The goal of this research was to develop calcium phosphate nanoparticles and ultrasmall gold nanoparticles that were tailored to their intended application by target surface modification, using established methods of coupling chemistry. Due to their structural similarity to human hard tissue, calcium phosphate nanoparticles can be used as bone tissue mimics. By employing click chemistry and thiol-maleimide coupling, the surface of the electrostatically stabilized nanoparticle was loaded with peptides or molecules that are known to bind to the bone surface. In the first part of this work, a range of calcium phosphate nanoparticles were used to develop a bone adhesive for surgical applications. The basic needs of a surgically applicable bone adhesive include (I) sterility and biocompatibility, (II) stabilization of the fracture, (III) no adverse effects on the natural healing process of the bone, (IV) easy application to hard-to-reach areas and (V) biodegradability of the adhesive without the formation of toxic metabolites after the healing process is completed. Calcium phosphate is a suitable starting material, as it fulfills the basic needs due to its biocompatibility in bone contact. The electrostatic stabilization of the calcium phosphate nanoparticles was achieved by the polymers PEI (cationic) and CMC (anionic). To modify the surface, a silica shell was applied under alkaline conditions using TEOS. In the subsequent step, the surface was modified with thiols and azides in order to enable covalent conjugation. In a further reaction step, peptides or catechols were coupled by thiol-maleimide-coupling or a copper-catalysed azide-alkyne-cycloaddition. The functionalization of the nanoparticles with peptides and catechols was successfully achieved. Neither the CaP/CMC nor CaP/PEI/SiO₂-N₃-DHBA nor CaP/PEI/SiO₂-S-Peptide nanoparticles gave a successful bone glueing in water. In order to reduce cytotoxicity, PEI was substituted for CMC. In mechanical tests of CaP/CMC/SiO₂-N₃-DHBA- and CaP/CMC/SiO₂-S-Peptide nanoparticles, it was demonstrated that neither peptide- nor catechol-functionalized particles significantly contributed to the fracture stability. The CaP/CMC paste material exhibited swelling in water. Consequently, the system was modified. CaP/CMC nanoparticles were substituted by CaP/CMC/SiO₂ nanoparticles. The addition of a silica shell prevented the observed swelling. Given that the water-based paste did not show any adhesive properties in an aqueous environment, a nanoparticle-filled hydrogel system was developed. Agarose and gellane gum were challenging to process because of their rapid gelling. In contrast, alginate was easily applicable and crosslinked in a surgical reasonable time frame of a few minutes. When filled with CaP/CMC/SiO₂ nanoparticles, the material fulfills the fundamental requirement of adhesion to bone in an aqueous environment. The stabilization is attributed to the alginate. Since alginate alone does not demonstrate long-term adhesion in an aqueous environment, the incorporation of CaP/CMC/SiO₂ nanoparticles is essential for achieving this property. The rheological testing demonstrated that the ratio of liquid-to-nanoparticle powder (L/P-ratio) and the alginate concentration influence the mechanical stability of the hydrogel. It was shown that a L/P-ratio of 3.6 together with an alginate solution of 2 wt% yielded in the most stable network. Shear-thinning properties were found in the paste. This allows the application with a syringe. Examination of the structure recovery showed a 60 – 70% recovery of the G’ modulus in a couple of minutes following shearing at high deformation in a thixotropy test. The sterility and biocompatibility are given by the starting materials i.e., calcium phosphate and alginate, and the synthesis conditions. The fracture stabilization of the paste is probably sufficient for body parts with low mechanical loads. In the case of fractures with high mechanical loads, e. g. the femur, the bone adhesive could be supported by a bone narrow nail in order to provide further stabilization. The nanoparticles in an alginate solution can be mixed into a smooth paste, which can be applied through a syringe, which allows the application into hard-to-reach areas. Hence, three of the five necessary criteria for a bone glue have been met. The second part of this thesis concerns the synthesis of ultrasmall gold nanoparticles for the diagnosis and treatment of brain tumors. Ultrasmall gold nanoparticles have a size of approximately 2 nm and the ability to transport fluorescent dyes and pharmaceutically active agents across the blood-brain barrier. The colloidal stabilization of the gold nanoparticle was achieved by the ligand tiopronin, which has a high affinity for gold due to its thiol group. The carboxylic acid group of the tiopronin was used for surface modification via the EDC/NHS coupling reaction. The ultrasmall size of AuTio was confirmed by DCS and HRTEM. The number of ligands present on the nanoparticle surface was determined to be 170 by NMR-spectroscopy with an - 178 - internal standard. The downfield shift and the signal expansion as compared to the unbound tiopronin indicate the proximity of the ligand to the metallic particle core. Therefore, the synthesis was successful. The successful coupling of doxorubicin via a peptide bond was confirmed by 1H-NMR spectroscopy. With UV-Vis-spectroscopy approximately 18 doxorubicin molecules were found on the particle surface. Under identical reaction conditions five molecules of the fluorescent dye Alexa-Fluor-647 were covalently coupled. The simultaneous coupling of doxorubicin and Alexa-Fluor647 for the synthesis of a theranostic particle yielded the coupling of approximately one molecule of each compound on every nanoparticle. The particles were easily taken up by cells, as evidenced in cell imaging investigations.