We designed a motif composed of a stem-loop probe (SP) hybridized to a fluoresecently labeled universal reporter (UR) for sensing unlabeled nucleic acids. At room temperature, SP-UR is in the closed-hairpin form in which the fluorophore of UR is in proximity to the G bases of the hairpin, where consequently the fluorescent emission is significantly quenched. Upon hybridization with target, SP-UR is trapped in the hairpin-opened configuration in which the fluorophore and the G-quenchers are apart. This turns off quenching, increases emission intensity, and signals the presence of target. Compared to the common approach that employs an oligonucleotide probe with a covalently linked fluorophore, the ultilization of a fluorescently labeled universal reporter strand hybridized to an unlabeled stem-loop probe provides a more efficient approach to the fabrication of nucleic acid sensors and microarrays.