To label individual cells, organelles, organs, or smallest components, nanoscaled luminophores are introduced into cell structures. The challenge consists in developing luminescent nanoparticles, whose luminescence is so intensive that they can be detected in spite of their small size. KIT scientists have developed novel luminophores with a small particle diameter of less than 100 nm.In contrast to classical nanoluminophores like cadmium selenide, a derivative of vitamin B2, flavinmononucleotide (FMN) is used as an organic fluorescent dye. Embedded in an inorganic zirconium oxide matrix, FMN forms a hybrid compound of high luminescence. This hybrid compound can be equipped variably with more or less vitamin B corresponding to the chemical formula of ZrO(HPO4)n(FMN)n-1 (n = 0 - 1) from a few percent up to molar amounts. Although B vitamins generally are not soluble in water, synthesis of the new hybrid luminophores in aqueous medium is simple and rapid, which highly facilitates the process. In first tests, no toxic or allergic reactions in cells or test mice were observed. The nanoparticles are biocompatible. Excitation of the hybrid luminophore is also achieved by blue LED that does not damage the cell contrary to UV light. This makes the new luminophore interesting for medical applications. As large amounts of hybrid luminophore can be produced aggregate-free in water and can be redispersed, it can be applied on a large scale, for example as an additive in plastics, or a safety feature in paper, for labeling or for advertisements. The nano-sized luminophore becomes visible in case of the respective excitation (UV light or blue LED) only. At daylight, it appears completely transparent.