Abstract
The geometry and dimension of a gold nanorod (GNR) are optimally designed to enhance the fluorescence intensity of a lanthanide-doped upconversion nanocrystal placed in close proximity to the GNR. A systematic study of the electromagnetic interaction between the upconversion emitter of three energy levels and the GNR shows that the enhancement effect arising from localized electric field-induced absorption can be balanced by the negative effect of electronic transition from an intermediate state to the ground state of the emitter. The dependence of fluorescence enhancement on the emitter-GNR separation is investigated, and the results demonstrate a maximum enhancement factor of 120 folds and 160 folds at emission wavelengths 650 and 540 nm, respectively. This is achieved at the emitter-GNR separation ranging from 5 to 15 nm, depending on the initial quantum efficiency of the emitter. The modified upconversion luminescence behavior by adjusting the aspect ratio of the GNR and the relative position of the emitter indicates the dominate role of excitation process in the total fluorescence enhancement. These findings are of great importance for rationally designing composite nanostructures of metal nanoparticles and upconversion nanocrystals with maximized plasmonic enhancement for bioimaging and sensing applications.
Highlights
Example, bioimaging of small animals[10,11,12,13,14] with significantly minimized photobleaching and unwanted non-specific fluorescence from live biological tissues when excited in the biologically transparent window at around 980 nm
To the best of our knowledge, the moderate enhancement in upconversion luminescence intensity can be attributed to fact that the plasmon resonance wavelengths of metallic nanostructures used in all previous studies matched either the emission or absorption wavelengths of upconversion nanocrystals (UCNCs)
The discrepancy can be attributed to the fact that the complexity of the energy level system of the UCNC may have not been fully considered in the theoretical model, and that the excited state absorption (ESA)-mediated upconversion mechanism cannot be excluded under confocal excitation configuration with relatively high pump intensity
Summary
Example, bioimaging of small animals[10,11,12,13,14] with significantly minimized photobleaching and unwanted non-specific fluorescence from live biological tissues when excited in the biologically transparent window at around 980 nm. It has been demonstrated that increasing excitation irradiance from 1.6 × 104 W/cm[2] to 2.5 × 106 W/cm[2] can alleviate concentration quenching in upconversion luminescence, significantly enhancing the luminescence signal from NaYF4:Yb3+/Tm3+ by a factor of 7023. Such high excitation radiance is clearly not favorable in fluorescence bioimaging because of the high-probability photodamage to biological tissues. We show that the overall fluorescence enhancement can be maximized by optimizing the emitter-GNR separation distance and adjusting the GNR aspect ratio
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