Highly-efficient luminol immobilization approach and exponential strand displacement reaction based electrochemiluminescent strategy for monitoring microRNA expression in cell. 
This work used 3,4,9,10-perylenetetracarboxylic acid-luminol composite (PTCA-luminol) as signal tag with improved ECL signal and applied cruciform DNA structure mediated exponential strand displacement reaction (SDR) to construct an ultrasensitive electrochemiluminescence (ECL) biosensor for microRNA-21 (miRNA-21) detection.  The novel luminol-based signal tags was synthesized utilizing the π-π stacking interaction between PTCA and luminol, realizing highly effective and stable immobilization of luminol and resulting in good stability and strong ECL response.  Meanwhile, target miRNA-21 triggered disaggregation of cruciform DNA structure was used to mediate exponential SDR for target recycling amplification.  Taking advantage of the novel luminol-based signal tag and exponential SDR, the proposed ECL biosensor achieved excellent sensitivity with wide linear range from 10 aM to 100 pM and detection limit was 2 aM. Moreover, this ECL biosensor was applied to estimate the expression level of miRNA-21 and pharmacodynamics of matrine in human breast cancer cells (MCF-7 cells).  The proposed biosensor provided a new opportunity for the preparation of ECL nanomaterials and exhibited great application potential in other biomarkers detection, clinical application and pharmacodynamics evaluation.