A system is provided which includes nanoparticle conjugates configured to bind with a tumor cell, the nanoparticle conjugate comprising a nanoparticle, at least one targeting entity bound to the nanoparticle, and at least one shielding entity that shields at the at least one targeting entity, the nanoparticle, or both; a body-mountable device mounted on an external surface of a living body and configured to detect a tumor cell binding response signal transmitted through the external surface, wherein the tumor cell binding response signal is related to binding of the nanoparticle conjugates with one or more tumor cells; and a processor configured to non-invasively detect the one or more tumor cells based on the tumor cell response signal. Nanoparticle conjugates and methods for use for treating or imaging tumor cells are also provided.