Herein,　a　novel　immunosensing　strategy　was　designed　for　the　sensitive　fluorescence/visual　detection　of　alpha-fetoprotein　(AFP),　by　combining　the　enzyme-controlled　formation　of　polydopamine　(PDA)　nanoparticles　with　the　dissolution　of　manganese　dioxide　(MnO2)　nanoflakes.　Initially,　a　sandwich-type　immunoreaction　was　fabricated　on　a　capture　antibody-coated　microplate　by　using　detection　antibody/ascorbate　oxidase-conjugating　gold　nanoparticles　(AOX-AuNP-Ab2)　as　the　detection　probe.　The　ascorbate　oxidase,　carried　within　the　immunocomplex,　oxidized　ascorbic　acid　(AA)　to　dehydroascorbic　acid　(DAA)　to　inhibit　the　dissolution　of　the　MnO2　nanoflakes.　This　resulted　in　an　increment　in　fluorescent　PDA　nanoparticles　from　the　oxidation　of　dopamine　(DA)　into　its　quinone　derivative　with　the　aid　of　the　MnO2　nanoflakes.　Under　optimal　conditions,　the　fluorescence　intensity　of　the　PDA　nanoparticles　increased　with　increasing　AFP　concentration　within　the　dynamic　working　range　of　0.05-20　ng　mL-1,　with　a　detection　limit　(LOD)　of　17.3　pg　mL-1.　The　sensor　also　showed　high　specificity　and　acceptable　accuracy　for　the　detection　of　target　AFP　against　other　potential　cancer　biomarkers.　Importantly,　this　study　demonstrated　for　the　first　time　that　fluorescent　PDA　nanoparticles　could　be　used　as　signal-generation　tags　and　applied　to　an　enzyme　immunoassay.　Although　this　methodology　focused　on　AFP　detection,　it　would　be　suitable　for　the　detection　of　other　cancer　biomarkers　if　the　corresponding　antibodies　were　employed.　©　2017　The　Royal　Society　of　Chemistry.