Controlled Synthesis, Surface Modification And Cell Imaging Of Upconversion Luminescence Nanomaterials

Since the abundant energy level and the intermediate long-lived energy states, the4f electrons of the rare earth elements (such as Er3+and Tm3+) can be successively excited by two or more pump photons and emit a higher-energy photon, which is named upconversion (UC) luminescence effects. Due to their upconversion ability of infrared radiation (NIR) into the visible light emission, rare earth doped nanocrystals (NCs) have attracted considerable attention in recent years. The advantages of using NIR as an excitation source in biomedical applications include its ability to penetrate deeply into tissue with little tissue damage, low autofluorescence and toxicity of the biological medium. The rare earth doped UC nanocrystals also exhibit a long luminescence lifetime, superior chemical stability and photostability, etc. So far, hexagonal sodium yttrium fluoride (p-NaYF4) nanomaterials are considered to be one of the most ideal systems for UC luminescence due to their relatively high chemical stability and low lattice phonon energies (-350cm-1). Thereafter, this paper mainly focuses on the controllable preparation and upconversion luminescence enhancement of the rare earth doped NaYF4NCs, as well as the surface modification of hydrophobic nanomaterials and their further biological applications. The details are as follows:A facile strategy has been successfully developed for the synthesis of rare-earth doped hexagonal phase NaYF4NCs with uniform shape and small particle size as well as strong photoluminescence, which expand the scope for inorganic nanomaterials synthesis. By controlling the reaction time, ratio of reagents and reaction temperature, the nanocrystals with different morphology (from a rod to a spherical particle) and sizes (from46nm to9.5nm) were obtained, respectively. We also find that the growth from ultra-small seed crystals (-2nm) to nanoparticles (-10nm) and the phase transition from cubic phase to hexagonal phase underwent an Ostwald-ripening process. As the Yb/Er/Tm triply doped NaYF4NCs exhibit strong emission peaks in the blue, green, red and NIR region when excited by980nm near-infrared (NIR) laser radiation, it is probable to change the relative intensities of the emission peaks. Hence, by adjusting the relative ratio of these doped ions, the observed color of the UC luminescence was tuned from bluish violet to orange-red gradually. The oleic acid coated NaYF4NCs were successfully transferred into aqueous solution by surface functionaliaztion of SiO2. The TEM imaging shows that no obvious aggregation was observed and the thickness of the SiO2layer was about2-3nm.Compared with the bulk materials or single-photon excited nanomaterials such as organic dyes and quantum dots, the upconversion luminescence nanomaterials exhibit relatively low quantum efficiency. Therefore, the luminescence quantum efficiency of the upconversion nanomaterials is one of the most important parameters that need improvement. In this paper, we developed a cation exchange method to greatly enhance the UC luminescent intensity of β-NaYF4NCs without obviously particle size and shape change. The cation-exchange reaction was operated by hot-injecting Gd3+precursors directly into the prepared NaYF4NPs solution without preseparation, the naked-eye visible UC emission of the NPs was enhanced about29times under980nm NIR excitation. STEM and HRTEM images indicated that the structure of the obtained composite materials is similar to that of the traditional core-shell NPs excepting little change in the particle size. The cation-exchange process was further identified by the case of NaYF4nanorods (NRs). After the cation-exchange by Gd3+precursor, obvious shrinkage defects were observed at the center of the short axis direction{100} and finally broken into two nanoparticles. The cation-exchanged hydrophobic UCNPs were then surface modified by amine-modified poly (amino acid) for further use. The TEM image and hydrodynamic diameter distribution indicated that the UCNPs were successfully encapsulated in the poly(amino acid) micelle without obviously aggregation. Additionally, these luminescence enhanced nanomaterials also exhibit a certain longitudinal relaxation (Ti) performance due to the presence of Gd3+(ri=0.515mM-1·S-1), which is expected to achieve in further application as dual-modality in vivo imaging.To render these hydrophobic nanocrystals reasonably water-stable and biocompatible, surface modification is necessary and prerequisite for biomedical applications. Via the ultrasonication assisted encapsulation technology, a facile and general strategy was successfully developed for the surface modification of different hydrophobic inorganic materials. The surface functionalization was attributed to the surface hydrophobic interaction between the amphiphilic phospholipids and alkyl chains of organic ligands. After removing the chloroform via evaporation, the amphiphilic phospholipids eventually self-assembled into a layer and surrounded the nanoparticle surface. As phospholipid is the main component of the cell membrane, the phospholipid modified NCs are supposed to display very good biocompatibility. In spite of large differences in shape, chemical compositions, and particle size, these nanocrystals were successfully immobilized in a single hydrophilic micelle with good water stability and their optical or magnetic property showed negligible change, indicating that this method has good versatility.The cytotoxicity of the hydrophilic NaYF4NPs modified with polysuccinimide (PSI) and ZnS:Mn2+QDs modified with amphiphilic phospholipids was investigated by methyl thiazolyl tetrazolium (MTT) cell proliferation assay, respectively. When the HepG2cancer cells were incubated with100μg/mL of UCNPs for24h or200μg/mL of UCNPs for48h, about92%and83%of the cells survived, respectively. To the ZnS:Mn2+quantum dots for24h and48h, the cell survival was94%and79%, respectively, when the particle concentration up to100μg/mL Therefore, the cytotoxicity of the two nanomaterials is relatively low. Finally, the two different luminescent materials were both conjugated with anti-fetoprotein antibody (anti-AFP) and were used for of HepG2cells imaging, which provides an example for biomedical research.