Size-Dependent Surface Enhanced Fluorescence of Gold Nanorod: Enhancement or Quenching

The size-dependent surface enhanced fluorescence (SEF) of a gold nanorod (GNR) on a vicinal molecule is studied systematically by using the multiple multipole method. The enhancement factors (EFs) of GNRs of different sizes (radius: a = 7 or 10 nm) with the same aspect ratio (AR), e. g., 3 or 3. 43, on the fluorescence of a molecule at different locations with various orientations excited at the longitudinal surface plasmon resonance (SPR) of a GNR are discussed in detail. The numerical results show that the EF of a GNR is sensitive not only to the molecular location and orientation but also to the size. The effective EF (EEF), the average of the EF over all possible orientations at a specific location, is further calculated. According to the EEF, the proximity of the end-cap of a GNR is a strong enhancing zone, while the waist area of the GNR is a relatively weak zone. Moreover, for the same AR, the EEF of a bigger GNR (a ≥ 10 nm) is larger than that of a smaller one. Hence, the SEF performance of a bigger GNR on the fluorescence can be a very strong enhancement if the molecule is close to the end-cup excited at the longitudinal SPR. On the contrary, the performance of a smaller GNR could be a quenching if the molecule is near the waist. In addition, the Stokes-shift effect of fluorescence on the EF is also discussed.