30 Single-Molecule Level Study and Control …
525
30.3 Enhancement of Energy Transfer by Localized
Plasmons of Metal Nanostructures in Hybrid
Materials
30.3.1 Selective Turn-on and Modulation of Resonant
Energy Transfer in Single Plasmonic Nanostructures
by the Polarization of Light
Förster resonant energy transfer (FRET) is a non-radiative process based on dipole–
dipole interactions, where excitation energy absorbed by a donor molecule is transferred to an acceptor molecule. The efficiency of FRET strongly depends on spectral overlap between the donor emission and the acceptor absorption, the donorto-acceptor distance, the relative orientation of the two transition dipoles, and the
quantum yield and extinction coefficient of the dyes. The donor–acceptor interaction distance R DA is typically on the order of nm and the energy transfer rate scales
as ~R
−6
DA . This strong distance dependence is utilized in applications of FRET as a
nanoruler in biological systems at molecular level. FRET is also at the center of photosynthesis where it ensures transport of the absorbed sunlight toward a reaction center,
and novel nature-inspired artificial light-harvesting systems are being explored for
potential use as light collectors and energy funnels in solar energy conversion
systems. FRET is also used in nanosensors or white light-emitting structures.
In many of these applications, the limited range of R DA may pose restrictions
on the design and efficiency of the systems. To increase sensitivity of FRET-based
nanosensors or improve the performance of FRET-based light-harvesting systems,
it is necessary to enhance FRET efficiencies over larger distances. Introducing plasmonic structures in the vicinity of an energy donor and/or energy acceptor can potentially lead to strong increase of FRET rate, efficiency, and range, especially when
the spectral range of energy transfer is overlapping with the plasmon resonance of
the plasmonic structure [24].
Here, we fabricated hybrid nanostructures composed of FRET dye pairs (donors
and acceptors) covalently attached to gold nanorods (GNR), Fig. 30.5a. Plasmon
resonances in GNR are highly sensitive to the polarization of the excitation light
due to the GNR anisotropic structure. In addition, the advantages of GNR are their
ease of manufacture and spectral tunability. We demonstrate on the level of a single
nanorod that the energy transfer process can be switched on and off by changing
the polarization of the excitation light relative to the long axis of the GNR [14].
Our study therefore provides an approach for controlling and modulating the FRET
process between different fluorescent species.
The hybrid nanoparticles are composed of BODIPY TR NHS dyes as donors
and BODIPY 650 NHS dye as acceptors. Both are attached at random positions
to a GNR via an alkane chain (Fig. 30.5a). Good overlap of the FRET parameters
of the dyes with the GNR plasmon resonance is achieved by using GNRs with a
longitudinal plasmon at 636 nm which coincides with the emission of the donor at
525
30.3 Enhancement of Energy Transfer by Localized
Plasmons of Metal Nanostructures in Hybrid
Materials
30.3.1 Selective Turn-on and Modulation of Resonant
Energy Transfer in Single Plasmonic Nanostructures
by the Polarization of Light
Förster resonant energy transfer (FRET) is a non-radiative process based on dipole–
dipole interactions, where excitation energy absorbed by a donor molecule is transferred to an acceptor molecule. The efficiency of FRET strongly depends on spectral overlap between the donor emission and the acceptor absorption, the donorto-acceptor distance, the relative orientation of the two transition dipoles, and the
quantum yield and extinction coefficient of the dyes. The donor–acceptor interaction distance R DA is typically on the order of nm and the energy transfer rate scales
as ~R
−6
DA . This strong distance dependence is utilized in applications of FRET as a
nanoruler in biological systems at molecular level. FRET is also at the center of photosynthesis where it ensures transport of the absorbed sunlight toward a reaction center,
and novel nature-inspired artificial light-harvesting systems are being explored for
potential use as light collectors and energy funnels in solar energy conversion
systems. FRET is also used in nanosensors or white light-emitting structures.
In many of these applications, the limited range of R DA may pose restrictions
on the design and efficiency of the systems. To increase sensitivity of FRET-based
nanosensors or improve the performance of FRET-based light-harvesting systems,
it is necessary to enhance FRET efficiencies over larger distances. Introducing plasmonic structures in the vicinity of an energy donor and/or energy acceptor can potentially lead to strong increase of FRET rate, efficiency, and range, especially when
the spectral range of energy transfer is overlapping with the plasmon resonance of
the plasmonic structure [24].
Here, we fabricated hybrid nanostructures composed of FRET dye pairs (donors
and acceptors) covalently attached to gold nanorods (GNR), Fig. 30.5a. Plasmon
resonances in GNR are highly sensitive to the polarization of the excitation light
due to the GNR anisotropic structure. In addition, the advantages of GNR are their
ease of manufacture and spectral tunability. We demonstrate on the level of a single
nanorod that the energy transfer process can be switched on and off by changing
the polarization of the excitation light relative to the long axis of the GNR [14].
Our study therefore provides an approach for controlling and modulating the FRET
process between different fluorescent species.
The hybrid nanoparticles are composed of BODIPY TR NHS dyes as donors
and BODIPY 650 NHS dye as acceptors. Both are attached at random positions
to a GNR via an alkane chain (Fig. 30.5a). Good overlap of the FRET parameters
of the dyes with the GNR plasmon resonance is achieved by using GNRs with a
longitudinal plasmon at 636 nm which coincides with the emission of the donor at