Gold nanostructures act as optical antennas : they couple efficiently in the near field to photon emitters in order to enhance their radiation the far field. A typical way to design nanoantennas is to couple the plasmonic modes of several nanoparticles positioned a few nanometers apart. We demonstrate here how gold nanoparticle dimers can be assembled around a DNA double-strand that features a single dye molecule. Using electrophoresis, we purify 36 nm diameter gold particles preferentially linked to a single DNA single strand. Hybridization of complementary sequences drives the assembly of dimers with spacings ranging between 7 and 18 nm. Shortening the DNA linker induces a clear red shift of the plasmon resonance wavelength as observed in scattering spectroscopy. A statisitcal analysis of scattering spectra is performed over several dozens of groupings and correlated with cryo-electron microscopy images and theoretical calculations. By adding a single molecule in the centre of the particle dimers, we observe accelerated single photon emission in time-correlated fluorescence measurements. Efficient coupling between the active photon source and the passive gold nanostructure enhances the spontaneous emission rate by up to two orders of magnitude. Ensebmle measurements in fluorescence correlation spectroscopy also indicate a one order of magnitude enhancement of the molar extinction coefficient and an increase of the fluorescence count rates for the shortest dimers while the quantum yield is significantly reduced. Dye molecule driven gold particle dimers thus operate effectively as antennas for light.
