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Abstract
Nd3+-sensitized nanoparticles have attracted attention recently due to their large absorption cross section and lower water absorption coefficient at 808 nm, which can potentially solve the laser-induced heating problem and is essential for labelling and imaging applications in living organisms. Here, we report a single-step hydrothermal synthesis of CaF2:Yb3+/Er3+/Nd3+ upconversion nanoparticles (UCNPs). The size of the as-prepared UCNPs decreases from ∼138 to ∼30 nm as the Nd3+ doping concentration increases from 0 to 3 mol%. Under the excitation of a 808 nm continuous-wave (CW) laser, these UCNPs exhibit typical UC emissions at 539 nm (green) and 656 nm (red) for Er3+, and the luminescence is strongest when Nd3+ doping concentration is 0.75 mol%. Moreover, with the increase of doped Nd3+ ions, the suppressing of green and the enhancing of red UC emissions can be observed, which leads to the luminescence color tuning from green to red. Further investigations suggest that the resonant cross-relaxations (CRs) between Er3+ and Nd3+ ions contribute to the UC luminescence color changing and the remarkable red UC emission for the highly Nd3+-doped UCNPs. These advances make the UCNPs potentially to be applied in vivo labelling, bioimaging and phototherapy.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
UCNPs are luminescent nanomaterials that absorb near-infrared (NIR) light and produce visible UC emissions through doping of the lanthanide ions [1]. Compared with the conventional organic dyes and semiconductor quantum dots nanomaterials, lanthanide-doped UCNPs show great superiorities in bioimaging and drug delivery applications, due to their outstanding chemical and optical properties [2]. Generally, these UCNPs are doped with sensitizer and activator ions simultaneously [3]. Based on the large absorption cross section at 980 nm, Yb3+ ions always act as sensitizer and co-dope along with the activator ions (such as Er3+, Tm3+ and Ho3+) [4–9]. These Yb3+/Er3+ (Ho3+ or Tm3+) co-doped UCNPs have been widely utilized in many fields, such as color display [10,11], photodynamic therapy [12], temperature sensing [13] and multimodal imaging [14]. Furthermore, Yb3+ doped nanoparticles are also widely used in laser cooling of solids [15–18]. However, owing to the high water absorption coefficient at 980 nm, Yb3+ ions suffer from the risk of laser-induced heating of biological samples in living organisms applications [19].
Comparing with the water absorption coefficient at 980 nm (0.48 cm−1), water absorption coefficient at 808 nm is much lower (0.02 cm−1) [20]. Since the excitation wavelength of Yb3+ and Nd3+ are 980 nm and 808 nm, the change of sensitizer ions can significantly reduce the overheat effect. In addition, Nd3+ ions also have a higher absorption cross section at 808 nm (1.2 × 10−19 cm2 [21]) than Yb3+ at 980 nm (1.2 × 10−20 cm2 [22]). Fortunately, Nd3+ and Yb3+ ions have highly resonant energy transfer (ET) process [22,23]. Therefore, the ET process of Nd3+→Yb3+→Ln3+ (Ln = Er, Tm or Ho) creates a doping system with new sensitizer ions. Nd3+-sensitized UCNPs have attracted wide attention recently. So far, Nd3+, Yb3+ and Ln3+ triply-doped UCNPs have been widely reported in NaYF4, CaIn2O4 and LuVO4 hosts, which have been applied in vivo bioimaging and phototherapy [24–33]. Recently, Qiu et al. have reported the convex-lens-like NaYF4 microcrystals doped with Nd3+, Yb3+ and Er3+, which illustrated the growth mechanism and photoluminescence properties clearly [34].
Benefited from the low phonon energy, large luminescence transparent region, high chemical stability, biocompatibility and great UC luminescence efficiency [35–37], lanthanide-doped CaF2 nanoparticles have been widely applied in drug delivery, bio-imaging and photoluminescence biosensing [38–41]. However, few works of Nd3+-sensitized CaF2 UCNPs have been reported [42,43]. To the best of our knowledge, there was no studies characterized the Nd3+/Yb3+/Er3+ triply-doped CaF2 UCNPs synthesized by single-step hydrothermal method. Therefore, it is important to know whether efficient UC emissions can be generated in these UCNPs. Furthermore, the lanthanide-doped CaF2 UCNPs at tens of nanometers can be easily synthesized through single-step hydrothermal method, without using the toxic trifluoroacetate (such as preparing the NaYF4 UCNPs) [44]. And finally, the shift of excitation wavelength from 980 to 808 nm will greatly expand the biological applications of CaF2 UCNPs.
In this paper, we firstly synthesized Nd3+, Yb3+, Er3+ triply-doped CaF2 UCNPs through a single-step hydrothermal method [45]. To overcome the problem of charge balance which may appear when doping trivalent rare earth ions into bivalent hosts, the total rare-earth doping concentration is kept below 25 mol% [46–50]. When the Nd3+ ions increase from 0 to 3 mol%, the size of these UCNPs decreases significantly from ∼138 to ∼30 nm, which can be contributed to the strong effect of the Nd3+ ions on nanoparticles growth rate. Under the excitation of 808 nm CW laser, the integral UC emission intensity is enhanced significantly after doped with Nd3+ ions, and the green-to-red (G/R) intensity ratios decrease from 6.25 to 0.39 as the Nd3+ ions gradually increase from 0 to 3 mol%, which means a 16–fold enhancement of the red UC luminescence proportion. The mechanism of the tunable multicolor is also demonstrated based on the CR process between Er3+ and Nd3+ ions.
2. Materials and methods
2.1 Materials
CaCl2·2H2O (99.99% metals basis), YbCl3·6H2O (99.9% metals basis) and NH4F (99.99% metals basis) were purchased from Aladdin Industrial Corporation. ErCl3 (99.9% metals basis), NdCl3 (99.9% metals basis) and Ethylenediaminetetraacetic acid (EDTA) (99%) were purchased from Alfa Aesar. All the chemicals were used as received, without further purification.
2.2 Synthesis of the CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs
Here, we chose the most commonly used concentration of Yb3+/Er3+ (20/2 mol%) as the typical concentration. The CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs were synthesized by a single-step hydrothermal method. Typically, 10 mL of CaCl2, YbCl3, ErCl3, and NdCl3 aqueous solutions (0.2 M) were mixed at their respective stoichiometric amounts in a beaker and were stirred for 0.5 hour. 10 mL of EDTA aqueous solution (0.5 M) was added to the beaker. Then, the NH4OH solution was added dropwise into the above mixtures under stirring, until the pH of the solution was 6. And the mixtures were stirred for 1 hour. Subsequently, 15 mL aqueous solutions containing 5 mmol NH4F was transferred to the chloride salts solution prepared above, and then, the solution was stirred for another 0.5 hour. Finally, the solution was transferred to a 50 mL Teflon-lined autoclave and heated for 30 hours at 180 °C. After the autoclave was cooled down to room temperature, the UCNPs were collected by centrifugation for 4 minutes at 8000 rpm, and then the precipitate was washed with deionized water and ethanol for three times to remove any possible remnants. The final CaF2 UCNPs were obtained after drying in air at 60 °C for 8 hours.
2.3 Instruments and measurements
X-ray diffraction (XRD) patterns of the UCNPs were measured using an X-ray diffractometer with Cu K radiation at 40 kV and 40 mA (Bruke D8 Advance). The morphology and size of the CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs were characterized by the transmission electron microscopy (TEM) (Tecnai G2 F20, FEI). For photoluminescence experiments, the 808 nm fiber coupled diode laser (BWT K808DAHFN-25.00 W) was used as the excitation source. The UC luminescence generated by the UCNPs was collected and detected by a monochromator (Zolix Omni-λ300i) and a photomultiplier (PMT). The photos of the UC luminescence color were taken by a CMOS sensor (Sony IMX519). All the above measurements were performed at room temperature.
3. Results and discussion
3.1 Structural and morphological characterization
The morphology and size of the CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs were characterized by TEM images. Figures 1(a)–(e) demonstrate the morphology of the as-prepared UCNPs doped with different concentrations of Nd3+ ions. Obviously, When the Nd3+ ions increase from 0 to 3 mol%, the size of these UCNPs decreases significantly from ∼138 to ∼30 nm. It can be attributed to the strong effect of the Nd3+ ions on UCNPs growth rate through surface charge modification [35]. The XRD patterns are demonstrated in Fig. 1(f). All the diffraction peaks of the as-prepared UCNPs are corresponding to the standard pure cubic-phase CaF2 (JCPDS no. 89-4794), which reveals that the doping of Nd3+ ions have no influence on the phase purity of the UCNPs. Furthermore, the local magnification of XRD patterns are also shown in Fig. 1(g), which indicates that, as Nd3+ concentration increases, the diffraction peaks shift slightly towards lower angle side as a result of the increase of the lattice constant of CaF2 host [45]. The lattice constant increases with Nd3+ concentration, because more substitution of bivalent Ca2+ ions by trivalent Nd3+ ions, more F− ions are needed for charge compensation in CaF2 host. And the strong charge repulsion between these F− ions results in the increase of lattice constant [35,51]. In addition, the increase of the lattice constant of the as-prepared UCNPs also indicated that the Nd3+ ions had been well embedded in the CaF2 crystal lattice rather than adsorbed on the surface of the CaF2 host [52].
Fig. 1. TEM images of CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs, (a) x = 0, (b) x = 0.25, (c) x = 1, (d) x = 1.5, (e) x = 3. (f) XRD patterns of CaF2:Yb3+/Er3+/Nd3+ UCNPs, (g) Local magnification of XRD patterns between 46.6°and 47.4°.
3.2 UC luminescence properties
The Nd3+ doping concentration will affect not only the size and the lattice constant, but also the UC luminescence properties of the UCNPs. In order to clarify the effect of Nd3+ concentration, the UC emission spectra of the CaF2:Yb3+/Er3+ (20/2 mol%) UCNPs doped with different concentrations of Nd3+ ions were measured under the excitation of 808 nm CW laser at a power density of 267 W cm−2, as shown in Fig. 2(a). The insert pictures which were taken by a CMOS sensor under the same excitation intensity, show the corresponding UC luminescence color. Obviously, the UC luminescence spectra consist of three emission bands centered at 523 nm (green), 539 nm (green) and 656 nm (red), corresponding to the typical Er3+ transitions of 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2. Furthermore, for Nd3+-free UCNPs, both the green and the red UC emissions are weak due to the low absorption cross section of Er3+ ions at 808 nm [53]. After Nd3+ ions are doped, the integral UC luminescence intensity increases significantly, which can be clearly seen in the insets of Fig. 2(a). The luminescence enhancement is basically because of the higher absorption cross section of Nd3+ at 808 nm (1.2 × 10−19 cm2) [21], comparing with the absorption cross section of Er3+ at 808 nm (5.5 × 10−22 cm2) [53]. And after the pump laser was absorbed efficiently, there is efficient ET process from Nd3+ to Yb3+ ions [22,23], leading to more electrons populating the exciting levels of Er3+ ions. The insets in Fig. 2(a) also show that the UC luminescence color changes significantly from green to red with the increasing of Nd3+ ions. And the normalized emission spectra acquired under the same excitation intensity in Fig. 2(a) reveal the same trend. To better illustrate the color tuning trend, the G/R intensity ratios are calculated and displayed in Fig. 2(b). As Nd3+ doping concentration increases from 0 to 3 mol%, the G/R ratios decrease from 6.25 to 0.39, which means a 16–fold enhancement of the red UC luminescence proportion. It is obvious that the Nd3+ ions co-doped into CaF2:Yb3+/Er3+ UCNPs will decrease the G/R ratio and suppress the green emission.
Fig. 2. (a) Normalized UC emission spectra of CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs under the excitation of 808 nm CW laser at an excitation intensity of 267 W cm−2. The insets are the corresponding UC luminescence color. (b) Intensity ratios of green (539 nm) to red (656 nm) UC luminescence for CaF2:Yb3+/Er3+ (20/2 mol%) UCNPs doped with different concentrations of Nd3+ ions.
In order to explore the principle of color tuning by varying the Nd3+ doping concentrations more clearly, we have demonstrated the energy levels for Yb3+, Er3+ and Nd3+ ions, as shown in Fig. 3. For Nd3+-free UCNPs, under the excitation of 808 nm CW laser, the Er3+ ions simultaneously absorb laser energy and emit UC emissions. The electrons of Er3+ in 4I15/2 level are excited to 4I9/2 level, then transfer to 4I11/2 by non-radiative transition. After that, sectional electrons in 4I11/2 level reach 4F3/2 through excited state absorption, and finally reach the luminescence upstate by non-radiative transition; residual electrons in 4I11/2 level reach 4I13/2 through non-radiative transition, and finally reach the luminescence upper states 2H11/2, 4S3/2 and 4F9/2 by excited state absorption and following non-radiative transition, resulting in the 523, 539 and 656 nm UC emissions. However, limited by the low absorption cross section of Er3+ at 808 nm, the UC luminescence of the Nd3+-free UCNPs is extremely weak. For Nd3+-doped UCNPs, compared to the Er3+ directly absorption, the ET process of Nd3+→Yb3+→Er3+ is the main energy absorption channel, benefiting from the large absorption cross section of Nd3+ at 808 nm and highly ET efficiency from Nd3+ to Yb3+. Specifically, the electrons of Nd3+ in 4I9/2 level are excited to 4F5/2 level by 808 nm pump laser. After non-radiative transition from 4F5/2 to 4F3/2, the ET occurs between 4F3/2 level of Nd3+ and 2F5/2 level of Yb3+, and the 2F5/2 level of Yb3+ is populated. Subsequently, two successive ETs from Yb3+ to Er3+ bring partial ground state electrons of Er3+ directly to 4F7/2 level. And after non-radiative transition, the luminescence upper states 2H11/2, 4S3/2 and 4F9/2 are populated. Besides successive ETs mentioned above, the electrons of Er3+ relaxed from 4I11/2 to 4I13/2 can also absorb a 980 nm photon and reach 4F9/2 level. And this transition process plays an important role in the phenomenon of color tuning by varying the concentrations of Nd3+ ions, which will be elaborated below.
Fig. 3. The energy levels diagram for Yb3+, Er3+ and Nd3+ ions under the excitation of the 808 nm CW laser. The principle of the ET processes, non-radiative transitions, UC luminescence, and CR transitions are also presented in the diagram.
There are three main resonant CR transitions between Nd3+ and Er3+, which are shown in Fig. 3 with purple dotted arrows [26]. The electrons in ground state of Nd3+ are excited to upper states of 4I13/2, 4I15/2 and 4F9/2, corresponding to resonant CR transitions of 4I11/2→4I13/2, 4I13/2→4I15/2 and 4F9/2→4I15/2 in Er3+ (CR1, CR2 and CR3 in Fig. 3). And the energy gaps of these three resonant CR transitions are 267 cm−1, 644 cm−1 and 936 cm−1 correspondingly. The small energy gap of CR1 process is comparable with CaF2 phonon energy (322 cm−1 [54]), indicating that the CR1 is more likely to happen than CR2 and CR3 in CaF2 host. Benefited from the high efficiency of CR1 process, electrons of Er3+ in 4I11/2 can relaxed to 4I13/2 fleetly, which reveals that the 4F9/2 level will be populated by more electrons than 2H11/2 and 4S3/2 levels. Being more specifically, for the main Nd3+→Yb3+→Er3+ ET process, the electrons in the ground state of Er3+ are pumped to 4I11/2 level firstly. And then, there are two main ET paths which tune the G/R ratios. In the first path, the electrons in 4I11/2 level are pumped to the 4F7/2, which leads to the green UC emissions (523 and 539 nm). In the second path, the electrons in 4I11/2 level are firstly relaxed to 4I13/2 level, and then pumped to 4F9/2, resulting in the red luminescence (656 nm). For Nd3+-free UCNPs, the green UC emission is much stronger than red UC emission, because there is a large energy gap (3609 cm−1) between 4I13/2 and 4I11/2 levels, and it is difficult to populate 4F9/2 level through the second path. With Nd3+ doped, the CR1 process accelerates the relaxation from 4I11/2 level to 4I13/2, so electrons pumped to 4F7/2 will be decreased and the electrons pumped to 4F9/2 will be increased. Therefore, as Nd3+ doping concentration increases from 0 to 3 mol%, the red luminescence will be enhanced greatly.
To further prove the ET processes, two double-logarithmic plots of the luminescence intensity versus excitation intensity for different concentrations of Nd3+ doped UCNPs are shown in Fig. 4. And the slopes of the lines fitted in the plots represent the number of infrared multi-photons involved in a single visible photon generation [55]. And combined with the ET mechanism, the number of infrared multi-photons involved can prove the ET processes. In Fig. 4(a), the slopes of the green and red UC luminescence are both 1.88, indicating that both emissions are two-photon absorption processes, which are consistent with the assumed ET processes shown above. As Fig. 4(b) shows, the slopes of green and red UC luminescence decrease to 1.45 and 1.46 when the doping Nd3+ ions increase to 3 mol%, which is due to the resonant CR transitions between Nd3+ and Er3+.
Fig. 4. UC emission intensity of CaF2:Yb3+/Er3+/Nd3+(20/2/x mol%) UCNPs as a function of excitation intensity. (a) x = 0 mol%, (b) x = 3 mol%.
In addition, we have also measured the time-decay curves of green (539 nm) and red (656 nm) UC luminescence, which are shown in Fig. 5(a) and Fig. 5(b). The green and red luminescence are ascribed to the transitions of 4S3/2→4I15/2 and 4F9/2→4I15/2 of Er3+, respectively. Through the time-decay curves, we get the lifetimes of the green and red UC emissions for different concentrations of Nd3+-doped UCNPs, which are shown in Fig. 5(d). For Nd3+-free UCNPs, the lifetimes of green and red UC emissions are 0.697 ms and 0.751 ms, which meets the trend of lifetime versus nanoparticle size in previous reports [51,52]. When Nd3+ ions are doped in low concentration (lower than 0.75 mol%), the lifetimes of 4S3/2 and 4F9/2 increase as the Nd3+ concentration increases. Due to the large absorption cross section of Nd3+ at 808 nm, more pump light can be absorbed, and more electrons can reach 4S3/2 level and 4F9/2 level, which increases the lifetimes significantly. Besides, low doping concentration of Nd3+ makes the resonant CR transitions inefficiently. However, when the UCNPs doped with high concentration of Nd3+ ions, there will be intense resonant CR transitions between Nd3+ and Er3+, except for more absorbed pump light. When Nd3+ doping concentration increases from 0.75 to 3 mol% the resonant CR transitions will decrease the lifetimes of both green and red UC luminescence. The absolute UC luminescence intensity of the UCNPs doped with different concentrations of Nd3+ ions are shown in Fig. 5(c). All the samples are excited by 808 nm CW laser at an excitation intensity of 267 W cm−2, and the dispersion concentrations are all 0.5 mg/ml. From Fig. 5(c), we can find that when Nd3+ doping concentration increases from 0 to 0.75 mol%, because of the additional ET process of Nd3+→Yb3+→Er3+, both the green and red luminescence of Er3+ increase greatly to the maximum. And 0.75 mol% is the optimal Nd3+ doping concentration for luminescence enhancing. When Nd3+ doping concentration increases to 1 mol%, due to the resonant CR transitions between Er3+ and Nd3+, both the green and red luminescence decrease. However, the declining proportion of red luminescence is smaller than that of green luminescence, because CR1 (see it in Fig. 3) will increase electrons pumped to the up state of red luminescence and decreased the electrons pumped to the up state of green luminescence. So, the G/R intensity ratio increases. When Nd3+ doping concentration increases from 1 to 3 mol%, because of the stronger effect of CR1, the green luminescence decreases continually, and the red luminescence increases slightly.
Fig. 5. Time-decay curve of luminescence with different Nd3+ doping concentrations, (a) 539 nm (b) 656 nm. (c) UC emission spectra of CaF2:Yb3+/Er3+/Nd3+ (20/2/x mol%) UCNPs with different concentrations of Nd3+ ions. (d) The lifetime trends of 4S3/2 and 4F9/2 levels as a function of the Nd3+ doping concentrations.
4. Conclusions
In conclusion, we have synthesized Nd3+-doped CaF2:Yb3+/Er3+ UCNPs through a single-step hydrothermal method. The TEM results obviously show that the average diameter of these UCNPs decrease significantly from ∼138 to ∼30 nm, when the Nd3+ doping concentrations increase from 0 to 3 mol%. And due to the strong charge repulsion between additional F− ions, the lattice constant increases as the Nd3+ ions increase. When Nd3+ ions are doped in low concentration, the luminescence of Er3+ ions can be greatly enhanced, and the optimum Nd3+ concertation is 0.75 mol%. In addition, the Nd3+ ions can also tune the UC luminescence color of these UCNPs. As the Nd3+ doping concentrations increase from 0 to 3 mol%, the G/R ratios decrease from 6.25 to 0.39, which means a 16–fold enhancement of the red luminescence proportion. Moreover, the probable mechanism of ET processes between Er3+ and Nd3+ are supported by the power dependence measurement of the UC luminescence intensity. The properties of nanoscale, 808 nm-excited and red emission enhancing make Nd3+-doped CaF2:Yb3+/Er3+ UCNPs more advantageous in bioimaging diagnosis and anticancer therapy.
Funding
State Key Laboratory of Laser Interaction with Matter Foundation (SKLLIM1708).
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