Abstarct: Part A: Herein, a novel, rapid, highly sensitive, and selective fluorescent biosensor is presented, which is designed baxsed on “sandwich-type” hybridization of oligonucleotides and the fluorescence resonance energy transfer (FRET) strategy. It senses and determines the MicroRNA-155 (miRNA-155) exxpression levels as a cancer biomarker. In this study, a modified La(III)-mextal–organic frxamework(MOF) and silver nanoparticles (Ag NPs) were used as the energy donor–acceptor pairs in fluorescence quenching through the FRET process. La(III)-MOF was synthesized and then modified by glutaraldehyde as a cross-lixnking agent. The Ag NPs were also prepared, and then, the surface of both was conjugated with different 5′-amino-labeled ssDNA strands (aptamers). These prepared nanoprobes were characterized by various physicochemical techniques such as X-ray diffraction, energy-dispersive X-ray spectrometry, Fourier transform infrared, field emission scanning electron microscopy, UV–vis spectroscopy, elemental mapping, and gel electrophoresis. To optimize the detection conditions, several factors affecting biosensor performance were assessed by one variable-at-a-time and central composite design methods. Under optimum conditions, this “turn-off” fluorescent biosensor could detect and determine as low as 0.04 ppb (ng. mL–1) or 5.5 fM of the miRNA-155 biomarker. Therefore, this biosensor provides highly promising potential for lung and breast cancer diagnosis.