Dual-Mode Long-Lived Luminescence of Mn2+-Doped Nanoparticles

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Functional Nanostructured Materials (including low-D carbon)

Dual-Mode Long-Lived Luminescence of Mn2+Doped Nanoparticles for Multilevel Anti-Counterfeiting Xiaowang Liu, Qiang Ji, Qiyan Hu, Cheng Li, Meiling Chen, Jian Sun, Yu Wang, Qiang Sun, and Baoyou Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09612 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

Dual-Mode Long-Lived Luminescence of Mn2+-Doped Nanoparticles for Multilevel Anti-Counterfeiting Xiaowang Liu,*,† Qiang Ji,† Qiyan Hu,‡ Chen Li,† Meiling Chen,† Jian Sun,† Yu Wang,# Qiang Sun,¶ and Baoyou Geng*,† †College

of Chemistry and Materials Science, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Key Laboratory of Electrochemical Clean Energy of Anhui Higher Education Institutes, Centre for Nano Science and Technology, Anhui Normal University, Wuhu 241000, P. R. China ‡School

of Pharmacy, Wannan Medical College, Wuhu 241002, PR China

#Engineering

Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ¶Center

for Functional Materials, NUS (Suzhou) Research Institute, Suzhou, Jiangsu, 215123, China

Supporting Information Placeholder

ABSTRACT: Luminescent nanoparticles with dual-mode long-lived luminescence are of great importance for their attractive applications in biosensing, bioimaging and data encoding. Herein, we report the realization of up- and down-conversion emission of Mn2+ dopants in multilayer nanoparticles of NaGdF4:Yb/Tm@NaGdF4:Ce/Mn@NaYF4 upon excitation at 980 and 254 nm, respectively. The dual-mode emission of the Mn2+ dopants at 531 nm have a long-lived lifetime up to ～ 30 ms as a result of the spin-forbidden optical transition of Mn2+ within the 3d5 configuration. After ceasing steady excitation at the two wavelengths, the long-lived feature of Mn2+ luminescence allows a longer persistent time than lanthanide emissions, thereby enabling the ease of data decoding by a cell phone camera under a burst mode. The long-lived green upconversion emission also permits the generation of a long green tail-emission upon dynamic excitation at 980 nm. These attributes make the asprepared Mn2+-doped multilayer nanoparticles particularly attractive for multilevel anti-counterfeiting. Keywords: long-lived luminescence; lanthanide-doped nanoparticles; upcovnersion; downconversion; anti-counterfeiting mode long-lived luminescence at the single-particle level is still a daunting challenge.18-23 Epitaxial growth of lanthanide-based multilayer nanoparticles with spatially confined doping of lanthanide sensitizers and transition metal activators may provide a much needed method.24-27 This is due to the fact that: (i) lanthanides are an excellent class of sensitizers for absorbing both UV and NIR photons, and the accumulated excitation energy can be transferred to neighboring transition metal activators. For example, Ce3+ and Yb3+ have proven effective as sensitizers for efficiently harvesting excitation energy at 254 and 980 nm, respectively;28-32 (ii) the spin-forbidden transition nature within the 3d5 configuration of the transition metal enables it to be a long-lived emission center within the lanthanide-based host lattices; (iii) in addition to utilizing short-distance sensitizer-to-activator energy transfer, long-distance energy migration from lanthanides to transition metals can also be exploited at the singleparticle level to alliviate the optical imcompatibility between the sensitizers and activators;33-38 and (iv) the

■ Introduction In recent years, there has been a surge of interest in the exploration of long-lived luminescent materials due to their widespread applications in optical sensing, bioimaging and document security.1-4 It has been well-documented that single-mode luminescence, either down- or up-conversion emission, can have lifetime longer than tens of milliseconds under near-infrared (NIR) or ultraviolet (UV) light excitation.5-7 Currently, the long-lived down-conversion luminescent materials mainly consist of carbon dots, pure organic compounds, organometallic complexes, metalorganic frameworks and transition metal-doped inorganic matrices;8-14 and the long-lived upconversion counterparts comprise lanthanide and transition metal-codoped inorganic matrices, such as NaGdF4:Yb/Tm/Mn and Zn3Ga2GeO8: Cr/Yb/Er.15-17 The long-lived nature of the two kinds of emissions enables the generation of a naked-eye visible afterglow after ceasing the corresponding excitation. Despite these remarkable advances in the field of singlemode long-lived luminescent materials, integration of dual1

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China). All the chemicals were used as received without further treatment. Physical Measurements. Transmission electron microscopy (TEM) was carried out on Tecnai G2 20 ST (FEI) with an acceleration voltage of 200 kV. The energy-dispersive X-ray (EDX) spectroscopy was recorded an Oxford INCA energy system operated at 200 kV. Powder X-ray diffraction (XRD) profiles were obtained on an a PANalytical X’Pert Pro MPD diffractometer using graphite-monochromatized CuKα radiation ( = 1.5406 Å). Opical characterization was performed on FSP920-C (Edinburgh) under excitation at 254 or 980 nm. The upconversion decay curves were acquired on the same spectrometer with the use of a tunable mid-band OPO laser (410-2400 nm, Vibrant 355II, OPOTEK) as an excitation source. The absolute quantum yield of the down-conversion emission of Mn2+ ions was evaluated via the spectrometer equipped with an integrating sphere. Synthesis of Ln(CF3COO)3 (Ln = Y, Gd, Yb, Tm, Tb and Eu). A mixture of Ln2O3 (5 mmol), deionized water (5.0 mL) and trifluoroacetic acid (5.0 mL) was first prepared. The resulting mixture was then heated at 80 oC until the formation of transparent solutions. The solutions were further dried at 100 oC to afford corresponding Ln(CF3COO)3 powders. The as-prepared Ln(CF3COO)3 powders were dissolved into deionized water (50 mL) to produce the lanthanide precursor solutions at a concentration of 0.2 M. Synthesis of Mn(CF3COO)2 and Ce(CF3COO)3. In brief, Mn(OH)2 precipitate was first prepared by addition of Na(OH) (2 M, 12 mL) into an aqueous solution of MnCl2 (0.4 M, 25 mL) under N2. The Mn(OH)2 was obtained by centrifugation, and washed with N2-degassed water several times to remove the unreacted NaOH. Thereafter, the purified Mn(OH)2 was quickly added into a mixture of water (5.0 mL) and trifluoroacetic acid (5.0 mL) under N2. The mixture was sonicated for 0.5 h to allow the formation of a pink transparent solution. Note that precipitates formed in this step needed to be removed centrifugation. Pink colored powder was finally obtained by drying the transparent solution at 100 oC under N2 for 5 h. This method was also used to prepare white powder of Ce(CF3COO)3. In this preparation, freshly prepared Ce(OH)3 was used as the precursor to react with CF3COOH to form colorless transparent solution. Synthesis of NaGdF4: Yb/Tm (49/1 mol%) Core Nanoparticles. A mixture of OA (6.2 mL), ODE (6.4 mL), Gd(CF3COO)3 (0.2 M, 2.5 mL), Yb(CF3COO)3 (0.2 M, 2.45 mL), Tm(CF3COO)3 (0.2 M, 0.05 mL), Na(CF3COO) (1.0 M, 1 mL) was first prepared in a 25-mL flask and then heated at 150 oC for 1 h to completely remove the water. Next, OM (3.2 mL) was added into the mixture at room temperature.

Figure 1. (a) Proposed energy transfer pathway to achieve dual-mode luminescence of the same Mn2+ upon NIR (980 nm) and UV (254 nm) excitation. In our design, the core nanoparticle of NaGdF4:Yb/Tm (49/1 mol%) is used to harvest NIR excitation energy in order to promote Gd3+ from 8S7/2 to 6P7/2, and then the excitation energy can be transferred through Gd-sublattice and finally trapped by Mn2+ dopants in the middle layer. Another function of the middle layer is to absorb UV excitation energy by Ce3+ ions at 254 nm, and then transfer the excitation energy to Mn2+ through Gd3+ ion arrays to achieve downconversion emission of the same Mn2+dopants. (b) Schematic representation of the generation of multiple optical signals for multilevel anti-counterfeiting based on the use of the long-lived luminescence of Mn2+ of the as-prepared multilayer nanoparticles under different excitation conditions.

optical properties of lanthanide-doped nanoparticles can by further tuned by two-wavelength excitation.39-41 In this work, we report the realization of dual-mode long-lived luminescence of Mn2+ dopants in a multilayer nanoparticle of NaGdF4:Yb/Tm@NaGdF4:Ce/Mn@NaYF4 upon excitation by NIR and UV light (Figure 1a), respectively. We find that low-level doping of Ce3+ ions into the middle layer of the nanoparticles has a marginal impact on the green upconversion emission of Mn2+ at 531 nm but enables the feasibility of achieving the green downconversion emission of the same Mn2+ dopants upon excitation at 254 nm. The long-lived nature allows the green emission to be still visualized by the naked eyes after ceasing the excitation at the two wavelengths. This attribute makes the dual-mode emission distinguished from the lanthanide emissions under the same excitation conditions.42-45 In addition, the long-lived upconversion emission of Mn2+ further permits the generation of a long tail emission upon dynamic excitation (Figure 1b). These features suggest the as-prepared Mn2+-doped multilayer nanoparticles to be a new class of optical materials particularly attractive for multilevel anti-counterfeiting without the need for complex time-gated decoding instrumentation.

■ Experimental Section Chemicals. Ln2O3 (99.9%, Ln = Y, Gd, Yb, Tm, Tb and Eu), CF3COONa (98%), MnCl2●H2O (99.9%), 1-octadecene (ODE, 90%) and oleic acid (OA, 90%) were purchased from Alfa Aesar. Oleylamine (OM, 80-90%) and trifluoroacetic acid (> 99.5%) were obtained from Aladdin Reagent (Shanghai, 2


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under protection of N2. Thereafter, the precursor solution for the outmost shell was quickly injected into the flask at 310 oC. The flask was kept at 310 oC for another 1 h. Multilayer nanoparticles were obtained and purified using a precudure similar to the one for the core nanoparticles. Synthesis of NaMnF3 and NaCeF4 Nanoparticles. Typically, a mixture of Mn(CF3COO)2 (1 mmol) and Na(CF3COO) (1 mmol) was first prepared in the presence of OA and ODE. The mixture was then heated at 310 oC for 0.5 h under a N2 atmosphere. NaCeF4 nanoparticles were synthesized by the same method except the use of Ce(CF3COO)3 to replace Mn(CF3COO)2. The as-prepared nanoparticles were collected by centrifugation, and washed with a mixture of ethanol and cyclohexane. After drying under vacuum, the weights of the as-prepared nanoparticles were obtained for evaluating the reactivity of the corresponding precursors in the formation of fluorides at high temperatures. Synthesis of NaGdF4: Ce/Tb or Eu (15/5 mol%)@NaYF4 Core@Shell Nanoparticles. In brief, a core precursor solution of OA (2.5 mL), OM (2.5 mL), Gd(CF3COO)3 (0.32 mmol), Ce(CF3COO)3 (0.06 mmol), Tb(CF3COO)3 or Eu(CF3COO)3 (0.02 mmol), and Na(CF3COO) (0.4 mmol) was first prepared. Meanwhile, a precursor solution for NaYF4 was prepared by the same way. After purging with N2, the mixture was heated at 310 oC for 0.5 h at which the shell precursor solution was injected.48 The resulting mixture was kept at the temperature for another 0.5 h. The core@shell nanoparticles were harvested and purified using a precudure similar to the one for the core nanoparticles. The as-prepared core@shell nanoparticles were stored in cyclohexane (5 mL) for further use. Synthesis of Ligand-Free Multilayer (or core@shell) Nanoparticles. The surface ligand of these nanoparticles was removed by slight modification of a previously reported method.49 In brief, hydrochloric acid (1 mL, 2 M) was added a mixture of ethanol (1 mL) and multilayer nanoparticle dispersion (1 mL) under sonication. The sonication treatment was kept for 5 min. The ligand-free multilayer or core@shell nanoparticles were obtained by centrifugation (6500 rpm, 20 min), washed several time with ethanol and finally dispersed into deionized water (1 mL).

Figure 2. (a) Scheme illustrating the synthesis of cubic-phased NaGdF4:Yb/Tm@NaGdF4:Ce/Mn@NaYF4 multilayer nanoparticles via a seed-mediated approach. Note that the growth of the outmost layer was carried out by direct injection of the corresponding precursor solution into the synthetic system at 310 oC after the growth of the second layer. (b) TEM image of the as-prepared NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4 multishell nanoparticles. Inset showing the HAADF-STEM image of the nanoparticles with a magnified resolution. (c-e) Elemental mapping analysis of the asprepared multilayer nanoparticles. (c) Yb, (d) Ce and (e) Y. (f) Overlap of the elements of Yb, Ce and Y. (g) HRTEM image of the as-prepared multilayer nanoparticles (inset showing the corresponding fast Fourier transform pattern).

After purging with N2, the mixture was heated to 310 oC at a temperature-rising rate of 12 oC/min, and then kept at the temperature for 0.5 h.46,47 The as-prepared core nanoparticles were collected by centrifugation, and washed with cyclohexane and ethanol for several times, and finally stored in cyclohexane (5 mL) for further use. Synthesis of NaGdF4: Yb/Tm (49/1 mol%)@NaGdF4: Ce/Mn (x/60 mol%, x = 0, 3, 9 and 12)@NaYF4 Multilayer Nanoparticles. The multilayer NPs were prepared by a hightemperature seed-mediated method.29 Briefly, a mixture of OA (5 mL), ODE (5 mL), Gd(CF3COO)3 (0.2 M, 1.0 mL) and Na(CF3COO) (1.0 M, 0.2 mL) was first heated at 150 oC for 1 h to remove the water. The resulting mixture was then further mixed with Mn(CF3COO)2 (100 mg) and Ce(CF3COO)3 (0, 8.4, 25.2 and 33.6 mg) at room temperature to afford a precursor solution for the first shell. Meanwhile, a precursor solution for outmost shell was also prepared by heating a mixture of OA (1.0 mL), ODE (1.0 mL), Y(CF3COO)3 (0.2 M, 0.5 mL) and Na(CF3COO) (1 M, 0.1 mL) at 150 oC for 0.5 h. After the addition of core nanoparticles (1.0 mL) in the precursor solution of the first shell, the resulting mixture was heated at 100 oC for 0.5 h to remove cyclohexane, and then heated at 310 oC for 0.5 h

■ Results and Discussion The synthetic scheme of the multilayer nanoparticles is presented in the Figure S2a. We began with the synthesis of NaGdF4:Yb/Tm (49/1 mol%) core nanoparticles via a reported thermal decomposition method. The choice of NaGdF4:Yb/Tm as core nanoparticles is governed by the consideration that they showed the ability to give a stronger emission of Gd3+ dopants at 311 nm at an optimal doping combination of Yb3+ and Tm3+ (Figure S1). The accumulated energy at the excited state of Gd3+ dopants facilitates the 3



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Figure 3. (a) Upconversion emission profiles of the as-prepared multishell nanoparticles of NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (x/60 mol%)@NaYF4 upon excitation at 980 nm. The emission profiles were normalized at 650 nm. The inset shows a photograph of a cyclohexane dispersion of NaGdF4:Yb/Tm(49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4 upon the excitation (2 W cm-2). (b) Time-resolved upconversion spectrum of the as-prepared multilayer nanoparticles of NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4. (c) Lifetime comparison of the upconversion emissions of Mn2+ (531 nm, 4T1 → 6A1) and Tm3+ (477 nm, 1D2 → 3H6) of the multilayer nanoparticles recorded in aqueous solution at room temperature. (d) Down-conversion emission profiles of the as-prepared multishell nanoparticles after being removed of surface ligand upon excitation at 254 nm. The emission profiles were normalized at 311 nm and the inset shows a photograph of an corresponding aqueous dispersion of NaGdF4:Yb/Tm(49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4 upon the excitation. (e) The relative emission enhancements of Mn2+ dopants of the multilayer nanoparticles at different doping level of Ce3+ after the removal of surface ligand. (f) Lifetime comparison of the down-conversion emission of Mn2+ (531 nm, 4T1 → 6A1) of the multilayer nanoparticles recorded in aqueous solution at room temperature. .

generation of a strong emission of other emitting ions in the middle layer of the multilayer nanoparticles via the energy migration mechanism.25 Notably, there is slight discrepancy between the doped and designed levels of the two ions as a result of different reactivity in their precursors in the formation of fluorides at high temperatures (Table S1). TEM showed an average particle size distribution of 15 nm and a polyhedron morphology of the as-prepared core nanoparticles (Figure S2). They are in pure cubic phase, as supported by XRD analysis (Figure S3a). These results are in line with previous reports.50 Next, we performed epitaxial growth of double layers of NaGdF4:Ce/Mn (3/60 mol%) and NaYF4 on the core nanoparticles using a successive thermal decomposition strategy. By comparison, XRD analysis showed no change in the phase purity of the as-prepared multilayer nanoparticles (Figure S3b and c). However, a prominent lower-angle shifting of the diffraction patterns was observed as a result of unit cell expansion induced by the partial substitution of Gd3+ (0.938 Å) with big-sized Ce3+ ions (1.01 Å). TEM studies revealed an obvious increase in the average size from 15 to 30 nm of the nanoparticles after growth the double shells (Figure 2b and S4), suggesting the

formation of multilayer nanoparticles. Instead of polyhedra, spherical multilayer nanoparticles were produced after the show growth step. The morphological change is likely due to an etching effect provided by oleic acid at high temperatures (Figure 2b, inset).51-53 The formation of core@shell@shell morphology was verified by elemental mapping analysis of Yb, Ce and Y within the target nanoparticles (Figure 2b-e). The elemental distribution analysis also suggests the thickness of the middle and outmost shells to be 4.8 and 3.8 nm, respectively. High-resolution TEM showed high crystalline nature of the surface layer of the multishell nanoparticles, as supported by the observation of measured d-spacings of 0.20 and 0.17 nm (Figure 2g).54 These values are consistent with the lattice spacings in the (002) and (311) planes of the cubicphased NaYF4 lattices (inset, Figure 2g). However, we also observed some grey striped areas in the middle parts of the nanoparticles, as indicated in Figure 2a (the inset). Highresolution TEM imaging of the striped region shows weak crystallinity nature relative to the outmost layer (marked area, Figure 2b). This observation is likely due to the presence of some degree of lattice strain within the 4


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multilayer nanoparticles induced by the mismatch in the ionic radius of the lanthanides. We further prepared other multilayer nanoparticles with varied doping concentrations of Ce3+ (0, 9 and 12 mol%) using the same method (Figure S5a-c). Notably, the annealing time at 310 oC at least 1 h after the injection of the outmost shell precursor is essential for the formation of multilayer nanoparticles. Otherwise, the final product is a mixture of the designed multilayer nanoparticles and separated NaYF4 nanoparticles (Figure S6). Next, we probed the practical doping levels of Ce3+ and Mn2+ in the multilayer nanoparticles by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results showed that the practical doping levels of Ce3+ ion are much higher than the designed concentration (Table S1). In addition, the increase in the doping level of Ce3+ from 7.51 to 28.44 mol% is accompanied by a decreased tendency in the doping concentration of Mn2+ from 2.95 to 0.58 mol%. These observations are attributed to a lower reactivity of Mn(CF3COO)2 relative to Ce(CF3COO)3 in the thermal decomposition procedure to produce corresponding fluorides.50 Indeed, the yields of NaMnF3 and NaCeF4 from Mn(CF3COO)2 and Ce(CF3COO)3 were estimated to be 38.4% and 69.2% at high temperatures in the presence of Na(CF3COO), respectively. In addition, the larger size and charge mismatch between Mn2+ and Gd3+ is likely to further increase the difficulty in doping of Mn2+ into the host lattices of NaGdF4.32 In a further set of experiments, we carried out upconversion luminescence characterization of the asprepared multilayer nanoparticles. Prior to the growth of the middle and the outmost layers, the core nanoparticles of NaGdF4:Yb/Tm (49/1 mol%) exhibited a typical emission of Tm3+ dopants, as evidenced by the appearance of multiband peaks at 345, 368, 450, 477 and 650 nm upon excitation at 980 nm (Figure S7). After passivation with the double layers, the resultant nanoparticles displayed a remarkable broad emission band centered at 531 nm in addition to the characteristic emission bands for Tm3+ (Figure 3a). The strong upconversion emissions for both Mn2+ and Tm3+ dopants render the colloidal solution with a whitish color output upon the excitation at a power density of 2 W cm-2 (Inset, Figure 3a). We further found that Ce3+ doping exhibits a negligible impact on the emission intensity of Mn2+ dopants at a concentration less than 3 mol%. However, an increased doping concentration of Ce3+ led to a gradual decrease trend in the emission intensity of Mn2+. These results are possibly ascribed to a decreased energy transfer efficiency from Gd3+ to Mn2+ due to presence of energy relay from Gd3+ to Ce3+ and to Mn2+ ions (Figure S8).30 Notably, it is difficult to achieve the upconversion emission of Mn2+ dopants when the multilayer nanoparticles were prepared with the use of hexagonal phased NaGdF4:Yb/Tm (49/1

mol%) as core nanoparticles (Figure S9). It is probably due to the difficulty of doping of Mn2+ ions into the hexagonal phase NaGdF4 lattices.50 The lifetime of Gd3+ emission at 311 nm was decreased from 1.52 to 0.52 ms after the addition of Mn2+ into the layer of NaGdF4 in the core@shell nanoparticles of NaGdF4:Yb/Tm@NaGdF4:Mn (Figure S8). This observation suggests the photon upconversion of Mn2+ dopants follows a Gd-sublattice-mediated energy migration mechanism.25 Time-resolved spectrum of the as-prepared NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4 multilayer nanoparticles suggests a much longer lifetime of the Mn2+ emission relative to those of Tm3+ emissions (Figure 3b). Indeed, a luminescence transition from white to green was observed after ceasing excitation at 980 nm during the optical measurement. The lifetime of Mn2+ emission at 531 nm was estimated to be about 32 ms, 81 times longer than that of Tm3+ emission at 477 nm (Figure 3c). More importantly, the lifetimes of Mn2+ upconversion emission in other multilayer nanoparticles were also found to be 32 ms, indicative of no quenching effect of Ce3+ on Mn2+ upconversion emission in these cases (Figure S10). We further studied the influence of surface oleate ligand on the upconversion emission profiles of the asprepared multishell nanoparticles. After removal of the surface ligand, a similar dependence of Ce3+ doping on the luminescence intensity of Mn2+ was observed (Figure S11). By comparison, the ligand-free multilayer nanoparticles showed enhanced emission for both Tm3+ and Mn2+ ions, and the increased degree is gradually decreased with increasing the emission wavelengths (Figure S12). This is attributed to a decreased photon absorption ability of the particle’s surface ligand with an increase in the wavelength (Figure S13).55,56 We also find an important role of the outmost protecting layer of NaYF4 in maintaining the strong upconversion emission of Mn2+ in the ligand-free nanoparticles (Figure S14a and b). Without the protecting effect, the accumulated excitation energy in the excited Gd3+ ions can be easily extracted by the surface defects and solvent molecules. Temperature-dependent upconversion emission properties of the ligand-free NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4 multilayer nanoparticles were studied in the temperature window from 308 to 138 K. Our results showed a strongest upconversion emission for the Mn2+ dopants at 258 K (Figure S15). This phenomenon can be understood by the fact an appropriate level of lattice vibration helps energy transfer from sensitizers and activators via reduction of energy mismatch between their transition energy levels.57 However, a strong stretching vibration of the host lattices at an elevated temperature can consume additional excitation 5



energy of the dopants, thereby giving rise to a decreased upconversion emission of the Mn dopants. Codoping of Ce3+ ions into the multilayer nanoparticles allows us to investigate the down-conversion luminescence of the same Mn2+ dopants. Without removing surface ligand, a weak green emission from the as-prepared Ce3+,Mn2+doped multishell nanoparticles was observed upon excitation at 254 nm (Figure S16). The luminescence spectroscopy showed dual emissions centered at 369 and 531 nm. The former can be attributed to the emission of the surface ligand (Figure S17), and the latter is a result of optical transition of 4T1 → 6A1 of Mn2+. The Mn2+ luminescence in the ligand-capped multilayer nanoparticles is increased gradually with enhancing the doping level of Ce3+ upon the UV excitation. We discovered that surface ligand showed a more profound impact on the down-conversion than the upconversion luminescence of the same Mn2+ dopants. This is due to the existence of a strong screen effect of the surface oleate layer to UV excitation light. Upon the removal of the surface ligand layer, the multilayer nanoparticles exhibited a remarkably enhanced green Mn2+ luminescence, accompanied by the emergence of two new emissions for Gd3+ at 311 nm and for Ce3+ at 331 nm (Figure 3d), respectively. We found a 31-fold enhancement in the Mn2+ luminescence for the ligand-free multilayer nanoparticles with a low doping concentration of Ce3+ (3 mol%). The absolute quantum yield of the down-conversion Mn2+ luminescence was estimated to be 35.5% via the use of following equation.58 QY 

N N

photon emitted

photon absorbed

where Nphoton emitted and Nphoton absorbed are denoted as the numbers of emitted and absorbed photons, respectively. Notably, a continued increase in the doping level of Ce3+ ions to 9 and to 12 mol% gave rise to lower enhancement factors of 9 and 11, respectively (Figure 3e). These results show that a high Ce3+ doping level in the multilayer nanoparticles can lead to deleterious Ce3+-Ce3+ crossrelaxation, thereby weakening the energy transfer efficiency of Ce3+ →Mn2+ or Ce3+ → Gd3+ → Mn2+. On a separate note, the spin-forbidden optical transition nature of 4T1 → 6A1 enables the down-conversion luminescence of the same Mn2+ dopants to have a comparable lifetime to the corresponding upconversion emission.59-62 More importantly, the lifetime of the green luminescence also is irrespective of the doping concentration of Ce3+ in the same layer (Figure 3f).

6


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Figure 4. (a) Demonstration of NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4 multilayer nanoparticles for multilevel anticounterfeiting. The patterns i, ii and iii were made by use of NaGdF4:Ce/Eu (15/5 mol%)@NaYF4, NaGdF4:Ce/Tb(15/5 mol%)@NaYF4 and NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%)@NaYF4. (a) Upon irradiation with UV light, the patterns exhibit typical color output of Eu3+, Tb3+ and Mn2+. When switching off the UV lamp, the pattern iii can still be visible after the complete disappearance of the patterns i and iii. (b) Upon dynamic excitation of the pattern iii by a 980 laser (64 W cm-2), a dual-color luminescence trace were produced. The head emission consists of a mixture emission of Tm3+ and Mn2+, while the tail only comprises the pure emission of Mn2+. Notably, the reproduced artwork was fixed on a spinspeed controllable rotor, and the spin speed was set at 500 rpm for the experiment. (c) Upon steady irradiation at 980 nm (6 W cm-2), the pattern iii first shows a whitish color of a mixed emission of Tm3+ and Mn2+, and then turns to green upon ceasing the excitation. Note that the time-dependent images were obtained by a cell phone camera under a burst mode after turning off the UV (a) and NIR excitation (c).

also observed in our multilayer nanoparticles. In the presence of surface ligand, the core@shell nanoparticles of NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Ce/Mn (3/60 mol%) can not give rise to a green emission upon excitation at 254 nm. A weak green emission for the Mn2+ dopants can be generated upon the growth of the protection layer of NaYF4. The difference is obviously due to the suppression of excitation energy leakage from the excited Gd3+ ions by the NaYF4 layer. After the removal of the surface ligand, both the core@shell and multilayer nanoparticles can enable bright green luminescence. When compared with the ligand-capped counterparts, the ligand-free core@shell and multilayer counterparts showed 240- and 34-fold enhancements in the Mn2+ luminescence, respectively (Figure S19a and b). A higher enhancement factor for the core@shell nanoparticles is a result of their weaker Mn2+ luminescence in the presence of the surface ligand. Taken together, these results suggest a similar energy protecting role of the outmost layer of NaYF4 in the down-conversion process. However, the protecting layer for the ligand-free multilayer nanoparticles exhibits a less important role in achieving bright down-conversion luminescence relative

The dependence of the down-conversion emission of the Mn2+ dopants on the temperature was studied by using the ligand-free NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4: Ce/Mn (3/60 mol%)@NaYF4 multishell nanoparticles as a model system. Similarly, an increased tendency in the green down-conversion emission was produced as the temperature rising from 138 to 308 K (Figure S18a). This observation suggests multiphonon-assisted energy transfer to be presented between Ce3+ and Mn2+.63 At low temperatures, the energy transfer from Ce3+ to Mn2+ is suppressed because the energy mismatch between the 4f5d band of Ce3+ and 4T1 energy level of Mn2+ cannot be consumed by the lattice vibration. However, low temperatures can effectively reduce the perturbation of the excited Mn2+ dopants from the surrounding environments, thereby resulting in a gradual increase in the lifetime of the Mn2+ luminescence from 29.5 to 34.1 ms when decreasing the temperature from 308 to 138 K (Figure S18b). A protecting effect of the outmost layer of NaYF4 on the down-conversion luminescence of Mn2+ dopants was 7



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luminescence of the same Mn2+ ions upon excitation at 980 nm. The dual-mode luminescence of the same Mn2+ dopants shows lifetimes longer than 30 ms upon the correspond excitation. The long-lived nature of the dual-mode Mn2+ luminescence benefits the ease of creation multiple up- and down-conversion time-domain optical features for multilevel anti-counterfeiting and multiplexing biolabeling.

to the upconversion of the same Mn2+ dopants. The ability of our Ce3+,Mn2+-codoped multilayer nanoparticles to emit dual-mode long-lived luminescence in response to NIR and UV light allows us to use them as security ink for multilevel anti-counterfeiting studies. Representative down-conversion phosphor particles, including NaGdF4:Ce/Eu (15/5 mol%)@NaYF4 and NaGdF4:Ce/Tb (15/5 mol%)@NaYF4 (Figure S20), were also prepared for performance comparison in document anti-counterfeiting. Notably, the lifetimes of the Eu3+ and Tb3+ emissions for the two phosphors at 616 and 543 nm were estimated to be 6.5 and 4.4 ms upon excitation at 254 nm, respectively (Figure S21 and 22). As a proof of demonstration, a two-dimensional covert pattern was created on a reproduced artwork by using Ce3+,Eu3+- (area i) and Ce3+,Tb3+-codoped phosphors (area ii), and Ce3+,Mn2+codoped multilayer nanoparticles (area iii) based on a stamping method. Upon steady excitation with a UV lamp (254 nm), red- and green-colored readout, typical luminescence for Eu3+, Tb3+ and Mn2+ ions, were produced as shown in Figure 4a. Notably, no obvious difference can be observed from the Tb3+ and Mn2+ luminescence upon the steady excitation. However, after turn-off of the excitation, the readout for Ce3+,Mn2+-codoped multilayer nanoparticles can persist for a longer time, enabling the production of an additional dimension of time-domain anti-counterfeiting code that can be facilely recorded by a cell phone camera under a burst mode.64 Next, the area iii was dynamically scanned by a 980nm laser (64 W cm-2) after mounting the artwork on a spincontrolled stage (Figure S23). The results showed a circleshaped green tail emission, together with a strong whitish head emission (Figure 4b).65 Obviously, the quick decay of the upconversion emission of Tm3+ leads to the generation of a pure green tail emission of Mn2+ at the end. Furthermore, color change of the area iii from white to green was also observed upon ceasing the steady excitation at 980 nm (Figure 4c). These findings suggest that the longlived dual-mode luminescence of Mn2+ allow it to be conveniently distinguished from corresponding lanthanide emission. This attribute make the as-prepared nanoparticles attractive for multi-level data storage not only due to the ease of creation of multiple time-domain codes under the UV or NIR excitation but also for the no need for complex time-gated instrumentation to proceed data decoding.

■ ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Additional characterization of as-prepared multilayer nanoparticles and the setup used for dynamic excitation of the multilayer nanoparticles

■ AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

■ Acknowledgment This work was supported by the Natural Science Foundation of Anhui Province for Distinguished Youth (1908085J06 and 1908085MB49), National Natural Science of China (21871005, 21701119 and 61705137) and the Science and Technology Project of Shenzhen (grant JCYJ20170817093821657).

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■ Conclusions In conclusion, we have demonstrated that at a low doping concentration of Ce3+ the as-prepared multilayers have shown the ability to be excited by the UV light to render green down-conversion of Mn2+ dopants while causing a marginal influence on the upconversion 8


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Dual-Mode Long-Lived Luminescence of Mn2+-Doped Nanoparticles

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