Exosomal microRNAs (miRNAs) play critical regulatory roles in many cellular processes, and so how to probe them has attracted increasing interest. Here we propose an aptamer-functionalized dimeric framework nucleic acid (FNA) nanoplatform for effective capture of exosomes and directly probing internal miRNAs with electrochemiluminescence (ECL) detection, not requiring RNA extraction in conventional counterparts. A CD63 protein-binding aptamer is tethered to one of the FNA structures, allowing exosomes to be immobilized there and release internal miRNAs after lysis. The target miRNA induces the formation of a Y-shaped junction on another FNA structure in a close proximity state, which benefits the loading of covalently hemin-modified spherical nucleic acid enzymes for enhanced ECL readout in the luminol-H2O2 system. In this facile way, the ultrasensitive detection of exosomal miR-21 from cancer cells is accomplished and then used for cell apoptosis analysis, indicating that the oncogene miR-21 negatively participates in the regulation of the apoptotic process; namely, downregulating the miR-21 level is unbeneficial for cancer cell growth.