Magnetic control and manipulation of bio-functionalized nanocrystals

Abstract

The main goal of this thesis is the design and characterization of functional magnetic nanomaterials to understand the mechanism of crystal growth and particle formation, which will shape up their intrinsic magnetic properties. The very basic idea is to exert control over the synthetic procedure to obtain magnetic nanoparticles, their manipulation and interactions in solution, and their assembly with various materials to render composites with specific characteristics, towards their final application in the bio-related field. Thus, we can divide this thesis into two main parts: Part I: Synthesis and characterization of magnetic nanocrystals. Chapter 2 describes the mechanism of formation of octahedron-shaped core@shell CoO@Co3O4 nanoparticles with cavities. First, the chemical thermodecomposition of the cobalt precursor leads to the production of CoO nanooctahedra, in which either internal voids or cracks form within the nanostructures, depending on if the precursor is injected at a certain temperature or added together with the remaining reactants. A careful analysis of the intermediate steps allowed to determine the crystal growing. Additionally, Raman spectroscopy experiments have been done to prove the spontaneous formation of the Co3O4 layer. The core@shell morphology promotes a Kirkendall process, responsible of the formation of cavities inside the nanostructures, due to the solid-solid diffusion of the constituent atoms. Chapter 3 focuses on the synthetic approach to obtain magnetic clusters of either magnetite (Fe3O4) or manganese-doped magnetite (MnxFe3-xO4) nanoparticles, by means of a solvothermal method. Thus, through an appropriated characterization of the products obtained at different reaction steps using (HR)TEM, XRD, and Raman spectroscopy, we were able to assess the mechanism by which the clusters form. It is worth highlighting the role of Raman spectroscopy as a valuable tool to discern between different intermediate phases of the oxides before attaining the final clusters. Moreover, we have studied the role of poly(ethylene glycol) as a key parameter to control both the crystallite and cluster size, allowing for a large library of different cluster sizes, depending on the concentration and length of the polymer used. Part II: Potential applications of magnetic composites in biomedicine. Chapter 4 describes the synthesis of magnetic nanoswimmers using polystyrene spheres coated with a shell of magnetic nanoparticles as substrates. The term micro- or nanoswimmer refers to those structures able to propel themselves, taking advantage of physical stimuli (e.g., magnetic fields) or chemical processes involving molecular fuels. Thus, we have assembled magnetic-based composites, such that their magnetophoretic mobility can be calculated and their magnetic guidance can be assessed in several aqueous media (i.e., ultrapure water, a glycerol/water mixture, and cell extract). Furthermore, the composites were tested in cells and zebrafish models, with the aim to evaluate the motion performance in more complex environments. Chapter 5 reports the assembly of diverse enzyme-powered nanoswimmers. First, simple swimmers that use glucose-oxidase/Pt nanoparticles, or trypsin were assembled. The mobility of these swimmers was independently tested in different microenvironments. Simultaneously, magnetic swimmers were designed and the mobility performance was tested using different magnetic field strengths. Next, the glucose-oxidase/Pt nanoparticles- and trypsin-based swimmers were combined in a single structure, so that synergies stemming from the two catalytic systems and the magnetic functionality was accomplished. Chapter 6 details the fabrication of collagenase-powered swimmers, together with magnetic nanoparticles. In a first step, the capacity of delivering heat by means of the magnetic nanoparticles was assessed. Then, the motion performance fostered by the collagenase motor was evaluated in fluid and viscous environments. Interestingly, collagenase-based swimmers were able to penetrate collagen gels, as a good example of how these architectures can propel not only in aqueous environments, but also in more rigid viscous media. These swimmers were also incubated with 3D-cell spheroids, that produce collagen to sustain the cell construct, and they exhibited a good penetration ability through the collagen matrix. Eventually, the controlled heat delivery upon applying an alternating magnetic field reduced the viscosity of the medium inside these spheroids, as a proof-of-concept of magnetic hyperthermia applied to self-propulsion.