The fluorescence resonance energy transfer (FRET) between two dye molecules is widely used technique to study the conformational dynamics of bio-molecules/polymers. It is often designated as a “spectroscopic ruler” because the strong distance dependence of the energy transfer rate provides us with a microscopic scale to measure the separations at a single molecule level. However, the conventional FRET technique employing dye molecules both as a donor and an acceptor suffers from several limitations, prominent among them is the restriction on the upper limit of separation (~ 80 Å) that can be monitored. This limitation has motivated a novel use of metallic nanoparticles having prominent absorption spectrum in the visible region as either the acceptor, or more recently, as both donor and acceptor in FRET. In such RET systems, separations upto 700 Å can be monitored, which are about 10-times larger than those obtained via conventional FRET. This unique feature makes a metal nanoparticle potentially an extremely useful marker in many bio-medical applications.
The rate of energy transfer (kDA) for conventional FRET systems where the donor and the acceptor are separated by a distance R (center to center distance), is analysed in terms of the well known Förster’s expression. According to Förster’s expression, the rate of energy transfer for such systems is proportional to 1/R6 (for more information on conventional FRET visit this site). A large number of theoretical and experimental studies exist on the rate of non-radiative energy transfer from a dye to both, a plane metallic surface and a nanoparticle. However, only a few of the studies explore the distance dependence of non-radiative energy transfer and still it remains unsettled.
Recently, we have addressed this aspect within the electro-hydrodynamic formalism where we study the distance dependence of the rate of excitation energy transfer from a dye to the surface plasmonic modes of a nanoparticle [J. Chem. Phys. 125, 181102 (2006)].
We observe the transfer rate to vary as 1/dσ (d is surface-to-surface distance, more relevant variable over R in context of the present problem), with σ =3-4 at intermediate distances, in partial agreement with recent experimental results [J. Am. Chem. Soc. 127, 3115 (2005)]. However, the Förster’s 1/d6 distance dependence is recovered at large separations.
Besides distance dependence we have explored the dependence of the rate of energy transfer on the orientation of the dye w.r.t. the distance vector d and the size of the nanoparticle.
Currently, we are looking at the same aspects of energy transfer for a donor-acceptor system consisting of two nanoparticles. The rate dependence on the shape factor of the nanoparticle forms another interesting aspect of the ongoing research.