Research Article | Open Access
Multicolor Emission Tuning and Red/Green Ratio Enhancement of Yb3+/Er3+ Codoped KGdF4 Upconversion Nanoparticles
Herein, a series of KGdF4:Yb3+/Er3+ upconversion nanoparticles (UCNPs) were synthesized through a one-pot hydrothermal method using polyethylene glycol (PEG) as capping ligands. The phase and microstructure studies show all these as-prepared UCNPs are pure cubic phase and uniformed nanoparticle shape by changing the doped Yb3+ concentration from 18% to 98%. The as-prepared UCNPs can realize the multicolor emissions from yellow to red and the red-to-green (R/G) ratio can be enhanced from 2.05 to 8.35 when Yb3+ varies from 18% to 98%. In addition, the proposed upconversion (UC) mechanisms of these PEGylated UCNPs are investigated in detail. The realization of multicolor tuning and enhanced R/G ratio by only increasing the doping concentration of Yb3+ ions in KGdF4 host indicates that the PEGylated KGdF4:Yb3+/Er3+ UCNPs can find their application on lighting devices, anticounterfeit technology, and even bioimaging field.
In recent years, lanthanides- (Ln-) doped UCNPs have triggered intense research interest and have been widely applied in lighting devices, magnets, anticounterfeit technology, and biomedical field [1–5]. This is due to the fact that their optical, magnetic, electronic, and chemical nature arises from the 4f shell of Ln ions [6–8]. Besides, Ln-doped UCNPs exhibit large antistocks effects, which can convert a longer wavelength light like near-infrared (NIR) to shorter wavelength luminescence (ultraviolet, visible light, and NIR). Among all the well-developed UCNPs, sodium rare-earth fluorides (NaREF4), such as NaYF4, NaLuF4, and NaGdF4, are considered as promising and the most efficient host matrix owing to their normally low photon energy, which can decrease the nonradiative radiation probability and increase the UC emission and quantum yield [9–11]. Boyer et al.  have reported that Yb3+/Er3+ and Yb3+/Tm3+ codoped NaYF4 colloidal UCNPs as the material with high upconversion efficiency. Wang et al.  have shown that NaYF4 nanocrystals can simultaneously realize size/phase and UC emission color tuning by doping trivalent Ln ions at defined concentrations. Recently, Tian et al.  and Zeng et al.  have demonstrated Mn2+ doping methods for multicolor tuning and enhanced red UC emission in NaREF4 systems with fixing the doped Yb3+ and Er3+ concentration. Nevertheless, how to achieve multicolor UC emission and enhanced red emission by only changing the doped Yb3+ concentration is still a huge challenge because Yb3+/Er3+ codoped UCNPs present green UC emission in general. Recently, Chen et al.  have reported that UC NIR emission centered at 800 nm can be enhanced in Yb3+/Tm3+ codoped NaYF4 UCNPs along with the increase of Yb3+ concentration. This finding inspires us to achieve UC emission color tuning from green to red and to enhance red luminescence in Yb3+/Er3+ codoped UCNPs.
Apart from the well-established NaREF4 UCNPs, KREF4 can also act as effect and ideal UC host lattice. For instance, Wong et al.  have synthesized small sized Yb3+/Er3+ codoped KGdF4 UCNPs via one-pot solvothermal process utilizing branched polyethyleneimine and 6-aminocaproic acid as capping ligands. However, the authors’ emphasis has not focused on multicolor tuning and red emission enhancement but the green emission and paramagnetic nature of these KGdF4:Yb3+/Er3+ UCNPs. Importantly, enhanced red emission, considered as “optical window” for live beings, is beneficial to bioimaging for deep tissue penetration. In addition, in spite of the perfect UC host, Gd3+-based nanomaterials can also act as promising -weighted contrast agents for magnetic resonance imaging . However, there are few literatures, up to now, concentrated on KGdF4-hosted UCNPs. Therefore, it is of significance to develop multifunctional Yb3+/Er3+ codoped KGdF4 UCNPs using a simple method for emission color tuning and enhanced red emission.
In this work, a series of KGdF4:Yb3+/Er3+ UCNPs were fabricated by a one-pot hydrothermal method using PEG as capping ligands. The phase compositions and microstructure of these as-prepared products were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM), respectively. The UC properties of these UCNPs were detected using emission spectra and the UC emission mechanisms were analyzed in detail. As excepted, multicolor tuning and enhanced red UC emission were achieved by only tuning the doped Yb3+ concentration in the host, which indicated that these as-prepared KGdF4:Yb3+/Er3+ UCNPs could expand their application from optics to biomedicine.
2. Experimental Section
2.1. Materials and Regents
RE(NO3)36H2O (RE = Gd3+, Yb3+, and Er3+, 99.99%) were purchased from Sigma-Aldrich. Ethylene glycol (EG), PEG, KF, and other reagents were purchased from Sinopharm Chemical Reagent Co., China. All of these chemicals were used as received without further purification.
2.2. One-Pot Method for Fabrication of KGdF4:Yb3+/Er3+ UCNPs
KGdF4:%Yb3+/2%Er3+ ( = 18, 38, 58, 78, and 98) UCNPs were synthesized through a simple one-pot hydrothermal method using PEG as a surface modifier . In a typical process, 1.0 g of PEG was dissolved into 15 mL of EG solution for the formation of aqueous solution. And then, a total amount (1 mmol) of RE(NO3)3 water solutions with as-designed molar ratio was added. After 10 min agitation, 6 mL of KF aqueous solution (1 mM) was added into the former mixture. After another 30 min agitation, the mixture was poured in a 50 mL Teflon-lined stainless steel autoclave. The system was sealed and kept at 200°C for 24 h. After reaction, the system was cooled to room temperature and the product was washed with absolute ethyl alcohol and deionized water and dried at 60°C for 24 h.
XRD analysis was measured by a D/max-γA System X-ray diffractometer at 40 kV and 250 mA with Cu-Kα radiation (λ = 1.54056 Å). Morphologies of these UCNPs were characterized by FE-SEM (FEI NanoSEM 450). UC photoluminescence spectra of these UCNPs were recorded by a Zolix analytical instrument (fluoroSENS 9000A) equipped with a 980 nm laser.
3. Results and Discussion
Crystal phases and compositions of these as-prepared KGdF4:Yb3+/Er3+ UCNPs were synthesized by a simple one-pot hydrothermal process utilizing PEG as capping ligands. These UCNPs were recorded by powder XRD patterns. As presented by the red line in Figure 1, the measured XRD pattern of the PEGylated KGdF4:18%Yb3+/2%Er3+ UCNPs is in agreement with that reported in previous literatures , which is similar to that of the cubic phase NaGdF4 (JCPDS Card: 27-0697) and space group Fm3m. It therefore can be seen that the KGdF4:18%Yb3+/2%Er3+ UCNPs crystallize in the fluorite structure known for cubic phase NaGdF4. When the doped Yb3+ concentration increases by 58%, the diffraction peaks shift towards high angel sides owing to the replacement of Gd3+ (1.193 Å) by relatively smaller Yb3+ ions (1.125 Å) in the host . After the total substitution of Gd3+ through Yb3+, the UCNPs show the pure cubic KYbF4 phase (JCPDS Card: 27-0462). As observed, all these as-prepared UCNPs are of pure and single cubic phase.
To further study the morphology of these PEGylated UCNPs, FE-SEM images were carried out. As shown in Figures 2(a)–2(e), all these as-prepared PEGylated UCNPs are uniformed nanoparticle shape and the high doping has not changed the crystal structures and morphology of the host materials. The average diameter of these UCNPs is measured to 86.7, 64.8, 60.8, 46.6, and 43.1 nm while the doping ions of Yb3+ vary from 18% to 98% based on FE-SEM images. In addition, the EDS of KGdF4:18%Yb3+/2%Er3+ UCNPs (Figure 2(f)) shows the major elements composition of K, Gd, and Yb. The detected C and Si come from the silicon pellet of FE-SEM. In addition, the absence signal of Er element is due to the low doping concentration and the accuracy of the detector. Figure 2(g) shows the EDS of KGdF4:98%Yb3+/2%Er3+ UCNPs, indicating the absolute substitution of Gd by Yb elements in the nanocrystals.
Figure 3 gives the UC emission spectra of KGdF4:%Yb3+/2%Er3+ ( = 18, 38, 58, 78, and 98) UCNPs with different doping concentration of Yb3+. The emission bands can be attributed to the 2H11/24I15/2, 4S3/24I15/2, and 4F9/24I15/2 transitions of Er3+, respectively [18, 19]. As observed in Figure 3, KGdF4:Yb3+/Er3+ UCNPs present the strong green and red emission when doping 18% Yb3+. However, the green emission has been inhibited and the R/G ratio (Figure 4) can be promoted from 2.05 to 2.28, 3.12, 3.48, and finally 8.35 by ranging from the doping concentration of Yb3+ from 18% to 98%. The corresponding CIE chromaticity of KGdF4:%Yb3+/2%Er3+ ( = 18, 38, 58, 78, and 98) UCNPs is shown in Figure 5. The output emission colors tune obviously from the yellow region to the red region with the increase of the doped Yb3+ concentration owing to the intensities variation of the green and red emissions.
To further understand the UC emission properties of the PEGylated KGdF4:Yb3+/Er3+ UCNPs by tuning the doped Yb3+ concentration, the proposed UC mechanisms should be investigated in detail. As illustrated in Figure 6, the first energy transfer from Yb3+ to Er3+ excites the 4I15/2 to 4I11/2 state followed by the redundant energy dissipated by phonons. As a consequence, Er3+ relaxes nonradiatively to the lower 4I13/2 state and then suffers a second energy transfer process to 4F9/2 state. The red emission is associated with the radiative transition from 4F9/2 state to ground state. In addition, another energy transfer from 4I11/2 to 4F7/2 state results in 2H11/2/4S3/2 to the ground state with the green emissions centered at 523 and 543 nm, respectively, followed by the corresponding nonradiative relaxations. However, the increase of the doped Yb3+ concentration into the host results in the inhibition of green emission and the enhancement of R/G ratio when ranging Yb3+ concentration from 18% to 98%. This is due to the decrease in average distance between Yb3+ and Er3+ with the increase in number of Yb3+ per Er3+, which can increase the energy transfer efficiency between Yb3+ and Er3+. However, the cross relaxation process 4F7/2 (Er3+) + 2F7/2 (Yb3+) 4I11/2 (Er3+) + 2F5/2 (Yb3+) from Er3+ to Yb3+ is favoured at the same time, which results in the decrease of 4F7/2 (Er3+) population and green emissions . As a consequence, through energy transfer process 2F5/2 (Yb3+) + 4I13/2 (Er3+) 2F7/2 (Yb3+) + 4F9/2 (Er3+), the excited Yb3+ can transfer its energy to Er3+, which contributes to the more population of Er3+ at 4F9/2 state and results in the enhancement of red emission.
In conclusion, KGdF4:%Yb3+/2%Er3+ ( = 18, 38, 58, 78, and 98) UCNPs were fabricated via a one-pot hydrothermal method using PEG as capping ligands. All these PEGylated UCNPs were cubic phase and uniformed nanoparticle shape. These UCNPs realized yellow to red color emission by tuning the Yb3+ concentration from 18% to 98%, as well as the R/G ratio enhancement from 2.05 to 8.35. The multicolor emission and enhanced R/G ratio revealed that these PEGylated KGdF4:Yb3+/Er3+ UCNPs can act as nanophosphors and expand their application on lighting devices, anticounterfeit technology, and even bioimaging field.
Conflict of Interests
The authors do not have any conflict of interests in their submitted paper.
This work was supported by the General Project of Hunan Provincial Education Department (11C1130).
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Copyright © 2015 Xionger Qiu and Tiejun Lin. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.