title: Development of ultra-stable nanomaterials for biological imaging applications
Creators: name: Wilson, Katie
Publisher: name: University of Liverpool Repository
Year: 2013

description: In order to use nanoparticles in biological applications, they need to be coated by a ligand shell (called biofunctionalisation) to provide stability in a physiological environment, including preventing non-specific binding, and to target the nanoparticle to areas of interest in the sample. One approach to synthesising ligand shells is to self-assemble a monolayer of small ligands on the surface of the nanoparticle. The ligand can be considered to consist of a ‘head’, ‘stem’ and ‘foot’. The ‘foot’ serves to anchor the ligand to the surface of the nanoparticle and, with the ‘stem’, drive self-assembly of the shell and seal off the core material from the environment. The environment is only exposed to the ‘head’ at the distal end of the ‘stem’. While the ‘stem’ and ‘head’ groups could be easily transposed to many different kinds of nanoparticles, the ‘foot’ must be adapted according to the surface properties of the nanoparticle. This approach has hitherto been successful with noble metal nanoparticles. In this thesis, it is tested with nanoparticles of very different materials: semiconductor nanoparticles or quantum dots (QDs) and superparamagnetic iron oxide nanoparticles (SPIONs). QDs are particularly useful for optical imaging due to their fluorescent properties, such as resistance to photobleaching, which give them important advantages over organic fluorophores. QDs have, therefore, been used to screen different ligand shells with a thiol as the “foot” and ethylene glycol (EG) as the “head”. A novel protocol, using EG alkanethiol ligands to biofunctionalise QDs, has been developed. These EG alkanethiol capped QDs are highly stable and soluble in physiological conditions according to a series of increasingly stringent stability tests. Crucially, they do not exhibit non-specific binding to cells and a controlled number of a specific recognition function can be introduced within the shell for targeting. These QDs have been used in live cell imaging. QDs monovalently functionalised with Tris-nitrilotriacetic acid (Tris-NTA) have been stoichiometrically coupled to fibroblast growth factor 2 (FGF2). The QDs allow direct visualisation of the interaction of FGF2 with its receptors on the surface of living cells. The approach has then been transposed to SPIONs. SPIONs, because of their magnetic properties, are particularly attractive materials for enhancing magnetic resonance imaging contrast in a variety of in vivo situations. The thiol “foot” of EG alkanethiol would not bind well to iron oxide, but phosphates bind strongly. Therefore, a new ligand was synthesised, in which a phosphoserine was placed at the foot on the ligand, to produce an EG alkane phosphoserine. A ligand exchange protocol was developed, which allowed capping of the SPIONs with this ligand. The resulting EG alkane phosphoserine capped SPIONs were, like the QDs, found to possess excellent stability in a series of tests of increasing stringency. The ligand shell also provided good protection of the SPION core against chelation by citrate at acid pH. The work presented in this thesis thus describes important advances in the mobilisation of the remarkable properties of nanoparticles for biology and medicine. Firstly, by directly synthesising small, highly stable QDs that can be stoichiometrically functionalised for imaging. Secondly, by synthesising the first ligand shell for SPIONs that provides sufficient stability to allow these materials to be used in experiments where longer term imaging is required, such as tracking transplanted stem cells in vivo. Thirdly, the strategy of synthesising ligand shells of small molecules that self assemble on the surface of nanoparticles has been extended to materials other than the noble metals. This aspect of the thesis highlights that this strategy is generic and, if a suitable “foot” unit can be identified, likely to be applicable to nanoparticles of any material.