Aptamers　have　many　advantages,　such　as　simple　synthesis,　good　stability,　high　binding　affinity　and　wide　applicability,　making　them　suitable　candidates　for　protein　detection.　Since　the　disease-related　protein　may　be　present　in　very　small　amounts　in　biological　samples,　the　development　of　amplification　paths　for　aptasensors　is　essential.　In　this　paper,　we　develop　a　simple　and　enzyme-free　amplified　aptasensor　for　protein　detection　via　target-catalyzed　hairpin　assembly.　This　aptasensor　contains　two　DNA　hairpins　termed　as　H1　and　H2.　H1,　which　is　modified　at　its　5＇　and　3＇　ends　with　a　fluorophore　and　a　quencher　respectively,　consists　of　the　aptamer　sequence　of　human　thrombin.　Meanwhile,　H2　is　partially　complementary　to　H1.　These　two　hairpins　H1　and　H2　interact　slowly　with　each　other.　Upon　the　addition　of　target　protein,　it　can　facilitate　the　opening　of　the　hairpin　structure　of　H1　and　thus　accelerate　the　hybridization　between　H1　and　H2,　resulting　in　the　significant　fluorescence　enhancement　of　the　system.　By　monitoring　the　change　in　fluorescence　intensity,　we　could　detect　the　target　protein　with　high　sensitivity.　The　detection　limit　of　this　aptasensor　is　20　pM,　which　is　more　than　two　orders　of　magnitude　lower　than　that　of　reported　unamplified　aptasensors.　Furthermore,　this　amplified　aptasensor　shows　high　selectivity　toward　its　target　protein.　Thus,　the　proposed　aptasensor　could　be　used　as　a　simple,　sensitive　and　selective　platform　for　target　protein　detection.　(C)　2012　Elsevier　B.V.　All　rights　reserved.