Improving 800 nm Triggered Upconversion Emission for Lanthanide

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Improving 800 nm Triggered Upconversion Emission for Lanthanide-Doped CaF Nanoparticles through Sodium Ion Doping 2

Bing Xu, Huimei He, Zhanjun Gu, Shengye Jin, Ying Ma, and Tianyou Zhai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05639 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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The Journal of Physical Chemistry

Improving 800 nm Triggered Upconversion Emission for Lanthanide-Doped CaF2 Nanoparticles through Sodium Ion Doping Bing Xu a, Huimei He a, Zhanjun Gu b, Shengye Jin c, Ying Ma *a, and Tianyou Zhai*a a

State Key Laboratory of Materials Processing and Die & Mould Technology, School of

Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China. b

Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High

Energy Physics and National Center for Nanosciences and Technology Chinese Academy of Sciences, Beijing 100049, P. R. China. c

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, P. R. China

ACS Paragon Plus Environment

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ABSTRACT Lanthanide ions (Ln3+) doped CaF2 luminescent nanoparticles exhibit unique optical properties and represent promising candidates for bio-applications. However, their inherent weak luminescent intensity greatly limits their application, especially for Nd3+ sensitized ones. Here, we have doped Ln3+ ions along with Na+ ion into CaF2 lattice to maintain charge neutrality and investigated the influence of Na+ ion on the 800 nm triggered upconversion process. As compared with Na+-free (shell layer) nanoparticles, the crystallinity of Na+ ion codoped ones has been improved greatly, and the average lifetimes of the excited Nd3+, Yb3+ and Er3+ states have been prolonged after Na+ ion codoping in the sensitizing shell. Thus the energy transfer efficiency from sensitizers (Nd3+) to activators (Er3+) has been increased by 67%. As a result, the upconversion luminescence (UCL) intensity of the nanoparticles with suitable Na+ doping reaches 67 times higher than Na+-free nanoparticles upon 800 nm irradiation although less Ln3+ ions were doped into CaF2 in this case. The as-synthesized CaF2: Yb/Er@CaF2: Nd/Yb coreshell structured (CS) UCNPs may be good candidates as excellent imaging nano-probes because of their good biocompatibility.


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INTRODUCTION Trivalent lanthanide ions (Ln3+) doped near-infrared (NIR)-to-visible upconversion nanoparticles (UCNPs) have attracted broad attention in recent years, benefitting from their long luminescence lifetimes, large anti-Stokes shifts, and sharp-band emissions.1-5 These materials with fluorides as lattice matrixes have found numerous bioapplications due to their tunable emission and high photostability.6,

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Unlike traditional organic-based bio-probes, metal complexes or inorganic

quantum dots, these UCNPs show superior features such as negligible auto-fluorescence background and deep tissue-penetrable excitation in vivo bio-applications, because their excitation wavelengths are usually located in near-infrared (NIR) light range (700~1000 nm), so called the biological transmission window.8,

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Particularly for vivo bioimaging, an ideal bio-

probe should have some rigorous features, such as low cytotoxicity, appropriate size and intense emission.8, 10-15 Among traditional fluorides, CaF2 may be a good choice as host lattice because of its excellent biocompatibility and unique optical property with absorption threshold of ca. 12 eV, making it a transparent matrix in the UV-visible-NIR region.14 More importantly, ultra-small sized CaF2 nanoparticles can be easily prepared, which enable them easy to pass through the cell membrane for intracellular imaging and better clearance renally.16 Nowadays CaF2 based UCNPs are commonly sensitized by Yb3+ ions and excited by 980 nm laser.6, 17-22 Many researchers have proven that substituting 980 nm with 800 nm laser as the excitation source for bio-application could greatly minimize laser-induced local overheating effect, and achieve an adequate light penetration for deep-tissue imaging.1, 5, 23-27 However, it remains a challenge to synthesize ultra-small CaF2:Ln nanoparticles with high upconversion emission owing to the surface quenching effect, especially for those sensitized by Nd3+ ions. Moreover, non-equivalent substitute Ln3+ for Ca2+ ions would bring about lots of lattice defects


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and Ln3+ clusters to balance the local charge in the CaF2 host lattice.28-30 These are harmful for an efficient upconversion process, and thus high upconversion emission. Doping Ln3+ ions along with Na+ ion may compensate the local charge in the CaF2 lattice and alleviate lattice defects. Herein, we synthesized CS CaF2:Yb/Er@CaF2:Nd/Yb nanoparticles with ~10 nm using Nd3+ ions as the sensitizer. Na+ ion was codoped with Nd3+ and Yb3+ ions for charge compensation in the CaF2:Nd/Yb sensitizing layer. The CS structure constructed here could not only isolate the core from the surrounding quenching centres, but also suppress the energy backtransfer from the activators Er3+ (in the core) to the sensitizers Nd3+ (in the shell) by separating them spatially.1, 5, 23, 31-33 The Yb3+ ions located in both the core and the shell could bridge the excitation energy from the sensitizers to the emitters.1, 34 Codoping of Na+ ion with the Ln3+ ions in the shell improved the crystallinity of the nanoparticles and prolonged the lifetimes of the excited states of both the sensitizer (Nd3+ ion) and the activator (Er3+ ion) markedly, thus the UCL intensity upon 800 nm irradiation was enhanced remarkably.

Experimental section Chemicals and Materials. Oleic acid (90%) (OA), 1-octadecene (90%) (ODE), YbCl3·6H2O (99.9%), ErCl3·6H2O (99.9%), Yb2O3 (99.9%), Nd2O3 (99.9%), sodium trifluoroacetate (NaTFA) (99.9%), CaCO3 (99%) were purchased from Sigma-Aldrich. CaCl2 (96%), NaF (98.0%), NaOH (96.0%), methanol (99.5%), ethanol (99.7%) and cyclohexane (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All other chemical reagents were of analytical grade and used directly without further purification. Deionized water was used throughout. Synthesis of CaF2: Yb/Er (20 mol%/2 mol %) nanoparticles. In a typical procedure, 1 mmol of chloride salts (0.78 mmol of calcium chloride, 0.2 mmol ytterbium chloride, 0.02 mmol erbium chloride) were added to a mixture of sodium hydroxide (1.2 g), ethanol (5 mL), deionized


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water (3 mL), and oleic acid (18 mL), and the solution was thoroughly stirred. Then 2 mmol of sodium fluoride aqueous solution (4 mL) was added with vigorous stirring until a milky colloidal solution was obtained. Subsequently, the above solution was transferred to a 50 ml Teflon-lined autoclave, and heated at 180 °C for 24 h. The system was then cooled to room temperature. The nanocrystals were collected by centrifugation at 10000 rpm for 5 min, washed with ethanol to remove any possible residuum and then dispersed in cyclohexane (0.2 mmol/mL) for photo luminescence test or the use as the seed crystal for the shell coating. Synthesis of CaF2:Yb/Er (20 mol%/2 mol%)@CaF2:Nd/Yb (30 mol%/5 mol%) CS structured nanocrystals. Ca(CF3COO)2, Yb(CF3COO)3, and Nd(CF3COO)3 dispersed in ethanol (0.2 mmol/mL) were first obtained according to the published literature, respectively.5 Subsequently, 5 ml of the mixture of the above trifluoroacetic acid salt solution (the molar ratio of Ca(CF3COO)2, Yb(CF3COO)3, and Nd(CF3COO)3 is 0.65 : 0.05 : 0.30) were added in a mixture of OA/ODE (2.5 mL/2.5 mL) in a three-necked flask at room temperature. The slurry was then heated to 150 °C under argon gas flow with vigorous magnetic stirring to remove water and oxygen for a period of about 30 min. The resulting solution was stored for standby. 5 mL of as-synthesized CaF2: Yb/Er core and NaTFA (0, 1, 2, 3, 4 mmol) were added to a solution of OA (15 mL), ODE (15 mL) in a three-necked flask. The slurry was heated to 110 °C under argon gas flow with vigorous magnetic stirring for about 30 min, after that a mixture of as-prepared Ca(CF3COO)2, Nd(CF3COO)3, Yb(CF3COO)3 (1 mmol) solution was added to the reaction solution, and then the solution was heated to 310 °C at a heating rate of 10 °C/min and maintained for 30 min under argon gas flow. After cooling to room temperature, excessive ethanol was added into the solution to separate the products. After centrifugation, the final products were collected and dispersed in cyclohexane (0.2 mmol/mL). The processes to obtain


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the samples with other doping concentration in the shell are the same except employing the mixture of the trifluoroacetic acid salt solution with different molar ratio of Ca2+, Nd3+, and Yb3+ (Ca: Nd: Yb=1-x-y: x: y, x=0.1, 0.2, 0.4, 0.5; y=0, 0.1, 0.15). Synthesis of CaF2: Nd and CaF2: Nd/Na nanocrystals. Cubic phased CaF2: Nd and CaF2: Nd/Na were obtained using the thermal-decomposition method. In a typical experiment, 0.7 mmol Ca(CF3COO)2, 0.3 mmol Nd(CF3COO)3 ethanol solution (and 0.6 mmol NaCF3COO for CaF2: Nd/Na synthesis) were added into a mixture of 1.6 mL OA, 1.6 mL OM, and 3.2 mL ODE in a three-necked flask (50 mL) at room temperature. Then the slurry was heated to 80 °C and maintained for 30 min to remove ethanol with vigorous stirring under argon gas flow, and then heated to 150 °C to remove water and oxygen for 30 min. Finally, the solution was heated to a temperature of 310 °C at a heating rate of 10 °C/min and kept for 30 min under argon atmosphere. After cooling to room temperature, the nanoparticles were precipitated by adding excess ethanol into the solution. The nanocrystals were collected by centrifugation at 10000 rpm for 5 min. Synthesis of CaF2: Nd/Yb and CaF2: Nd/Yb/Na nanocrystals. The process to obtain the CaF2: Nd/Yb and CaF2: Nd/Yb/Na nanocrystal is the same as the synthetic procedures of CaF2: Nd and CaF2: Nd/Na except that the precursor solution is a mixture of 0.65 mmol Ca(CF3COO)2, 0.3 mmol Nd(CF3COO)3, and 0.05 mmol Yb(CF3COO)3 ethanol solution (and 0.7 mmol NaCF3COO for CaF2: Nd/Yb/Na synthesis). Characterization methods. X-ray diffraction (XRD) patterns were recorded on a D/MAXRB Xray diﬀractometer operated at 12 kW with Cu-Ka radiation (l = 1.5418 Å). Transmission electron microscopy (TEM) images were taken with a Tecnai G220 transmission electron microscope operating at 200 kV. High-resolution scanning transmission electron microscopy (HRTEM) and


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energy dispersive X-ray (EDX) spectra were performed on a Tecnai G2 F30 transmission electron microscope operating at 300 kV. The UCL properties were recorded using an Edinburgh FLS 980 spectrofluorometer with a red photomultiplier (PMT), in conjunction with a continuouswave (CW) 800 nm diode laser (30 W cm-2). The downconversion near-infrared photoluminescence were obtained upon CW 800 nm diode laser irradiation with a near-infrared PMT. Time-resolved fluorescence measurement is performed by using a fluorescence lifetime measurement system based on time-correlated single photon counting (TCSPC). Excitation of the sample is achieved with a femtosecond laser of 800 nm, 1 kHz repetition rate and ~35 fs pulse width. The downconversion luminescence (DCL) lifetimes of Nd3+ at 864 nm for the CaF2: Nd and CaF2:Yb/Er@CaF2: Nd/Yb samples and the UCL lifetime of Er3+ at 540 nm for CaF2:Yb/Er@CaF2: Nd/Yb sample were determined by single-exponential fit to the decays. The PL lifetime of Yb3+ at 975 nm for CaF2: Nd/Yb samples were determined by double-exponential fit to the decays, using the following equation35. y = ∑ ∙

+

(1)

The luminescent photographs were taken with a Nikon D3100 digital camera. The mole percentage of the elements (Ca/Nd/Yb/Er/Na) were obtained by Inductively Coupled PlasmaAtomic Emission Spectrometry (ICP-AES) on Optima 4300DV (Perkin-Elmer). Infrared absorption spectra were obtained by ultraviolet and visible spectrophotometer (Hitachi U-2910). The measurement of upconversion quantum yield was similar to the published literature.1,

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Fluorescence spectroscopy (Edinburgh FLS980) equipped with a red PMT was used to detect the excitation light from 800-nm CW laser and UCL from Er3+ (400- 700 nm). An integrating sphere was used to measure the efficiency data. The excitation light was attenuated by a neutral density


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filter to avoid saturating the detector before being detected. According to the equation (2), the quantum yield of UCL emission of the nanocrystal was calculated.

QY =

# #

=

!"#$%&' ()'*')'+,' ("#$%&'

(2)

Where QY is the quantum yield, Lsample is the emission intensity, Ereference and Esample are the intensities of the excitation light not absorbed by the reference sample (cyclohexane in this case) and the upconversion nanoparticles, respectively. RESULTS AND DISCUSSIONS In a typical experiment, cubic-phased CaF2: Yb/Er nanoparticles were synthesized by a wellestablished hydrothermal method at first.17,

20, 37

TEM image shows that the as-synthesized

nanoparticles are roughly cubic with the side length of (7.6±1) nm (Figure 1a). Corresponding Xray diffraction (XRD) pattern matches well with the standard cubic CaF2 crystal structure data (Figure S1). High resolution transmission electron microscopy (HRTEM) image shows the high crystallinity of the nanoparticles with a clear d spacing of 0.31 nm (Figure 1d), which is in good agreement with the lattice spacing of the (111) plane of cubic-phased CaF2. The CaF2:Nd/Yb shell was grown on the core epitaxially through a thermal decomposition route. TEM image of CS UCNPs shows an increase in the particle size by coating the CaF2: Nd/Yb shell (Figure 1b). EDX result of the CS UCNPs illustrates the presence of Ca2+ and Nd3+ ions in a single particle (Figure 1c). HRTEM image reveals the single-crystalline nature of the CS nanocrystals (Figure 1e). The side length of the resulting particles is about 10.3 nm, indicating that the thickness of the CaF2:Nd/Yb shell is approximately 1.3 nm (Figure 1f).


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Figure 1. TEM images of (a) CaF2:Yb/Er core and (b) CaF2:Yb/Er@CaF2:Nd/Yb core-shell (CS) structured UCNPs; (c) EDX spectrum collected from one CS UCNP. Inset: STEM image of the corresponding nanoparticles; HRTEM images of (d) core and (e) CS UCNPs; (f) Size distributions of CaF2: Yb/Er core-only and CS UCNPs by surveying 100 particles in the TEM images. Molar ratio of Na+ ion to Ln3+ ions in the shell layer of CS UCNPs is 2:1. In this CS UCNPs, Nd3+ ions in the outer-layer are used to harvest excitation photons. The 800 nm excitation energy captured by Nd3+ ions is transferred to Er3+ ions through the Yb sublattice crossing the core and shell regions. The UCL intensity upon 800 nm irradiation increases continuously as the Nd3+ ions doping concentration in the shell is elevated from 10 to 50 mol% (Figure 2a). As the doping concentration exceeds 30 mol%, the UCL intensity increases gently because the cross-relaxation energy transfer between Nd3+ ions enhances remarkably, resulting in a non-radiative depopulation of the excitation energy1, 5, 23, 32, 38-41. As shown in Figure 2b, when the Yb3+ doping concentration in the shell is 5 mol%, the UCL intensity upon 800 nm irradiation is the highest. A high doping level of Yb3+ ions in the shell would facilitate energy migration from the sensitizer in the shell not only to the activator in the core but also to surface quenchers, which suppress the UCL of Er3+ ions obviously42. The as-


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synthesized CaF2: Yb/Er@CaF2: Nd/Yb UCNPs were hydrophobic and could be dispersed and stored in nonpolar organic solvents, such as cyclohexane. The resulting solution displays intense green luminescence upon 800 nm and 980 nm continuous wave (CW) laser excitation (Inset in Figure 3a).

Figure 2. Emission spectra of Er3+-activated core-shell nanoparticles as a function of (a) Nd3+ and (b) Yb3+ molar concentration in the shell under 800 nm excitation. Inset in (a) and (b): Normalized UCL emission intensity at 540 nm.

Figure 3. (a) UCL emission spectra of CaF2:Yb/Er@CaF2:Nd/Yb NPs synthesized at different molar ratios of Na+/Ln3+ upon excitation at 800 nm. (b) PL decay curves by monitoring the emission of Er3+ at 540 nm for the CaF2:Yb/Er@CaF2:Nd/Yb nanoparticles codoped with and without Na+ under 800 nm excitation. Insets in (a): normalized UC emission intensity at 540 nm


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(top) and UCL photographs of the as-prepared nanoparticles with and without Na+ codoping upon 980 and 800 nm irradiation, respectively (bottom, 0.1 mmol·mL-1, the laser power density used for excitation is 5 W·cm-2). As we expected, codoping Na+ ion with Ln3+ ions could enhance the UCL intensity greatly. Figure 3a shows the UCL emission spectra of CaF2:Yb/Er@CaF2:Nd/Yb UCNPs synthesized at different molar ratios of Na+/Ln3+ upon 800 nm excitation. The more Na+ ions added, the higher the UCL intensity of Er3+ displayed. Typically, the UCL intensity of the sample prepared at Na+/Ln3+ ratio of 2:1 was about 68 times as that of the nanoparticles prepared in Na+-free condition. Correspondingly, the absolute quantum yield, defined as the ratio of the number of emitted photons to the number of absorbed photons, increased from 0.01% to 0.13%. The lifetime of 4S3/2 state of Er3+ ions in the core increased from 51 µs in Na+-free nanoparticles to 118 µs in nanoparticles synthesized at Na+/Ln3+ = 2:1 (Figure 3b). However, when the CaF2:Nd/Yb shell grew at higher Na+/Ln3+ ratios, the resultant UCNPs showed slightly higher UCL than those acquired at Na+/Ln3+ = 2:1. There was no obvious change in the morphology of the nanoparticles prepared at Na+/Ln3+ ratios below 2:1 (Figure 4a, b and Figure 1b). When more Na+ ions were added, the average sizes and size-polydispersity increased greatly, as shown in Figure 4c, d. Besides, at high Na+/Ln3+ ratios, hexagonal NaNdF4 phase formed as marked by red circles in addition to cubic CaF2 crystalline (Figure 4c, d and Figure S2). The generation of NaNdF4 phase might change the monomer concentration in the solution, and thus the growth kinetics of CaF2 nanoparticles, resulting in increased average sizes and size-polydispersity. The increase of particle sizes may also make a contribution to the enhanced UCL of these nanoparticles. The UCNPs prepared at Na+/Ln3+ ratio of 2:1 were chosen thereafter if not specified in this work. Interestingly, codoping of Na+ ion along with Ln3+ ions in the shell could also enhance Yb3+ sensitized UCL intensity of the UCNPs although most sensitizers (Yb3+ ion)


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exist in the core instead of the shell. Upon 980 nm irradiation, the strongest UCL was found in those nanoparticles synthesized at Na+/Ln3+ ratios of 1:1 (Figure S3) and the UCL intensity increased by a factor of 3 as compared with Na+-free ones, different from that for Nd3+ sensitized UCL. This is reasonable since the majority of Yb3+ ions are located in the CaF2: Yb/Er core and insensitive to the variation of the shell layer. With the addition of more Na+ ions, the UCL intensity reduced gradually.

Figure 4. TEM images of CaF2:Yb/Er@CaF2:Nd/Yb (30/5 mol%) UCNPs prepared at Na+/Ln3+ ratios of 0/1 (a), 1/1 (b), 3/1 (c), and 4/1 (d). Hexagonal NaNdF4 particles in c, d are marked with red circles. Insets: Size distribution of core/shell NPs obtained by surveying 100 particles in the TEM images. To find out the reason for the enhancement of the UCL intensity of Na+-codoped NPs upon 800 nm irradiation, the microstructures of UCNPs fabricated in Na+-containing and Na+-free solutions were investigated in detail. As shown in Figure 5, the intensity of XRD peaks increases


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gradually with Na+ concentration increasing from 0:1 to 3:1, which indicates that codoping with Na+ ions have improved the crystallinity of the UCNPs. As we know, it’s easy to dope Ln3+ ions into the CaF2 lattice since Ca2+ and Ln3+ ions have similar ionic sizes. But Ln3+ doping in CaF2 lattice is always accompanied by the formation of crystal defects such as interstitial fluorine ions or cation vacancies to maintain the charge neutrally upon replacement of Ca2+ with Ln3+ ions.29, 30

Such defects would consume the excitation energy greatly as quenching centres.28 When Na+

ions were introduced into the system, a couple of Na+ ion and Ln3+ ion would occupy the lattice sites of two adjacent Ca2+ ions in the CaF2 crystal to balance the local charge.43 Thus the defects mentioned above could be suppressed greatly and the crystallinity of UCNPs would be improved. Meanwhile, XRD diffraction peaks shift towards smaller angles with the Na+ increasing, which means a lattice expansion. Hence the distance between neighboring Nd3+ ions in the CaF2 lattice would be enlarged and the cross relaxation of energy between them might be suppressed. Thus, both defect reduction and lattice expansion after Na+ codoping are favorable for lessening energy loss and promoting upconversion luminescence. In contrast, absorption cross-section of Nd3+ in the spectral range from 710 to 850 nm belonging to the Na+ codoped sample decreased obviously in comparison to that of the Na+-free one (Figure S4). This excludes the contribution of promoted absorbance to UCL improvement of these Na+ codoped nanoparticles. It could be also clearly observed that the absorption peaks of Nd3+ slightly blue shifted from 742 nm and 800 nm to 736 nm and 796 nm, respectively, after Na+ ion was codoped in the sensitizing layer. Meanwhile, new absorption shoulders (marked with arrows) appeared at shorter wavelengths. Such spectral differences further confirm the variation of local environment around Nd3+ ions after codoping with Na+ ions as the charge compensator.


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Figure 5. XRD patterns of samples synthesized at different molar ratios of Na+/Ln3+; the shift of the peaks to smaller angle indicates the lattice expansion of CaF2 codoped with Na+. All patterns were recorded under identical conditions. The lines in the bottom are the standard diffraction patterns of cubic CaF2 crystals (JCPDS card no. 35-0816).

Figure 6. (a) Room-temperature downconversion luminescence spectra and (b) Decay curves of CaF2:Nd nanoparticles with and without Na+ codoping upon 800 nm irradiation. The emission at 860-895 and 1049-1057 nm are the results of 4F3/2 → 4I9/2, and 4F3/2 → 4I11/2 transitions of Nd3+, respectively. (c) Room-temperature luminescence spectra and (d) Decay curves of CaF2:Nd/Yb nanoparticles with and without Na+ codoping upon 800 nm irradiation. The emission at about 975 nm is attributed to the 2F5/2 to 2F7/2 transition of Yb3+. (All of the samples were dispersed in cyclohexane with 0.1 mmol·mL-1)


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To investigate the effects of Na+ ions codoping on the sensitizing process, we also prepared CaF2:Nd and CaF2:Nd/Na nanoparticles for comparison. Corresponding XRD patterns and TEM images confirm their cubic phase and nearly uniform appearances (Figure S5a-c). Much clearer SAED rings were observed in Na+ codoped nanoparticles than in Na+ free ones, directly verifying the crystallinity of CaF2 nanoparticles was obviously improved after Na+ codoping. Under excitation at 800 nm, the nanoparticles codoped with Na+ ion emitted stronger downconversion luminescence than Na+-free ones peaking at around 865-895 and 1049-1057 nm, which are attributable to transitions from 4F3/2 to 4I9/2, and 4F3/2 to 4I11/2 of Nd3+, respectively (Figure 6a). The PL decay curves at 864 nm excited by 800 nm pulsed laser are shown in Figure 6b. With Na+ codoping, the average lifetime of 4F3/2 state of Nd3+ was prolonged from 0.39 µs to 1.52 µs, which agrees well with the enhanced crystallinity and suppressed cross relaxation between Nd3+ ions after charge compensating doping. Such an extended lifetime is no doubt favourable for subsequent energy transfer from 4F3/2 state of Nd3+ to 2F5/2 state of Yb3+ ions. Consequently, CaF2:Nd/Yb/Na nanoparticles emitted greatly enhanced luminescence at ~975 nm upon 800 nm irradiation as compared with CaF2:Nd/Yb nanoparticles (Figure 6c). It can be also noted that emission at 975 nm attributed to the transition from 2F5/2 to 2F7/2 of Yb3+ increases by nearly 10 times, while emission at 1049-1057 nm attributed to the transition from 4F3/2 to 4I11/2 of Nd3+ only increases by 2 times after codoping with Na+ ions. This indicates that the extended lifetime of excited states may have an important effect on energy transfer. Moreover, we used a pulsed 800 nm excitation to validate the non-steady mechanism of the energy transfer process from Nd3+ to Yb3+ (Figure 6d). Without Na+ codoping, the 2F5/2 state of Yb3+ was sharply populated and began to decay shortly after the pulsed excitation stopped. In remarkable contrast, for the nanoparticles with Na+ codoping there is an obvious rise stage and the saturation time of


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the 2F5/2 state of Yb3+ markedly prolonged due to a waiting period for the energy transferring from energy donor (Nd3+ ions) to energy acceptor (Yb3+ ions).44 An obvious rise stage in the upconversion transient after short-pulse excitation is a feature of an efficient energy transfer process among rare earth ions.45 The phenomena demonstrate that codoping with Na+ ions could improve the energy transfer from Nd3+ to Yb3+ efficiently. Corresponding average lifetime of 2

F5/2 state of Yb3+ was prolonged from 21.73 µs to 38.80 µs with Na+ ions doping. The long

lifetime of 2F5/2 state of Yb3+ favors the energy transfer to Er3+ emitter locating in the core region and thus enhances the UCL intensity. The efficiency of a nonradiative energy transfer can be quantified using the following equation32, 46 ET = 1 −

123 12

(3)

Where ET stands for energy transfer efficiency, τDA and τD are the effective lifetimes of the energy donor in the presence and absence of an energy acceptor (Er3+ ion herein), respectively. Following this equation and measuring the decay curves of Nd3+ ions at 864 nm, the energy transfer efficiency from Nd3+ ions to Er3+ ions was determined to be respectively ~43% and ~72% for CaF2: Yb/Er@CaF2:Nd/Yb UCNPs before and after Na+ doping in the sensitizing shell (Figure S6, Table S1). This result confirms the great influence of Na+ ions as the charge compensator for non-equivalent doping on the upconversion emission upon 800 nm irradiation. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) results indicate less Ln3+ ions were doped into CaF2 lattice after Na+ ions were introduced in the precursors for shell growth (Table 1). Na+ ions detected in the Na+-free sample come from the CaF2: Yb/Er core prepared by hydrothermal process in which excess NaF was added. Reduced Nd3+ concentration agrees well with the decreased absorbance of Nd3+ in the range from 710 to 850 nm discussed


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above. Detected molar ratio of Na+ ions to Ln3+ ions in UCNPs increases from 0.29, to 0.65, and to 0.77, when the Na+/Ln3+ ratio in the precursor solution for shell growth increases. The two samples codoped with Na+ ions have the similar doping concentrations of Nd3+ and Yb3+ ions, but the one with more Na+ doping in the shell (Na+/Ln3+ = 2:1) exhibited much stronger UC emission than the one with less Na+ doping (Na+/Ln3+ = 1:1), suggesting the key roles Na+ ions played for charge compensation. Table 1. Elemental analysis of the CaF2: Yb/Er@CaF2: Nd/Yb nanoparticles by ICP-AES after dissolving the nanoparticles in diluted HNO3/HCl solution (Mole percent). Na/Ln

Ca

Nd

Yb

Er

Na

0/1

62.7

21.5

7.0

0.5

8.3*

1/1

62.8

16.0

6.0

0.5

14.7

2/1

59.6

16.5

6.1

0.4

17.4

*Na+ ions come from the CaF2: Yb/Er core.

Figure 7. UCL spectra of CaF2:Yb/Er@CaF2:Nd/Yb CS structured UCNPs without alkali metal (AM) doping, and with Li, Na, K codoping in the shell (the ratios of AM ions to Ln3+ ions are all 2:1) upon 800 nm irradiation.


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It has been reported that many Ln3+ clusters exist in the CaF2 crystal due to non-equivalent doping, especially at dopant concentrations around and exceeding 0.5 mol%.29, 30 Herein, the doping concentration of Nd3+ in the shell is far more than 0.5 mol%, so Nd3+ clusters could be easily formed in the CaF2 lattice. And it is well known that the cross relaxation process of 4

F3/2+4I9/2→24I15/2 can nonradiatively depopulate the excited 4F3/2 level if the adjacent Nd3+ ions

are too close, thus the excited energy captured by Nd3+ ions for Na+-free sample might be consumed greatly. As discussed above, by using Na+ ions as the charge compensator, the distribution of Ln3+ ions in CaF2 crystal might become more uniform, and the formation of Ln3+ clusters might be suppressed, which would improve the energy transfer from Nd3+ ions to Yb3+ ions. Moreover, lattice expansion induced by Na+ codoping may also contribute to inhibiting the cross relaxation process. In addition to Na+ ion, other alkali metal ions, such as Li+ and K+ were also used for charge compensators in the synthesis of Ln3+ doped CaF2 UCNPs, similar enhancement in UCL was also observed (Figure 7). The enhanced factors of UCL intensity at 540 nm are 10, and 23 times for Li+, and K+ codoped samples, respectively, in comparison with those without alkali metal ions doping. The difference in the UCL enhancement may be attributed to the different sizes of the alkali metal ions. On the one hand, the ionic diameter of Na+ is more close to that of Ca2+ and thus it is easier for Na+ ions to dope into CaF2 lattice. On the other hand, alkali metal ions with different diameters may have different influences on the local ligand environment around the sensitizer, Nd3+ ion.43

CONCLUSION In summary, we have developed 800 nm laser triggered upconversion nanoparticles with size of about 10 nm using biocompatible CaF2 as the host lattice. The defects in CaF2 lattice caused by non-equivalent doping were greatly suppressed and the lattice were expanded through codoping


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with Na+ ion, which significantly improved the crystallinity of the nanoparticles and increased the lifetimes of the sensitizer (Nd3+ ion) and the activator (Er3+ ion). As a result, the energy transfer efficiency from the sensitizer to the activator were increased from 43% to 72%, and the UCL intensity upon 800 nm irradiation was about 68 times as that of Na+-free samples. These 800 nm excited nanoparticles may have potential applications in biolabelling due to their ultrasmall size and good biocompatibility. ASSOCIATED CONTENT Supporting Information. Additional decay curves for different samples, XRD patterns, TEM images, and near-infrared absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Ma). *E-mail: [email protected] (T.Y. Zhai). ACKNOWLEDGMENT We acknowledge the support from National Natural Science Foundation of China (21673090). Here the authors also want to thank the technical support from the Analytical and Testing Center in Huazhong University of Science and Technology. REFERENCES (1) Zhong, Y.T.; Tian, G.; Gu, Z. J.; Yang, Y. J.; Gu, L.; Zhao, Y. L.; Ma, Y.; Yao, J. N., Elimination of photon quenching by a transition layer to fabricate a quenching-shield sandwich structure for 800 nm excited upconversion luminescence of Nd3+-sensitized nanoparticles. Adv. Mater. 2014, 26, 2831-2837.


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TOC Graphic


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Improving 800 nm Triggered Upconversion Emission for Lanthanide

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