Modification of NaYF4: Yb, Er@ SiO2 Nanoparticles with Gold

Feb 9, 2011 - Modification of NaYF4:Yb,Er@SiO2 Nanoparticles with Gold Nanocrystals for Tunable Green-to-Red Upconversion Emissions. Zhengquan Li†* ...
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Modification of NaYF4:Yb,Er@SiO2 Nanoparticles with Gold Nanocrystals for Tunable Green-to-Red Upconversion Emissions Zhengquan Li,†,* Limin Wang,† Zeye Wang,† Xinghui Liu,† and Yujie Xiong‡,* †

Department of Materials Physics and Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, P. R. China, and ‡ School of Engineering and Applied Science and Nano Research Facility, Washington University in St. Louis, St. Louis, Missouri 63130, United States ABSTRACT: Lanthanide-doped upconversion nanocrystals which can convert near-infrared lights to visible lights have attracted growing interest because of their great potentials in biomedical engineering. However, it remains a grand challenge to maneuver the intensity ratio between different emission lines and enable tunable upconversion functions. Herein, we present a facile method to integrate NaYF4:Yb, Er upconversion nanocrystals with gold nanocrystals for constructing NaYF4:Yb,Er@SiO2@Au hybrid nanostructures, in which a silica layer on NaYF4:Yb,Er nanocrystals serves as an interface for gold decoration. The green-to-red emissions of the upconversion nanocrystals can be conveniently tuned by altering the amount of surrounding gold nanocrystals. We further demonstrate the capability of utilizing both green and red emission lines for spontaneous signaling in an emitter-quencher-based bioassay by implementing this novel hybrid nanostructure. It is anticipated that the controllability in upconversion fluorescence in this hybrid nanostructure may provide a platform for widely exploring applications in biological imaging, detection, and sensing.

1. INTRODUCTION Inorganic nanocrystals that exhibit a wide range of sizedependent properties have been intensively explored in the past decades. Recently, considerable attention has been paid to interaction of multiple nanocrystals with different intrinsic properties into functional conformation because hybrid nanostructures can be endowed with tunable physicochemical properties originating from interactions among nanocrystals.1,2 To investigate the coupling between excitation and plasmonics at nanoscale, many self-assemblies of nanocrystals of semiconductors and noble metals have been developed in past years.3,4 In addition, a number of multifunctional nanoparticles in which semiconductor nanocrystals are combined with magnetic nanoparticles have been prepared for exploiting their biomedical applications.5,6 As a promising luminescent probe for biological species, upconversion nanocrystals (UCNs) have attracted growing interest owing to their unique optical properties which allow minimization of autofluorescence and photodamage as well as enable high penetration depth in biospecies when excited with a near-infrared (NIR) light source.7-10 A great deal of research effort has been focused on the synthesis and surface-modification of UCNs in the past few years;11-15 however, the integration of UCNs with plasmonic nanocrystals remains largely unexplored, particularly for controlling the fluorescent properties of UCNs with plasmonic metals. Theoretically, two alternative interactions may occur when a noble metal approaches to a phosphor: a fluorescence enhancement effect and nonradiative quenching. The former effect enhances fluorescence of the phosphor by increasing the incident electromagnetic field arising from the surface plasmon resonance r 2011 American Chemical Society

(SPR) of the metal. The fluorescence of UCNs has been improved along this mechanism, such as decorating UCNs on silver nanowires or coating a gold shell on them.16,17 The latter reduces fluorescence of the phosphor by nonradiative energy transfer from the phosphor to the metal via SPR absorption. This process could be utilized for emitter-quencher-based bioassays and has been demonstrated as a highly sensitive and powerful tool in bioassays when the phosphor and metal are employed as energy emitter and quencher, respectively.18 Although luminescent UCNs and gold nanocrystals have been recognized as an effective emitter-quencher pair in a few examples of biomolecule detection, reports about the synthesis of hybrid assemblies of UCNs and gold nanocrystals with controllable upconversion fluorescence are still rare. Fluorescence of lanthanide-doped UCN originates from the inner f-f transitions of lanthanide atoms, and several emissions with a certain intensity ratio generally appear at one time. It has been proven difficult to maneuver the intensity ratio between different emission lines of the UCNs, since transition possibilities of these emissions are relatively constant. Although adjustment of the lanthanide doping level in the host nanocrystals offers a way to tune the intensity ratio, the narrow window of doping level for favoring optimal fluorescence is a major limitation because overdoping mostly results in heavy fluorescence quenching.19 In comparison, altering the surrounding environment may provide a more powerful and versatile means to control the Received: November 5, 2010 Revised: January 5, 2011 Published: February 09, 2011 3291

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The Journal of Physical Chemistry C intensity ratio between different emissions of the lanthanidedoped UCNs. In this work, we present a rational route to design and synthesize a binary assembly of NaYF4:Yb,Er UCNs and gold nanocrystals using a modified silica layer as an interface. We have been able to maneuver the green-to-red emissions of the UCNs by altering the number of gold nanocrystals on their surface and by taking advantage of surface plasmonic features that gold nanocrystals exhibit a strong band in the green region. Furthermore, we have demonstrated the potentials in utilizing both green and red emissions of the NaYF4:Yb,Er UCNs for emitter-quencher-based bioassays by employing the silicamodified NaYF4:Yb,Er UCNs and gold nanocrystals as an emitter-quencher pair. In contrast to a traditional fluorescent emitter such as quantum dots and organic dyes in an emitter-quencherbased process in which only monoemission is used for signaling, the two emissions for simultaneous signaling demonstrated in our system will definitely improve detection accuracy and provide reliable measurement for quantitative analysis. It is expected that the controllable UC fluorescence and unique features in the emitter-quencher-based process offered by this novel nanostructure may lead to broad applications in future biomedical engineering.

2. EXPERIMENTAL SECTION Synthesis of NaYF4:Yb,Er@SiO2 Nanoparticles and Gold Nanocrystals. Monodisperse NaYF4:Yb20%, Er2% UCNs of

20 nm diameter were synthesized using a user-friendly method we developed previously.14 In brief, powders of three lanthanide chlorides with a stoichiometric ratio were dissolved in a mixed solution of oleic acid and octadecene, which acted as surfactant and solvent, respectively. After a methanol solution of NH4F and NaOH were added, the mixed solution was slowly heated to evaporate methanol, degassed, and then maintained at 300 °C for 1 h. After precipitating from the solution and being washed three times, the hydrophobic NaYF4:Yb,Er UCNs were finally dispersed in cyclohexane. Coating of the NaYF4:Yb,Er UCNs with a 8 nm silica layer was performed by a microemulsion method.14 The microemulsion was formed by mixing a certain amount of the UCNs in cyclohexane, ammonia, and CO-520 surfactant under ultrasonic treatment. After tetraethyl orthosilicate (TEOS) was added into the microemulsion and rotated for 2 days, a uniform silica layer appeared on the surface of the NaYF4:Yb,Er UCNs. Gold nanocrystals with a diameter of 5 nm were prepared by citrate reduction methods in the presence of tannic acid as coreductant, which has been reported in detail in the literature.20 Preparation of NaYF4:Yb,Er@SiO2@Au Hybrid Nanostructures. The surface of the NaYF4:Yb,Er@SiO2 nanoparticles was initially modified with amino groups before conjugating to gold nanocrystals. In a typical process, 0.02 mmol of NaYF4:Yb,Er@ SiO2 nanoparticles were dispersed in 10 mL of water containing 200 μL of acetic acid. Then, 20 μL of (3-trimethoxysilylpropyl)diethylenetriamine was added to the solution and stirred for 4 h.21 The amino-modified nanoparticles were collected by centrifugation; washed with DI water twice; and finally, dispersed in DI water. To prepare NaYF4:Yb,Er@SiO2@Au hybrid nanoparticles (for example, mole ratio of NaYF4:Yb,Er/Au = 0.4/1), a 1 mL suspension of amino-modified NaYF4:Yb,Er@SiO2 nanoparticles (0.004 M) was swiftly added into a 1 mL suspension of gold nanoparticles (0.01 M). After being vigorously rotated for 5 min, the mixed solution was sealed in a 2 mL plastic tube and

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slowly rotated for another 2 h. A series of NaYF4:Yb,Er@SiO2@ Au hybrid nanoparticles with a target NaYF4/Au ratio (such as 0.2/1 and 0.6/1) were prepared by a similar process except changing the added amount of NaYF4:Yb,Er@SiO2 nanoparticles. Characterizations. Transmission electron microscopy (TEM) images were recorded on a JEOL 2010F TEM. The TEM samples were prepared by dropping a suspension of nanoparticles on a carbon-film-coated copper grid. X-ray powder diffraction (XRD) was carried out on a Japan Rigaku D/max rA X-ray diffractometer equipped with Cu KR radiation. UV-vis extinction spectra were collected on a Shimadzu UV-vis spectrophotometer (UV-2450). Fluorescence spectra were acquired on a SpectroPro 2150i fluorescence spectrometer equipped with a commercial 980 nm NIR laser. The zeta potential of the nanoparticles was performed on a Malvern Zetasizer Nano ZS dynamic light scattering system. DNA Quantification Using NaYF4:Yb,Er@SiO2 Nanoparticles and Gold Nanocrystals. Synthetic DNA modified with a thiol linker at the 50 end was purchased from Integrated DNA Technologies. The DNA has a short single strand of five bases, and its sequence is shown as follows: 50 -(thiol)-TTTTT. To prepare Au-DNA conjugates, 100 nM of the DNA was mixed with the gold nanocrystals (10 μM) in a 0.5 TBE buffer containing 50 mM NaCl. After incubation for 24 h, the conjugates were collected by centrifuge and washed with TBE buffer twice. To demonstrate the capability of using NaYF4:Yb,Er@SiO2 and Au nanocrystals for emitter-quencher-based quantification, aliquots of amino-modified NaYF4:Yb,Er@SiO2 nanoparticles (1 μM) were slowly added into 1 mL of Au-DNA conjugates at a certain molar ratio (e.g., DNA/NaYF4:Yb,Er = 0.01/1, 0.02/1, 0.03/1, etc.), and the mixed solution was then diluted to 2 mL. By measuring the fluorescence spectra of these solutions under the same conditions, we could draw two individual lines by plotting the emission intensity of the 525 and 655 nm peaks of each solution against the DNA concentration.

3. RESULTS AND DISCUSSION Figure 1A shows a typical TEM image of the as-synthesized NaYF4:Yb,Er nanocrystals with an average diameter of 20 nm. These nanocrystals are uniform in size and bear a hydrophobic surface due to the capping of oleic acid on their surface. To make these nanocrystals water-dispersible and suitable for conjugation, a uniform silica layer is coated on their surface by following a microemulsion method.5,14 The layer consists of amorphous SiO2 with a thickness of around 8 nm, as shown in the TEM image (Figure 1B). High-resolution TEM observation suggests that crystal lattice fringes from core nanocrystals are clearly captured in the presence of a silica coating layer. Meanwhile, no lattice fringe can be observed from the shell (Figure 1C). Taken together, the high crystallization of NaYF4:Yb,Er nanocrystals and the amorphous nature of silica layer are confirmed. After aminosilane is grafted onto the silica layer, the morphology of the core-shell nanoparticles is well maintained (Figure 1D), which suggests that the amino modification process has no side effect on the uniformity of the NaYF4:Yb,Er@SiO2 nanoparticles. During the amino modification process, the surface charge of the core-shell nanoparticles is altered from negative (ζ = -20 mV) to positive (ζ = þ35 mV), confirming that the amino groups have been successfully grafted onto these nanoparticles. The existence of amino groups provides a powerful claw for the NaYF4:Yb,Er@SiO2 nanoparticles to grab gold nanocrystals, thanks to the strong coordination between the gold and the -NH2 group. 3292

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Figure 1. Schematic illustration for synthetic approach to NaYF4:Yb, Er@SiO2@Au nanostructures. A uniform silica layer is coated on the NaYF4:Yb,Er nanocrystals and then modified with amino groups. Gold nanocrystals are attached to the NaYF4@SiO2 core-shell nanoparticles through coordination between the gold and the -NH2 groups and electrostatic attraction. Figure 3. (A) XRD pattern of the amino-modified NaYF4:Yb,Er@ SiO2 nanoparticles. (B) Upconversion spectra of pure NaYF4:Yb, Er nanocrystals and amino-modified NaYF4:Yb,Er@SiO2 core-shell nanoparticles.

Figure 2. TEM images of prepared NaYF4:Yb,Er nanocrystals (A) before silica coating and (B) after coated with a 8 nm silica layer. (C) High-resolution TEM image of a single NaYF4@SiO2 nanoparticle. (D) TEM image of NaYF4@SiO2 nanoparticles after being modified with amino groups.

XRD patterns of the amino-modified NaYF4:Yb,Er@SiO2 nanoparticles are shown in Figure 2A. All the peaks can be indexed to pure hexagonal-phase NaYF4 crystal (JCPDS standard card No. 28-1192). Due to the amorphous nature, no defined peak from the silica shell is observed except a small uplift on the baseline at the low angle region, which may be caused by

the diffraction between irregular molecule layers of amorphous SiO2. Upconversion fluorescence spectra of the pure NaYF4:Yb, Er nanocrystals is show in Figure 3B. Two green peaks (525 and 543 nm) and a red peak (655 nm) are ascribed to the energy transitions from 2H11/2, 4S3/2, and 4F9/2 to 4I15/2 of Er3þ ions, respectively,12 when excited under a commercial continuous wave diode NIR laser (power density ≈ 500 mW cm-2). After the NaYF4:Yb,Er nanocrystals were coated with a silica layer and modified with amino groups, their fluorescence decreased to some extent because of the light-scattering effect on both emission and incident light by the silica layer. Despite the decrease in fluorescence, the NaYF4:Yb,Er@SiO2 nanoparticles still show naked-eye emissions under a NIR laser, which remains strong enough for various biomedical applications such as imaging, detection, and sensing.8,9 After the surface is modified with amino groups, the NaYF4: Yb,Er@SiO2 nanoparticles show strong binding to gold nanoparticles due to two contributions. First, the amino groups have strong interactions with the Au nanocrystals due to the coordination between the gold and the -NH2.22 Second, the aminomodified NaYF4:Yb,Er@SiO2 nanoparticles are positively charged while the prepared gold nanocrystals have negative surface charges, and as such, a strong electrostatic attraction occurs between these two types of nanoparticles. Once they are mixed in solution, small gold nanocrystals will be swiftly attached to the NaYF4:Yb,Er@SiO2 nanoparticle surface to form dual shellstructured NaYF4:Yb,Er@SiO2@Au hybrid nanoparticles. A typical TEM image of the NaYF4:Yb,Er@SiO2@Au nanostructures (molar ratio of NaYF4/Au = 0.4/1) is shown in Figure 4A. From this image, one can see that most Au nanocrystals have been attached to the NaYF4:Yb,Er@SiO2 nanoparticles and no free Au nanocrystal has been found nearby. By 3293

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Figure 4. TEM image of the as-prepared NaYF4:Yb,Er@SiO2@Au nanostructures with different molar ratio of NaYF4:Yb,Er to Au: (A) 0.4 and (B) 0.8. (C, D) High-resolution TEM images of a single NaYF4: Yb,Er@SiO2@Au nanostructure (NaYF4/Au = 0.4) at different magnifications.

adjusting the molar ratio of two types of nanoparticles during the assembly process, one can prepare a series of hybrid nanostructures with a similar structure but different decoration amount. For example, a TEM image of NaYF4:Yb,Er@SiO2@Au nanostructures (molar ratio = 0.8/1) is shown in Figure 4B. Their structures are very similar to those in Figure 4A, except that fewer Au nanocrystals are attached on the NaYF4:Yb,Er@SiO2 nanoparticles. High-resolution TEM images of the hybrid nanostructures at different magnifications are shown in Figure 4C and D, respectively. Crystal lattices from the core nanocrystal can be clearly indexed to the (001) plane of hexagonal NaYF4, and those from the exterior nanocrystals match well with the (111) plane of face-centered cubic gold. These results confirm the high quality and detailed structure of the prepared hybrid nanostructures. Upconversion fluorescence spectra of the NaYF4@SiO2@Au hybrid nanostructures with different NaYF4/Au ratios are shown in Figure 5A. Spectral profiles of these nanoparticles are significantly varied in terms of intensity, which can be attributed to the difference ratio of NaYF4:Yb,Er@SiO2 to gold, although the locations of the two characteristic emissions are consistent. Specifically, the green emission (525 nm) of these hybrid nanostructures is enhanced much faster than the red emission (655 nm) when the NaYF4/Au ratio is increased. When plotting the intensity ratio of emission peak at 525 nm to that at 655 nm against the NaYF4/Au ratio (Figure 5B), one can find that the green emission becomes stronger than the red emission once the NaYF4/Au ratio is above 0.6, whereas the red emission is dominant when the NaYF4/Au ratio is less than 0.4. These results suggest that one can maneuver the green-to-red emissions of the hybrid nanostructures by simply adjusting the molar ratio of NaYF4:Yb,Er/Au. In the size regime of gold nanocrystals in our studies, they have a strong SPR band in the green region while there is resonance

Figure 5. (A) Upconversion fluorescence spectra of the NaYF4:Yb, Er@SiO2@Au nanoparticles prepared using different molar ratios of NaYF4 to Au. (B) Curve of plotting the intensity ratio between emission lines at 525 and 655 nm versus the molar ratio of NaYF4 to Au, showing that green-to-red emissions could be tuned by the NaYF4/Au ratio. (C) Normalized upconversion spectra of NaYF4:Yb,Er nanocrystals (green line) and extinction spectra of gold nanocrystals (red line).

in the red region (Figure 5C). It is hypothesized that the function of gold nanocrystals in the present system works mainly through absorption mechanism, given that green light is suppressed rather than enhanced by Au. Note that the surface plasmonics of gold nanocrystals is a sum of absorption and scattering. Thus, green upconversion emission of NaYF4:Yb,Er@SiO2 nanoparticles will be absorbed more than the red emission when gold nanocrystals are attached to their surface. As such, the intensity ratio of green to red emissions in the NaYF4@SiO2@Au nanostructures could be tuned by altering the ratio of NaYF4/Au. We have attempted to graft gold nanocrystals of different sizes (e.g., 10 nm) to NaYF4@SiO2 nanoparticles. Our preliminary results show that gold nanocrystals with larger size can tune the greed-to-red emissions in their upconversion spectra more efficiently than small ones at the same molar concentration of gold, because larger gold nanocrystals have a larger absorption 3294

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Figure 6. Plots of upconversion emission intensity of the peaks at 525 and 655 nm versus DNA concentration, respectively. It shows a linear relationship between the fluorescence peak intensity and the DNA concentration (below 100 nM) on both emission lines. Above a concentration of 100 nM, both emissions show a higher intensity than that linearly corresponding to the DNA concentration as a result of insufficient Au nanocrystals that are attached to NaYF4@SiO2 nanoparticles.

coefficient in the green region as compared with their small counterparts. We have measured the fluorescence intensity of amino-modified NaYF4:Yb,Er@SiO2 before and after being decorated with gold nanocrystals of different sizes. When 5-nm gold nanocrystals were used, the quenching efficiency was ∼76% to the green emission and 28% to the red emission. As we quenched fluorescence with 10-nm gold nanocrystals, the quenching efficiency was 85% (green) and 36% (red), respectively. A more careful study of the upconversion spectra in Figure 5A shows that both the green and red emissions linearly increased with the concentration of the UCNs. This suggests that it is possible to use NaYF4:Yb,Er@SiO 2 nanoparticles and gold nanocrystals as an emitter-quencher pair for biomolecule quantification when biomolecules are bonded on either type of nanoparticles. To demonstrate the potential in using these two nanoparticles for bioassays, we attached a short singlestrand DNA to the gold nanocrystals and then slowly added different amounts of amino-modified NaYF4@SiO2 to the Au-DNA conjugates. Figure 6 displays the relationship between the DNA concentration and the emission intensity of peaks at 525 and 655 nm, respectively. The intensity of either fluorescence peak linearly increases along with the DNA concentration in a range from 10 to 100 nM, indicating that either of the emissions can be utilized for the quantification of DNA. However, when the concentration of DNA reaches above 120 nM (namely, a NaYF4/Au ratio beyond 1.2:1), both green and red emissions show slightly higher intensity than that in the linear relationship with DNA concentration. It should be attributed to the fact that the ratio of NaYF 4@SiO2 to Au is too high so that there are insufficient Au nanocrystals to quench fluorescence. Note that the number of DNA strands is varied on each gold nanocrystal in a typical conjugation process. The number of DNA strands per gold nanocrystal does not affect the assay as long as all the DNA strands in solution can be conjugated to gold nanocrystals. Au-DNA conjugates remain negatively charged, so they can be attached to the positively charged NH2NaYF4@SiO2 nanoparticles through electrostatic attraction, regardless of whether the gold nanocrystals are capable of coordinating with amino groups in addition to the DNA coverage. As

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such, the accuracy of this assay is not dependent on the DNA concentration per gold nanocrystal. In previous emitter-quencher-based bioassays, quantum dots or fluorescent dyes have been used as fluorescent emitters. As a result, only monoemission could be used for the quantification of biomolecules.23,24 As such, its accuracy is largely affected by the fluorescence stability of phosphors during experimental process. In most cases, biological environments, particularly in in vivo studies, are much more complex, which introduces many factors affecting fluorescence stability. In our system, in addition to the advantage that fluorescence of lanthanide nanocrystals is relatively stable in various environments, the NaYF4:Yb,Er UCNs offer two strong emissions to act as fluorescent emitters simultaneously. Notably, two emissions for simultaneous signaling will greatly improve the detection accuracy and provide a platform for reliable measurements in bioassays. It is worth pointing out that the distance between the fluorescent emitter and acceptor is important in a typical emitter-acceptor-based system. In our case, we employed a microemulsion technique that can form a very uniform silica layer on individual hydrophobic NaYF4 nanocrystals. As compared with the traditional sol-gel process (i.e., so-called “st€ober method”), this technique greatly reduces the size of the resultant nanoparticles and thus facilitates their future applications in biomedicine and other applications. To this end, we specifically chose the microemulsion route instead of the st€ober method in our process. However, the thickness of the silica layer could not be finely tuned with this technique, although we have attempted to control the thickness of the silica shell by adding different amounts of TEOS. For instance, a loose layer of silica with similar thickness was obtained when reducing the amount of TEOS to half; many free silica nanospheres were produced along with the NaYF4@SiO2 nanoparticles as the amount of TEOS was doubled. For this reason, it makes it not feasible to achieve controllable thickness of silica shell in our system, particularly in the range of 1-10 nm. As a result, it could not provide a valid platform for us to investigate the distance effect. Our future efforts will further modify the silica coating process and allow us to tune the silica thickness and perform a systematic investigation of this aspect.

4. CONCLUSIONS In summary, we have developed a facile method to synthesize a binary assembly of NaYF4:Yb,Er UCNs and gold nanocrystals. Gold nanocrystals could be swiftly attached to the surface of NaYF4:Yb,Er@SiO2 nanoparticles by coating a silica layer and modifying it with amino groups for interfacing UCNs and gold. The prepared NaYF4:Yb,Er@SiO2@Au hybrid nanostructures have showed controllable green-to-red emissions that are achieved by simply adjusting the NaYF4/Au ratio during the assembly process. This work represents a new strategy for maneuvering the fluorescence of lanthanide nanocrystals by altering their surrounding environment. We have also demonstrated the capability of utilizing both green and red emissions of UCNs for spontaneous signaling in emitter-quencher-based bioassays in which NaYF4:Yb,Er@SiO2 nanoparticles and gold nanocrystals are used as fluorescent emitters and quenchers, respectively. It is expected that the unique properties and functions offered by this novel binary assembly will enable its wide implementation in biological applications such as imaging, detection, and sensing. 3295

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.L.). *E-mail: [email protected] (Y.X.).

’ ACKNOWLEDGMENT The authors acknowledge financial support from National Nature Science Foundation of China (No. 20901068) and Zhejiang Qianjiang Talent Project (No. 2010R10028). Y.X. is supported in part by the National Nanotechnology Infrastructure Network (NNIN) (funded by the US NSF under award no. ECS-0335765) and a pilot grant from the St. Louis Institute of Nanomedicine (funded by Missouri Life Science Research Board). ’ REFERENCES (1) Talapin, D. V.; Shevchenko, E. V.; Bodnarchuk, M. I.; Ye, X. C.; Chen, J.; Murray, C. B. Nature 2009, 461, 964. (2) Kim, J.; Piao, Y.; Hyeon, T. Chem. Soc. Rev. 2009, 38, 372. (3) Shevchenko, E. V.; Ringler, M.; Schwemer, A.; Talapin, D. V.; Klar, T. A.; Rogach, A. L.; Feldmann, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2008, 130, 3274. (4) Lee, J. S.; Shevchenko, E. V.; Talapin, D. V. J. Am. Chem. Soc. 2008, 130, 9673. (5) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990. (6) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620. (7) Chatterjee, D. K.; Rufalhah, A. J.; Zhang, Y. Biomaterials 2008, 29, 937. (8) Jalil, R. A.; Zhang, Y. Biomaterials 2008, 29, 4122. (9) Idris, N. M.; Li, Z. Q.; Ye, L.; Sim, E. K. W.; Mahendran, R.; Ho, P. C. L.; Zhang, Y. Biomaterials 2009, 30, 5104. (10) Xiong, L. Q.; Yang, T. S.; Yang, Y.; Xu, C. J.; Li, F. Y. Biomaterials 2010, 31, 7078. (11) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (12) Li, Z. Q.; Zhang, Y. Angew. Chem., Int. Ed. 2006, 45, 7732. (13) Chen, Z. G.; Chen, H. L.; Hu, H.; Yu, M. X.; Li, F. Y.; Zhang, Q.; Zhou, Z. G.; Yi, T.; Huang, C. H. J. Am. Chem. Soc. 2008, 130, 3023. (14) Li, Z. Q.; Zhang, Y.; Jiang, S. Adv. Mater. 2008, 20, 4765. (15) Wu, S. W.; Han, G.; Milliron, D. J.; Aloni, S.; Altoe, V.; Talapin, D. V.; Cohen, B. E.; Schuck, P. J. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10917. (16) Feng, W.; Sun, L. D.; Yan, C. H. Chem. Commun. 2009, 4393. (17) Zhang, H.; Li, Y. J.; Ivanov, I. A.; Qu, Y. Q.; Huang, Y.; Duan, X. F. Angew. Chem., Int. Ed. 2010, 49, 2865. (18) Wang, L. Y.; Yan, R. X.; Hao, Z. Y.; Wang, L.; Zeng, J. H.; Bao, H.; Wang, X.; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 6054. (19) Zeng, J. H.; Su, J.; Li, Z. H.; Yan, R. X.; Li, Y. D. Adv. Mater. 2005, 17, 2119. (20) Handley, D. A. Colloidal Gold: Principles, Methods and Applications: Academic Press: San Diego, 1989. (21) Wang, L.; Zhao, W. J.; O’Donoghue, M. B.; Tan, W. H. Bioconjugate Chem. 2007, 18, 297. (22) Fang, C. L.; Qian, K.; Zhu, J. H.; Wang, S. B.; Lv, X. X.; Yu, S. H. Nanotechnology 2008, 19, 125601. (23) Oh, E.; Hong, M. Y.; Lee, D.; Nam, S. H.; Yoon, H. C.; Kim, H. S. J. Am. Chem. Soc. 2005, 127, 3270. (24) Pons, T.; Medintz, I. L.; Sapsford, K. E.; Higashiya, S.; Grimes, A. F.; English, D. S.; Mattoussi, H. Nano Lett. 2007, 7, 3157.

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