Electrochemiluminescence　(ECL)　integrates　the　advantages　of　electrochemical　detection　and　chemiluminescent　techniques.　The　method　has　received　particular　attention　because　it　is　highly　sensitive　and　selective,　has　a　wide　linear　range　but　low　reagent　costs.　The　use　of　nanomaterials　with　their　unique　physical　and　chemical　properties　has　led　to　new　kinds　of　biosensors　that　exhibit　high　sensitivity　and　stability.　Compared　to　other　nanomaterials,　DNA　nanostructures　are　more　biocompatible,　more　hydrophilic,　and　thus　less　prone　to　nonspecific　adsorption　onto　the　electrode　surface.　We　describe　here　a　label-free　and　ultrasensitive　ECL　biosensor　for　detecting　a　cancer-associated　microRNA　at　a　femtomolar　level.　We　have　designed　two　auxiliary　probes　that　cause　the　formation　of　a　long-range　self-assembly　in　the　form　of　a　mu　m-long　1-dimensional　DNA　concatamer.　These　can　be　used　as　carriers　for　signal　amplification.　The　intercalation　of　the　ECL　probe　Ru(phen)(3)　(2+)　into　the　grooves　of　the　concatamers　leads　to　a　substantial　increase　in　ECL　intensity.　This　amplified　sensor　shows　high　selectivity　for　discriminating　complementary　target　and　other　mismatched　RNAs.　The　biosensor　enables　the　quantification　of　the　expression　of　microRNA-21　in　MCF-7　cells.　It　also　displays　very　low　limits　of　detection　and　provides　an　alternative　approach　for　the　detection　of　RNA　or　DNA　detection　in　diagnostics　and　gene　analysis.