The present application is directed to the preparation and use of a class of nanoparticles that contain a single Quantum Confined dopant. A QCA nanocrystal comprises of a plurality of host atoms in a nanocrystal of a size of less than 10 nm with a single atom of a dopant (or activator). This single QCA dopant, when confined, becomes polarized and creates a large magnetic-moment in a nanosize host that contains atoms of unpaired spins. The quantum confined atom (QCA) which is now pinned, triggers the alignment of the host atoms resulting in nanosize magnetic domain. Engineering of nanomagnets based on QCA nanoparticles can be used in different applications such as: sensors, drug delivery, bio-tagging, cell/DNA tagging, magnetic memories and others.