Abstract
Abstract Förster Resonance Energy Transfer (FRET) is a widely applied technique in biology to accurately measure intra- and inter-molecular interactions at the nanometre scale. FRET is based on near-field energy transfer from an excited donor to a ground state acceptor emitter. Photonic nanoantennas have been shown to modify the rate, efficiency and extent of FRET, a process that is highly dependent on the near-field gradient of the antenna field as felt by the emitters, and thus, on their relative distance. However, most of the experiments reported to date focus on fixed antennas where the emitters are either immobilized or diffusing in solution, so that the distance between the antenna and the emitters cannot be manipulated. Here, we use scanning photonic nanoantenna probes to directly modulate the FRET efficiency between individual FRET pairs with an unprecedented nanometric lateral precision of 2 nm on the antenna position. We find that the antenna acts as an independent acceptor element, competing with the FRET pair acceptor. We directly map the competition between FRET and donor-antenna transfer as a function of the relative position between the antenna and the FRET donor-acceptor pair. The experimental data are well-described by FDTD simulations, confirming that the modulation of FRET efficiency is due to the spatially dependent coupling of the single FRET pair to the photonic antenna.
Highlights
Metallic nanostructures, called photonic nanoantennas, can convert propagating electromagnetic waves into localized fields at the nanometre scale, and vice versa [1]
The fluorescence emitted from the sample is collected through a 1.3 NA objective and split towards two singlephoton counting avalanche photodiodes (SPADs) to discriminate the light emitted from the acceptor and donor molecules
In this work we have used nanoantennas mounted on a near-field optical set-up to achieve full 3D control of the antenna position with respect to single Förster Resonance Energy Transfer (FRET) pairs
Summary
Called photonic nanoantennas, can convert propagating electromagnetic waves into localized fields at the nanometre scale, and vice versa [1]. Metallic nanoantennas enhance and confine electromagnetic fields much below the wavelength of light This property has been used for many purposes, ranging from super-resolution microscopy [2] or biosensing at high concentrations [3] to detection of dynamic events at the nanometre scale [4], and, in particular, to enhance the fluorescence of single emitters placed in their vicinity by manipulating both excitation and emission processes [1, 5]. Photonic antennas modify the local density of states (LDOS) in their vicinity, which in turn affects the total (ktot kr + knr), radiative (kr) and non radiative (knr) decay rates of nearby quantum emitters [6] This has the effect of reducing the fluorescence lifetime τ of the emitters (τ (kr + knr)−1), and modifying their quantum yield φ(φ kr/(kr + knr)). The emitted fluorescence can be enhanced by the photonic antenna, if the quantum yield and/or the excitation field are increased by the nanostructure [8]
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