Investigating the distance limit of a metal nanoparticle based spectroscopic ruler

Conventional Förster resonance energy transfer (FRET) processes involving a pair of fluorophore and organic quencher are restricted to an upper distance limit of ~10 nm. The application of a metal nanoparticle as a quencher can overcome the distance barrier of the traditional FRET technique. However, no standard distance dependence of this resonance energy transfer (RET) process has been firmly established. We have investigated the nonradiative energy transfer process between an organic donor (fluorescein) and gold nanoparticle quencher connected by double stranded (ds) DNA. The quenching efficiency of the gold nanoparticle as a function of distance between the donor and acceptor was determined by time-resolved lifetime analyses of the donor. Our results showed a 1/d(4) distance dependence for the RET process for longer distances (>10 nm) and 1/d(6) distance dependence for shorter distances (<10 nm). Our results clearly indicate the applicability of metal nanoparticle based quenchers for studying systems that exceed the 10 nm FRET barrier.