the evaluation of a novel storage method to retain long-term colloidal stability of NPs suspensions.

I) Addressing the need of cheap devices capable of detecting bacteria, here we investigated the use of NPs for bacterial detection. An "on-off" fluorescent substrate was firstly synthesised by glycosylic bond formation between a carbohydrate and a switchable fluorophore and its efficiency in detecting the presence of microorganisms was evaluated. 'Nanoprobes' were then developed by functionalising the surface of particles (silica, gold nanoparticles and streptavidin-coated magnetic nanoparticle) with the synthesised substrates. The nanoprobes successfully detected bacteria as proof-of-concept and the developed detection strategy provides a platform upon which further improvements in bacterial detection can be made in the future.

II) The second research challenge proposes to take advantage of the pH-sensitivity of SiNPs for the design of drug delivery systems suitable for oral administration. SiNPs were synthesised from tetraethyl orthosilicate (TEOS) and ethyl triethoxysilane (ETEOS) using different ratios of the two precursors. The degradation and model-drug release profiles from the NPs were analysed in the context of oral drug delivery. The NPs were exposed to pH's found in the GI tract and indicated that silica NPs can be suitable carriers as they avoided degradation at pH4 found in the fed stomach and began releasing model-drug in small intestine-like pH's (pH6 and pH7.4). It was also shown that covalently binding model-drug (fluorescein isothiocyante) inside the NPs led to greater molecule retention inside the NPs over time compared to those physically entrapped in the particle matrix (fluorescein sodium salt and methylene blue). In addition, the dissolution of the NPs was also proved to proceed via different mechanisms depending on the TEOS:ETOS formulation, which presents future avenues for drug release studies.

III) Finally, we addressed the colloidal stability issues that are still limiting the real application of nanoparticles on the market. With this aim, a novel approach to stabilise nanoparticles during storage was explored. By trapping the NPs in the matrix of a reversible hydrogel we showed that the physico-chemical proprieties are better maintained over time compared to NPs stored in solution. In vitro and ex ovo assays proved that the gel-NPs were non-toxic and suggested that NPs stored in gel can be used for biological experimentations without require further purification steps. Studies of gel strength allow considering the proposed method as a promising alternative to the storage techniques currently available, such as surface-coating or lyophilisation methods. The evaluation of the efficiency and versatility of the proposed storage method has been recently published.

Overall the applicability of NPs as platform for bacterial detection, for oral drug delivery have been evaluated and a novel versatile and efficient approach to stabilise NPs in suspension during storage have been developed and evaluated. The findings will hopefully contribute positively to the field of nanomedicine and helping the translation of materials from the laboratory bench to the market.