Understanding the Invisible Handshake: FRET Microscopy and FRET-Based Assays in Molecular Diagnosis

Förster Resonance Energy Transfer (FRET) microscopy has evolved from a specialized biophysical assay into a versatile platform for investigating molecular interactions and dynamics at the nanometer scale. FRET exploits nonradiative energy transfer between donor and acceptor fluorophores to enable real-time detection of protein–protein interactions, conformational dynamics, and enzymatic activity—all of which lie at the heart of the molecular basis of disease. These capacities bring valuable insights into molecular mechanisms involved in disease pathogenesis. This review discusses the clinical diagnostic possibilities that FRET microscopy holds in oncology and infectious diseases concerning nucleic acid and antigen detection. We detail the main technological aspects of key advances, such as cutting-edge biosensors, ratiometric imaging, and fluorescence lifetime imaging microscopy (FLIM-FRET). The persistent clinical translation challenges posed by photobleaching, spectral overlap, sample heterogeneity, and instrumentation cost are critically discussed alongside promising future solutions such as near-infrared fluorophore pairs and multiplex FRET assays. In conclusion, FRET microscopy is a mature technique, but its clinical diagnostic application remains emerging and largely confined to research and preclinical settings. Still, its unique ability to report molecular-scale events directly in situ makes it an attractive addition to the diagnostic toolbox for early disease detection, particularly where conventional approaches lack sufficient sensitivity.