Study On Synthesis,Physical Property And Application Of NaErF₄ Based Upconversion Nanoparticles

In recent years,lanthanide-doped upconversion nanoparticles?UCNPs?has attracted more and more attention attributed to their outstanding physical and chemical properties.According to present research results,UCNPs have shown promising potentials in diverse fields,such as solar energy utilization,panel display,fluorescence coding security and anti-counterfeiting.Especially in field of medical and biological applications,they have presented many favorable advantages like deeper tissue penetration depth?the excitation light is exactly located within the bio-optical window from 660 nm to 1300 nm?,lower background noises and no auto-fluorescence from bio-tissues.Besides,the surface of UCNPs has the capacity to be modified by or load other molecules.With surface modification,the toxicity of UCNPs is minimized to a very safe level for living organism and better bio-comparability.By loading functional molecules,it enables UCNPs a kind of competitive nanocarrier for functional composite nanomaterials applied in multi-mode imaging,bio-labeling as well as targeting,controllability and visualization for diagnosis and therapy.However,at present,the development of UCNPs is faced with limitations such as low upconversion luminescence efficiency,fixed excitation wavelength,and so on.In order to solve the issues mentioned above,it is urgent for researchers to design and develop novel UCNPs.The development of novel lanthanide-doped upconversion nanosystem and deepening the comprehension of upconversion mechanism are of great significance in promoting applications of UCNPs.To date,most of the UCNPs studied and applied are based on sensitizer-activator co-doping system.Because of the so-called“concentration quenching effect”,the doping concentration of activators,like Er³⁺,are usually restricted as lower than 3%mol and for other activators like Tm³⁺and Ho³⁺,the doping concentrations are even lower.The low activator concentration within the UCNP has made the low upconversion luminescence efficiency an essential problem and difficult to overcome.Given the issues above,in this paper,we have developed a novel system of UCNPs with high-concentration activators(Er³⁺)benefiting from the new interpretation towards the“concentration quenching effect”mechanism.The novel nanosystem are optimized as the structure of NaErF₄@NaYF₄.It can produce highly efficient quasi-monochromatic red emission,which is different from the conventional opinions.Furthermore,we have applied the novel UCNP structure into the bio-application for imaging and therapy.The obtained creative outcomes are listed in details as follows:I)We have successfully designed a novel upconversion nanosystem:the highly Er³⁺doped UCNPs with unique optical propertiesAs known by traditional opinions,the traditional activator-sensitizer co-doping upconversion system is limited by“concentration quenching effect”,the optimal doping concentration for the activator,Er³⁺,always remains at a low level,no more than 3%mol.Even if in some certain Er³⁺singly doped system,the applied doping concentration only reaches to 20%³0%mol.In this work,we have completely overcome the“concentration quenching effect”of Er³⁺by core shell epitaxial growth to deactivate the defects on the surface of UCNPs?the main source for defects within a single UCNP?.In this way,we have successfully improved the optimal doping concentration of Er³⁺to 100%mol with efficient upconversion luminescence and unique optical properties.The UCNPs synthesized are demonstrated to have uniformed size and morphology,pure hexagonal-phase structures and clear core shell structures,which provides a favorable condition for further study on their properties and mechanism.II)We have revealed the new principle and mechanism of the unique efficient quasi-monochromatic red emission from highly Er³⁺doped UCNPsAccording to our finding,the“concentration quenching effect”of Er³⁺within UCNPs is related to the numbers of defects?namely quenching sites?.By analyzing the steady state luminescence spectra and dynamic luminescence spectra of bare core and core shell structure of UCNPs,we have demonstrated that it is possible to establish an area of“zero defect”within a single UCNP by deactivating the surface defects with inert shells.We found that after avoiding the influence of surface defects on activators,the 100%mol Er³⁺doped nanomaterial?for example:NaErF₄@NaYF₄?not only is able to generate efficient upconversion luminescence but also has some other new and unique properties like multi-wavelength excitation?⁸00 nm,⁹80nm and¹530 nm?and quasi-monochromic red emission?⁶60 nm?.These unique property of Er³⁺highly doped core shell nanostructure are of significance in enriching more options for upconversion luminescence system,deepening the comprehension of upconversion luminescence mechanism and promoting the application of UCNPs in fields like energy utilization,anti-counterfeiting,medicine and biology.III)We have established a novel photoswitchable upconversion nanoplatform based on NaErF₄ system in application of separating imaging and therapy and confirmed the feasibility to be applied in tumor diagnosis and photodynamic therapySince Er³⁺highly doped system possesses some unique optical properties like multi-wavelength excitation and efficient quasi-monochromatic emission,we combine it with a kind of traditional structure,Yb³⁺/Tm³⁺co-doping system,to establishanefficientphotoswithableupconversionnano-system:NaErF₄@NaYF₄@NaYbF₄:0.5%Tm@NaYF₄.With the shift of the excitation lasers,800 nm/980 nm,this photoswitchable nanosystem is able to achieve the“on/off”effect of UV-blue upconversion emission.Compared with traditional lanthanide-doped UCNPs involved photoswtichable nanosystem,our system has two obvious advantages:1)the novel nanosystem is free from the limitation of Nd³⁺sensitizer which simplifies the nanostructure of UCNPs,reduces the difficulty of synthesis and decreases the size of UCNPs.2)We solved the problem of being not able to completely“turn off”the UV-blue emission and accomplished a more strict spectra tuning effect during the process of photoswtiching.Furthermore,we have combined this nanosystem together with the photosensitizer TiO₂ to establish UCNP@TiO₂photosensitizing nanoplatform and applied it to the study of real-time imaging and photodynamic therapy?PDT?.The experiments are conducted on cancer cells and tumor bearing mice.The experimental results indicate that we can precisely control the separation of imaging?800 nm excitation?and PDT?980 nm?during the entire process from the nanoplatform injection to the therapy triggering.This provides a feasible and promising strategy for real-time image-guided precise treatment in vivo.In summary,in this paper,we have successfully developed a kind of novel activator Er³⁺highly doped UCNPs,NaErF₄@NaYF₄,with the feature of efficient luminescence,multi-wavelength excitation?⁸00 nm,⁹80 nm,and¹530 nm?and quasi-monochromic emission?⁶60 nm?.By further studying this nanomaterial,it is beneficial that we can better understand the physical process of upconversion luminescence and promote this nanomaterial to diverse applications,especially in fields of medicine and biology.Furthermore,by applying the NaErF₄@NaYF₄ system,wehaveconstructedthenovelphotoswitchablenanosystem,NaErF₄@NaYF₄@NaYbF₄:0.5%Tm@NaYF₄,and successfully achieved the goal of precise control in separating real-time imaging and therapy from cell level to mice level.This highlights the promising potential of upconversion functional nanomaterial being applied in medicine and biology.