Surface State Control And Application Of Upconversion Nanoparticles

Upconversion luminescent materials are capable of converting near infrared excitation radiation into shorter visible or ultraviolet wavelengths through a photon upconversion process.In recent years,lanthanide-doped upconversion nanocrystals(UCNPs)have been developed as a new class of luminescent optical labels because of their low autofluorescence background,large anti-Stokes shifts,sharp emission bandwidths,high stability.As a result,lanthanide-doped upconversion nanocrystals have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging.As nanoparticles are usually smaller than 100 nm,high surface-to-volume ratio means that large fractions atoms are located on the surface of nanoparticles.Therefore,surface states play an important role in the luminescence properties of UCNPs and it is of great importance to explore the relationship between surface states and luminescence properties.The understanding of the behavior of different surface adsorbents and their influence on the upconversion process will allow for a better control and utilization of the surface state of nanoparticles,We expect our work will provide important guidelines for practical applications of luminescent nanoparticles.on the influence of surface states and give a further insight on the mechanism as well.Our work consists of four parts,which are as follows:(1)Large-scale synthesis of monodisperse nanocrystalsCurrent synthetic methods based on co-deposition for the nanoparticles are usually limited by the small scale.In this part,through investigation of experimental parameters including the concentration of lanthanide ions,heating program and heat preservation period,we successfully realize large-scale synthesis of monodisperse lanthanide fluoride nanocrystals with good dispersity.(2)Controlling luminescent behavior through core-shell structureWith the surface of nanoparticles converted from hydrophobic to hydrophilic,the surfaces are usually terminated by hydroxyl groups,which strongly absorb the 980 excitation light and reduce the number of photons which are otherwise transferred to sensitizer ions.In addition,the high phonon energy of hydroxyl groups around 3200?3500 cm-1 also enhances the rate of nonradiative relaxation for excited ions which then attenuate the emission intensity.In this chapter,the NaYF4:20Yb,0.5Tm nanoparticles are surface-passivated by an inert layer of NaYF4.which separate the emitting core from the surface adsorbents.By controlling the thickness of this layer,we reveal the upconversion process of the core-shell nanoparticles.(3)Enhanced upconversion luminescence based on surface states controlIn this chapter,to gain a deeper insight into the influence of nanoparticle surface on the luminescent behavior,we synthesis ultra-small nanoparticles with maximized the specific surface area.By controlling the measurement temperature,we examine the abnormal temperature dependence of the upconversion emission in ultra-small NaGdF4:20Yb3+,0.5Tm3+nanoparticles.We reveal that the nanoparticles capped with oleic acid and hydroxyl groups exhibit quite different upconversion behavior at high temperatures.To understand the mechanisms,we characterize the surface and interior structures of the nanoparticle and model the experimental results by numerical method.We clarify that the abnormal upconversion behavior at high measurement temperatures can be ascribed to the sintering of nanoparticles and volatilization of surface adsorbents.(4)Detect Cu2+ ions by utilizing surface stateCu2+ is an essential trace element for human beings and animals.We attach Cu2+ ions onto the surface of nanoparticles base on the complexation between Cu2+ and PEI.The luminescence of upconversion nanoparticles can be quenched effectively by Cu2+ on the surface,and,as a result,Cu2+ concentration can be measured according to the luminescence intensity.Based on single particle spectroscopy,we achieve a detection limit down to 20.7×10-9 M,which are promising for diagnosis for various diseases.