The present invention provides a new and improved oligonucleotide detection method based on the nanopore technology with a probe containing a complementary sequence to the target oligonucleotide and a terminal extension at the probe's 3′ terminus, 5′ terminus, or both termini. The improved nanopore sensor with the probe enables sensitive, selective, and direct detection, differentiation and quantification of target oligonucleotides such as miRNAs. The inventive detection method may also be employed as a non-invasive and cost-effective diagnostic method for cancer detection based on miRNA levels in the patient's blood sample.