NIR-II Upconversion Photoluminescence of Er3+ Doped LiYF4 and NaY(Gd)F4 Core-Shell Nanoparticles

The availability of colloidal nano-materials with high efficiency, stability, and non-toxicity in the near infrared-II range is beneficial for biological diagnosis and therapy. Rare earth doped nanoparticles are ideal luminescent agents for bio-applications in the near infrared-II range due to the abundant energy level distribution. Among them, both excitation and emission range of Er³⁺ ions can be tuned into second biological window range. Herein, we report the synthesis of ∼15 nm LiYF₄, NaYF₄, and NaGdF₄ nanoparticles doped with Er³⁺ ions and their core-shell structures. The luminescent properties are compared, showing that Er³⁺ ions with single-doped LiYF₄ and NaYF₄ nanoparticles generate stronger luminescence than Er³⁺ ions with doped NaGdF₄, despite the difference in relative intensity at different regions. By epitaxial growth an inert homogeneous protective layer, the surface luminescence of the core-shell structure is further enhanced by about 5.1 times, 6.5 times, and 167.7 times for LiYF₄, NaYF₄, and NaGdF₄, respectively. The excellent luminescence in both visible and NIR range of these core-shell nanoparticles makes them potential candidate for bio-applications.

Introduction

The 4f-4f transition of rare earth elements endows them with outstanding optical properties. Specifically, the energy of photons can be converted through rational designed transition among their abundant energy levels (Auzel, 2004; Wen et al., 2018). Furthermore, the photo-luminescence of rare earth is extremely stable, and it could not be affected by blinking or photobleaching (Shao et al., 2018; Song et al., 2019). These advantages make rare earth elements perfect candidates for light-triggered diagnostics and therapy (Chen et al., 2014; Shao et al., 2016; Qiang and Wang, 2019; Wang et al., 2020). As an unusual anti-stokes process, photon upconversion (UC) refers to converting several low-energy photons to one high-energy photon (Zhou et al., 2015; Wei et al., 2016). Due to the non-linear instincts, UC materials are well-developed for bio-imaging (Gonzalez-Bejar et al., 2016; Chen et al., 2021), photo-dynamics therapy(Lucky et al., 2015; Liu et al., 2019; Jia et al., 2020), and single particle imaging (Gargas et al., 2014; Liu et al., 2017; Zhou et al., 2020b; Dong et al., 2021).

The penetration depth of photons with different energies is significantly varied due to the scattering and absorption of biological tissue (Hemmer et al., 2016; Li and Wang, 2018). Compared to the visible range, where the auto-fluorescence of most tissue components is located, the near-infrared (NIR) range generally exhibits better spatial resolution and sharper contrast (Kenry et al., 2018). Recent studies have demonstrated that probes utilized in biological window II (1000–1700 nm, NIR-II) work more effectively than that in biological window I (700–900 nm, NIR-I) (He et al., 2015; Zhong et al., 2017; Fan et al., 2018; Huang et al., 2020; Lin et al., 2021; Lv et al., 2021; Yu et al., 2021). However, most reports on NIR probes are realized in the first biological window (Zhou et al., 2020a). Meanwhile, upconversion and down-shifting of rare earth with emission in biological window II have gained considerable interest recently. Zhong et al. have designed an Er-based nanoparticle (NPs) doped with Ce³⁺, which enhanced the down-shifting emission at 1550 nm under excitation at 980 nm (Zhong et al., 2017). They acquired high spatiotemporal resolution cerebral vasculatures in the NIR-II window with short exposure time. Liu et al. synthesized Er and Ho co-doped NPs with both excitation and emission in the NIR-II window as a sensor for inflammation dynamic detection (Liu et al., 2018). The time-gate imaging technique in the NIR-II window was realized in a NaYF₄: Yb, Nd@CaF₂ core shell structure with outstanding brightness (Tan et al., 2020). Er³⁺ ion is known as an outstanding imaging agent for NIR-II range, since it can be excited around 1550 nm due to the ⁴I_15/2 → ⁴I_13/2 transition (Chen et al., 2011; Shao et al., 2014; Cheng et al., 2018). Moreover, the emission peaks are at 550 nm (⁴S_3/2 → ⁴I_15/2) and 660 nm (⁴F_9/2 → ⁴I_15/2) in the visible range, 800 nm (⁴I_9/2 → ⁴I_15/2) in the NIR-I range, and 1000 nm (⁴I_11/2 → ⁴I_15/2) in the NIR-II range, respectively. The two-photon NIR-II to NIR-II UC process is rather unique, which have been investigated widely. However, how the matrix determines the luminescence remains unknown and deserves our efforts.

Herein, we reported the successful synthesis of three types of fluoride matrix, LiYF₄, NaYF₄, and NaGdF_4, each one doped with Er³⁺ ions. Their luminescence properties were compared, showing different relative intensity within each matrix. With epitaxial growth an inert homogeneous protect layer, the luminescence of each kind of UCNPs achieve much improvement due to the surface passivation. A proper mechanism was proposed based on the emission intensity dependence on excitation power density.

Materials and Methods

Materials

Yttrium (III) chloride hexahydrate (99.99%), erbium (III) chloride hexahydrate (99.9%), gadolinium (III) chloride hexahydrate (99.9%), yttrium (III) oxide (99.999%), gadolinium (III) oxide (99.9%), sodium trifluoroacetate (98%), lithium trifluoroacetate (95%), trifluoroacetic acid (98%), 1-octadecene (90%), and oleic acid (90%) were purchased from Sigma-Aldrich. Sodium hydroxide (98%), lithium hydroxide (98%), and ammonium fluoride (98%) were purchased from Aladdin Co. All chemicals were used as received.

Instruments

The size and morphology of the resulting nanoparticles were characterized by Hitachi 7700 electron microscope. Upconversion emission spectra in visible and NIR range were recorded using Horiba Fluoromax–4. Laser excitation at 1532 nm, which was purchased from Changchun New Industries Optoelectronics Technology Co., Ltd. The emission signal from the sample in the cuvette was collected at 90° relative to the exciting laser beam. The powder X-ray diffraction (XRD) patterns were recorded by a Bruker D8 Advance diffractometer using Cu Kα radiation. The 2θ angle of the XRD spectra was recorded at a scanning rate of 5°/min.

Synthesis of LiYF₄:10%Er³⁺ (NaYF₄:10%Er³⁺ and NaGdF₄:10%Er³⁺)

In a typical procedure, a total of 0.1°mmol ErCl₃•H₂O and 0.9°mmol YCl₃•H₂O were added into a 100°ml flask containing 6°ml oleic acid and 15°ml octadecene. The mixture was heated to 160°C for 60°min then cooled down to room temperature. Subsequently, a solution of 4°mmol NH₄F and 2.5°mmol LiOH (2.5°mmol NaOH for NaYF₄:10%Er³⁺ and NaGdF₄:10%Er³⁺) in 10°ml methanol was added and stirred for 30°min. The reaction mixture was then heated at 100°C for 30°min to remove the methanol, followed by heating up to 290°C (300°C for NaYF₄:10%Er³⁺ and NaGdF₄:10%Er³⁺) and keeping for 60°min before cooling down. A syringe needle was used to let the argon gas out during the synthesis. The mixture was cooled to room temperature and precipitated by excess ethanol and collected by centrifugation. The precipitate was washed with ethanol several times, and the nanocrystals were finally dispersed in hexane.

Synthesis of LiYF₄:10%Er³⁺@LiYF₄ (NaYF₄:10%Er³⁺@NaYF₄, NaGdF₄:10%Er³⁺@NaGdF₄)

The LiYF₄:10%Er³⁺@LiYF₄ core-shell nanoparticles are synthesized via a thermal decomposition method. A total of 0.5°mmol of Y₂O₃ was dissolved in 50% trifluoroacetic acid at 95°C in a three-neck flask. Then, the solutions were evaporated to dryness under an argon gas purge. Next, 10°ml of oleic acid, 10°ml of 1-octadecene, 1.5°mmol lithium trifluoroacetate, and 1°mmol LiYF₄:10%Er³⁺ core (2°mmol sodium trifluoroacetate and 1°mmol NaYF₄:10%Er³⁺ core for NaYF₄:10%Er³⁺@NaYF₄, 2°mmol sodium trifluoroacetate and 1°mmol NaGdF₄:10%Er³⁺ core for NaGdF₄:10%Er³⁺@NaGdF₄) were added into the flask. The resulting solution was then heated at 120°C with magnetic stirring for 45°min to remove water and oxygen. The brown solution was then heated to 310°C at a rate of about 12°C per min under argon gas protection and kept at this temperature under vigorous stirring for 45°min. A syringe needle was used to let the argon gas out during the synthesis. The mixture was cooled to room temperature and precipitated by excess ethanol and collected by centrifugation. The precipitate was washed with ethanol several times, and the nanocrystals were dispersed in hexane.

Results and Discussion

In order to evaluate the emission intensity impacted by matrix, 10% Er³⁺ was doped into three matrix LiYF₄, NaYF₄ and NaGdF₄ according to a previous synthesis procedure (Wang et al., 2010b). These three matrix types have been investigated extensively due to their low phonon energy (∼400°cm^–1), which are beneficial for efficient UC luminescence. As shown in Figure 1, three types of UCNPs were all monodisperse in hexanes solvents. LiYF₄ was shaped with spindle morphology (Figure 1A), while NaYF₄ (Figure 1B) and NaGdF₄ (Figure 1C) were shaped with the spheric shape, respectively. The size of LiYF₄ is determined to be around 16 nm, NaYF₄ around 19 nm, and NaGdF₄ 14 nm (Supplementary Figures S1A–C), which makes them comparable since the luminescences of NPs are significantly effected by their sizes. The X-ray diffraction (XRD) pattern in Supplementary Figure S2 showed the LiYF₄ possessed tetragonal phase, which was accordance with previous report. However, a hexagonal phase was identified for NaYF₄, and a mixed hexagonal and cubic phase for NaGdF₄ as the peaks at ∼27.5° and ∼32.5° referred to the (111) and (200) plane of the cubic phase of NaGdF₄. Previous reports have concluded doping ions with larger size will efficiently reduce the energy and time for transformation from cubic to hexagonal phase (Wang et al., 2010a). Considering the reverse, a smaller size dopant is supposed to increase the energy barrier of transformation and reduce the stability of the hexagonal phase. In our case, the dopant size of Er³⁺ ion is 1.144 Å, which is smaller than Gd³⁺ (1.193 Å) and Y³⁺ (1.159 Å). The differential between Gd³⁺ and Er³⁺ is more remarkable than that of Y³⁺ and Er³⁺. As a result, the NaGdF₄ requires more energy to complete the phase transfer. Then we further grew a homogeneous shell on each type of core UCNP as protect layer since the inert epitaxial shell growth could enhance the UC luminescence by overcoming the concentration quenching effects. The Transmission electron microscopy (TEM) imaging shows the sizes of three types of NPs grow larger, and the morphologies are much more uniform and monodisperse after coating the shell layer (Figure 1D–F). The size increase in the core-shell NPs is shown in Supplementary Figures S1D–F, and this proved the successful coating of shell on the core NPS. The phase of these core-shell UCNPs remain the same with core NPs, except NaGdF₄ change from the fixed cubic and hexagonal phase to pure hexagonal phase (Supplementary Figures S1G–I). We believe the shell coating process provides enough energy for the phase transfer of NaGdF₄ as well as epitaxial growth.

FIGURE 1

FIGURE 1. TEM images of (A) LiYF₄:10%Er³⁺, (B) NaYF₄:10%Er³⁺, (C) NaGdF₄:10%Er³⁺, (D) LiYF₄:10%Er³⁺@LiYF₄, (E) NaYF₄:10%Er³⁺@NaYF₄, (F) NaGdF₄:10%Er³⁺@NaGdF₄ nanoparticles; and XRD pattern of (G) LiYF₄:10%Er³⁺@LiYF₄, (H) NaYF₄:10%Er³⁺@NaYF₄, (I) NaGdF₄:10%Er³⁺@NaGdF₄ nanoparticles.

The UC properties of three core NPs under 1532 nm excitation were compared. As shown in Figure 2, three types of core NPs all emit four characteristic peaks assigned to the ⁴S_3/2 → ⁴I_15/2 (550 nm), ⁴F_9/2 → ⁴I_15/2 (660 nm), ⁴I_9/2 → ⁴I_15/2 (800 nm), and ⁴I_11/2 → ⁴I_15/2 (1000 nm), respectively. The optimized concentration of Er³⁺ ions is determined to be 10% according to integration of the total emission intensity (Supplementary Figure S3). The emission intensity of NaYF₄ core UCNPs is almost the same with LiYF₄, which is about 50 times stronger than NaGdF₄ UCNPs (Supplementary Figure S4). We further integrated the emission intensities of each peak for all three core NPs, respectively, as illustrated in Figure 2B. Notably, the ⁴F_9/2 → ⁴I_15/2 transition (660 nm) of NaYF₄ is about 1.7 times stronger than LiYF₄, while on the contrary, the ⁴I_11/2 → ⁴I_15/2 transition (1000 nm) of LiYF₄ is about twice of the NaYF₄. As we all know, the luminescence of rare earth can be ascribed to the local field splitting, which is highly depended on the crystal symmetry. Then it is understandable the tetragonal and hexagonal phases with different symmetry structures will favor variable energy transition, corresponding to the diverse dominant luminescence character. This explains the intensities of different peaks are not identical for the NaYF₄ and LiYF₄ matrix, and so the NIR-II emission bands of LiYF₄:Er³⁺ are more suitable than NaYF₄:Er³⁺ NPs. As for the NaGdF₄, it contains both cubic and hexagonal phase. For the parity forbidden essence, the phase with lower symmetry exhibits stronger emission intensity than the phase with higher symmetry (Liu, 2015). The luminescence of hexagonal phase NaGdF₄ is always much stronger than the cubic phase, so the mixed phase NaGdF₄ emits less efficient than pure hexagonal phase NaYF₄ (Damasco et al., 2014).

FIGURE 2

FIGURE 2. (A) UC emission spectra and (B) integration of all peaks of LiYF₄:10%Er³⁺, NaYF₄:10%Er³⁺, NaGdF₄:10%Er³⁺ nanoparticles excited at 1532 nm.

For rare earth nanomaterials, the protection shell architecture can suppress the surface-related quenching effects. Especially for Er³⁺ heavily doped nanoparticles, the core-shell architecture can significantly enhance both the UC and down-shifting luminescence. The concentration quenching of bare core is contributed mainly by surface quenching of defects and high phonon ligand instead of common misconceptions of cross-relaxation of rare earth (Johnson et al., 2017). Indeed, by coating a homogeneous shell, the luminescent intensity is unanimously enhanced for all the UCNPs, specifically, 5.1 times for LiYF₄, 6.5 times for NaYF₄, and 167.7 times for NaGdF₄, as shown in Figure 3. The enhancement of LiYF₄ nanoparticles and NaYF₄ nanoparticles is similar, which illustrates surface passivation is the dominant factor rather than the crystal phase, since the sizes of core LiYF₄ and NaYF₄ UCNPs are almost the same. Compared to LiYF₄ and NaYF₄, the increase in the luminescence of NaGdF₄ is more remarkable. As mentioned above, the shell growth on NaGdF₄ includes two processes: complete phase transfer from the mixed phase to pure hexagonal phase together with the epitaxial growth of protection shell. Two processes all result in considerable emission enhancement to the extent that the total luminescence of NaGdF₄:Er³⁺@NaGdF₄ is as high as that of LiYF₄:Er³⁺@LiYF₄ core-shell UCNPs. The luminescence of NaYF₄:Er³⁺@NaYF₄ is slightly higher than the other two types of UCNPs by about 1.6 times. Moreover, the emission intensity around 1000 nm of LiYF₄:Er³⁺@LiYF₄ core-shell UCNPs is slightly higher than NaYF₄:Er³⁺@NaYF₄ UCNPs. All three types of core-shell UCNPs exhibit excellent emission in visible and NIR range, showing great potential for NIR-II range bio-applications.

FIGURE 3

FIGURE 3. UC emission spectra of (A) LiYF₄:10%Er³⁺ and LiYF₄:10%Er³⁺@LiYF₄, (B) NaYF₄:10%Er³⁺ and NaYF₄:10%Er³⁺@NaYF₄, (C) NaGdF₄:10%Er³⁺ and NaGdF₄:10%Er³⁺@NaGdF₄ nanoparticles, and (D) the integration of their emission peaks excited at 1532 nm.

Then, we measured the pump power dependences of four bands for the colloidal LiYF₄:Er³⁺@LiYF₄ core-shell UCNPs. As shown in Figure 4A, all the bands show a slope around 1, which is deviated from the reported UC mechanism. It is well established when the UC rate exceeds the decay rate for the intermediated states, the dependence on the excitation power P will decrease from Pⁿ to P¹. In our case, the saturation of ⁴I_13/2 energy levels will cause such phenomenons, which explains the smaller measured photon numbers. These data are well accordance with previous literature, revealing similar energy transfer UC mechanism (Chen et al., 2011; Shao et al., 2014). Briefly, directly excitation of Er³⁺ ions by 1532 nm laser promotes the transition from ⁴I_15/2 to ⁴I_13/2. Then three-photon energy transfer process generates both green (⁴S_3/2) and red (⁴F_9/2) emission bands as well as the two-photon process of 800 nm (⁴I_9/2) and 980 nm (⁴I_11/2) emission peaks.

FIGURE 4

FIGURE 4. (A) Dependence of four peaks versus pump power, and (B) the proposed energy transfer upconversion mechanism between Er³⁺ ions.

Conclusion

In conclusion, we have observed four intensive emission bands for three colloidal and uniform LiYF₄:Er³⁺@LiYF₄, NaYF₄:Er³⁺@NaYF₄, and NaGdF₄:Er³⁺@NaGdF₄ core-shell UCNPs under excitation at 1532 nm. Homogeneous epitaxial growth of the shell layer promoted the luminescence intensity efficiently. Notably, NaGdF₄ completed the phase transfer during the shell coating, which enhanced the core luminescence intensity about 167.7 folds. Taking LiYF₄:Er³⁺@LiYF₄ as example, the intensities of four UC peaks showed linear dependence on the excitation power, which can be explained by a saturation effect of ⁴I_13/2 energy level. These core-shell UCNPs are pronounced as ideal NIR-II agent for biological related applications.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

QF, WZ, JP conducted the synthesis and optical measurement under supervision of WS and QC.

Funding

This work is supported by the National Natural Science Foundation of China (51802281), and the Zhejiang Provincial Natural Science Foundation (LY21B010006).

Conflict of Interest

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.2021.690833/full#supplementary-material

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