Biocompatible inorganic nanocrystals for fluorescence and CT imaging

The enhancement of existing and development of new nanoparticular probes is a promising field of research, emanating from the increasing demand on nanoparticular contrast agents for luminescence and CT imaging. The first developed system is an ultra small inorganic semiconductor based nanoparticle with unique optical properties, ranging from size tunable emission to very high photostability. The second investigated nanoparticulate system is a lot less prominent on the field of molecular imaging till now. Nevertheless, the inherent properties of gadolinium oxide nanoparticles are as unique as those of the quantum dots. These nanoparticles are not only applicable in fluorescence imaging, but open up the way to combined multi-modal imaging and therapy. Following this evolution, herein, an organometallic synthesis for long-wavelength emitting quantum dots with an effective surface passivation of the semiconductor core was developed. The synthesis parameters were optimized with respect to the emission wavelength and quantum yield. Moreover the most prominent influence factors could be dedicated. The further modification of these particles rendering them water-soluble was achieved using different surface ligands. The developed procedure allowed the preparation of long-term stable, water-soluble quantum dots in a simple ligand exchange reaction. Furthermore, it could be shown that the coated nanocrystals have very small diameters of sub-10 nm. Moreover, the functionalization of the nanocrystal surface with amino groups, and therefore the prerequisite for further modification with e.g. targeting functions, was achieved. Furthermore, a way to tune the amount of amino ligands, and hence, functionizable groups on the surface was developed. The examination of the cytotoxicity of as well the quantum dots, the ligands, as free cadmium ions, showed that the observed toxicity can be attributed to the attached ligands, and not to the released cadmium ions. However, the tested dilutions of quantum dots exceeded the usually applied ones about three orders of magnitude, suggesting that the quantum dots can be regarded as non toxic for cellular experiments. Therefore, this preparation method turned out to be an easy way to stable biocompatible and functionizable quantum dots for biomedical applications. Concerning the second nanoparticulate system, two different routes were applied o synthesize those particles. One route yields bigger particles for CT contrast agent use only, the other synthesis procedure leads to particles for bimodal fluorescence- and CT-imaging. The “polyol” mediated synthesis yielded nanoparticles of bigger sizes. However, the coating step was already included into the particle formation. Herein, four ligands giving different particle properties were applied. All those particle species were investigated in terms stability after high pressure homogenization, freeze drying and redispersion in different media, to fabricate a stable, storable and redisperable powder for CT imaging applications. One of those as-coated nanoparticle systems was investigated in an animal experiment, resulting in very good contrast properties in vivo. The precipitation synthesis, on the other hand, gave smaller nanoparticles with very good luminescent properties. The luminescence was achieved by doping the nanocrystals with different lanthanide elements, yielding blue, green and red emission wavelengths. The parameters for optimal doping amount and annealing temperature were investigated. Moreover, the influence of the reaction time, reducing agent concentration and overall concentration on the nanoparticle size was studied. Therefore, highly luminescent and, independently size tunable, particles could be successfully prepared. The coating of these nanocrystals was achieved using a bisphosphonate functionalized PEG ligand, giving very good stabilization against aggregation of the particles. These coated particles were applied in an in vitro CT experiment, testing the X-ray attenuation of the nanocrystals in aqueous environment. This resulted in a very good CT contrast of about 60 HU per milligram of gadolinium. The testing of the cytotoxicity of all coated particles proved that there is no observable toxicity. Therefore, a successful preparation of stable, biocompatible, size-tunable, luminescence-tunable and CT active nanoparticles was achieved.