Multifunctional　nanocomposites　have　the　potential　to　integrate　sensing,　diagnostic,　and　therapeutic　functions　into　a　single　nanostructure.　Herein,　we　synthesize　Fe3O4@polydopamine　core-shell　nanocomposites　(Fe3O4@PDA　NCs)　through　an　in　situ　self-polymerization　method.　Dopamine,　a　melanin-like　mimic　of　mussel　adhesive　proteins,　can　self-polymerize　to　form　surface-adherent　polydopamine　(PDA)　films　onto　a　wide　range　of　materials　including　Fe3O4　nanoparticles　used　here.　In　such　nanocomposites,　PDA　provides　a　number　of　advantages,　such　as　near-infrared　absorption,　high　fluorescence　quenching　efficiency,　and　a　surface　for　further　functionalization　with　biomolecules.　We　demonstrate　the　ability　of　the　Fe3O4@PDA　NCs　to　act　as　theranostic　agents　for　intracellular　mRNA　detection　and　multimodal　imaging-guided　photothermal　therapy.　This　work　would　stimulate　interest　in　the　use　of　PDA　as　a　useful　material　to　construct　multifunctional　nanocomposites　for　biomedical　applications.　©　2014　American　Chemical　Society.