Abstract: The present invention provides a TNA-based probe for detecting and imaging a target miRNA in living cells. TNA-based probe is composed of a fluorophore-labeled TNA reporter strand partially hybridizing to a quencher-labeled TNA recognition strand which is designed to be antisense to the target RNA transcript via pair pairing. Upon cellular entry without the need of harmful transfection treatment, the quencher-labeled TNA recognition strand binds to targeted transcript, and these target binding events displace the reporter strand from the quencher, resulting in a discrete “turning-on” of the fluorescence. The extent of fluorescence enhancement is quantifiably related to the target RNA expression level. Additionally, the TNA-based probe shows rapid detection response, excellent selectivity and specificity toward target miRNAs and is able to distinguish the target molecules with 1-2 base mismatches.