Colloidal Nanoparticles of a Europium Complex with Enhanced

Jun 6, 2008 - window for the EuIII luminescence of Eu(tta)3dpbt nanoparticles, extending up .... digital oscilloscope (HP 54503A, HP) connected to a p...
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Langmuir 2008, 24, 6932-6936

Colloidal Nanoparticles of a Europium Complex with Enhanced Luminescent Properties Xiaofan Wen,† Manyu Li,‡ Yuan Wang,*,† Jianping Zhang,*,‡ Limin Fu,‡ Rui Hao,† Yan Ma,† and Xicheng Ai‡ Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China, Department of Chemistry, Renmin UniVersity of China, Beijing 100872, China, and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed March 23, 2008 We report an alternative approach, that is, forming Eu(tta)3dpbt (dpbt ) 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5dimethylpyrazol-1-yl)-1,3,5-triazine, tta ) thenoyltrifluoroacetonato) nanoparticles in water/methanol mixtures, to satisfy the combined requirements of good dispersibility in water solutions and efficient long-wavelength sensitization for EuIII complexes to be used in biological applications. The size of Eu(tta)3dpbt colloidal particles with very high luminescent capabilities can be modulated to some extent by changing the preparation conditions. The optical excitation window for the EuIII luminescence of Eu(tta)3dpbt nanoparticles, extending up to 475 nm, is wider than that of Eu(tta)3dpbt molecules dissolved in toluene. This is the first example for obviously extending the sensitization window of luminescent lanthanide materials to the long-wavelength region by forming nanoparticles of a lanthanide complex. Quantum yields of EuIII luminescence of the prepared Eu(tta)3dpbt colloidal particles, with an average diameter of 33.1 nm, are 0.27, 0.27, 0.24, 0.19, 0.14, and 0.01 upon excitation at 402, 420, 430, 440, 450, and 475 nm, respectively. The Eu(tta)3dpbt nanoparticles exhibited excellent two-photon sensitization performance with a highest δΦ value of 3.2 × 105 GM (1 GM ) 10-50 cm4 s photo-1 particle-1) at the excitation wavelength of 832 nm, which is about 7 times higher than the highest value reported for the CdSe/ZnS core-shell quantum dots. The favorable luminescent properties and the good dispersibility in water solutions of the Eu(tta)3dpbt nanoparticles are very promising for the development of new luminescent nanoprobes for bioanalysis.

Introduction Intensive studies have been conducted on the synthesis and photophysical properties of luminescent lanthanide (III) complexes because of the scientific interests in exploring the photosensitization mechanism of the coordinated chromophores, and because of the great anticipation in their new technological applications,1 such as fluorescent labels in life science,2–5 nonlinear optical materials,6,7 white-light emission materials,8 temperature-sensitive probes,9 and high color-purity emitters in optoelectronics.10 For application in biosensing or bioimaging, great efforts have been focused on the development of novel luminescent lanthanide (III) complexes by designing various chromophore ligands. One of the manifold purposes for this molecular engineering * To whom correspondence should be addressed. E-mail: wangy@ pku.edu.cn (Y.W.); [email protected] (J.Z.). Telephone: +86-106275-7497 (Y.W.); +86-10-6251-6604 (J.Z.). Fax: +86-10-6276-5769 (Y.W.); +86-10-6251-6444 (J.Z.). † Peking University. ‡ Renmin University of China and Chinese Academy of Sciences.

(1) Bu¨nzli, J.-C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048–1077. (2) Handl, H. L.; Vagner, J.; Yamamura, H. I.; Hruby, V. J.; Gillies, R. J. Anal. Biochem. 2004, 330, 242–250. (3) Santos, M.; Roy, B. C.; Goicoechea, H.; Campiglia, A. D.; Mallik, S. J. Am. Chem. Soc. 2004, 126, 10738–10745. (4) Terai, T.; Kikuchi, K.; Iwasawa, S.-Y.; Kawabe, T.; Hirata, Y.; Urano, Y.; Nagano, T. J. Am. Chem. Soc. 2006, 128, 6938–6946. (5) Ziessel, R. F.; Ulrich, G.; Charbonnie`re, L.; Imbert, D.; Scopelliti, R.; Bu¨nzli, J.-C. G. Chem.sEur. J. 2006, 12, 5060–5067. (6) Tancrez, N.; Feuvrie, C.; Ledoux, I.; Zyss, J.; Toupet, L.; Bozec, H. L.; Maury, O. J. Am. Chem. Soc. 2005, 127, 13474–13475. (7) Wong, K. L.; Law, G. L.; Kwok, W. M.; Wong, W. T.; Phillips, D. L. Angew. Chem., Int. Ed. 2005, 44, 3436–3439. (8) Coppo, P.; Duati, M.; Kozhevnikov, V. N.; Hofstraat, J. W.; Cola, L. D. Angew. Chem., Int. Ed. 2005, 44, 1806–1810. (9) Borisov, S. M.; Wolfbeis, O. S. Anal. Chem. 2006, 78, 5094–5101. (10) Kuriki, K.; Koike, Y.; Okamoto, Y. Chem. ReV. 2002, 102, 2347–2356.

investigation is to increase their solubility and stability in water, usually by the incorporation of chromophores into a multidentate chelating ligand containing carboxyl or amide groups with strong coordination abilities to the lanthanide ions.1 Most of the complexes synthesized according to this strategy could only be efficiently excited by ultraviolet radiation,11–14 although some water-soluble lanthanide complexes with low luminescence quantum yields (Φ < 5.5 × 10-2) have a wide optical excitation window extending up to 450 nm.15 Another purpose is to meet the demand for luminescent lanthanide(III) complexes capable of being efficiently sensitized by long-wavelength light. The design and synthesis of appropriate ligand chromophores for this purpose, however, are rather challenging.16 Recently, we reported an interesting complex, Eu(tta)3dpbt (dpbt ) 2-(N,Ndiethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine, tta ) thenoyltrifluoroacetonato) (Scheme 1).17 The EuIII fluorescence of Eu(tta)3dpbt can be very efficiently sensitized by visible light through a ligand-to-EuIII singlet excitation energy transfer pathway. Moreover, this complex possesses quite large two-photon excitation action cross sections (δΦ) with a maximal value of 82 GM at 808 nm, which may be useful for developing (11) Zucchi, G.; Ferrand, A.-C.; Scopelliti, R.; Bu¨nzli, J.-C. G. Inorg. Chem. 2002, 41, 2459–2465. (12) Petoud, S.; Cohen, S. M.; Bu¨nzli, J.-C. G.; Raymond, K. N. J. Am. Chem. Soc. 2003, 125, 13324–13325. (13) Weibel, N.; Charbonnie`re, L. J.; Guardigli, M.; Roda, A.; Ziessel, R. J. Am. Chem. Soc. 2004, 126, 4888–4896. (14) Beeby, A.; Bushby, L. M.; Maffeo, D.; Williams, J. A. G. J. Chem. Soc., Perkin Trans. 2 2000, 1281–1283. (15) Dadabhoy, A.; Faulkner, S.; Sammes, P. G. J. Chem. Soc., Perkin Trans. 2 2002, 348–357. (16) Werts, M. H. V.; Duin, M. A.; Hofstraat, J. W.; Verhoeven, J. W. Chem. Commun. 1999, 799–800. (17) Yang, C.; Fu, L. M.; Wang, Y.; Zhang, J. P.; Wong, W. T.; Ai, X. C.; Qiao, Y. F.; Zou, B. S.; Gui, L. L. Angew. Chem., Int. Ed. 2004, 43, 5010–5013.

10.1021/la800903s CCC: $40.75  2008 American Chemical Society Published on Web 06/06/2008

Eu(tta)3dpbt Colloidal Nanoparticles Scheme 1. Molecular Structure of Eu(tta)3dpbt Complex

high-sensitive, deep-penetrating, and less-harmful fluorescent labels of biological samples.18 A common problem for the practical use of the luminescent lanthanide complexes capable of being efficiently sensitized by visible light in bioanalysis is their poor stability and/or solubility in water. Actually, it has long been a challenge to simultaneously match the demands for good stability against water and efficient long-wavelength sensitization for luminescent lanthanide complexes. On the other hand, nanoparticles composed of inorganic lanthanide compounds or doped with organic lanthanide complexes have attracted increasing interest for their potential application in fluoroimmunoassay, bioimaging, or other techniques.19–21 However, luminescent nanoparticles of lanthanide compounds which can be efficiently excited by visible light have so far been scarce. Herein, we report the preparation of stable colloidal solutions of Eu(tta)3dpbt nanoparticles in water/methanol mixtures. It was found that the excitation window of these nanoparticles extended up to ∼475 nm and the luminescent capability (defined as εΦ, ε ) extinction coefficient) of Eu(tta)3dpbt molecules in the nanoparticles sensitized at long wavelengths was much higher than that of Eu(tta)3dpbt molecules dissolved in toluene. To the best of our knowledge, this is the first example for significantly expanding the optical excitation window of lanthanide luminescence by forming nanoparticles. This finding opens an alternative way to match the combined demands for excellent dispersibility in a water solution and high luminescent capability upon long-wavelength optical excitation of lanthanide complexes.

Experimental Section Materials and Methods. Eu(tta)3dpbt was synthesized essentially according to the method reported previously.17 1H NMR (CDCl3, 400 MHz, 25 °C, tetramethylsilane (TMS)) data of a precipitate obtained by adding water into a methanol solution of Eu(tta)3dpbt are as follows: δ ) 25.012 (s, 6H; Pz-CH3), 12.043 (s, 2H; Pz-H), 7.698 (d, 3J(H,H) ) 8.8 Hz, 2H; Ph-H), 6.857 (d, 3J(H,H) ) 5.0 Hz, 3H; Th-H), 6.386 (d, 3J(H,H) ) 8.8 Hz, 2H; Ph-H), 6.019 (t, 3J(H,H) ) 0.1 Hz, 3H; Th-H), 5.007 (s, 3H; Th-H), 4.823 (s, 6H; Pz-CH3), 3.325 (q, 3J(H,H) ) 9.9 Hz, 4H; NCH2CH3), 1.055 (t, 3J(H,H) ) 7.1 Hz, 6H; NCH CH ), -0.148 (s, 3H; CH) ppm. 2 3 A colloidal solution of Eu(tta)3dpbt nanoparticles with an average diameter of 33.1 nm was prepared by dropwise adding 6.5 mL of (18) Fu, L. M.; Wen, X. F.; Ai, X. C.; Sun, Y.; Wu, Y. S.; Zhang, J. P.; Wang, Y. Angew. Chem., Int. Ed. 2005, 44, 747–750. (19) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256–3260. (20) Hai, X. D.; Tan, M. Q.; Wang, G. L.; Ye, Z. Q.; Yuan, J. L.; Matsumoto, K. Anal. Sci. 2004, 20, 245–246. (21) Peng, H. S.; Wu, C. F.; Jiang, Y. F.; Huang, S. H.; McNeill, J. Langmuir 2007, 23, 1591–1595.

Langmuir, Vol. 24, No. 13, 2008 6933 water into a methanol solution of Eu(tta)3dpbt (3 mL, 3.33 × 10-5 mol L-1) under magnetic stirring. After further stirring for 5 min, an aqueous solution of cetyltrimethyl ammonium bromide (CTAB) (0.5 mL, 2 × 10-2 mol L-1) was added into the system. Finally, a methanol/water mixture (volume ratio 3:7) was added to increase the volume of the colloidal solution to 10 mL. A colloidal solution of Eu(tta)3dpbt nanoparticles with an average diameter of 46.7 nm was prepared by dropwise adding a methanol solution of Eu(tta)3dpbt (2 mL, 5 × 10-5 mol L-1) into 7.5 mL of water under magnetic stirring. After further stirring for 5 min, an aqueous solution of CTAB (0.5 mL, 2 × 10-2 mol L-1) was added into the system, and then a methanol/water mixture (volume ratio 2:8) was added to increase the volume of the colloidal solution to 10 mL. Instrument Analysis. Transmission electron microscopy (TEM) images were taken using a JEM 2000FX electron microscope. UV–vis absorption and photoluminescence measurements were carried out on an absorption spectrometer (CARY 1E, Varian) and a fluorescence spectrophotometer (F-4500, Hitachi), respectively, at ambient conditions using 10 mm cells. Luminescence quantum yields (Φ) of Eu(tta)3dpbt in toluene and the prepared Eu(tta)3dpbt colloidal particles were determined at 10 °C according to the method described by Demas and Grosby,22 using 4-dicyanomethylene-2-methyl-6p-dimethylaminostyryl-4H-pyran (DCM) in n-propanol (Φ ) 0.57 ( 0.02) as the reference.23 Two-photon absorption (TPA) measurements on the colloidal solution of Eu(tta)3dpbt nanoparticles (dav ) 33.1 nm) were conducted according to the method reported previously,18 using Rhodamine B as a standard with known TPA cross sections. For recording kinetics of the luminescence, the excitation wavelength at 430 nm was supplied by an optical parametric oscillator (MOPO-SL, Spectra-Physics); a photomultiplier tube (PMT, R298, Hamamatsu) attached to a polychromator (Spectrapro2300i, Acton) was used for detection, and the signal was sent to a digital oscilloscope (HP 54503A, HP) connected to a personal computer.

Results and Discussion The colloidal solution of Eu(tta)3dpbt nanoparticles was prepared by dropwise adding water to a methanol solution of Eu(tta)3dpbt, or the reverse process, under stirring to reach a water-to-methanol volume ratio of 7-8. After further stirring for 5 min, an aqueous solution of cetyltrimethyl ammonium bromide (CTAB) was added into the colloidal solution. The final concentrations of Eu(tta)3dpbt and CTAB in the colloidal solutions were adjusted to be 1 × 10-5 and 1 × 10-3 mol L-1, respectively. The prepared colloidal solutions of Eu(tta)3dpbt nanoparticles were quite stable; no precipitate was observed after standing for a month. The particle size of the prepared Eu(tta)3dpbt colloidal particles could be modulated to some extent by changing the preparation conditions such as the concentration of Eu(tta)3dpbt in methanol or the methanol-to-water ratio in the precipitation process of Eu(tta)3dpbt molecules. Figure 1 shows the TEM images and the size distributions of the Eu(tta)3dpbt nanoparticles with average diameters (dav) of 33.1 and 46.7 nm. IR spectroscopy (Supporting Information, Figure S1) and 1H NMR data (see the Experimental Section) of a precipitate obtained by adding water into a methanol solution of Eu(tta)3dpbt (1 × 10-3 mol L-1) proved that it is pure Eu(tta)3dpbt, suggesting that the nature of the prepared nanoparticles is Eu(tta)3dpbt. CTAB added into the colloidal solutions in the present manner exhibited an obvious stabilization effect on the nanoparticles, while it caused only a slight change in the sizes of the prepared nanoparticles (Supporting Information, Figure S2). (22) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024. (23) Bondarev, S. L.; Knyukshto, V. N.; Stepuro, V. I.; Stupak, A. P.; Turbana, A. A. J. Appl. Spectrosc. 2004, 71, 194–201.

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Figure 1. TEM images of Eu(tta)3dpbt colloidal particles with different sizes.

The UV-vis absorption spectra of the prepared colloidal solutions of Eu(tta)3dpbt nanoparticles with different average particle sizes, a toluene solution of Eu(tta)3dpbt, and a methanol solution of Eu(tta)3dpbt are shown in Figure 2a. Compared with the absorption spectrum of Eu(tta)3dpbt molecules in methanol (curve 1, λmax ) 394 nm), the longer-wavelength absorption peaks of the Eu(tta)3dpbt colloidal nanoparticles with average sizes of 33.1 and 46.7 nm red-shifted to 420 nm. The absorption windows of the nanoparticles are much wider than that of Eu(tta)3dpbt in toluene (curve 4). Figure 2b shows the luminescence excitation spectra of Eu(tta)3dpbt nanoparticles with different average diameters and Eu(tta)3dpbt molecules dissolved in methanol and in toluene. The fluorescence excitation spectrum of Eu(tta)3dpbt molecules in methanol exhibits a single broadband peaked at 337 nm which is derived from the sensitization of coordinated tta ligands, while the band near 400 nm due to the sensitization effect of coordinated dpbt was not observed. The fluorescence quantum yield of free dpbt molecules in ethanol is 0.0218 and less than 0.01 in methanol as measured in this work. These phenomena suggest that alcohols such as ethanol and methanol can quench the electronic excitations of dpbt molecules, which should be a cause of the weak luminescence of Eu(tta)3dpbt molecules dissolved in methanol. It is interesting to find that the Eu(tta)3dpbt nanoparticles in very dilute colloidal solutions (Eu(tta)3dpbt ) 1 × 10-5 mol L-1) exhibit a wider excitation window for the EuIII luminescence, with the red edge extending up to ∼475 nm, while Eu(tta)3dpbt in toluene with the same concentration exhibits its excitation window red edge at ∼441 nm, although the excitation window for Eu(tta)3dpbt in toluene with a concentration of 1 × 10-2 mol L-1 extends up to 460 nm.17 The peak positions and shapes of the fluorescence excitation spectra of Eu(tta)3dpbt nanoparticles with average diameters of 33.1 and 46.7 nm are quite similar, suggesting that the energy levels dominating the excitation energy transfer to EuIII are very similar in the nanoparticles with different sizes. Some photophysical data of the colloidal nanoparticles (dav ) 33.1 nm) and Eu(tta)3dpbt molecules in toluene are listed in Table 1. The overall

Figure 2. UV-vis absorption spectra (a), fluorescence excitation spectra (λem ) 614 nm) (b), and photoluminescence spectra (λex ) 430 nm) (c). (1) Eu(tta)3dpbt in methanol; (2) colloidal solution of Eu(tta)3dpbt nanoparticles with dav ) 33.1 nm; (3) colloidal solution of Eu(tta)3dpbt nanoparticles with dav ) 46.7 nm; and (4) Eu(tta)3dpbt free in toluene. The Eu(tta)3dpbt concentration in all of these samples was 1 × 10-5 mol L-1. Table 1. Luminescent Capability for Eu(tta)3dpbt Molecules in the Nanoparticles (Average Diameter, 33.1 nm) and That Dissolved in Toluene Excited at Different Wavelengths (λex)a εΦ (×103 M-1 cm-1) λex (nm)

Eu(tta)3dpbt nanoparticle

Eu(tta)3dpbt free in toluene

402 420 430 440 450 475

9.8 11.8 9.9 6.5 3.4 0.1

37.8 9.1 2.4 n.d. n.d. n.d.

a

The uncertainty of Φ is 10%.

quantum yields for the EuIII emission of Eu(tta)3dpbt in toluene were measured to be 0.54 upon the excitation in the wavelength range of 402-430 nm at 10 °C. Under the same conditions, the quantum yields for the EuIII emission of the Eu(tta)3dpbt colloidal particles (dav ) 33.1 nm) were measured to be 0.27, 0.27, 0.24, 0.19, 0.14, and 0.01 upon the excitation at 402, 420, 430, 440, 450, and 475 nm, respectively. The wavelength-dependent quantum yields of the nanoparticles can be attributed to the presence of different stacking structures of Eu(tta)3dpbt molecules in nanoparticles, which may alter the energetic structures and thereby the processes of excitation energy transfer among ligands and/or to the EuIII luminescent centers. Figure 2c shows the fluorescence spectrum (λex ) 430 nm) of the colloidal solution of Eu(tta)3dpbt nanoparticles (dav ) 33.1 nm) and that of a toluene solution of Eu(tta)3dpbt molecules with the same Eu(tta)3dpbt concentration. It is seen that, upon optical excitation at 430 nm, the EuIII luminescence intensity at

Eu(tta)3dpbt Colloidal Nanoparticles

614 nm of the Eu(tta)3dpbt colloidal solution is much higher than that of the toluene solution of Eu(tta)3dpbt. Eu(tta)3dpbt molecules in the colloidal particles possess a εΦ value of 9.9 × 103 M-1 cm-1 (λex ) 430 nm), which is about 4 times higher than that of Eu(tta)3dpbt molecules free in toluene. With a further increase in the excitation wavelength, the superiority in the luminescent capacity of the Eu(tta)3dpbt complex in nanoparticles over Eu(tta)3dpbt molecules free in toluene is more remarkable (Table 1). X-ray diffraction and electron diffraction measurements on the Eu(tta)3dpbt nanoparticles proved that their structures are amorphous. The remarkable bathochromic shifts in absorption and excitation spectra as well as the unique luminescent properties of the amorphous Eu(tta)3dpbt nanoparticles possibly originate from the formation of J-type aggregates of the polar Eu(tta)3dpbt molecules. It is well-known that the formation of J-aggregates of organic dye molecules will result in a red-shift in the UV-vis absorption bands.24 Ligands coordinated to lanthanide ions may stack via π-π interaction to form J-aggregates, which may lead to a red-shift in the optical absorption band of the ligands.25 However, reports on the extension of excitation (sensitization) windows for lanthanide ion luminescence of lanthanide complexes to a long-wavelength region by forming J-aggregates have so far been scarce. Zhang et al. reported that Eu(tta)3 molecules adsorbed on Ag nanoparticles showed a red-shifted absorption band relative to Eu(tta)3 molecules in solution, which was attributed to the formation J-aggregates of the adsorbed Eu(tta)3 molecules. However, the excitation windows for the EuIII luminescence of Eu(tta)3 molecules in solution and that adsorbed on the Ag particles are very similar, with a red edge at ∼400 nm.26 Very recently, Manseki and Yanagida investigated the absorption spectra and luminescent features of two nonanuclear Tb(III) clusters, Tb9(Hesa)16(µ-OH)10(NO3) (TNS-He, H-Hesa ) hexyl salicylate) and the analogous compound Tb9(Mesa)16(µOH)10(NO3) (TNS-Me, H-Mesa ) methyl salicylate).27 With respect to TNS-Me, TNS-He exhibited a remarkable enhancement in the molar extinction coefficient and luminescent intensity upon optical excitation at 380 nm as well as an obvious change in the shape of the absorption band. These phenomena were rationalized as being due to the enhanced J-type π-π stacking of the ligands in TNS-He. However, the red edges of the optical absorption windows of the two complexes locate at almost the same position of about 400 nm. On the contrary, the formation of Eu(tta)3dpbt nanoparticles led to remarkable extensions of both the absorption window and excitation window for EuIII luminescence to the long-wavelength region, and resulted in a significantly enhanced luminescent capability at long excitation wavelengths (420-475 nm). Time-resolved photoluminescence spectra of Eu(tta)3dpbt nanoparticles in colloidal solutions and Eu(tta)3dpbt molecules in toluene were investigated in this work (λex ) 430 nm). For the colloidal particles (dav ) 33.1 nm), the photoluminescence decay kinetics at the probing wavelength of 616 nm, corresponding to the 5D0 f 7F2 transition of Eu(tta)3dpbt, could be well accounted for by a three-exponential decay model function, which yielded the apparent decay time constants of 416, 221, and 69 µs. The (24) Ro¨sch, U.; Yao, S.; Wortmann, R.; Wu¨rthner, F. Angew. Chem., Int. Ed. 2006, 45, 7026–7030. (25) Huang, C. H.; Wang, K. Z.; Xu, G. X.; Zhao, X. S.; Xie, X. M.; Xu, Y.; Liu, Y. Q.; Xu, L. G.; Li, T. K. J. Phys. Chem. 1995, 99, 14397–14402. (26) Sun, Y. Y.; Jiu, H. F.; Zhang, D. G.; Gao, J. G.; Guo, B.; Zhang, Q. J. Chem. Phys. Lett. 2005, 410, 204–208. (27) Manseki, K.; Yanagida, S. Chem. Commun. 2007, 12, 1242–1244. (28) Werts, M. H. V.; Nerambourg, N.; Pe´le´gry, D.; Grand, Y. L.; B-Desce, M. Photochem. Photobiol. Sci. 2005, 4, 531–538.

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Figure 3. Decay curve of luminescence at 616 nm of the colloidal nanoparticles (dav ) 33.1 nm) in nondeuterated (red line) and deuterated (black line) solvents (λex ) 430 nm).

photoluminescence kinetics for Eu(tta)3dpbt molecules in toluene obeys monoexponential decay with a time constant of 480 µs as previously reported.17 To understand the multiphasic decay of EuIII luminescence in the colloidal particles, we prepared a colloidal solution of Eu(tta)3dpbt nanoparticles under similar conditions using deuterated water and deuterated methanol. The decay curve for the luminescent intensity at 616 nm of the Eu(tta)3dpbt nanoparticles in the deuterated solvents was measured to be identical to that in the nondeuterated solvents (Figure 3). Moreover, the quantum yield of EuIII luminescence of the nanoparticles dispersed in the deuterated solvents upon excitation at 430 nm was measured to be 0.23, which is very close to that of the nanoparticles in the corresponding nondeuterated solvents. It is then concluded that the H- or D-related vibration mode of solvent molecules contributes little to the nonradiative deactivation of the electronic excitations in the Eu(tta)3dpbt nanoparticles. Since the 616 nm luminescence kinetics of the nanoparticles exhibited three apparent time constants with the longest one (416 µs) close to that of Eu(tta)3dpbt free in toluene (480 µs), it is likely that the additional shorter-lived components originated from the static quenching centers in the nanoparticles, which should be partially responsible for the relatively low luminescence quantum yield with respect to Eu(tta)3dpbt free in solution. The identities of the quenching centers are unclear at present. Two-photon-sensitized luminescence of lanthanide (III) complexes offers a very promising manner for detecting labeled biomolecules with the advantages of high sensitivity, deep penetration, and less photodamage to biological samples, in which luminescence is induced via two-photon excitation (TPE) of a light-harvesting ligand and subsequent excitation energy transfer (EET) to the metal ions.1,18,28,29 The maximal two-photon excitation action cross section (δΦ) of Eu(tta)3dpbt molecules in the prepared nanoparticles (dav ) 33.1 nm) is 37.4 GM at 832 nm (Figure 4, 1 GM ) 10-50 cm4 s photo-1 molecule-1), a wavelength that is red-shifted by 24 nm compared to that for Eu(tta)3dpbt molecules dissolved in toluene.18 Although the maximal TPA action cross section is only about half of that for Eu(tta)3dpbt molecules in toluene, it is still a very high value among the reported lanthanide compounds.28 Moreover, the density of the Eu(tta)3dpbt solid was measured to be 0.92 g cm-3. From the density, it can be estimated that a Eu(tta)3dpbt nanoparticle (dav ) 33.1 nm) contains about 8.5 × 103 Eu(tta)3dpbt molecules. This estimation yields the maximal TPA action cross section of the Eu(tta)3dpbt nanoparticles (dav ) 33.1 nm) of 3.2 × 105 GM (1 GM ) 10-50 cm4 s photo-1 particle-1), which is (29) Picot, A.; Malvolti, F.; Guennic, B. L.; Baldeck, P. L.; Williams, J. A. G.; Andraud, C.; Maury, O. Inorg. Chem. 2007, 46, 2659–2665.

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Figure 4. Two-photon excitation action cross sections (δΦ) (b) and one-photon excitation spectrum (λem ) 614 nm) (solid line) for the Eu(tta)3dpbt nanoparticles (dav)33.1 nm). The experimental uncertainty for the two-photon excitation action cross sections is about 15%.

about 7 times higher than the highest value reported for the CdSe/ZnS core-shell quantum dots (dav ) ∼4.5 nm) determined by a similar method.30,31

Conclusion In summary, we have demonstrated an alternative approach, that is, forming Eu(tta)3dpbt nanoparticles in water/methanol (8:2 or 7:3) mixtures, to satisfy the combined requirements of good dispersibility in water solutions and efficient longwavelength sensitization for EuIII complexes to be used in biological applications. The size of Eu(tta)3dpbt colloidal particles (30) Larson, D. R.; Zipfel, W. R.; Williams, R. M.; Clark, S. W.; Bruchez, M. P.; Wise, F. W.; Webb, W. W. Science 2003, 300, 1434–1436. (31) Komoto, A.; Maenosono, S.; Yamaguchi, Y. Physica E 2004, 24, 74–77.

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with very high luminescent capabilities can be modulated to some extent by changing the preparation conditions. The optical excitation window for the EuIII luminescence of Eu(tta)3dpbt nanoparticles, extending up to 475 nm, is much wider than that of Eu(tta)3dpbt molecules dissolved in toluene. Quantum yields of EuIII luminescence of the prepared Eu(tta)3dpbt colloidal particles, with an average diameter of 33.1 nm, are 0.27, 0.27, 0.24, 0.19, 0.14, and 0.01 upon the excitation at 402, 420, 430, 440, 450, and 475 nm, respectively. The Eu(tta)3dpbt nanoparticles exhibited excellent two-photon sensitization performance with a highest δΦ value of 3.2 × 105 GM (1 GM ) 10-50 cm4 s photo-1 particle-1) at the excitation wavelength of 832 nm. The favorable luminescent properties and the good dispersibility in water solutions of the Eu(tta)3dpbt nanoparticles are very promising for the development of new luminescent nanoprobes for bioanalysis. Studies on the detailed sensitization mechanism of the EuIII luminescence of the Eu(tta)3dpbt nanoparticles and development of new nanoprobes based on the present Eu(tta)3dpbt nanoparticles are going on. Acknowledgment. This work is jointly supported by NSFC (20433010, 90606017, 20673144, 50521201, 20573005), NKBRSF (G2006CB806102) and 863 program from the Chinese Ministry of Science and Technology, and RFDP of the Ministry of Education of China. Supporting Information Available: The TEM image of Eu(tta)3dpbt colloidal particles prepared without CTAB surfactant and FT-IR spectrum of a precipitate obtained by adding water into a methanol solution of Eu(tta)3dpbt. This material is available free of charge via the Internet at http://pubs.acs.org. LA800903S