There remains a great challenge in the sensitive and selective detection of microRNAs (miRNAs) in cells. In this work, a label-free and enzyme-free assay for ultrasensitive detection of miRNAs using polyethylenimine (PEI)/SiO2 nanoparticles has been developed. The PEI/SiO2 nanoparticles, which serve as an indicator to discriminate availably between single-stranded DNA (ss-DNA) and double helix structure DNA (ds-DNA), are self-assembled on the Nafion/carbon nanotubes (CNT)/Ru(bpy)(3)(2+) modified electrode surface by supramolecular effect. What is more, the polymer PEI in the composite nanoparticles has a remarkable enhancing effect on the electrochemiluminescence (ECL) signals of Ru(bpy)(3)(2+) immobilized in advance on the Nafion/CNT modified electrode surface by electrostatic interaction. In addition, the enhancement of ECL signals has exactly good linear relationships with the concentration of Let-7a in miRNAs. Finally, this approach paves a way for simple and convenient ECL detection of miRNAs in 116 Colon Cancer Cells with very high sensitivity and good selectivity. (C) 2016 Elsevier B.V. All rights reserved.