Molecular oxygen plays an important role in many of the chemical reactions involved in the synthesis of biological life. In this review, we explore the interaction between O-2 and silicon nanocrystals, which can be employed in the photosynthesis of singlet oxygen. We demonstrate that nanoscale Si has entirely new properties owing to morphological and quantum size effects, i.e., large accessible surface areas and excitons of variable energies and with well-defined spin structures. These features result in new emerging functionality for nanoscale silicon: it is a very efficient spin-flip activator of O-2, and therefore, a chemically and biologically active material. This whole effect is based on energy transfer from long-lived electronic excitations confined in Si nanocrystals to surrounding O-2 via the exchange of single electrons of opposite spin, thus enabling the spin-flip activation Of O-2. Further, we discuss the implications of these findings for physics, chemistry, biology, and medicine.