The project aims to optimize the methodology of Correlative Light Electron Microscopy (CLEM), starting with imaging of living cells using a confocal scanning laser microscope, through fixation of samples, preparation of ultrathin sections of selected cells, and correlation imaging of the nanoparticles inside ultrastructures of cells using a transmission electron microscope. Two types of inorganic nanoparticles of similar size but with different chemical composition and morphology, quantum dots and gold nanoparticles will be used as research models. This will allow the determination of the nanoparticle migration pathway inside cancer cells at the ultrastructural level. In addition, selected types of nanoparticles will be characterized in detail in terms of size, structure, chemical composition, and biocompatibility, which, correlated with the analysis of the penetration mechanism and their precise intracellular location, may contribute to the development of new low-toxicity biomarker systems.