Carbon-based　dots　(CDs)　exhibiting　excellent　fluorescence　(FL)　properties　have　been　well　applied　in　sensing.　In　particular,　many　FL　sensors　based　CDs　have　been　developed　for　various　metal　ions.　However,　the　sensors　based　on　the　CDs　without　special　functionalization　usually　show　poor　selectivity　and　sensitivity.　In　this　work,　cobalt(II)　ion　(Co2+)　is　taken　as　the　model　to　introduce　a　novel,　sensitive　and　selective　FL　sensing　mode　for　metal　ions　based　on　CDs　without　special　functionalization.　A　kind　of　highly　luminescent　nitrogen　and　sulfur　co-doped　CDs　(N,S-CDs)　are　used　as　the　probes.　The　FL　signal　of　N,S-CDs　is　insensitive　to　Co2+　itself,　but　is　quenched　sensitively　and　selectively　by　Co2+　in　the　presence　of　cysteamine.　Actually,　cysteamine　reacts　with　Co2+　to　form　a　positively　charged　and　absorbent　complex　(Co(cys)(3)(2+)),　whose　absorption　band　covers　completely　the　emission　spectrum　of　N,S-CDs.　Then,　a　FL　resonance　energy　transfer　(FRET)　system　is　formed,　in　which　N,S-CDs　act　as　the　donor　and　Co(cys)(3)(2+)　acts　as　the　acceptor.　Furthermore,　the　electrostatic　interaction　between　the　positivity　charged　Co(cys)(3)(2+)　and　the　negatively　charged　N,S-CDs　increases　the　collision,　enhancing　the　FRET　effects.　As　a　result,　the　FL　signal　of　N,S-CDs　can　be　dynamically　quenched　by　Co(cys)(3)(2+).　Finally,　a　sensitive　and　selective　FL　sensor　is　developed　for　Co2+.　More　importantly,　this　paper　provides　CDs　a　novel　FL　sensing　mode,　which　may　be　applied　in　many　other　metal　ions.　(C)　2016　Published　by　Elsevier　B.V.