Nanodiamonds for biological applications: Synthesis by laser ablation and sensing of local magnetic environment by optical spectroscopy of NV centers

Nanodiamonds (NDs) are the subject of intense investigation for their unique physical and chemical properties. Due to high hardness, optical transparency and biocompatibility, NDs find applications in tribology, catalysis and drug delivery. When enriched with nitrogen-vacancy (NV) centers, NDs can be used in bioimaging and biosensing. While the field is progressing rapidly, a number of problems are still open. In this dissertation I have tackled two important aspects for the development of NDs as biosensors: 1) production of NDs with controlled size and properties; 2) characterization and optimization of commercial fluorescent NDs as probes of paramagnetic species. In the first part of my thesis, I report a novel synthesis route for NDs by pulsed laser ablation (PLA) in water. PLA can directly produce diamonds on a nanoscopic scale, with potential advantages over alternative methods, like grinding of bulk crystals or detonation techniques. Specifically, I demonstrate synthesis of nanometric diamond crystals by PLA in an aqueous environment and investigate the thermodynamics of this process. Indeed, the synthesis of NDs by PLA is related to a drastic change in the thermodynamic state of the target upon laser irradiation. Fast laser-induced heating results in melting and superheating of the target, followed by a strong boiling, a process named “phase explosion†, and then by a fast cooling of the molten material in water. I provide a theoretical description of both superheated and undercooled liquids and of the mechanism of phase explosion. The investigation of the link between the metastable liquids (superheated or undercooled) and the synthesis of nanoparticles is carried out by theoretical analyses, computer simulations and comparison of our experimental data with previous literature. In the second part of the thesis I turn to commercial NDs enriched with (NV) centers. The purpose of the investigation is to explore the use of fluorescent NDs for sensing of paramagnetic species of biological interest. To this end, I explored the effects of size and surface coating on the optical properties and sensing capabilities of fluorescent NDs. Following a theoretical introduction to the basic properties of the NV centers and to the ground state spin dynamics of these color centers, I describe the set up used for the experimental characterization of the NDs. All NDs used in my experiments, characterized by different sizes and coatings, presented high fluorescence levels, the result of a relatively high concentration of NV center. In all NDs, I observed a fast loss in coherence due to interactions between the NV centers and with the external environment. The most striking and unexpected result concerns the dynamics of the spin-lattice relaxation time T1. Differently from previous reports, spin dynamics after polarization of NV centers could not be described by a single exponential decay, but showed a sharp signal increase that I attribute to charge dynamics and charge conversion between the negative and neutral forms of the NV center. Unexpectedly, I found that coupled charge and spin dynamics are strongly affected by paramagnetic interactions, yielding unprecedented sensitivity to subnanomolar concentrations of gadolinium, a strong paramagnetic contrast agent. The connection between relaxation dynamics and concentration of paramagnetic species can open new perspectives in biosensing and in bioimaging. As a demonstration of a practical application, I tested the sensitivity of NDs in the detection of deoxyhemoglobin, an endogenous paramagnetic species in blood.