Surface Modification of ZrO2:Er3+ Nanoparticles to Attenuate

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J. Phys. Chem. C 2008, 112, 2836-2844

Surface Modification of ZrO2:Er3+ Nanoparticles to Attenuate Aggregation and Enhance Upconversion Fluorescence Qiang Lu1 ,*,†,‡ FengYun Guo,† Liang Sun,† Aihua Li,§ and LianCheng Zhao† Department of Material Physics and Chemistry, School of Material Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China, Center of Electron Microscope Technology, Mudanjiang Medical College, Mudanjiang, 157011, People’s Republic of China, and Center for the Condensed Matter Science and Technology, Department of Applied Physics, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China ReceiVed: September 17, 2007; In Final Form: NoVember 20, 2007

ZrO2:Er3+ nanoparticles are synthesized and further modified via a ligand-capped/ligand-exchanging method with TEOS, APTES, and SA. Antiaggregation investigations using TEM and FT-IR indicate that severe aggregation can be reduced by adhering ammonic or carboxylic functional groups to the nanoparticle surfaces. The upconversion fluorescence spectra of nonmodified and the modified nanoparticles with the same peak splitting and positions show that local crystalline environments in which the Er3+ ions are embedded are identical before and after modifying the surfaces. The remarkable upconversion fluorescence enhancements of 4.7 and 1.5 times for amine- and carboxyl-modified nanoparticles are observed under the same excitation power densities, respectively. An enhancement mechanism of upconversion luminescence, in which an asymmetric association crystalline field from both the degenerated crystalline field of the host interface and a complementary crystal field of the SiO2 shell can make the ‘dormant’ rare earth ions on nanoparticle surfaces be activated, is presented. In addition, the improved spontaneous emission rate of Er3+ ions due to the enhanced local classical density of states and organic ligands with high vibrational energy on the nanoparticle surfaces are also considered. Thus, intense upconversion fluorescence and hydrophilicity via ammonic or carboxylic functional groups will provide the doped core-shell nanoparticles great potential as biolabels in the future.

Introduction Rare earth (RE) doped nanoparticles will eventually become excellent competitors as biological fluorescence labels because of their unique merits, such as low photobleaching, multicolor labeling, and excitation under near-infrared (NIR) light.1-5 To obtain successful and reproducible detection of biological targets, the prerequisite for biological applications of rare earth doped nanoparticles is to gain water-soluble, biocompatible, and photostable nanoparticles. However, most of the doped nanoparticles are hydrophobic in physiological salt solutions and do not present functional groups on the nanoparticle surfaces for attachment of biomolecules. Thus, the following surface modifications with different functional groups are required to prevent the nanoparticles from aggregating if the doped nanoparticles will be used for biological applications. Surface modifications with active functional groups can be obtained to increase the general solubility of the doped nanoparticles in various solvents.6-8 Moreover, although the biological applications of the nanoparticles are preliminary, surface-modified nanoparticles can be more readily conjugated with biomolecules and used as highly fluorescent, sensitive, and reproducible biolabels in bioanalytical and biomedical applications.9-11,12 Another one of preconditions for surface modifications is that upconversion fluorescence properties of the doped nanoparticles * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Material Physics and Chemistry, Harbin Institute of Technology. ‡ Mudanjiang Medical College. § Department of Applied Physics, Harbin Institute of Technology.

cannot be largely reduced after modifying surfaces because the degenerated fluorescence properties might limit their biological applications. Fortunately, some researchers12-15 recently have found that upconversion fluorescence can be improved via the core-shell structure, which provides a core structure with inside crystalline environment for rare earth ions to emit upconversion fluorescence efficiently and a shell structure for surface modification to make them water soluble and biocompatible for specific targeting. This is of significance for their practical applications. It has been suggested that the organic ligands with high-energy C-H and C-C vibrational oscillators around rare earth ions on the nanoparticle surfaces are fluorescence quenchers.2 Shell structures with low phonon energy can greatly suppress energy transfer to the particle surface to reduce the quenching.13-15 In this paper, we report the synthesis of ZrO2:Er3+ nanoparticles via the Pechini process. Subsequently, these nanoparticles are modified with different functional groups to improve the solubility in solvents. Another enhancement mechanism of upconversion fluorescence via the core-shell treatment is presented to develop the doped core-shell nanoparticles with excellent properties. Spontaneous emission rate of rare earth ions due to the enhanced local classical density of states and effect of shell thickness on upconversion fluorescence are discussed, respectively. Experimental Section 1. Chemicals. All reagents used were purchased from Sinopharm Chemical Reagents Co., Ltd., P.R. China, and used

10.1021/jp077498c CCC: $40.75 © 2008 American Chemical Society Published on Web 02/01/2008

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Figure 1. Upconversion emission spectra of the nonmodified (bottom) and amine- (top)/carboxyl-modified (middle) ZrO2:Er3+ nanoparticles under excitation power density of 6.8 × 102 W/cm2.

SCHEME 1: Schematic Surface Modification Procedure of ZrO2:Er3+ Nanoparticles

without further purification. Reagents were of analytical grade, except for erbium oxide (Er2O3, 99.99%). 2. Nanoparticle Synthesis. ZrO2 colloidal suspension doped with Er3+ ions was synthesized according to the Pechini type sol-gel method described by B. L. Cushing16 and Cuikun Lin17 with a slight modification: 0.0252 g of Er2O3 and 6.0132 g of ZrOCl2‚8H2O with molar ratios Er:Zr ) 0.7:99.3 were dissolved in dilute hydrochloric acid solution (1:1 by volume). Citric acid (70.0 g) (C6H8O7‚H2O) and 90 mL of glycol (C2H6O2) were added, respectively. After this procedure, 5.0 g of polyethylene glycol 20000 (HO(CH2CH2O)nH, molecular weight g20000) as cross-linking agent was also added. The turbid suspension was heated at 80 °C and stirred for 8 h to form a gel. The obtained gel was preheated at 300 °C for 2 h and then sintered at 800 °C for 1 h in air to remove organic materials and form final powder samples. 3. Surface Modification. The doped nanoparticles modified with amine (NH2) and carboxylate (COOH) were used to study the effect of surface modification of nanoparticles on attenuating aggregation and enhancing upconversion fluorescence. The schematic surface modification procedure is illustrated in Scheme 1. (1) Surface modification with amine group. ZrO2:Er3+ nanoparticles (0.2887 g) as core and 100 mL of isopropyl alcohol as solvent were added into a 200 mL flask. The flask was shaken with ultrasonic oscillation for 30 min to disperse the nanoparticles. Under vigorous stirring, 9 mL of deionized water and 12 mL of hydrous ammonia (25%) were mixed with the solution

and heated for 10 min at 34 °C. Then 289 µL of tetraethyl orthosilicate (TEOS) was added into the flask, and the reaction was allowed to continue for another 30 min under the same conditions. Subsequently, 3 mL of chloroform and 300 µL of 3-aminopropyltriethoxysilane (APTES) were added into the flask and vigorously stirred under the same conditions. After 60 min, reaction mixture was taken out and precipitated by centrifuge. The supernatant was discarded, and the nanoparticles were rinsed twice with deionized water to form the reaction product. A part of reaction product was used as the amine-modified sample for further investigation. (2) Surface modification with carboxyl group. Succinic anhydride (SA) (65 mg) was dissolved into 100 mL of PB buffer solution (pH ) 11.0) in a 200 mL flask. The other reaction product was added into the flask and dispersed for 3 min by ultrasonic oscillation. Under vigorous stirring, the reaction was allowed to continue for another 7 h at room temperature. Subsequently, the reaction mixture was taken out and precipitated by centrifuge. The supernatant was discarded, and the nanoparticles were rinsed twice with PB buffer solution (pH ) 6.4) and deionized water, respectively. The reaction products were dried at 110 °C for 24 h. 4. Characterization. The crystalline phases of nonmodified and amine-/carboxyl-modified ZrO2:Er3+ nanoparticles were identified with X-ray diffraction (XRD). The XRD patterns of samples were obtained with a Rigaku D/max-γB X-ray diffractometer system in a conventional 2θ reflection geometry using

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Figure 2. Energy levels of Er3+ with Stark splitting of 4S3/2 and 4I15/2 levels in cubic ZrO2:Er3+nanoparticles and the upconversion mechanism.

response) with an external diode laser (976.1 ( 2.9 nm, 1.3 W, continuous wave with 1 m fiber (N.A: 0.22 and Ø100 µm), Beijing Hi-Tech Optoelectronics Co., Ld, P.R. China) as the excitation source in place of the xenon lamp in the spectrometer. The measurements were performed at room temperature. Results and Discussion

Figure 3. The calculated p values of ZrO2:Er3+ nanoparticles between different energy levels.

Cu KR radiation (λ ) 0.15418 nm). The crystallite sizes of the nanoparticles were calculated using the Scherrer equation:

Dhkl ) Kλ/β cos θ

(1)

where K ) 0.9; λ is the wavelength of Cu KR radiation, β is the corrected half width of the diffraction peak, and Dhkl represents the size along (hkl) direction. Fourier transform infrared (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the potassium bromide (KBr) pellet technique. In making the KBr pellets, 1 mg of sample was diluted with approximately 100 mg of KBr powder. Each FT-IR spectrum was collected from 300 to 4000 cm-1. Absorption spectrum was measured using a Varian Cary 100 Conc UV-visible spectrophotometer. The sample morphologies were inspected with a transmission electron microscope (TEM, JEOL-1010, accelerating voltage 100 kV, resolution of point image 0.4 nm). The samples for TEM observation were prepared via dropping dilute solutions of the isolated ZrO2:Er3+ nanoparticles in ethanol onto 300-mesh carbon-coated copper grids and immediately evaporating the solvent in air. The samples were stable under the electron beam and did not degrade within the typical observation time. Upconversion emission spectra were recorded by the Hitachi F-2500 fluorescence spectrophotometer (instrument parameters: 2.5 nm for excitation slit, 2.5 nm for emission slit, 400 V for PMT voltage, and 0.04 s for

1. Upconversion Luminescence. A pure green emission has been observed even with the naked eye during a 976 nm CW diode laser irradiation. Figure 1 presents the emission spectra of nonmodified and amine-/carboxyl-modified ZrO2:Er3+ nanoparticles. Since the emission spectra for all samples manifest similar characteristics, it is reasonable to ascribe the emissions to the same origins. Upconversion mechanisms are identical and described as follows: the energy level diagram of Er3+ ion is used to interpret the upconversion mechanism, as shown in Figure 2. The wavelength of the 976 nm diode laser resonantly matches well with the transition of Er3+ ions between the ground state 4I15/2 and the excited state 4I11/2; thus, Er3+ ion at the ground state 4I15/2 is excited to the excited state 4I11/2 by absorbing a photon. Energy transfer (ET) takes place from an excited state Er3+ ion to another excited state Er3+ ion nearby.

Er3+:4I15/2 + hV f Er3+:4I11/2 (GSA) Er3+:4I11/2 + Er3+:4I11/2 f Er3+:4I15/2 + Er3+:4F7/2 (ET) Although excited-state absorption (ESA) of Er3+ also occurs, this process is inappreciable due to less efficiency.

Er3+:4I11/2 + hV f Er3+:4F7/2 (ESA) It is well-known that the lifetime of the excited-state is determined by radiative and nonradiative depopulation processes. The radiative rate constants are largely determined by the electronic structure of rare earth ion. Nonradiative depopulation processes, such as multiphonon relaxation (MR), depend on the maximum phonon energy of the host matrix. Multiphonon relaxation is dominant according to the energy-gap law when p ) ∆E/pω e5,18 where the electronic energy gap is ∆E and pω is maximum phonon energy of the host matrix. The calculated p values of ZrO2:Er3+ nanoparticles between parts of energy levels are shown in Figure 3. Thus, population at level Er3+: 4F 2 4 7/2 can nonradiatively decay to the level H11/2 and S3/2 due

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Figure 4. Double logarithmic plots of the integrated intensities of the upconversion emission from level 2H11/2 and 4S3/2 to level 4I15/2 obtained under 976 nm laser excitation.

Figure 5. TEM photographs of the nonmodified (a) and amine-(b)/carboxyl-modified (c) ZrO2:Er3+ nanoparticles.

to less than five times the phonon energy of the ZrO2 lattice (470 cm-119). However, the nonradiative contribution to the population at level 4F9/2 from the above excited level 4S3/2 can be neglected due to more than six times the phonon energy of the ZrO2 lattice. The level 4F9/2 is also not populated by the above level 2H11/2 due to more than eight times the phonon energy of the ZrO2 lattice. Therefore, Er3+ ion at the excited state 2H11/2 and 4S3/2 radiatively returns to the ground state 4I15/2, emitting the green upconversion luminescence. Upconversion emission bands at 525 and 548 nm are assigned to transitions from level 2H11/2 and 4S3/2 to level 4I15/2. Red upconversion emission from level 4F9/2 to 4I15/2 can hardly occur, which agrees well with our experimental results. Obviously, the green upconversion luminescence is a twophoton process of energy transfer. To better understand the upconversion mechanism and further demonstrate the twophoton process, we have investigated the upconversion emission intensities of dependence on laser pump power. For an unsaturated upconversion process, upconversion emission intensity Iup depends on the incident pump power Ppump according to the relation of Iup ∝ Ppumpn, where n is the number of photons involved in the pumping process.20 The upconversion emission intensities of nonmodified and the modified ZrO2:Er3+ nanoparticles were integrated for different pump powers, respectively. Figure 4 shows double logarithmic plots of the integrated intensities of the upconversion emissions as a function of laser pump power. Their slopes are 1.89, 1.82, and 1.76, respectively.

These slope values for nonmodified and amine-/carboxylmodified nanoparticles further confirm our analysis that the green upconversion luminescence indeed is a two-photon upconversion process. 2. Antiaggregation Investigations of the Modified ZrO2: Er3+ Nanoparticles. The most obvious evidence confirming the effect of surface modification on attenuating aggregation of nanoparticles comes from TEM photographs. Figure 5 shows the TEM photographs of nonmodified and amine-/carboxylmodified ZrO2:Er3+ nanoparticles. The average particle size of ZrO2:Er3+ nanoparticles is about 20-30 nm. The morphologies of these nanoparticles in Figure 5a mainly consisted of tightly agglomerated particles with irregular shapes. The aggregation of these nanoparticles has limited their biological applications. The aggregation phenomenon is mainly attributed to the larger specific surface area of nanoparticles and less hydrophilic functional groups on the nanoparticle surfaces in solutions. Water dispersibility and bioconjugation with active functional groups of the nanoparticles have become important problems for their applications as bioprobes. Surface modifications with high hydrophilic or active functional groups are needed, which can make the nanoparticles water soluble, biocompatible, and connectible with specific groups. Thus, an important factor in the design of surface-modified nanoparticles for biological applications is the controlled covalent attachment of desired functional groups on the nanoparticle surfaces. As shown in Figure 5b and 5c, one- and multicore are coated by shell

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Figure 6. FT-IR spectra of nonmodified (a) and amine- (b) /carboxylmodified (c) ZrO2:Er3+ nanoparticles.

structures. Average sizes of amine-/carboxyl-modified ZrO2: Er3+ nanoparticles are about 100-200 nm, and the coated shell thicknesses are about 40-50 nm. In addition, the well-dispersed nanoparticles show that the amine-/carboxyl-groups bound to the nanoparticle surfaces can indeed prevent the nanoparticles from aggregating. The hydrophobic nanoparticles were rendered hydrophilic by coating with the amine or carboxyl groups and easily dispersed in polar solvents such as ethanol and water to form stable solutions. The other evidence confirming the existence of amine-/ carboxyl-groups on the nanoparticle surfaces comes from FTIR vibrational spectroscopy. The FT-IR spectra of nonmodified and amine-/carboxyl-modified ZrO2:Er3+ nanoparticles are shown in Figure 6. Similarities and differences are clearly obtained by comparing these spectra. All samples exhibit a broad absorption band at 3430 cm-1, which are attributed to the nanoparticle surface hydroxyl group and strongly absorbed molecular water. In particular, for the nonmodified nanoparticle, hydroxyl group adhered to the particle surface provides the opportunity to link covalently organic ligands to the nanoparticle surface. For the amine-modified nanoparticle, because the broad absorption band of hydroxyl group at 3430 cm-1 fully covers over the narrow absorption band of N-H, the absorption band of N-H cannot be distinguished easily. For the carboxylmodified nanoparticle, the absorption band at 1739 cm-1, corresponding to the stretching vibration of CdO, with the addition of O-H at 3430 cm-1 shows that there are carboxyl groups available on the nanoparticle surfaces. The results above confirm the formation of amine or carboxyl groups on the nanoparticle surface. It is worth noting that several characteristic absorption bands at 1090, 953, 741, and 515 cm-1 are clearly observed. They are attributed to Si-O-Si,21 Si-O-Zr,22-24 and Zr-O25 vibrations, respectively. An absorption band at 953 cm-1 indicates that Si-O-Zr chemical bond has been established between the SiO2 shell and the ZrO2 core surfaces. This means that the nonmodified nanoparticles link amine- or carboxyl-groups via a SiO2-coated shell structure to their surfaces. TEM photographs are also consistent with the result. Thus, we conclude that the amine-/carboxyl-groups via the SiO2 interlayer are grafted on the nanoparticle surfaces. To better understand the effect of surface modification on the phase structures of nanoparticles, the crystalline structures of nonmodified and amine-/carboxyl-modified ZrO2:Er3+ nanoparticles are determined using XRD, and the XRD patterns are shown in Figure 7. For nonmodified and amine-/carboxylmodified nanoparticles, the diffraction peaks at 2θ ) 24.020

Lu¨ et al. (011, M), 24.480 (-110, M), 28.140 (-111, M), 30.160 (111, T, the strongest), 31.360 (111, M), 34.440 (002, T), 35.180 (200, T), 50.160 (-220, M), 50.720 (-122, M), 59.260 (311, T), 62.800 (311, M), 74.680 (400, M), 81.760 (331, T), 82.480 (204, T), and 84.860 (420, T) can be well indexed to the JCPDS card 17-0923 for Tetragonal ZrO2 and 13-0307 for monoclinic ZrO2. The results show a high crystallinity of the nanoparticles and also indicate that surface modification with amine-/carboxylgroups does not affect phase structures of the nanoparticles. The mean particle sizes of 21.899 nm for the nonmodified ZrO2 nanoparticles, 18.424 nm for the amine-modified nanoparticles and 16.670 nm for carboxyl-modified nanoparticles are calculated from different half widths of the diffraction peaks at 2θ ) 30.160 by using the Scherrer formula assuming spherical nanoparticles. The calculated results agree well with TEM photograph of the nonmodified nanoparticles but are not consistent with the TEM photographs of amine-/carboxylmodified nanoparticles. It is very suprising because TEM photographs and FT-IR vibrational spectroscopy have clearly confirmed the formation of SiO2 shell structure on the nanoparticle surfaces. From XRD patterns of amine-/carboxylmodified nanoparticles, however, no characteristic diffraction peaks of SiO2 are detected. Further experiments, in that pure SiO2 fine powder is synthesized at the same modification condition, confirm that the SiO2 shell structure on the nanoparticle surfaces is noncrystalline, as shown in Figure 7. Thus, the differences between the calculated results and TEM photographs come from a fact that core structures are highly crystalline, but SiO2 shell materials are noncrystalline and lower contents. 3. Enhancement of Upconversion Luminescence. It is wellknown that applications of rare earth doped nanoparticles as biolabels are limited because of their very low solubility in water and unsuitable surface properties. Surface modification, which can provide nanoparticles with high solubility in water/organic solvents and a desirable surface for the conjugation of biomolecules, can solve the aforementioned problems. Thus, we applied ligand-capped/ligand-exchanging method for modifying ZrO2: Er3+ nanoparticle surfaces with chemical reagents, such as TEOS, APTES, and SA. As shown in Figure 1, upconversion emission spectra of nonmodified and amine-/carboxyl-modified nanoparticles have the same peak splitting and positions, indicating that local crystalline environments in which Er3+ ions are embedded are identical before and after modifying surfaces. This means that the upconversion mechanism has not been affected by surface modification processes. Moreover, it is worth noting that the integrated intensities of all emission bands for amine-/carboxylmodified nanoparticles in our experiments are 4.7 and 1.5 times larger than that of nonmodified nanoparticles under the same excitation power density of 6.8 × 102 W/cm2, respectively. This is of importance for biological applications of rare earth doped nanoparticles as biolabels. Recently, some researchers had reported the improved upconversion fluorescence of the core/ shell nanoparticles. Haase et al.13 first reported the quantum yield enhancement from 53% for CePO4:Tb nanoparticles to 80% for CePO4:Tb/LaPO4 core-shell nanoparticles. They attributed the significant enhancement of the quantum yield to a shell around each doped nanoparticle, which can suppress the energy-loss process at the nanoparticle surface. Veggel et al.14 ascribed the improved quantum yield of LaF3/Ce, Te nanoparticles from 24 to 54% to LaF3 shells around the nanoparticles. Guang-Shun Yi et al.15 also reported that upconversion fluorescence for NaYF4:Yb/Er(Tm)/NaYF4 core-shell nanoparticles

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Figure 7. XRD patterns of the nonmodified and amine-/carboxyl-modified ZrO2:Er3+ nanoparticles, with the addition of SiO2 shell material.

Figure 8. Absorption spectra of non- (a) and amine- (b)/carboxylmodified (c) ZrO2:Er3+ nanoparticles.

were 7.4 and 29.6 times larger than noncoated samples, respectively. They suggested that upconversion emission enhancements greatly originated from the NaYF4 shell with low phonon energy to reduce the quenching fluorescence process. However, another possibility to enhance the upconversion fluorescence mechanism can be considered here, because there are no obvious absorption bands for amine-/carboxyl-modified nanoparticles at the visible spectral range showing in Figure 8. It is well-known that the 4f n electrons of rare earth ions are

screened by the outer electron shells, yielding a number of discrete energy levels. Due to the forbidden character of the 4f n transitions, the direct excitation of rare earth ions is a relatively inefficient process. Compared to free rare earth ions, the discrete energy levels of rare earth ions are perturbed by the crystalline field of the host matrix, resulting in the decrease of the selection rules of radiation transition. Practically, upconversion luminescence of rare earth ions originates from intra-4f n transitions perturbed by static crystalline fields and does not involve valence electrons.26,27 Thus, the crystalline fields are prerequisite to achieving upconversion luminescence of rare earth ions. Rare earth ions on crystal surfaces, which are similar to free rare earth ions, have very low quantum yield due to almost no perturbation by the degenerated crystalline fields from outside surfaces of host matrix. This might be the aforementioned quenching fluorescence process. That means that rare earth ions on the nanoparticle surfaces are not likely to emit fluorescence. The effects of decreasing the nanoparticle sizes on upconversion fluorescence had previously been reported for Y2O2S: Yb/Er phosphors.28 When Y2O2S:Yb/Er phosphor sizes decreased to 30 nm, upconversion efficiency was reduced to 22% of the bulk counterpart. When particle sizes decreased from 30 to 2 nm, the efficiency rapidly decreased. What is the influence of quantum size effect on the upconversion fluorescence of the doped nanoparticles? Here, we use a simple model to interpret it, as shown in Scheme 2. First, we assume that nanoparticles are regarded as spherical, and their radii are noted as r. Rare earth ions and host material ions are regarded as spherical, and they have same radii as d. Nanoparticles have an approxima-

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SCHEME 2: Schematic Diagram of Molar Ratio of Rare Earth Ions of Outside Spherical Shell to Entire Sphere

tively close-packed structure. Rare earth ions have randomly distributed within the nanoparticles. The concentration of doped rare earth ions is noted as n. Moreover, assuming that the outside spherical shell is the first ion layer of nanoparticle surfaces. Parts of rare earth ions within the first ion layers are beyond nanoparticle surfaces. When neglecting errors, molar ratio (MR) of rare earth ions of outside spherical shell to entire sphere can be given by

4 4 πr - π(r - 2d) ] × n [ 3 3 2d ) 1 - (1 - ) MR ) 4 r πr × n 3

3

3

(2)

coated shells, are noncrystalline. This means that the SiO2-coated shell can provide the ‘dormant’ rare earth ions with parts of the asymmetric association crystalline environment or crystalline field. The asymmetric crystalline environment is very beneficial to rare earth ions luminescence. With the help of the asymmetric association crystalline field, part or all rare earth ions on the nanoparticle surfaces will be aroused or activated and will become new luminescence centers. Due to the additional luminescence, upconversion emission enhancements can easily be achieved. This is well consistent with our experiments, of which the integrated intensities of upconversion fluorescence of amine-/carboxyl-modified nanoparticles in Figure 1 are 4.7 or 1.5 times larger than that of nonmodified nanoparticle. In our experiments, compared with the result reported by GuangShun Yi et al.,15 the enhanced degree of upconversion fluorescence is slightly low. The difference may originate from the fact that the NaYF4 shell is crystalline, whereas the SiO2 shell is noncrystalline. However, amine- and carboxyl-modified nanoparticles have the same core structure and a similar SiO2 shell. How can the difference of the enhanced degree of upconversion fluorescence between amine-modified nanoparticles (4.7 times higher) and carboxyl-modified nanoparticles (1.5 times higher) be interpreted? On the one hand, we ascribe a part of the difference to a result that long-chain organic ligands have more high-energy C-H and C-C vibrational oscillators than short-chain ligands on the noncrystalline SiO2 shells,2 which can reduce the fluorescence. On the other hand, we also believe that the other part of the difference is due to a result that longchain organic ligands on carboxyl-modified nanoparticle surfaces have a larger absorption effect on incident pump light than that of short-chain ligands on amine-modified nanoparticle surfaces. Next, we also attribute another part of the upconversion fluorescence enhancement to the improved spontaneous emission rate of rare earth ions, due to the enhanced local classical density of states arisen by the change of refractive index at the interface between core and shell,30 as shown in Scheme 2. The spontaneous emission rate is given by

3

3

Considering the nanoparticle size in our experiments, we take d ) 0.5 nm, and r ) 10.9 nm, thus, MR ) 25.1%. This means that about 25% of rare earth ions are on the nanoparticle surfaces. In other words, about one-fourth of rare earth ions are ‘dormancy’ according to our model. Thus, this leaves us certain space to investigate how to enhance upconversion fluorescence of the doped nanoparticles. When nanoparticle size decreases further, MR will rapidly increase, resulting in very low upconversion fluorescence. Considering the thickness of the outside spherical shell29 and the same r value in our experiments, we obtain MR ) 1 - (1 - 4.5/r)3 ) 79.8% . That means that only one-fifth rare earth ions can practically participate in the upconversion fluorescence process. After an undoped shell was coated, ‘the naked’ rare earth ions on the nanoparticle surfaces are shielded from outside environment and synchronously perturbed by an asymmetric association crystalline field from both the degenerated crystalline field of host interface and a complementary crystal field of SiO2 shell. Thus, ‘the naked’ rare earth ions become ‘the substituted dopants’. Evidence for SiO2-coated shells can be clearly discerned by TEM photographs and the FT-IR vibrational spectra in our experiments. Especially, FT-IR absorption band at 953 cm-1 indicates Si-O-Zr linkage between the threedimensional Si-O-Si network and ZrO2 surfaces has formed, although the three-dimensional Si-O-Si network, i.e., SiO2-

W(r) )

πω |〈a|µˆ |b〉|2F(ω,r) p(r)

(3)

where W(r) is spontaneous emission rate, ω is transition frequency, (r) is dielectric constant positioned at r, |〈a|µˆ |b〉|2 is the transition matrix elements of dipole moment operator, and F(ω,r) is the local classical density of states. The same peak splitting and positions of emission bands before and after modifying the surface indicate that the coated shells have provided the doped rare earth ions on the nanoparticle surfaces with the same local crystalline fields, and |〈a|µˆ|b〉|2 is determined by local crystalline fields around rare earth ions. Thus, |〈a|µˆ |b〉|2 is constant. We can know from eq 3 that the spontaneous emission rate of rare earth ions is directly relative to the local classical density of states. Dielectric interfaces between core and shell can make local optical density fluctuations within nanoparticles. Subsequently, when the coated shell thickness is appropriate, the improved local optical density can enhance the local classical density of states within nanoparticles, resulting in enhancement of the spontaneous emission rate of rare earth ions. Finally, we also attribute the other part of the upconversion fluorescence enhancement to suppressing the quenching fluorescence centers from organic ligands with high vibrational energy on the nanoparticle surfaces.2 4. The Coated Shell Thicknesses Influence. According to our model and relevant interpretation, we could deduce that

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Figure 9. TEM photographs of the silica-coated ZrO2:Er3+ nanoparticles during different reaction times.

TABLE 1: Ratios of the Integrated Intensities of the Upconversion Emission of the Modified to the Nonmodified ZrO2:Er3+ Nanoparticles during Different Reaction Times under the Same Excitation Power Density of 1.2 × 103 W/cm2 reaction time (min) shell thicknesses (nm) ratio (Imod/Inon)

Figure 10. Upconversion emission spectra of the silica-coated ZrO2: Er3+ nanoparticles during different reaction times under same excitation power density of 1.2 × 103 W/cm2.

further growth of the coated shells no longer has obvious effects on upconversion fluorescence improvement when the coated shell has provided enough crystalline environment for the ‘dormant’ lanthanide ions to be activated. At the same time, thicker shells can decrease the excited light transmission because of the coated shells’ absorption effect, so upconversion fluorescence enhancement is practically limited. To further verify the influence of the coated shell thicknesses on upconversion luminescence, we modify the surfaces of nanoparticles with a certain amount of TEOS and APTES via different reaction times. The shell thicknesses shown in Figure 9a-d are about 20-25 nm during 30 min, 30-35 nm during 60 min, 70-75 nm during 90 min, and 50-60 nm during 120 min. From upconversion emission spectra of SiO2-coated ZrO2: Er3+ nanoparticles during different reaction times in Figure 10, we can know that the integrated intensities of upconversion fluorescence gradually increase during 30 min, reach the biggest intensity during 60 min, and then decrease during 90 and 120 min. Ratios of the integrated intensities of upconversion emission of SiO2-coated nanoparticles during different reaction times to nonmodified nanoparticles are shown in Table 1. The result indicates that the SiO2-coated shell provides a deficient crystalline environment for the ‘dormant’ rare earth ions on the nanoparticle surfaces to be activated when the shell is thinner; the SiO2 shell can provide the most congruent crystalline environment for these ‘dormant’ rare earth ions to be fully activated when the shell thickness is appropriate. Although the ‘dormant’ rare earth ions on the nanoparticle surfaces have been fully activated, the excitation power density on the interface between core and shell might decrease because of the SiO2

30

60

90

120

20-25

30-35

70-75

50-60

1.2

3.3

2.3

1.3

shell’s absorption effect when the SiO2 shell is very thick, resulting in the decrease of upconversion fluorescence. The experimental results are consistent with our deduction. Moreover, the Veggel et al.14 report confirmed that the growth of more shell material has a smaller influence on the luminescence lifetime, which is also consistent with our deduction. This indicates that the SiO2-coated shell indeed plays an important role in changing upconversion fluorescence of the modified nanoparticles. Conclusions ZrO2:Er3+ nanoparticles are synthesized via the Pechini method. The nanoparticle surfaces are modified via the ligandcapped or ligand-exchanging method with chemical reagents, such as TEOS, APTES, and SA. Antiaggregation investigations using TEM and FT-IR indicate that severe aggregation can be reduced by adhering amine-/carboxyl-groups to the nanoparticle surfaces. The green upconversion fluorescence spectra of nonmodified and the modified nanoparticles with the same spectral characteristics are obtained. The remarkable upconversion fluorescence enhancements for amine-/carboxyl-modified nanoparticles are 4.7 and 1.5 times larger than that of nonmodified nanoparticles under excitation power densities of 6.8 × 102 W/cm2, respectively. An enhancement mechanism of upconversion luminescence, in which an asymmetric association crystalline field from both the degenerated crystalline field of the host interface and a complementary crystal field of the SiO2 shell can make the ‘dormant’ rare earth ions on nanoparticle surfaces be activated, is presented. Moreover, the improved spontaneous emission rate of rare earth ions due to the enhanced local classical density of states and suppressing the quenching fluorescence centers from organic ligands with high vibrational energy on the nanoparticle surfaces are also considered. Intense upconversion fluorescence and hydrophilicity via ammonic or carboxylic functional groups will provide the doped core-shell nanoparticles great potential as biolabels in the future.

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