Understanding the Role of Yb3+ in the Nd/Yb Coupled 808-nm-Responsive Upconversion

Introduction

Recently, substantial attention has been devoted to the lanthanide-doped nanoparticles due to their great infrared-to-visible photon upconversion performance (Auzel, 2004; Haase and Schafer, 2011; Zhou et al., 2015), which shows potential applications ranging from bioimaging (Zhu et al., 2017) to photodynamic therapy (Xu et al., 2017), 3D display (Deng et al., 2015), security (Lu et al., 2014), anti-counterfeiting (Li et al., 2016), and super-resolution nanoscopy (Liu et al., 2017). The unique 4f electronic configuration with abundant energy levels allows us to easily realize multi-wavelength upconversion emission bands upon infrared excitations (Chen et al., 2014; Dong et al., 2015; Zheng et al., 2015). By taking advantage of strategies including development of new host materials (Lei et al., 2017), control of local structure (Fischer et al., 2016), mechanistic exploration of new pathways using energy migration (Wang et al., 2011; Chen et al., 2017), cross-relaxation (Liu et al., 2017) and interfacial energy transfer (Zhou et al., 2016, 2018), efficient upconversion from a set of lanthanides such as Er³⁺, Tm³⁺, Ho³⁺, Tb³⁺, and Eu³⁺ was obtained. However, the most used upconversion systems are based on a Yb-sensitized design with 980 nm excitation, which makes the upconversion nanoparticles unsuitable for biological application because of the strong absorption of water at this wavelength region (Weissleder, 2001; Kobayashi et al., 2011; Zhu et al., 2017). Therefore, developing new classes of upconversion materials would be of great importance for their biomedical applications.

Interestingly, recent works suggest that Nd³⁺ is a possible sensitizer to move the excitation wavelength from 980 to 808 nm due to its ⁴F_5/2 ← ⁴I_9/2 absorption transition at this wavelength region (Liu et al., 2017; Xie et al., 2017) together with efficient energy transfer from Nd³⁺ to Yb³⁺ (Parent et al., 1986). More importantly, the absorption of excitation energy by water in biological tissues can be effectively minimized. To date, the 808 nm pumped upconversion from a series of lanthanides (e.g., Er and Tb) has been realized (Zhong et al., 2014; Zhou et al., 2018). In the Nd-sensitized upconversion system, Yb³⁺ is also employed to facilitate the energy transfer from Nd³⁺ to the lanthanide emitter, and the typical processes involved in this system can be schematically illustrated in Figure 1A. So far, there are four typical core-shell schemes can be used to obtain the upconversion, and typical sample forms for Er³⁺ are NaYF₄:Yb/Er/Nd(20/2/1 mol%)@NaYF₄, NaYF₄:Yb/Er/Nd(20/2/1 mol%)@NaYF₄:Nd(30 mol%), NaYF₄:Yb/Er@NaYF₄:Nd(30 mol%), and NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(30/10 mol%), among which the last one shows the best upconversion behavior (Figure 1B). However, few studies have explored the role of Yb³⁺, which is necessary in the 808 nm pumped upconversion systems, and the mechanism of the luminescence physics occurring in the Nd/Yb coupled upconversion is still not clear.

FIGURE 1

Figure 1. (A) Schematic illustration of energy transportation processes involving the Nd/Yb coupled upconversion system. The typical upconverted visible transitions from Er³⁺, Tm³⁺, and Ho³⁺ are marked in the right panel. ET and ETU stand for energy transfer and energy transfer upconversion, respectively. (B) Upconversion emission spectra from the four types of core-shell samples of NaYF₄:Yb/Er/Nd(20/2/1 mol%)@NaYF₄, NaYF₄:Yb/Er/Nd(20/2/1 mol%)@NaYF₄:Nd(30 mol%), NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd(30 mol%), and NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(30/10 mol%) under 808 nm excitation.

In this study, we performed a mechanistic investigation on the role of Yb³⁺ in the Nd/Yb coupled 808 nm responsive upconversion. We demonstrated that energy migration over the Yb-sublattice plays a key role in facilitating the energy transportation from the Nd³⁺ sensitizer to the lanthanide emitter. More importantly, we showed that the interfacial energy transfer from Yb³⁺ in the shell to the lanthanide emitter in the core across the core-shell interface also contributes to the upconversion. Such an 808 nm responsive upconversion can be markedly enhanced by optimizing the sample structure together with surface anchoring the infrared indocyanine green (ICG) dye. Our results present an in-depth understanding of the luminescence mechanism involving the Nd/Yb coupled upconversion nanomaterials, which would contribute to both fundamental research and practical applications of lanthanide-doped luminescent materials.

Experimental

Materials

The materials including yttrium(III) acetate hydrate (99.9%), erbium(III) acetate hydrate (99.9%), ytterbium(III) acetate hydrate (99.99%), thulium(III) acetate hydrate (99.9%), holmium(III) acetate hydrate (99.9%), neodymium(III) acetate hydrate (99.9%), oleic acid (90%), 1-octadecene (90%), sodium hydroxide (NaOH; >98%), and ammonium fluoride (NH₄F; >98%) were all purchased from Sigma-Aldrich. The nitrosonium tetrafluoroborate (NOBF₄; 98%) was purchased from Alfa Aesar and indocyanine green (C₄₃H₄₇N₂NaO₆S₂), N,N-dimethylformamide (DMF; anhydrous, 99.8%) were purchased from Energy Chemical. All these materials were used as received unless otherwise noted.

Sample Synthesis

The core-shell based nanoparticles were synthesized using a coprecipitation chemical method, which was shown to be a good way for the preparation of core only and core-shell based nanoparticles (Wang et al., 2011). The core nanoparticle samples were pre-synthesized as the seeds for growth of the core-shell structure. In a typical procedure for the synthesis of NaYF₄:Yb/Er core nanoparticles, to a 50-mL flask containing oleic acid (3 mL) and 1-octadecene (7 mL) was added a water solution containing Y(CH₃CO₂)₃, Yb(CH₃CO₂)₃, and Er(CH₃CO₂)₃ at designed ratios (e.g., 78:20:2 mol%) with a total amount of 0.4 mmol. The resulting mixture was heated at 150°C for 1 h and then cooled down to room temperature. Subsequently, a methanol solution containing NaOH (1 mmol) and NH₄F (1.6 mmol) was added and stirred at 50°C for 0.5 h, and then heated at 290°C under an argon flow for 1.5 h before cooling down to room temperature. The resulting core nanoparticles were collected by centrifugation, washed with ethanol, and finally dispersed in cyclohexane. Other control core nanoparticles were synthesized using a similar procedure except for the use of different lanthanide precursors.

Next, the core-shell nanoparticles were prepared with a two-step coprecipitation method using the pre-synthesized core nanoparticles as seeds for shell layer growth. Typically, for the synthesis of NaYF₄:Yb/Er@NaYF₄:Nd/Yb core-shell nanoparticles, to a 50-mL flask containing oleic acid (3 mL) and 1-octadecene (7 mL) was added a water solution containing Y(CH₃CO₂)₃, Nd(CH₃CO₂)₃, and Yb(CH₃CO₂)₃ at designed ratios (e.g., 40:50:10 mol%) with a total amount of 0.4 mmol. The resulting mixture was heated at 150°C for 1 h and then cooled down to room temperature. Subsequently, the pre-synthesized NaYF₄:Yb/Er particles were added as seeds along with a methanol solution containing NaOH (1 mmol) and NH₄F (1.6 mmol) was added and stirred at 50°C for 0.5 h, and then heated at 290°C under an argon flow for 1.5 h before cooling down to room temperature. The resulting nanoparticles were collected by centrifugation, washed with ethanol, and finally dispersed in cyclohexane. The core-shell-shell nanoparticles were prepared through a three-step coprecipitation method with a similar procedure except for using pre-synthesized core-shell particles as the seeds for the outermost shell layer growth.

The following synthetic procedure was used to prepare dye-decorated nanoparticles. Firstly, the sub-nanometer ligands of NOBF₄ were used to exchange the oleic acid ligands for the nanoparticles by coprecipatition method. The nanoparticles despersed in cyclohexane were mixed with the DMF solution of NOBF₄ (0.1 M) at room temperature, and the mixture was shaken gently for minutes to extract nanoparticles from upper cyclohexane layer into the bottom DMF layer. The bottom layer solution was then centrifuged at 11,000 rpm for 25 min, and the precipitated nanoparticles were weighted and re-dispersed in DMF (~60 mg/mL) for NIR dye sensitization experiment. Subsequently, a suitable amount of the ICG dyes dissolved DMF solution (1 μg/mL) was added to the nanoparticle dispersed DMF solution. This mixture was stirred overnight at room temperature to produce the ICG dyes sensitized nanoparticles in DMF solution.

Characterization

The powder X-ray diffraction (XRD) data were recorded on a Philips Model PW1830 X-ray powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). The upconversion emission spectra and infrared emission spectra were measured by a Jobin-Yvon Triax 320 spectrofluorometer equipped with an 808-nm laser diode with power density of 27.9 W/cm². A 980-nm laser diode with identical power density was also used for the excitation of control samples. The decay curves were measured with the same spectrofluorometer using the pulsed lasers as excitation sources. The low- and high-resolution transmission electron microscopy (TEM) measurements, together with elements mappings, were performed on a JEM 2100F with an acceleration voltage of 200 kV. The upconversion emission photographs were taken with a digital camera.

Results and Discussion

We firstly made an optimization of the upconversion from the NaYF₄:Yb/Er@NaYF₄:Nd/Yb core-shell nanoparticles at 808 nm excitation. Here the typical NaYF₄:Yb/Er(20/2 mol%) nanoparticles were used as core seeds because of their good upconverted luminescence (Figure S1). The as-synthesized core-shell nanoparticles showed good monodispersive characteristic (Figures 2A,B) and are in hexagonal phase according to the XRD diffraction profile (Figure S2). As shown in Figure 2C, the increment of Nd³⁺ concentration in the shell layer contributes to an enhancement of the visible upconversion of Er³⁺ and the optimal concentration of Nd³⁺ is found to be 50 mol%. It is easily understood that heavier dopant concentration of sensitizer can result in a higher absorption of the incident excitation irradiation. The upconversion as a function of Yb³⁺ concentration in the shell was also investigated, and the result is shown in Figure 2D. The presence of Yb³⁺ in the shell layer indeed leads to the enhancement of upconversion; however, a higher dopant concentration can cause a serious luminescence quenching, and a concentration of 10 mol% is found to be the optimized value with 12.3 times enhancement (Table S1). Thus, the optimal concentration of dopants in the shell layer for Nd-Yb pair is determined to be 50 and 10 mol%, respectively. The upconversion performance from Tm³⁺ and Ho³⁺ was also measured by preparing the NaYF₄:Yb/A(A=Tm,Ho)@NaYF₄:Nd/Yb core-shell samples, and typical upconversion emission profiles were obtained (Figure S3). Because Er³⁺ exhibits the most intensive upconversion emission, it was used in the following experiments.

FIGURE 2

Figure 2. (A) TEM image of the as-synthesized NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(50/10 mol%) core-shell nanoparticles. Inset shows the high-resolution TEM image of a single nanoparticle. (B) STEM image of the (A) sample and the element mappings of Y, Nd, and F. (C,D) Upconversion emission spectra from (C) NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(x/10; x = 30~60 mol%) and (D) NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(40/y; y = 0~60 mol%) core-shell nanoparticles under 808 nm excitation.

In order to investigate the possible energy migration involving the Nd/Yb coupled upconversion system, we propose a core-shell-shell trilayer nanostructure by inserting the Yb-doped interlayer into the NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(50/10 mol%) core-shell nanostructure, as schematically shown in Figure 3A. In this case, the upconversion of Er³⁺ from the core should be closely dependent on the content of Yb³⁺ in the interlayer since it acts as a bridge to facilitate the energy transportation from the outermost shell layer to the core under 808 nm irradiation. These nanoparticles were successfully prepared using the three-step co-precipitation method (Figure 3B) and their upconversion emission spectra are shown in Figure 3C. It is clearly observed that the upconverted emission from Er³⁺ produces an initial increase and then a decline with the increase of Yb³⁺ content. More importantly, almost no Er³⁺ upconversion is observed for the sample without Yb³⁺ doping in the interlayer. These results clearly confirmed the occurrence of energy migration among Yb-sublattice, and the optimal Yb³⁺ content is at around 20 mol%. On the other hand, intense infrared emission bands of Yb³⁺ were also recorded, see Figure 3D. This indicates that the spontaneous ²F_5/2→²F_7/2 transition is also a leading de-excitation channel for Yb³⁺ ions apart from energy migration.

FIGURE 3

Figure 3. (A) Schematic of proposed NaYF₄:Yb/Er@NaYF₄:Yb@NaYF₄:Nd/Yb core-shell-shell nanostructure for investigating the role of energy migration involving Yb-sublattice. (B) TEM image of the as-synthesized NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Yb(20 mol%)@NaYF₄:Nd/Yb(50/10 mol%) core-shell-shell nanoparticles. (C) Upconversion emission spectra from the (A) samples with a fine tuning of Yb³⁺ concentration under 808 nm excitation. (D) Near infrared emission spectra from (C) samples under 808 nm excitation.

We recently discovered that Yb-mediated interfacial energy transfer is an efficient process for enabling the upconversion from lanthanides (Zhou et al., 2018). Thus, there might exist a channel to activate the lanthanide emitter through a way of interfacial energy transfer in addition to the energy migration involving Yb-sublattice, as schematically illustrated in Figure 4A. Then two control core-shell samples of NaYF₄:Er@NaYF₄:Nd/Yb and NaYF₄:Er@NaYF₄:Nd were synthesized to check the role of Yb³⁺ in the shell layer. Note that no Yb³⁺ was incorporated into the core aiming to remove the possible interference of Yb³⁺ from the core on the resultant upconversion. Interestingly, the upconverted emission of Er³⁺ was markedly enhanced for the core-shell sample after the presence of Yb³⁺ in the shell layer (Figure 4B). This observation confirmed that the Yb³⁺ in the shell layer plays a key role in transporting the excitation energy from the shell layer to the core, and more importantly, it verified that the interfacial energy transfer from the Yb³⁺ in the shell to the Er³⁺ in the core indeed occur. Therefore, it can be concluded that both processes of energy migration and interfacial energy transfer contribute to the observation of efficient upconversion from the Nd/Yb coupled nanosystem.

FIGURE 4

Figure 4. (A) Schematic of possible energy migration (EM; Route A) and interfacial energy transfer (IET; Route B) in the design of NaYF₄:Yb/Er@NaYF₄:Nd/Yb core-shell nanostructure. Ex and Em stand for excitation and emission, respectively. (B) Upconversion emission spectra from control samples of NaYF₄:Er(2 mol%)@NaYF₄:Nd/Yb(50/10 mol%) and NaYF₄:Er(2 mol%)@NaYF₄:Nd(50 mol%) under 808 nm excitation.

Such an 808 nm enabled upconversion allows for a further enhancement of the upconversion by introducing the infrared dyes which have much higher absorption ability than Nd³⁺ at 808 nm wavelength region (Figure 5A), and the subsequent energy transfer from dye to Nd³⁺ could help greatly enhance the upconversion in this system (Zou et al., 2012; Wu et al., 2016; Wang et al., 2017). Considering the high absorption cross section (~6 × 10⁻¹⁶ cm²) of indocyanine green (ICG) dye which is ~5,000 times higher than that of Nd³⁺ (1.2 × 10⁻¹⁹ cm²) at around 800 nm (Kushida et al., 1968; De Boni and Mendonca, 2011), here we used it to sensitize the upconversion from the present NaYF₄:Yb/Er@NaYF₄:Nd/Yb core-shell nanoparticles (Wang et al., 2018). As shown in Figures 5B,C, the emission intensity is markedly enhanced when using the ICG dye amount of 150 μL (1 μg/mL), confirming the effectiveness of the construction in Figure 5A. The detail of energy transportation was shown in Figure S4. It should be pointed out that this dye-sensitized upconversion is 210 times more enhanced than that from NaYF₄:Yb/Er/Nd@NaYF₄:Nd core-shell nanoparticles (Figure 5D) when ICG used is 150 μL, at which the number of ICG per particle is estimated to be 32.8 given a full attachment of them at the surface. This result would greatly contribute to the diversity of frontier biological applications.

FIGURE 5

Figure 5. (A) Schematic of enhancing upconversion through using the infrared ICG dye. (B) Upconversion emission spectra from the (B) NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(50/10 mol%) core-shell nanoparticles sensitized with different amount of ICG dyes (1 μg/mL) under 808 nm excitation. (C) Dependence of upconversion emission intensity on the content of ICG. (D) A comparison of the upconversion from the NaYF₄:Yb/Er/Nd(20/2/1 mol%)@NaYF₄:Nd(30 mol%) (sample A) and NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(50/10 mol%) (sample B), and the ICG dye sensitized NaYF₄:Yb/Er(20/2 mol%)@NaYF₄:Nd/Yb(50/10 mol%) (sample C) core-shell-shell nanoparticles. Inset shows their emission photographs under 808 nm irradiation.

On the other hand, recent studies showed that a control of energy migration involving Yb-sublattice present a novel and efficient approach to tuning and enhancing upconversion performance of lanthanides (Chen et al., 2016; Liu et al., 2018). In the present work, the presence of Yb³⁺ into the shell layer indeed leads to a decline of the upconversion (Figure S5). We then designed a NaYF₄:Yb/Er@NaNdF₄:Yb@NaYF₄:Nd core-shell-shell nanostructure to improve the upconversion at 808 nm irradiation by making a fine tuning of the Yb³⁺ concentration in the Nd-sublattice (Figure 6A). It was found that 50 mol% Yb³⁺ in the NaNdF₄ interlayer is the optimal value for balancing the absorption and further transportation of the incident 808 nm excitation energy. Note that this Yb³⁺ content (50 mol%) in the interlayer is much higher than that from the NaYF₄:Yb/Er@NaYF₄:Nd/Yb core-shell nanostructure (10 mol%), revealing that there might exist a possibility of energy migration over to the surface which can quench the upconversion. In this case, the designs with high doping of migratory lanthanides need an optically inert shell layer to isolate the interactions between lanthanide emitter and surface quencher (Liu et al., 2018; Yan et al., 2018). Notably, the upconversion emission intensity is much weaker for these samples without the outermost shell layer (Figure S6). We further investigated the role of Nd³⁺ in the outermost shell layer. And the spectral result shows that a doping of it in the shell layer caused a slight decline in the upconversion intensity (Figure 6B) and lifetime (Figure S7). It should be noted that the near infrared emission from Yb³⁺ can be well improved (Figure S8).

FIGURE 6

Figure 6. Upconversion emission spectra from the (A) NaYF₄:Yb/Er(20/2 mol%)@NaNdF₄:Yb(0~100 mol%)@NaYF₄:Nd(50 mol%) and (B) NaYF₄:Yb/Er(20/2 mol%)@NaNdF₄:Yb(50 mol%)@NaYF₄:Nd(0~70 mol%) core-shell-shell samples under 808 nm excitation. Insets show the structure of the trilayer samples.

Conclusions

In conclusion, the role of Yb³⁺ in the Nd/Yb coupled upconversion system was mechanistically investigated. By designing a NaYF₄:Yb/Er@NaYF₄:Yb@NaYF₄:Nd/Yb core-shell-shell nanostructure with a Yb³⁺ content tuneable interlayer, we have confirmed that the energy migration over the ytterbium sublattice plays a critical role in facilitating the energy transportation from the sensitizer in the shell to the lanthanide emitter in the core. Interestingly, the direct interfacial energy transfer from Yb³⁺ in the shell to the emitter in the core also contributes to the upconversion. By further sensitization through using the infrared dyes, the upconversion luminescence intensity was markedly enhanced. These results on dynamics in the 808 nm pumped upconversion systems provide an in-depth mechanistic understanding of the energy interactions occurred in lanthanides, and more importantly, the markedly enhanced upconversion performance shows great promise in the diversity of biological applications.

Author Contributions

BZ conceived and designed the experiments. NS, LY, and JH performed the experiments. BZ and QZ supervised the project. BZ wrote the manuscript with input from all authors.

Funding

This work is supported by the National Natural Science Foundation of China (51702101 and 51472088), the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137), and the Fundamental Research Funds for the Central Universities (2017MS001, SCUT).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00673/full#supplementary-material

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