Versatile Spectral and Lifetime Multiplexing Nanoplatform with

of Polymers, Fudan University, 220 Handan Road, Shanghai 200433, China. ACS Nano , 2017, 11 (3), pp 3289–3297. DOI: 10.1021/acsnano.7b00559. Pub...
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Versatile Spectral and Lifetime Multiplexing Nanoplatform with Excitation Orthogonalized Upconversion Luminescence Hao Dong,† Ling-Dong Sun,*,† Wei Feng,‡ Yuyang Gu,‡ Fuyou Li,‡ and Chun-Hua Yan*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Department of Chemistry & Institutes of Biomedical Sciences & State Key Laboratory of Molecular Engineering of Polymers, Fudan University, 220 Handan Road, Shanghai 200433, China S Supporting Information *

ABSTRACT: Optical encoding together with color multiplexing benefits on-site detection, and enriching the components with narrow emissions from lanthanide could greatly increase the coding density. Here, we show a typical example to combine emission color and lifetime that are simultaneously integrated in a single lanthanide nanoparticle. With the multicompartment core/shell structure, the nanoparticles can activate different emitting pathways under varied excitation. This enables the nanoparticles to generate versatile excitation orthogonalized upconversion luminescence in both emission colors and lifetimes. As a typical example, green emission of Er3+ and blue emission of Tm3+ can be triggered with 808 and 980 nm lasers, respectively. Moreover, with incorporation of Tb3+, not only is emission from Tb3+ introduced but also the lifetime difference of 0.13 ms (Er3+) and 3.6 ms (Tb3+) is yielded for the green emission, respectively. Multiplexed fingerprint imaging and time-gated luminescence imaging were achieved in wavelength and lifetime dimensions. The spectral and lifetime encoding ability from lanthanide luminescence greatly broadens the scope of luminescent materials for optical multiplexing studies. KEYWORDS: multiplexing, upconversion, lanthanide, core/shell nanoparticle, energy transfer multicolor luminophores.17−20 Besides integration and enhancement of intrinsic emissions with optically active and inert shells,21−24 extra excitation can also be afforded as reported with a Nd3+ and Yb3+ co-doped shell,25−27 and corresponding emission spectra are well maintained in wavelength and intensity ratio.25 To further correlate emission with excitation, as well as incorporate lifetime for two-dimensional multiplexing studies, upconversion nanoparticles to incorporate these potentials simultaneously should be investigated. Herein, we develop a versatile multiplexing nanoplatform (Scheme 1) to get excitation orthogonalized emission, in both spectral wavelength and lifetime. The nanoplatform is based on a core/shell structure and composed of two noninterfering luminescent regions with distinctive excitation and emission characters. To inhibit interactions between these two regions, an optically inert layer is sandwiched. This endows the nanoplatform with excitation correlated distinctive emission

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ultiplexing applications in chemistry, biology, and security fields require code-intensive materials.1,2 Incorporating multiple information into an individual encoding unit should be of great significance for intensifying multiplexing.3,4 Inherited by intraconfigurational transitions, upconversion luminescence from lanthanides are endowed with large anti-Stokes shift, narrow emission band, long decay time, excellent photostability, and low background interference.5,6 Moreover, since the energy migration and photon transition can be confined in the nanodomain, lanthanide nanoparticles with tunable multicolor emissions are more facile. This makes them more attractive for multiplexing studies compared with broad and small Stokes emitted quantum dots and organic dyes.7−9 Recently, substantial efforts have been devoted to confining excitation and emission for tailoring spectral characters,10−16 but there is still a lack of experience in tuning the lifetime characters, and simultaneously controlling the emission spectra and lifetime, in particular, employing these two features as dual codes for multiplexing is still a challenge. A core/shell structure, with layers releasing emissions separately, has been developed as a promising platform for © 2017 American Chemical Society

Received: January 25, 2017 Accepted: February 25, 2017 Published: February 26, 2017 3289

DOI: 10.1021/acsnano.7b00559 ACS Nano 2017, 11, 3289−3297

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As a proof-of-concept (Figure 1a), dual lanthanide sensitizers, Yb3+ and Nd3+, are incorporated into the two luminescent regions to respond to dual excitation at 980 and 808 nm, respectively, and dual activators, Er3+ and Tm3+, are separately doped to emit distinctive emissions. As Nd3+ is involved in only region II, noninterfering energy transfer pathways can be activated. Inner region I is efficeint to respond to 980 nm excitation, while the ourter region II would be luminous under 808 nm excitation. This enables the nanoparticle to emit specific upconversion emission depending on the excitation wavelength. Following this principle, two types of nanoparticles were constructed and studied by exchanging the arrangement of activators (Figure 1b). Furthermore, to differentiate the lifetime, a Tb3+-containing layer is introduced, which is activated through sequential energy migration from Tm3+ and Gd3+ in neighboring layers (Figure 1c,d). The long lifetime of Tb3+ in milliseconds would contrast greatly with the short lifetime of Er3+ in microseconds. This facilitates twodimensional signals, color and lifetime, to be effectively comprised in one nanoparticle. These as-developed nanoparticles show decent performance in multiplexing.

Scheme 1. Schematic illustration for the nanoplatform showing excitation orthogonalized spectral wavelength (λ) and lifetime (τ). Note that emission should be recorded at the same wavelength in lifetime multiplexing, and different colors are used to highlight the distinctive emission.

RESULTS AND DISCUSSION Spectral multiplexing was first studied in the nanoplatform. Hexagonal phased NaGdF4 and NaYF4 were employed to construct the core/shell nanoparticle for their isomorphism and

spectra and lifetime information. Compared with nanoparticle mixtures for multiplexing,28 the multilayered nanoplatform integrated with different luminescent components is more significant in imaging resolution.3

Figure 1. Schematic illustration for the lanthanide nanoplatform with excitation orthogonalized multicolor upconverison luminescence for spectral and lifetime multiplexing. 3290

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Figure 2. (a) TEM, (b) HRTEM image, and (c) corresponding FFT pattern, (d) HAADF-STEM and (e) linear EDX scanning image of the Er@Y@Tm@Nd@Y nanoparticle. (f, g) Upconversion emission spectra under excitation of 808 nm (f) and 980 nm (g) lasers (3 W/cm2). Structure-correlated emission spectra with varied radial thickness of an optically inert NaYF4 interlayer (h, i) and NaYF4:Yb,Tm layer (j−n) under 808 and 980 nm excitations. Spectra in (h) and (i) are normalized at 450 nm, and spectra in (j)−(n) are normalized at maximum intensity.

low phonon energy.29,30 To minimize surface quenching,24 an optically inert NaYF 4 shell was grown outermost. A NaGdF4:Yb,Er@NaYF4@NaYF4:Yb,Tm@NaYbF4:Nd@NaYF4 (Er@Y@Tm@Nd@Y) nanoparticle, with Er3+ in region I and Tm3+ in region II, was synthesized with a modified thermal decomposition method.31,32 Transmission electron microscopy (TEM) images (Figure 2a and Figure S1) indicate uniform core and core/shell nanoparticles with a narrow size distribution. Morphology evolution from sphere to quasi-hexagon as well as a notable size increase from 20.1 ± 1.1 nm to 73.5 ± 2.6 nm in diameter and to 36.3 ± 1.9 nm in height accompanied by core/ shell formation was observed.33 Legible lattice fringes of 0.52 nm in the high-resolution TEM (HRTEM) image (Figure 2b) are correlated with the (101̅0) plane of hexagonal NaYF4, which is also verified by fast Fourier transform (FFT) result (Figure 2c). Powder X-ray diffraction (XRD) patterns (Figure S2) further suggest the hexagonal structure for all nanoparticles. Techniques including high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), linear energy dispersive spectroscopy (EDX) scanning, and EDX mapping were employed to confirm the core/shell structure. Figure 2d shows a typical HAADF-STEM image of an individual nanoparticle; five domains (1−5) can be identified via image contrast. The bright parts 1 and 4 correspond to heavier lanthanide elements in the NaGdF4:Yb,Er core and NaYbF4:Nd subshell, respectively. Both the dark regions 2 and 5 refer to layers with lighter elements, Y, in NaYF4 layers. The image contrast of region 3 is between these two due to the

coexistence of heavier lanthanide and lighter Y elements in NaYF4:Yb,Tm. This confirms the successful formation of the as-designed five-layered nanoparticle. In the linear EDX scanning (Figure 2e) and EDX mapping (Figure S3) analyses, Gd3+ is concentrated in the core, Nd3+ is mainly distributed in the secondary outer layer, and the location of Yb3+ and Y3+ also corresponds well with the HAADF-STEM result, which discloses the core/shell structure. Subsequently, upconversion spectral characters of Er@Y@ Tm@Nd@Y nanoparticles under 808 and 980 nm excitation were investigated. As shown in Figure 2f and g, the nanoparticles exhibit apparent excitation-dependent spectral profiles. Under 808 nm irradiation, ultraviolet and blue emissions from Tm3+ via five-photon 1I6 → 3F4 (345 nm), four-photon 1D2 → 3H6, 3F4 (365 nm, 450 nm), and threephoton 1G4 → 3H6, 3F4 (475 nm, 645 nm) transitions (Scheme S1) remarkably dominated the spectrum. Negligible green emission of Er3+ was observed for the elemental diffusion of lanthanides during high-temperature synthesis.34 In sharp contrast, visible 525 nm (2H11/2 → 4I15/2), 545 nm (4S3/2 → 4 I15/2), and 655 nm (4F9/2 → 4I15/2) emissions of Er3+ dominate the spectrum under 980 nm excitation (Figure 2g). Although Tm3+ emission is partially activated, it is very weak in spectra. This should be attributed to the fact that two-photon emission of Er3+ is more efficient under low excitation power density.35,36 The distinctive spectral response suggests that the energy transfer pathways in the two luminescent regions differ greatly and are noninterfering with each other, consistent with the 3291

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relaxations (Figure S11). Nanoparticles for further studies were based on the optimized structure. With the platform of a core/shell structure, activators in the two luminescent regions can be exchanged, resulting in a reversed spectral response. Tm@Y@Er@Nd@Y nanoparticles (Scheme S2, Figures 3a−c, S12, S13) were prepared with the

proposed mechanism in Scheme 1. Furthermore, the excitation orthogonalized emissions can be maintained with an increasing power density of more than 60 W/cm2 (Figure S4), ensuring extended spectral multiplexing. As lanthanides would diffuse in a NaGdF4 and NaYF4 lattice during high-temperature synthesis,34 the thickness of the sandwiched NaYF4 interlayer should be an important factor to inhibit energy migration between the two luminescent regions. We then assessed a series of Er@Y@Tm@Nd@Y nanoparticles with varied thickness of the NaYF4 interlayer (Figure S5). As shown in Figure 2h, by depositing a ca. 1 nm NaYF4 interlayer (thickness hereinafter refers to the radial thickness), emissions of Er3+ from lanthanide diffusion can be largely suppressed under 808 nm excitation, and the blocking efficacy is enhanced as the NaYF4 interlayer is thickened. When the thickness of the NaYF4 interlayer was over ca. 12 nm, the emission of Er3+ from lanthanide diffusion was very weak. In the meanwhile, the red emission ranging from 630 to 690 nm exhibits a clear transformation from spectral fingerprints of Er3+ to that of Tm3+. Throughout tuning the thickness, emissions of Tm3+ were scarcely affected. When these nanoparticles were excited at 980 nm, Tm3+ emissions were almost unchanged as well (Figure 2i). By contrast, emissions of Er3+ enhanced as the NaYF4 interlayer thickened for the more prominent elimination of surface quenching.23 Therefore, a ca. 12 nm optically inert NaYF4 interlayer is efficient to inhibit the energy migration between the two luminescent regions, although lanthanide diffusion exists. The relative proportion of active layers, NaGdF4:Yb,Er to NaYF4:Yb,Tm, is also important in determining the output, because the emission intensity is positively correlated with the number of luminescent layers. A series of Er@Y@Tm@Nd@Y nanoparticles (Figure S6) with varied thickness of the NaYF4:Yb,Tm layer were evaluated. As the thickness was ca. 1 nm, emissions of Tm3+ were quite weak under 808 nm excitation (Figure 2j), while Er3+ emissions from lanthanide diffusion dominated the spectrum. With increasing thickness of the NaYF4:Yb,Tm layer, Tm3+ emission enhanced progressively (Figure 2k−n), and it dominated the spectrum as the thickness increased to more than ca. 7 nm (Figure 2m,n). On the other hand, irradiated with 980 nm light, all the spectra were dominated by Er3+ emissions, but noticeable emissions of Tm3+ emerged as the thickness reached ca. 9 nm. Hence, Er@Y@ Tm@Nd@Y nanoparticles with a ca. 7 nm thick NaYF4:Yb,Tm layer are ideal to release excitation wavelength correlated emissions. It has been verified that Er@Y@Tm@Nd@Y nanoparticles can generate distinct spectral codes for multiplexing. Next, a series of control experiments were conducted to certify the functionality of specific layers in the nanoparticles. Both Er@ Tm@Y nanoparticles (Figure S7) comprising one type of sensitizer and Er@Nd@Y nanoparticles (Figure S8) comprising one type of activator fail in the differentiation of excitation, suggesting the necessity of simultaneous incorporation of dual sensitizers, Yb3+ and Nd3+, and dual activators, Er3+ and Tm3+. Yet the incorporation of Nd3+ and spatial arrangement of the NaYbF4:Nd layer should affect the upconversion efficiency via nonradiative relaxations (Figure S9).37,38 To get an optimized multiplexed nanoparticle, Nd3+ is preferred for introducing into an isolated layer compared to codoping with Er3+/Tm3+ (Figure S10), and the NaYbF4:Nd layer should be placed outside the two luminescent regions to avoid prominent cross-

Figure 3. (a) TEM, (b) HRTEM, (c) and images of Tm@Y@Er@ Nd@Y nanoparticle. Because NaYF4:Yb,Er layers were doped with only 20% Yb and 2% Er, their image contrast to the secondary layers from inside (NaYF4) is not significant. Upconversion emission spectra of the nanoparticles under 808 nm (d) and 980 nm (e) excitations (3 W/cm2). High-resolution fingerprint imaging using Er (f, g, h), Er@Y@Tm@Nd@Y (i, j, k), and Tm@Y@Er@ Nd@Y (l, m, n) nanoparticles.

same strategy. Spectral profiles in Figures 3d,e indicate the good correlation between emission spectra and excitation wavelength. Visible emissions of Er3+ and Tm3+ dominated under 808 nm (Figure 3d) and 980 nm excitation (Figure 3e), respectively, and the spectral difference was also well maintained with increasing power density (Figure S14). In this structure, the relative proportion between the two luminescent regions is investigated (Figures S15, S16), and the optimized thickness of the NaYF4:Yb,Er layer is ca. 7 nm to correlate excitation with emission spectra. The core/shell nanoparticles were then demonstrated for spectral multiplexing. A fingerprint, with significance in personal identification, was employed for excitation and emission orthogonal imaging.39,40 NaGdF4:Yb,Er (Er) nanoparticles, a traditional upconversion marker, were used as a 3292

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Figure 4. (a) TEM, (b) HRTEM, and (c) HAADF-STEM images of Tm@Tb@Y@Er@Nd@Y nanoparticles. The image contrast between the inner Tm3+ and Tb3+ layers is not obvious due to the composition with heavier lanthanides. Upconversion emission spectra (d, e) and decay curves (f) of the nanoparticles under 808 and 980 nm excitation. Schematic illustration (g) and experimental tailing decay luminescence imaging (h) under 980 and 808 nm excitation (15 W/cm2).

prepared. Similar to that shown in Figure 3d, the Tb3+containing layer does not change the overall emission under 808 nm excitation (Figure 4d), but strong emissions from Tb3+ as well as Tm3+ were detected under 980 nm excitation (Figure 4e). This ensures distinctive orthogonal spectra (Figure S18). Moreover, the green emission in Figure 4d and e differs a lot in lifetimes. As shown in Figure 4f, emission of Er3+ under 808 nm excitation showed a lifetime of 0.13 ms. By contrast, much prolonged green emission lifetimes, 3.64 ms from Tb3+, were obtained under 980 nm excitation. Because of the existence of Er3+ emissions, a fast decay process with a lifetime of 0.15 ms can also be observed. This suggests the identifiable lifetime could be realized in a single nanoparticle. Moreover, by adjusting the concentration of Tb3+ in the NaGdF4:Tb layer, the long lifetime of Tb3+ can be tuned in the range of 0.82− 4.21 ms (Figure S19), which could be differentiated well from that of Er3+ (0.11−0.13 ms), and this is much broader than previously reported.1 By exchanging the arrangement of Er3+ and Tb3+/Tm3+ as Er@Y@Tb@Tm@Nd@Y (Scheme S4, Figure S20), excitation correlated spectra and emission lifetimes are also confirmed (Figures S21, S22). In addition, on replacing the Tb3+ layer with a Eu3+-containing one (Scheme S5), distinctive emission spectra and lifetimes can also be noticed with varied excitation (Figures S23, S24). Emissions of Er3+ and Tm3+/Eu3+ are dominant under 808 and 980 nm excitation,

control. Greenish patterns including island (1) and termination (2) can be visualized among well-resolved ridge flows (Figure 3f) under 980 nm excitation. However, they were cloaked under 808 nm excitation (Figure 3g). Strikingly, with Er@Y@ Tm@Nd@Y nanoparticles encoded fingerprints, greenish fingerprint patterns can be vividly imaged under 980 nm excitation (Figure 3i), and identical patterns but in blue color can be identified under 808 nm excitation (Figure 3j). In a similar fashion, with Tm@Y@Er@Nd@Y nanoparticles, distinctive fingerprints in blue (Figure 3l) and green (Figure 3m) can be discriminated with 980 and 808 nm irradiation, respectively. All the luminescent images can be well merged with the bright field one (Figure 3h,k,n). This shows that asdeveloped core/shell nanoparticles are promising for spectral multiplexing. Next, emission lifetime information was considered. Although the lifetime could be tuned with doping concentration of lanthanides,1 it is on the same order of microseconds in Er3+/Tm3+ systems. A long-lifetime part, Tb3+ with lifetime in milliseconds,41−43 was introduced to compensate for and differentiate with that of Er3+. The incorporation of Tb3+ was set in a separate NaGdF 4 :Tb layer adjacent to the NaGdF4:Yb,Tm layer, to be activated by energy transfer from Tm3+ and Gd3+. According to this principle, Tm@Tb@Y@Er@ Nd@Y (Scheme S3, Figures 4a−c, S17) nanoparticles were 3293

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Figure 5. (a) Lifetime multiplexing with the time-gated technique. (b) Pseudocolored upconversion luminescence images of Er and Tm@Tb@ Y@Er@Nd@Y nanoparticles under steady-state and time-gated state of 980 and 808 nm excitation, respectively.

multiplexed Tm@Tb@Y@Er@Nd@Y nanoparticles were added to encode MILES. In the time-gated mode, short lifetime signal SMILE was filtered, while MILES with long lifetime emission from Tb3+ was detected, and the same pattern could also be visualized with 808 nm excitation. The overall SMILES pattern appeared with 980 nm steady-state irradiation, and the overlay part of MILE came from emission of both Er3+ and Tb3+. These demonstrations highlight the long lifetime and excitation dependent features of the typical multiplexed nanoparticles. The lifetime encoded multiplexing also showed good contrast for logos (Figure S26).

respectively, and the red emission lifetime differs a lot, 0.17 ms (Er3+) and 4.73 ms (Eu3+) with 808 and 980 nm excitation, respectively. This further confirms the versatility of the nanoplatform in getting excitation orthogonalized emission in wavelength and lifetime. We then made a smart design to make the lifetime difference more intuitionistic. Nanoparticles were loaded on a rotating plate under near-infrared illumination. The upconversion luminescence would decay gradually as it rotates far away from the excitation spot. Therefore, the trajectory of rotated nanoparticles can be tracked via its upconversion luminescence, and the tailing length is well correlated with the decay rate (Figure 4g). As shown in Figure 4h, apparently short tailing from Er3+ can be noticed with 808 nm irradiation. In sharp contrast, a short blueish and long greenish luminescent trajectory, from Tm3+ and Tb3+, respectively, can be observed under 980 nm excitation. As rotating speeds up, tailing lengthens, but the luminescent track with 808 nm irradiation is still shorter than that of 980 nm, consistent with the decay profiles. Lifetime multiplexing was demonstrated with time-gated luminescence (Figure 5a).44 The delay time is set with the combination of pulse synchronizer and chopper to filter faster decaying luminescence. Er3+ luminescence from a NaGdF4:Yb,Er nanoparticle showed a lifetime of 0.09 ms (Figure S25), which decays much faster than that of Tb3+ (3.64 ms), which is used to highlight the nanoparticles with Tb3+ emission in the time-gated imaging. To differentiate these two kinds of nanoparticles, their luminescence is highlighted with different pseudocolors. The pattern of SMILES is used for encoding and decoding together with time-gated luminescence (Figure 5b). Er nanoparticles were tested as shown in the upper part of Figure 5b. SMILE can be observed as steady-state luminescence under 980 nm irradiation. However, no signals can be collected with the time-gated mode or 808 nm irradiation. Then,

CONCLUSIONS In summary, we have described the simultaneous tuning of upconversion spectra and lifetime in a versatile nanoplatform based on core/shell structure. By constructing noninterfering luminescent regions in a nanoparticle, the spectral and lifetime characters can correlate orthogonally with excitation. Multiplexed fingerprint and time-gated luminescent imaging were realized in both spectral and lifetime dimensions. Compared with conventional markers, the as-developed nanoparticles show great advantage in encoding densified information. This work not only deepens photon upconversion modulation but also sheds light on multiplexing applications in bi- and multidimensions with luminescent materials. EXPERIMENTAL SECTION Materials. Rare earth oxides were bought from China Rare Earth Online Co., Ltd. Oleic acid (OA, 90%) and trifluoroacetic acid (99%) were purchased from Sigma-Aldrich. Oleylamine (OM, 90%) and octadecene (ODE, 90%) were procured from J&K. Trifluoroacetic acid sodium salt (99%) was obtained from Alfa Aesar. Ethanol (AR) and cyclohexane (AR) were acquired from Beijing Chemical Works. All the reagents were used as received without further purification. Instrumentation. Copper grids were used to support colloidal nanoparticles for TEM examination. Low-resolution TEM images were 3294

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ACS Nano taken with a JEOL JEM-2100 TEM operated at 200 kV. HRTEM, HAADF-STEM, linear EDX scanning, and EDX mapping images were collected with a JEOL JEM-2100F TEM operated at 200 kV. XRD patterns were recorded on the Bruker D2 PHASER diffractometer (Germany), using Cu Kα radiation (λ = 1.5406 Å). Upconversion emission spectra were measured on a Hitachi F-4500 spectrometer. The 808 and 980 nm lasers are high-power multimode pump lasers (Hi-tech Optoelectronic Co. Ltd.). Upconversion emission lifetime was measured on a customized UV to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh Instrument) equipped with a tunable midband OPO pulse laser as excitation source (410−2400 nm, 10 Hz, pulse width ≤5 ns, Vibrant 355II, OPOTEK). Colloidal upconversion luminescence pictures were taken with a Nikon D3000 camera (5 s exposure time). Fingerprint imaging pictures were taken with an iPhone 4S. A GRB3 filter was added for all pictures taken under 808 nm excitation. Synthetic Procedures for the Multiplexing Nanoplatform. The Er@Y@Tm@Nd@Y nanoparticle, as an example, was synthesized as follows. a. Er Nanoparticles. Precursors (1 mmol CF3COONa, 0.78 mmol (CF 3 COO) 3 Gd, 0.2 mmol (CF 3 COO) 3 Yb, and 0.02 mmol (CF3COO)3Er) were added to a 100 mL three-necked flask containing 10 mmol of OA, 10 mmol of OM, and 20 mmol of ODE. The slurry was heated to 110 °C to remove water and oxygen. Then, a clear solution formed and was heated to 310 °C for 15 min under a N2 atmosphere (15 °C/min heating rate). After cooling to room temperature, an excess amount of ethanol was added to precipitate the nanoparticles. The final products were collected by centrifugation at 7800 rpm for 10 min. Nanoparticles were dispersed in 10 mL of cyclohexane. A 5 mL amount of as-prepared nanoparticle colloidal solution (nominal 0.5 mmol) was added into a 100 mL three-necked flask containing precursors (0.5 mmol of CF3COONa, 0.39 mmol of Gd(CF3COO)3, 0.1 mmol of Yb(CF3COO)3, and 0.01 mmol of Er(CF3COO)3) and solvents (20 mmol of OA and 20 mmol of ODE). The removal of cyclohexane, water, and oxygen was required before heating to 310 °C under a N2 atmosphere. The reaction was maintained at 310 °C for 30 min, and the aftertreatments were identical to that in the first step. Resulting NaGdF4:Yb,Er nanoparticles were stocked in 10 mL of cyclohexane. b. Er@Y Nanoparticles. A 2.5 mL amount of hexagonal NaGdF4:Yb,Er solution (nominal 0.25 mmol), used as seeds for epitaxial growth, was added into the 100 mL three-necked flask containing precursors of shell (1 mmol of CF3COONa and 1 mmol of Y(CF3COO)3) and solvents (20 mmol of OA and 20 mmol of ODE). Reaction conditions and aftertreatments are the same as those in the first step to get NaGdF4:Yb,Er@NaYF4 nanoparticles. c. Er@Y@Tm Nanoparticles. (c) A 10 mL amount of as-prepared Er@Y colloidal solution (nominal 0.25 mmol) was added into a 100 mL three-necked flask containing precursors (1 mmol of CF3COONa, 0.49 mmol of Y(CF3COO)3, 0.5 mmol of Yb(CF3COO)3, and 0.01 mmol of Tm(CF3COO)3) and solvents (20 mmol of OA and 20 mmol of ODE). The removal of cyclohexane, water, and oxygen was required at 110 °C. Then, the reaction system was heated to 310 °C for 30 min under a N2 atmosphere. Aftertreatments were similar to those before. d. Er@Y@Tm@Nd Nanoparticles. A 5 mL amount of as-prepared Er@Y@Tm colloidal solution (nominal 0.125 mmol) was added into a 100 mL three-necked flask containing precursors (0.5 mmol of CF3COONa, 0.25 mmol of Nd(CF3COO)3, 0.25 mmol of Yb(CF3COO)3) and solvents (20 mmol of OA and 20 mmol of ODE). The removal of cyclohexane, water, and oxygen was performed at 110 °C. The reaction system was then heated to 310 °C for 30 min under a N2 atmosphere. Aftertreatments were similar to those before. e. Er@Y@Tm@Nd@Y Nanoparticles. A 5 mL amount of asprepared Er@Y@Tm@Y colloidal solution (nominal 0.0625 mmol) was added into a 100 mL three-necked flask containing precursors (0.25 mmol of CF3COONa, 0.25 mmol of Y(CF3COO)3) and solvents (20 mmol of OA and 20 mmol of ODE). The removal of cyclohexane, water, and oxygen was performed at 110 °C. Simultaneously, the reaction system was heated to 310 °C for 30

min under a N2 atmosphere. Aftertreatments were similar to those before. The final products were dispersed in 5 mL of cyclohexane for further characterizations. Tailing Decay Luminescence Imaging. Multiplexed nanoparticles were loaded on a rotating plate by spin coating. Then, they were irradiated by a near-infrared laser. Once the nanoparticles are irradiated, upconversion luminescence will emerge and then decay gradually as the plate rotates. An iPhone 4S is used to collect the tailing decay luminescence imaging signals. Time-Gated Luminescence Imaging. Time-gated luminescence imaging pictures were taken with a time-gated imaging setup, which is equipped with a fast switchable 980 nm diode laser, a high-speed optical chopper, and an EMCCD camera (Andor iXon Ultra 897). The excitation and delay time are set as 0.1 and 0.03 ms, respectively.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00559. More experimental details, TEM images, XRD patterns, upconversion spectra, decay files, and lifetime multiplexed patterns (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (L.-D. Sun). *E-mail: [email protected] (C.-H. Yan). ORCID

Fuyou Li: 0000-0001-8729-1979 Chun-Hua Yan: 0000-0002-0581-2951 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by NSFC (Nos. 21425101, 21321001, 21371011, 21331001) and MOST of China (2014CB643800). We thank Prof. Xueyuan Chen and Dr. Datao Tu at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, for their valuable help in emission lifetime measurements. REFERENCES (1) Lu, Y.; Zhao, J.; Zhang, R.; Liu, Y.; Liu, D.; Goldys, E. M.; Yang, X.; Xi, P.; Sunna, A.; Lu, J.; Shi, Y.; Leif, R.; Huo, Y.; Shen, J.; Piper, J. A.; Robinson, J. P.; Jin, D. Tunable Lifetime Multiplexing Using Luminescent Nanocrystals. Nat. Photonics 2014, 8, 32−36. (2) Gorris, H. H.; Wolfbeis, O. S. Photon-Upconverting Nanoparticles for Optical Encoding and Multiplexing of Cells, Biomolecules, and Microspheres. Angew. Chem., Int. Ed. 2013, 52, 3584−3600. (3) 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. (4) Lee, J.; Bisso, P. W.; Srinivas, R. L.; Kim, J. J.; Swiston, A. J.; Doyle, P. S. Universal Process-Inert Encoding Architecture for Polymer Microparticles. Nat. Mater. 2014, 13, 524−529. (5) Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling Upconversion Nanocrystals for Emerging Applications. Nat. Nanotechnol. 2015, 10, 924−936. (6) Chan, E. M. Combinatorial Approaches for Developing Upconverting Nanomaterials: High-Throughput Screening, Modeling, and Applications. Chem. Soc. Rev. 2015, 44, 1653−1679. (7) Xiang, W.; Zhang, Y.; Takle, K.; Bilsel, O.; Li, Z.; Lee, H.; Zhang, Z.; Li, D.; Fan, W.; Duan, C.; Chan, E. M.; Lois, C.; Xiang, Y.; Han, G. Dye-Sensitized Core/Active Shell Upconversion Nanoparticles for 3295

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DOI: 10.1021/acsnano.7b00559 ACS Nano 2017, 11, 3289−3297

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ACS Nano (43) Senden, T.; Rabouw, F. T.; Meijerink, A. Photonic Effects on the Radiative Decay Rate and Luminescence Quantum Yield of Doped Nanocrystals. ACS Nano 2015, 9, 1801−1808. (44) Zheng, X.; Zhu, X.; Lu, Y.; Zhao, J.; Feng, W.; Jia, G.; Wang, F.; Li, F.; Jin, D. High-Contrast Visualization of Upconversion Luminescence in Mice Using Time-Gating Approach. Anal. Chem. 2016, 88, 3449−3454.

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DOI: 10.1021/acsnano.7b00559 ACS Nano 2017, 11, 3289−3297