Preparation Of High Performance Lanthanide-Doped Fluorescent Nanocrystals By Microfluidic Reactors With Control Of Nucleation And Growth Process

Inorganic fluorescent nanocrystals(NCs) show great potential in applications at research fields of immunobiology and clinical examination.Quantum dots(QDs) and lanthanide-doped fluorescent NCs are the two main kinds in these NCs.The QDs, such as CdS,CdSe,CdTe,are mostly prepared in organic phase with harsh conditions. What's more,they could hardly be used in bio-labeling because of the emissiontwinkling and heavy metal toxicity.The lanthanide-doped fluorescent NCs exhibit huge potential applications in bio-labeling because of the low-toxicity,wide Stokes-shift,high resistance to the photobleaching,emission-twinkling and photodegradation.However,the lanthanide-doped NCs produced by the conventional methods usually have wide size distribution,large diameter,poor dispersibility and low emission yield.They could hardly meet the requirements of bio-labeling,which need excellent redispersibility,well water-solubility,and strong emission intensity.In this paper,LaF₃/LaPO₄:Ln³⁺(Ln³⁺=Ce³⁺,Tb³⁺,Eu³⁺) fluorescent NCs were prepared by microfluidic reactor in aqueous phase.The nucleation and growth processes were controlled in the microspace by microfluidic reactor combined with microfluid dynamics.The redispersibility,water-solubility and the fluorescent intensity were enhanced by optimizing the process in microreactor.The yields of NCs synthesized by microfluidic reactor were improved obviously by microwave irradiation.The studies finished are as follows:(1) The lanthanide-doped fluorescent NCs were synthesized by a microfluidic reactor made by a Teflon microcapillary with an inside diameter of 300μm.The ethylene glycol(EG) and the diethylene glycol(DEG) were used as the solvents,the reaction temperature is from 80℃to 140℃and the residence time in microcapillary is from 5 s to 60 s.X-Ray Diffraction(XRD),Transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FT-IR), UV-vis absorption spectroscopy and Photo-luminescence(PL) spectra were used to characterize the NCs.The results confirmed that the crystalinity and the performances were improved by microfluidic reactor.(2) Based on the classic crystallization theory,including the momentary nucleation at a high temperature and the slower growth at a lower temperature,the microfluidic reactor was designed as a two-steps path for preparing the lanthanide-doped fluorescent NCs.The results displayed the LaF₃:Ce,Tb NCs prepared by two-step microreactor had a narrow size distribution,about 4～5 nm,excellent redispersibility and water-solubility.The emission intensity was improved obviously.Another attractive improvement was that the quantum yield of the NCs has been improved to 49%,much higher than that by conventional methods.(3) The microwave(MW) irradiation was used to heat the microfluidic reacting section to control the nucleation and growth processes.The lanthanide-doped fluorescent NCs were obtained by one-step synthesis in microfluidic reactor.The obtained NCs revealed great improvements in crystallinity,dispersibility and shape control by microfluidic reactor under MW irradiation.The monodispersed LaF₃:Ce,Tb nanoparticles had a mean diameter of 4.5 nm,and the LaPO₄:Ce,Tb nanorods had a mean length of 65 nm and diameter of 12 nm.The two systems showed great redispersibility and water-solubility,with obvious improved fluorescent intensity and quantum yields compared to those by oilbath(OB) heating.What's more,the difficulty of how to improve the low yield in microreactor was handled by MW irradiation,LaF₃:Ce,Tb NCs showed a yield of 72.3%and the LaPO₄:Ce,Tb NCs 91.2%.For LaPO₄:Eu NCs,MW irradiation had special effects during the crystallization process.The monodispersed NCs-clusters in which 7～8 little NCs linked together as one had the same tropism.The size was from 25 nm to 30 nm.(4) LaF₃:Eu@LaF₃ core@ shell NCs were synthesized in a continual microfluidic reactor targeted at the difficulty of how to improve the fluorescent intensity of Eu³⁺ doped LaF₃ NCs.As a result,about a 1.5 nm LaF₃ host material was coated around a LaF₃:Eu core particle with a mean diameter of 4.5 nm,to form a core @ shell structure in size of about 6 nm.The crystallinity,water-solubility and the fluorescent intensity were improved obviously.