Disclosed herein is a nanosensor of miRNA activity in a target cell, and methods of use, for detection and diagnostic applications. The nanosensor comprises a delivery particle comprising an iron oxide crystal coated with a polymer； and a sensor oligonucleotide covalently attached to the polymer. The sensor oligonucleotide comprises a seed region, comprising a nucleic acid sequence that is completely complementary to the target miRNA and comprises a cleavage site which can be engaged and cleaved by the target miRNA.
