  In metal-enhanced fluorescence (MEF), the localized surface plasmon
resonances of metallic nanostructures amplify the absorption of excitation
light and assist in radiating the consequent fluorescence of nearby molecules
to the far-field. This effect is at the base of various technologies that have
strong impact on fields such as optics, medical diagnostics and biotechnology.
Among possible emission bands, those in the near-infrared (NIR) are
particularly intriguing and widely used in proteomics and genomics due to its
noninvasive character for biomolecules, living cells, and tissues, which
greatly motivates the development of effective, and eventually multifunctional
NIR-MEF platforms. Here we demonstrate NIR-MEF substrates based on Au nanocages
micropatterned with a tight spatial control. The dependence of the fluorescence
enhancement on the distance between the nanocage and the radiating dipoles is
investigated experimentally and modeled by taking into account the local
electric field enhancement and the modified radiation and absorption rates of
the emitting molecules. At a distance around 80 nm, a maximum enhancement up to
2-7 times with respect to the emission from pristine dyes (in the region 660
nm-740 nm) is estimated for films and electrospun nanofibers. Due to their
chemical stability, finely tunable plasmon resonances, and large light
absorption cross sections, Au nanocages are ideal NIR-MEF agents. When these
properties are integrated with the hollow interior and controllable surface
porosity, it is feasible to develop a nanoscale system for targeted drug
delivery with the diagnostic information encoded in the fluorophore.
