Guided mode resonance enhanced upconversion fluorescence of rare earth nanoparticles in aqueous solution using a low refractive index resonant waveguide grating

We demonstrated that upconversion fluorescence (UCF) produced from rare earth upconversion nanoparticles (UCNPs) in aqueous solution could be greatly enhanced using a low refractive index resonant waveguide grating (low-n... [ view full abstract ]

We demonstrated that upconversion fluorescence (UCF) produced from rare earth upconversion nanoparticles (UCNPs) in aqueous solution could be greatly enhanced using a low refractive index resonant waveguide grating (low-n RWG). The low-n RWG comprised of a low refractive index porous silica (n=1.22) sinusoidal grating layer in the bottom and a thin high refractive index TiO₂ waveguide layer on the top (see Fig. 1 (a)). The surface of the TiO₂ layer was biotinylated and then covered with aqueous solution (PBS). After drop-coating of streptavidin-conjugated UCNPs on the biotinylated TiO₂ surface, streptavidin-coated UCNPs were immobilized on biotinylated surface. Strong local field was built near the interface between TiO₂ and PBS aqueous solution, when the low-n RWG was illuminated with a near IR laser with an incident angle matching with the guided mode resonance (GMR) angle of the low-n RWG. Figure 1(b) depicts the UCF spectra of the low-n RWG sample obtained with different excitation angles. As displayed, UCF intensities from all emission peaks were dramatically increased when the low-n RWG was illuminated under GMR condition, i.e., q = 1.5°. As the excitation angle was shifted to q = 2.0°, UCF intensities collapsed and became barely higher than those from a non-RWG area. We also compared UCF spectra of biotinylated (labelled) and non-biotinylated (non-labelled) low-n RWG samples under resonant excitation. As shown in Fig. 1(c), the UCF emissions of the labelled sample were much stronger than those of non-labelled one, indicating higher numbers of UCNPs immobilized on the TiO₂ surface of the labelled low-n RWG, and thus validating the biotinylated functionalization process workable for our low-n RWG structured surface. The result confirms that the low-n RWG is powerful for enhancing UCF of UCNPs which paves a good foundation of using the low-n RWG in UCF biosensing and bioimaging applications.