Tracers for oil recovery, particularly fluorescent nanotracers conservative in aqueous phases. The tracer comprises a core-shell nanoparticle tailored according to the operation to be traced. It contains a fluorescent core that allows the detection thereof in the field and a functionalized polymeric shell that provides increased stability in high salinity aqueous phases. A method for preparing said nanotracer. Given the nanotracer versatility, it can be used both for tracing fracking steps as well as meshes of secondary and tertiary recovery.