Compositions and methods for determining the origin location of a subterranean sample are provided. Compositions include a nanoparticle tag including a superparamagnetic iron oxide core, an intermediate layer including a fluorescent dye, and a polymer shell. The nanoparticles can be synthesized by functionalizing a superparamagnetic iron oxide nanoparticle core and covalently bonding a fluorescent dye to the functionalized nanoparticle core. In some implementations, a polymer is covalently bonded to the functionalized, fluorescent superparamagnetic iron oxide nanoparticle core. The nanoparticle tag can be used to determine the origin location of a subterranean sample by mixing the nanoparticle tag into a fluid, flowing the fluid into a subterranean formation, recovering subterranean samples from the subterranean formation, and separating tagged samples from untagged samples using a magnet. The origin location of the subterranean sample can be determined by analyzing the fluorescent signal of the nanoparticle tag.