The　synthesis　of　hydrophilic　lanthanide-doped　nanocrystals　(Ln3+-NCs)　with　molecular　recognition　ability　for　bioimaging　currently　remains　a　challenge.　Herein,　we　present　an　effective　strategy　to　circumvent　this　bottleneck　by　encapsulating　Ln3+-NCs　in　graphene　oxide　(NCs@GO).　Monodisperse　NCs@GO　was　prepared　by　optimizing　GO　size　and　core–shell　structure　of　NaYF4:Yb,Er@NaYF4,　thus　combining　the　intense　visible/near-infrared　II　(NIR-II)　luminescence　of　NCs　and　the　unique　surface　properties　and　biomedical　functions　of　GO.　Such　nanostructures　not　only　feature　broad　solvent　dispersibility,　efficient　cell　uptake,　and　excellent　biocompatibility　but　also　enable　further　modifications　with　various　agents　such　as　DNA,　proteins,　or　nanoparticles　without　tedious　procedures.　Moreover,　we　demonstrate　in　proof-of-concept　experiments　that　NCs@GO　can　realize　simultaneous　intracellular　tracking　and　microRNA-21　visualization,　as　well　as　highly　sensitive　in　vivo　tumor-targeted　NIR-II　imaging　at　1525　nm.　©　2019　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim