MicroRNAs　(miRNAs),　a　kind　of　endogenous,　noncoding　RNAs　(19-24　nucleotides),　play　vital　roles　in　regulating　gene　expression　and　cellular　processes.　In　recent　years,　it　has　been　found　that　circulating　miRNAs　are　differentially　expressed　in　patients　and　healthy　controls.　This　leads　to　the　suggestion　that　circulating　miRNAs　are　promising　biomarkers　for　cancer　classification　and　prognosis.　However,　it　is　still　difficult　to　detect　circulating　miRNAs　directly　from　real　samples　such　as　human　serum　without　prior　extraction　and　purification.　In　this　work,　we　developed　an　ultrasensitive　electrochemical　biosensor　for　detection　of　cancer-associated　circulating　miRNAs　based　on　DNA　concatamers　amplification.　The　proposed　biosensor　showed　a　high　sensitivity　for　target　miRNA-21　in　a　concentration　range　from　100　aM　to　100　pM　with　a　detection　limit　of　100　aM.　Furthermore,　the　biosensor　was　successfully　employed　for　direct　detection　of　circulating　miRNAs　in　human　serum.　Due　to　the　high　sensitivity,　good　selectivity　and　stability,　the　proposed　electrochemical　biosensor　might　have　potential　clinical　application　for　circulating　miRNAs　in　relation　to　diagnosis　and　prognosis.　(C)　2013　Elsevier　B.V.　All　rights　reserved.