A method is described for modifying the surface of upconverting microparticles (UC mu Ps) and nanoparticles (UCNPs) such that they become amenable to click chemistry. Respective reagents are presented and used in both kinds of particles, either directly or in combination with tetraethoxysilane. The particles also were labeled by using the click reaction, a) with fluorophores to yield materials that have emission colors that depend on the wavelength of excitation; b) with maleinimido groups (so to obtain labels for thiols), and c) with biotin (to make them useful for affinity studies based on the biotin streptavidin system). We believe that both the UC mu Ps and UCNPs have the potential of being used in numerous areas including up-conversion imaging, biolabeling, and derivatization, but also in encoding and security.