Covalent Surface Functionalization of Calcium Phosphate Nanoparticles with Fluorescent Dyes by Copper‐Catalysed and by Strain‐Promoted Azide‐Alkyne Click Chemistry

Abstract Spherical calcium phosphate nanoparticles with a solid core diameter around 90 nm (from scanning electron microscopy, SEM) were coated with a silica shell and then covalently functionalized by azide groups. To these azide groups, all kinds of alkyne‐carrying molecules can be covalently attached by copper‐catalysed azide‐alkyne cycloaddition (CuAAC) and by strain‐promoted azide‐alkyne cycloaddition (SPAAC) at a very high density. This was demonstrated for a number of dyes (FAM, TAMRA, Cy5, Alexa Fluor™ 488, and an aromatic thioether with aggregation‐induced emission (AIE) properties). It was also possible to attach more than one molecule to the surface of one particle by two‐step click reaction, permitting the synthesis of multimodal nanoparticles that are stable under biological conditions. The nanoparticles have a hydrodynamic diameter of around 200 nm (from dynamic light scattering, DLS), which makes them suitable for uptake by cells. The strongly fluorescing nanoparticles were easily taken up by cells as demonstrated by fluorescence microscopy, confocal laser scanning microscopy (CLSM), and structured illuminated microscopy (SIM).