Fluorescence enhancement dependence on refractive index variation in a nanostructured plasmonic surface

The more recent progress and technological improvement in nanostructured plasmonic surfaces has gathered huge interest in applications related to actual sensing challenges. In particular, fluorescence has been one of the most exploited measurement techniques within such a context. Specifically, resorting to custom structures supporting suitable plasmonic resonances, we studied their influence on the emission efficiency of the commercial ATTO700 fluorophore. An extended optical analysis in combination with a complementary investigation through finite-difference time-domain (FDTD) simulations have been performed to understand the coupling mechanism between the excitation of the plasmonic modes and the fluorescence absorption and emission processes. In addition, within this framework, we focused on the use of a buffer solution, flowing across the grating active surface, to mimic a real measurement. The refractive index of the surrounding medium is therefore altered, with a consequent modification of the resonance conditions. The result is a shift of the emission spectral features characterized by a reshaping of the fluorescence curve in terms of spectral weight and direction.

Therefore, the strongest signature of the plasmonic modes determines the spectral region characterized by the largest relative enhancement, as shown by both the optical measurements and the FDTD findings.

Besides, the electric field corresponding to the localized modes at the interface with the substrate carries and favors the fluorophore emission toward its side.