In　this　work,　o-phenylenediamine　is　utilized　as　a　precursor　to　synthesize　the　fluorescent　emission　wavelength　switchable　carbon　dots　(o-CDs).　Our　investigation　reveals　that　ferrous　ions　(Fe2+)　can　effectively　induce　fluorescence　quenching　of　o-CDs　by　chelation　and　aggregation.　After　the　addition　of　hydrogen　peroxide　(H2O2),　the　fluorescence　of　o-CDs　recovers　and　the　fluorescent　color　changes　from　yellow　to　green.　As　far　as　we　know,　o-CDs　are　the　first　reported　CDs　with　switchable　fluorescence　emission　wavelength.　In　order　to　fabricate　an　enzyme-free　immunosensor,　an　amino-functionalized　dendritic　mesoporous　silica　nanoparticle　(DMSN)-gold　nanoparticle　(Au　NP)　nanostructure　was　fabricated　as　a　glucose　oxidase　mimetic　nanoenzyme　by　in　situ　coating　of　the　Au　NPs　on　the　surface　of　the　DMSNs.　Then,　the　functionalized　DMSN-Au　NPs　were　modified　on　the　detection　antibody　and　hydrolyzed　with　glucose　to　produce　H2O2.　This　immune　induced　recognition　strategy　combines　with　the　o-CDs+Fe2+　signal　generation　system　to　achieve　specific　and　sensitive　detection　of　the　target.　The　replacement　of　glucose　oxidase　by　DMSN-Au　NPs　not　only　reduces　the　cost　but　also　provides　significantly　amplified　signals　due　to　DMSNs　haing　a　high　specific　surface　area.　We　show　the　detection　of　carcinoembryonic　antigen　(CEA)　as　an　example　target　to　evaluate　the　analytical　figure　of　merits　of　the　proposed　strategy.　Under　the　optimal　conditions,　two-photon-based　o-CDs　displayed　excellent　performances　for　CEA　and　the　limit　of　detection　as　low　as　74.5　pg/mL　with　a　linear　range　from　0.1　to　80　ng/mL.　The　proposed　fluorescent　immunosensor　provides　an　optional　and　potential　scheme　for　low　cost,　high　sensitivity,　and　versatile　discovery　of　clinical　biomarkers.