(374d) Facile Synthesis and Optical Characterization of Hybrid Upconverting and Plasmonic NaGdF4: Yb3+, Er3+/Silica/Gold Nanoparticles

We present a simple process for preparing water dispersable hybrid NaGdF₄: Yb³⁺, Er³⁺/silica/gold nanoparticles. Monodispersed upconversion nanoparticles of NaGdF₄:2% Er, 20% Yb capped by oleic acid were first synthesized via a thermal-decomposition method. The hydrophobic UC NPs were then capped with silica via a reverse-micelle method to make them hydrophilic. Amino-propyltriethoxysilane (APTES) was then used to functionalize the silica-capped UC NPs with amine groups. Finally, gold NPs capped with PVP were mixed with the silica-coated UC NPs. Amine groups on the UC NPs displaced the PVP from the AuNP surface, yielding AuNP-decorated UC NPs that were dispersible in water or ethanol. The emission intensity and color of the upconverting cores are modulated by the plasmonic absorbance and field enhancement of the gold nanoparticles. Energy transfer happened in the hybrid nanoparticles from UC core to the decorating gold nanoparticles, especially at the wavelength of 530 nm at which gold nanoparticles have strong absorption. The lifetime analysis shows that the plasmon resonance effected the emission of upconversion nanoparticles. The applicability of hybrid NPs for multi-modal bioimaging probes was also illustrated with confocal microscopy of living cancer cells in vitro.