Plasmon-enhanced Fӧrster resonance energy transfer in Langmuir-Blodgett films based on organic dyes

Nanoparticles and island films of metals are of particular interest for nanotechnology. The intermolecular electronic excitation energy transfer underlies in many important physical and photochemical processes. Despite the... [ view full abstract ]

Nanoparticles and island films of metals are of particular interest for nanotechnology. The intermolecular electronic excitation energy transfer underlies in many important physical and photochemical processes. Despite the available studies, the mechanism of intermolecular energy transfer in the presence of plasmon nanoparticles remains poorly understood.

Here the results of studying of the interlayer energy transfer (FRET) between organic dyes on the surface of silver island films are presented.

The amphiphilic derivatives of Rhodamine B (HERB) and Nile red (NR) were chosen as an energy donor and acceptor. Island silver films (SIF) were deposited onto glass substrates by the method of thermal vacuum deposition. Then films of donor or acceptor were deposited onto SIF by Langmuir-Blodgett (LB) method on the KSV Nima trough. The distance from the dye film to the SIF was varied by monolayers of stearic acid (SA, molecule length ~ 0.2 nm). The absorption and fluorescence spectra of the films were measured on the Cary and Eclipse (Agilent) spectrometers, correspondingly. The fluorescence lifetimes of the donor and acceptor were recorded by using of TCSPC system (Becker&Hickl) at λ_ex=488 nm. The energy transfer efficiency was estimated by the Förster formula.

Studying of plasmon effect on pure donor and acceptor films have shown that increase in the fluorescence intensity of both HEBR and NR is approximately 25% was registered at a distance from the dye to the SIF of ~6 nm (Figures 1 and 2).

In FRET studies the fluorescence quenching of the donor and the appearance of sensitized fluorescence of the acceptor molecules were registered. Spectral-kinetic measurements of donor-acceptor films without SIF and in their presence have shown that the efficiency of energy transfer in the presence of silver nanoparticles was increased. At the same time, when the donor-acceptor film is separated from the SIF, the energy transfer efficiency was decreased (Table).

Thus, it was shown that, in the presence of an SIF, the efficiency of the FRET can be increased. In this case, the increase in E_ET can be associated with an increase in the fluorescence quantum yield of the energy donor upon direct contact with the SIF.