An　electrochemiluminescence　(ECL)　based　biosensor　is　described　for　determination　of　microRNAs　in　the　A549　cell　line.　Firstly,　graphene　oxide　(GO)　is　dripped　onto　a　glassy　carbon　electrode　surface　to　form　an　interface　to　which　one　end　of　the　capture　probe　(with　a　stem-loop　structure)　can　be　anchored　through　-interaction　via　dangling　unpaired　bases.　The　other　end　of　the　capture　probe　is　directed　away　from　the　GO　surface　to　make　it　stand　upright.　Target　microRNAs　can　open　the　hairpin　structure　to　form　a　double-stranded　DNA-RNA　structure.　Two　auxiliary　probes,　generating　a　hybridization　chain　reaction,　are　used　to　elongate　the　DNA　duplex.　Finally,　doxorubicin-modified　cadmium　telluride　quantum　dot　nanoparticles　(Dox-CdTe　QD)　are　intercalated　into　the　base　pairs　of　the　hybrid　duplexes　to　act　as　signalling　molecules.　The　ECL　signal　of　the　Dox-CdTe　QD　increases　proportionally　with　the　concentration　of　microRNAs,　specifically　for　microRNA-21.　The　assay　covers　a　wide　linear　range　(1　fM　to　0.1nM),　has　a　low　detection　limit　for　microRNA-21　(1　fM),　and　is　selective,　reproducible,　and　stable.