J. Phys. Chem. C 2010, 114, 17535–17541
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Up-conversion FRET from Er3+/Yb3+:NaYF4 Nanophosphor to CdSe Quantum Dots Artur Bednarkiewicz,*,† Marcin Nyk,‡ Marek Samoc,‡ and Wieslaw Strek† Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okolna 2, 50-422 Wroclaw, Poland, and Institute of Physical and Theoretical Chemistry, Department of Chemistry, Wroclaw UniVersity of Technology, Wybrzeze Wyspian´skiego 27, 50-370 Wroclaw, Poland ReceiVed: July 2, 2010; ReVised Manuscript ReceiVed: September 17, 2010
Fo¨rster resonance energy transfer (FRET) between nanoparticles of up-conversion lanthanide phosphor as donors and quantum dots as acceptors is demonstrated. Fluoride (NaYF4) nanocrystals (ca. 30 nm size) codoped with the Er3+ and Yb3+ ions were synthesized with a high-pressure solvothermal microwave assisted technique and dispersed in organic solvent. Up-converted luminescence from the rare-earth ions doped into fluoride nanomatrix was achieved with optical pumping in NIR (976 nm) region. The green erbium upconversion emission at 540 nm is efficiently quenched by quantum dots (QDs) leading to color change of the mixture for different relative concentrations. Simultaneously, orange emission from quantum dots appears due to energy transfer from Er3+/Yb3+:NPs donors to QDs acceptors. The concomitant decrease of the average lifetime of donor emission at 540 nm (from ∼153 to ∼130 µs) indicates that the excitation of CdSe QDs occurs not only through reabsorption but also through Fo¨rster Resonance Energy Transfer. Acceptor luminescence lifetime mimics that of the donor and reaches hundreds of microseconds. The Fo¨rster radius (R0) was calculated to be short (∼15 Å), mostly due to low quantum yield of the multilevel emitting donor. This short-range interaction proves that in our system the FRET occurs mostly through Er3+ ions proximate to the surface, resulting in efficiency of energy transfer equal to η ) 14.8%. Introduction Fo¨rster Resonance Energy Transfer (FRET) is a nonradiative energy transfer process between donor and acceptor fluorophores, which is widely used in bioassays,1 biosensing, and bioimaging2,3 in studying conformational changes of proteins,4 DNA hybridization, binding sites characterization, as a spectroscopic ruler,5,6 or in photodynamic therapy.7,8 Organic dyes have been most frequently used for FRET detection and imaging as donors or acceptors.9 They are small, demonstrate good biocompatibility, bioselectivity, and relatively high-fluorescence intensity. However, with typically excitation in UV/blue region, the dyes suffer from severe photobleaching and hampered multiplexing capabilities. Semiconductor nanocrystals (quantum dots, QDs) have been considered as alternate FRET energy donors,5,10,11 or acceptors6,12-14 because of their large absorption/emission cross sections, emission tuneability, and multiplexing capabilities. Another class of luminophores, Tb3+ and Eu3+ complexes has been also extensively studied and used as FRET donors.5,13,14 However, all these donors require photoexcitation in UV/blue spectral region, which limits their use to complex heterogeneous immunoassays. Simple homogeneous assays performed directly in tissue or whole blood, would be of high interest for biodetection or imaging.15,16 Only recently, a new class of biocompatible luminescent labels and FRET donors has been proposed. Lanthanide doped nanoluminophores, especially near-infrared (NIR) to visible upconverting nanoparticles (UCNPs) exhibit a number of advantages over down-converting organic fluorophores, quantum dots, or lanthanide chelates and possess great potential either for bioimaging or as efficient energy donors in FRET biodetection * To whom correspondence should be addressed. E-mail: a.bednarkiewicz@ int.pan.wroc.pl. † Polish Academy of Sciences. ‡ Wroclaw University of Technology.
schemes. Large Stokes shifts (>100 nm) together with very narrow absorption/emission band widths ( 0.99. Results and Discussion The UCNPs and QDs could be well-dispersed in chloroform to form stable colloidal solutions directly without further surface modification, due to the presence of oleic acid and TOPO capped layer on their surfaces. The chloroform dispersed CdSe QDs and fluoride UCNPs were uniform in size with a very narrow
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Figure 2. Up-conversion emission spectra and fluorescence image of 2%Er3+/20%Yb3+R-NaYF4 UCNPs without (green curve, UCNPs region on the picture) and with (red curve, UCNP+QDs region on the picture) presence of CdSe QDs. The absorption coefficient (RCdSe, light gray) and emission (I(λ)CdSe, dark gray) spectra under 405 nm excitation for pristine CdSe QDs are presented for comparison.
size distribution, as shown by the TEM picture (parts a and b of Figure 1). Part c of Figure 1 shows the XRD patterns of the UCNPs showing diffraction lines that can be ascribed to the face-centered cubic NaYF4 structure (JCPDS no. 77-2042). Figure 2 shows the up-converted emission spectra with and without the presence of CdSe QDs. Whereas red erbium emission at 650-670 nm (4F9/2f4I15/2) serves as the reference, green erbium emission at 520-550 nm (2H11/2,4S3/2 f 4I15/2) is effectively transferred to or absorbed by 555 nm absorption band of the CdSe QD acceptor, leading to 580 nm emission. Absorption RCdSe and emission I(λ) spectra under 405 nm excitation for pristine CdSe QDs are presented for comparison. The absorption coefficient of the QDs colloidal solution used in the studies is equal to ∼24 cm-1 at 560 nm, and the RCdSe values are plotted divided by 2000 to fit the normalized scale. The absorption coefficient of Yb3+ ions (at 976 nm) in the ∼10% colloidal solution (20 mg/mL) 2%Er3+/20%Yb3+:NaYF4 was equal to ∼0.0093 cm-1. There are sufficient conditions for UCNPs f CdSe QDs FRET or energy reabsorption by CdSe QDs to take place because the up-converted green luminescence of the donor overlaps perfectly with the strong absorption bands of the acceptor. However, acceptor emission due to solely reabsorption of donor emission in similar UCNP - SYBR green I system has been regarded as negligible.11 The following experiment allows one to draw conclusions about the dominating mechanism. On changing the relative content of the acceptor versus the constant donor concentration, visual change of up-converted color was observed (Figure 3) under 976 nm NIR excitation. Both green (2H11/2 + 4S3/2f4I15/2) to red (4F9/2f4I15/2) emission ratio (GRR) of erbium ions and QDs emission band to red (4F9/2f4I15/2) emission ratio (QRR) varied predictably versus the concentration of QDs in the blend. The error bars for GRR and QRR correspond to 20% variation of input green, red, and QD emission integrals. The GRR was proven to vary with the surface-to-volume ratio, excitation power as well as the size and shape of the hexagonal β-NaYF4 nanocrystals.27 Similar behavior may be expected in cubic nanocrystals as well. However, both the excitation intensity and UCNP/QDs size were kept constant during all of the experiments. Thus, the variation
Figure 3. GRR and QRR ratios as well as digital camera photos of the thin films excited with 976 nm NIR radiation measured for different relative concentration of the acceptor (QDs) vs the constant concentration of the luminescent UCNPs donor.
of GRR and QRR observed in Figure 2 results only from the changes in the relative concentration ratio between the NPs. The red spectrum in Figure 2 originates from UCNP + QDs system. The QDs admixed to UCNP introduced three different effects. QDs filtered (attenuated) the green erbium emission in relation to the unaffected red erbium emission. Second, the QDs were FRET acceptors of 2H11/2 + 4S3/2 energy of excited Er3+ ions, which caused the presence of weak and broad yellow emission centered at 585 nm (Figure 2, red curve). Finally, the QDs served as blue filter for their own emission, even in the case of thin samples. Because the IR excitation radiation (976 nm) penetrated deeply into the sample and excited Er/Yb:UCNP, which in turn transferred their energy to QDs, the FRET originated QDs emission was attenuated. However, because of the edge of QDs absorption located at ∼572 nm, only the blue tail of QDs emission was effectively filtered out resulting in the apparent 576 f 584 nm red shift in comparison to QDs emission under UV excitation. The red shift may be also explained by close packaging of the QDs during drying of the sample on the slide. This is because the solvent evaporation may lead to the loss of the capping agent resulting in the formation of collective electronic states between interacting QDs nanocrystals.28
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Figure 4. Luminescence decays of 4S3/2 level of Er3+ donor with (black) or without (red) the presence of CdSe QDs. The emission of the acceptor (green) was monitored at 585 nm.
The GRR ratio behavior supports the emission/reabsorption mechanism hypothesis, whereas QRR changes suggest resonant energy transfer to be present as well. Luminescence lifetimes of donor alone as well as those of donor and acceptor in the mixed system provide further details. The presence of acceptor (QDs) admixture leads to a decrease of average Er3+ luminescence lifetime (λmon ) 540 nm) from 153.1 ( 0.2 µs (χ2 ) 6.29 × 10-5, R2fit ) 0.99541) down to 130.4 ( 0.3 µs (χ2 ) 7.46 × 10-5, R2fit ) 0.99057) when excited with short 10 ns pulses at 976 nm (Figure 4). The luminescence lifetime of QDs (81.8 ( 0.2 µs, χ2 ) 2.36 × 10-5, 0.99051) recorded at 585 nm (the green curve of Figure 4) imitates donor emission rate of the 2H11/2, 4S3/2 levels of Er3+ ion, which is irrefutable evidence that FRET takes place. This is also in contrast to typically short luminescence lifetimes of QDs, which are on the order of nanoseconds. The CdSe QDs did not exhibit any emission under 976 nm excitation light used in the studies. In the recent article of Yan et al.,25 the increased photoconductivity of UCNP - CdSe QD nanoheterostructures under NIR photoexcitation was also explained by the FRET energy transfer from UCNP to QDs. The generated excitons dissociated in the external bias field and created charge carriers (hν f e- + h+). The observed processes are schematically illustrated in Figure 5, where higher energy levels of Er3+ ions (2H11/2 + 4S3/2 and 4 F9/2) are populated through the up-conversion of 976 nm radiation. The energy of these excited levels fits the energy gap between the conduction and valence bands of the CdSe QDs. The energy may be transferred either by resonant energy transfer (RET) or photons reabsorption (PR). On the basis of the present results, we are convinced that both mechanisms are present in our system, however the reabsorption is mainly responsible for GRR variation whereas QRR corresponds to resonant energy transfer. We suppose, these two mechanisms may originate from the relatively small size (∼25 nm) of the UCNPs and their high surface-to-volume ratio. Even though the Fo¨rster distance for QDs (as donors) or lanthanide downconverting chelate-QD pair has been assessed to reach 100 Å,29,14 the critical distance Ro for UCNPs as donors and QDs as acceptors may be lower. The Fo¨rster radius (Ro [Å]) is defined as a distance at which the energy transfer efficiency between donor and acceptor is
Figure 5. Energy transfer up-conversion (ETU) scheme of NIR 976 nm radiation to populate 2H11/2+4S3/2 and 4F9/2 levels of Er3+ ions in Er/Yb:NaYF4 nanocrystals. The emitting levels are populated either by nonradiative de-excitation (gray wave arrows) or cross-relaxations (CR1). The UCNPs energy is transferred to QDs either through FRET or photon reabsorption (PR). The corresponding conduction (CB) and valence (VB) bands of CdSe QDs are presented.
50%. The R0 (eq 2) depends on fluorescence quantum yield of the donor in the absence of the acceptor (Q0), the overlap integral (J) of the donor emission spectrum with the acceptor absorption spectrum and their mutual molecular orientation (κ2) and index of refraction (nd) as expressed by the following equation
R60
)
9Q0κ2J(ln 10) 128π5n4dNA
(2)
NA is the Avogadro number. The spectral overlap integral J is calculated as
J)
∫ fD(λ)εA(λ)λ4dλ
(3)
The normalized donor emission fD(λ) (2H11/2 + 4S3/2 f 4I15/2) overlaps perfectly the CdSe 560 nm absorption band, which is
Er3+/Yb3+:NaYF4 Nanophosphor to CdSe Quantum Dots an advantage when the J value is considered. However, beside green up-conversion emission, erbium ions emit in the red (4F9/2 f 4I15/2) and IR (i.e., major transitions are 4S3/2 f 4I13/2 at 850 nm, 4S3/2 f 4I11/2 at 1.23 µm, 4S3/2 f 4I9/2 at 1.7 µm) spectra ranges with higher efficiency resulting in only a fraction of energy able to be resonantly transferred to the acceptor. The fluorescence quantum yield (QY) of the donor in the absence of the acceptor (Qo) for the green emission is not very high. The QY values in the range of 0.005% to 0.3% were measured for several hexagonal NaYF4:2%Er3+, 20%Yb3+ nanoparticles with particle sizes ranging from 10 to 100 nm, whereas a QY of 3% was measured for a bulk sample.30 The spectral properties and quantum yield of Er3+-Yb3+ codoped materials are more complex functions of light density, temperature, size, and structural phase of the crystallites31 and in fact they vary considerably. The low QY observed for nanocrystallites may be attributed to the increase of surface-to-volume ratio for smaller nanoparticles leading to increased relative amount of doping ions (Yb3+ sensitizer and Er3+ emitter/donor) close to the surface of the nanocrystallite. This in turn results in the increase of nonradiative relaxation of the emitting and intermediate energy levels due to presence of solvent molecules in close proximity to the surface. Additionally, the increase of surface defects close to the outer shell of the NP may affect the superficial ions as another cause for reduced energy transfer QY. These facts underline the need for developing core-shell UCNPs to reduce the de-excitation mechanism for RE doped NPs as luminescent (bio)labels. A 300% increase in the measured QY was observed for 30 nm 2%Er3+/20%Yb3+:NaYF4 covered with undoped shell in comparison to bare core 30 nm 2%Er3+/20%Yb3+:NaYF4 UCNP, which is roughly similar to the QY comparison between 30 and 100 nm UCNPs.30 Another solution to increase QY would require the application of large energy gap lanthanides. The Eu3+ and Tb3+ excitation have been observed in bulk crystals through the indirect NIR excitation of codoping Yb3+ ions.32,33 To calculate R0 we need to know the κ2 value, which in most cases is assumed to be 2/3 when the donor and acceptor dyes are freely rotating and can be considered to be isotropically oriented during the excited state lifetime. In our case, neither donor or acceptor reorient on a time scale that is faster than their fluorescence lifetime which would require to set the k value in the 0 e κ2 e 4 range. Index of refraction of the medium between spacing the donor and acceptor was assumed to be n ) 1.46, which is an approximate value close to those for oleic acid n ) 1.459 and TOP (tri-n-octylphosphine) n ) 1.468 indexes. The surfactants are also spacers between donor and acceptor molecules and may increase the D-A distance up to a few nanometers. However, a number of studies has demonstrated closed packing of colloidal nanomaterial by the removal or exchange of the capping molecules28,34 due to thermal treatment or simple drying like in the present case. The close packing of the NCs may be supported by the red shift of QDs emission observed in Figure 2 as discussed earlier. One finds the Ro value is equal to ∼15 Å at κ2 ) 4 and Qo ) 0.01 ( 0.01% (Figure 6). This low Ro value results mainly from low Qo obtained for up-conversion in multilevel emission donor nanoparticle. Suyver et al31 achieved 23% of QY for micrometer sized 2%Er3+,18%Yb3+:β-NaYF4. Adoption of such a value would increase our R0 by 3.6 fold to reach 54 Å. However low Qo is justified because we are applying nanometer colloidal solution of the R-NaYF4 nanocubes, which is a less efficient phosphor than the β hexagonal phase. Low QY may limit the application of Er3+/Yb3+ UCNPs donors to short-range
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Figure 6. (a) Amount of erbium ions expressed as number of ions (Ni) and in percentage terms (Ni/NTOT) calculated for different distance from NC surface (bars with solid red dot and empty dot respectively). Cumulative number (ΣNi/NTOT) of erbium ions (cyan) as well as theoretical energy transfer efficiency for Fo¨rster distances R0 ) 100, 50, 25, and 12.5 Å as a function of distance from NC surface are presented. Right hand graphs present variation of R0 vs (b) κ2 and (c) QY.
FRET. However, even though the relatively large size of these NPs may prevent elaborated studies of protein conformation changes, these are bioassays and deep tissue LRET imaging that may profit when unique properties of UCNPs are taken into account. In the first approximation, the erbium emission from single UCNPs originates from all available Er3+ excited ions. However only superficial ions (around 12% of all available ions within 12 Å, Figure 6) are capable, due to the distance, to resonantly transfer their energy to acceptors. Having in mind that the superficial erbium ions are susceptible to nonradiative deexcitation by IR vibrations of organic/inorganic solvent molecules, the number of FRET donors further decreases in comparison to the number of erbium ions present in the core of the nanocrystallite. These proportions are reflected in low efficiency of the energy transfer and only minor changes in donor’s lifetime. To further quantify the presence of FRET one may calculate the transfer efficiency by relating the donor fluorescence intensities with (FD′) and without an acceptor (FD) according to the equation η ) 1 - FD’/FD. This approach is not reliable in our case due to attenuation of green erbium emission by QDs. The FRET efficiency may be alternatively quantified by comparison of the donor lifetimes in the presence (τ′D) and absence (τD) of the acceptor and it may be related to the Fo¨rster radius R0 and the distance R between the D-A pair:
η ) 1 - τD′ /τD ) R60 /(R60 + R6)
(4)
According to the decay measurements, the energy transfer efficiency equals ca. 14.8 ( 0.3%. The resulting R equal to ∼20 Å confirms our suspicions that in our system the FRET occurs mostly through Er3+ ions proximate to the surface. The low total efficiency of energy transfer calculated with eq 4 may also result from the fact that the donor luminescence originates from all erbium ions evenly distributed in the volume of the UCNP, whereas only superficial ions take part in the energy transfer to QDs. It is worth mentioning that, due to long luminescence lifetime of the 2H11/2 + 4S3/2 levels of Er3+, the QDs emission lifetimes become apparently much longer than those in pristine QDs.
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Considering 30 nm diameter of R-NaYF4 nanocrystallites (cubic elementary cell with a ) 5.47 Å, 8 F-, 7Na+, 7Y3+ ions per unit cell) doped with 2% of Er3+ and 20% of Yb3+ (which substitute Y3+ ions), one may calculate the volume of the crystallite (14.1 × 106 Å3) and of the cell itself (163.7 Å3). With the assumption of uniform distribution of Er3+/Yb3+ dopants in the whole volume of the nanocrystallite, the Er3+ and Yb3+ ions may be statistically found in one unit cell per 50 and 5, respectively. Within a single cubiclike nanocrystallite one may find 95 468 unit cells and ca. 1909 Er3+ ions available, one per 50 unit cells of 8185 Å3 volume, and an average distance of 25 Å between Er3+ ions. These estimations in comparison to Ro ) ∼15 Å suggest that one erbium ion (donor) to one QD (acceptor) is a dominating energy transfer mechanism; even though many donors exist within single UCNP and many acceptors may locate on the surface of the UCNP. These estimations allow also assessing the amount of Er3+ ions (donors) as a function of distance from the surface of the NC. Figure 6 shows, that within the outermost 11 and 21 Å NC layers, correspondingly 12.5 and 24% (i.e., 240 and 459 ions) of total erbium ions present in the whole NC volume may be found. Only these ions may be effective energy donors to QD acceptors (ηFRET ) ∼15%). Simultaneously, these superficial ions may be susceptible to parasitic emission quenching at the surface of UCNPs. Most of the Er3+ ions (1450 ions, i.e. 76% of total) are responsible for the unquenched (due to either FRET or de-excitation) emission. The green emission coming from these ions may be however reabsorbed by QDs all around and thus give rise to the GRR variation. The unperturbed erbium emission originating from the core of the NCs gives rise to the total erbium luminescence observed from the sample and is therefore responsible for the minor change in the luminescence lifetime and thus lower energy transfer quantum yield observed for the D-A pair. One may conclude that the improvement in ηFRET could be observed either for undoped core/doped shell NPs or smaller UCNPs as donors. The latter will unfortunately lead to a decreased amount of active ions in the donor NPs hindering the luminescence recording. Even though the absorption/ emission cross section of forbidden f-f transitions in RE doped materials is a few orders of magnitude lower than that in the organic fluorophores or QDs, the UCNPs are perfectly photostable, provide long luminescence lifetimes, and may be excited in the NIR spectral region. In consequence, extremely long observation times, time gated detection, and absence of sample autofluorescence make these UCNPs highly promising materials as biolabels and FRET donors for bioassays and deep tissue imaging. Summary In summary, a new approach is presented to produce FRET/ LRET systems based on fluoride up-conversion nanophosphors as donors and CdSe QDs as acceptors. We have demonstrated that this system possesses the essential features required for FRET applications. These include high D-A spectral overlap described by J(λ) and satisfactory quantum yield Qo of the donor with no acceptor present. Both acceptor emission and donor luminescence lifetime decrease were experimentally demonstrated and were in good agreement with the theoretical R0 value. The main advantage of using UCNPs doped with lanthanides as luminescent labels or FRET energy donors comes from the ability of using NIR excitation. This allows eliminating autofluorescence of biological samples and drastically reducing background signal because no endogenous chromophores absorb NIR excitation. Two-photon excitation is feasible; however, up-
Bednarkiewicz et al. conversion in RE ions is a few orders of magnitude more efficient. Additionally, due to long living luminescence of lanthanides, time gated/delayed luminescence detection or imaging is easily achievable, leading to further reduction of the background emission. Finally, the lanthanides’ luminescence is practically not susceptible to photobleaching and lanthanidedoped nanocrystals are barely prone to photochemical decomposition/alterations. These three factors are critical, when sensitivity is to be considered either for protein/DNA assaying or biopsy screening. Thus, small sample volumes with little or no sophisticated sample (e.g., biopsy) preparation may satisfy the requirements of rapid screening tests. Additionally, the sample labeled with these kinds of luminescent probes may be examined for extended periods of time, which is especially useful when protein distribution/transportation within cell volume/tissue is studied. Thus small animals imaging could possibly gain by employing these UCNPs-QDs FRET systems when attached to proper drug delivering systems. Both high photostability and high depth of NIR light penetration into tissues may guarantee high-contrast bioimaging. Finally the donor’s NIR-to-VIS up-conversion emission demonstrates characteristic multiwavelength narrow absorption/ emission spectral lines excitable in the NIR range (976 nm), which may provide fundamentals for ratio-metric multicolor multiplexed imaging, when many biotargets are to be reached within the same sample volume. Together with their low toxicity, good chemical resistance and excellent photostability make these nanolabels new frontiers in the biodetection field. Acknowledgment. A.B. acknowledges support from the MNiSW under Grant No. N N507 584938. M.N. and M.S. acknowledge support from the Foundation for Polish Science and the MNiSW under Grant No. N N507 599038. The authors are grateful to L. Kepinski and L. Krajczyk for performing electron microscope imaging of CdSe QDs. References and Notes (1) Klostermeier, D.; Sears, P.; Wong, C. H.; Millar, D. P.; Williamson, J. R. Nucleic Acids Res. 2004, 32, 2707. (2) Jares-Erijman, E. A.; Jovin, T. M. Nat. Biotechnol. 2003, 21, 1387. (3) Prasad, P. N. Introduction to Biophotonics; Wiley-Interscience: New York, 2003. (4) Heyduk, T. Curr. Opin. Biotechnol. 2002, 13, 292. (5) Hildebrandt, N.; Lohmannsroben, H.-G. Curr. Chem. Biol. 2007, 1, 167. (6) Beck, M.; Hildebrandt, N.; Lohmannsroben, H. G. In Biophotonics and New Therapy Frontiers; Grzymala, R., Haeberle, O., Eds.; 2006; Vol. 6191, p X1910. (7) Samia, A. C. S.; Chen, X. B.; Burda, C. J. Am. Chem. Soc. 2003, 125, 15736. (8) Bakalova, R.; Ohba, H.; Zhelev, Z.; Ishikawa, M.; Baba, Y. Nat. Biotechnol. 2004, 22, 1360. (9) Truong, K.; Ikura, M. Curr. Opin. Struct. Biol. 2001, 11, 573. (10) Lee, J.; Govorov, A. O.; Kotov, N. A. Nano Lett. 2005, 5, 2063. (11) Liu, W.; Howarth, M.; Greytak, A. B.; Zheng, Y.; Nocera, D. G.; Ting, A. Y.; Bawendi, M. G. J. Am. Chem. Soc. 2008, 130, 1274. (12) Clapp, A. R.; Medintz, I. L.; Fisher, B. R.; Anderson, G. P.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 1242. (13) Hildebrandt, N.; Charbonniere, L. J.; Lohmannsroben, H. G. J. Biomed. Biotechnol. 2007, 2007, 79169. (14) Charbonniere, L. J.; Hildebrandt, N.; Ziessel, R. F.; Loehmannsroeben, H. G. J. Am. Chem. Soc. 2006, 128, 12800. (15) Kuningas, K.; Pakkila, H.; Ukonaho, T.; Rantanen, T.; Lovgren, T.; Soukka, T. Clin. Chem. 2007, 53, 145. (16) Rantanen, T.; Jarvenpaa, M. L.; Vuojola, J.; Kuningas, K.; Soukka, T. Angew. Chem., Int. Ed. 2008, 47, 3811. (17) Nyk, M.; Kumar, R.; Ohulchanskyy, T. Y.; Bergey, E. J.; Prasad, P. N. Nano Lett. 2008, 8, 3834. (18) Kumar, R.; Nyk, M.; Ohulchanskyy, T. Y.; Flask, C. A.; Prasad, P. N. AdV. Funct. Mater. 2009, 19, 853. (19) Wang, M.; Hou, W.; Mi, C. C.; Wang, W. X.; Xu, Z. R.; Teng, H. H.; Mao, C. B.; Xu, S. K. Anal. Chem. 2009, 81, 8783.
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