Preparation Of Upconversion Nano-materials And Its Sensing Application

Upconversion nanomaterials have been developed and applied to ions or small molecular detection, biological labling, cell imaging and in vivo analysis, due to their superior luminescence features including high luminescence efficiency, long photoluminescence lifetime, narrow special emission, large stokes shift, excellent emission stability, low damage to analytes, low autofluorescence interference, and nearly no photobleaching. Compare to the traditional down-conversion nanomaterials, this upconversion nanomaterials have the especial luminescence mechanism, in which the low-energy near infrared light (typical,980nm) is converted into a higher energy visible emission through multiple-photon absorption or energy transfer. So upconversion nanoparticles are perfect fluorescence materials, including small hart to biological tissue and strong penetration. To realize the trace detection of nitrite ion, upconversion ratiometric fluorescence method is established in this paper, while is successfully applied in the determination of nitrite in real samples.In chapter1, we introduce the upconversion nanomaterials, including the luminescence mechanism, the synthetic process, the surface modification, and the abroad application in sensors.In chapter2, hexagonal phase NaYF4:Yb,Er (upconversion nanoparticles, UCNPs) were prepared according to the cothermolysis of rare-earth chlorides. Due to oleic acid (OA) on their surface, the UCNPs can be well-dispersed in organic solvent such as cyclohexane, chloroform and methylbenzene. Therefore, through a surface ligand-exchange with polyacrylic acid (PAA), the hydrophobic OA-capped UCNPs were converted into hydrophilic PAA-capped UCNPs. Meanwhile, we also prepared the NaYF4：Yb,Tm and NaYF4:Yb,Ho, which have blue and green emission, respectively. Additionally, the as-synthesized UCNPs were characterized by transmission electron microscope (TEM), selected area electron diffraction (SED), X-ray powder diffraction (XRD), fourier transform infrared (FT-IR), and photoluminescence spectrometer. The results indicated that the as-prepared NaYF4:Yb,Er nanoparticles are highly uniform, monodisperse, relatively high luminescence efficiency and stability.In chapter3, using the UCNPs as probe and an efficient reaction between neutral red and nitrite, we developed a selective upconversion switching method for the ratiometric fluorescence detection of nitrite, in which the green emission (λem=539 nm) of NaYF4:Yb,Er nanoparticles can be selectively turned on while its red emission (λem=654nm) keeps unchanged. The results demonstrated that the ratio of emission intensities (I539/I654) is quantitatively correlated to the concentration of nitrite ions with the detection limit to be0.2ppm. Under excitation with980-nm laser, it can realize the visual color changes from red to orange yellow and final green. Moreover, the developed method has been successfully applied to real sample detection such as nitrite in drinking water, natural water and meat foods...