Synthesis Of Rare Earth Upconversion Nanocrystals By Heterogeneous-core-induced Method

Upconversion (UC) means the emitted photon energy is higher than the photon energyof the excitation light. In the late1990s, with the development of nanotechnology, therare earth UC nanomaterials had been extensively studied by researchers. As a novelbiological fluorescent probe, the rare earth UC nanomaterials are expected to replacetraditional organic fluorescent dyes and semiconductor quantum dots, for they havemany virtues, including absence of background noise, large penetrated depth inbiological tissue, narrow emission peaks, good optical stability, large stocks shift, andso on. As UC fluorescence probes in biological detections and imaging, these rare earthdoped UC nanoparticles should emit strong UC luminescence on infrared excitation toachieve high sensitivity and resolution. In the most of cases, the UC nanoparticles arealso required to have both small size (sub-50nm) and hydrophilic surface in order tomeet the post biological functionalization.Researches have shown that among many UCnanomaterials, NaYF4and NaLuF4were recognized as the best UC matrix materials.With the same rare earth dopants, hexagonal phase NaREF4exhibit higher UCluminescence than cubic ones.Therefore, large numbers of researches have beenfocused on the synthesis of water soluble, small-sized hexagonal NaREF4nanoparticleswith strong UC luminescence.In this thesis, we exploited a novel strategy, heterogeneous core induced mothed.Our strategy is based on using small cubic phase NaREF4cores to induce the growth ofheterogeneous hexagonal NaRE’F4shells (RE and RE’ are different elements). Throughcontrolled synthesis, we successfully prepared water soluble, small size (<30nm), lowpumping threshold, high UC luminescent β-NaREF4.We presumed that theheterogeneous interface between the core and shell owing to cation exchange was thekey factor, which caused the shell to grow into hexagonal phase. Then we selected cubicNaGdF4nanocrystals as cores to induce the growth of hexagonal NaYF4shellsaccording to the heterogeneous core/shell strategy. Furthermore, we combined theheterogeneous core shell nanocrystals with rare earth complexes in a single particle toproduce dual mode emission nanocomposites which could be used as fluorescencecoding. This thesis possesses a certain degree of innovation in the synthesis of water soluble β-NaREF4nanocrystals. The specific contents are as flollows:(1)We have developed a new strategy based on the core/shell heterostructure toprepare sub-30-nm water dispersed NaREF4nanoparticles in hexagonal phase. CubicNaREF4cores were used to induce heterogeneous growth of hexagonal phase shells.Co-doping sensitizer and activator ions into the hexagonal shells yielded thenanophosphors with more intense upconversion luminescence compared with thecorresponding cubic phase nanocrystals of identical sizes and morphologies.Two typesof core/shell nanoparticles, α-NaLuF4/β-NaYF4:Yb,Er and α-NaYF4/β-NaLuF4:Yb,Erwere prepared. A gradual increase in diffraction peak intensities for thehexagonal phase counterpart is observed as increased reaction timefrom6hours to24hours, with the particles sized increased. This strategy not onlyprovides a convenient route for facile synthesis of small water soluble β-NaREF4UCnanophosphors without complex surface modification, but also helps to understand theformation mechanism of small core/shell heterostructure, which may finally promotethe applications of lanthanide-based luminescent probes in biological studies.(2)A heterogeneous core/shell structure was constructed, in which cubic NaGdF4nanocrystals, serving as cores, induced the growth of the heterogeneous hexagonalNaYF4shells co-doped with Yb3+and Er3+ions. We prepared a set of heterogeneous α-NaGdF4/β-NaYF4:Yb,Er core/shell crystals by varying the shell growth time from2to24hours. The characterizations of XRD, TEM, HRTEM, EDX and local elementalmapping results were performed. With the growth time increasing from2hours, to6hours, to12hours, finally to24hours, the diffraction peaks of hexagonal NaYF4in theXRD patterns became more and more dominant, and the crystals shapes changed fromnanosquare, to nanopolyhedron, finally to hexagonal prism shape, and the particle sizeincreased from28nm, to33nm,38nm, finally to115nm×125nm, respectively.Moreover, in the local elemental mapping images, Gd element was localized inthecenter, and Y, Yb elements uniformly covered across the whole outer layer of thenanocrystals. All the results confirmed the successful formation of the uniqueheterogeneous α-NaGdF4/β-NaYF4:Yb,Er core/shell crystals. Furthermore, both theUC emission intensities and the measured lifetimes of several levels were graduallyincreased with the reaction time being prolonged. An interesting observation of Gd3+UC emission perhaps inferred a hetero interface existence. In our reasoning, wedemonstrated that the hetero interface between the core and shell, which were producedby cation exchanges that caused a lower lattice symmetric structure, probably attributed to the heterogeneous hexagonal shell growth.(3)We have successfully prepareda novel type of core-shellrare earth nanoprobesfor biological fluorescence encoding. The characteristic dual emission bandsenable a novel spectral encoding strategy.In these nanoprobes, Yb3+, Er3+(and/orTm3+) codoped heterogeneous NaYF4/NaLuF4nanocrystals served as cores andamorphous SiO2embedded with DC Eu (or Tb)complexes(Eu(DBM)3phen,Tb(SA)3) are coated as shells. Excited by both980nm NIR light and354nm UVlight, these core-shell hybrid NPs exhibited dual emissions carrying UC and DCcharacteristic optical signals. The characteristic emission bandsin both UC andDC luminescence spectra were highlydistinguishable.Through finely tuningofvarious Ln3+doping,we can control the relative emission intensity ratiosofresolvable multiple bands to getdistinct codes.In our dual emission encodingstrategy, the number of fluorescence codes can significantly increase comparingwith single-mode excitation strategy via combination of UC and DCluminescence in a single particle. In addition, these codes were water soluble andtheir sizes were about30nm. The all above properties enable these encodingnanoprobes have potential applications inmultiplexed biological detection.(4) We prepared the NaYF4:Yb3+, Tm3+core-only crystals and the homogeneousNaYF4:Yb3+, Tm3+/x mmol NaYF4(x=0.1,0.5, and1) core-shell crystals by ahydrothermal method. The core crystals were hexagonal plate shape with the size of300nm×115nm. After coating an inert NaYF4shell with amount of0.1,0.5and1mmol, the crystal shape changed to hexagonal prism gradually, with the particle sizeincreased to320nm×144nm,350nm×208nm, and400nm×277nm, respectively.The high-order UCL (1I6and1D2radiative transitions) of all core-shell crystals wereenhanced by several times compared with core-only crystals. Moreover, when theamount of the shell precursor was0.1mmol, the increased ratio of the emissions from1I6and1D2were7.44and5.63times, respectively. With the amount of the shellprecursor increased to0.5and1mmol, the high-order UCL decreased, that theincreased ratio of the emissions of1I6and1D2were reduced to2.47,3.02times and1.53,2.41times, respectively. We discussed that the enhancement of high-order UCLthrough coating a homogeneous shell on the core crystals was due to the well-knownsurface passivation effect of the shell. And, we considered that when the shell thicknessincreased to a certain extent, it perhaps tended to be unfavorable for the high-order UCLowing to the reduction of received pumping power which must transmit through the whole inert shell, which was the reason why the high-order UCL slightly declined whenthe amount of the shell precursor increased to0.5and1mmol. The furtherphotoluminescence decay results were consistent with the UCL spectra and justsupported our statements.