A　nitrogen-doped　carbon　quantum　dots　(N-CQDs)-based　＂on-off＂　fluorescent　nanosensor　was　profitably　synthe-sized　through　one-pot　hydrothermal　treatment　of　citric　acid　and　o-phenylenediamine　for　the　determination　of　Fe3+　and　imaging　towards　cancerous　cells　in　vitro.　The　synthesized　N-CQDs　exhibited　excellent　photo-luminescence　and　favorable　biocompatibility.　The　excitation-dependent　fluorescence　emission　spectra　were　observed　and　the　maximum　emission　wavelength　was　483　nm　at　the　excitation　wavelength　of　420　nm.　The　fluorescent　N-CQDs　had　a　selective　and　sensitive　response　to　Fe3+　with　a　limit　of　detection　of　0.177　mu　M　and　an　allowable　detection　range　of　0-110　mu　M.　The　sensor　was　applied　for　the　determination　of　Fe3+　ions　in　tap　water　with　satisfactory　results.　Moreover,　the　N-CQDs　provide　a　practical　and　promising　method　for　cellular　imaging　due　to　their　excellent　fluorescence　potency　and　negligible　cytotoxicity.