Colorimetric Effect of Gold Nanocup Arrays on Fluorescence Amplification

Without changing underlying plasmonic substrate designs, tunable fluorescence enhancement is achieved by varying surrounding fluidic refractive indices on a colorimetric plasmonic substrate of periodic Au nanocup arrays. When comparing the emission characteristics of two fluorophores on the plasmonic surface, HEX with green emission and TEX with red emission, tuning the plasmonic resonance wavelength matched with the emission band of fluorophores is verified as a key factor for the largest fluorescence amplification. Even though the plasmonic nanocup array substrate is already capable of enhancing fluorescence emission compared to that on the Au film, further enhancement by 6.93-fold for HEX at 1.36 refractive index unit (RIU) and 7.12-fold for TEX at 1.45 RIU, both from 1 RIU, is accomplished on the same nanoplasmonic device. The numerical calculations of the quantum efficiency and the radiative decay rate show the same response with the experimental fluorescence emission variations under fluidic refractive index variations. The lifetime reduction of HEX and TEX from solution states proves the enhanced photostability on the plasmonic surface. These findings on optofluidically tuned fluorescence enhancement guide a pathway on multiplexed detection of local molecular binding on targets tagged with fluorescence dyes through a subsequent refractive index change on the nanoplasmonic substrate.