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Intense red upconversion luminescence in Er3+-sensitized particles through confining the 1532 nm excitation energy
Journal of Luminescence 216 (2019) 116731
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Intense red upconversion luminescence in Er3+-sensitized particles through confining the 1532 nm excitation energy
T
Hao Lina,∗, Dekang Xuc, Yongjin Lib, Lu Yaob, Liqin Xua, Ying Maa, Shenghong Yangb, Yueli Zhangb,∗∗ a
School of Physics and Electronic Engineering, Guangzhou University, Guangzhou, 510006, PR China State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, School of Physics, Sun Yat-sen University, Guangzhou, 510275, PR China c School of Chemistry and Materials Engineering, Huizhou University, Huizhou, 516007, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Upconversion Intense red emission Er3+-sensitized particles 1532 nm NIR-II excitation Deep optical imaging
Lanthanide-doped upconversion (UC) materials are commonly excited by 700–1000 nm laser because of Yb3+ (980 nm excitation) and Nd3+ (808 nm excitation) are typically used as sensitizers. However, the excitation band located in the second near-infrared window (NIR-II, 1000–1700 nm) demonstrates the deeper penetration in biological tissue with minimum tissue scattering and autofluorescence. Herein, intense red UC emission in Er3+sensitized NaLuF4 particles under 1532 nm NIR-II excitation is presented. Red UC luminescence is significantly enhanced and purified through confining the 1532 nm excitation energy by Ho3+ and Tm3+ mediators, via an energy back transfer pathway (Er3+ → Ho3+ → Er3+ and Er3+ → Tm3+ → Er3+). Furthermore, when the particles injected into the fresh beef, red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation, implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission. Our results suggest that bright red UC luminescence from Er3+-sensitized NaLuF4 crystals with 1532 nm NIR-II excitation may be suitable for deep optical imaging.
1. Introduction Lanthanide (Ln)-doped upconversion (UC) materials demonstrate diverse advantages containing long luminescence lifetimes, narrow band emission, low toxicity and high photochemical stability [1–3], which make them have great potential applications in biological imaging, three-dimensional flat-panel displays, solar energy conversion, optical temperature sensors and so on [4–9]. Photon UC can transduce near-infrared (NIR) excitation into visible emission through an antiStokes process [10]. The excitation energy can be absorbed by sensitizers and subsequently transferred to activators (emitting ions, such as Er3+, Ho3+ and Tm3+) [11,12]. As for traditional UC nano/microparticles, the widely used sensitizers are Yb3+ and Nd3+, in which the excitation bands (~980 nm for Yb3+ and ~808 nm for Nd3+) are located in the first NIR biological window (NIR-I, 700–1000 nm) [13]. However, the excitation band located in NIR-I window is not optimal, due to the existence of photon scattering and substantial background noise [14]. Moreover, the excitation energy at ~980 nm overlaps well with the strong absorption peak of water molecules, which would cause
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the deleterious heating effects, resulting in serious damage to biological tissues and cells [12,14]. Compared to NIR-I window, optical imaging in the second NIR biological window (NIR-II, 1000–1700 nm) displays deeper imaging depth owing to the decrease of photon scattering and auto-fluorescence [13,15]. Recent experimental results show that Er3+ can be efficiently excited by NIR excitation sources at 808 nm, 980 nm and 1532 nm, which correspond to the NIR absorption bands of Er3+: 4 I15/2 → 4I9/2 (808 nm), 4I15/2 → 4I11/2 (980 nm) and 4I15/2 → 4I13/2 (1532 nm), respectively [16,17]. Thus, Er3+-sensitized UC particles with 1532 nm NIR-II excitation have currently aroused significant attention due to their great advantages in deep optical imaging. For instance, Huang's group demonstrated Er3+-sensitized nanocrystals with UC emission ranging from 400 to 1200 nm under 1532 nm NIR-II excitation [12]. Zhang et al. reported Er3+-sensitized NaErF4:Ln3+@ NaYF4 UC nanoparticles (Ln3+ = Ho3+ or Nd3+) with both excitation (1532 nm) and emission (Ho3+:1180 nm or Nd3+:1060 nm) located in NIR-II window [13]. Relative to NIR-I and NIR-II UC emission, red UC emission (600–700 nm) can be easily detected [18]. Besides, red emission falls into the visible window (600–700 nm) in biological
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Lin), [email protected] (Y. Zhang).
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https://doi.org/10.1016/j.jlumin.2019.116731 Received 26 May 2019; Received in revised form 19 August 2019; Accepted 4 September 2019 Available online 11 September 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 216 (2019) 116731
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tissue, which also demonstrates the deep tissue penetration [18]. Therefore, Er3+-sensitized red-emissive UC crystals with 1532 nm NIRII excitation can act as ideal bioprobes for deep optical imaging. As is known, conventional UC materials usually perform multi-peak emission profiles owing to Ln ions have more than one metastable excited state [18–23]. Thus, a strategy to achieve high pure red UC emission is urgently needed. In general, Er3+-enriched UC particles possess a high red to green (R/G) ratio when excited by NIR light [18,24–26]. Due to the high Er3+ doping, treating Er3+ as both sensitizer and activator is an effective way to obtain high purity of red UC luminescence. However, high-content doping of Er3+ inevitably gives rise to the energy migration loss and cross-relaxation quenching, leading to serious luminescence quenching [26–28]. Recently, Liu's group demonstrated Er3+host-sensitized NaErF4:Tm (0.5 mol%)@NaYF4 nanoparticles with bright red UC luminescence through the use of Tm3+ mediator, and the effect of concentration quenching could be suppressed in Er3+-enriched nanoparticles after adding Tm3+ mediator [17]. Yang et al. obtained bright red UC emission in NaErF4:Tm@NaGdF4:Yb nanocrystal through an energy back transfer (Er3+-Tm3+-Er3+) pathway after introducing Tm3+ mediator upon 980 nm excitation [18]. Zhang's group reported the enhanced red UC emission in NaErF4@NaYF4 nanoparticles under multiple (808 nm, 980 nm and 1532 nm) excitation, and the adding of Ho3+, Tm3+ and Yb3+ could effectively contribute to the population in the red emitting state [16]. Despite some obvious progress, it is still a challenge to simultaneously suppress the energy migration loss and cross-relaxation quenching in Er3+-sensitized UC crystals. Compared with NaErF4 host matrix, NaLuF4 has been regarded as more efficient host matrix for UC process, owing to its high refractive index and low phonon energy [29–31]. However, there is no report about the intense high-pure red UC luminescence in Er3+-sensitized NaLuF4 particles under 1532 nm NIR-II excitation. Besides, the mechanism of the enhanced red UC emission in Er3+-sensitized particles with 1532 nm NIRII excitation is rarely studied. In our previous report, Er3+-sensitized NaLuF4 particles with red UC emission were investigated under 808 nm and 980 nm excitations [32]. In this work, Er3+-sensitized red-emissive NaLuF4 UC crystals with 1532 nm NIR-II excitation is presented. Intense high-pure red UC emission can be obtained through introducing Ho3+ and Tm3+ mediators under 1532 nm NIR-II excitation. The mechanism of the dominant red emission feature is discussed in detail. For comparison, Yb3+/ Er3+ co-sensitized NaLuF4, Er3+-host-sensitized NaErF4 and Er3+-sensitized NaYF4 crystals were prepared and their UC emission spectra under 1532 nm NIR-II excitation were measured. Furthermore, relevant biological experiment has been done to display the deep penetration of 1532 nm NIR-II excitation and red UC emission (Er3+: 660 nm).
(1 M), Er(NO3)3 (1 M), Ho(NO3)3 (0.1 M)] were added to the above mixture and then stirred for 30 min to form the Ln-Cit3– complex. Subsequently, 16 mL of aqueous solution containing 10 mmol of NaF (1 M, 10 mL) and 6 mL of deionized water were added into the chelated Ln-Cit3– complex to form a colloidal suspension and kept stirring for another 30 min. Finally, the suspension was transferred into a 50 mLTeflon vessel, sealed in an autoclave and maintained at 200 °C for 10 h. After the reaction, the autoclave was allowed to cool down naturally. The final products were separated by centrifugation, washed with ethanol and deionized water several times, and then dried in air at 60 °C overnight. NaLuF4:Er, Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 50 mol%), NaLuF4:Er, Tm, 0.5 mol% Ho particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er, Tm and Ho is 50 mol%, Lu is 50 mol%), NaLuF4:49.5 mol% Er, Yb, 0.5 mol% Ho particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), NaLuF4:49 mol% Er, Yb, 1 mol% Tm particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), NaLuF4:Er, 10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 40 mol%), NaLuF4:Er, 10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 40 mol%), NaErF4:Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 100 mol%), NaErF4:Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 100 mol%), NaErF4:10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 90 mol%), NaErF4:10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 90 mol%) and NaYF4:49.5 mol% Er, 0.5 mol% Ho particles were synthesized by a similar procedure as described above only by adjusting the Ln3+ concentration. 2.3. Characterization The phase of the as-prepared samples was confirmed by powder Xray diffraction (XRD) patterns using the D-Max 2200VPC XRD from Rigaku Company (Cu-Kα radiation, λ = 1.5418 Å). Morphology and energy dispersive spectrum (EDS) were characterized by using an Oxford Quanta 400F Termal Field Emission environmental Scanning Electronic Microscope (SEM) equipped with the Energy Dispersive Xray Spectrometer. UC photoluminescence spectra and lifetime measurements were carried out on an Edinburgh Instrument Company FLS980 combined fuorescence lifetime and steady-state fuorescence spectrometer equipped with a 2 W 1532 nm diode laser. Optical micrograph of UC photoluminescence at the single particle level was obtained by using an upright Olympus BX51 microscope equipped with a 2 W 1532 nm diode laser and Nikon D700 camera.
2. Experimental 2.1. Materials
3. Results and discussion NaOH solution (4 M), NaF solution (1 M) and citric acid solution (2 M) were prepared by dissolving the corresponding amount of NaOH, NaF and citric acid in deionized water with an ultrasonic cleaning bath. Lu(NO3)3 (1 M), Er(NO3)3 (1 M), Yb(NO3)3 (0.5 M), Ho(NO3)3 (0.1 M), and Tm(NO3)3 (0.1 M) stock solutions were prepared by dissolving the corresponding Ln oxide (99.99% trace metals basis, Aladdin) in dilute nitric acid (30%) at elevated temperature.
A series of Er3+-sensitized NaLuF4 crystals were synthesized through a hydrothermal method using citric acid as a chelating agent. The XRD patterns and the corresponding SEM images of NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%) are shown in Figs. S1 and S2. As can be seen, homogeneous short hexagonal microprisms with pure hexagonal phase NaLuF4 (JCPDS 27–0726) are obtained. In Fig. S3a, the EDS of NaLuF4:Er, Ho particles demonstrates that the Na, F, Lu, Er and Ho elements are clearly present. UC emission spectra of (a) NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%) and (b) NaLuF4:Er, Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 50 mol%) under excitation with a 1532 nm diode laser are displayed in Fig. 1. As expected, two typical UC emission bands of Er3+ are detected in the range
2.2. Synthesis All samples were synthesized based on our previously reported procedures [32–36]. As for the synthesis of NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%), 3 mmol of citric acid (2 M, 1.5 mL), 5 mmol of NaOH (4 M, 1.25 mL) and 10 mL of deionized water were mixed and stirred for 10 min. Then 1 mmol of Ln(NO3)3 [Lu(NO3)3 2
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Fig. 1. UC emission spectra of (a) NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%) and (b) NaLuF4:Er, Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 50 mol%) under excitation with a 1532 nm diode laser. Fig. 2. (a) Energy loss processes: energy migration to crystal defects and cross-relaxation quenching among Er3+. (b) Efficient strategy to simultaneously suppress the energy migration loss and cross-relaxation quenching among Er3+ through the introduction of mediators. The light purple space in (b) means the confined excitation energy domain.
energy migration loss and cross-relaxation quenching among Er3+. As displayed in Fig. 3a and b, the energy value of Er3+:4I11/2 state is slightly higher than that of Ho3+:5I6 state and Tm3+:3H5 state, which favors the energy transfer from Er3+ to Ho3+/Tm3+ (Er3+:4I11/2 → Ho3+:5I6 and Er3+:4I11/2 → Tm3+:3H5). Subsequently, an energy-backtransfer process from Ho3+/Tm3+ to Er3+ (Ho3+:5I6 → Er3+:4I13/2 and Tm3+:3H5 → Er3+:4I13/2) takes place, due to the energy level of Er3+:4I13/2 state is slightly lower than that of Ho3+:5I6 state and Tm3+:3H5 state. Consequently, the population inversion between Er3+:4I11/2 state and Er3+:4I13/2 state arises after the addition of Ho3+/ Tm3+ mediators. The results are similar to the previous report by Liu's group [17]. As a result, Er3+:4F9/2 state (660 nm red emission) is directly populated, which induces the huge enhancement of red UC emission (660 nm). Meanwhile, R/G ratio is obviously increased owing to the increasing intensity difference between 660 nm red emission and 520/540 nm green emissions. From the above analysis, it can be concluded that the intense high-pure red UC emission (660 nm) in Er3+sensitized NaLuF4 crystals is mainly attributed to the effective confinement of 1532 nm excitation energy by Ho3+/Tm3+ mediators. Based on the above results, the doping concentration of Ho3+ was fixed at 0.5 mol%, and NaLuF4:Er, Tm, 0.5 mol% Ho particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er, Tm and Ho is 50 mol%, Lu is 50 mol%) were synthesized (Fig. S4), which Ho3+ and Tm3+ are simultaneously used as mediators in Er3+-sensitized NaLuF4 UC particles. As can be seen from Fig. S5, the optimal doping content of Tm3+ is 2 mol%, which shows the strongest 660 nm red UC emission in NaLuF4:Er, Tm, 0.5 mol% Ho crystals under 1532 nm NIR-II excitation. As is known, Yb3+/Er3+ co-doping pair is typically used as sensitizer-activator pair for UC process, due to the large absorption crosssection of Yb3+ sensitizer. Consequently, it's necessary to investigate the UC emission property of Yb3+/Er3+ co-sensitized NaLuF4 crystals
of 400–710 nm. Green emissions at approximately 520/540 nm and red emission at approximately 660 nm are attributed to the transitions of 2 H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 of Er3+, respectively. Obviously, the intensity of 660 nm red UC emission is much higher than that of 520/540 nm green UC emissions. Relative to green emission, red emission is greatly increased after adding Ho3+ or Tm3+, which is favorable for the achievement of bright red UC luminescence with a high R/G ratio. As shown in Fig. 1a and b, the optimal doping contents of Ho3+ and Tm3+ are determined to be 0.5 mol% and 1 mol%, respectively. Compared with un-doped Ho3+/Tm3+ samples, 660 nm red UC emission in NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles and NaLuF4:49 mol% Er, 1 mol% Tm particles is increased by 13.1 and 6.0 times, respectively. The maximum R/G ratio is simultaneously obtained in the samples doped with 0.5 mol% Ho3+ (R/G = 33.4) and 1 mol% Tm3+ (R/G = 15.2), which is enhanced by 12.4 and 5.6 times relative to Ho3+/Tm3+-free samples (R/G = 2.7), respectively. The above results indicate that concentration quenching effect is effectively suppressed in Er3+-enriched NaLuF4 particles after introducing Ho3+ or Tm3+, which acts as energy mediators. As presented in Fig. 2a, the energy loss processes of luminescence quenching usually include energy migration loss and cross-relaxation quenching among Er3+ in Er3+enriched UC particles. The introduction of energy mediators, which can efficiently confine the excitation energy of Er3+ activators, is an effective strategy to simultaneously suppress the energy migration loss and cross-relaxation quenching among Er3+ (Fig. 2b). As for Er3+heavily-doped system, Er3+ can serve as both the sensitizers and activators. The 1532 nm NIR-II excitation energy can be absorbed by Er3+ sensitizers, and then emit visible light from Er3+ activators through multiphoton absorption. Ho3+ and Tm3+ as the energy mediators can temporarily store the excitation energy and subsequently return the energy to Er3+ activators, which is beneficial for the reduction of 3
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Fig. 3. Proposed UC processes after adding Ho3+ and Tm3+ energy mediators in (a, b) Er3+-sensitized and (c, d) Yb3+/Er3+ co-sensitized NaLuF4 crystals under the excitation of 1532 nm.
(Figs. S9 and S10), NaErF4:Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 100 mol%), NaErF4:10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 90 mol%) (Figs. S11 and S12) and NaErF4:10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 90 mol%) were further investigated under 1532 nm NIR-II excitation, as shown in Fig. 6. It can be seen that the optimal doping contents of Ho3+ and Tm3+ for UC emission in NaErF4:Ho particles, NaErF4:Tm particles, NaErF4:10 mol% Yb, Ho particles and NaErF4:10 mol% Yb, Tm particles with 1532 nm NIR-II excitation were determined to be 2 mol% (Fig. 6a), 1 mol% (Fig. 6b), 2 mol% (Fig. 6c) and 1 mol% (Fig. 6d), respectively. The EDS of NaErF4:Ho particles (Fig. S3c) demonstrates that the Na, F, Er and Ho elements are clearly present, and Fig. S3d reveals the presence of doped elemental Na, F, Yb, Er and Ho in NaErF4:Yb, Ho particles. Fig. 7a shows the comparison results of 660 nm red UC emission intensity in NaLuF4:49.5 mol% Er, 0.5 mol% Ho (Sample A), NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho (Sample B), NaLuF4:49 mol% Er, 1 mol% Tm (Sample C), NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm (Sample D) and NaLuF4:47.5 mol% Er, 2 mol% Tm, 0.5 mol% Ho crystals (Sample E) under 1532 nm NIR-II excitation. As can be seen, red emission intensity in Sample B is lower than that in Sample A; and Sample D is lower than that in Sample C, thus the emission efficiency in Yb3+/Er3+ co-sensitized NaLuF4 crystals is lower than that in Er3+-sensitized NaLuF4 crystals under 1532 nm NIR-II excitation. From the comparison result between Sample A, C and E, we can draw a conclusion that the confining efficiency of 1532 nm excitation energy in Er3+-sensitized NaLuF4 crystals with Ho3+/Tm3+ simultaneously used as energy mediators becomes lower compared to that with single energy mediator (Ho3+ or Tm3+). The main reason is that, relative to single energy mediator (Ho3+ or Tm3+), double energy
with 1532 nm NIR-II excitation. Firstly, we found that the optimal doping content of Yb3+ for UC emission in NaLuF4:49.5 mol% Er, Yb, 0.5 mol% Ho particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%) (Figs. S6 and S7) and NaLuF4:49 mol% Er, Yb, 1 mol% Tm particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%) under 1532 nm NIR-II excitation was determined to be 10 mol% (Fig. 4a and b), which the doping content of Ho3+ and Tm3+ were fixed at 0.5 mol% and 1 mol%, respectively. Next, the doping concentration of Yb3+ was fixed at 10 mol%, and it is found that the optimal doping contents of Ho3+ and Tm3+ for UC luminescence in NaLuF4:Er, 10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol % (Er and Ho is 50 mol%, Lu is 40 mol%) (Fig. S8) and NaLuF4:Er, 10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 40 mol%) under 1532 nm NIR-II excitation were determined to be 1 mol% and 1 mol%, respectively, as shown in Fig. 4c and d. The EDS of NaLuF4:Er, Yb, Ho crystals demonstrates that the Na, F, Lu, Yb, Er and Ho elements are clearly present (Fig. S3b). Proposed UC processes of NaLuF4:Er, Yb, Ho and NaLuF4:Er, Yb, Tm under 1532 nm NIR-II excitation is displayed in Fig. 3c and d. The above-mentioned energy transfer processes (Fig. 3) were further confirmed by power density-dependent luminescence studies. As presented in Fig. 5, both 660 nm red UC emission and 520/ 540 nm green UC emissions in NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho, NaLuF4:49 mol% Er, 1 mol% Tm and NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm crystals are two-photon processes under 1532 nm NIR-II excitation, which are in accord with our proposed mechanism (Fig. 3). As for Er3+-sensitized UC particles, there are some reports about Er3+-host-sensitized NaErF4 particles [13,16–18]. For comparison, UC luminescence properties of NaErF4:Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 100 mol%) 4
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Fig. 4. UC emission spectra of (a) NaLuF4:49.5 mol% Er, Yb, 0.5 mol% Ho particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), (b) NaLuF4:49 mol% Er, Yb, 1 mol% Tm particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), (c) NaLuF4:Er, 10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 40 mol%) and (d) NaLuF4:Er, 10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 40 mol%) under excitation with a 1532 nm diode laser.
mediators (Ho3+/Tm3+) with the higher optimal doping content increases the interaction between Er3+ and energy mediators, leading to more non-radiation loss [37]. Besides, from Sample A and C, it can be seen that the confining efficiency of 1532 nm excitation energy of Tm3+ energy mediator is notably lower than that of Ho3+ energy mediator. This conclusion can be also demonstrated in Er3+-host-sensitized NaErF4 particles (Fig. 7b), which may be attributed to the emission peak at 696 nm (Fig. 6b) from Tm3+:3F3 to Tm3+:3H6, implying the
leakage of the confined 1532 nm excitation energy [38]. But unlike Er3+-sensitized NaLuF4 crystals, the emission efficiency in NaErF4 crystals with Yb3+ doping is higher than that without Yb3+ doping under 1532 nm NIR-II excitation, as displayed in Fig. 7b. Furthermore, red UC emission intensity from NaLuF4:49.5 mol% Er, 0.5 mol% Ho crystals is higher than that from NaErF4:2 mol% Ho (Fig. 7c) and NaYF4:49.5 mol% Er, 0.5 mol% Ho (Fig. S13) crystals under 1532 nm NIR-II excitation. Consequently, the emission efficiency in Er3+-
Fig. 5. (a, b) Power density dependence of red and green emissions at 660 nm and 520/540 nm in NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho, NaLuF4:49 mol% Er, 1 mol% Tm and NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm crystals under excitation with a 1532 nm diode laser. 5
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Fig. 6. UC emission spectra of (a) NaErF4:Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 100 mol%), (b) NaErF4:Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 100 mol%), (c) NaErF4:10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 90 mol%) and (d) NaErF4:10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 90 mol%) under excitation with a 1532 nm diode laser.
sensitized NaLuF4 particles is higher than that in Er3+-host-sensitized NaErF4 particles and Er3+-sensitized NaYF4 particles with 1532 nm NIR-II excitation. In addition, the R/G ratio in NaErF4:2 mol% Ho crystals is only 6.8, which is much lower than that in NaLuF4:49.5 mol% Er, 0.5 mol% Ho crystals (R/G = 33.4). Based on the above analysis, it can be concluded that, Er3+-sensitized NaLuF4 particles is more suitable for obtaining intense high-pure red UC emission with 1532 nm NIR-II excitation. From the absorption spectrum of NaLuF4:Er, Ho and NaErF4:Ho particles in Fig. 7d, it can be clearly seen that 808 nm, 980 nm and 1532 nm NIR photons can be simultaneously absorbed by Er3+-sensitized UC particles, indicating Er3+-sensitized UC particles can be efficiently excited by three NIR excitation sources (808 nm, 980 nm and 1532 nm) rather than single-band excitation (such as 980 nm excitation for Yb3+-sensitized UC crystals or 808 nm excitation for Nd3+-sensitized UC crystals), which exhibits great potential applications in fields such as solar cells and anti-counterfeiting. Moreover, the absorption peak at 1532 nm is strongest relative to the peaks at 808 nm and 980 nm (Fig. 7d), thus Er3+-sensitized red-emissive NaLuF4 UC crystals with 1532 nm NIR-II excitation can be more efficiently applied in deep optical imaging. As shown in Fig. 8a, the luminescence lifetime of Er3+:4F9/2 → 4I15/2 transition (red emission) in Sample A, B, C and D is decreased from 0.702 ms, 0.625 ms, 0.386 ms–0.347 ms under 1532 nm NIR-II excitation. It is well known that the inverse of lifetime is equal to the sum (Ir+nr = Ir + Inr) of radiative (Ir) and nonradiative (Inr) transitions. As for the red-emitting state (Er3+:4F9/2), the decreasing lifetime indicates the increasing Ir+nr (Er3+:4F9/2). It can be seen from Fig. 7a, red UC emission intensity [Ir (Er3+:4F9/2)] is decreased in Sample A, B, C and D
under 1532 nm NIR-II excitation. Thus, the increasing Ir+nr (Er3+:4F9/2) should come from the dramatic increase of Inr (Er3+:4F9/2), which means the increase magnitude of Inr must be higher than the decrease magnitude of Ir, implying the obvious decrease of red UC emission. By the same token, as for the green-emitting state (Er3+:4S3/2), the increasing lifetime in Sample A, B, C and D under 1532 nm NIR-II excitation (Fig. 8b) indicates the decreasing Ir+nr (Er3+:4S3/2), which means the increase of green UC emission. Consequently, 660 nm red UC emission and R/G ratio in Sample A, B, C and D are reduced with 1532 nm NIR-II excitation, thus Sample A (NaLuF4:49.5%Er/0.5%Ho crystals) has the highest red UC emission intensity (660 nm) and R/G ratio, which is in accordance with the above UC emission spectra of Fig. 7a. Furthermore, Er3+:4I11/2 state is depopulated after introducing Ho3+/Tm3+ mediators (Fig. S14), which indicates that Ho3+/Tm3+ mediators play a role in trapping energies from Er3+:4I11/2 state. Fig. 9a displays the model of optical microscope setup for characterization of luminescence property at the single particle level. As can be seen from the optical micrograph of Fig. 9b (left), red UC luminescence from single NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles under 1532 nm NIR-II excitation demonstrates the high crystallinity and uniform doping in single particles. From the photograph of UC photoluminescence on the right side of Fig. 9b (one is obtained in a bright environment without 1532 nm NIR-II excitation, another is obtained in a dark environment with 1532 nm NIR-II excitation), intense high-pure red UC emission can be clearly observed with the naked eye, which is consistent with the above UC emission spectra that red emission at 660 nm dominates the UC process under the 1532 nm irradiation. Furthermore, when NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles 6
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Fig. 7. UC emission spectra of (a) NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho, NaLuF4:49 mol% Er, 1 mol% Tm, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm and NaLuF4:47.5 mol% Er, 2 mol% Tm, 0.5 mol% Ho crystals, (b) NaErF4:2 mol% Ho, NaErF4:1 mol% Tm, NaErF4:10 mol% Yb, 2 mol% Ho and NaErF4:10 mol% Yb, 1 mol% Tm crystals, (c) NaErF4:2 mol% Ho and NaLuF4:49.5 mol% Er, 0.5 mol% Ho crystals under 1532 nm excitation. Absorption spectrum of (d) NaLuF4:49 mol% Er, 1 mol% Ho and NaErF4:1 mol% Ho crystals.
660 nm red UC emission and 1532 nm excitation in biological tissue.
injected into the fresh beef (Fig. 10a,c), red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation (Fig. 10d), implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission in biological tissue (Fig. 10b). Fig. 10a exhibits the model of NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles injected into the fresh beef under excitation with a 1532 nm diode laser, and the model in Fig. 10b shows the deep penetration of
4. Conclusions In summary, intense high-pure red UC emission is obtained in Er3+sensitized NaLuF4 particles through introducing Ho3+ and Tm3+ mediators under 1532 nm NIR-II excitation. The enhanced 660 nm red
Fig. 8. (a, b) Decay curves of the 4F9/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ in NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol % Ho, NaLuF4:49 mol% Er, 1 mol% Tm and NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm crystals under the excitation of 1532 nm. The decay lifetime was calculated according to the function: τ = ʃI(t)dt/IP, where IP represents the peak intensity in the decay curve, and I(t) represents the emission intensity at time t. 7
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UC emission and R/G ratio are mainly attributed to the effective confinement of 1532 nm excitation energy by Ho3+/Tm3+ mediators: the energy transfer from Er3+ to Ho3+/Tm3+ (Er3+:4I11/2 → Ho3+:5I6 and Er3+:4I11/2 → Tm3+:3H5) and energy-back-transfer process from Ho3+/ Tm3+ to Er3+ (Ho3+:5I6 → Er3+:4I13/2 and Tm3+:3H5 → Er3+:4I13/2). In addition, it is found that the red UC emission intensity in Er3+sensitized NaLuF4 crystals is stronger than that in Er3+-host-sensitized NaErF4, Er3+-sensitized NaYF4 and Yb3+/Er3+ co-sensitized NaLuF4 crystals under 1532 nm NIR-II excitation. Furthermore, when the particles injected into the fresh beef, red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation, implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission. The results suggest that Er3+-sensitized red-emissive NaLuF4 UC crystals with 1532 nm NIR-II excitation can be efficiently applied in deep optical imaging.
Acknowledgments This work was supported by a start-up foundation from Guangzhou University under Grant No. 69-18ZX10334, Young Innovative Talents Project of Guangdong Province under Grant No. 2018KQNCX193, National Natural Science Foundation of China under Grant No. 61172027, Science and Technology Planning Project of Guangdong Province under Grant No. 2017A010103035. Fig. 9. (a) Model of the optical microscope setup for characterization of luminescence property at the single particle level. (b) Optical micrograph (left) and the photograph (right) of UC photoluminescence of NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles under excitation with a 1532 nm diode laser.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116731. Fig. 10. (a) Model of NaLuF4:49.5 mol% Er, 0.5 mol % Ho particles injected into the fresh beef under excitation with a 1532 nm diode laser. (b) Model showing the deep penetration of 660 nm red UC emission and 1532 nm excitation in biological tissue. (c) Fresh beef. (d) The photograph of UC photoluminescence after NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles injected into the fresh beef under the excitation of 1532 nm.
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References [20]
[1] F. Zhang, G.B. Braun, Y.F. Shi, Y.C. Zhang, X.H. Sun, N.O. Reich, D.Y. Zhao, G. Stucky, Fabrication of Ag@SiO2@Y2O3:Er nanostructures for bioimaging: tuning of the upconversion fluorescence with silver nanoparticles, J. Am. Chem. Soc. 132 (2010) 2850–2851. [2] F. Zhang, S.S. Wong, Ambient large-scale template-mediated synthesis of high-aspect ratio single-crystalline, chemically doped rare-earth phosphate nanowires for bioimaging, ACS Nano 4 (2010) 99–112. [3] Z.Q. Li, Y. Zhang, Monodisperse silica-coated polyvinylpyrrolidone/NaYF4 nanocrystals with multicolor upconversion fluorescence emission, Angew. Chem. Int. Ed. 45 (2006) 7732–7735. [4] J. Zhou, Q. Liu, W. Feng, Y. Sun, F.Y. Li, Upconversion luminescent materials: advances and applications, Chem. Rev. 115 (2015) 395–465. [5] M.Y. Ding, D.Q. Chen, S.L. Yin, Z.G. Ji, J.S. Zhong, Y.R. Ni, C.H. Lu, Z.Z. Xu, Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage, Sci. Rep. 5 (2015) 12745. [6] G. Murali, R.K. Mishra, J.M. Lee, Y.C. Chae, J. Kim, Y.D. Suh, D. Lim, S.H. Lee, Aspect-ratio controlled synthesis and tunable luminescence of YF3:Yb3+/Er3+ upconversion nanocrystals, Cryst. Growth Des. 17 (2017) 3055–3061. [7] J.Y. Liao, Z.W. Yang, J.B. Sun, S.F. Lai, B. Shao, J. Li, J.B. Qiu, Z.G. Song, Y. Yang, Preparation and upconversion emission modifcation of crystalline colloidal arrays and rare earth fuoride microcrystal composites, Sci. Rep. 5 (2015) 7636. [8] Y. Vyborna, M. Vybornyi, A.V. Rudnev, R. Haner, DNA-grafted supramolecular polymers: helical ribbon structures formed by self-assembly of pyrene-DNA chimeric oligomers, Angew. Chem. Int. Ed. 54 (2015) 7934–7938. [9] Y.Y. Li, J.J. Guo, X.H. Liu, T. Aidilibike, W.P. Qin, White upconversion luminescence in CaF2:Yb3+/Eu3+ powders via the incorporation of Y3+ ions, Phys. Chem. Chem. Phys. 18 (2016) 16094–16097. [10] H. Lin, D.K. Xu, A.M. Li, Z.R. Qiu, S.H. Yang, Y.L. Zhang, Enhanced red upconversion emission and its mechanism in Yb3+-Er3+ codoped α-NaLuF4 nanoparticles, New J. Chem. 41 (2017) 1193–1201. [11] L.P. Tu, X. Liu, F. Wu, H. Zhang, Excitation energy migration dynamics in upconversion nanomaterials, Chem. Soc. Rev. 44 (2015) 1331–1345. [12] X.W. Cheng, Y. Pan, Z. Yuan, X.W. Wang, W.H. Su, L.S. Yin, X.J. Xie, L. Huang, Er3+ sensitized photon upconversion nanocrystals, Adv. Funct. Mater. 28 (2018) 1800208. [13] L. Liu, S.F. Wang, B.Z. Zhao, P. Pei, Y. Fan, X.M. Li, F. Zhang, Er3+ sensitized 1530 nm to 1180 nm second near-infrared window upconversion nanocrystals for in vivo biosensing, Angew. Chem. Int. Ed. 57 (2018) 7518–7522. [14] W. Shao, G.Y. Chen, A. Kuzmin, H.L. Kutscher, A. Pliss, T.Y. Ohulchanskyy, P.N. Prasad, Tunable narrow band emissions from dye-sensitized core/shell/shell nanocrystals in the second near-infrared biological window, J. Am. Chem. Soc. 138 (2016) 16192–16195. [15] A.M. Smith, M.C. Mancini, S. Nie, Second window for in vivo imaging, Nat. Nanotechnol. 4 (2009) 710–711. [16] L. Yan, B. Zhou, N. Song, X.L. Liu, J.S. Huang, T. Wang, L.L. Tao, Q.Y. Zhang, Selfsensitization induced upconversion of Er3+ in core–shell nanoparticles, Nanoscale 10 (2018) 17949–17957. [17] Q.S. Chen, X.J. Xie, B.L. Huang, L.L. Liang, S.Y. Han, Z.G. Yi, Y. Wang, Y. Li, D.Y. Fan, L. Huang, X.G. Liu, Confining excitation energy in Er3+-sensitized upconversion nanocrystals through Tm3+-mediated transient energy trapping, Angew. Chem. Int. Ed. 56 (2017) 7605–7609. [18] J.T. Xu, D. Yang, W. Han, S.M. Dong, T. Jia, F. He, H.T. Bi, S.L. Gai, L. Li, P.P. Yang, A novel strategy for markedly enhancing the red upconversion emission in Er3+/ Tm3+ cooperated nanoparticles, J. Mater. Chem. C 6 (2018) 7533–7540. [19] X. Chen, W. Xu, H. Song, C. Chen, H. Xia, Y. Zhu, D. Zhou, S. Cui, Q. Dai, J. Zhang, Highly efficient LiYF4:Yb3+, Er3+ upconversion single crystal under solar cell
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28] [29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
9
spectrum excitation and photovoltaic application, ACS Appl. Mater. Interfaces 8 (2016) 9071–9079. Y. Liu, S. Zhou, Z. Zhuo, R. Li, Z. Chen, M. Hong, X. Chen, In vitro upconverting/ downshifting luminescent detection of tumor markers based on Eu3+-activated core–shell–shell lanthanide nanoprobes, Chem. Sci. 7 (2016) 5013–5019. L. Xiong, T. Yang, Y. Yang, C. Xu, F. Li, Long-term in vivo biodistribution imgaing and toxicity of polyacrylic acid-coated upconversion nanophosphors, Biomaterials 31 (2010) 7078–7085. J. Yang, Y. Deng, Q. Wu, J. Zhou, H. Bao, Q. Li, F. Zhang, F. Li, B. Tu, D. Zhao, Mesoporous silica encapsulating upconversion luminescence rare-earth fluoride nanorods for secondary excitation, Langmuir 26 (2010) 8850–8856. N. Moller, A. Ruhling, S. Lamping, T. Hellwig, C. Fallnich, B.J. Ravoo, F. Glorius, Stabilization of high oxidation state upconversion nanoparticles by N-heterocyclic carbenes, Angew. Chem. Int. Ed. 56 (2017) 4356–4360. F. Wang, X.G. Liu, Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles, J. Am. Chem. Soc. 130 (2008) 5642–5643. Q. Ju, X. Chen, F. Ai, D. Peng, X. Lin, W. Kong, P. Shi, G. Zhu, F. Wang, An upconversion nanoprobe operating in the first biological window, J. Mater. Chem. B 3 (2015) 3548–3555. J. Zuo, Q.Q. Li, B. Xue, C.X. Li, Y.L. Chang, Y.L. Zhang, X.M. Liu, L.P. Tu, H. Zhang, X.G. Kong, Employing shells to eliminate concentration quenching in photonic upconversion nanostructure, Nanoscale 9 (2017) 7941–7946. N.J.J. Johnson, S. He, S. Diao, E.M. Chan, H. Dai, A. Almutairi, Direct evidence for coupled surface and concentration quenching dynamics in lanthanide-doped nanocrystals, J. Am. Chem. Soc. 139 (2017) 3275–3282. M. Haase, H. Schafer, Upconverting nanoparticles, Angew. Chem. Int. Ed. 50 (2011) 5808–5829. Z. Chen, X. Wu, S. Hu, P. Hu, H. Yan, Z. Tang, Y. Liu, Multicolor upconversion NaLuF4 fluorescent nanoprobe for plant cell imaging and detection of sodium fluorescein, J. Mater. Chem. C 3 (2015) 153–161. Z. Chen, X. Wu, S. Hu, P. Hu, H. Yan, Z. Tang, Y. Liu, Upconversion NaLuF4 fluorescent nanoprobes for jellyfish cell imaging and irritation assessment of organic dyes, J. Mater. Chem. C 3 (2015) 6067–6076. W. Li, C. Tan, Y. Zhang, Simultaneous phase and shape control of monodisperse NaLuF4:Yb, Er microcrystals and greatly enhanced upconversion luminescence from their superstructures, Opt. Commun. 295 (2013) 140–144. H. Lin, D.K. Xu, Y.J. Li, L. Yao, L.Q. Xu, Y. Ma, S.H. Yang, Y.L. Zhang, Enhanced red emission in Er3+-sensitized NaLuF4 upconversion crystals via energy trapping, Inorg. Chem. 57 (2018) 15361–15369. H. Lin, D.K. Xu, D.D. Teng, S.H. Yang, Y.L. Zhang, Shape-controllable synthesis and enhanced upconversion luminescence of Li+ doped β-NaLuF4:Yb3+, Ln3+ (Ln = Tm, Ho) microcrystals, New J. Chem. 39 (2015) 2565–2572. H. Lin, D.K. Xu, A.M. Li, D.D. Teng, S.H. Yang, Y.L. Zhang, Tuning of structure and enhancement of upconversion luminescence in NaLuF4:Yb3+, Ho3+ crystals, Phys. Chem. Chem. Phys. 17 (2015) 19515–19526. H. Lin, D.K. Xu, A.M. Li, D.D. Teng, S.H. Yang, Y.L. Zhang, Morphology evolution and pure red upconversion mechanism of β-NaLuF4 crystals, Sci. Rep. 6 (2016) 28051. H. Lin, D.K. Xu, A.M. Li, D.D. Teng, S.H. Yang, Y.L. Zhang, Simultaneous realization of structure manipulation and emission enhancement in NaLuF4 upconversion crystals, J. Mater. Chem. C 3 (2015) 11754–11765. E.M. Chan, D.J. Gargas, P.J. Schuck, D.J. Milliron, Concentrating and recycling energy in lanthanide codopants for efficient and spectrally pure emission: the case of NaYF4:Er3+/Tm3+ upconverting nanocrystals, J. Phys. Chem. B 116 (2012) 10561–10570. Y.F. Shang, S.W. Hao, W.Q. Lv, T. Chen, L. Tian, Z.T. Lei, C.H. Yang, Confining excitation energy of Er3+-sensitized upconversion nanoparticles through introducing various energy trapping centers, J. Mater. Chem. C 6 (2018) 3869–3875.
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Intense red upconversion luminescence in Er3+-sensitized particles through confining the 1532 nm excitation energy
T
Hao Lina,∗, Dekang Xuc, Yongjin Lib, Lu Yaob, Liqin Xua, Ying Maa, Shenghong Yangb, Yueli Zhangb,∗∗ a
School of Physics and Electronic Engineering, Guangzhou University, Guangzhou, 510006, PR China State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, School of Physics, Sun Yat-sen University, Guangzhou, 510275, PR China c School of Chemistry and Materials Engineering, Huizhou University, Huizhou, 516007, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Upconversion Intense red emission Er3+-sensitized particles 1532 nm NIR-II excitation Deep optical imaging
Lanthanide-doped upconversion (UC) materials are commonly excited by 700–1000 nm laser because of Yb3+ (980 nm excitation) and Nd3+ (808 nm excitation) are typically used as sensitizers. However, the excitation band located in the second near-infrared window (NIR-II, 1000–1700 nm) demonstrates the deeper penetration in biological tissue with minimum tissue scattering and autofluorescence. Herein, intense red UC emission in Er3+sensitized NaLuF4 particles under 1532 nm NIR-II excitation is presented. Red UC luminescence is significantly enhanced and purified through confining the 1532 nm excitation energy by Ho3+ and Tm3+ mediators, via an energy back transfer pathway (Er3+ → Ho3+ → Er3+ and Er3+ → Tm3+ → Er3+). Furthermore, when the particles injected into the fresh beef, red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation, implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission. Our results suggest that bright red UC luminescence from Er3+-sensitized NaLuF4 crystals with 1532 nm NIR-II excitation may be suitable for deep optical imaging.
1. Introduction Lanthanide (Ln)-doped upconversion (UC) materials demonstrate diverse advantages containing long luminescence lifetimes, narrow band emission, low toxicity and high photochemical stability [1–3], which make them have great potential applications in biological imaging, three-dimensional flat-panel displays, solar energy conversion, optical temperature sensors and so on [4–9]. Photon UC can transduce near-infrared (NIR) excitation into visible emission through an antiStokes process [10]. The excitation energy can be absorbed by sensitizers and subsequently transferred to activators (emitting ions, such as Er3+, Ho3+ and Tm3+) [11,12]. As for traditional UC nano/microparticles, the widely used sensitizers are Yb3+ and Nd3+, in which the excitation bands (~980 nm for Yb3+ and ~808 nm for Nd3+) are located in the first NIR biological window (NIR-I, 700–1000 nm) [13]. However, the excitation band located in NIR-I window is not optimal, due to the existence of photon scattering and substantial background noise [14]. Moreover, the excitation energy at ~980 nm overlaps well with the strong absorption peak of water molecules, which would cause
∗
the deleterious heating effects, resulting in serious damage to biological tissues and cells [12,14]. Compared to NIR-I window, optical imaging in the second NIR biological window (NIR-II, 1000–1700 nm) displays deeper imaging depth owing to the decrease of photon scattering and auto-fluorescence [13,15]. Recent experimental results show that Er3+ can be efficiently excited by NIR excitation sources at 808 nm, 980 nm and 1532 nm, which correspond to the NIR absorption bands of Er3+: 4 I15/2 → 4I9/2 (808 nm), 4I15/2 → 4I11/2 (980 nm) and 4I15/2 → 4I13/2 (1532 nm), respectively [16,17]. Thus, Er3+-sensitized UC particles with 1532 nm NIR-II excitation have currently aroused significant attention due to their great advantages in deep optical imaging. For instance, Huang's group demonstrated Er3+-sensitized nanocrystals with UC emission ranging from 400 to 1200 nm under 1532 nm NIR-II excitation [12]. Zhang et al. reported Er3+-sensitized NaErF4:Ln3+@ NaYF4 UC nanoparticles (Ln3+ = Ho3+ or Nd3+) with both excitation (1532 nm) and emission (Ho3+:1180 nm or Nd3+:1060 nm) located in NIR-II window [13]. Relative to NIR-I and NIR-II UC emission, red UC emission (600–700 nm) can be easily detected [18]. Besides, red emission falls into the visible window (600–700 nm) in biological
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Lin), [email protected] (Y. Zhang).
∗∗
https://doi.org/10.1016/j.jlumin.2019.116731 Received 26 May 2019; Received in revised form 19 August 2019; Accepted 4 September 2019 Available online 11 September 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
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tissue, which also demonstrates the deep tissue penetration [18]. Therefore, Er3+-sensitized red-emissive UC crystals with 1532 nm NIRII excitation can act as ideal bioprobes for deep optical imaging. As is known, conventional UC materials usually perform multi-peak emission profiles owing to Ln ions have more than one metastable excited state [18–23]. Thus, a strategy to achieve high pure red UC emission is urgently needed. In general, Er3+-enriched UC particles possess a high red to green (R/G) ratio when excited by NIR light [18,24–26]. Due to the high Er3+ doping, treating Er3+ as both sensitizer and activator is an effective way to obtain high purity of red UC luminescence. However, high-content doping of Er3+ inevitably gives rise to the energy migration loss and cross-relaxation quenching, leading to serious luminescence quenching [26–28]. Recently, Liu's group demonstrated Er3+host-sensitized NaErF4:Tm (0.5 mol%)@NaYF4 nanoparticles with bright red UC luminescence through the use of Tm3+ mediator, and the effect of concentration quenching could be suppressed in Er3+-enriched nanoparticles after adding Tm3+ mediator [17]. Yang et al. obtained bright red UC emission in NaErF4:Tm@NaGdF4:Yb nanocrystal through an energy back transfer (Er3+-Tm3+-Er3+) pathway after introducing Tm3+ mediator upon 980 nm excitation [18]. Zhang's group reported the enhanced red UC emission in NaErF4@NaYF4 nanoparticles under multiple (808 nm, 980 nm and 1532 nm) excitation, and the adding of Ho3+, Tm3+ and Yb3+ could effectively contribute to the population in the red emitting state [16]. Despite some obvious progress, it is still a challenge to simultaneously suppress the energy migration loss and cross-relaxation quenching in Er3+-sensitized UC crystals. Compared with NaErF4 host matrix, NaLuF4 has been regarded as more efficient host matrix for UC process, owing to its high refractive index and low phonon energy [29–31]. However, there is no report about the intense high-pure red UC luminescence in Er3+-sensitized NaLuF4 particles under 1532 nm NIR-II excitation. Besides, the mechanism of the enhanced red UC emission in Er3+-sensitized particles with 1532 nm NIRII excitation is rarely studied. In our previous report, Er3+-sensitized NaLuF4 particles with red UC emission were investigated under 808 nm and 980 nm excitations [32]. In this work, Er3+-sensitized red-emissive NaLuF4 UC crystals with 1532 nm NIR-II excitation is presented. Intense high-pure red UC emission can be obtained through introducing Ho3+ and Tm3+ mediators under 1532 nm NIR-II excitation. The mechanism of the dominant red emission feature is discussed in detail. For comparison, Yb3+/ Er3+ co-sensitized NaLuF4, Er3+-host-sensitized NaErF4 and Er3+-sensitized NaYF4 crystals were prepared and their UC emission spectra under 1532 nm NIR-II excitation were measured. Furthermore, relevant biological experiment has been done to display the deep penetration of 1532 nm NIR-II excitation and red UC emission (Er3+: 660 nm).
(1 M), Er(NO3)3 (1 M), Ho(NO3)3 (0.1 M)] were added to the above mixture and then stirred for 30 min to form the Ln-Cit3– complex. Subsequently, 16 mL of aqueous solution containing 10 mmol of NaF (1 M, 10 mL) and 6 mL of deionized water were added into the chelated Ln-Cit3– complex to form a colloidal suspension and kept stirring for another 30 min. Finally, the suspension was transferred into a 50 mLTeflon vessel, sealed in an autoclave and maintained at 200 °C for 10 h. After the reaction, the autoclave was allowed to cool down naturally. The final products were separated by centrifugation, washed with ethanol and deionized water several times, and then dried in air at 60 °C overnight. NaLuF4:Er, Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 50 mol%), NaLuF4:Er, Tm, 0.5 mol% Ho particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er, Tm and Ho is 50 mol%, Lu is 50 mol%), NaLuF4:49.5 mol% Er, Yb, 0.5 mol% Ho particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), NaLuF4:49 mol% Er, Yb, 1 mol% Tm particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), NaLuF4:Er, 10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 40 mol%), NaLuF4:Er, 10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 40 mol%), NaErF4:Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 100 mol%), NaErF4:Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 100 mol%), NaErF4:10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 90 mol%), NaErF4:10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 90 mol%) and NaYF4:49.5 mol% Er, 0.5 mol% Ho particles were synthesized by a similar procedure as described above only by adjusting the Ln3+ concentration. 2.3. Characterization The phase of the as-prepared samples was confirmed by powder Xray diffraction (XRD) patterns using the D-Max 2200VPC XRD from Rigaku Company (Cu-Kα radiation, λ = 1.5418 Å). Morphology and energy dispersive spectrum (EDS) were characterized by using an Oxford Quanta 400F Termal Field Emission environmental Scanning Electronic Microscope (SEM) equipped with the Energy Dispersive Xray Spectrometer. UC photoluminescence spectra and lifetime measurements were carried out on an Edinburgh Instrument Company FLS980 combined fuorescence lifetime and steady-state fuorescence spectrometer equipped with a 2 W 1532 nm diode laser. Optical micrograph of UC photoluminescence at the single particle level was obtained by using an upright Olympus BX51 microscope equipped with a 2 W 1532 nm diode laser and Nikon D700 camera.
2. Experimental 2.1. Materials
3. Results and discussion NaOH solution (4 M), NaF solution (1 M) and citric acid solution (2 M) were prepared by dissolving the corresponding amount of NaOH, NaF and citric acid in deionized water with an ultrasonic cleaning bath. Lu(NO3)3 (1 M), Er(NO3)3 (1 M), Yb(NO3)3 (0.5 M), Ho(NO3)3 (0.1 M), and Tm(NO3)3 (0.1 M) stock solutions were prepared by dissolving the corresponding Ln oxide (99.99% trace metals basis, Aladdin) in dilute nitric acid (30%) at elevated temperature.
A series of Er3+-sensitized NaLuF4 crystals were synthesized through a hydrothermal method using citric acid as a chelating agent. The XRD patterns and the corresponding SEM images of NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%) are shown in Figs. S1 and S2. As can be seen, homogeneous short hexagonal microprisms with pure hexagonal phase NaLuF4 (JCPDS 27–0726) are obtained. In Fig. S3a, the EDS of NaLuF4:Er, Ho particles demonstrates that the Na, F, Lu, Er and Ho elements are clearly present. UC emission spectra of (a) NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%) and (b) NaLuF4:Er, Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 50 mol%) under excitation with a 1532 nm diode laser are displayed in Fig. 1. As expected, two typical UC emission bands of Er3+ are detected in the range
2.2. Synthesis All samples were synthesized based on our previously reported procedures [32–36]. As for the synthesis of NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%), 3 mmol of citric acid (2 M, 1.5 mL), 5 mmol of NaOH (4 M, 1.25 mL) and 10 mL of deionized water were mixed and stirred for 10 min. Then 1 mmol of Ln(NO3)3 [Lu(NO3)3 2
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Fig. 1. UC emission spectra of (a) NaLuF4:Er, Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 50 mol%) and (b) NaLuF4:Er, Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 50 mol%) under excitation with a 1532 nm diode laser. Fig. 2. (a) Energy loss processes: energy migration to crystal defects and cross-relaxation quenching among Er3+. (b) Efficient strategy to simultaneously suppress the energy migration loss and cross-relaxation quenching among Er3+ through the introduction of mediators. The light purple space in (b) means the confined excitation energy domain.
energy migration loss and cross-relaxation quenching among Er3+. As displayed in Fig. 3a and b, the energy value of Er3+:4I11/2 state is slightly higher than that of Ho3+:5I6 state and Tm3+:3H5 state, which favors the energy transfer from Er3+ to Ho3+/Tm3+ (Er3+:4I11/2 → Ho3+:5I6 and Er3+:4I11/2 → Tm3+:3H5). Subsequently, an energy-backtransfer process from Ho3+/Tm3+ to Er3+ (Ho3+:5I6 → Er3+:4I13/2 and Tm3+:3H5 → Er3+:4I13/2) takes place, due to the energy level of Er3+:4I13/2 state is slightly lower than that of Ho3+:5I6 state and Tm3+:3H5 state. Consequently, the population inversion between Er3+:4I11/2 state and Er3+:4I13/2 state arises after the addition of Ho3+/ Tm3+ mediators. The results are similar to the previous report by Liu's group [17]. As a result, Er3+:4F9/2 state (660 nm red emission) is directly populated, which induces the huge enhancement of red UC emission (660 nm). Meanwhile, R/G ratio is obviously increased owing to the increasing intensity difference between 660 nm red emission and 520/540 nm green emissions. From the above analysis, it can be concluded that the intense high-pure red UC emission (660 nm) in Er3+sensitized NaLuF4 crystals is mainly attributed to the effective confinement of 1532 nm excitation energy by Ho3+/Tm3+ mediators. Based on the above results, the doping concentration of Ho3+ was fixed at 0.5 mol%, and NaLuF4:Er, Tm, 0.5 mol% Ho particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er, Tm and Ho is 50 mol%, Lu is 50 mol%) were synthesized (Fig. S4), which Ho3+ and Tm3+ are simultaneously used as mediators in Er3+-sensitized NaLuF4 UC particles. As can be seen from Fig. S5, the optimal doping content of Tm3+ is 2 mol%, which shows the strongest 660 nm red UC emission in NaLuF4:Er, Tm, 0.5 mol% Ho crystals under 1532 nm NIR-II excitation. As is known, Yb3+/Er3+ co-doping pair is typically used as sensitizer-activator pair for UC process, due to the large absorption crosssection of Yb3+ sensitizer. Consequently, it's necessary to investigate the UC emission property of Yb3+/Er3+ co-sensitized NaLuF4 crystals
of 400–710 nm. Green emissions at approximately 520/540 nm and red emission at approximately 660 nm are attributed to the transitions of 2 H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 of Er3+, respectively. Obviously, the intensity of 660 nm red UC emission is much higher than that of 520/540 nm green UC emissions. Relative to green emission, red emission is greatly increased after adding Ho3+ or Tm3+, which is favorable for the achievement of bright red UC luminescence with a high R/G ratio. As shown in Fig. 1a and b, the optimal doping contents of Ho3+ and Tm3+ are determined to be 0.5 mol% and 1 mol%, respectively. Compared with un-doped Ho3+/Tm3+ samples, 660 nm red UC emission in NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles and NaLuF4:49 mol% Er, 1 mol% Tm particles is increased by 13.1 and 6.0 times, respectively. The maximum R/G ratio is simultaneously obtained in the samples doped with 0.5 mol% Ho3+ (R/G = 33.4) and 1 mol% Tm3+ (R/G = 15.2), which is enhanced by 12.4 and 5.6 times relative to Ho3+/Tm3+-free samples (R/G = 2.7), respectively. The above results indicate that concentration quenching effect is effectively suppressed in Er3+-enriched NaLuF4 particles after introducing Ho3+ or Tm3+, which acts as energy mediators. As presented in Fig. 2a, the energy loss processes of luminescence quenching usually include energy migration loss and cross-relaxation quenching among Er3+ in Er3+enriched UC particles. The introduction of energy mediators, which can efficiently confine the excitation energy of Er3+ activators, is an effective strategy to simultaneously suppress the energy migration loss and cross-relaxation quenching among Er3+ (Fig. 2b). As for Er3+heavily-doped system, Er3+ can serve as both the sensitizers and activators. The 1532 nm NIR-II excitation energy can be absorbed by Er3+ sensitizers, and then emit visible light from Er3+ activators through multiphoton absorption. Ho3+ and Tm3+ as the energy mediators can temporarily store the excitation energy and subsequently return the energy to Er3+ activators, which is beneficial for the reduction of 3
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Fig. 3. Proposed UC processes after adding Ho3+ and Tm3+ energy mediators in (a, b) Er3+-sensitized and (c, d) Yb3+/Er3+ co-sensitized NaLuF4 crystals under the excitation of 1532 nm.
(Figs. S9 and S10), NaErF4:Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 100 mol%), NaErF4:10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 90 mol%) (Figs. S11 and S12) and NaErF4:10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 90 mol%) were further investigated under 1532 nm NIR-II excitation, as shown in Fig. 6. It can be seen that the optimal doping contents of Ho3+ and Tm3+ for UC emission in NaErF4:Ho particles, NaErF4:Tm particles, NaErF4:10 mol% Yb, Ho particles and NaErF4:10 mol% Yb, Tm particles with 1532 nm NIR-II excitation were determined to be 2 mol% (Fig. 6a), 1 mol% (Fig. 6b), 2 mol% (Fig. 6c) and 1 mol% (Fig. 6d), respectively. The EDS of NaErF4:Ho particles (Fig. S3c) demonstrates that the Na, F, Er and Ho elements are clearly present, and Fig. S3d reveals the presence of doped elemental Na, F, Yb, Er and Ho in NaErF4:Yb, Ho particles. Fig. 7a shows the comparison results of 660 nm red UC emission intensity in NaLuF4:49.5 mol% Er, 0.5 mol% Ho (Sample A), NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho (Sample B), NaLuF4:49 mol% Er, 1 mol% Tm (Sample C), NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm (Sample D) and NaLuF4:47.5 mol% Er, 2 mol% Tm, 0.5 mol% Ho crystals (Sample E) under 1532 nm NIR-II excitation. As can be seen, red emission intensity in Sample B is lower than that in Sample A; and Sample D is lower than that in Sample C, thus the emission efficiency in Yb3+/Er3+ co-sensitized NaLuF4 crystals is lower than that in Er3+-sensitized NaLuF4 crystals under 1532 nm NIR-II excitation. From the comparison result between Sample A, C and E, we can draw a conclusion that the confining efficiency of 1532 nm excitation energy in Er3+-sensitized NaLuF4 crystals with Ho3+/Tm3+ simultaneously used as energy mediators becomes lower compared to that with single energy mediator (Ho3+ or Tm3+). The main reason is that, relative to single energy mediator (Ho3+ or Tm3+), double energy
with 1532 nm NIR-II excitation. Firstly, we found that the optimal doping content of Yb3+ for UC emission in NaLuF4:49.5 mol% Er, Yb, 0.5 mol% Ho particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%) (Figs. S6 and S7) and NaLuF4:49 mol% Er, Yb, 1 mol% Tm particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%) under 1532 nm NIR-II excitation was determined to be 10 mol% (Fig. 4a and b), which the doping content of Ho3+ and Tm3+ were fixed at 0.5 mol% and 1 mol%, respectively. Next, the doping concentration of Yb3+ was fixed at 10 mol%, and it is found that the optimal doping contents of Ho3+ and Tm3+ for UC luminescence in NaLuF4:Er, 10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol % (Er and Ho is 50 mol%, Lu is 40 mol%) (Fig. S8) and NaLuF4:Er, 10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 40 mol%) under 1532 nm NIR-II excitation were determined to be 1 mol% and 1 mol%, respectively, as shown in Fig. 4c and d. The EDS of NaLuF4:Er, Yb, Ho crystals demonstrates that the Na, F, Lu, Yb, Er and Ho elements are clearly present (Fig. S3b). Proposed UC processes of NaLuF4:Er, Yb, Ho and NaLuF4:Er, Yb, Tm under 1532 nm NIR-II excitation is displayed in Fig. 3c and d. The above-mentioned energy transfer processes (Fig. 3) were further confirmed by power density-dependent luminescence studies. As presented in Fig. 5, both 660 nm red UC emission and 520/ 540 nm green UC emissions in NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho, NaLuF4:49 mol% Er, 1 mol% Tm and NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm crystals are two-photon processes under 1532 nm NIR-II excitation, which are in accord with our proposed mechanism (Fig. 3). As for Er3+-sensitized UC particles, there are some reports about Er3+-host-sensitized NaErF4 particles [13,16–18]. For comparison, UC luminescence properties of NaErF4:Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 100 mol%) 4
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Fig. 4. UC emission spectra of (a) NaLuF4:49.5 mol% Er, Yb, 0.5 mol% Ho particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), (b) NaLuF4:49 mol% Er, Yb, 1 mol% Tm particles doped with different Yb concentrations of 5, 10, 20, 30, 50 mol% (Lu and Yb is 50 mol%), (c) NaLuF4:Er, 10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 50 mol%, Lu is 40 mol%) and (d) NaLuF4:Er, 10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 50 mol%, Lu is 40 mol%) under excitation with a 1532 nm diode laser.
mediators (Ho3+/Tm3+) with the higher optimal doping content increases the interaction between Er3+ and energy mediators, leading to more non-radiation loss [37]. Besides, from Sample A and C, it can be seen that the confining efficiency of 1532 nm excitation energy of Tm3+ energy mediator is notably lower than that of Ho3+ energy mediator. This conclusion can be also demonstrated in Er3+-host-sensitized NaErF4 particles (Fig. 7b), which may be attributed to the emission peak at 696 nm (Fig. 6b) from Tm3+:3F3 to Tm3+:3H6, implying the
leakage of the confined 1532 nm excitation energy [38]. But unlike Er3+-sensitized NaLuF4 crystals, the emission efficiency in NaErF4 crystals with Yb3+ doping is higher than that without Yb3+ doping under 1532 nm NIR-II excitation, as displayed in Fig. 7b. Furthermore, red UC emission intensity from NaLuF4:49.5 mol% Er, 0.5 mol% Ho crystals is higher than that from NaErF4:2 mol% Ho (Fig. 7c) and NaYF4:49.5 mol% Er, 0.5 mol% Ho (Fig. S13) crystals under 1532 nm NIR-II excitation. Consequently, the emission efficiency in Er3+-
Fig. 5. (a, b) Power density dependence of red and green emissions at 660 nm and 520/540 nm in NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho, NaLuF4:49 mol% Er, 1 mol% Tm and NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm crystals under excitation with a 1532 nm diode laser. 5
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Fig. 6. UC emission spectra of (a) NaErF4:Ho particles doped with different Ho concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Ho is 100 mol%), (b) NaErF4:Tm particles doped with different Tm concentrations of 0, 0.5, 1, 2, 5, 10 mol% (Er and Tm is 100 mol%), (c) NaErF4:10 mol% Yb, Ho particles doped with different Ho concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Ho is 90 mol%) and (d) NaErF4:10 mol% Yb, Tm particles doped with different Tm concentrations of 0.5, 1, 2, 5, 10 mol% (Er and Tm is 90 mol%) under excitation with a 1532 nm diode laser.
sensitized NaLuF4 particles is higher than that in Er3+-host-sensitized NaErF4 particles and Er3+-sensitized NaYF4 particles with 1532 nm NIR-II excitation. In addition, the R/G ratio in NaErF4:2 mol% Ho crystals is only 6.8, which is much lower than that in NaLuF4:49.5 mol% Er, 0.5 mol% Ho crystals (R/G = 33.4). Based on the above analysis, it can be concluded that, Er3+-sensitized NaLuF4 particles is more suitable for obtaining intense high-pure red UC emission with 1532 nm NIR-II excitation. From the absorption spectrum of NaLuF4:Er, Ho and NaErF4:Ho particles in Fig. 7d, it can be clearly seen that 808 nm, 980 nm and 1532 nm NIR photons can be simultaneously absorbed by Er3+-sensitized UC particles, indicating Er3+-sensitized UC particles can be efficiently excited by three NIR excitation sources (808 nm, 980 nm and 1532 nm) rather than single-band excitation (such as 980 nm excitation for Yb3+-sensitized UC crystals or 808 nm excitation for Nd3+-sensitized UC crystals), which exhibits great potential applications in fields such as solar cells and anti-counterfeiting. Moreover, the absorption peak at 1532 nm is strongest relative to the peaks at 808 nm and 980 nm (Fig. 7d), thus Er3+-sensitized red-emissive NaLuF4 UC crystals with 1532 nm NIR-II excitation can be more efficiently applied in deep optical imaging. As shown in Fig. 8a, the luminescence lifetime of Er3+:4F9/2 → 4I15/2 transition (red emission) in Sample A, B, C and D is decreased from 0.702 ms, 0.625 ms, 0.386 ms–0.347 ms under 1532 nm NIR-II excitation. It is well known that the inverse of lifetime is equal to the sum (Ir+nr = Ir + Inr) of radiative (Ir) and nonradiative (Inr) transitions. As for the red-emitting state (Er3+:4F9/2), the decreasing lifetime indicates the increasing Ir+nr (Er3+:4F9/2). It can be seen from Fig. 7a, red UC emission intensity [Ir (Er3+:4F9/2)] is decreased in Sample A, B, C and D
under 1532 nm NIR-II excitation. Thus, the increasing Ir+nr (Er3+:4F9/2) should come from the dramatic increase of Inr (Er3+:4F9/2), which means the increase magnitude of Inr must be higher than the decrease magnitude of Ir, implying the obvious decrease of red UC emission. By the same token, as for the green-emitting state (Er3+:4S3/2), the increasing lifetime in Sample A, B, C and D under 1532 nm NIR-II excitation (Fig. 8b) indicates the decreasing Ir+nr (Er3+:4S3/2), which means the increase of green UC emission. Consequently, 660 nm red UC emission and R/G ratio in Sample A, B, C and D are reduced with 1532 nm NIR-II excitation, thus Sample A (NaLuF4:49.5%Er/0.5%Ho crystals) has the highest red UC emission intensity (660 nm) and R/G ratio, which is in accordance with the above UC emission spectra of Fig. 7a. Furthermore, Er3+:4I11/2 state is depopulated after introducing Ho3+/Tm3+ mediators (Fig. S14), which indicates that Ho3+/Tm3+ mediators play a role in trapping energies from Er3+:4I11/2 state. Fig. 9a displays the model of optical microscope setup for characterization of luminescence property at the single particle level. As can be seen from the optical micrograph of Fig. 9b (left), red UC luminescence from single NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles under 1532 nm NIR-II excitation demonstrates the high crystallinity and uniform doping in single particles. From the photograph of UC photoluminescence on the right side of Fig. 9b (one is obtained in a bright environment without 1532 nm NIR-II excitation, another is obtained in a dark environment with 1532 nm NIR-II excitation), intense high-pure red UC emission can be clearly observed with the naked eye, which is consistent with the above UC emission spectra that red emission at 660 nm dominates the UC process under the 1532 nm irradiation. Furthermore, when NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles 6
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Fig. 7. UC emission spectra of (a) NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Ho, NaLuF4:49 mol% Er, 1 mol% Tm, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm and NaLuF4:47.5 mol% Er, 2 mol% Tm, 0.5 mol% Ho crystals, (b) NaErF4:2 mol% Ho, NaErF4:1 mol% Tm, NaErF4:10 mol% Yb, 2 mol% Ho and NaErF4:10 mol% Yb, 1 mol% Tm crystals, (c) NaErF4:2 mol% Ho and NaLuF4:49.5 mol% Er, 0.5 mol% Ho crystals under 1532 nm excitation. Absorption spectrum of (d) NaLuF4:49 mol% Er, 1 mol% Ho and NaErF4:1 mol% Ho crystals.
660 nm red UC emission and 1532 nm excitation in biological tissue.
injected into the fresh beef (Fig. 10a,c), red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation (Fig. 10d), implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission in biological tissue (Fig. 10b). Fig. 10a exhibits the model of NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles injected into the fresh beef under excitation with a 1532 nm diode laser, and the model in Fig. 10b shows the deep penetration of
4. Conclusions In summary, intense high-pure red UC emission is obtained in Er3+sensitized NaLuF4 particles through introducing Ho3+ and Tm3+ mediators under 1532 nm NIR-II excitation. The enhanced 660 nm red
Fig. 8. (a, b) Decay curves of the 4F9/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ in NaLuF4:49.5 mol% Er, 0.5 mol% Ho, NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol % Ho, NaLuF4:49 mol% Er, 1 mol% Tm and NaLuF4:49 mol% Er, 10 mol% Yb, 1 mol% Tm crystals under the excitation of 1532 nm. The decay lifetime was calculated according to the function: τ = ʃI(t)dt/IP, where IP represents the peak intensity in the decay curve, and I(t) represents the emission intensity at time t. 7
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UC emission and R/G ratio are mainly attributed to the effective confinement of 1532 nm excitation energy by Ho3+/Tm3+ mediators: the energy transfer from Er3+ to Ho3+/Tm3+ (Er3+:4I11/2 → Ho3+:5I6 and Er3+:4I11/2 → Tm3+:3H5) and energy-back-transfer process from Ho3+/ Tm3+ to Er3+ (Ho3+:5I6 → Er3+:4I13/2 and Tm3+:3H5 → Er3+:4I13/2). In addition, it is found that the red UC emission intensity in Er3+sensitized NaLuF4 crystals is stronger than that in Er3+-host-sensitized NaErF4, Er3+-sensitized NaYF4 and Yb3+/Er3+ co-sensitized NaLuF4 crystals under 1532 nm NIR-II excitation. Furthermore, when the particles injected into the fresh beef, red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation, implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission. The results suggest that Er3+-sensitized red-emissive NaLuF4 UC crystals with 1532 nm NIR-II excitation can be efficiently applied in deep optical imaging.
Acknowledgments This work was supported by a start-up foundation from Guangzhou University under Grant No. 69-18ZX10334, Young Innovative Talents Project of Guangdong Province under Grant No. 2018KQNCX193, National Natural Science Foundation of China under Grant No. 61172027, Science and Technology Planning Project of Guangdong Province under Grant No. 2017A010103035. Fig. 9. (a) Model of the optical microscope setup for characterization of luminescence property at the single particle level. (b) Optical micrograph (left) and the photograph (right) of UC photoluminescence of NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles under excitation with a 1532 nm diode laser.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116731. Fig. 10. (a) Model of NaLuF4:49.5 mol% Er, 0.5 mol % Ho particles injected into the fresh beef under excitation with a 1532 nm diode laser. (b) Model showing the deep penetration of 660 nm red UC emission and 1532 nm excitation in biological tissue. (c) Fresh beef. (d) The photograph of UC photoluminescence after NaLuF4:49.5 mol% Er, 0.5 mol% Ho particles injected into the fresh beef under the excitation of 1532 nm.
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References [20]
[1] F. Zhang, G.B. Braun, Y.F. Shi, Y.C. Zhang, X.H. Sun, N.O. Reich, D.Y. Zhao, G. Stucky, Fabrication of Ag@SiO2@Y2O3:Er nanostructures for bioimaging: tuning of the upconversion fluorescence with silver nanoparticles, J. Am. Chem. Soc. 132 (2010) 2850–2851. [2] F. Zhang, S.S. Wong, Ambient large-scale template-mediated synthesis of high-aspect ratio single-crystalline, chemically doped rare-earth phosphate nanowires for bioimaging, ACS Nano 4 (2010) 99–112. [3] Z.Q. Li, Y. Zhang, Monodisperse silica-coated polyvinylpyrrolidone/NaYF4 nanocrystals with multicolor upconversion fluorescence emission, Angew. Chem. Int. Ed. 45 (2006) 7732–7735. [4] J. Zhou, Q. Liu, W. Feng, Y. Sun, F.Y. Li, Upconversion luminescent materials: advances and applications, Chem. Rev. 115 (2015) 395–465. [5] M.Y. Ding, D.Q. Chen, S.L. Yin, Z.G. Ji, J.S. Zhong, Y.R. Ni, C.H. Lu, Z.Z. Xu, Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage, Sci. Rep. 5 (2015) 12745. [6] G. Murali, R.K. Mishra, J.M. Lee, Y.C. Chae, J. Kim, Y.D. Suh, D. Lim, S.H. Lee, Aspect-ratio controlled synthesis and tunable luminescence of YF3:Yb3+/Er3+ upconversion nanocrystals, Cryst. Growth Des. 17 (2017) 3055–3061. [7] J.Y. Liao, Z.W. Yang, J.B. Sun, S.F. Lai, B. Shao, J. Li, J.B. Qiu, Z.G. Song, Y. Yang, Preparation and upconversion emission modifcation of crystalline colloidal arrays and rare earth fuoride microcrystal composites, Sci. Rep. 5 (2015) 7636. [8] Y. Vyborna, M. Vybornyi, A.V. Rudnev, R. Haner, DNA-grafted supramolecular polymers: helical ribbon structures formed by self-assembly of pyrene-DNA chimeric oligomers, Angew. Chem. Int. Ed. 54 (2015) 7934–7938. [9] Y.Y. Li, J.J. Guo, X.H. Liu, T. Aidilibike, W.P. Qin, White upconversion luminescence in CaF2:Yb3+/Eu3+ powders via the incorporation of Y3+ ions, Phys. Chem. Chem. Phys. 18 (2016) 16094–16097. [10] H. Lin, D.K. Xu, A.M. Li, Z.R. Qiu, S.H. Yang, Y.L. Zhang, Enhanced red upconversion emission and its mechanism in Yb3+-Er3+ codoped α-NaLuF4 nanoparticles, New J. Chem. 41 (2017) 1193–1201. [11] L.P. Tu, X. Liu, F. Wu, H. Zhang, Excitation energy migration dynamics in upconversion nanomaterials, Chem. Soc. Rev. 44 (2015) 1331–1345. [12] X.W. Cheng, Y. Pan, Z. Yuan, X.W. Wang, W.H. Su, L.S. Yin, X.J. Xie, L. Huang, Er3+ sensitized photon upconversion nanocrystals, Adv. Funct. Mater. 28 (2018) 1800208. [13] L. Liu, S.F. Wang, B.Z. Zhao, P. Pei, Y. Fan, X.M. Li, F. Zhang, Er3+ sensitized 1530 nm to 1180 nm second near-infrared window upconversion nanocrystals for in vivo biosensing, Angew. Chem. Int. Ed. 57 (2018) 7518–7522. [14] W. Shao, G.Y. Chen, A. Kuzmin, H.L. Kutscher, A. Pliss, T.Y. Ohulchanskyy, P.N. Prasad, Tunable narrow band emissions from dye-sensitized core/shell/shell nanocrystals in the second near-infrared biological window, J. Am. Chem. Soc. 138 (2016) 16192–16195. [15] A.M. Smith, M.C. Mancini, S. Nie, Second window for in vivo imaging, Nat. Nanotechnol. 4 (2009) 710–711. [16] L. Yan, B. Zhou, N. Song, X.L. Liu, J.S. Huang, T. Wang, L.L. Tao, Q.Y. Zhang, Selfsensitization induced upconversion of Er3+ in core–shell nanoparticles, Nanoscale 10 (2018) 17949–17957. [17] Q.S. Chen, X.J. Xie, B.L. Huang, L.L. Liang, S.Y. Han, Z.G. Yi, Y. Wang, Y. Li, D.Y. Fan, L. Huang, X.G. Liu, Confining excitation energy in Er3+-sensitized upconversion nanocrystals through Tm3+-mediated transient energy trapping, Angew. Chem. Int. Ed. 56 (2017) 7605–7609. [18] J.T. Xu, D. Yang, W. Han, S.M. Dong, T. Jia, F. He, H.T. Bi, S.L. Gai, L. Li, P.P. Yang, A novel strategy for markedly enhancing the red upconversion emission in Er3+/ Tm3+ cooperated nanoparticles, J. Mater. Chem. C 6 (2018) 7533–7540. [19] X. Chen, W. Xu, H. Song, C. Chen, H. Xia, Y. Zhu, D. Zhou, S. Cui, Q. Dai, J. Zhang, Highly efficient LiYF4:Yb3+, Er3+ upconversion single crystal under solar cell
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28] [29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
9
spectrum excitation and photovoltaic application, ACS Appl. Mater. Interfaces 8 (2016) 9071–9079. Y. Liu, S. Zhou, Z. Zhuo, R. Li, Z. Chen, M. Hong, X. Chen, In vitro upconverting/ downshifting luminescent detection of tumor markers based on Eu3+-activated core–shell–shell lanthanide nanoprobes, Chem. Sci. 7 (2016) 5013–5019. L. Xiong, T. Yang, Y. Yang, C. Xu, F. Li, Long-term in vivo biodistribution imgaing and toxicity of polyacrylic acid-coated upconversion nanophosphors, Biomaterials 31 (2010) 7078–7085. J. Yang, Y. Deng, Q. Wu, J. Zhou, H. Bao, Q. Li, F. Zhang, F. Li, B. Tu, D. Zhao, Mesoporous silica encapsulating upconversion luminescence rare-earth fluoride nanorods for secondary excitation, Langmuir 26 (2010) 8850–8856. N. Moller, A. Ruhling, S. Lamping, T. Hellwig, C. Fallnich, B.J. Ravoo, F. Glorius, Stabilization of high oxidation state upconversion nanoparticles by N-heterocyclic carbenes, Angew. Chem. Int. Ed. 56 (2017) 4356–4360. F. Wang, X.G. Liu, Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles, J. Am. Chem. Soc. 130 (2008) 5642–5643. Q. Ju, X. Chen, F. Ai, D. Peng, X. Lin, W. Kong, P. Shi, G. Zhu, F. Wang, An upconversion nanoprobe operating in the first biological window, J. Mater. Chem. B 3 (2015) 3548–3555. J. Zuo, Q.Q. Li, B. Xue, C.X. Li, Y.L. Chang, Y.L. Zhang, X.M. Liu, L.P. Tu, H. Zhang, X.G. Kong, Employing shells to eliminate concentration quenching in photonic upconversion nanostructure, Nanoscale 9 (2017) 7941–7946. N.J.J. Johnson, S. He, S. Diao, E.M. Chan, H. Dai, A. Almutairi, Direct evidence for coupled surface and concentration quenching dynamics in lanthanide-doped nanocrystals, J. Am. Chem. Soc. 139 (2017) 3275–3282. M. Haase, H. Schafer, Upconverting nanoparticles, Angew. Chem. Int. Ed. 50 (2011) 5808–5829. Z. Chen, X. Wu, S. Hu, P. Hu, H. Yan, Z. Tang, Y. Liu, Multicolor upconversion NaLuF4 fluorescent nanoprobe for plant cell imaging and detection of sodium fluorescein, J. Mater. Chem. C 3 (2015) 153–161. Z. Chen, X. Wu, S. Hu, P. Hu, H. Yan, Z. Tang, Y. Liu, Upconversion NaLuF4 fluorescent nanoprobes for jellyfish cell imaging and irritation assessment of organic dyes, J. Mater. Chem. C 3 (2015) 6067–6076. W. Li, C. Tan, Y. Zhang, Simultaneous phase and shape control of monodisperse NaLuF4:Yb, Er microcrystals and greatly enhanced upconversion luminescence from their superstructures, Opt. Commun. 295 (2013) 140–144. H. Lin, D.K. Xu, Y.J. Li, L. Yao, L.Q. Xu, Y. Ma, S.H. Yang, Y.L. Zhang, Enhanced red emission in Er3+-sensitized NaLuF4 upconversion crystals via energy trapping, Inorg. Chem. 57 (2018) 15361–15369. H. Lin, D.K. Xu, D.D. Teng, S.H. Yang, Y.L. Zhang, Shape-controllable synthesis and enhanced upconversion luminescence of Li+ doped β-NaLuF4:Yb3+, Ln3+ (Ln = Tm, Ho) microcrystals, New J. Chem. 39 (2015) 2565–2572. H. Lin, D.K. Xu, A.M. Li, D.D. Teng, S.H. Yang, Y.L. Zhang, Tuning of structure and enhancement of upconversion luminescence in NaLuF4:Yb3+, Ho3+ crystals, Phys. Chem. Chem. Phys. 17 (2015) 19515–19526. H. Lin, D.K. Xu, A.M. Li, D.D. Teng, S.H. Yang, Y.L. Zhang, Morphology evolution and pure red upconversion mechanism of β-NaLuF4 crystals, Sci. Rep. 6 (2016) 28051. H. Lin, D.K. Xu, A.M. Li, D.D. Teng, S.H. Yang, Y.L. Zhang, Simultaneous realization of structure manipulation and emission enhancement in NaLuF4 upconversion crystals, J. Mater. Chem. C 3 (2015) 11754–11765. E.M. Chan, D.J. Gargas, P.J. Schuck, D.J. Milliron, Concentrating and recycling energy in lanthanide codopants for efficient and spectrally pure emission: the case of NaYF4:Er3+/Tm3+ upconverting nanocrystals, J. Phys. Chem. B 116 (2012) 10561–10570. Y.F. Shang, S.W. Hao, W.Q. Lv, T. Chen, L. Tian, Z.T. Lei, C.H. Yang, Confining excitation energy of Er3+-sensitized upconversion nanoparticles through introducing various energy trapping centers, J. Mater. Chem. C 6 (2018) 3869–3875.