J. Phys. Chem. C 2008, 112, 2353-2358
2353
Tb3+- and Eu3+-Doped Lanthanum Oxysulfide Nanocrystals. Gelatin-Templated Synthesis and Luminescence Properties Zhigang Liu,†,‡ Xudong Sun,*,† Shukun Xu,§ Jingbao Lian,† Xiaodong Li,† Zhimeng Xiu,† Qiang Li,† Di Huo,† and Ji-Guang Li† Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, and Department of Chemistry, Northeastern UniVersity, Shenyang 100004, and School of Materials, Hebei Polytechnic UniVersity, Tangshan 063009, China ReceiVed: August 11, 2007; In Final Form: NoVember 16, 2007
Tb3+- and Eu3+-doped lanthanum oxysulfide nanocrystals (NCs) with homogeneous grain size have been prepared by a gel-network coprecipitation method using gelatin as the template and ammonium sulfate as the sulfurizing agent and a following reduction process at relatively low temperatures (750 °C) in a H2 atmosphere. The products were characterized using thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The gelatin template can act as a “nanoreactor”, which is beneficial to the dispersion and size control of the coprecipitation product. The high decomposition temperature (about 482 °C) of the gelatin network is advantageous to prevent aggregation of the newly formed (LaO)2SO4 NCs during calcination of the coprecipitation product. Single hexagonal phase La2O2S NCs were obtained by reduction of the (LaO)2SO4 powder, and the NCs are equiaxial in shape with particle sizes ranging from 40 to 50 nm. The photoluminescence spectra and time-resolved spectra of the La2O2S:Tb(Eu) in phosphate buffer were investigated. The results show that the La2O2S:Tb NCs have a strong luminescence at 544 nm, whose intensity decreases along with decreasing particle size. Concentration quenching occurs when the Tb concentration reaches 8 mol %. The fluorescence color of the La2O2S:Eu NCs depends on the concentration of Eu3+ ions. The red emission (at 625 nm) from 5D0 transition has a much longer decay time than the green emission from 5D1 transition of the La2O2S:Eu NCs. The Tb3+- and the Eu3+-doped La2O2S NCs have respective fluorescence lifetimes of 1.10 and 0.41 ms in phosphate buffer, allowing their potential applications in biological labeling.
Introduction (Eu3+,
Tb3+)
In recent years, trivalent lanthanide doped inorganic nanocrystals (NCs), including oxides, phosphates, vanadates, etc., have attracted great interest due to their promising applications as novel biological labels and their unique physical properties distinctive to the bulk.1 These NCs are suitable for luminescence detection in bioassays due to their excellent spectral characteristics such as narrow line-shaped emission bands, large Stokes shifts, long-lived luminescence (approximately 1-2 ms), and inherent photostability. In contrast with semiconductor quantum dots,2,3 the emission wavelength of NCs is independent of particle size, and hence, the demand for monodispersity is lower. Since luminescence arises from electronic transitions of the inner lanthanide ions, surface modification has smaller effects on optical properties of the NCs when compared with quantum dots. Compared with the lanthanide oxides, lanthanide oxysulfide is a more efficient phosphor with a wider excitation band. Eu3+doped yttrium oxysulfide ground from its original 6 µm size to an average size of 0.1-0.3 µm by ball milling was successfully used as a luminescent immunocytochemical marker in the 1980s.4 However, little attention was given to lanthanide * To whom correspondence should be addressed. Phone: 86-2483687787. Fax: 86-24-23906316. E-mail:
[email protected]. † Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University. ‡ Hebei Polytechnic University. § Department of Chemistry, Northeastern University.
oxysulfide phosphors as luminescent biological labels. One of the reasons is that the oxysulfide NCs cannot be prepared easily.5 The conventional way to synthesize bulk lanthanide oxysulfide is the solid-state reaction of lanthanide oxide with elemental sulfur and reflux (Na2CO3/K3PO4/Na2S2O3) at high temperature (about 1200 °C).6 Other methods are also used such as the sulfuration of rare earth oxides by H2S or CS2,7 the combustion method,8 and the solvothermal pressure relief method.9 Synthesis of lanthanide oxysulfide NCs is difficult, and there are only a few reports available. Dhanaraj et al.10 proposed a two-step solgel polymer thermolysis method to synthesize Y2O2S:Eu3+ NCs, in which sodium thiosulfate (Na2S2O3) was used to sulfurize the first synthesized Y2O3:Eu3+ NCs. Pires et al.11 reported a method to synthesize nanosized spherical particles of yttrium oxysulfide doped with Yb and Er or Yb and Tm. In this method, polymeric and basic carbonate precursors were reacted with sulfur vapor by using argon as a drag gas at 750 °C for 4 h. Zhao et al.12 obtained Eu2O2S NCs by the thermal decomposition of single molecular precursors [Eu(phen)(ddtc)3] (phen ) 1,10-phenanthroline; ddtc ) diethyldithiocarbamate) in air. Peng et al.5 reported a route to fabricate La2O2S:Eu3+ NCs using gel thermolysis, in which thiourea (CS(NH2)2) was employed as the sulfuration source and poly(vinyl alcohol) (PVA) as the dispersing medium to limit the agglomeration of the NCs. Although lanthanide oxysulfide NCs have been synthesized by the above-mentioned methods, some problems still exist, for example, the possible introduction of sodium impurities which may influence luminescent properties when using sodium
10.1021/jp0764687 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/25/2008
2354 J. Phys. Chem. C, Vol. 112, No. 7, 2008 thiosulfate for sulfuration, the existence of remaining phase impurities of La2O3 or La2O2SO4 in the final products for the gel thermolysis method, aggregation of NCs in the sulfuration process when using sulfur vapor as the sulfurizing agent owing to the slow reaction rate, and the high cost of organic agents such as phen and ddtc. In this study, we propose the preparation of Tb3+- and Eu3+doped La2O2S NCs via a gel-network coprecipitation method using gelatin as the template and ammonium sulfate as the sulfurizing agent at relatively low temperatures (750 °C) in a H2 atmosphere. During the synthesis process, La(OH)3 was precipitated in the homogeneous gelatin network like microemulsion cells with diameters varying in the range 10-100 nm according to the water content in the gel. The gelatin network can limit effectively the agglomeration of La(OH)3 nanoparticles during the drying and heating processes due to its relatively high decomposition temperature (450-500 °C). Phase development, the particle size and morphology, and the optical properties of the thus-synthesized La2-2xO2S:2xTb and La2-2xO2S: 2xEu NCs were investigated.
Liu et al.
Figure 1. TG-DTA curves of the La2O2S precursor. Data were taken in stagnant air under a constant heating rate of 10 °C/min.
Experimental Section Tb3+- and Eu3+-doped lanthanum oxysulfide NCs were prepared via a gel-network coprecipitation method using gelatin as the template. The lanthanum, terbium, and europium sources for the synthesis were lanthanum oxide (99.99% pure, Baotou Hengyitong Rare Earth Co. Ltd., Baotou, China), terbium oxide (Tb4O7; 99.99% pure, Conghua Jianfeng Rare Earth Co. Ltd., Conghua, China), and europium oxide (99.99% pure, Conghua Jianfeng Rare Earth Co. Ltd.), respectively. All the solvents and other reagents were of analytical grade and purchased from Shenyang Chemical Reagent Factory (Shenyang, China). Stock solutions of (RE)(NO3)3 (RE ) La, Tb, Eu) were prepared by dissolving the above lanthanide oxides with minimum amounts of dilute nitric acid (8 M). The starting solution for powder synthesis was then obtained by mixing under magnetic stirring the stock solutions according to the desired Tb(Eu)/La molar ratio. A proper amount of gelatin was dissolved in the above mixed nitrate solution under violent stirring in an 80 °C water bath till a translucent gelatin sol was obtained. Upon cooling to 0 °C in an ice-water bath, the sol turned into a translucent gel, which was then cut into small pieces and soaked in 6 M NH3‚H2O solution for 24 h at 0 °C. During the soaking period, the gel gradually sucked in the NH3‚H2O, and then hydroxides were coprecipitated in the gel network. The gel was washed with cooled distilled water and then heated to 80 °C in a water bath. After violent stirring, the gel turned to sol again, to which a 0.5 M ammonium sulfate solution (SO42-/Ln3+ ) 1/2, mol/ mol) was added under violent stirring in the 80 °C water bath for 1 h. The sol was then dried in a vacuum oven at 110 °C, and the resultant dry gel (the precursor) was preheated at 500 °C in stagnant air for 2 h to obtain a powder product. After being gently crushed with an agate mortar and pestle, the powder was reduced in a tube furnace in flowing H2 (100 mL/min) at various temperatures from 650 to 800 °C for 1 h. Thermogravimetry-differential thermal analysis (TG-DTA) of the precursor was recorded using an SDT 2960 simultaneous thermal analyzer (TA Instruments, New Castle, DE). Phase identification and cell parameter calculation were performed by X-ray diffractometry (XRD; mode PW3040/60, Philips, Eindhoven, The Netherlands) using nickel-filtered Cu KR radiation. The lattice constants and cell volume were calculated on the basis of the XRD patterns using the software package X’Pert HighScore Plus version 2.0 (PANanalytical B.V., Almelo, The
Figure 2. XRD patterns of the La2O2S precursor preheated in air at 450 °C for 2 h.
Netherlands). The morphology of the NCs was observed by transmission electron microscopy (TEM; model 200CX, JEOL, Tokyo, Japan). Photoluminescence (PL) properties were measured using an LS-55 fluorescence spectrophotometer (PerkinElmer, Shelton, CT) at room temperature. The concentration of nanoparticles in the phosphate buffer (pH 7.0) is 50 mg/mL for PL measurements and 20 mg/mL for time-resolved emission spectral analysis. Results and Discussion TG-DTA curves of the precursor are shown in Figure 1. The significant weight losses (up to 60%) are due to the removal of the adsorbed moisture and dehydration of the precursor hydroxide (eq 1). Strong exothermic behavior can be seen at
La(OH)3 f LaOOH + H2O
(1)
about 482 °C due to the ignition of gelatin. The combustion of gelatin leads to a violent release of gaseous products and a sharp weight loss in the narrow temperature range 450-500 °C. Lanthanum oxysulfate, (LaO)2SO4, is produced gradually by the solid-state reaction (eq 2) between LaOOH and (NH4)2SO4
2LaOOH + (NH4)2SO4 f (LaO)2SO4 + 2NH3 + 2H2O (2) during the weight loss stage from 250 to 450 °C, which is evidenced by the XRD pattern (Figure 2) of the precursor preheated at 450 °C for 2 h; that is, only (LaO)2SO4 with space group C2/c was detected after the preheating. It is expected that the newly formed (LaO)2SO4 NCs can be protected from agglomeration by the gelatin network owing to the high
Synthesis/Luminescence of Lanthanum Oxysulfide NCs
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2355
Figure 3. XRD patterns of the preheated precursor reduced at various temperatures in a H2 atmosphere for 1 h.
TABLE 1: Crystallite Sizes of La2O2S Prepared at Various Temperatures As Assayed from the Scherrer Equation heat heat treatment av size of treatment av size of sample temp (°C) La2O2S (nm) sample temp (°C) La2O2S (nm) 1 2
650 700
30 39
3 4
750 800
45 53
TABLE 2: Influence of Doping Concentrations of Tb3+ and Eu3+ on the Lattice Constants and Cell Volumea La2-2xO2S:2xTb x value a ) b (Å) 0 0.02 0.05 0.08 0.11
4.0497 4.0477 4.0385 4.0302 4.0264
La2-2xO2S:2xEu
c (Å)
cell vol (Å3)
a ) b (Å)
c (Å)
cell vol (Å3)
6.9463 6.9442 6.9368 6.9247 6.9258
98.6574 98.5302 97.9783 97.4058 97.2376
4.0497 4.0464 4.0400 4.0345 4.0260
6.9463 6.9446 6.9350 6.9285 6.9210
98.6574 98.4736 98.0257 97.6639 97.1509
Figure 4. TEM analysis of the La1.90O2S:0.10Tb NCs reduced at 750 °C for 1 h, with (a) the overall morphology and (b) the lattice image of a single nanocrystallite. The inset in (a) is the corresponding SAED pattern, while the interplanar spacing of 0.335 nm in (b) corresponds to the (101) plane of the hexagonal structured nanocrystallites.
a JCPDS no. 27-0263: a ) b ) 4.0510 Å, R ) β ) 90°, γ ) 120°, c ) 6.9440 Å, cell volume 98.69 Å3.
decomposition temperature of the latter. PVA was used to avoid aggregation of La2O2S:Eu3+ NCs.5 However, the relatively low decomposition temperature of PVA (about 300 °C) indicates that PVA is not as effective as gelatin to avoid agglomeration of NCs. XRD patterns of the preheated precursor reduced at various temperatures in a H2 atmosphere for 1 h are shown in Figure 3. At 650 °C, the dominant phase is hexagonal structured La2O2S (space group P3hm1, JCPDS no. 27-0263), with the presence of a small amount of monoclinic structured (LaO)2SO4 phase. Almost all the (LaO)2SO4 impurity converts into La2O2S at 750 °C, and pure La2O2S can also be obtained by extending the reduction time to 2 h at 700 °C. Thus, phase-pure La2O2S NCs can be synthesized by this method at comparatively low crystallization temperatures. Assuming a homogeneous strain across the nanocrystallites, the average crystallite size can be estimated from the full width at half-maximum (fwhm) values of diffraction peaks at an angle of 2θ ) 28.58° using the Scherrer formula, and the results are summarized in Table 1. XRD analysis was also made on the La2-2xO2S:2xTb(Eu) (x ) 0, 2, 5, 8, 11) NCs synthesized at the reduction temperature of 750 °C in the 2θ scan range 20-100° with a scan step size of 0.03°. The change of lattice constants and cell volume with Tb3+ and Eu3+ concentration in the host of La2O2S NCs is shown in Table 2. Since the ionic radius of Tb3+ (0.0923 nm) or Eu3+ (0.0950 nm) is smaller than that of La3+ (0.1061 nm), the lattice constants and cell volume decrease with increasing Tb3+ or Eu3+ doping concentration. This trend proves that Tb3+
Figure 5. Excitation and emission spectra of the La1.90O2S:0.10Tb NCs in phosphate buffer (pH 7.0).
and Eu3+ can easily enter the host lattice of La2O2S NCs via this synthetic method. The La1.90O2S:0.10Tb NCs synthesized at 750 °C are uniform in size, well-dispersed, and nearly equiaxial in particle shape, as can be seen from the TEM micrograph (Figure 4a). The thussynthesized NCs show excellent crystallinity, as evidenced by the selected area electron diffraction (SAED) pattern (Figure 4a, inset) and the well-resolved lattice fringes (Figure 4b), where the spacing of 0.335 nm corresponds to the (101) plane of the hexagonal crystal structure of the nanocrystallites. The particle size ranges from 40 to 50 nm, which is consistent with the average crystallite size (45 nm) obtained via broadening analysis. The NCs with such morphology are suitable for biological labeling because of their easier conjugation with biological molecules, better mobility, and less aggregation tendency in a liquid phase. Figure 5 shows the excitation and emission spectra of the as-synthesized La1.90O2S:0.1Tb NCs in a phosphate buffer (pH 7.0). The electronic configuration of terbium ion (Tb3+) is 4f8, and the broad excitation band at 256 nm is due to the 4f8 to
2356 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Figure 6. PL intensity (λex ) 250 nm) from the 5D4 f 7F5 transition of the La1.90O2S:0.10Tb NCs in phosphate buffer (pH 7.0) and the ratio F6/F5 as a function of Tb concentration. F6/F5 denotes the emission intensity from the 5D4 f 7F6 transition divided by that from the 5D4 f 7 F5 transition.
Liu et al.
Figure 8. Excitation and emission spectra of the La1.90O2S:0.10Eu NCs in phosphate buffer (pH 7.0).
Figure 7. Emission spectra of the La1.90O2S:0.10Tb NCs obstained at different reducing temperatures in phosphate buffer (pH 7.0). The inset is for the dried NCs (λex ) 250 nm).
4f75d1 transition. The emission spectrum, consisting of four lines, originates from the 5D4 to 7Fj (j ) 3, 4, 5, and 6) transitions of the Tb3+ ions. The 5D4 f 7F6 transition (488.5 nm) is known to be worked by the electric-dipole mechanism and the 5D4 f 7F transition (544 nm) by the magnetic-dipole mechanism. In 5 the La1.90O2S:0.10Tb NCs, emissions from 5D3 to 7Fj were not found, which is consistent with the previous reports.13,14 It was reported that the relative intensity of the 5D3 emission increases dramatically as the lanthanide element goes from La to Gd to Y and then to Lu.13 When Tb-doped lanthanide oxysulfide NCs are excited by UV, the density of each exited state depends on the crossover between the 4f-5d absorption energy level and each excited level (5Dj). This crossover position depends on the host material. Therefore, the fact that the emission from 5D3 is not observed in La1.90O2S:0.10Tb NCs can be understood. The intensity of the 5D4 f 7F5 transition of the La1.90O2S: 0.10Tb NCs is shown in Figure 6 as a function of the Tb3+ concentration, along with the peak height ratio F6/F5, which is defined as the emission intensity from the 5D4 f 7F6 transition divided by that from the 5D4 f 7F5 transition. The intensity of the 5D4 f 7F5 emission reaches its maximum at 8 mol % Tb3+ ions, after which it decreases quickly owing to the concentration quenching effect. The F6/F5 ratio only decreases slightly with increasing Tb3+ content, because the electric-dipole transition of Tb3+ ions, unlike Eu3+ ions, is less affected by the variation of the crystal structure and chemical surroundings.
Figure 9. Emission spectra of the La2-2xO2S:2xEu NCs (λex ) 250 nm), where x is 0.5 for (A), 1.0 for (B), 2.5 for (C), and 5.0 for (D).
Emission spectra of the La1.90O2S:0.10Tb NCs obstained at different reducing temperatures are compared in Figure 7, from which it can be seen that the relative intensity of the 5D4 f 7Fj luminescence is affected strongly by the reducing temperature. The higher the reducing temperature, the bigger the mean grain size (Table 1) and the higher the luminous intensity. The lower luminous intensity for the finer NCs cannot be solely ascribed to the larger contacting interface with water, which may quench luminescence of the phosphor. It is believed that a greater number of defects both on the surface and inside the finer NCs or the more impurity phase arising from the lower treatment temperature may have dominant effects on the luminescence intensity. Figure 8 shows excitation and emission spectra of the La1.90O2S:0.10Eu NCs synthesized at 750 °C. The broad excitation bands ranging from 220 to 360 nm can be ascribed to charge transfer. The one around 320 nm is attributable to S2- f Eu3+ transition and the 250 nm one to O2- f Eu3+ transition. In the PL spectrum, emissions from the excited 5DJ (J ) 0, 1) levels to 7FJ (J ) 1, 2, 3, and 4) levels of Eu3+ ions were observed. The strongest red emission which splits into two peaks at 625 and 615 nm arises from the forced electric-dipole transition of 5D0 f 7F2. The emission peaks at 538, 556, and
Synthesis/Luminescence of Lanthanum Oxysulfide NCs TABLE 3: Influence of Eu3+ Concentration on the Fluorescence Intensity Ratio of 5D0 f 7F2/5D1 f 7FJ (J ) 1, 2, and 3) Eu3+ ion concn (mol %) 0.5 1.0 2.5 5.0
5
D0 f 7F2/ D1 f 7F1 I623/I538
5
5
5
5
D0 f 7F2/ D1 f 7F2 I623/I556
5
0.221 0.257 0.692 1.357
0.730 1.073 2.360 4.206
D0 f 7F2/ D1 f 7F3 I623/I587 0.312 0.423 1.035 2.115
587 nm originate from 1 f J (J ) 1, 2, and 3) transitions, respectively, which are strongly affected by the concentration of Eu3+ ions in the La2O2S:Eu NCs. Figure 9 shows emission spectra of the La2O2S:Eu NCs with various concentrations of Eu3+ ions heat treated at 750 °C in H2 for 1 h. It is obvious that the relative intensities of the emission peaks vary remark5D
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2357 ably with the concentration of Eu3+. Fluorescence intensity ratios of 5D0 f 7F2/5D1 f 7FJ (J ) 1, 2, and 3) are summarized in Table 3 as a function of Eu3+ concentration. The ratio increases with increasing Eu3+ addition, indicating that the emission from the higher energy level of 5D1 is quenched but that from the lower energy level of 5D0 enhanced. Such a phenomenon can be attributed to the cross-relaxation between Eu3+ energy levels according to the following expression:
Eu3+(5D1) + Eu3+(7F0) f Eu3+(5D0) + Eu3+(7F3)
7F
Since the process of cross relaxation depends on the distance between the Eu3+ centers, the effect of cross-relaxation can be revealed at a relatively high concentration of Eu3+ ions. Time-resolved emission spectra of the La1.90O2S:0.10Tb and La1.90O2S:0.10Eu NCs are shown in Figure 10 (insets). There
Figure 10. Decay curves of the La1.90O2S:0.10Tb (a) and La1.90O2S:0.10Eu (b) NCs in phosphate buffer (pH 7.0). The insets are the corresponding time-resolved emission spectra under excitation at 250 nm. The delay times for La1.90O2S:0.10Tb labeled from 1 to 8 are 0.02, 0.08, 0.2, 0.4, 0.8, 1.2, 1.4, and 1.6 ms, while those for La1.90O2S:0.10Eu labeled from 1 to 7 are 0.06, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 ms, respectively.
2358 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Liu et al.
is no new emission peak emerging with the increase of the delay time, but the emissions from the 5D1 transition of La1.90O2S: 0.10Eu NCs decay much faster than those from the 5D0 transitions. The luminescence decay curves of the La1.90O2S: 0.10Tb NCs from the 5D4 f 7F5 transition and of the La1.90O2S: 0.10Eu NCs from the 5D0 f 7F2 transition can be fitted to single exponentials according to eq 3, and the results are presented in Figure 10. τR is the fluorescence lifetime, I the relative intensity, and t the delay time, and A and B are constants.
I ) Ae-(t/τR) + B
(3)
The fitting yields τR,Tb ) 1.10 ms, A ) 1165.09 au, and B ) -123.30 au for La1.90O2S:0.10Tb and τR,Eu ) 0.41 ms, A ) 588.22 au, and B ) 278.42 au for the La1.90O2S:0.10Eu NCs. It is known that autofluorescence and the fixative-induced fluorescence of cells and tissues as well as fluorescences from FITC (fluorescein isothiocyanate), TRITC (tetramethylrhodamine isothiocyanate), and phycobiliproteins are relatively rapid processes with fluorescence decay times in the range of 1-100 ns.4 The fluorescent lifetimes of Tb3+- and Eu3+-doped La2O2S NCs synthesized in this work are thus long enough to eliminate the background fluorescence in highly sensitive time-resolved fluorescence bioassay. Conclusions La2O2S:Tb and La2O2S:Eu NCs have been synthesized using a gel-network coprecipitation method, with ammonium sulfate as the sulfurizing agent, at relatively low temperatures (750 °C) in a H2 atmosphere. Single hexagonal phase La2O2S:Tb(Eu) NCs were obtained. TEM observation shows that the doped La2O2S NCs are equiaxial particles about 40-50 nm in diameter. The La2O2S:Tb NCs exhibit strong emissions from 5D4 f 7Fj transitions with no observation of 5D3 emissions. The relative luminescence intensity was observed to decrease with decreasing particle size. Concentration quenching occurs when the Tb concentration reaches 8 mol %. The fluorescence color of the La2O2S:Eu NCs depends on the concentrations of Eu3+ ions. The emissions from 5D0 transition of the La1.90O2S:0.10Eu NCs have a much longer decay time than those from 5D1 transitions. Fluorescence lifetimes of the as-synthesized La1.90O2S:0.10Tb
and La2O2S:0.10Eu NCs are 1.10 and 0.41 ms in a phosphate buffer, respectively. The as-synthesized La2O2S:Tb(Eu) NCs may potentially be used in biological labeling because of their equiaxial particle shape, uniform particle size, strong luminescence intensity, and long fluorescence lifetime. Acknowledgment. This work was supported by the Program for New Century Excellent Talents in University (Grant NCET25-0290), the National Science Fund for Distinguished Young Scholars (Grant 50425413), and the National Natural Science Foundation of China (Grant 50672014). References and Notes (1) (a) Nichkova, M.; Dosev, D.; Perron, R.; Gee, S. J.; Hammock, B. D.; Kennedy, I. M. Anal. Bioanal. Chem. 2006, 384, 631. (b) Louis, C.; Bazzi, R.; Marquette, C. A.; Bridot, J. L.; Roux, S.; Ledoux, G.; Mercier, B.; Blum, L.; Perriat, P.; Tillememt, O. Chem. Mater. 2005, 17, 1673. (c) Meiser, F.; Cortez, C.; Caruso, F. Angew. Chem., Int. Ed. 2004, 43, 5954. (d) Guo, H.; Dong, N.; Yin, M.; Zhang, W. P.; Lou, L.; Xia, S. D. J. Phys. Chem. B 2004, 108, 19205. (e) Yan, R. X.; Li, Y. D. AdV. Funct. Mater. 2005, 15, 763. (f) Beaurepaire, E.; Buissette, V.; Sauviat, M. P.; Giaume, D.; Lahlil, K.; Mercuri, A.; Casanova, D.; Huignard, A.; Martin, J. L.; Gacoin, T.; Boilot, J. P.; Alexandrou, A. Nano Lett. 2004, 4, 2079. (2) Chan, W. C. W.; Maxwell, D. J.; Gao, X. H.; Bailey, R. E.; Niu, S. M. Curr. Opin. Biotechnol. 2002, 13, 40. (3) Riegler, J.; Nann, T. Anal. Bioanal. Chem, 2004, 379, 913. (4) Beverloo, H. B.; van Schadewijk, A.; van Gelderen-Boele, S.; Tanke, H. J. Cytometry 1990, 11, 784. (5) Peng, H. S.; Huang, S. H.; You, F. T.; Chang, J. J.; Lu, S. Z.; Cao, L. J. Phys. Chem. B 2005, 109, 5774. (6) (a) Pham-Thi, M.; Morell, A. J. Electrochem. Soc. 1991, 138, 1100. (b) Kottaisamy, M.; Jagannathan, R.; Rao, R. P.; Avudaithai, M.; Srinivasan, L. K.; Sundaram, L. K. J. Electrochem. Soc. 1995, 142, 3205. (c) Kottaisamy, M.; Horikawa, K.; Kominami Aoki, H. T.; Azuma, N.; Nakamura, T.; Nakanishi, Y.; Hatanaka, Y. J. Electrochem. Soc. 2000, 147, 1612. (7) Haynes, J. W.; Brown, J. J. J. Electrochem. Soc. 1968, 115, 1060. (8) Mishenina, L. N.; Kazarbina, T. V.; Kozik, V. V. Inorg. Mater. 1994, 30, 733. (9) Yu, S. H.; Han, Z. H.; Yang, J.; Zhao, H. Q.; Xie, Y.; Qian, Y. T. Chem. Mater. 1999, 11, 192. (10) Dhanaraj, J.; Geethalakshmi, M.; Jagannathan, R.; Kutty, T. R. N. Chem. Phys. Lett., 2004, 387, 23. (11) Pires, A. M.; Serra, O. A.; Davolos, M. R. J. Alloys Compd. 2004, 374, 181. (12) Zhao, F.; Yuan, M.; Zhang, W. J.; Gao, S. J. Am. Chem. Soc. 2006, 128, 11758. (13) Bang, J.; Abboudi, M.; Abrams, B.; Holloway, P. H. J. Lumin. 2004, 106, 177. (14) Struck, C. W.; Fonger, W. H. J. Appl. Phys. 1971, 42, 4515.
