Both　long　wavelength　excitation　and　emission　are　highly　desired　for　fluorescent　carbon　dots　in　biorelated　applications.　According　to　semiconductor　theory,　heteroatom　doping　is　an　effective　way　to　tune　the　band　gap　of　a　semiconductor.　In　this　work,　we　developed　an　exceedingly　simple　and　mild　one-pot　method　to　synthesize　selenium　and　nitrogen　co-doped　carbon　dots　(Se&N-CDs)　by　using　low　cost　citric　acid,　ethane　diamine　and　selenium　powder　as　the　starting　materials.　After　doping　the　Se　element　into　N-CDs,　the　absorption　of　Se&N-CDs　was　expanded　from　UV　to　the　visible　light　region　(similar　to　497　nm).　More　important　is　that　the　Se&N-CDs　show　a　strong　green　light　emission　with　a　high　quantum　yield　of　B52%,　attributed　to　the　new　Se-state　introduced　in　the　Se&N-CDs　due　to　the　low　electronegativity　of　Se.　Furthermore,　we　successfully　demonstrate　that　Se&N-CDs　have　an　excellent　performance　as　a　dye　for　fluorescein　fundus　angiography　(FFA).　After　injection,　a　clear　vascular　image　in　the　retina　and　detailed　images　of　the　capillary　bed　were　exhibited　in　the　FFA　image.　This　will　provide　a　great　advantage　for　bio-imaging　and　diagnosis.