Porous silicon (porSi), which contains luminescent Si nanocrystal assemblies, is a promising semiconductor material for the photosensitized formation of singlet oxygen, 1 O 2 , for biological applications. However, because as-prepared luminescent Si nanocrystals are H-terminated and therefore hydrophobic, the first step to create a prototype of Si nanocrystalbased photosensitizer should be a modification of the Si nanocrystal surface with the aim to make it hydrophilic and able to work in a biological ambient. Such surface modification by surfactants should not, however, result in a decrease of the nanocrystals' durability that may, in principle, take place because of surfactant-induced weakening of surface tension and the consequent increase of interaction with H 2 O molecules. Surfactants should not also decrease the ability of the nanocrystals to transfer the excitation energy to acceptors (first of all molecular oxygen) on their surface. These rather contradictory tasks have been largely solved in the present work by use of nonionic surfactants physically adsorbed on the porSi surface.One of the strategic objectives of nanotechnology is the development of new materials of nanometer size that have entirely new physical properties, and, therefore, new functionality. Over the last decade, tailoring of material characteristics by size control has been demonstrated for many types of semiconductors. Optical and electronic properties of semiconductor nanocrystals can be simply engineered by changing their size and composition. When electrons and holes are squeezed into a dimension that approaches a critical size, quantum-confinement effects become apparent. This effect can be seen experimentally as a widening of the semiconductor nanocrystal bandgap.[1]