Metal‐Coupled Fluorescence Resonance Energy Transfer in Layer‐by‐Layer Assemblies for Dual Modality Fluorescence Enhancement

Fluorescence resonance energy transfer (FRET) has attracted a great deal of attention in chemical and biological analysis because fluorescence intensity of given fluorophores (acceptor) can be amplified by placing energy‐harvesting molecules (donor) in their vicinity. However, FRET‐based fluorescence has a certain limitation in terms of signal amplification because it inherently relies on the intrinsic properties of donors and acceptors as well as the donor‐to‐acceptor distance. To overcome this limitation, metal nanoparticles (NPs) are introduced to a FRET system to engineer the dipole–dipole interaction in FRET using the localized surface plasmon resonance (LSPR) of metal NPs. A 63.1‐fold fluorescence enhancement is observed using an LSPR‐coupled FRET process based on layer‐by‐layer (LbL) assemblies composed of a pair of donor–acceptor fluorescent dyes and metal NPs. It is discovered that by a) optimizing LSPR‐coupled excitation enhancement in donors as well as emission enhancement in acceptors and b) opening FRET channels between LSPR‐coupled donors and acceptors, dual enhancement mechanisms based on FRET and LSPR can be coupled to induce strong fluorescence. Interestingly, it is noticed that the fluorescence enhancement is achieved with reduced FRET efficiency, which is explained by competitive near‐field interactions in the LSPR‐coupled FRET system. Layer‐by‐Layer assemblies composed of a pair of donor‐acceptor fluorescent dyes and metal nanoparticles (NPs) are prepared. In the assembled multilayers, the localized surface plasmon resonance (LSPR) of metal NPs alters the excitation and relaxation processes of donors and acceptors to induce strong fluorescence enhancement based on LSPR‐coupled energy transfer mechanism.