Synthesis and Application of Nanoparticles for Medical Imaging and Cancer Therapy

Abstract

Nanoparticles are a highly intriguing prospect as cutting-edge materials for biomedical procedures. Their physical properties allow them to be controlled via external stimuli, which is an attractive option when compared to invasive and costly surgical procedures which are currently used in the clinic. These physical properties are highly tunable via tuning and optimizing the methods used to synthesize the nanoparticles. This thesis is concerned with the organic solution-phase synthesis, characterization and application of metallic nanoparticles for biomedical applications. The focus is on two classes of nanoparticles; magnetic and plasmonic. Magnetic nanoparticles are emerging as ultra-efficient MRI agents for early-stage imaging of cancers, as alternatives to materials currently used for tumour imaging. Plasmonic nanoparticles have been widely applied in photothermal therapy, which is a therapy type in which nanoparticles are heated via laser light and kill tumour DNA through heat-mediated stress.

Chapter 1 describes the theoretical principles of nanoparticle synthesis, the concepts of magnetic and plasmonic nanoparticles, and describes the biomedical applications which were investigated in this research. Chapter 2 describes in detail the experimental methods and characterization techniques which were used in this thesis, with relevant examples from either the literature or this research to illustrate their purpose.

Chapter 3 describes experiments performed in order to synthesize iron nanoparticles with tunable size and magnetic properties. Seed-mediated synthesis was used as a synthetic route to controlling the size, using the scalable and low-toxicity iron (II) complex Fe(C₅H₅)(C₆H₇) as a precursor material. The surfactant concentration during growth stage was found to have the highest impact on quality of nanoparticle growth. Once optimal reaction conditions were identified for seed-mediated growth of the nanoparticles, the synthetic protocol was performed several times on the same nanoparticles, showing size increase of the nanoparticles at each step. Subtle changes in iron nanoparticle size resulted in highly pronounced changes in nanoparticle magnetism, and a range of magnetic behaviours was observed, from superparamagnetic to ferromagnetic.

Chapter 4 carries on from the work in Chapter 3, and investigates the surface coating of iron nanoparticles with a polyelectrolyte coating, in order to perform phase transfer into aqueous media for MRI application. The polyelectrolyte coating was performed via a facile sonication process, which displaced hydrophobic surfactant molecules from the surface of iron/iron oxide nanoparticles, to which the phosphonate-grafted polymer PolyM3 was then bound. The PolyM3 polymer was able to bind multiple nanoparticles into one “assembly”. When PolyM3 with different molecular weights (i.e. different chain lengths) was used to coat the nanoparticles, their performance in T₂-weighted MRI was found to be proportionate to the chain length of the polymer used to stabilize the nanoparticles. It was concluded that larger-chain length polymer coatings were better at stabilizing the magnetic core of iron/iron oxide nanoparticles, and preserving their magnetic properties accordingly.

Chapter 5 described experiments performed with the aim of synthesis of core/shell iron/palladium nanoparticles, via seed-mediated synthesis on pre-synthesized iron nanoparticles. The coating of the reactive iron nanoparticles with a noble metal palladium shell was predicted to protect the magnetic properties of the iron core, which could enhance their performance in biomedical applications requiring magnetic nanoparticles. Experiments were performed using Fisher Porter bottle reactions, which were only successful in growing small palladium islands on the iron nanoparticle surface. Experiments were then performed based on a transmetallation process described in the literature, and nanoparticles showed a morphology indicating a crystalline iron core with a polycrystalline palladium coating. The major driving forces for the palladium shell formation were thought to be thermal decomposition of a palladium-trioctylphosphine complex, followed by Pd(II) reduction to Pd(0) by iron atoms at the surface of the iron seed particles.

Chapter 6 investigated the synthesis and application of novel, plasmonic palladium-gold nanoparticles in photothermal therapy. This chapter expanded preliminary work performed by Dr. John Watt and Lucy Gloag. Palladium-gold nanoparticles were synthesized with gold nanoparticles grown of branched palladium seeds. The size of the gold nanoparticles was tunable with the amount of precursor added. A growth mechanism for palladium-gold nanoparticles was proposed based on several control experiments carried out to determine the dependence of the synthesis on various reaction parameters. The nanostructures with larger gold nanoparticles showed much better heating performance under an 808 nm continuous-watt laser than for those with smaller gold nanoparticles. The heating performance of the nanostructures under laser irradiation and their biocompatibility after coating with poly(ethylene glycol) was investigated, in order to inform protocols for their application in therapy. Part of the work from this chapter has been published in the journal ACS Nano.

Chapter 7 describes general conclusions reached from the research performed in this thesis, including insights gained on seed-mediated nanoparticle growth and the control over physical properties of the nanoparticles synthesized. Ideas for future experiments to optimize nanoparticles’ design, preparation or application are proposed, based on the observations and results from this research.