Influence of the TGA Modification on Upconversion Luminescence of

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J. Phys. Chem. C 2010, 114, 8219–8226

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Influence of the TGA Modification on Upconversion Luminescence of Hexagonal-Phase NaYF4:Yb3+, Er3+ Nanoparticles Dan Li, Biao Dong, Xue Bai, Yu Wang, and Hongwei Song* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China ReceiVed: January 29, 2010; ReVised Manuscript ReceiVed: March 31, 2010

Fluorescent nanoparticles used for biomedical applications should be small with narrow size distribution, and water-soluble with high luminescent efficiency. In this paper, uniform hexagonal-phase NaYF4:20%Yb, 2%Er nanocrystals (NCs) were synthesized in high boiling solvent oleylamine at 320 °C. The upconversion luminescence (UCL) intensity of the hexagonal-phase NaYF4:Yb, Er NCs in this work is much higher than that of other cubic-phase NCs. These hydrophobic NCs were further rendered hydrophilic using the ligand thioglycollic acid (TGA). The UCL properties of the surface-modified NCs were studied in contrast to the unmodified NCs under 980 nm laser diode excitation. After surface modification, the UCL revealed not only an enhancement of the red (4F9/2-4I15/2) emission with respect to the green (2H11/2, 4S3/2-4I15/2) one, but also an increase of the blue (2H9/2-4I15/2) due to the involving of surface vibration bonds -COOH, while the overall intensity had rarely changed. UCL dynamics were also studied, which indicate that the decay time constants of the green and the red emissions had only a little decrease, while the rising time constants of them decreased considerably. The rate equations for the red and green emissions were set up and solved to analyze the luminescent dynamics. The present results demonstrate that the modification of TGA can both make the NaYF4 NCs realize biofunctionality and efficient UCL. 1. Introduction Recently, fluorescence-based techniques have gained considerable attention due to their current and potential applications in the field of biology, which mainly include cellular labeling,1 in vivo imaging,2,3 FRET-based biosensors,4,5 and photodynamic therapy.6,7 Currently, the commonly used fluorescent nanoprobes are organic dyes and quantum dots (QDs),6-8 because they can be very small in size and have high aqueous solubility. However, both of them are downconversion fluorescent materials. The biological tissue can be photodamaged after long-term ultraviolet (UV) irradiation. Besides, organic dyes and QDs have their intrinsic disadvantages such as photobleaching, broad emission band, chemical instability, background noise, and toxicity to living organisms.3,5,6 The rapid development of materials science has brought us some newer and striking lanthanide (III)-doped upconverted fluorescent labels,9-12 which have unique antiStokes optical property with the excitation of noninvasive 980 nm near-infrared (NIR) light. The intrinsic properties of the lanthanide (III)-doped upconversion (UC) nanocrystals (NCs), such as high chemical stability, high quantum yield, and low background noise, can overcome many problems existing in the earlier classes of fluorescent nanoprobes. Among all the lanthanide (III)-doped upconverted NCs, NaYF4 is a perfect host material for green and blue phosphors owing to its low lattice phonon energy13-18 and thereby can minimize nonradiative loss and maximize the radiative emission. Moreover, crystal structure of the fluoride plays an important role in controlling the optical properties.19-22 Due to site symmetry and multisite character of the structure of the hexagonal phase, the UCL intensity of hexagonal-phase NaYF4:Yb, Er NCs is at least 1 order of magnitude more than the cubic-phase NCs.13 These consider* To whom correspondence should be addressed. E-mail: hwsong2005@ yahoo.com.cn. Phone and fax: 86-431-85155129.

ations have led to intensive research on the synthesis of hexagonal-phase NaYF4 NCs with free carboxylic acid groups or amino groups on their surfaces which allow conjugation with biomolecules for further biomedical applications.15,16 Many efforts have been devoted to the synthesis of hexagonalphase NaYF4 NCs with controllable size and morphology in recent years.17,18 Yan et al.17 and Capobianco et al.18 have prepared NaYF4 NCs based on the thermal decomposition of sodium trifluoroacetate and corresponding rare earth trifluoroacetate precursors in high boiling solvents such as oleic acid (OA), octadecene (ODE), and oleylamine (OM). In this work, we obtained hexagonal-phase NaYF4 monodisperse nanoparticles codoped with Er3+ ions and Yb3+ ions in an efficient and user-friendly method in which rare earth nitrates were used instead of trifluoroacetates. According to Ghosh et al.,20 crystal phase can be modified by temperature of heating and varying the Re3+/F- ratio. In our present work, we fixed the ratio of Re3+/F- as a constant of 1/4 to ensure that there were no excessive fluoride reactants. This is because at high temperatures, the excessive fluoride reactants were easily hydrolyzed to produce HF gas which may raise safety concerns.23 When the temperature of heating in our system was lower than 320 °C, the hexagonal phase would be impure, and the intensity of cubic-phase NCs increased with decreasing the temperature of heating. In addition, as we know for bioapplications, water solubility of the NCs is crucial, and numerous efforts have been done to transfer the NCs from hydrophobic to hydrophilic.5,16,24-26 The most popular strategy was surface silanization.24,25 However, direct silica coating on the nanoparticles was difficult to control; one silica shell may contain several nanoparticles inside, making them too big in size and therefore not suitable for further applications in biolabiles. Chen et al.26 have reported the conversion of oleic acid stabilized hydrophobic NaYF4 NCs into water-soluble NCs with Lemieux-von Rudloff reagent. Chow

10.1021/jp100893k  2010 American Chemical Society Published on Web 04/21/2010

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and co-workers16 demonstrated the coating of core/shell NCs of NaYF4 with poly(acrylic acid) (PAA). Wang et al.5 developed a layer-by-layer (LBL) assembly strategy based on electrostatic interactions to generate water-soluble UC nanoparticles. Here we address the problem by introducing thioglycollic acid (TGA) as the ligand to render NaYF4 nanoparticles well-dispersed in aqueous solutions or other biorelevant media. TGA has been successfully used as the stabilizer in synthesizing water-soluble semiconductor QDs.27-29 To the best of our knowledge, it has not been used for the surface modifications of lanthanide ion doped nanoparticles. Our goal was to utilize its sulfhydryl group (-SH) coordinated with lanthanide ions, and its carboxylic acid group (-COOH) stretched out, which facilitated further conjugation with bimolecules such as antibodies and antigens. And more, previous studies mostly concentrated on the preparation and charatererization of such compositions. The influence of surface modification on UC processes was rarely studied. Here, we systemically discussed the influence of TGA modification on UCL properties of NaYF4:Yb3+, Er3+ NCs and their origins. It is interesting to observe that after the surface modification the red (4F9/2-4I15/2) and blue (2H9/2-4I15/2) emission was considerably enhanced with respect to the green (2H11/2, 4S -4I 3/2 15/2) one, and there is no notable reduction in the overall luminescence under the excitation of 980 nm. 2. Experimental Section 2.1. Synthesis of Hexagonal-Phase NaYF4:Yb3+(20%), Er3+(2%) Nanoparticles. Oleylamine (OM) was purchased from Acros and used without further purification. In a typical synthesis of hexagonal-phase NaYF4:Yb3+, Er3+ nanoparticles, 0.78 mmol of Y(NO3)3 · 5H2O, 0.20 mmol of Yb(NO3)3 · 5H2O, and 0.02 mmol of Er(NO3)3 · 5H2O were dissolved in 1 mL of methanol in a three-necked flask at room temperature. After vigorous magnetic stirring, 15 mL of OM was added, and the mixture was then heated to 120 °C in a temperature-controlled electromantle to form an optically transparent solution and then cooled down to room temperature. Five milliliters of methanol solution containing NaOH (2.5 mmol) and NH4F (4 mmol) were slowly added into the three-necked flask, and the solution became a bit turbid. Next the slurry was heated to 100 °C and maintained at this temperature for 20 min to remove methanol and degas. Then the mixed solution was heated to 320 °C and maintained for 1 h under nitrogen atmosphere. When it cooled down to the room temperature, an excess amount of ethanol was poured into the solution. The resultant mixture was centrifugally separated and washed with absolute ethanol/ deionized water (1:1 v/v) three times, and the final products were collected. The as-prepared NCs could be easily redispersed in various nonpolar organic solvents, such as hexane and cyclohexane. 2.2. Synthesis of Hydrophilic NaYF4:Yb3+, Er3+/Tm3+ Nanoparticles. NaYF4:Ln3+ nanoparticles (0.2 mmol) dissolved in 5 mL of cyclohexane and 1 mL of TGA in 10 mL of ethanol were mixed and stirred for 48 h; the nanoparticles were isolated by centrifugation, washed several times with deionized water and ethanol, and then redispersed in deionized water to form a transparent colloidal solution without any noticeable precipitation after two weeks. Under 980 nm excitation, multicolor emission bands can be observed for different codopants, which are particularly useful in multiplexed labeling.30 Figure 1A-C shows UCL photographs of NaYF4:20%Yb3+, 2%Er3+, NaYF4: 70%Yb3+, 2%Er3+, and NaYF4:20%Yb3+, 0.5%Tm3+ NCs dispersed in deionized water, respectively. Obviously, bright green, pink, and blue emissions can be observed under the excitation of a 980 nm laser diode.

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Figure 1. Photographs of the upconversion luminescence of hexagonalphase NaYF4 nanocrystals excited with 980 nm laser dispersed in deionized water: (A) NaYF4:20%Yb3+, 2%Er3+; (B) NaYF4:70%Yb3+, 2%Er3+; (C) NaYF4:20%Yb3+, 0.5%Tm3+ samples.

2.3. Measurements and Characterization. Field emission scanning electron microscopy (FE-SEM) images were taken on S-4800 (Hitachi Company) electron microscopes. The crystalline structure of samples were characterized by X-ray diffraction (XRD) (Rigaku D/max-rA power diffractometer using Cu KR radiation (λ) 1.541 78 Å). The UCL spectra were detected with a Hitachi F-4500 fluorescent spectrometer. In UCL measurement, a 980 nm diode laser was used to pump the samples. The Fourier transform infrared (FTIR) absorption spectra were measured using a Shimadzu DT-40 model 883 IR spectrophotometer. In the measurement of UCL dynamics, a tunable Nd:YAG laser pumped OPO laser was used as excitation source. It had a pulse duration of 10 ns, repetition frequency of 10 Hz, and line width of 4-7 cm-1 and was fixed at 980 nm. A charge coupled device (CCD) detector combined with a monochromator was used for signal collection. 3. Results and Discussion 3.1. Crystal Structure and Morphology. The crystalline structure of the NaYF4 nanoparticles was characterized by the powder X-ray diffraction (XRD) technique. As shown in Figure 2, both the patterns of the unmodified NCs (Figure 2a) and the TGA-modified NCs (Figure 2b) can be well-indexed as pure hexagonal-phase (JCPDS 28-1192), and their detail origins are labeled in the figure. Based on the XRD data, both of the samples were well-crystallized. The average sizes of the asprepared and the surface-modified TGA/NaYF4 NCs were determined from Scherrer’s formula to be 18.2 and 18.3 nm, respectively, which are consistent with the FE-SEM images of the corresponding samples. Figure 3 shows the FE-SEM and size distribution histogram of the samples. It can be seen that both the as-prepared (Figure 3A) and the modified (Figure 3B) NaYF4 nanoparticles were uniform in size and morphology and well-dispersed. We can clearly see from the size distribution histogram of the samples (Figure 3C and D) that they both had a relatively narrow size

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Figure 2. XRD patterns of the unmodified (a) and TGA-modified (b) NaYF4:Yb3+, Er3+ nanoparticles. The hexagonal-phase NaYF4 pattern from the JCPDS database is also shown for reference. Figure 5. Scheme of surface modification of NaYF4 nanoparticles via TGA.

Figure 6. Room-temperature upconversion emission spectra of the unmodified (a) and TGA-modified (b) NaYF4:20%Yb3+, 2%Er3+ under the same 980 nm NIR excitation power density (2 W/mm2).

Figure 3. FE-SEM images of the unmodified (A) and TGA-modified (B) NaYF4:Yb3+, Er3+ nanoparticles and corresponding size distribution of the unmodified (C) and TGA-modified (D) NaYF4:Yb3+, Er3+ nanoparticles.

Figure 4. FTIR spectra of the unmodified (a) and TGA-modified (b) NaYF4:Yb3+, Er3+ samples.

distribution. So it should be noted that our strategy is very simple and no obvious adverse effects on the morphology phase are observed. 3.2. FTIR Analysis. As is well-known, Fourier-transform infrared (FTIR) spectroscopy is a powerful tool for identifying different types of chemical bonds in a molecule, since each bond produces a unique infrared absorption spectrum.31 As we can see from Figure 4, compared with the as-prepared NaYF4 samples (Figure 4a), the TGA-modified samples (Figure 4b) exhibit a broad band at around 3500 cm-1, corresponding to the -OH stretching vibrations.32 The peak at 1620 cm-1 refers to CdO stretching vibrations.33 The two peaks mentioned above come from the -COOH in TGA, whose molecular formula is

HS-CH2-COOH. The band at about 2936 cm-1 was assigned to the -CH2 stretching vibrations.34 It should be noticed that the spectra (Figure 4b) do not show the characteristic peak -SH at 2600 cm-1,35 suggesting -SH has been destroyed. A similar phenomenon was also observed in previous literature.35 There are two possibilities for the realization of water solubility in TGA-modified NaYF4 NCs. First, -SH is coordinated to Y3+ ion, and second, formed disulfides, HOOCCH2S-SCH2COOH, are coordinated to Y3+ ion with one carboxylate. Here, the inexistence of typical SdS bond around 650 cm-1 excludes the second possibility. The peak at 1380 cm-1 has a slight split, which can be attributed to the intensive coordinating interactions existing between -SH bond and Y3+ ions and further confirms that the TGA has been successfully modified on the surface of NaYF4 NCs by this strategy. The scheme of surface modification of NaYF4 nanoparticles via TGA was illustrated in Figure 5. 3.3. UCL and Population Mechanism in NaYF4:Yb3+, Er3+ NCs. In Yb3+ and Er3+ codoped systems, Yb3+ ions act as sensitizers and Er3+ ions as activators. Figure 6 shows the room-temperature UCL spectra of the NaYF4:20%Yb3+, 2%Er3+ (Figure 6a) and TGA/NaYF4:20%Yb3+, 2%Er3+ NCs (Figure 6b) under the same power density 980 nm laser excitation (2 W/mm2). In the figure, four groups of emission lines appear, peaking at 409, 525, 542, and 655 nm in turn, which are assigned to the 2H9/2-4I15/2 (blue), 2H11/2-4I15/2 (green), 4S 3/2-4I15/2 (green), and 4F9/2-4I15/2 (red) transitions of Er3+ ions, respectively. From Figure 6, it is interesting to observe that not only the relative intensity of the red (4F9/2-4I15/2), blue (2H9/24I 2 4 4 15/2) to the green ( H11/2, S3/2- I15/2) but also the intensity ratio of 2H11/2-4I15/2 to the 4S3/2-4I15/2 increase after surface modification. The intensity ratios R(r/g) of the red (4F9/24I 2 4 4 15/2) to the green ( H11/2, S3/2- I15/2) emission at different excitation power densities were also obtained, and R(r/g) as a function of power density was drawn as Figure 7. It can be observed that (1) the value of R(r/g) in the surface-modified sample is larger than that of the unmodified one all the time;

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Figure 7. Intensity ratio R(r/g) of red (4F9/2-4I15/2) emission to green (2H11/2, 4S3/2-4I15/2) emission as the increase of excitation power density.

Figure 8. Upconversion population processes in the Yb3+, Er3+ codoped system under 980 nm laser-diode excitation.

(2) R(r/g) increases obviously with the increasing excitation power density in the range of 0.5-2.0 W/mm2 and gradually approaches a saturation value as the power density increases continuously. It should be noted that within the experimental error ((20%) the absolute emission intensity for the green emission rarely had changed. Therefore, we can conclude that the involving of extra -COOH vibration modes in the TGAmodified samples has little influence on UCL intensity of the green emission. Unlike the present result, obvious quenching

Li et al. of the UCL was observed in the silica-coated NaYF4:Yb3+, Er3+ nanocrystals.36 The schematics of the populating and UCL processes for the blue, green, and red emissions were drawn in Figure 8. First, the Er3+ ion is excited from the ground state 4I15/2 to the excited state 4I11/2 via ET of neighboring Yb3+ and Er3+. Subsequent nonradiative relaxations of 4I11/2-4I13/2 populate the 4I13/2 level. In the second-step excitation, the same laser pumps the excited state atoms from the 4I11/2 to the 4F7/2 level via ET, or from the 4I 4 4 13/2 to F9/2 state via phonon-assisted ET. The populated F7/2 2 may mostly nonradiatively relax to two lower levels: H11/2 and 4S , which produce two green upconversion emissions. The 3/2 populated 4F9/2 level of the Er3+ ion most relaxes radiatively to the ground state 4I15/2 level, which causes red emissions. On the other hand, part of the populated 4F9/2 level may be excited to the 4G11/2 level via phonon-assisted ET, while the populated 4G 2 11/2 level nonradiatively relaxed to the H9/2 state, which caused blue emission (2H9/24I15/2). Besides, the CR process (4F7/2 f 4F9/2 and 4I11/2 f 4F9/2 of Er3+ ion) should also exist because the doping concentrations of Yb3+(20%) and Er3+(2%) are not low.37 The energy gap ∆E between 4I11/2 and 4I13/2 is ∼3600 cm-1. In the traditional bulk fluoride materials the largest phonon energy of the hosts is much smaller than this gap; in this case, according to the theory of multiphonon relaxation, the nonradiative relaxation of 4I11/2-4I13/2 hardly happens; therefore, the red emission is negligible and the green emission is dominant. While in the NCs, a large amount of OH- and CO32- is involved due to the surface adsorption, the existence of these bonds with large phonon energy can effectively bridge the gap between 4I 4 11/2- I13/2 through one or several photons, and thus the red emissions of 4F9/2-4I15/2 effectively generate, which depends strongly on the particle size of the NCs.38 In the TGA-modified NaYF4:Yb3+, Er3+ NCs, as has been observed from FTIR absorption spectra, some large phonon modes such as CdO, -CH2, and -OH were purposely involved, which leads the nonradiative relaxation probability of 4I11/2-4I13/2 to increase more than that in the unmodified NCs, resulting in the increase of 4F9/2-4I15/2 red emissions.38 As for the increase of the ratio of 2H11/2-4I15/2 to the 4S3/2-4I15/2 in the modified NCs, it can be attributed to the increased local thermal effect, which will be discussed in detail in section 3.5.

Figure 9. UC decay dynamics of the blue (a) and (b), green (c) and (d), and red (e) and (f) emissions in the unmodified NaYF4:20%Yb3+, 2%Er3+ and TGA/NaYF4:20%Yb3+, 2%Er3+ samples, respectively.

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dNYb2 ) -KNYb2 - RYbNYb2 dt dN6 ) K36NYb2 - γ65N6 - CN6 dt dN5 ) γ65N6 - R5N5 dt dN4 ) K24NYb2 - R4N4 + CN6 dt

Figure 10. UC rising dynamics of green (a) and (b) and red (c) and (d) emissions in the unmodified NaYF4:20%Yb3+, 2%Er3+ and TGA/ NaYF4:20%Yb3+, 2%Er3+ samples, respectively.

3.4. Upconversion Luminescent Dynamics. Figure 9 shows the luminescence decay dynamics of the blue (2H9/2-4I15/2), green (2H11/2-4I15/2, 4S3/2-4I15/2), and red (4F9/2-4I15/2) emissions in the unmodified and TGA-modified samples at room temperature. It is clear that for all the emissions the intensity decayed exponentially. The exponential lifetime constants in the unmodified and the modified samples were deduced, respectively, to be 137 and 227 µs for the blue, 203 and 201 µs for the green, and 370 and 356 µs for the red. We can clearly see that the lifetime constants of the green and red emissions had only a slight decrease (4%), while that of the blue emission remarkably increased (66%) after surface modification. Because in the unmodified and TGA-modified samples the crystalline structure is the same, we have reason to consider that the radiative transition rates of all the f-f transitions do not change. Therefore, the slight decrease of lifetime constants for the red and the green emissions could be attributed to the slight increase of the nonradiative transition rates. The slight increase of nonradiative transition rate hardly quenches the UCL. As to the increase of the lifetime constant of the blue 2H9/2-4I15/2 emission, its origin has not been clarified. We propose that as one excitation pulse ended, in the TGA-modified sample more electrons populated on the media states of 2H9/2, such as 4F9/2 and 4G11/2, which could continuously supply electron populations to the 2H9/2, postponing the decay of 2H9/2. The rising dynamics for the red and green emissions in the TGA-modified sample were also studied in contrast to the unmodified one, as shown in Figure 10. All curves can be well fitted by the following equation

I(t) ) I0 exp(-t/τ0) - I1 exp(-t/τ1)

(1)

where I(t) denotes the emission intensity as a function of time, and I0 and I1 are both positive constants. By fitting, the rising time constants τ1 in the unmodified and TGA-modified samples were respectively deduced to be 17.0 and 5.9 µs for the green emission and 45.5 and 25.2 µs for the red emission. Obviously, the rising time constants in the TGA-modified sample became shorter in contrast to the unmodified sample, for both the green and red emissions. In order to better understand the luminescent dynamics, the rate equations were set up based on the model drawn in Figure 8.

(2) (3) (4) (5)

where NYb2, RYb, and K denote, respectively, the population density of excited state Yb3+ at any time, the total transition rate of Yb3+, and the total ET rate from Yb3+ to Er3+. Ni (i ) 4, 5, 6) represents the population of Er3+ on level |i〉. Ri (i ) 4, 5) is the total decay rate of Er3+ on level |i〉. Kij is the branch ET rate from Yb3+ to Er3+, corresponding to the populating process from level |i〉 to |j〉 of Er3+ ion. γ65 is the nonradiative relaxation rate from level |6〉 to |5〉 of Er3+. C is the CR rate of 4F 4 4 4 3+ 7/2 + I11/2 f F9/2 + F9/2 for Er . It should be noted that in the rate equations Kij (K ) ∑Kij) is proportional to the Er3+ population density on the |i〉 level; for simplification, here we have assumed that K and Kij are both proportional to the total density of Er3+.39 Similarly, the cross-relaxation rate C has also been proposed to be proportional to the total density of Er3+. Based on eqs 2-5, the population densities for the green and the red levels at any time t can be deduced as

N5(t) ) R0 · e-R5 · t - R1 · e-[(1+γ65+C)(1+K+RYb)-1] · t

(6)

N4(t) ) R2 · e-R4 · t - R3 · e-(K+RYb) · t R4 · e-[(1+γ65+C)(1+K+RYb)-1] · t (7) with

R0 )

γ65 · K36 · NYb2(0) (1 + γ65 + C)(1 + K + RYb) - (1 + R5)

+ N5(0)

(8) R1 ) R2 )

γ65 · K36 · NYb2(0) (1 + γ65 + C)(1 + K + RYb) - (1 + R5)

(9)

C · K36 · NYb2(0)

+ (1 + γ65 + C)(1 + K + RYb) - (1 + R4) K24 · NYb(0) + N4(0) (10) K + RYb - R4 R3 )

C · K36 · NYb2(0) (1 + γ65 + C)(1 + K + RYb) - (1 + R4) R4 )

K24 · NYb2(0) K + RYb - R4

(11) (12)

Based on the solutions in eqs 6-12, we can conclude that for both the green and the red emissions, there exists only one decay time constant. For the green emission, there exists one rising time constant, τ1 ) 1/((1 + γ65 + C)(1 + K + RYb) 1), which depends not only on the total ET rate from Yb3+ to Er3+ and the transition rate of Yb3+ but also on the relaxation rate γ65 and the cross-relaxation rate. For the red emissions, there are two rising time constants, τ1 and τ2 ) 1/(K + RYb) (τ2 > τ1). In Figure 9, the experimental result shows that the rising dynamics for the red emissions can be well-fitted by one rising time constant, suggesting that one rising process may contribute dominantly to the UCL of the red.

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TABLE 1: Variation of the Rise and Decay Time Constants with Different Yb3+ Concentrations in the As-Prepared NaYF4:Yb3+, Er3+ NCs Yb3+ concentration (mol %) green red

τ0(µs) τ1(µs) τ0(µs) τ1(µs)

0

2

5

10

20

267 0 508 0

717 76.2 1240 156.2

466 37.5 623 66.4

241 18.9 405 55.3

203 17.0 370 45.5

Comparing the theoretical result with the experimental, we can draw the following three conclusions: (1) The decay time constants R4 and R5 rarely change after surface modification, implying that the nonradiative transition rates for the green and red levels rarely increase, and the radiative transition rates contribute dominantly to the decays of the red and the green levels. (2) Because of the contribution of rising time constant τ2 (τ2 > τ1) to the red emissions, it is easy to understand that in the experiment the rising time of the red emissions is always longer than that of the green ones. (3) In the modified sample, any increase of γ65, K, C, or RYb all would lead to the shortness of the rising time constants. We tend to consider that in the surface-modified sample γ65 would increase considerably, because of the low energy gap between 4F7/2 and 2H11/2/4S3/2 (∼1500 cm-1) and the involving of large vibration bonds CdO. Considering the spectral broadening, the vibration energy of CdO closely matches the energy gap of 4F7/2-2H11/2. In addition, the involvement of extra large vibration modes probably increases the phonon-assisted ET processes. The CR process is concentration dependent, and in our case, after TGA surface modification, the doping concentrations of Yb3+ and Er3+ ions are not changed, so the CR rate should have little variation. As to the electronic transition rate RYb of 2F5/2-2F7/2 for Yb3+ should rarely change; the reason is that similar decay time constants of the red and green emissions for Er3+ did not change either. In order to deeply understand the UCL processes, we also measured the UCL dynamics in the as-prepared NaYF4:Yb3+, Er3+ samples with a variety of Yb3+ doping concentrations, and Er3+ concentration was fixed at 2% (mol). Table 1 lists the variation of the rise as well as the decay time constants with different Yb3+ concentrations for the green and red emissions. It can be seen that in Er3+ single doped sample, the rise process does not appear in the studied time scale, for both the red and green emissions. This implies that in the Yb3+ and Er3+ codoped samples the rising process is absolutely related to the ET from Yb3+ to Er3+. As the Yb3+ doping concentration increases, the rise time constant becomes shorter. As the Yb3+ ion concentration increases, the ET excitation for Er3+ becomes effective, and at the same time, the CR process (4F7/2 f 4F9/2 and 4I11/2 f 4F9/2 of Er3+ ion) particularly increases, leading to the increase of K and C and the decrease of rise time constants. In Table 1, it can be also seen that the decay time constants for the red and the green emissions both largely decrease with the increasing Yb3+ concentration, which could mainly be attributed to the back ET from Er3+ to Yb3+.40 In the setting up of rate equations, the back ET processes from Er3+ to Yb3+ were not considered. 3.5. Local Thermal Effect. In nanosized materials, the electron-phonon interaction becomes stronger in comparison to that in the bulk. The thermal effect caused by the exposure of the 980 nm diode laser has to be considered. For Er3+ ions, the energy separation between the nearest excited states 2H11/2 and 4S3/2 is only several hundred wave numbers, and the population distribution on 2H11/2 and 4S3/2 should be dominated by thermal distribution. Elevated temperature leads to rapid

Figure 11. Intensity ratio (RHS) of 2H11/2-4I15/2 to 4S3/2-4I15/2 as a function of excitation power.

population of 4S3/2-2H11/2. Therefore, the intensity ratio (RHS) of 2H11/2-4I15/2 to 4S3/2-4I15/2 is sensitive to temperature, and RHS is a critical parameter to discuss the temperature change in UCL processes. Figure 11 shows RHS as a function of excitation power. It is obvious that RHS increased with the increase of excitation power density. The intensity ratio of RHS dominated by Boltzmann’s distribution can be written as

RHS ) RHS(0) exp(-∆E/kT)

(13)

where RHS(0) is a constant, ∆E is the energy separation between the 2H11/2 and the 4S3/2 levels (740 cm-1), k is Boltzmann’s constant, and T is the absolute temperature. In our previous work,41 we have successfully speculated that at the irradiated spot the temperature increase is proportional to the adsorbed IR power density; therefore, RHS as a function of excitation power density can be expressed as

RHS ) RHS(0) exp[-∆E/k(RI+β)]

(14)

where R represents the rate of temperature increase as the increase of excitation power density, I denotes the power density, and β is a temperature constant. As the excitation power is zero, the sample temperature is room temperature, so β can be determined to be ∼293 K. In Figure 11, the dots are experimental data and the solid curves are fitting functions based on eq 14. By fitting, we got R ) 11.6 (K0mm2/W) for previouslyprepared NaYF4 samples, while R ) 17.5 (K0mm2/W) for the TGA-modified NaYF4 samples. By fitting, we can also deduce the dependence of practical temperature at the irradiated spot on excitation power density in the unmodified and TGAmodified NCs, as follows:

T ) 11.6*I + 293 T ) 17.5*I + 293

(unmodified NCs) (modified NCs)

We attributed the significant enhancement of R and increased local thermal effect in the TGA/NaYF4 samples to the involving of numerous surface defects with high phonon energy, such as -OH bonds, CdO bonds, and -CH2 bonds. As a consequence, electron-photon coupling processes became stronger and thus more electronic energies were transferred to the lattices of the host, leading to the larger increase of temperature. 3.6. Unusual Power Dependence of UCL. At low excitation power densities, for nearly any unsaturated UC, the visible upconverted intensity (If) is proportional to some power (n) of the infrared excitation (Ip) power density:42,43 If ∝ Ipn, where n is the number of IR photons absorbed per visible photon emitted. For the two-photon green and red emissions of Er3+ ions, n is approximately equal to 2 in general cases. However, at high pump power densities, the fluorescence intensity becomes independent of the excitation power density. Ghosh et al. demonstrated this phenomenon by solving the rate equations

NaYF4:Yb3+, Er3+ Nanoparticles

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8225 thermal effect in the TGA-modified sample, it is not difficult to understand that in the TGA-modified sample the slopes in the ln-ln plot of power dependence of UCL intensity deviated more from the typical n values for two-photon or three-photon processes in contrast to the unmodified sample. 4. Conclusion

Figure 12. Logarithm plots of the emission intensity of the (a) unmodified NaYF4:20%Yb3+, 2%Er3+ and (b) TGA/NaYF4:20%Yb3+, 2%Er3+ NCs as a function of excitation power density.

based on the excitation and emission to analyze the probabilities that lead to the unusual power-dependent UCL behavior.43 Some previous studies indicated that the power dependencies of the Er3+ UCL became linear (n ) 1) at high excitation densities due to “saturation” of the UC processes.44,45 In the present NaYF4:Yb3+, Er3+ NCs, we also observed that the UCL intensities for both the red and green emissions of Er3+ did not obey the traditional power law (If ∝ Ipn) in the measured power density range. In order to determine the number of photons responsible for the UC mechanism, the intensities of the UCL were recorded as a function of the 980 nm excitation intensity (Figure 12). As seen in Figure 12, the green and red Er3+ UCL intensities demonstrated quadratic power dependencies at low excitation densities. In the as-prepared NaYF4 NCs, the blue emission yielded a slope of 1.77, while the green and the red emissions yielded slopes of 1.50 and 1.51, respectively. After TGA modification, all of the slopes decreased. For blue emissions, it decreased to 1.25, and for green and red emissions, it decreased to 1.10 and 1.19, respectively. It can be seen that for all the emissions, the slopes largely deviated from the typical values of 2 or 3 for two-photon or three-photon UC population processes but was closest to the value of 1. This can be partly attributed to the competition between the linear decay and the UC processes for the depletion of the intermediate excited states, which is described by M. Pollnau et al. theoretically,44,45 the so-called “saturation” phenomenon. Here, we should emphasis that the local thermal effect probably contributed to the slope deviation also. As the exposure power increases, the local thermal effect will lead to the gradual increase of the temperature at the irradiated spot, which will lead to the increase of nonradiative transition and the decrease of luminescent quantum yield. Theoretically, we have

If ∝ Inp*[WR /(WR + WNR)]

(15)

In eq 15, if the nonradiative transition rate that depends on temperature is considered as a multiphonon relaxation process, and the temperature at the irradiated spot is proportional to the adsorbed IR power density, after one-order Taylor approximation we can deduce that

If ∝ Inp /(RI + β)P

(16)

with P ) ∆E/hw, where ∆E is the energy separation between the emission level and the nearest down-level and hw is the phonon energy. According to eq 16 and the increased local

In summary, we have succeeded in preparing upconverted lanthanide-doped NaYF4 monodisperse NCs. Thioglycollic acid (TGA) was introduced as the ligand to render NaYF4 nanoparticles well-dispersed in aqueous solutions. The conversion procedure is very simple and does not change the size and shape of particles. We also systematically compared the UCL properties of the unmodified and the TGA-modified NaYF4:Yb3+, Er3+ NC under 980 nm excitation. The results demonstrate that the blue (2H9/2-4I15/2) and red (4F9/2-4I15/2) emission with respect to the green (2H11/2, 4S3/2-4I15/2) emission increased after surface modification, while the overall UCL intensity did not change. The luminescent dynamics show that the decay time constants of the green and the red emissions had only a little decrease, while the rising time constants of them decreased considerably. Rate equations were also set up to analyze the UCL dynamics. In the modified NCs, local thermal effect induced by laser irradiation was enhanced due to the involvement of large phonon bonds. This work is helpful for physically understanding the dependence of UCL properties on surface modification. Acknowledgment. The authors are thankful for the financial support of the High-Tech Research and Development Program of China (863) (Grant No. 2007AA03Z314) and the National Natural Science Foundation of China (Grant Nos. 50772042 and 10704073). References and Notes (1) Chan, W.; Nie, S. Science 1998, 281, 2016. (2) Gao, X.; Cui, Y.; Richard, M. L.; Leland, W. K. C.; Nie, S. Nat. Biotechnol. 2004, 22, 969. (3) Chatterjee, D. K.; Rufaihah, A. J.; Zhang, Y. Biomaterials 2008, 29, 937. (4) Sadhu, S.; Patra, A. ChemPhysChem. 2008, 9, 2052. (5) Wang, L. Y.; Yan, R. X.; Huo, Z. Y.; Wang, L.; Zeng, J. H.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 6054. (6) Chatterjee, D. K.; Zhang, Y. Nanomedicine 2008, 3, 73. (7) Ayman, K.; Hitesh, H.; Mao, G. Z.; Jayanth, P. Y. Eur. J. Pharm. Biopharm. 2009, 71, 214. (8) Wu, X.; Liu, H.; Liu, J. Nat. Biotechnol. 2003, 21, 412. (9) Yi, G. S.; Sun, B. Q.; Yang, F. Z.; Chen, D. P.; Zhou, Y. X.; Cheng, J. Chem. Mater. 2002, 14, 2910. (10) Matsuura, D. Appl. Phys. Lett. 2002, 81, 4526. (11) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. J. Phys. Chem. B 2002, 106, 1909. (12) Vetrone, F.; Boyer, J. C.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2003, 107, 1107. (13) Yi, G. S.; Chow, G. M. AdV. Funct. Mater. 2006, 16, 2324. (14) Kra¨mer, K. W.; Biner, D.; Frei, G.; Gu¨del, H. U.; Hehlen, M. P.; Lu¨thi, S. R. Chem. Mater. 2004, 16, 1244. (15) Li, Z. Q.; Zhang, Y. Angew. Chem., Int. Ed. 2006, 45, 7732. (16) Chow, G.; Yi, G. S. Chem. Mater. 2007, 19, 341. (17) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (18) Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. (19) Ghosh, P.; Patra, A. J. Phys. Chem. C 2008, 112, 3223. (20) Shan, J. N.; Uddi, M.; Wei, R.; Yao, N.; Ju, Y. G. J. Phys. Chem. C 2010, 114, 2452. (21) Ghosh, P.; Patra, A. J. Phys. Chem. C 2008, 112, 19283. (22) Ghosh, P.; Kar, A.; Patra, A. J. Phys. Chem. C 2010, 114, 715. (23) Li, Z. Q.; Zhang, Y. Nanotechnology 2008, 19, 345606. (24) Jalil, R. A.; Zhang, Y. Biomaterials 2008, 29, 4122. (25) Liu, Z.; Yi, G.; Zhang, H.; Ding, J.; Zhang, Y.; Xue, J. Chem. Commun. 2008, 6, 694.

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