808 nm Excited Multicolor Upconversion Tuning through Energy

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C: Plasmonics, Optical Materials, and Hard Matter

808 nm Excited Multicolor Up-Conversion Tuning Through Energy Migration in Core-Shell-Shell Nanoarchitecture Tao Wang, Haifeng Zhou, Zhichao Yu, Guangjun Zhou, Juan Zhou, Dapeng Huang, Leilei Sun, Peng Gao, Yuzhen Sun, and Jifan Hu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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The Journal of Physical Chemistry

808 nm Excited Multicolor Up-Conversion Tuning through Energy Migration in Core-Shell-Shell Nanoarchitecture Tao Wang,† Haifeng Zhou,‡ Zhichao Yu,† Guangjun Zhou,*,†,ǁ Juan Zhou,*,§ Dapeng Huang,† Leilei Sun, † Peng Gao,† Yuzhen Sun,† and Jifan Hu† † State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China ‡ School of Materials Science and Engineering, Qilu University of Technology, Jinan 250353, P. R. China § Center for Disease Prevention and Control of Jinan Military Command, Jinan 250014, P. R. China ǁ Shenzhen Research Institute of Shandong University, Shenzhen 518057, P. R. China.

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 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

ABSTRACT NaGdF4: A (A=Eu, Tb)@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd core-shell-shell nanoarchitecture is designed to achieve 808 nm excited up-conversion emission tuning. Base on above core-shellshell nanostructure, intense up-conversion emission has been realized for activators without long-lived intermediate states (Eu3+, Tb3+) through Gd3+-mediated energy migration under 808 nm irradiation, enriching the emission colors. The spatial separation, where sensitizer (Nd3+), accumulator (Tm3+), activator (Eu3+, Tb3+) are doped into separated layers, effectively suppresses nonradiative decays so that the doping concentration of Nd3+ can reach to 40%, vastly enhancing the luminescence intensity. Notably, when Gd3+ ions are replaced by Nd3+ or inert Y3+ in NaGdF4: Yb, Nd outershell, without Gd3+-mediated energy migration, the deleterious energy transfer from Tm3+ in interlayer to surface quenchers is suppressed and, thus, more active energy is trapped by activators, which induces the further change of up-conversion emission color. Furthermore, the multicolor up-conversion tuning can also be realized via Tb3+-mediated energy migration. 808 nm excited multicolor up-conversion tuning, overcoming low tissue penetration and overheating effect under 980 nm excitation, improves the feasibility of up-conversion nanoparticles in multicolor imaging and multiplexed detection areas.

KEYWORDS: 808 nm excitation; multicolor; energy migration; spatial separation; surface suppression;


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1. INTRODUCTION Lanthanide-doped up-conversion nanoparticles (UCNPs) have attracted extensively attention as a key research topic, owing to their distinct optical properties, such as sharp emission peaks, high photostability, low toxicity and deep penetration depth, which make them become ideal candidates for application in biological labeling and imaging areas.1-9 Tuning emission color of UCNPs is of particular importance for multicolor bio-labeling and multiplexed detection.10-12 In recent years, tunable up-conversion emissions spanning from ultraviolet to visible and nearinfrared regions have been realized by the control of emission peaks and relative emission intensities through manipulating the host-dopant combinations and doping concentration.13-16 For instance, with increasing Yb3+ doping concentration, the emission color of cubic NaYF4: Yb, Er nanocrystals (NCs) varies from yellow to red. With appropriate doping ratio, Yb3+, Er3+, Tm3+ tri-doped cubic NaYF4 NPs exhibit white up-conversion emission.17 With low Yb3+ doping concentration, the KMgF3: Yb, Er NCs already yield single-band red up-conversion emission.18 However, the excessive doping concentration and co-doping of different activators in the same layer will induce the deleterious interaction (e.g., cross-relaxation) between doped ions, severely quenching luminescence.19 On the other hand, the most common activators, applied to achieve efficient up-conversion emission, are restricted to Er3+, Tm3+ and Ho3+, due to the fact that their ladder-like energy levels ensure effective sensitization by Yb3+ ions.20 In contrast, a series of lanthanide ions without long-lived intermediate states can’t be utilized as activators to accomplish their own intrinsic emission.21 In 2011, Liu group proposed energy migrationmediated up-conversion (EMU) mechanism to realize photon up-conversion in NaGdF4: Yb, Tm@ NaGdF4: A (A3+= Tb3+, Eu3+, Dy3+ and Sm3+) core-shell (CS) nanoarchitecture.22 The Gd3+-mediate energy migration bridges the energy transfer from accumulator (Tm3+) to activator



(Tb3+, Eu3+, Dy3+ and Sm3+). In consequence, tunable up-conversion emissions are realized for these activators without long-lived intermediate states. Moreover, the spatial separation between sensitizer/accumulator and activator minimizes deleterious nonradiative decays, such as crossrelaxation. However, up-conversion emission tuning through Gd3+-mediated energy migration can only be realized at 980 nm laser excitation, on basis of abovementioned NaGdF4: Yb, Tm@ NaGdF4: A CS nanostructure. Unfortunately, the 980 nm excitation source is largely overlapped with the absorption peak of water molecules. As a result, once biological tissues are irradiated by 980 nm laser continuously, the high absorption of water will induce local overheating of tissues and low tissue penetration.23-26 Therefore, it’s necessary to turn excitation wavelength into the biological NIR window, which is suitable for biological applications. In contrast to 980 nm excitation wavelength, the water absorption at 808 nm is relatively low (less than 1/20 of that at 980 nm).2729

Meanwhile, the absorption cross section around 808nm of Nd3+ ions is about 10 times higher

than that of Yb3+ ions at 980 nm.23, 30 Together with efficient energy transfer from Nd3+ to Yb3+, Nd3+ could be a promising candidate as sensitizer to realize intense up-conversion luminescence.31-36 Huang group ever fabricated NaGdF4: Nd, Yb, Tm@ NaGdF4: Eu/Tb CS NPs to realize dual-model up-conversion luminescence. In contrast to intense Eu3+, Tb3+ emission under 980 nm excitation, no obvious characteristic emission peaks of Eu3+, Tb3+ were detected, excited by 808 nm laser.37 Herein, we realized up-conversion emission tuning through Gd3+-mediated energy migration under 808 nm irradiation in rationally designed NaGdF4: A (A= Eu, Tb)@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS nanoarchitecture. In contrast to NaGdF4: Yb, Tm@ NaGdF4: A CS NP designed by Liu group, we fabricated NaGdF4: A@ NaGdF4: Yb, Tm NPs as seeds to deposit


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NaGdF4: Yb, Nd sensitizing layer. The immediate contact between NaGdF4: Yb, Tm and NaGdF4: Yb, Nd layer ensures efficient energy transfer from Nd3+ to Tm3+ through Yb3+mediated energy migration. The role of Gd3+ ions as migrators and corresponding Tm3+→Gd3+→Gd3+→A3+ energy migration process in NaGdF4: A@ NaGdF4: Yb, Tm CS nanoarchitecture were clearly clarified. Nd3+ ions in the sensitizing layer were selected as sensitizers to sufficiently absorb 808 nm photons. The spatial separation, where sensitizer (Nd3+), accumulator (Tm3+), activator (Eu3+, Tb3+) are separately doped into CSS nanostructure at different layers, vastly reduces radiative decays, such as cross-relaxation, the energy back transfer from Tm3+ to Nd3+. As a result, the intense Eu3+, Tb3+ emission can be achieved at 808 nm excitation. Notably, the variation of Gd3+ doping content in NaGdF4: Yb, Nd sensitizing layer can further tune up-conversion emission, since that the decrease of Gd3+ ions suppresses Gd3+ assisting energy transfer from Tm3+ to surface quenchers. Furthermore, the up-conversion tuning through Tb3+-mediated energy migration also was realized in NaTbF4: Eu@ NaYbF4: Tb@ NaGdF4: Yb, Nd nanoarchitecture. 2. EXPERIMENTAL SECTION 2.1 Materials. Gadolinium(III) acetate hydrate (99.9%), yttrium(III) acetate hydrate (99.9%), ytterbium(III) acetate hydrate (99.9%), thulium(III) acetate hydrate (99.9%), neodymium(III) acetate hydrate (99.9%), europium(III) acetate hydrate, and terbium (III) acetate hydrate (99.9%) were purchased from Jining Tianyi New Material Company. Oleic acid (AR), and 1-octadecene (90%) were purchased from Aladdin. Sodium hydroxide (NaOH; >96%), ammonium fluoride (NH4F; >96%), and cyclohexane were purchased from Sinopharm Chemical Reagent Company. All chemicals were used as received without further purification.



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2.2 Synthesis of NaGdF4: (0-20%)Eu, NaGdF4: (0-20%)Tb, NaGdF4: 5%Eu, 10%Tb core nanoparticles: The core nanoparticles were prepared using a co-precipitation method. In a typical experiment for synthesis of NaGdF4: 15%Eu core nanoparticles, 0.85 mmol Gd(CH3COOH)3, 0.15 mmol Eu(CH3COOH)3 were mixed with 10 ml oleic acid and 15 ml 1octadecene in a 100 ml four-neck round-bottom flask. The resulting mixture was then heated at 150 oC for 40 min under a gentle nitrogen flow. After cooling down to room temperature, 10 ml methanol containing 2.5 mmol NaOH and 4.0 mmol NH4F was slowly added into flask and stirred at 50 oC for 40 min. Subsequently, the solution was heated at 310 oC under nitrogen flow for 60 min before cooling down to room temperature. The as-prepared nanoparticles were precipitated with ethanol, collected by centrifugation and washed with ethanol for several times. Finally, the core nanoparticles were dispersed in 3 ml cyclohexane. The synthetic procedure for other core nanoparticle of NaGdF4: (0, 5, 10, 20%)Eu, NaGdF4: (0-20%)Tb, NaGdF4: 5%Eu, 10%Tb core nanoparticles was identical to the synthesis of NaGdF4: 15%Eu core nanoparticles except for the use of different lanthanide precursors. 2.3 Synthesis of NaGdF4: 15%Eu@ NaGdF4: (39-59%)Yb, (0.5-1.5%)Tm, NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm and NaGdF4: 5%Eu, 10%Tb@ NaGdF4: 49%Yb, 1%Tm,

NaGdF4@NaGdF4:

49%Yb,

1%Tm,

NaGdF4@NaGdF4:

40%Nd

and

NaGdF4@NaGdF4: 40%Nd, 5%Yb core-shell nanoparticles core-shell nanoparticles: The core-shell nanoparticles were synthesized by co-precipitation method with the as-prepared core nanoparticles used as seeds. In a typical synthetic procedure of NaGdF4: 15%Eu@ NaGdF4: 49%Yb,

1%Tm

core-shell

nanoparticles,

0.50

mmol

Gd(CH3COOH)3,

0.49

mmol

Yb(CH3COOH)3, 0.01 mmol Tm(CH3COOH)3 were mixed with 10 ml oleic acid and 15 ml 1octadecene in a 100 ml four-neck round-bottom flask. The resulting mixture was then heated at


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150 oC for 40 min under a gentle nitrogen flow to form lanthanide oleate complexes. After cooling down to room temperature, the as-prepared NaGdF4: 15%Eu nanoparticles were added as seeds, together with 10 ml methanol solution of 2.5 mmol NaOH and 4.0 mmol NH4F. The reaction mixture was stirred at 50 oC for 40 min, and then heated at 310 oC under nitrogen flow for 60 min before cooling down to room temperature. The as-prepared nanoparticles were precipitated with ethanol, collected by centrifugation and washed with ethanol for several times. Finally, the core-shell nanoparticles were dispersed in 3 ml cyclohexane. The synthetic procedure for other core-shell nanoparticle of NaGdF4: 15%Eu@ NaGdF4: (39, 59%)Yb, (0.5, 1.5%)Tm, NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm and NaGdF4: 5%Eu, 10%Tb@ NaGdF4: 49%Yb, 1%Tm was identical to that of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm core-shell nanoparticles except for the use of different shell lanthanide precursors. 2.4 Synthesis of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: (0-20%)Yb, (080%)Nd, NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd, NaGdF4: 5%Eu, 10%Tb@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd, NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGd0.7-xYxF4: 10%Yb, 20%Nd, NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4 and NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaYF4 core-shellshell nanoparticles: The core-shell-shell nanoparticles were prepared using a co-precipitation method with the as-prepared core-shell nanoparticles used as seeds. In a typical experiment for synthesis of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd core-shellshell nanoparticles, 0.55 mmol Gd(CH3COOH)3, 0.05 mmol Yb(CH3COOH)3, 0.40 mmol Nd(CH3COOH)3 were mixed with 10 ml oleic acid and 15 ml 1-octadecene in a 100 ml fourneck round-bottom flask. The resulting mixture was then heated at 150 oC for 40 min under a gentle nitrogen flow. After cooling down to room temperature, 3 ml cyclohexane solution of the



as-prepared NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm core-shell nanoparticles was added into above solution along with 10 ml methanol solution of 2.5 mmol NaOH and 4.0 mmol NH4F. The reaction mixture was stirred at 50 oC for 40 min, and then heated at 310 oC under nitrogen flow for 60 min before cooling down to room temperature. The as-prepared nanoparticles were precipitated with ethanol, collected by centrifugation and washed with ethanol for several times. Finally, the core-shell-shell nanoparticles were dispersed in 3ml cyclohexane. The synthetic procedure for other core-shell-shell nanoparticle of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: (0, 2, 10, 15, 20%)Yb, (0, 10, 20, 60, 80%)Nd, NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd, NaGdF4: 5%Eu, 10%Tb@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd, NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGd0.7-xYxF4: 10%Yb, 20%Nd, NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4 and NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaYF4 core-shell-shell nanoparticles was identical to that of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd core-shell-shell nanoparticles except for the use of different shell lanthanide precursors. 2.5 Synthesis of NaYF4:15%Eu@ NaYF4: 49%Yb, 1%Tm core-shell nanoparticles: The synthetic procedure for NaYF4: 15%Eu@ NaYF4: 49%Yb, 1%Tm core-shell nanoparticles was identical to that of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm core-shell nanoparticles except that the precursor Gd(CH3COOH)3 were substituted by Y(CH3COOH)3. 2.6 Measurments. The luminescence spectra were collected at room temperature on an Edinburgh FLS980 spectrometer equipped with power adjustable 980 and 808 nm diode lasers as excitation sources. The decay curves were measured with the same spectrometer and pulse modulator with adjustable pulse length from 3 to 300µs. UV-Vis absorption spectra were recorded on UV-2600 spectrophotometer (Shimadzu). Low-resolution transmission


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electron microscope (TEM) measurement was performed on a JEM-1400 transmission electron microscope operated at 100 kV accelerating voltage. High-resolution TEM images, electron energy loss spectroscopy (EELS) and elemental mapping were carried out on JEM-2100F transmission electron microscope. X-ray diffraction (XRD) patterns were recorded on Germany Bruker Axs D8-Focus powder diffractometer using Cu Kα radiation (λ = 1.5418 Å nm). The effective lifetime τ was calculated based on the following equation: ஶ

߬ = ‫׬‬଴ ‫ܫ‬ሺ‫ݐ‬ሻ݀‫ݐ‬/ ‫ܫ‬଴ Where I0 represents the maximum emission intensity and I(t) denotes the time-dependent UC emission intensity. 3. RESULTS AND DISCUSSION 3.1 Structure and morphology Monodisperse NaGdF4: A (A=Eu, Tb)@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS NPs were synthesized by a co-precipitation method, involving a layer-by-layer growth process that NaGdF4: A core NPs were first fabricated and then use as seeds to successively deposit two shells of NaGdF4:Yb, Tm and NaGdF4:Yb, Nd. The as-synthesized core and multishell samples were confirmed to be hexagonal phase by powder X-ray diffraction (XRD) analysis (Figure S1). It is observed that the diffraction peaks tend to be sharpened with the addition of outer shells, verifying the increasing of mean grain size. The representative transmission electron microscope (TEM) imagines for core, CS, CSS NPs (Figure 1a, b, c, Figure S2) display uniform size distributions with the mean sizes of 19.5 ± 1.1 nm, 23.9 ± 1.0 nm, 33.7 ± 1.0 nm ( length) × 26.1 ± 1.2 nm (width), respectively. The thickness of interlayer is approximately 2.2 nm. The axial and radial thicknesses of outer shell are 4.9 nm and 1.1 nm, respectively. An increase in grain



size indicates the successful coating of outer shells. The High-resolution TEM (HRTEM) image for an individual CSS NP (Figure 1d) exhibits clear lattice fringes with a typical d-spacing of about 0.30 nm, corresponding to the (110) plane for hexagonal NaGdF4. In addition, the selected area electron diffraction (SAED) pattern of multiple CSS NPs, shown in Figure 1e, demonstrates that the d-spacing calculated by the diffraction rings from inside out are 0.303 nm, 0.215 nm, 0.175 nm, respectively, agreeing well with (110), (201) and (211) plane for hexagonal β-NaGdF4. All these evidences reveal that the NPs are pure hexagonal phase. The CSS nanoarchitecture were validated by EDS elemental mappings for an individual NaGdF4: 15%Eu@NaGdF4: 49%Yb, 1%Tm@NaGdF4: 5%Yb, 40%Nd (Figure 1g), displaying that Gd crosses from core to shell, and Eu , Yb/Tm, Yb/Nd mostly locate in the core, interlayer, outer shell, respectively. Moreover, the corresponding electron energy loss spectroscopy (EELS) (Figure 1f) also illustrates that the content distributions of Gd and Yb, where the higher Gd content in inner region and higher Yb content in the middle region, are consistent with the design compositions for the CSS nanoarchitecture.


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Figure 1. TEM of (a) NaGdF4: 15%Eu core (b) NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm CS and (c) NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd CSS NPs. (d) The HRTEM of individual CSS NP. (e) SAED pattern of multiple CSS NPs. (f) EELS line scan and (g) STEM-HAADF image with EDS elemental mapping for an individual NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd CSS NP. 3.2 808 nm excited multicolor and emission enhancement of NaGdF4: A@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS nanostructure NaGdF4: A@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS nanoarchitecture is design to realize up-conversion tuning through energy migration under 808 nm irradiation. As illustrated in Figure 2, at 808 nm excitation, Nd3+ sensitizers in the outershell are excited and transfer energy to Yb3+ ions. Subsequently, excitation energy in the 2F5/2 state enters the interlayer through migration between Yb3+ ions. And then, the 1I6 state of Tm3+ is populated by five-photon energy transfer processes from Yb3+ to Tm3+ (Figure S3). Afterward, the excitation energy at 1I6 state is extracted by Gd3+ and stay at 6P7/2. Eventually, the energy enters into the core through Gd3+mediated energy migration and trapped by Eu3+, Tb3+ in the core to realize intrinsic luminescence. As expected, the characteristic up-conversion emissions, belong to Eu3+, Tb3+, are clear observed under the excitation of 808 nm laser (Figure 3a, b). For example, the emission bands of Eu3+ centered at 591 nm, 615 nm and 695 nm originate from Eu3+ 5D0→7F1, 5D0→7F2, 5

D0→7F4 transitions (Figure S5d). In addition, at optimized sensitizing layer components, the

luminescence intensity of Eu3+, Tb3+ doping NaGdF4: A@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS NPs enhance 5.9, 7.7 times, respectively, compared with CS NPs without NaGdF4: Yb, Nd sensitizing layer coating. The similar intensity enhancement also emerges in Eu3+, Tb3+ co-doped CSS NPs (Figure S4).



Figure 2. Proposed up-conversion mechanism through Gd3+-mediated energy migration under 808 nm excitation in NaGdF4: A@NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS NPs.

Figure 3. (a) Up-conversion emission spectra of NaGdF4: 15%Eu @NaGdF4: 49%Yb, 1%Tm (Eu@ Tm) CS NPs and NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd (Eu@ Tm@ Nd) CSS NPs. (b) Up-conversion emission spectra of NaGdF4: 15%Tb @NaGdF4: 49%Yb, 1%Tm (Tb@ Tm) CS NPs and NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd (Tb@ Tm@ Nd) CSS NPs. Inset shows the corresponding emission photograph. The emission spectra of CS NPs (1 wt%) dispersed in cyclohexane were obtain under 980 nm excitation (the power density: 20 W/cm2), that of CSS NPs (1 wt%) dispersed in cyclohexane were obtain under 808 nm excitation at the same power density.


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In the whole CSS nanoarchitecture, NaGdF4: A@ NaGdF4: Yb, Tm CS structure serves as modulator to tune the emission color through Gd3+-mediated energy migration. At 980 nm excitation, the NaGdF4: A@ NaGdF4: Yb, Tm CS NPs exhibits abundant emission colors, varying from blue to violet or cyan (Figure S5a, b, c, S6a, b). Notably, when the doping concentration of Eu3+, Tb3+ reach at 15%, the 615 nm (Eu3+) /450 nm (Tm3+) and 540 nm (Tb3+) /450 nm (Tm3+) intensity ratio can be up to 92.0%, 81.3%, respectively. In contrast, no emission peak of Eu3+ is found in luminescence spectrum of NaYF4: Eu@ NaYF4: Yb, Tm NPs, with Gd3+ replaced by inert Y3+ ions. As shown in the schematic diagram inset in Figure 4a, in absence of Gd3+, the energy transfer between Tm3+ and Eu3+ can’t be accomplished. In addition, compared with distinct emission peak at 290 nm of NaYF4@ NaYF4: Yb, Tm CS NPs, attributed to 1

I6→3H6 transition of Tm3+, the relative intensity of 290 nm reduce obviously and the emission

peak of Gd3+ at 311 nm emerges with NaYF4 matrix replaced by NaGdF4, verifying Tm3+→ Gd3+ energy transfer process (Figure 4b). Also, with Eu3+ ion doped, the lifetime of Gd3+ 6P7/2 state changes from 1367 µs to 469 µs, undoubtedly revealing the energy transfer from Gd3+ ions to activators (Figure 4c). These results together confirmed that Gd3+ ions, which bridge the energy transfer from Tm3+ ions to activators throughout entire CS nanoarchitecture, play vital role in achieving activators emission.



Figure 4. (a) Up-conversion emission spectra of NaGdF4@ NaGdF4: 49%Yb, 1%Tm (top) and NaYF4@ NaYF4: 49%Yb, 1%Tm CS NPs (bottom) in the ultraviolet spectral regions. The insets show the Schematic CS structure and the related energy transfer processes. (b) Up-conversion emission spectra of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm (top) and NaYF4: 15%Eu@ NaYF4: 49%Yb, 1%Tm CS NPs (bottom). The insets show the Schematic CS structure and the related energy transfer processes. (c) A comparison of up-conversion luminescence lifetimes of Gd3+ and Tm3+ in NaGdF4@ NaGdF4:49%Yb, 1%Tm and NaGdF4: 15% Eu@ NaGdF4: 49%Yb, 1%Tm CS NPs excited by pulse 980 nm laser. To demonstrate the necessity of NaGdF4: A@ NaGdF4: Yb, Tm CS nanoarchitecture to realize efficient luminescence, especially excited by 808 nm laser, we carry out a series of contrast experiments. The luminescence spectra, shown in Figure S7, display that the emission of NaGdF4: Eu@ NaGdF4: Yb, Tm CS NPs is far more intense than Eu3+, Yb3+, Tm3+ triply doped


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NaGdF4: Eu, Yb, Tm NPs, verifying that spatial separation of the dopants by CS structure can minimize the deleterious nonradiative decays, such as cross-relaxation. In order to further certify the influence of CS structure on luminescence under 808 nm irradiation, we coated abovementioned NPs with NaGdF4: Yb, Nd sensitizing layer to synthesis NaGdF4: Eu@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS NPs and NaGdF4: Eu, Yb, Tm@ NaGdF4: Yb, Nd CS NPs. As anticipated, the luminescence intensity of CS NPs is far lower than CSS NPs at 808 nm excitation. In addition, the relative intensity of Eu3+ to Tm3+, belong to CS NPs, reduce clearly, compared with CSS NPs (Figure 5a,d). In contrast to NaGdF4: Yb, Tm@ NaGdF4: A nanoarchitecture, the reason why we select NaGdF4: A@ NaGdF4: Yb, Tm as seed to deposit NaGdF4: Yb, Nd shell is that the better luminescence can be achieved by the latter, excited by 808 nm laser. As shown in Figure 5b, the intensity of NaGdF4: Eu@ NaGdF4: Yb, Tm@NaGdF4: Yb, Nd CSS NPs is 3.9 times as high as that of NaGdF4: Yb, Tm@ NaGdF4: Eu@ NaGdF4: Yb, Nd CSS NPs. Furthermore, only weak emission peaks of Eu3+ are detected (Figure 5d). The weak emission, in particular Eu3+, of NaGdF4: Yb, Tm@NaGdF4: Eu@NaGdF4: Yb, Nd NPs , can be ascribed to the ineffective energy migration from Yb3+ in the outer shell to Yb3+ in the core, caused by the suppression of NaGdF4: Eu interlayer. NaGdF4: Eu@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd nanoarchitecture resolves this problem availably. Therefore, tuning up-conversion effectively through energy migration can be realized at 808 nm excitation.



Figure 5. Up-conversion emission spectra of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd (Eu@ Tm@ Nd) CSS NPs as well as (a) NaGdF4: 15% Eu, 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd (Eu, Tm@ Nd) CS NPs, (b) NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 15% Eu@ NaGdF4: 5%Yb, 40%Nd (Tm@ Eu@ Nd) CSS NPs and (C) NaGdF4: 1%Nd, 49%Yb, 1%Tm@ NaGdF4: 15%Eu (Nd, Tm@ Eu) CS NPs. (d) Up-conversion emission spectra of abovementioned samples, normalized at maximum intensity. The emission spectra were obtained in cyclohexane solutions comprising 1 wt% particles under 808 nm excitation (the power density: 20 W/cm2). For NaGdF4: A@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS nanoarchitecture, the NaGdF4: Yb, Nd outershell is selected as sensitizing layer to realize up-conversion tuning under 808 nm excitation. Nd3+ ions in the sensitizing layer are efficient sensitizers to absorb 808 nm laser,


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which is much higher than absorption of Yb3+ for 980 nm laser (Figure S8a). The energy transfer from Nd3+ to Yb3+ is demonstrated by the luminescence spectra showed in Figure S9. Compared with NaGdF4@ NaGdF4: 40%Nd CS NPs, the new emission peak centered at 980 nm (Yb3+: 2

F5/2 → 2F7/2) emerges, along with original intense emission centered at 1060 nm (Nd3+: 4F3/2 →

4

I11/2) severely weakening after introducing 5% Yb3+ ions in outer shell under 808 nm excitation.

The adequate absorbance and high energy transfer from Nd3+ to Yb3+ (up to 75%) ensure intense up-conversion luminescence.32 Yb3+ ions in the sensitizing layer can bridge the interlayer to improve energy transfer efficiency. As shown in Figure S8b, without Yb3+ in sensitizing layer, only weak luminescence can be detected. In contrast, when merely 2% Yb3+ ions are doped into sample, obvious enhancement can be observed.

Figure 6. Up-conversion emission spectra of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: Yb, Nd CSS NPs with (a) different Nd3+ doping concentration, the content of Yb3+ is



fixed at 10% and (b) different Yb3+ doping concentration, the content of Nd3+ is fixed at 40% ,excited by 808 nm laser at 20 W/cm2. Insets of (a, b) show the corresponding variation trend. Up-conversion emission spectra of the same CSS NPs with (c) different Nd3+ doping concentration, the content of Yb3+ is fixed at 10% and (d) different Yb3+ doping concentration, the content of Nd3+ is fixed at 40% under 980 nm excitation at 20 W/cm2. Insets of (c, d) show the corresponding variation trend. For NaGdF4: Yb, Nd sensitizing layer, the optimal doping concentration for Yb3+, Nd3+ ions to realize efficient luminescence are 5%, 40%, respectively (Figure 6a, b). With the doping concentration increasing, the absorbance gradually rises, especially for Nd3+ (Figure S10). Nevertheless, excessive doping level leads to the deleterious quenching of luminescence, caused by energy back transfer from Tm3+ to Yb3+, Nd3+. As shown in Figure 6c, d, the luminescence intensity of CSS NPs constantly decrease with the increasing Yb3+, Nd3+ doping content under 980 nm excitation, verifying the existence of energy back transfer. Even so, the CSS structure, where Nd3+, Tm3+ are doped in outer shell and interlayer, respectively , still vastly suppresses energy back transfer, due to spatial separation of the dopants. As a result, the doping content can be up to 40%, in favour of enhancing up-conversion luminescence. As a contrast, for NaGdF4: Nd, Yb, Tm@ NaGdF4: A CS nanostructure, the doping concentration of Nd3+ is restrict in 1% , to reduce quenching effect.37 As shown in Figure 5c, d, the intensity of NaGdF4: Eu@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS NPs is about 70 times higher than aforementioned CS NPs. Besides, for the latter, the relative intensity of Eu3+ is lower, compared with CSS NPs. Surface passivation is another reason for up-conversion luminescence enhancement after NaGdF4: Yb, Nd sensitizing layer coating, affirmed by the decay curves change, provided in Figure 7. Compared with NaGdF4: Eu@ NaGdF4: Yb, Tm CS NPs, significant increase in Tm3+


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lifetimes( 1I6, 1D2 and 1G4 states ) was observed after sensitizing layer coating. In stark contrast, the lifetime for 5D0 state of Eu3+, located in core, is essentially unaltered by NaGdF4: Yb, Nd layer coating, ascribed to pre-existing Surface passivation of NaGdF4: Yb, Tm interlayer. Indeed, the addition of NaGdF4: Yb, Nd layer protect exposed luminescence centers (Tm3+) from harsh quenching effect, originated from surface defects, solvent molecules, etc.38 Therefore, surface luminescence centers are recovered and luminescence intensity increases.

Figure 7. Decay curves of (a) Tm3+: 1I6, (b) Tm3+: 1D2, (c) Tm3+: 1G4 and (d) Eu3+: 5D0 states in NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm (Eu@ Tm) CS NPs (excited by pulse 980 nm laser) and NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd (Eu@ Tm@ Nd) CSS NPs (excited by pulse 808 nm and 980 nm laser, respectively ). It is expected that tuning excitation wavelength from 980 nm to 808 nm would effectively minimize the heating effect caused by the intense water absorption for 980 nm laser. The CSS



NPs were transferred into water phase according to a literature procedure.27 As shown in Figure S11, under 808 nm laser continuous irradiation even at 2 W power, there is no obvious change happened in temperature of sample (10 mg CSS NPs dispersed in 2 ml deionized water). As a contrast, with excitation power and irradiation time increasing, the temperature of sample distinctly rises under 980 nm excitation. In addition, the power dependent temperature rise profiles were recorded (Figure S12). It’s observed that the temperature rises 11.2 oC under irradiation of a 980 nm laser at 2 W power for 5 min, in contrast to 1.3 oC under 808 nm irradiation, revealing the low heating effect under 808 nm excitation. 3.3 The suppression of Gd3+-mediated energy migration to surface quencher and further emission color tuning in NaGdF4: Yb, Nd sensitizing layer The coating of NaGdF4: Yb, Nd sensitizing layer not only accomplishes the luminescence enhancement at 808 nm excitation, variable component of sensitizing layer also has a significant impact on emitting color. As shown in Figure 8a, b, at 808 nm irradiation, the relative emission intensity of Eu3+ at 591 nm, 615 nm gradually increase, with the decrease of Gd3+ doping concentration, replaced by Nd3+ ions. The same trend is observed in CSS NPs, excited by 980 nm laser (Figure S13a, b). It is worth mentioning that the 615 nm (Eu3+) /450 nm (Tm3+) intensity ratio of CSS NPs reach to 1.79, when the Gd3+ is completely replaced by Nd3+. As a flagrant contrast, for NaGdF4: Eu@ NaGdF4: Yb, Tm CS NPs, the intensity ratio is merely 0.92.


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Figure 8. (a) Up-conversion emission spectra of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 10%Yb, x%Nd CSS NPs with different Nd3+ doping concentration excited by 808 nm laser, normalized at 450 nm, as well as (b) corresponding CIE chromaticity diagram (the points ,in turn, belong to 10%, 20%, 40%, 60%, 80% Nd3+ doping concentration from left to right). (c) Up-conversion emission spectra of NaGdF4: 15%Eu@ NaGdF4: 49%Yb, 1%Tm@ NaYXGd0.7XF4:

10%Yb, 20%Nd CSS NPs with different Y3+ doping concentration under 808 nm excitation,

normalized at 450 nm, as well as (d) corresponding CIE chromaticity diagram (the points ,in turn, belong to 0%, 35%, 70% Y3+ doping concentration from left to right). The excitation power density is 20 W/cm2.



To confirm that the emitting color change is attributed to the variation of Gd3+ doping concentration in sensitizing layer, we further conducted a set of verifying experiments, where Gd3+ was gradually substituted by optically inert Y3+ ions, Yb3 +, Nd3+ doping content were fixed. As expected, with Y3+ ions doping , a steady enhancement of Eu3+ emission is observed, excited by either 808 nm or 980 nm laser (Figure 8c, d, S13c, d). For NaGdF4: A @ NaGdF4: Yb, Tm@NaGdF4: Yb, Nd CSS NPs, with high Gd3+ doping concentration, only limited energy is transferred from Gd3+ to activator to realize activator emission. This can be ascribed to deleterious surface quenching effects.39 As shown in Figure S14, Since the energy stored in Gd3+ can migrate through NaGdF4: Yb, Nd sensitizing layer, the partial Gd3+ excitation energy is trapped by surface defects, surface ligands, solvent molecules, etc. However, with Gd3+ replaced by inert Y3+, Gd3+ in sensitizing layer assisting energy transfer is suppressed, so that more energy stored in Gd3+ is transferred to activator. Therefore, the activator emission strengthens.

Figure 9. Up-conversion emission spectra of NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaYF4 (top) and NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4 CSS NPs (bottom) in the ultraviolet spectral regions. The insets show the Schematic CSS structure and the related energy transfer processes.


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To validate the outer shell, with Gd3+ ions replaced, plays a vital role in protecting Gd3+ excitation energy, we carried out a series of contrast experiments to compare the luminescence intensity and lifetime of Gd3+ emission at 311 nm for NaGdF4@ NaGdF4: Yb, Tm with NaGdF4 and NaYF4 coating. In contrast to severely quenching of Gd3+ emission at 311 nm after NaGdF4 coating, the luminescence intensity of NaYF4-coated sample at 311 nm vastly strengthen, verifying that NaYF4 shell can effectively protect energy stored in Gd3+ from being trapped by surface defect, solvent molecules, etc (Figure 9). The result can also be demonstrated by lifetime comparison. As for lifetime of Gd3+ emission at 311 nm, the lifetime of NaGdF4-coated sample (504µs) is significantly shorter than that of NaYF4-coated counterpart (1943µs) (Figure 10a). In contrast, the lifetimes of Tm3+ 1I6, 1D2, 1G4 all obviously decrease when the coating layer changes from NaGdF4 to NaYF4, attributed to lattice mismatch between NaGdF4 and NaYF4 (Figure 10b, c, d). Without the influence of lattice mismatch, the lifetime of Gd3+ emission for NaYF4-coated sample would be longer. Moreover, the decay curves of NaGdF4: Eu@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd also were investigated (Figure S15). With the Gd3+ in sensitizing layer gradually replaced by Y3+, the lifetime of Gd3+ emission is basically unchanged, indicating that high activator doping concentration assures the adequate extraction for Gd3+ excitation energy, which is protected from being trapped by the surface quenchers.



Figure 10. Decay curves of (a) Gd3+: 6P7/2, (b) Tm3+: 1I6, (c) Tm3+: 1D2, (d) Tm3+: 1G4 states in NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4 and NaGdF4@ NaGdF4: 49%Yb, 1%Tm@ NaYF4 CSS NPs under pulse 980 nm excitation. 3.3 808 nm excited up-conversion tuning through Tb3+-mediated energy migration Especially to deserve to be mentioned, 808 nm excited up-conversion tuning can also be realized through Tb3+-mediated energy migration in NaTbF4: Eu@ NaYbF4: Tb@ NaGdF4: Yb, Nd nanoarchitecture. It is observed that the emission peaks at 591 nm, 615 nm, 695 nm, belong to Eu3+, are far higher than Tb3+ emission peaks, revealing the effective energy transfer from Tb3+ to Eu3+ (Figure 11a). Furthermore, the intensity of CSS NPs enhance 7.6 times after NaGdF4: Yb, Nd layer coating. In order to further enrich 808 nm excited up-conversion emission colors, we fabricated NaTbF4: Eu@ NaGdF4: Tb@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSSS


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nanostructure to accomplish Tm3+, Eu3+, Tb3+ common luminescence via both of Gd3+ and Tb3+ migrators assisting energy migration. Indeed, the obvious emission peaks of Tm3+, Eu3+, Tb3+ were observed under 808 nm excitation (Figure 11b). Compared with aforementioned NaGdF4: Eu, Tb@ NaGdF4: Yb, Tm@ NaGdF4: Yb, Nd CSS NPs, Tb3+ emission is reserved in CSSS NPs, due to the fact that spatial separation weakens the quenching of Tb3+ emission in the interlayer (Fig S16) .

Figure 11. (a) Up-conversion emission spectra of NaTbF4: 20%Eu@ NaYbF4: 30%Tb (Eu@ Tb) CS NPs (excited by 980 nm laser at 20 W/cm2) and NaTbF4: 20%Eu@ NaYbF4: 30%Tb@ NaGdF4: 5%Yb, 40%Nd (Eu@ Tb@ Nd) CSS NPs (excited by 808 nm laser at the same power density). Inset shows the emission photograph of CSS NPs. (b) Up-conversion emission spectra of NaTbF4: 20%Eu@ NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm (Eu@ Tb@ Tm) CSS NPs (excited by 980 nm laser at 20 W/cm2) and NaTbF4: 20%Eu@ NaGdF4: 15%Tb@ NaGdF4: 49%Yb, 1%Tm@ NaGdF4: 5%Yb, 40%Nd (Eu@ Tb@ Tm@ Nd) CSSS NPs (excited by 808 nm laser at the same power density). Inset shows the emission photograph of CSSS NPs. 4. CONCLUSIONS In summary, we have realized tunable up-conversion emission through Gd3+-mediated energy migration upon excitation of 808 nm in elaborate NaGdF4: A (A=Eu, Tb)@ NaGdF4: Yb, Tm@ 25 Environment ACS Paragon Plus


NaGdF4: Yb, Nd CSS nanoarchitecture. The Gd3+-mediated energy migration ensures upconversion emission for the activators without long-lived intermediate states (Eu3+, Tb3+). In contrast to NaGdF4: A@ NaGdF4: Yb, Tm CS NPs, The coating of NaGdF4: Yb, Nd sensitizing layer not only vastly enhances the up-conversion luminescence intensity (up to 5.9, 7.7 times for Eu3+,Tb3+ doped sample, respectively), also turns excitation wavelength from 980 nm to 808 nm, which will significantly reduce local overheating of tissues. The spatial separation, induced by CSS nanostructure, plays a vital role in intense up-conversion emission, ascribed to the decrease of nonradiative decays. Notably, with Gd3+ replaced by Nd3+ or inert Y3+ in sensitizing layer, the 615(Eu3+)/ 450(Tm3+) intensity ratio constantly increases (up to 1.79), which further tunes upconversion emission colors. Furthermore, Tb3+-mediated energy migration also was successfully used to tune up-conversion emission color. The tunable emission colors of CSS UCNPs under 808 nm irradiation make them ideal candidates for applications in multicolor imaging and multiplexed detection areas. ASSOCIATED CONTENT Supporting Information XRD patterns, absorption spectra, decay curves, CIE chromaticity diagram and additional upconversion emission spectra (Figures S1-S16) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (G. J. Zhou). * E-mail: [email protected] (J. Zhou).


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ORCID Guangjun Zhou: 0000-0002-4447-2048 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Shenzhen Science and Technology Research and Development Funds (JCYJ20160331173823401), National Science Foundation of Shandong Province (ZR2016EMM20, ZR2016EMQ09) and the National Natural Science Foundations of China (51472150). REFERENCES (1) Sun, L. D.; Wang, Y. F.; Yan, C. H. Paradigms and Challenges for Bioapplication of Rare Earth Upconversion Luminescent Nanoparticles: Small Size and Tunable Emission/ Excitation Spectra. Acc. Chem. Res. 2014, 47, 1001-1009. (2) Xu, J. T.; Gulzar, A.; Liu, Y. H.; Bi, H. T.; Gai, S. L.; Liu, B.; Yang, D.; He, F.; Yang, P. P. Integration of IR-808 Sensitized Upconversion Nanostructure and MoS2 Nanosheet for 808 nm NIR Light Triggered Phototherapy and Bioimaging. Small 2017, 13, 1701841. (3) Dong, H.; Sun, L. D.; Yan, C. H. Energy Transfer in Lanthanide Upconversion Studies for Extended Optical Applications. Chem. Soc. Rev. 2015, 44, 1608-1634. (4) Chen, D. Q.; Liu, L.; Huang, P.; Ding, M. Y.; Zhong, J. S.; Ji, Z. G. Nd3+-Sensitized Ho3+ Single-Band Red Upconversion Luminescence in Core-Shell Nanoarchitecture. J. Phys. Chem. Letter. 2015, 6, 2833-2840.



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TOC GRAPHICS


808 nm Excited Multicolor Upconversion Tuning through Energy

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