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The fluorescence of organic fluorophore molecules is enhanced when they areplaced in contact with certain metals (Al, Ag, Cu, Au, etc.) whose surfaceplasmon waves couple into the radiative modes of the molecules and increase theradiative efficiency. Here, we report a hitherto unknown size dependence ofthis metal enhanced fluorescence (MEF) effect in the nanoscale. When themolecules are deposited in nanoporous anodic alumina films with exposedaluminum at the bottom of the pores, they form organic nanowires standing onaluminum nanoparticles whose plasmon waves have much larger amplitudes. Thisincreases the MEF strongly, resulting in several orders of magnitude increasein the fluorescence intensity of the organic fluorophores. The increase inintensity shows an inverse super-linear dependence on nanowire diameter becausethe nanowires also act as plasmonic 'waveguides' that concentrate the plasmonsand increase the coupling of the plasmons with the radiative modes of themolecules. Furthermore, if the nanoporous template housing the nanowires hasbuilt-in electric fields due to space charges, a strong molecule-specific red-or blue-shift is induced in the fluorescence peak owing to a renormalization ofthe dipole moment of the molecule. This can be exploited to detect minuteamounts of target molecules in a mixture using their optical signature(fluorescence) despite the presence of confounding background signals. It canresult in a unique new technology for bio- and chemical-sensing.

nano letters

Giant increase in the metal-enhanced fluorescence of organic molecules  in nanoporous alumina templates and large molecule-specific red/blue shift of  the fluorescence peak