MicroRNAs　(miRNAs)　have　attracted　enormous　interest　due　to　their　involvement　in　complex　regulatory　functions　and　for　being　closely　linked　to　some　cancers.　Thus,　qualitative　and　quantitative　detection　of　miRNAs　has　become　an　urgent　necessity.　In　this　study,　we　report　an　enzyme-free　fluorescent　biosensor　for　miRNA-21　detection　based　on　MnO2　nanosheets　and　catalytic　hairpin　assembly　amplification.　Results　showed　that　the　detection　limit　for　miRNA-21　was　0.33　nM　(S/N　=　3)　and　the　linearity　range　was　from　1　to　50　nM.　Compared　with　traditional　methods,　such　strategy　is　convenient,　rapid,　and　low-cost　and　it　can　have　a　promising　potential　for　miRNA　detection　in　cancer.　©　The　Royal　Society　of　Chemistry.