Effective Enhancement Factors Research Articles

Microfluidic-based metal enhanced fluorescence for capillary electrophoresis by Ag nanorod arrays

As metal nanorods show much higher metal enhanced fluorescence (MEF) than metal nanospheres, microfluidic-based MEF is first explored with Ag nanorod (ND) arrays made by oblique angle deposition. By measuring the fluorescein isothiocyanate (FITC) solution sandwiched between the Ag NDs and a piece of cover slip, the enhancement factors (EFs) are found as 3.7 ± 0.64 and 6.74 ± 2.04, for a solution thickness at 20.8 μm and 10 μm, respectively. Because of the strong plasmonic coupling between the adjacent Ag NDs, only the emission of the fluorophores present in the three-dimensional NDs array gets enhanced. Thus, the corresponding effective enhancement factors (EEFs) are revealed to be relatively close, 259 ± 92 and 340 ± 102, respectively. To demonstrate the application of MEF in microfluidic systems, a multilayer of SiO2 NDs/Ag NDs is integrated with a capillary electrophoresis device. At a microchannel depth of 10 μm, an enhancement of 6.5 fold is obtained for amino acids separation detection. These results are very encouraging and open the possibility of MEF applications for the Ag ND arrays decorated microchannels. With the miniaturization of microfluidic devices, microfluidic-based MEF by Ag ND arrays will likely find more applications with further enhancement.

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.

Theoretical investigation of plasmonic enhancement of silica-coated gold nanorod on molecular fluorescence

The plasmonic enhancement or quenching effects of a silica-coated gold nanorod (GNR@SiO2) on the fluorescence of a molecule doped in the silica layer are studied using the multiple multipole method. The enhancement factors (EF) of a GNR with a typical aspect ratio of 3 coated by a 13nm silica layer upon the fluorescence of a molecule embedded at different locations with various orientations irradiated by a plane wave are analyzed, particularly at the longitudinal surface plasmon resonance (SPR) of GNR. The numerical results show that the EF of a GNR@SiO2 on the fluorescence is sensitive to the molecular location and orientation. Furthermore, an effective EF (EEF), which is an average of EF over all possible orientations at a specific location, is calculated. According to EEF, the proximities of the end-caps of a GNR are strong enhancing zones. In contrast, the waist area is the weak zone. Moreover, a bigger GNR (a=10nm) possesses a higher EEF than a smaller one (a=7nm) for the same aspect ratio and the molecular relative location. Hence, a strong enhancement on the fluorescence is obtained using bigger GNR, if the molecule is near the end-cup and the dipole orientation is along the long axis. On the contrary, the consequence could be quenching, if the molecule is near the waist of a small GNR. The Stokes shift of fluorescence can also affect the EF, except the excitation wavelength.