It　is　highly　desirable　and　a　great　challenge　for　red　light　emission　of　carbon　dots　under　long　wavelength　excitation.　Here,　we　developed　a　facile　route　to　synthesize　carbon　dots　with　red　emission　due　to　the　doping　effect　of　S　and　N　elements,　borrowing　from　the　concept　of　the　semiconductor.　The　maximum　emission　locates　at　594　nm　under　560　nm　excitation.　The　absolute　photoluminescence　(PL)　quantum　yield　(QY)　is　as　high　as　29%　and　22%　in　ethanol　and　water,　respectively.　XPS　and　FTIR　spectra　illustrated　that　there　exist　-SCN　and　-COOH　groups　on　the　surface　of　the　carbon　dots.　They　endow　the　carbon　dots　with　high　sensitivity　for　ion　detection　of　Fe3+.　The　quenched　PL　emission　of　Fe3+-S,N-CDs　can　be　recovered　by　adding　ascorbic　acid　to　release　the　-COOH　and　-SCN　group　due　to　Fe2+　formation　in　the　presence　of　ascorbic　acid.　High　PL　QY　of　red　emission　is　beneficial　to　application　in　bioimaging.　Doxorubicin　was　loaded　onto　carbon　dots　through　pi-pi　stacking　to　form　a　theranostic　agent.　When　the　CD-Dox　was　injected　into　the　tumor　site,　a　strong　PL　emission　was　observed.　The　PL　intensity　indicates　the　concentration　of　the　theranostic　agent.　After　7　times　injection,　both　the　tumor　size　and　weight　clearly　decrease.　The　results　demonstrate　that　the　S,N-CDs　are　a　potentially　excellent　bioimaging　component　in　the　theranostic　field.