A nanoparticle is defined as a particle which is less than 100 nm at least in one dimension. Nanotechnology enables the integration of entities with complementary or tailored properties into a single functional unit. In this thesis, we demonstrate how nanoprobes can be rationally designed to target and image biomedical structures and sense intracellular pH.

Cerium is biocompatible element and cerium oxide (CeO_x) nanoparticles are widely used in biomedical applications; however, they do not inherently generate a magnetic resonance (MR) signal. Gadolinium provides excellent contrast in magnetic resonance imaging (MRI) and is extensively used in clinical practice. Both cerium and gadolinium-based materials can also provide contrast in computed tomography (CT).

In this work, we aim to develop the next-generation dual-mode contrast agents for combined MRI and CT imaging by incorporating gadolinium into the cerium oxide lattice. In paper I we focus on the nanoparticle core and provide thorough characterization of a cerium oxide nanoparticle doped with gadolinium. We synthesized cerium oxide nanoparticles containing 5-20% gadolinium within the crystal lattice and evaluated their ability to enhance MRI contrast via relaxivity measurements. The resulting nanoparticles exhibit higher relaxivity than commercially used contrast agents.

The aim of paper II is to develop a nanoprobe capable of tracking the pH fluctuations that naturally occur in lysosomes. We synthesize SiO_x shell nanoparticles loaded with pH sensitive fluorophores. The emission wavelength of the first fluorophore falls within the excitation range of the second fluorophore, enabling Förster resonance energy transfer. The second fluorophore has a ring structure that will open at low pH, this makes the compound fluorescent. By measuring the ratio between the two emission maxima we can determine the pH. The cellular uptake of the pH sensitive nanoprobes is significantly increased using a cyclic disulfide.

In paper I, we show that there is great potential for cerium oxide nanoparticles with integrated gadolinium to act as contrast agent in both CT and MRI. Our aim in paper III is to further develop nanomaterials with enhanced targeting capabilities for contrast agent applications. Here we use the CeO_x core with 5% Gd to create epidermal growth factor receptor targeting nanoprobes. Poly acrylic acid (PAA) is used as a capping agent to provide colloidal stability and biocompatibility. The PAA is prefunctionalized with a fluorophore and a clickable moiety. The nanoparticles were then conjugated to the monoclonal antibody cetuximab via click chemistry. The nanoprobes were evaluated with respect to core and coating characteristics as well as targeting efficacy and cellular uptake.

The aim of paper IV is to explore a novel technique that could enable future upscaling of the nanoparticle synthesis. We use the same nanoparticle formulation as in paper I and further coat them using plasma enhanced chemical vapor deposition (PE-CVD). This creates multi-core nanoparticles with an organic coat. The obtained particles are thoroughly characterized and further functionalized using hydrazide and click chemistry. The polymeric thin film obtained using PE-CVD is characterized with respect to position in reaction chamber, electrode size and shape using Au surfaces. We show that using a small bottom electrode and positioning the samples close to the reaction chamber opening enables the incorporation of ketones in the organic matrix.

Thrombin is a multifunctional enzyme involved in the coagulation of blood. One role of thrombin is to bind and activate blood platelets to induce blood coagulation. As thrombin has a central role in blood coagulation it is a common target for anti-coagulant treatment. Paper V investigates the effect of direct thrombin inhibitor dabigatran on thrombin platelet binding. Thrombin contains two exosites that mediate interactions with multiple targets, including platelets. Herein, we show that although dabigatran attenuates thrombin binding to platelets, the binding affinities of exosites I and II are unaffected. Demonstrating the complexity of thrombin binding and that the exosites likely require synergistic binding for the thrombin-platelet interaction.

Characterization in this thesis is performed using dynamic light scattering (DLS), infrared spectroscopy (IR), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), fluorescence spectroscopy and microscopy and surface plasmon resonance (SPR).

In summary, this thesis explores a series of nanoparticle syntheses and nanoprobe formulations to develop tools for targeted biomedical imaging.