Shell Structured Lanthanide Doped

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Elemental Migration in Core/Shell Structured Lanthanide Doped Nanoparticles Lu Liu, Xiaomin Li, Yong Fan, Changyao Wang, Ahmed Mohamed ElToni, Mansour Saleh Alhoshan, Dongyuan Zhao, and Fan Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01348 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Chemistry of Materials

Elemental Migration in Core/Shell Structured Lanthanide Doped Nanoparticles Lu Liu,1 Xiaomin Li,1* Yong Fan,1 Changyao Wang,1 Ahmed Mohamed El-Toni,2 Mansour Saleh Alhoshan,3 Dongyuan Zhao,1 Fan Zhang1* 1Department

of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, State Key Laboratory of Molecular Engineering of Polymers and iChem, Fudan University, Shanghai 200433, P. R. China. 2King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia 3Department of Chemical Engineering, King Saud University, Riyadh 11421, Saudi Arabia ABSTRACT: Lanthanide doped core/shell structured nanoparticles have being widely studied because of their unique optical properties and promising applications in many fields. However, the elemental migration of the lanthanide ions in the core/shell nanoparticles still lacks sufficient understanding, which may influence the optical properties of the nanoparticles. By monitoring the variation of optical properties during the post annealing progress in the solution at high temperature, the elemental migration of Er3+ in the core/shell structured NaErF4@NaYF4 luminescent nanoparticles are investigated, which are influenced by the annealing temperature, relative ion radius of lanthanide elements and doping concentration differential between two adjacent layers. Based on the dopants migration in the core/shell structured nanoparticles, the emission profile of the luminescent nanoparticles can be well tuned by the post annealing process. The findings described here suggest a general insight into constructing lanthanide doped core/shell luminescent nanomaterials with controllable dopant ions spatial distributions and energy migration in the core/shell nanostructure.

■ INTRODUCTION Lanthanide based luminescent nanomaterials, including upconversion nanoparticles, down-shifting nanoparticles, etc. have gained increasing attention for applications in the forefront of materials science and biological fields.[1-10] Different lanthanide ions that produce distinct optical properties can be easily doped in a single nanoparticle to realize the highly designable emission profiles.[11-15] The luminescence of the lanthanide doped nanoparticles are greatly influenced by species, concentration and spatial distribution of the dopants inside the host lattice.[16-19] Recently, the construction of core/shell structured nanoparticles by integrating and manipulating the complex interaction of lanthanide dopants in each layer has been a useful strategy to construct novel lanthanide doped nanoparticles. The core/shell structure engineering can be used to easily access to luminescent nanoparticles with well-defined crystal structure, diameter, morphology, surface properties and optical properties.[20-24] However, although many synthetic methods can be used to embed different lanthanide dopants into separated layers of the core/shell nanostructure, the structural integrity and elemental migration in the core/shell nanoparticles is still indeterminate, especially in the core/shell structured nanoparticles involving heavily doped layers, e. g. NaErF4@NaYF4, NaYF4@NaYbF4:Tm@NaYF4, NaErF4:Ho@NaYF4.[25-27] Wang’s research group present an indirect approach to investigate the integrity of core/shell nanostructures by using Tb3+ and Ce3+ dopant ions as luminescent probes.[28] When the concentration differential between two layers is 20 %, the heat treatment at 350 °C can promote migration of dopant ions. However, it is still lack direct evidences for the elemental migrations in the lanthanide doped

luminescent nanoparticles, especially in the heavily doped core/shell nanoparticles with superior optical properties.

Figure 1. (a) Schematic illustration of elemental migration in the core/shell nanoparticles, which is related to the annealing temperature, ion radius and doping concentration differential between two adjacent layers. (b-d) TEM, HAADF-STEM, and HRTEM images of the typical NaErF4@NaYF4 core/shell structured nanoparticles. (e) Schematic of energy migration in the core/shell structured nanoparticles before and after post annealing treatment. (f) 1530 nm emission spectra under 980-nm excitation and (g) XPS spectra of the NaErF4@NaYF4 core/shell nanoparticles before and after post annealing progress in the solution at 280 °C for 12 h. (h) Normalized intensity of the emission at 1530 nm of NaErF4@NaYF4 nanoparticles with different shell thickness at different post annealing time.

In this work, the migration of Er3+ in core/shell structured NaErF4@NaYF4 luminescent nanoparticles with high concentration gradient layers was systematically investigated

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by directly monitoring the optical properties of the nanoparticles. It is found that the ions migrations are easily occurred in the NaErF4@NaYF4 core/shell nanoparticles during the post annealing progress in the solution, even when the annealing temperature as low as 280 °C (much lower than the literature reported 350 °C [28]). Besides the annealing temperature, the migration efficiency are also influenced by the relative ion radius of lanthanide elements, doping concentration gradient between two adjacent layers etc. (Figure 1a). Based on this dopants migration in the nanoparticles at low temperature, the emission profile of the luminescent nanoparticles can be well tuned by the post annealing process.

■ MATERIALS AND METHODS Materials. Erbium chloride hexahydrate (ErCl3·6H2O, 99.9%), holmium chloride hexahydrate (HoCl3·6H2O, 99.9%), yttrium chloride hexahydrate (YCl3·6H2O, 99.9%), ytterbium chloride hexahydrate (YbCl3·6H2O, 99.9%), gadolinium chloride hexahydrate (GdCl3·6H2O, 99.9%), neodymium chloride hexahydrate (NdCl3·6H2O, 99.9%), thulium chloride hexahydrate (TmCl3·6H2O, 99.9%), terbium chloride hexahydrate (TbCl3·6H2O, 99.9%), lutetium chloride hexahydrate (LuCl3·6H2O, 99.9%), sodium trifluoroacetate (Na-TFA, 98%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH, 96%), ammonium fluoride (NH4F, 96%), ethanol, methanol and cyclohexane were purchased from Beijing Chemical Reagents Co. Ltd. All chemicals were used as received without any further purification. Preparation of Shell Precursors. Ln-OA host precursor (Ln = Y, Gd, Lu, 0.10 M): 2.50 mmol of LnCl3 , 10.0 mL of OA, and 15.0 mL of ODE was loaded in a three-necked bottle. The mixture was heated to 140 °C under magnetic stirring and vacuum for 0.5 h. Finally, the 0.10 M Ln-OA precursors was obtained. Gd/Tb-OA dopants precursor: The synthesis of Gd,15%TbOA (0.10 M) was carried out all the same as that of Ln-OA precursor except 2.50 mmol of Gd-Tb chloride (2.125 mmol of GdCl3 and 0.375 mmol of TbCl3) were used instead of LnCl3. Nd/Yb-OA (0.05 M) dopants precursor: a mixture of NdCl3 (1.4 mmol), YbCl3 (0.6 mmol), OA (8.0 mL), and ODE (32.0 mL) was loaded in a reaction container and heated at 140 °C under vacuum with magnetic stirring for 30 min to remove residual water and oxygen. Then the Nd,30%Yb-OA precursor solution was obtained. Na-TFA-OA (0.40 M) precursor: 4.0 mmol of Na-TFA and 10 mL of OA was loaded in a three-necked bottle at room temperature under vacuum and magnetic stirring. Then, the colorless Na-TFA-OA precursor solution was obtained, sealed and used for the shell coating. Synthesis of β-NaErF4 Core Nanoparticles. Hexagonal phase NaErF4 nanocrystals were synthesized following a previously reported thermolysis method.[29] In a typical synthesis, 1.0 mmol of ErCl3·6H2O, 6.0 mL of OA and 15.0 mL of ODE were mixed and heated to 140 °C under vacuum until a clear solution formed, after that, the solution was cooled down to room temperature. Then, a methanol solution (10.0 mL) of

ammonium fluoride (4.0 mmol) and sodium hydroxide (2.5 mmol) was added and stirred for 1 h. The reaction mixture was then heated to 70 °C and maintained for half an hour to remove the methanol. Afterward, the solution was heated to 290 °C (∼10 °C/min) and maintained for 60 min under a gentle argon flow. Then, the solution was cool down to room temperature. The nanoparticles were centrifuged and washed twice with ethanol and finally dispersed in 10 mL of cyclohexane for further use. The synthesis of NaY(Er)F4 with different Er3+ doping concentrations (5, 20, 100 mol%), NaYF4:30%Gd,25%Yb,2%Ho, NaYF4:30%Gd,25%Yb,2%Er and NaGdF4:25%Yb,0.5%Tm core nanoparticles were similar to that of NaErF4 nanoparticles (see the Supporting Information for the detailed synthesis process). Synthesis of NaErF4@NaYF4 Core/Shell Nanoparticles. The core/shell nanoparticles were fabricated by using the onepot successive layer-by-layer (SLBL) protocol, which was developed by our group.[30] The purified NaErF4 core nanoparticles solution (0.25 mmol) were mixed with 4.0 mL of OA and 6.0 mL of ODE. The flask was heated at 70 °C for 0.5 h under vacuum and magnetic stirring to remove residual cyclohexane and air. Subsequently, the system was switched to inert gas flow (Ar or N2). The reaction mixture was further heated to 280 °C at a rate of 20 °C/min. Then pairs of Y-OA (0.10 M, 1.0 mL) and Na-TFA-OA (0.40 M, 0.50 mL) precursors were alternately introduced by dropwise addition at 280 °C and the time interval between each injection was 15 min. The shell thickness can be well tuned by changing the addition times or the amount of the shell precursors. The synthesis of the NaY(Er)F4@NaYF4, NaYF4:Gd,Yb,Ho@NaYF4, NaYF4:Gd,Yb,Er@NaYF4, NaGdF4:Yb,Tm@NaYF4 core/shell nanoparticles were similar to that of the NaErF4@NaYF4 core/shell nanoparticles except NaY(Er)F4, NaYF4:Gd,Yb,Ho, NaYF4:Gd,Yb,Er, NaGdF4:Yb,Tm were used as core instead of NaErF4. The synthesis of the NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb, NaYF4:Gd,Yb,Er@NaYF4@NaNdF4:Yb core/shell/shell nanoparticles were similar to that of the NaErF4@NaYF4 core/shell nanoparticles except NaYF4:Gd,Yb,Ho@NaYF4, NaYF4:Gd,Yb,Er@NaYF4 nanoparticles were used as core instead of NaErF4. Nd/Yb-OA (0.10 M) was used as shell coating precursor. The synthesis of the NaGdF4:Yb,Tm@NaYF4@NaGdF4:Tb core/shell/shell nanoparticles were similar to that of the NaErF4@NaYF4 core/shell nanoparticles except NaGdF4:Yb,Tm@NaYF4 nanoparticles were used as core instead of NaErF4. Gd/Tb-OA (0.10 M) was used as shell coating precursor. In order to avoid the dissolution of lanthanide ions on the surface of the core/shell nanoparticles into the solution, the mother solution of the core/shell nanoparticles is directly used for the post annealing process. In this mother solution, a dynamic equilibrium has been formed between the dissolution and deposition of lanthanide ions on the surface of the nanoparticles. All the samples are re-dispersed in cyclohexane after annealing and calibrated by the Er3+ absorption at 1530 nm for the photoluminescence measurement.


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Figure 2. TEM images and evolution of NIR emission spectra of NaErF4@NaYF4 (a, d), NaErF4@NaLuF4 (b, e), NaErF4@NaGdF4 (c, f), core/shell nanoparticles (core size: 19.6 nm; shell thickness: 5.3 nm) during the post annealing progress in the solution at 280 °C. (g) Normalized intensity of the emission at 1530 nm of NaErF4@NaGdF4, NaErF4@NaLuF4, NaErF4@NaYF4 core/shell nanoparticles at different post annealing time.

■ RESULTS AND DISCUSSION The NaErF4 core nanoparticles with an average diameter of 19.6 nm are synthesized and followed by the epitaxial deposition of a NaYF4 shell with controllable thickness (1.2 nm, 5.3 nm, 10.2 nm) by using layer-by-layer epitaxial growth strategy (Figure S1, S2). It can be seen that the as-prepared core/shell structured nanoparticles are very uniform in size and morphology (Figure 1b). The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is employed to identify the formation of the core/shell structure, in which the brighter regions correspond to NaErF4 cores and the darker parts correspond to the NaYF4 shell (Figure 1c). Furthermore, high resolution TEM (HRTEM) image (Figure 1d) of the resultant core/shell nanoparticles show that the nanoparticles are hexagonal crystalline phase without any significant impurity phases, which is consistent with the result of the powder X-ray diffraction patterns (Figure S3). It has been demonstrated that the major quenching process for erbium-enriched NaErF4 nanoparticles is the energy migration to the surface quenchers.[25,27,31] In the NaErF4@NaYF4 core/shell nanoparticles, the migration of Er3+ from NaErF4 core to NaYF4 shell can induce the quenching of the emission according to the energy transfer from the activators in the inner core to the surface defects (Figure 1e). So, the intensity of emission dominated by Er3+ ions can provide a sensitive probe of elemental migration of Er3+ in the NaErF4@NaYF4 core/shell host lattice during the post annealing progress in the solution at 280 °C. Figure 1f shows representative down-shifting luminescence spectra of the NaErF4@NaYF4 core/shell nanoparticles under the excitation of 980 nm before and after the post annealing for 12 h. Although the size and morphology of the core/shell nanoparticles are maintained (Figure S5), the down-shifting emission at 1530 nm dominated by 4I13/2-4I15/2 transition of Er3+ is greatly decreased

by about 47 %, so do the upconversion emissions at 650 nm and 540 nm (Figure S6). The surface compositions of NaErF4@NaYF4 core/shell nanoparticles are characterized by XPS before and after post annealing treatment (Figure 1g). The amount of Er3+ in the outermost surface of the nanoparticles is greatly increased, which means that the Er3+ migrate from the NaErF4 inner core to NaYF4 shell. So, the quenching of Er3+ dominated luminescence can be attributed to the Er3+ migration, which will further induce the break over of the energy transfer between the inside Er3+ activators and surface quenchers (Figure 1e). It is found that this cross talk between the emission activators and the surface quenchers can be greatly inhibited by increasing the thickness of the NaYF4 passivation shell, and vice versa (Figure 1h). When the thickness of the NaYF4 shell increased from 5.3 nm to 10.2 nm, the Er3+ dominated emission at 1530 nm is decreased less than 10 % after post annealing at 280 °C for 12 h (Figure S8, S9). It is worth to mention that the Er3+ ions can also be detected in the outer surface of the NaErF4@NaYF4 core/shell nanoparticles before the annealing process (Figure 1g), which can be attributed to the dissolution of NaErF4 core during the shell coating process.[32] Our research object in this paper is the as-made core/shell nanoparticles, not the nanoparticles before or during the shell coating. The pre-existing Er3+ in the shell of the as-made core/shell nanoparticles cannot influence the further elemental migration between the core and shell during the post annealing process. In order to avoid the dissolution of lanthanide ions on the surface of the NaErF4@NaYF4 core/shell nanoparticles into the solution, the mother solution of the core/shell nanoparticles is directly used for the post annealing process. In this mother solution, a dynamic equilibrium has been formed between the dissolution and deposition of lanthanide ions on the surface of the nanoparticles.



Figure 3. TEM images and evolution of down-shifting emission spectra of NaErF4@NaYF4 core/shell nanoparticles during the post annealing progress in the solution at 200 °C (a, d), 280 °C (b, e) and 300 °C (c, f). (g) Normalized intensity of the emission at 1530 nm of NaErF4@NaYF4 core/shell nanoparticles at different temperature and post annealing time.

Hexagonal phase NaLnF4 (Ln = Gd, Lu, Y) was often chosen as the host matrix and inert protecting shell for its weak electron-phonon interactions with 4fn electrons of the lanthanide dopant ions. Due to the considerable discrepancy on the ionic radius of different lanthanide ions (Er3+: 1.062 Å; Gd3+: 1.107 Å; Lu3+: 1.032 Å; Y3+: 1.075 Å),[33] the migration speed of Er3+ in the different inert host crystal lattices maybe different. The NaErF4@NaGdF4, NaErF4@NaLuF4, NaErF4@NaYF4 with the same morphology, core size (19.6 nm) and shell thickness (5.3 nm) (Figure 2a-c) are fabricated and post annealed at 280 °C for 12 h. It is found that the morphologies of the nanoparticles are all maintained very well (Figure 2a-c) after post annealing treatment. However, the evolutions of 1530 nm emission along with post annealing progress are quite different for the three kinds of core/shell structured nanoparticles. Compared with NaErF4@NaYF4 nanoparticles (Figure 2d), the descending speed of the downshifting emission from NaErF4@NaLuF4, NaErF4@NaGdF4 nanoparticles are much slower (Figure 2g). Especially for the NaErF4@NaGdF4, the emission intensity is most stable one in the three. These results indicate that the migration of Er3+ in the NaGdF4 host crystal is more difficult than that of in the NaLuF4 and NaYF4. By comparing the ionic radius of Er3+ with Gd3+, Lu3+ and Y3+, the discrepancies between Er3+ and Gd3+, Lu3+, Y3+ are 4.2 %, 2.8 % and 1.2 %, respectively. It has been demonstrated that the vacancies exist in the NaREF4 crystal host.[34] The elemental migration within the crystal lattice occurs according to the position exchange between the vacancies and the lanthanide atoms.[35] It means that not only the lanthanide atoms, but also the vacancies are movable in the host crystal. The movement of an atom within the core/shell crystal is governed by the kinetic energy of the atom within the crystal lattice that can be affected by the temperature, the ionic radius, the concentration gradient and so on. In the case of ions migration in the core/shell structured crystal host with similar ionic radius, the activation energy for migration is decreased since the ions can be well accommodated in the pre-existing vacancies. So, we can conclude that the elemental migration is more likely to occur when the ionic radius discrepancy is smaller between two adjacent layers.

Besides the ionic radius, the Er3+ migration in the core/shell structured NaErF4@NaYF4 nanoparticles can also be influenced by annealing temperature. After the post annealing treatment at 200 °C, 280 °C and 300 °C, the morphology of the core/shell nanoparticles maintain very well (Figure 3a-c). The intensity of down-shifting emission at 1530 nm dropped off only 8% after the post annealing at 200 °C for 12 h (Figure 3d). When the annealing temperature was increased to 280 °C and 300 °C, the intensity of the emission sharply decreased by 47 % and 91 %, respectively (Figure 3e, f). The similar tendency can also be observed for the upconversion emission in visible region dominated by Er3+ activators (Figure S10). As shown in Figure 3g, the 1530 nm emission intensity of the nanoparticles decreased much faster when annealing at relative higher temperature, indicating that the migration speed of Er3+ from the NaErF4 inner core to NaYF4 shell is faster at a higher temperature. Recently, high doping concentrations of sensitizers or activators have been developed to realize super bright emissions under mild excitation.[36-39] As another important factor influencing the elemental migration, the doping concentration differential between two adjacent layers should also be considered, especially in the core/shell structured nanoparticles with high doping amount. The NaYF4:x%Er@NaYF4 (x = 5, 20, 100) nanoparticles with different Er3+ doping concentrations in the inner core are synthesized and post annealed at 280 °C for 12 h. Both the down-shifting and upconversion emission of NaYF4:5%Er@NaYF4 nanoparticles are very stable during the post annealing progress (Figure 4a, S11). In stark contrast, for the nanoparticles with 100 % Er3+ doping in the inner core, the intensity of the down-shifting emission at 1530 nm sharply decreased by 47 % after post annealed at 280 °C for 12 h (Figure 4b, c), so do the upconversion emissions in the visible region (Figure S11). So, we consider that the high concentration discrepancy between two adjacent layers can promote the elemental migration in the core/shell structure.


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Figure 4. (a-c) The evolution of NIR emission spectra of NaYF4:x%Er@NaYF4 core/shell nanoparticles with different Er3+ doping concentrations (a, 5 %; b, 20 %; c, 100 %) in the inner core during the post annealing progress at 280 °C. (d) Normalized intensity of the emission at 1530 nm of NaYF4:x%Er@NaYF4 core/shell nanoparticles at different post annealing time.

It is interesting that the emission intensity of NaYF4:20%Er@NaYF4 core/shell nanoparticles increased about 20 % after annealing at 280 °C for 6 h, and then decreased with the following annealing process. The increasing of the emission intensity can be attributed to the relative alleviation of cross relaxation among the Er3+, which is induced by the migration of Er3+ from core to shell. In comparison, when the doping concentration is 5 %, cross relaxation among the Er3+ is much lower and the elemental migration is depressed because of low doping concentration. So, the emission is very stable during the post annealing progress. When the doping concentration is 100 %, the migration speed of Er3+ are greatly promoted. Although the cross relaxation among the Er3+ can also be relatively alleviated, the surface quenching effect can rapidly influence the emission and induced the sharp decreasing of the emission. In addition, it is found that elemental migration can also be influenced by the size of core nanoparticles. As shown in Figure S15, different sized NaErF4 core nanoparticles are synthesized and coated with similar thickness of NaYF4 shell (~ 5.3 nm). The emission intensity at 1530 nm of the nanoparticles with smaller core (19.6 nm) decreased faster during the post annealing process than that of the core/shell nanoparticles with bigger core (28.2 nm), indicating that the elemental migration in the core/shell nanoparticles with smaller core size is faster than the bigger one. The faster elemental migration at the interface between the core and shell of the core/shell nanoparticles with smaller core can be attributed to the relative lager amount of surface defects (e. g. vacancies) for the small cores. It has been demonstrated that the distribution of the lanthanide dopants in the core/shell structured nanoparticles has a great influence on the migration of energy within the core/shell structure. So, we hypothesize that the optical properties of the nanoparticles can also be manipulated based on above mentioned elemental migration in the core/shell structure. To verify this assumption, the NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb core/shell/shell

structured nanoparticles was fabricated and post annealed at 280 °C for 12 h. As shown in Figure 6a, Nd3+ ions in the outer most layer are used as sensitizer to harvest the 808 nm excitation light. The Yb3+ ions are introduced to extract the excitation energy from Nd3+ ions. NaYF4 interlayer are designed as the transition layer between the outer Nd3+ sensitizer and inner Ho3+ activators. The presence of NaYF4 transition layer is crucial for regulating the emission profile of the nanoparticles, because the dopants in both outer shell and inner core can migrate into this transition layer to mediate the energy transfer between the outer shell and inner core. It can be seen that the monodispersed NaYF4:Gd,Yb,Ho core nanoparticles grown from 20 nm to 25 nm after uniformly coating with NaYF4 transition layer (Figure 5a, b). Due to preferential orientation growth of NaNdF4 on the (001) crystal face of NaYF4:Gd,Yb,Ho@NaYF4 nanoparticles,[32] the NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb nanoparticles with short-rod morphology was obtained after coating with NaNdF4:Yb outer most shell. The size and morphology of the obtained core/shell/shell nanoparticles are maintained after the post annealing at 280 °C for 12 h.

Figure 5. TEM images of the obtained NaYF4:Gd,Yb,Ho core nanoparticles (a), NaYF4:Gd,Yb,Ho@NaYF4 core/shell nanoparticles (b), NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb core/shell/shell nanoparticles (c), and the post annealed NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb nanoparticles (d).

Initially,

the

NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb

core/shell/shell structured nanoparticles can generate three

upconversion emission bands at ~ 540 nm (green), ~ 645 nm (red), and ~ 580 nm (yellow) under the excitation of 808 nm, which can be attributed to the 4F4 → 5I8, 4F5 → 5I8 transitions of Ho3+ and 2H11/2 → 4I9/2 transition of Nd3+, respectively (Figure 6b). The ratio of the three emission bands can be tuned by the post annealing process. It can be seen that the intensity of green and red upconversion emission bands dominated by Ho3+ increased as the prolonging of annealing time, while the yellow emission band dominated by Nd3+ decreased. So, for this core/shell structured nanoparticles with a fixed composition, the emission colour can be tuned by post annealing treatment (Figure 6c). The variation of emission intensities for different emission bands should be attributed to the change in composition of NaYF4 transition layer. During the post annealing process, Yb3+ ions in NaYF4:Gd,Yb,Ho core and NaNdF4:Yb outer layer gradually migrate into the inert NaYF4



transition layer, which result in the enhanced energy transfer from Nd3+ in the outer layer to the Ho3+ ions in the core (Figure 6a), and further induced increasing of emissions dominated by Ho3+ and decreasing of the emissions dominated by Nd3+. When the Ho3+ activators in the inner core are replaced with Er3+ (NaYF4:Gd,Yb,Er@NaYF4@NaNdF4:Yb, Figure S12), similar phenomenon can also be observed. The Er3+ dominated upconversion emission at 540 and 654 nm under the excitation of 800 nm can be tuned by controlling the post-annealing time.

nanoparticles. It is demonstrated that the ions migration indeed occur in the NaErF4@NaYF4 core/shell structured nanoparticles during the post annealing progress in the solution, even when the annealing temperature is as low as 280 °C. The migration of the doping ions are influenced by the annealing temperature, relative ion radius of lanthanide elements and doping concentration differential between two adjacent layers. The dopants migration in the core/shell nanoparticles can be used as a valuable strategy for tuning of the emission profile of the NIR luminescent nanoparticles. However, to avoid harmful elemental migration, the low temperature reaction, thick insulation layer, optimized doping concentration and spatial distributions of the dopants are indispensable. Our results provide new fundamental insights into elemental migration in core/shell structured lanthanide doped nanoparticles, which will provide a general guideline for the synthesis of lanthanide doped nanoparticles with controllable dopant ions spatial distributions and energy migration in the core/shell nanostructure.

■ ASSOCIATED CONTENT

Figure 6. (a) Scheme illustration of the energy transfer in the core/shell/shell structured NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb nanoparticles. (b) The evolution of upconversion emission spectra of NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb nanoparticles during the post annealing progress in the solution at 280 °C. (c) The corresponding emission colour of the samples in b on the chromaticity diagram.

Similarly, the emission profile of the NaGdF4:Yb,Tm@NaYF4@NaGdF4:Tb core/shell/shell nanoparticles can also be manipulated by the post-annealing induced elemental migration (Figure S13). Yb3+ ions in the core are used as sensitizer to harvest the 980-nm excitation light. The Tm3+ ions are introduced to extract the excitation energy from Yb3+ and generate the high-lying energy states. A Gd3+ migrator extracts the excitation energy from high-lying energy states of Tm3+, followed by transferring the energy to Tb3+. Then, Tb3+ dominated upconversion emissions can be obtained. However, in the NaGdF4:Yb,Tm@NaYF4@NaGdF4:Tb core/shell/shell nanoparticles, the energy transfer from Gd3+ to Tb3+ is inhibited because of the presence of the inert NaYF4 middle separation layer. During the post annealing process, Gd3+ migrator in NaGdF4:Yb,Tm core and NaGdF4:Tb outer layer gradually migrate into the inert NaYF4 transition layer, which result in the enhanced energy transfer from Tm3+ in the inner core to the Tb3+ ions in the outer shell, and further induced the increasing of intensity and life time of the upconversion emissions dominated by Tb3+.

Supporting Information. Detailed process for the synthesis of NaY(Er)F4, NaYF4:Gd,Yb,Ho, NaYF4:Gd,Yb,Er, NaGdF4:Yb,Tm, core nanoparticles; TEM, XRD, PL measurement of the obtained nanoparticles. Time decay curves of NaErF4@NaYF4 nanoparticles. High-angle annular dark field scanning transmission electron microscopy, elemental mapping, and energy dispersive Xray analysis of NaYF4:Gd,Yb,Ho@NaYF4@NaNdF4:Yb nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Funding Sources The work was supported by the National Key R & D Program of China (2017YFA0207303, 2018YFA0209400), National Natural Science Foundation of China (21875043, 21725502, 21701027), Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (17JC1400100), Natural Science Foundation of Shanghai (18ZR1404600), and Shanghai Sailing Program (17YF1401000). The authors extend their appreciation to the International Scientiﬁc Partnership Program ISPP at King Saud University for funding this research work through ISPP#0100.

Notes The authors declare no competing financial interest.

■ REFERENCES

■ CONCLUSIONS

(1) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning upconversion through energy migration in coreshell nanoparticles. Nat. Mater. 2011, 10, 968-973.

The migration of Er3+ in the NaErF4@NaYF4 core/shell nanoparticles have been systematically investigated by directly monitoring the variation of optical properties of the

(2) Zhang, F.; Braun, G. B.; Pallaoro, A.; Zhang, Y.; Shi, Y.; Cui, D.; Moskovits, M.; Zhao, D.; Stucky, G. D. Mesoporous multifunctional upconversion luminescent and magnetic “Nanorattle” materials for targeted chemotherapy. Nano Lett. 2012, 12, 61-67.


Page 6 of 9

Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials (3) Heer, S.; Kömpe, K.; Güdel, H.; Haase, M. Highly Efficient Multicolour Upconversion Emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals. Adv. Mater. 2004, 16, 21022105. (4) Wang, Y.; Liu, G.; Sun, L.; Xiao, J.; Zhou, J.; Yan, C. Nd3+sensitized upconversion nanophosphors: efficient in vivo bioimaging probes with minimized heating effect. ACS Nano 2013, 7, 7200-7206. (5) Wolfbeis, O. S. An overview of nanoparticles commonly used in fluorescent bioimaging. Chem. Soc. Rev. 2015, 44, 4743-4768. (6) Chen, G.; Ågren, H.; Ohulchanskyya, T. Y.; Prasad, P. N. Light upconverting core-shell nanostructures: nanophotonic control for emerging applications. Chem. Soc. Rev. 2015, 44, 1680-1713. (7) Garfield, D. J.; Borys, N. J.; Hamed, S. M.; Torquato, N. A.; Tajon, C. A.; Tian, B.; Shevitski, B.; Barnard , E. S.; Suh, Y. D.; Aloni, S.; Neaton, J. B.; Chan, E. M.; Cohen, B. E.; Schuck, P. J. Enrichment of molecular antenna triplets amplifies upconverting nanoparticle emission. Nat. Photonics 2018, 12, 402-407. (8) Yuan, D.; Tan, M. C.; Riman, R. E.; Chow, G. M. Comprehensive study on the size effects of the optical properties of NaYF4:Yb,Er nanocrystals. J. Phys. Chem. C 2013, 117, 13297-13304. (9) Swabeck, J. K.; Fischer, S.; Bronstein, N. D.; Alivisatos, A. P. Broadband sensitization of lanthanide emission with indium phosphide quantum dots for visible to near-infrared downshifting. J. Am. Chem. Soc. 2018, 140, 9120-9126. (10) Zhong, Y.; Ma, Z.; Zhu, S.; Yue, J.; Zhang, M.; Antaris, A. L.; Yuan, J.; Cui, Wan, R. H.; Zhou, Y.; Wang, W.; Huang, N. F.; Luo, J.; Hu, Z.; Dai, H. Boosting the down-shifting luminescence of rare-earth nanocrystals for biological imaging beyond 1500 nm. Nat. Commun. 2017, 8, 737. (11) Wen, H.; Zhu, H.; Chen, X.; Hung, T. F.; Wang, B.; Zhu, G.; Yu, S. F.; Wang, F. Upconverting near-infrared light through energy management in core-shell-shell nanoparticles. Angew. Chem. Int. Ed. 2013, 52, 13419-13423.

(21) DiMaio, J. R.; Sabatier, C.; Kokuoz, B.; Ballato, J. Controlling energy transfer between multiple dopants within a single nanoparticle. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 1809-1813. (22) Vetrone, F.; Naccache, R.; Mahalingam, V.; Morgan, C. G.; Capobianco, J. A. The active-core/active-shell approach: a strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles. Adv. Funct. Mater. 2009, 19, 2924-2929. (23) Qian, H.; Zhang, Y. Synthesis of hexagonal-phase core-shell NaYF4 nanocrystals with tunable upconversion fluorescence. Langmuir 2008, 24, 12123-12125. (24) Li, X.; Zhou, L.; Wei, Y.; El-Toni, A. M.; Zhang, F.; Zhao, D. Anisotropic growth-induced synthesis of dual-compartment janus mesoporous silica nanoparticles for bimodal triggered drugs delivery. J. Am. Chem. Soc. 2014, 136, 15086-15092. (25) Johnson, N. J.; He, S.; Diao, S.; Chan, E. M.; Dai, H.; Almutairi, A. Direct evidence for coupled surface and concentration quenching dynamics in lanthanide-doped nanocrystals. J. Am. Chem. Soc. 2017, 139, 3275-3282. (26) Chen, X.; Jin, L.; Kong, W.; Sun, T.; Zhang, W.; Liu, X.; Fan, J.; Yu, S. F.; Wang, F. Confining energy migration in upconversion nanoparticles towards deep ultraviolet lasing. Nat. Commun. 2016, 7, 10304. (27) Liu, L.; Wang, S.; Zhao, B.; Pei, P.; Fan, Y.; Li, X.; Zhang, F. Er3+ Sensitized 1530 nm to 1180 nm second near-Infrared window upconversion nanocrystals for in vivo biosensing. Angew. Chem. Int. Ed. 2018, 57, 7518-7522. (28) Chen, B.; Peng, D.; Chen, X.; Qiao, X.; Fan, X.; Wang, F. Establishing the structural integrity of core-shell nanoparticles against elemental migration using luminescent lanthanide probes. Angew. Chem. Int. Ed. 2015, 54, 12788-12790. (29) Wang, F.; Deng, R.; Liu, X. G. Preparation of core-shell NaGdF4 nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes. Nat. Protoc. 2014, 9, 1634-1644.

(12) Li, X.; Guo, Z.; Zhao, T.; Lu, Y.; Zhou, L.; Zhao, D.; Zhang, F. Filtration shell mediated power density independent orthogonal excitations-emissions upconversion luminescence. Angew. Chem. Int. Ed. 2016, 55, 2464-2469.

(30) Li, X.; Shen D.; Yang, J.; Yao, C.; Che, R.; Zhang, F.; Zhao, D. Successive layer-by-layer strategy for multi-shell epitaxial growth: shell thickness and doping position dependence in upconverting optical properties. Chem. Mater. 2013, 25, 106-122.

(13) Deng, R.; Qin, F.; Chen, R.; Huang, W.; Hong, M.; Liu, X. Temporal full-colour tuning through non-steady-state upconversion. Nat. Nanotechnol. 2015, 10, 237-242.

(31) Chen, Q.; Xie, X.; Huang, B.; Liang, L.; Han, S.; Yi, Z.; Wang, Y.; Li, Y.; Fan, D.; Huang, L.; Liu, X. Confining excitation energy in Er3+-sensitized upconversion nanocrystals through Tm3+-mediated transient energy trapping. Angew. Chem. Int. Ed. 2017, 56, 7605-7609.

(14) Zuo, J.; Sun, D.; Tu, L.; Wu, Y.; Cao, Y.; Xue, B.; Zhang, Y.; Chang, Y.; Liu, X.; Kong, X.; Buma, W. J.; E.; Meijer, J.; Zhang, H. Precisely tailoring upconversion dynamics via energy migration in core-shell nanostructures. Angew. Chem. Int. Ed. 2018, 57, 3054-3058. (15) Liu, X.; Yan, C.; Capobianco, J. A. Photon upconversion nanomaterials. Chem. Soc. Rev. 2015, 44, 1299-1301. (16) Liu, Y.; Lu, Y.; Yang, X.; Zheng, X.; Wen, S.; Wang, F.; Vidal, X.; Zhao, J.; Liu, D.; Zhou, Z.; Ma, C.; Zhou, J.; Piper, J. A.; Xi, P.; Jin, D. Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy. Nature 2017, 543, 229-233. (17) Wang, F.; Liu, X. Multicolor tuning of lanthanide-doped nanoparticles by single wavelength excitation. Acc. Chem. Res. 2014, 47, 1378-1385. (18) Shen, J.; Chen, G.; Vu, A. M.; Fan, W.; Bilsel, O. S.; Chang, C. C.; Han, G. Engineering the upconversion nanoparticle excitation wavelength: cascade sensitization of tri-doped upconversion colloidal nanoparticles at 800 nm. Adv. Opt. Mater. 2013, 1, 644-650. (19) Chen, G.; Qiu, H.; Prasad, P. N.; Chen, X. Upconversion nanoparticles: design, nanochemistry, and applications in theranostics. Chem. Rev. 2014, 114, 5161-5214. (20) Mai, H.; Zhang, Y.; Sun, L.; Yan, C. Highly efficient multicolor up-conversion emissions and their mechanisms of monodisperse NaYF4: Yb, Er core and core/shell-structured nanocrystals. J. Phys. Chem. C 2007, 111, 13721-13729.

(32) Liu, D.; Xu, X.; Du, Y.; Qin, X.; Zhang, Y.; Ma, C.; Wen, S.; Ren, W.; Goldys, E. M.; Piper, J. A.; Dou, S.; Liu, X.; Jin, D. Threedimensional controlled growth of monodisperse sub-50 nm heterogeneous nanocrystals. Nat. Commun. 2016, 7, 10254. (33) Shannon, R. Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides. Acta Cryst. 1976, A32, 751. (34) Sun, M. Dong, H. Dougherty, A. W. Lu, Q. Peng, D. Wong, W. Huang, B. Sun, L. Yan, C. Nanophotonic energy storage in upconversion nanoparticles. Nano Energy 2019, 56, 473-481. (35) Weisberg, L. R. Blanc, J. Diffusion with InterstitialSubstitutional Equilibrium. Zinc in GaAs. Physical Review 1963, 131, 1548-1552. (36) Zhan, Q.; Qian, J.; Liang, H.; Somesfalean, G.; Wang, D.; He, S.; Zhang, Z.; Andersson-Engels, S. Using 915 nm laser excited Tm3+/Er3+/Ho3+-doped NaYbF4 upconversion nanoparticles for in vitro and deeper in vivo bioimaging without overheating irradiation. ACS Nano 2011, 5, 3744-3757. (37) Shi, R.; Ling, X.; Li, X.; Zhang, L.; Lu, M.; Xie, X.; Huang, L.; Huang, W. Tuning hexagonal NaYbF4 nanocrystals down to sub-10 nm for enhanced photon upconversion. Nanoscale, 2017, 9, 13739-13746. (38) Chen, G.; Shen, J.; Ohulchanskyy, T. Y.; Patel, N. J.; Kutikov, A.; Li, Z.; Song, J.; Pandey, R. K.; Ågren, H.; Prasad, P. N.; Han, G. (α-NaYbF4:Tm3+)/CaF2 core/shell nanoparticles with efficient near-



infrared to near-infrared upconversion for high-contrast deep tissue bioimaging. ACS Nano 2012, 6, 8280-8287.

and anticancer drug delivery to overcome multi-drug resistance. Biomaterials 2015, 40, 107-116.

(39) Tian, G.; Zheng, X.; Zhang, X.; Yin, W.; Yu, J.; Wang, D.; Zhang, Z.; Yang, X.; Gu, Z.; Zhao, Y. TPGS-stabilized NaYbF4:Er upconversion nanoparticles for dual-modal fluorescent/CT imaging


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Table of Contents


9

Shell Structured Lanthanide Doped

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