Single-molecule-nanometal pairs for resonance energy transfer sensing of live-cell events: Principles, progress, and prospects

Light-matter interaction commonly acts as an optical switch for sensitive molecular sensing, and its photophysics is essentially a process of energy conversion or transfer. Resonance energy transfer (RET) is related to the transfer of energy from an excited donor (D) to an acceptor (A). Beyond the classical Förster RET (FRET), the nanometal-involved RET models, such as NSET, PRET, and MEF, exhibit distinctive distance-dependent photophysics, enabling advanced applications in tracking, imaging, and sensing. A large gap remains to be filled between the state-of-the-art new RET strategies and their systematic differences, connections, challenges, and opportunities. Here, this review systematically decouples these emerging RET mechanisms, examining their principles, efficiencies, and analytical performance, with emphasis on single-molecule donor or acceptor contexts. It covers advancements from solution-phase studies to in-cell and in vivo systems, highlighting capabilities in resolving single-cell, single-particle, and single-molecule events, including target identification, spatial distances, binding conformations, and subunit interactions. We also discuss the challenges of these RET analytical strategies and try to figure out the trends of future developments regarding key problems of measuring spatial resolution, dynamic range, stability, accuracy, and photobleaching. Finally, we propose innovative RET-based modeling strategies, highlighting their transformative potential across the interdisciplinary fields of biology, chemistry, and spectroscopy.