CdxZn1–xS Quantum Dots Coated with

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Highly Luminescent CdSe/CdxZn1 xS Quantum Dots Coated with Thickness-Controlled SiO2 Shell through Silanization Ping Yang, Masanori Ando, and Norio Murase* Health Research Institute, National Institute of Advanced Industrial Science and Technology, Midorigaoka, Ikeda-city, Osaka 563-8577, Japan ABSTRACT: A silanization technique of hydrophobic quantum dots (QDs) was applied to SiO2-coated CdSe/CdxZn1 xS QDs to precisely control the SiO2 shell thickness and retain the original high photoluminescence (PL) properties of the QDs. Hydrophobic CdSe/CdxZn1 xS core shell QDs with PL peak wavelengths of 600 and 652 nm were prepared by a facile organic route by using oleic acid (OA) as a capping agent. The QDs were silanized by using partially hydrolyzed tetraethyl orthosilicate by replacing surface OA. These silanized QDs were subsequently encapsulated in a SiO2 shell by a reverse micelles synthesis. The silanization plays an important role for the QDs to be coated with a homogeneous SiO2 shell and retain a high PL efficiency in water. Transmission electron microscopy observation shows that the shells are 1 9 nm with final particle sizes of 10 25 nm, depending on the initial QD size. In the case of short reaction time (6 h), the QDs were coated with a very thin SiO2 layer because no visible SiO2 shell was observed but transferred into the water phase. The silica coating does not change the PL peak wavelength of the QDs. The full width at half-maximum of PL was decreased 4 nm after coating for QDs emitting at both 600 and 652 nm. The PL efficiency of the SiO2-coated is up to 40%, mainly determined by the initial PL efficiency of the underlying CdSe/CdxZn1 xS QDs.

’ INTRODUCTION Luminescent quantum dots (QDs) have found important applications such as biological imaging, energy conversion, and environmental remediation due to their unique properties compared with organic dyes, such as narrow photoluminescence (PL) spectra, single excitation wavelength for tunable PL, and high stability against photobleaching.1 3 Among these, QDs with CdSe core and ZnS or CdZnS alloy shell have been the most frequently used optical material for bioconjugation. However, the QDs are typically synthesized in high boiling point, nonpolar organic solvents to obtain high quality, monocrystalline particles with narrow size distributions.4 To make the application possible in biological sensors and labels, the surface of the QDs has to be modified for water-soluble and biocompatible nature.5 7 It is also important to retain the initial PL properties of the QDs after surface modification. The ideal biocompatible QDs must be homogeneously dispersed and colloidally stable in aqueous solvents, exhibit pH and salt stability, show low levels of nonspecific binding to biological components, maintain high PL efficiency, and have a small diameter. Several successful approaches including ligand exchange, amphiphilic polymer coating, and SiO2 layer growth have been used for transferring organic QDs to aqueous solvents.8 10 Poly(maleic anhydride-alt-1-octadecene) was used to create water-soluble magnetic, semiconductor, and metallic nanoparticles. Another particularly attractive material is SiO2 particles encapsulating luminescent QDs because SiO2-coated QDs have shown no r 2011 American Chemical Society

cytotoxic effects in cell lines for concentrations as high as 30 μM.11 For the bioapplications of QDs, silica is an ideal matrix because of its good mechanical and optical properties compared with polymers.12 There have been several reports describing hydrophobic QDs encapsulated into SiO2 particles.13,14 These particles of several tens of nanometers contained a single QD at their center, but the SiO2 shell drastically reduced the PL efficiency of the QDs. Despite advances toward this direction, much work is still required. Reverse micelle synthesis is a convenient and advantageous technique to coat a silica layer on QDs. This synthesis can easily yield more uniformly sized silica particles in the range of 30 150 nm in diameter15 because the size of the water pool, which is a reaction field for the formation of silica particles, is rather uniform. Furthermore, it is reported that the reverse micelle synthesis normally does not depend very much on reaction conditions, leading to the formation of small silica particles with relatively low density of defects, and tunable size.16 In this regard, several groups have developed different reverse micelle approaches to form QD-SiO2 composite particles. For example, Koole and co-workers reported on hydrophobic QDs encapsulated in SiO2 particles with a mean size of 40 nm in diameter and a PL efficiency of 35%.9 Ying’s group reported on robust, noncytotoxic, Received: April 2, 2011 Revised: June 5, 2011 Published: July 06, 2011 9535

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Langmuir silica-coated CdSe QDs with 20% PL efficiency.8 However, these SiO2-coated QDs had a thick SiO2 shell (more than 15 nm) because of their preparation method. The size of QD-based bioprobes is considered to have a major limitation for their broad applications in biological imaging. Conventional QDs coated with silica or polymers are typically 15 20 nm in diameter, which is too bulky for efficient labeling of fine subcellular features.17 Although much research has been focused on biocompatible QDs with small diameter by ligand exchange,15 biocompatible SiO2-coated QDs with small diameter such as less than 15 nm are anticipated We previously prepared SiO2 particles encapsulating multiple hydrophilic CdTe QDs by using a reverse micelle method and controlled sol gel processes.18,19 The QDs in the resulting particles retained high PL efficiency. These particles exhibited a hollow or egg-yolk structure after optimization of the preparation procedures. These hydrophilic QDs were incorporated into SiO2 particles without ligand (thioglycolic acid) exchange on their surface. However, the particle size distribution was quite broad, ranging from several tens of nanometers to several micrometers in diameter. The QDs were distributed homogeneously throughout these particles. Subsequently, multiple aqueous CdTe QDs were impregnated in SiO2 particles by using ligand exchange and St€ober synthesis.20 Those particles exhibited tunable mean size from 12 to 50 nm. Recently, we found out that a partially hydrolyzed silane agent such as tetraethyl orthosilicate (TEOS) has the ability to replace organic amine groups on the surface of CdSe/ ZnS QDs in organic solvents without deteriorating their initial PL efficiency. After this silanization, multiple hydrophobic CdSe/ZnS QDs were encapsulated in SiO2 particles with bright luminescence and no cytotoxicity.21 In this Article, we have developed a method consisting of two steps for hydrophobic CdSe/CdxZn1 xS core shell QDs to be coated with a SiO2 shell having adjustable thickness. In step 1, ligand exchange, a useful technique for making luminescent hydrophobic QDs biocompatible,5,6 occurs during the silanization. The initially hydrophobic ligands (OA) on the QDs are replaced by partially hydrolyzed TEOS. The silanized QDs are coated homogeneously with a SiO2 shell with tunable shell thickness by a subsequent reverse micelle synthesis. The minimum thickness of the SiO2 shell should be several SiO2 layers. These QDs with an adjusted SiO2 shell thickness exhibit a PL efficiency of 40%. Systematic experiments were done for investigating the deposition process of SiO2 monomers on the QDs during reverse micelle synthesis.

’ EXPERIMENTAL SECTION Chemicals. Cadmium oxide (99.99%), selenium (99.5%, 100 mesh), sulfur (99.98%, powder), trioctylphosphine (TOP, 90%), OA (90%), cadmium acetate dihydrate (Cd(Ac)2 3 2H2O, 98%), zinc acetate (Zn(Ac)2, 99.99%), octadecylphosphonic acid (ODPA, 97%), trioctylamine (TOA, 98%), TEOS (98%), and Igepal CO-520 were purchased from Sigma Aldrich. All the chemicals were used directly without any further purification except for TOP. The pure water was obtained from a Milli-Q synthesis system. Synthesis of CdS/CdxZn1 xS Core Shell QDs. All reactions were conducted under N2 atmosphere. CdSe cores were synthesized by modifying the published method.22 In a typical synthesis of the CdSe cores, CdO (0.54 mmol), 180 mg of ODPA, and 5 mL of TOA were first placed in a three-neck round-bottom flask under N2 flow and then stirred at 300 °C until the CdO completely dissolved. Se powder (1 mmol) was

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dissolved in 1 mL of TOP. The TOPSe solution was then injected into the cadmium precursor solution with rapid stirring and kept at 300 °C for 2 min, followed by cooling down to room temperature. Volumes of 10 mL of hexane and 60 mL of ethanol were added to precipitate the CdSe nanocrystals. The product was then washed with copious ethanol, redispersed in 20 mL of toluene, and centrifuged to remove the sludge. Next, the CdSe cores were precipitated with ethanol and redispersed in 10 mL of toluene for subsequent shell coating. To prepare the core with different size, the reaction time and injection speed of TOPSe were adjusted. The injection speed and reaction temperature of core 1 were 20 mL/min and 300 °C, respectively. In contrast, the injection speed and reaction temperature of core 2 were 0.25 mL/min and 325 °C, respectively. In a typical synthesis of the CdxZn1 xS shell, Cd(Ac)2 3 2H2O (0.05 mmol), Zn(Ac)2 (0.05 mmol), 2 mL of OA, and 5 mL of TOA were placed in a three-neck round-bottom flask under Ar flow and then stirred at 300 °C until the Cd and Zn salts were completely dissolved. S powder (0.19 mmol) was dissolved in 0.5 mL of TOP. The toluene solution of CdSe cores (3 mL) was injected with vigorous stirring, followed by injection of the TOPS solution. The mixture was kept at 300 °C with stirring for further certain time (5 70 min), followed by cooling down to room temperature. The products were precipitated, washed twice with ethanol, and redispersed in 10 mL of toluene. Different Cd/Zn molar ratios were used to investigate their effect on the PL properties of CdSe/CdxZn1 xS core shell QDs.

CdSe/CdxZn1 xS Core Shell QDs Coated with SiO2 Shell. CdSe/CdxZn1 xS core shell QDs were coated with a SiO2 shell by using a two-step synthesis (step 1: silanization of the QDs; step 2: reverse micelle synthesis of sialnized-QDs). For step 1, as-prepared CdSe/CdxZn1 xS core shell QDs were redispersed in 0.3 mL of anhydrous toluene. A total of 1.5 μL of TEOS was added with stirring for 20 h to get silanized QDs. Anhydrous toluene is crucial for retaining a high PL efficiency because it resulted in a slow hydrolysis of TEOS. For step 2, Igepal CO-520 (1.0 g) was added to cyclohexane (10 mL) while stirring until the solution became clear to obtain the stock solution. An amount of 0.3 mL of silanized-QDs was added to the stock solution with stirring. An ammonia solution (6.25 wt %, 0.3 mL) was then added, followed by the injection of 1.5 8.5 μL of TEOS. Normally, the reaction time during step 2 was 3 24 h. SiO2-coated QDs was separated at 22 000 rpm for 30 min, washed for three times with ethanol, and then redispersed in H2O for further characterization. The preparation conditions and PL properties of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell are summarized in Table 1. For comparison, we prepared SiO2-coated CdSe/CdxZn1 xS QDs by a traditional reverse micelle method and without using a silanization process. Namely, 0.3 mL of QD toluene solution was added in the stock solution prepared by mixing 1.0 g of Igepal CO-520, 10 mL of cyclohexane, and 0.3 mL of NH3 (6.25 wt %). An amount of 5 10 μL of TEOS was added with stirring for 24 h to make a SiO2 shell coating on the QDs. The SiO2-coated CdSe/CdxZn1 xS QDs were separated and washed by centrifuging for further characterization. Apparatus. Transmission electron microscopy (TEM) observation was carried out by mainly using a Hitachi EF-1000 electron microscope. The absorption and PL spectra were recorded using conventional spectrometers (Hitachi U-4000 and F-4500, respectively). The PL efficiencies of the emitting SiO2 particles and the QDs in solution were estimated with a method previously reported.23 Briefly, the PL and absorption spectra of a standard quinine solution (quinine in 0.1 N H2SO4 solution; PL efficiency η0 of 55%) were measured in a 1 cm quartz cell as a function of its concentration. The emission intensity P0 (in units of the number of photons) is expressed as P0 ≈ Kη0a010 0.5a0, where a0 is absorbance at the excitation wavelength (365 nm) and K is the apparatus function. After measurement of the absorbance and PL intensity of the sample using the same apparatus parameters, the PL efficiency of the sample was derived by comparing the PL intensity of 9536

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Table 1. Preparation Conditions and Properties of CdSe Cores, CdSe/CdxZn1 xS Core Shell QDs, and Core Shell QDs Coated with SiO2 Shell PL peak total TEOS sample amount/μL

reaction

mean

PL

wavelength/ efficiency fwhm/

time/h size/nmb

nm

(%)

nm 24

core 1

N/A

N/A

2.7 ( 0.2

543

9

QD 1

N/A

N/A

6.7 ( 0.4

600

80

32

1a 2a

10 5

24 24

25 ( 3.0 18 ( 1.5

600 600

15 19

28 28

3

5

24

18 ( 1.6

600

29

28

4

3

15

12 ( 1.2

600

40

28

core 2

N/A

N/A

8.5 ( 0.4

624

1

25

QD 2

N/A

N/A

10 ( 0.5

652

60

28

5

3

6

10 ( 0.8

652

31

24

6 7

3 3

9 15

11 ( 0.7 14 ( 1.0

652 652

32 35

24 24

8

3

24

19 ( 1.2

652

34

24

a

Samples 1 and 2 were prepared without QD silanization before reversed micelle synthesis. Other samples were prepared by QD silanization and subsequent reverse micelle synthesis. b The mean size was obtained by TEM observation. samples with that of the standard quinine solution. The error in the PL efficiency is estimated to be within 10% by comparing the results using two standards including quinine and R6G.

Figure 1. Absorption and PL spectra of CdSe cores and CdSe/ CdxZn1 xS core shell QDs shown in Table 1. (a) Core 1 and core 2; (b) QD 1 and QD 2.

’ RESULTS AND DISCUSSION CdSe Cores and CdSe/CdxZn1 xS Core Shell QDs. The PL properties of CdSe cores and CdSe/CdxZn1 xS core shell QDs are summarized in Table 1. QDs 1 and 2 were prepared by using cores 1 and 2 with reaction time of 15 min, respectively. QD 1 is ca. 7 nm while QD 2 is 10 nm in diameter as explained later. The shell coating drastically increased the PL efficiency and redshifted PL peak wavelength. Figure 1 shows the absorption and PL spectra of CdSe cores and CdSe/CdxZn1 xS core shell QDs shown in Table 1: (a) core 1 and core 2; (b) QD 1 and QD 2. QD 1 and QD 2 were prepared by using a molar ratio of Cd/Zn of 1/1 for the growth of CdxZn1 xS shell. The PL peak wavelength of CdSe cores used to prepare QDs 1 and 2 are 543 and 625 nm, respectively. The full width at half-maximum (fwhm) of PL spectra was increased after coating with a CdxZn1 xS shell. These QDs exhibited high PL efficiencies and narrow PL spectra. Figure 2 shows the TEM images of CdSe cores (Core 1 and 2) and CdSe/CdxZn1 xS core shell QDs (QD 1 and 2) shown in Table 1. The well-developed lattice fringes of the core and core shell QD are observed in the insets in Figure 2c and d. The mean sizes of QDs 1 and 2 are 6.7 and 10 nm in diameter, while the mean diameters of cores 1 and 2 are 2.7 and 8.5 nm, respectively. These QDs exhibited a narrow size distribution which resulted in narrow PL spectra. Core 2 exhibited rod morphology. Because the band gap of QDs may be tuned to a precise energy depending on the dimensionality and degree of confinement, QDs with rod morphology yield a stronger degree of electronic confinement and thus a wider range of tunability in the band gap that brings a tunable emission. Core Shell QDs Encapsulated in SiO2 Particles without Silanization. The incorporation of QDs in silica particles has

Figure 2. TEM images of CdSe cores and CdSe/CdxZn1 xS core shell QDs shown in Table 1. (a) Core 1; (b) QD 1 prepared from core 1; (c) core 2; (d) QD 2 prepared from core 2. Insets in (c) and (d) show well-developed lattice fringes of the core and core shell QD.

been accomplished using a water-in-oil reverse microemulsion system, where small water droplets are stabilized by a nonionic surfactant. Following such traditional synthesis procedure, we 9537

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Scheme 1. Fabrication Procedure of CdSe/CdxZn1 xS Core Shell QDs Coated with a SiO2 Shell by Silanization and Subsequent Reverse Micelle Synthesis

Figure 3. TEM images of CdSe/CdxZn1 xS core shell QDs encapsulated in SiO2 particles (shown in Table 1) prepared by a traditional reverse micelle method without silanization in advance. (a) Sample 1; (b) sample 2. Several QDs are not located in the center of SiO2 particles as shown in (a) and (b). The mean diameters of sample 1 and 2 were 25 and 18 nm, respectively. Arrows show the QDs located not in the center of the particles.

have incorporated 6.7 nm CdSe/CdxZn1 xS core shell QDs (QD 1 shown in Table 1) in silica particles. Figure 3 shows the TEM images of CdSe/CdxZn1 xS core shell QDs encapsulated in SiO2 particles shown in Table 1. The resulting SiO2 particles of 25 and 18 nm are highly monodispersed and have exactly one QD incorporated in one particle. However, several QDs are not located in the center of the particle (Figure 3). This is ascribed to the surface properties of the QDs. Previous reports have incorporated hydrophobic QDs prepared by tri-n-octylphosphine oxide (TOPO) or organic amine (as capping agents) in the center of SiO2 particles.9,16 In our experiments, CdSe cores were prepared by ODPA as capping agent and CdSe/CdxZn1 xS core shell QDs were stabilized by OA during preparation. As shown in Figure 2, the core shell QDs exhibited a high crystalline property. In other words, the QDs we used have well-developed facets. Different crystalline facets have different chemical activities, where some facets have lower chemical affinity to SiO2. Therefore, the SiO2 shell cannot be deposited homogeneously on the surface of the QDs because the amount of SiO2 monomers generated by the hydrolysis of TEOS resulted in the monomer with quick deposition. The not centered QDs would decrease the stability of the SiO2 particles in applications. Core Shell QDs Coated with a SiO2 Shell with Silanization. To realize homogeneous SiO2 coating, a controlled sol gel process is therefore necessary. As shown in Scheme 1, the incorporation of hydrophobic QDs in silica particles by a reverse micelle method includes three steps: QD silanization, phase transfer from toluene to water, and SiO2 shell growth. The ligand exchange occurs by using partially hydrolyzed TEOS. Since TEOS with a purity of 98% is hydrolyzed to some extent by H2O from the atmosphere, the partially hydrolyzed part in TEOS attached to the QDs during ligand exchange. Similar results for the silanization of QDs have been reported in our previous paper.21 In that case, the QD silanization was performed in toluene with TEOS and without the addition of H2O. Accompanied by the hydrolysis of TEOS, the QDs were transferred into the water phase and subsequently coated with a SiO2 shell in the present case. Figure 4 shows the TEM images of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell shown in Table 1: (a) sample 3; (b) sample 4; (c) sample 5. After silanization, the QDs were coated with a homogeneous SiO2 shell, namely, one QD is located in the center of the SiO2 particle.

Figure 4. TEM images of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell shown in Table 1. (a) Sample 3; (b) Sample 4; (c) sample 5. After silanization, the QDs were coated with a homogeneous SiO2 shell (the QD in the center of the SiO2 particle). For sample 5, no SiO2 shell was observed by TEM as shown in (c).

For investigating the effect of preparation conditions on the thickness of SiO2 shell, Figure 5 shows the TEM images of CdSe/ CdxZn1 xS core shell QDs with a mean diameter of 10 nm coated with a SiO2 shell shown in Table 1, namely, (a) sample 6, (b) sample 7, and (c) sample 8. The result indicates the shell thickness can be precisely adjusted by changing reaction time. For sample 5 shown in Figure 4c, the SiO2 shell on the QDs should be very thin (perhaps, several SiO2 layers) due to short reaction time (6 h). We assured that the coating was done because 9538

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Figure 6. Absorbance and PL spectra of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell (samples 4 and 7 shown in Table 1).

Figure 5. TEM images of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell shown in Table 1. (a) Sample 6; (b) sample 7; (c) sample 8. The thickness of SiO2 shell increased with reaction time.

the QDs were redispersed in pure water without any aggregation and significant deterioration of the PL properties. The result indicates that the silanized QDs act as nucleation centers. Partially hydrolyzed TEOS was a primer for a homogeneous SiO2 coating during the reverse micelle synthesis. Because of this primer, the QDs were coated homogeneously with a SiO2 shell of various thicknesses. This should be crucial for further application because small probes were requested for the efficient labeling of fine subcellular features and a high performance single-photon emitter.24 To study the effect of the QD silanization on the PL properties of the QDs after coating with a SiO2 shell, Figure 6 shows the absorbance and PL spectra of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell (samples 4 and 7 shown in Table 1). The PL properties of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell are summarized in Table 1. Under same reaction time, the PL efficiencies of samples 3 and 8 are 64 and 44% of their initial value, respectively. Therefore, large QDs (QD 2) revealed a high stability compared with small QDs (QD 1) during bead preparation. After coating with a SiO2 shell, the PL peak wavelength of the QDs did not exhibit any change. However, the fwhm of PL spectra was decreased both in QDs with 600 and 652 nm of PL peak wavelengths. This is ascribed to the increase of surface defects of the QDs during silanization because the OA on QD surface was replaced by TEOS. The QDs exhibited a PL

efficiency of 40% after coating with a SiO2 shell (sample 4). In addition, the PL efficiency of the QDs decreased with increasing reaction time as shown in Table 1. This was associated with the surface deterioration of the QDs in a long reaction time. These narrow PL spectra and high PL efficiency show clear advantage of the silica-coated QDs for supersensitive biodetection. The PL efficiency of CdSe/CdxZn1 xS core shell QDs after coating with a SiO2 shell depended on the SiO2 shell thickness. The QDs have not exhibited significant decrease of their PL efficiency with increasing the SiO2 shell thickness (in a range of 1 5 nm) when silanizized in advance. Namely, the PL efficiency of CdSe/CdxZn1 xS core shell QDs coated with a SiO2 shell when silanized in advance is 29% (sample 3) while the PL efficiency is 19% (sample 2) in the case of no silanization before a reverse micelle synthesis. This indicates the silanization process plays an important role for retaining a high PL efficiency. We will further explain the reason for getting a high PL efficiency of QDs. Because of the poor solvent properties of toluene with respect to H2O, only a very small amount of H2O dissolved in the toluene from atmospheric conditions. The hydrolysis of TEOS was thus very slow, resulting in a well-ordered arrangement of the TEOS on the QD surface and thus proper passivation of the surface. Previously, we investigated this mechanism by measuring the PL efficiencies of CdSe/ZnS QDs in toluene with pure TEOS and water-containing TEOS.21 We found out that a small amount of H2O resulted in the regular attachment of TEOS on the surface of QDs, where the PL efficiency of the QDs encapsulated in SiO2 particles was maintained. In the present case, the silanization of QDs during reverse micelle synthesis is quick. This results in the decrease of PL efficiency if the QDs are not silanized in advance. Decoration of a SiO2 shell with any kind of functional group enables greater control in bioconjugation protocols. Because of the small size and a high PL efficiency of these SiO2 particles, we will focus their applications.

’ CONCLUSIONS We have successfully extended the direct silica coating approach developed in our laboratory to hydrophobic (OA-capped) CdSe/ CdxZn1 xS core shell QDs of two kinds of sizes. The procedure included QD silanization and a subsequent reverse micelle synthesis. Namely, two kinds of hydrophobic core shell QDs with high PL efficiencies (80% for the QDs with a PL peak wavelength of 600 nm and 60% for the QDs with a PL peak 9539

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wavelength of 652 nm) were used as initial QDs. The silanized QDs after coating with a SiO2 shell still retain a PL efficiency of 40% accompanied by the narrow PL spectra and retain initial PL peak wavelength. QD silanization is a key step to retain high PL efficiency, and it makes QDs coated with a homogeneous SiO2 shell with tunable thickness. Importantly, these SiO2-coated QDs with a very thin SiO2 layer are still monodispersed in pure water. In addition, the surface modification of the particles became easy with reverse micelle synthesis. Because of high PL efficiency, narrow PL spectra, and small size, we will next focus the application of these SiO2-coated QDs as bioprobes.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +81-72-751-9637.

’ ACKNOWLEDGMENT This work was supported in part by Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Agency (JST). ’ REFERENCES (1) Anderson, R. E.; Chan, W. C. W. ACS Nano 2008, 2, 1341. (2) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisators, A. P. Science 1998, 281, 2013. (3) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (4) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (5) Dorokhin, D.; Tomczak, N.; Han, M.; Reinhoudt, D. N.; Velders, A. H.; Vancso, G. J. ACS Nano 2009, 3, 661. (6) Talapin, D. V.; Rogach, A. L.; Mekis, I.; Haubold, S.; Kornowski, A.; Haase, M.; Weller, H. Coll. Colloids Surf., A 2002, 202, 145. (7) Yang, J.; Dave, S. R.; Gao, X. J. Am. Chem. Soc. 2008, 130, 5286. (8) Selvan, S. T.; Tan, T. T.; Ying, J. Adv. Mater. 2005, 17, 1620. (9) Koole, R.; Schooneveld, M. M.; Hilhorst, J. C.; Donega, M.; Hart, D. C.; Blaaderen, A.; Vanmaekelbergh, D.; Meijerink, A. Chem. Mater. 2008, 20, 2503. (10) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676. (11) Kirchner, C.; Liedl, T.; Kudera, S.; Pellegrino, T.; Javier, A. M.; Gaub, H. E.; Stolzle, S.; Fertig, N.; Parak, W. J. Nano Lett. 2005, 5, 331. (12) Murase, N. In Nanomaterials for the Life Sciences; Kumar, C., Ed.; Wiley-VCH: Weinheim, Germany, 2010; Vol. 6, p 393. (13) Rossi, L. M; Shi, L.; Quina, F. H.; Rosenzweig, Z. Langmuir 2005, 21, 4277. (14) Mulvaney, P.; Liz-Marzan, L. M.; Giersig, M.; Ung, T. J. Mater. Chem. 2000, 6, 1259. (15) Zhelev, Z.; Ohba, H.; Bakalova, R. J. Am. Chem. Soc. 2006, 128, 6324. (16) Darbandi, M.; Thomann, R.; Nann, T. Chem. Mater. 2005, 17, 5720. (17) Zheng, Y.; Yang, Z.; Li, Y.; Ying, J. Y. Adv. Mater. 2008, 20, 3410. (18) Yang, P.; Ando, M.; Murase, N. New J. Chem. 2009, 33, 561. (19) Yang, P.; Ando, M.; Murase, N. New J. Chem. 2009, 33, 1457. (20) Yang, P.; Murase, N. ChemPhysChem 2010, 11, 815. (21) Yang, P.; Murase, N.; Suzuki, M.; Hosokawa, C.; Kawasaki, K.; Kato, T.; Taguchi, T. Chem. Commun. 2010, 46, 4595. (22) Jun, S.; Jang, E.; Lim, J. E. Nanotechnology 2006, 17, 3892. (23) Murase, N.; Li, C. L. J. Lumin. 2008, 128, 1896. (24) Yuan, C. T.; Yu, P.; Ko, H. C.; Huang, J.; Tang, J. ACS Nano 2009, 3, 3051.

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