Concentric FRET: a review of the emerging concept, theory, and applications

Concentric Förster resonance energy transfer (cFRET) is an emerging concept for single-vector multiplexed bioanalysis and imaging. It features a network of competitive and sequential energy transfer pathways, which, to date, has been assembled with a central semiconductor quantum dot (QD) and biomolecular linkers to multiple copies of multiple types of concentrically-arranged fluorescent dyes. In this review, we provide a first-hand account of the concept and development of cFRET, starting from its place in the broader context of FRET probes and assemblies. Topics of discussion include materials for cFRET, with a focus on the enabling properties of QDs and the ideal properties of nominal acceptor dyes; characterization and analysis of cFRET configurations via photoluminescence intensity, emission ratio, lifetime, and photobleaching measurements; semi-empirical modeling to determine the rates and efficiencies of competitive and sequential FRET pathways from overall quenching efficiencies; and archetypical examples of cFRET configurations and their application in bioanalysis and imaging. Most of the latter examples demonstrate multiplexed detection of protease activity or nucleic acid targets. Examples of atypical and cFRET-like configurations are also discussed, including those that utilize time-gated FRET relays and charge-transfer quenching. We conclude with a perspective on challenges and directions for future research with cFRET. Although still emerging as a method, many exciting opportunities in bioanalysis, imaging, and beyond are envisioned for cFRET.