Nanoparticles are characterized by a particularly large ratio between coordinatively unsaturated surface atoms and atoms inside the particle. This gives them a reactivity that is similar to that of a molecule. In addition, nanoparticles have physical properties (magnetism, absorption) which, depending on their size, are located between the solid and the molecule. The chemical reactivity can be used to change the shape and crystalline phase of the nanoparticles via concentration, temperature, reaction time, and pressure. The choice of stabilizer (so-called capping agents) is of elementary importance for the long-term stability of the nanoparticles in dispersion since otherwise, the Ostwald ripening can lead to an increase in the size of the seeds with subsequent sedimentation. We investigate the influence of the above-mentioned synthesis parameters on the shape-property relationship of the nanoparticles and study their application in biomedical applications, i.a. for cancer therapy and diagnosis.