Synthesis And Application Of NaErF₄@NaREF₄ Core/shell Upconversion Nanoparticles

Lanthanide-doped upconversion nanomaterials have been widely applied in fields like illumination and display,photovoltaics,bio imaging and sensing,ascribed to their unique optical and magnetic properties.Especially for upconversion nanoparticles?UCNPs?,which can convert long wavelength photon to short wavelength photon.Compared to traditional fluorescent labelling materials?organic dyes and quantum dots?,UCNPs have outstanding advantages like infrared light excitation,low toxicity,chemical stability,high resistance to photobleaching,large Stokes shift and so on,attracting attentions from researchers as a competitive candidate of novel multi-functional bio-labelling material.However,the limited upconversion efficiency is a keynote factor hindering the further applications of UCNPs.Because of the concentration quenching effect,the doping concentration of activators is restricted to a low level,which is one of the issues limiting the upconversion efficiency.To overcome shortcomings brought by low concentration of activators,we have designed core-shelled nanostructure of rare earth fluoride?NaREF₄?,accomplishing a heavy doping effect of activators in UCNPs.This nanostructure shows promising potentials in many practical applications.In this paper,we have applied it to 1)the determination of trace water molecules and 2)multi-modal imaging and photodynamic therapy?PDT?of tumor.The outline is listed as follows:1.We have studied different synthesis approaches of NaErF₄@NaREF₄ core-shell nanoparticles.Uniform hexagonal-phased nanoparticles with good morphology have been successfully obtained.We employed solvothermal method?chloride?and trifluoroacetate decomposition method to obtain NaErF₄@NaREF₄ core-shell nanoparticles,respectively.It is found that the as-synthesized nanoparticles by trifluoroacetate decomposition method are not chemically stable.After long time preservation,precipitate will occur,which limits further applications of UCNPs.We studied the upconversion luminescence?UCL?of UCNPs with different shell thicknesses,where we elaborate to reveal the role of surface defects in concentration quenching effect.Due to rich ladder-like energy levels of Er³⁺,the as-obtained NaErF₄@NaREF₄ core-shell nanoparticles are capable of being excited by three different near infrared laser?800 nm,980 nm and1530 nm?and emitting quasi-monochromatic red light.We researched the UCL properties under each excitation and explain the mechanisms.2.We have prepared the NaErF₄@NaYF₄ nanoprobe for the determination of trace water molecules.The probe achieves an ultrasensitive measurement with the moisture content reaching to ppm?parts per million?level.The nanoprobe used for the determination of trace water molecules is on the basis of the sensitivity of UCL from NaErF₄@NaYF₄ nanoparticles towards their surrounding environment.Compared with 980 nm excitation,the application of 800nm excitation is able to avoid the heating effect caused by water absorption in solutions.According to experimental results,we found that the UCL changed as the nanoprobe being placed in DMF and water.In aqueous solution,the UCL of the nanoprobe was strongly quenched,demonstrating the feasibility to determine trace water molecules in DMF.Then we studied the relationship between UCL of the nanoprobe and the water content in DMF.The results show that with different water contents,nanoprobes with different shell thicknesses have different sensitivity towards water molecules.When the water content is below 0.1 vol%,the sensitivity increases as the shell thickness decreasing.When the water content is above 0.1 vol%,the thicker shell,the higher sensitivity.After calculation,the nanoprobe with 1 nm shell has the limit of detection?LOD?reaching to 5 ppm.By analyzing the lifetime changes of various energy levels of Er³⁺,we have revealed the mechanism of how water molecules quench the UCL.It turns out that the electron population of ⁴F_9/2/2 and⁴I_11/21/2 was influenced by water molecules mostly.Electrons in ⁴F_9/2/2 and ⁴I_11/21/2 energy level transfer energy to water molecules and then the energy is consumed for multiphonon relaxation,greatly depressing the UCL.3.We have constructed the NaErF₄@NaLu F₄-Ce6/PEG multifunctional upconversion nanoplatform used for multimodal imaging and PDT of tumor,presenting the feasibility of its theranostic application in cancer treatment.By combining the features of Er³⁺?strong T₂-MR and CT signals?and Lu³⁺?strong CT signals?,the nanoplatform is designed to be applied as a promising contrast agent for multimodal imaging?CT and MR imaging?.Meanwhile,the intensive UCL of each near infrared laser excitation?800 and 980 nm?can be used for PDT and photoluminescence?PL?imaging.In this way,a theranostic nanoplatform capable of tri-modal imaging?PL,CT and MR imaging?and PDT is obtained.This nanoplatform features with advantages of 1)800 nm excitation,effectively avoiding the overheating caused by 980 nm excitation;2)overcoming the difficulties in optimizing multimodal imaging integration?PL intensity,CT contrast and MR contrast?within a single particle.On the basis of the consideration above,we have optimized the NaLuF₄ shell thickness to 5 nm by constructing three NaErF₄@NaLuF₄ nanoparticles with same NaErF₄ core and different NaLuF₄ shell thicknesses.Then the as-obtained nanoparticles were further covalently conjugated with chlorin e6 photosensitizer followedbythesurfacemodificationwithm PEG.Thisbiocompatible NaErF₄@NaLuF₄-Ce6/PEG nanoplatform was further applied to cancer cells?in vitro?and mice?in vivo?for PDT.The results indicated that the cancer cells were inhibited effectively with the performed treatment.In addition,the imaging results showed that this nanoplatform possessed an enhanced contrast ability in CT and MR imaging.Our study shed light on a new potential strategy to realize multimodal imaging-guided PDT.