Systematic Study of the Properties of CdSe Quantum Dots

Aug 22, 2008 - with a long-chain alkane octadecene (ODE) if a fatty acid, oleic acid (OA), was used as the ligand to stabilize the nanocrystals.12. Wo...
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J. Phys. Chem. C 2008, 112, 14318–14323

Systematic Study of the Properties of CdSe Quantum Dots Synthesized in Paraffin Liquid with Potential Application in Multiplexed Bioassays Bin Xing, Wanwan Li, Hongjing Dou, Pengfei Zhang, and Kang Sun* State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong UniVersity, 800 Dongchuan Road, Shanghai 200240, P. R. China ReceiVed: May 23, 2008; ReVised Manuscript ReceiVed: July 14, 2008

High-quality CdSe quantum dots (QDs) were synthesized in a mixture of paraffin liquid and oleic acid (OA), and the effects of the synthetic conditions, including the growth temperature, precursor concentration, and initial OA:Cd molar ratio, on the optical properties of the as-prepared CdSe QDs were systematically investigated. The thermal stability and the effects of the nanocrystal (NC) purification on the optical properties of the as-prepared CdSe QDs were also studied for their further bioapplication. The results demonstrate that the CdSe QDs have good thermal stability and a high photoluminescence quantum yield (PLQY) after NC purification, which is very important for their bioapplication. Moreover, optically encoded microbeads have been successfully prepared by directly embedding the as-prepared oil soluble CdSe QDs into the carboxyl group-capped porous polystyrene (PS) beads through our route, which suggests their promising application in multiplexed bioassays. 1. Introduction In the past decade, fluorescent semiconductor nanocrystals known as quantum dots (QDs) have attracted intensive attention because of their unique size-dependent optical properties and their application in solar cells,1 light-emitting devices,2 biological fluorescent labeling,3,4 photovoltaic,5 and lasers.6 Cadmium selenide (CdSe) QDs are some of the most attractive QDs because of their size-dependent emission in a visible region as well as their high photoluminescence quantum yield. For more than a decade, it has been possible to synthesize high-quality II-VI quantum dots by using solvents with high boiling points to achieve the desired growth temperatures.7 However, the typical methods require very restricted conditions because of the as-used toxic, unstable, and expensive organometallic precursors which may explode at elevated temperatures.8,9 In the past few years, the nonorganometallic precursors such as cadmium oxide (CdO)10,11 have proven to be excellent substitutes for dimethylcadmium, but expensive and toxic phosphines were still used as coordinating solvents in the synthesis procedure, which often limit the production of QDs in organics to milligram quantities. Therefore, works examining the synthesis of CdSe QDs in phosphine-free organics have become more and more attractive. Recently, the solvent trioctylphosphine oxide (TOPO), traditionally used as the reaction medium, has been substituted with other low-cost, less hazardous, and more air-stable solvents.12-16 Peng et al. reported that TOPO could be replaced with a long-chain alkane octadecene (ODE) if a fatty acid, oleic acid (OA), was used as the ligand to stabilize the nanocrystals.12 Wong et al. synthesized CdSe QDs in two heat transfer fluids of Dowtherm A and Therminol 66 instead of TOPO; they also had high boiling points but were much cheaper than TOPO.13 However, trioctylphosphine (TOP) was still used to form the Se precursor of TOPSe. Paul et al. developed a phosphine-free route for preparing CdSe QDs, by which Se can be dissolved * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-21-34202743. Fax: +86-21-34202745.

in ODE at high temperatures.14 This breakthrough not only eliminated the use of TOPO but also eliminated the need for TOP to form the Se precursor, greatly reducing the cost and toxicity of the reagents, and making it possible for large-scale and clean synthesis of CdSe QDs. However, ODE still accounted for 90% of the materials cost in preparing one batch of CdSe QDs. Sapra et al.15 proposed another low-cost route in olive oil and presented a different mechanism in which a stable Se species was believed to be the Se precursor besides H2Se16 during the synthetic process; however, the as-prepared CdSe QDs had relatively poor PLQY (e15%), and the stable Se species needed further identification. Tang et al. reported another non-TOPbased route for obtaining high-quality CdSe QDs by using a cheaper long chain alkane of paraffin liquid and oleic acid (OA) as the solvent and ligand at relatively low temperatures (200-240 °C), which significantly simplified the process of clean and low-cost synthesis of the CdSe QDs.16 However, there was little discussion about the effects of different synthetic conditions on the optical properties of the as-prepared CdSe QDs; the stability and subsequent application of the CdSe QDs were not reported, either. In addition, the precipitation process for the CdSe QDs they presented proved not to be an effective because paraffin liquid was insoluble in methanol. In this work, we systematically studied the growth and evolution of the optical properties of the CdSe QDs under different reaction conditions (the growth temperature, precursor concentration, and OA:Cd molar ratio) in the paraffin liquid/ OA synthetic system. The thermal stability of the as-prepared CdSe QDs from 1 to 60 °C and the effects of NC purification on the PL properties of the CdSe QDs were also investigated, which were important properties for their further bioapplication. Moreover, the as-prepared CdSe QDs were directly embedded into the carboxyl group-capped porous polystyrene (PS) beads17 to fabricate optically encoded beads, which have great potential application in multiplexed bioassays.

10.1021/jp8045577 CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

Properties of CdSe Quantum Dots

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2. Experimental Section 2.1. Chemicals. CdO (99.5%), Se powder (99.99%), and rhodamine 6G were purchased from Alrich. Oleic acid (chemical grade), paraffin liquid (chemical grade), n-hexane (analytical grade), methanol (analytical grade), and alcohol (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the reagents were used as received. 2.2. A Typical Fabrication of CdSe QDs. Two millimoles of CdO was dissolved in a mixture of 4 mmol of oleic acid (OA) and 8 mL of liquid paraffin in a three-neck flask at 150 °C for the preparation of Cd precursor solution, and then 0.4 mmol of Se was dissolved in 20 mL of paraffin liquid at 220 °C (above the melting temperature of Se) with vigorous stirring. Then 4 mL of a solution that contained 0.8 mmol of Cd precursors was quickly injected into the Se precursor solution with vigorous stirring, and the growth temperature was kept at 200 °C. Aliquots were taken at different time intervals and immediately put into n-hexane to prevent further growth. 2.3. NC Purification of the As-Prepared CdSe QDs. The NC purification included precipitation with methanol from paraffin liquid, centrifugation, and redissolving the CdSe QDs in CHCl3. At first, the CdSe QD solution was mixed with CHCl3 to form a well-distributed CdSe QD solution, and the methanol was added to precipitate CdSe QDs. Then the precipitated CdSe QDs were separated by centrifugation, further washed with methanol three times, and dried in a vacuum for XRD characterization or redissolved in CHCl3 for TEM analysis and the preparation of optically encoded beads. 2.4. Preparation of the Optically Encoded Beads. A chloroform solution of CdSe QDs (0.5 mL) was added to 0.5 mL of a chloroform solution containing ∼107 PS beads.17 Then the solution was stirred in atmosphere at 25 °C until the chloroform was thoroughly evaporated. Beads were washed with butanol and ethanol three times and then were collected by centrifugation. Finally, CdSe QD-tagged beads were dispersed in 10 mL of deionized water. 2.5. Characterization. UV-vis absorption and fluorescence data were obtained by using UV-2550 Shimadzu UV-vis spectrophotometer and RF-5301PC Shimadzu spectrofluorophotometer at room temperature. All the aliquots were measured without any size sorting. The PL spectra were collected between 400 and 800 nm using an excitation wavelength of 400 nm with a slit width of 5.0 nm. The UV-vis absorption spectra were collected between 300 and 800 nm with a slit width of 2.0 nm. The PLQY of the as-prepared CdSe QDs was obtained by comparing the integrated PL intensities of the CdSe QDs with that of rhodamine 6G (PLQY ) 95%). The spectra of temperature-dependent CdSe QDs were acquired using previously reported methods.18 Transmission electron micrographs (TEM) and high-resolution TEM (HRTEM) samples were prepared by depositing a drop of CdSe QDs diffusing in CHCl3 onto the carbon-coated copper grids and evaporating the solvent immediately. Transmission electron micrographs (TEM) were recorded on a JEOLJEM 2010 electron microscope operating at an accelerating voltage of 200 kV. High-resolution TEM (HTEM) was obtained on a JEM-2100F transmission electron microscope. X-ray diffraction spectra were recorded with a BRUKER-AXS X-ray diffractometer. The fluorescent microscope images of the optically encoded CdSe QD PS beads were recorded with an inverted Olympus microscope (IX-70) equipped with a digital color camera.

Figure 1. Temporal evolution of UV-vis absorption spectra (a) and PL spectra (b) of the CdSe QDs prepared at 200 °C.

3. Results and Discussion 3.1. Influence of the Reaction Conditions on the PL Properties of CdSe QDs. The temporal evolution of photoluminescence (PL) and UV-vis absorption spectra of the CdSe QDs prepared at 200 °C are shown in Figure 1. The obvious red shift of both PL and absorption spectra toward longer wavelengths could be observed with an increase in growth time. The sharp absorption peaks and narrow full width at halfmaximum (fwhm) of PL spectra (30-45 nm) indicate the narrow size distribution of the as-prepared CdSe QDs. The “nonresonant” Stokes shift of the as-prepared CdSe QDs, defined as the energy difference between the first peak of the absorption spectra and the emission peak of the CdSe QDs, became smaller with an increase in particle size, which was consistent with the previously published data of Bawendi et al.19,20 According to the predicted band edge structure model of the CdSe QDs proposed by Bawendi et al., the nonresonant Stokes shift originated from the difference in energy between the mean position of three optically active fine structure states ((1L, (1U, 0U) and the net energy of the (2L dark exciton state,19-21 and the distance between the first optically active state and the optically forbidden ground exciton state increased with a decrease in size, leading to an increase in the Stokes shift in the luminescence.19 As shown in Figure 1, the size of the CdSe QDs increased significantly in the first 3 min, and then the growth rate began to decrease steadily until the CdSe QDs reached a constant particle size at 90 min, which revealed the relatively slower growth rate of CdSe QDs in our route versus the conventional routes.14-16,22,23 Similar results were also observed at different reaction temperatures and Cd precursor concentrations. However, because paraffin liquid lacked the ability to coordinate the CdSe particle, the resultant CdSe QDs are relatively smaller than those synthesized in TOPO.22 The growth and optical properties of the as-prepared CdSe QDs were found to be related to the reaction conditions, including the growth temperature, precursor concentration, and initial OA: Cd molar ratio in precursors. The emission peak positions of the CdSe QDs taken from the solution at different times for all three temperatures (200, 220, and 240 °C) are illustrated in Figure 2a. A high reaction temperature caused a fast growth rate and thus the formation of larger CdSe QDs. However, it was difficult to execute a growth reaction when the growth temperature was beyond 240 °C, which was different from the synthetic TOPO/TOP system.22 It was probably due to the higher growth temperature (>240 °C) inducing the deactivation of the paraffin liquid/Se precursor (polymeric Se),14 and the polymeric Se turning into long and

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Figure 2. Emission peak wavelength (a), PLQY (b), and PL fwhm (c) as a function of growth time for the synthesis of CdSe QDs in the paraffin liquid/OA system at different temperatures. For all three reactions, the initial OA:Cd:Se molar ratio was fixed at 3:1:0.5 and the Cd precursor concentration was fixed at 57.2 mM.

Figure 3. Emission peak wavelength (a), PLQY (b), and PL fwhm (c) as a function of growth time for the synthesis of CdSe QDs in the paraffin liquid/OA system at different Cd precursor concentrations. For all three reactions, the OA:Cd:Se molar ratio was fixed at 3:1:0.5 and the growth temperature was fixed at 220 °C.

Figure 4. Emission peak wavelength (a), PLQY (b), and PL fwhm (c) as a function of growth time for the synthesis of CdSe QDs in the paraffin liquid/OA system with different initial OA:Cd molar ratios in precursors. For all three reactions, the Cd:Se molar ratio was fixed at 2:1, the temperature was fixed at 220 °C, and the Cd precursor concentration was fixed at 57.2 mM.

random chains, inhibiting the nucleation of CdSe QDs, which was consistent with the phenomenon in the ODE system.14 The reaction temperature also had an obvious effect on the PLQY of the as-prepared CdSe QDs, as shown in Figure 2b. When the temperature rose from 200 to 220 °C, the PLQY was improved slightly. However, the temperature of 240 °C provided a striking boost of the PLQY (up to 50%) of the as-prepared CdSe QDs, showing 240 °C was an appropriate growth temperature. The polymeric Se provided a steady source of Se monomers for the surface ordering and reconstructions at 240 °C, thereby resulting in the CdSe QDs with high PLQY. It is worth pointing out that the temporal evolution of the PLQY of the CdSe QDs in our route was different from the regulation discussed by Peng et al.23 which considered that their PLQY increased from a small value to a maximum (bright point), and then gradually decreased. There were two peak values (one of them was the maximum) during the evolution of the PLQY in our route; one existed at the start moment, and the other existed in the following growth stage, which was further confirmed by Figures 3b and 4b. In principle, the instantaneous increase in the level of oversaturation of the monomers via injection of the precursors made the critical size shift to a very small value and the CdSe QDs with a size close to the critical size possessed smooth and defect-free surfaces.24 Therefore, it was possible that the average size of the CdSe QDs was close to the critical size after the injection of precursors in the poor coordinating solvent system of paraffin liquid and oleic acid, which resulted in the CdSe QDs with high PLQY. Then, the PLQY of the CdSe

QDs had a quick drop at the start moment because the relatively fast growth rate made the particle size increase to a value larger than the critical size rapidly and produced more surface defects. During the further growth, the growth rate slowed, the critical size increased gradually as the monomer concentration decreased due to the growth,25 and the PLQY of the CdSe QDs would reach the other high value when the average size was close to the critical size again. The PL fwhm of the emission peak was plotted versus growth time in Figure 2c. After the injection of precursor, the CdSe QDs had a broad size distribution; however, the critical size was very small, and most CdSe QDs had the positive growth rate which narrowed their size distribution due to the smaller nanocrystals in the distribution growing faster than the larger ones.25 During the further growth process, the critical size became larger than the average size, and the distribution broadened because some smaller nanocrystals were shrinking and eventually disappeared while larger ones were still growing. At higher temperatures, the PL fwhm was broader than that at lower temperatures, because of the increase in the critical size at the fast depletion rate of the monomers. The dependence of the optical properties of the CdSe QDs on the Cd precursor concentration is summarized in Figure 3. The emission peak position depended strongly on the precursor concentration, as shown in Figure 3a; the emission peak wavelength of the CdSe QDs synthesized at high precursor concentrations was relatively small after nucleation, but it increased rapidly during the following growth stage. In comparison to those prepared at lower precursor concentrations, the

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Figure 5. TEM (a) and histogram of size distribution (b) of the as-prepared CdSe QDs with the excitonic absorption peak at 580 nm. The insets in panel a are the SAEM pattern and HRTEM of the as-prepared CdSe QDs. (c) X-ray powder diffraction patterns of CdSe QDs prepared at three different temperatures.

initial critical size is smaller at high precursor concentrations after the injection of precursors;25 thus, the CdSe QDs are relatively small after nucleation, and the small sized CdSe QDs can grow into larger ones because of the abundant residual monomers. Figure 3b shows that the CdSe QDs prepared in diluted precursors had a relatively high PLQY, which might result from the formation of CdSe QDs with fewer surface defects.26 However, an overly low precursor concentration was disadvantageous in obtaining a narrow size distribution, because of the strong Ostwald ripening. The CdSe QDs prepared at high precursor concentrations have a relatively broad size distribution after nucleation, but the residual monomers were present in a larger amount and result in a narrower fwhm because of the smaller critical size at high monomer concentrations (Figure 3c). Oleic acid, as the stabilizer in the synthesis of CdSe QDs, also influences the optical properties of the as-prepared CdSe QDs. Figure 4a shows that the increase in the concentration of oleic acid reduced the nucleation rate26 and promoted the formation of larger sized CdSe QDs because of the abundant residual monomers. The abundant residual monomers also increased the amount of surface defects and caused the decline of PLQY (Figure 4b). If too few nuclei formed, the growth rate of the CdSe QDs would be too fast to be controlled, which broadened the size distribution of the as-prepared CdSe QDs (Figure 4c). Moreover, the evolution of the PL fwhm at the initial OA:Cd molar ratio of 5:1 was found to differ from those at the other two ratios (Figure 4c) at which nucleation happened soon after injection, and clear focusing followed; finally, defocusing occurred. However, as for the initial OA:Cd molar ratio of 5:1, the size defocusing of the CdSe QDs followed for a short time after the nucleation, and then size focusing occurred; a similar result was also observed in the ODE system with TOPO acting as the secondary ligand.27 It was reported that surface ligands played a much more important role in the stability of the small CdSe nanocrystals than in the stability of the larger ones because small nanocrystals were primarily composed of surface atoms.28,29 Therefore, we presumed that a large excess of oleic acid bound to the surface of CdSe QDs soon after nucleation when the initial OA:Cd molar ratio was kept at 5:1, and the CdSe QDs would be transformed to smaller crystals with a lower emission wavelength. Compared with the CdSe QDs prepared in the ODE system with an (OA+TOPO): Cd ratio of 10:1,27 the resultant CdSe QDs possessed a relatively broader size distribution, which probably resulted from the weaker coordinating ability of OA versus that of TOPO. 3.2. Shape and Crystal Structure of As-Prepared CdSe QDs. TEM and HRTEM images of the CdSe QDs with the excitonic absorption peak at 580 nm are shown in Figure 5a,

which shows that the as-prepared CdSe QDs have a narrow size distribution and high shape uniformity (spherical). The wellresolved lattice fringes (the inset in Figure 5a) confirm the good crystallinity of the CdSe QDs. Lattice parameters (311), (220), and (111) derived from the selected area electron diffraction (SAED) of the CdSe QDs (the inset in Figure 5a) illustrate that the CdSe QDs have a cubic zinc blende structure, which is confirmed by the results of XRD analysis (Figure 5c). The average diameter of as-prepared CdSe QDs is ∼4.0 nm from the size distribution image (Figure 5b), which is in a good accordance with the result calculated from the first absorption peak in UV-vis absorption spectra.30 The powder XRD patterns of CdSe QDs prepared at three different temperatures are shown in Figure 5c, and three distinct diffraction peaks are observed at 2θ values of 25.5°, 42.4°, and 50.0°, corresponding to the (111), (220), and (311) crystalline planes, respectively, of cubic CdSe. The CdSe QDs prepared at three different temperatures have a single crystal phase and a typical zinc blende structure, which may be caused by their low growth temperatures. Growth temperature is a critical factor in determining the crystal structures, and the low growth temperatures (below 300 °C) will favor the formation of zinc blende structure in the paraffin liquid/OA system.16 From top to bottom, the sizes of CdSe QDs are 4, 3.7, and 2 nm, respectively, calculated from the first absorption peak in the UV-vis absorption spectra, and the broadening of XRD peaks can be observed because of the decrease in size of the CdSe QDs. 3.3. Thermal Stability of the As-Prepared CdSe QDs. It is necessary to investigate the temperature-dependent photoluminescence of QDs on the 1-80 °C scale, which is widely required by their bioapplications. The PL properties of both oilsoluble QDs31,32 and water-soluble QDs18 have proven to be very sensitive to temperature. Here we investigated the thermal stability of the as-prepared oil-soluble CdSe QDs from 1 to 60 °C (below the boiling point of the organic solution) for their further application. Figure 6 shows the absorption and fluorescence spectra of various sized CdSe QDs at different temperatures from 1 to 60 °C. As shown, with the temperature increasing from 1 to 60 °C, the PL intensity in each chart declined irreversibly with a slight red shift in the PL emission position, but without any change in the absorption spectra. The decline in PL intensity might result from nonradiative emission of carriers that were in deep trap sites when the temperature increased,33 and the PL red shift was attributed to inter-QD dipole-dipole interactions.34 Via comparison of the thermal stability of the CdSe QDs with different sizes, it should be stressed that the influence of the increasing temperature was less significant for larger CdSe QDs, which might result from the larger surface areas bound by relatively more ligands to fill

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Figure 6. Variation of spectra of temperature-dependent CdSe QDs with different sizes: (a) 2.3, (b) 3, and (c) 3.6 nm.

Figure 7. Variation of the PL spectra of different-sized CdSe QDs before (s) and after (---) purification: (a) 2.4, (b) 3.0, and (c) 4.7 nm.

Figure 8. (a) Seven distinguishable emission colors of different-sized CdSe QDs in n-hexane. (b) True color fluorescence images of the singlecolor CdSe QD-encoded porous PS beads (∼5.5 µm diameter) with emitting signals at 470 (blue), 537 (green), 573 (yellow), and 610 nm (red) and their mixture. (c) PL spectra of the 573 nm QD-encoded PS beads in aqueous solution (s) and after aging for 50 days in the air (---).

surface vacancies. However, the declining degrees of the PL intensity (by 40-60%) were lower than those of the water-soluble CdSe/ZnS QDs,18 the PL intensity of which declined by nearly 90% when the temperature increased from 7 to 60 °C. Obviously, the CdSe QDs synthesized in paraffin liquid exhibit better thermal stability than water-soluble CdSe/ZnS QDs. 3.4. Effects of NC Purification on the PL Properties of the As-Prepared CdSe QDs. Usually, the as-prepared CdSe QDs need to be separated from the paraffin liquid before characterization or further application. Thus, it is necessary to investigate the effects of NC purification on the PL properties of the CdSe QDs. However, we failed to separate the CdSe QDs from the paraffin liquid according to the method reported by Tang16 because methanol was insoluble in paraffin liquid. Thus, we used a modified method for the NC purification (see section 2.3). As shown in Figure 7, the effects of NC purification on the three different-sized CdSe QDs were investigated. The PL emission peak and PL fwhm of these CdSe QDs remained

almost unchanged after the NC purification, which was better than the CdSe QDs synthesized in TOPO and TOP, whose PL maxima had a red shift of 6 nm after NC purification.35 However, the NC purification process caused a notable decline in the PLQY of the as-prepared CdSe QDs, which was similar to the previous report.35 As mentioned in section 2.3, the CdSe QDs were precipitated from the mixture of CHCl3/paraffin liquid via addition of methanol followed by centrifugation. The decline of the PL QY of the CdSe QDs might result from the binding of added CH3OH to the vacancies on the CdSe QDs surfaces, which were exposed for the loss of the ligand of oleic acid after the purification process.35 However, the degree of decline was much smaller than that of the CdSe QDs synthesized in TOPO and TOP, whose PLQY declined by ∼90% after NC purification.35 It was possible that OA was less soluble in the methanol/ CHCl3 mixtures than TOP, and their detachments from the surfaces of CdSe QDs introduced fewer additional surface defects. The degree of decline was 57% when the diameter of

Properties of CdSe Quantum Dots the CdSe QDs was 2.4 nm and decreased to 24% when their diameter increased to 4.7 nm. Because the larger CdSe QDs were more prone to aggregate after the addition of methanol and can be more easily separated from the paraffin liquid than the smaller ones, less ligand oleic acid was lost for the larger CdSe QDs after the purification process. 3.5. Fabrication of the CdSe QD-Encoded Microbeads. Possessing unique, excellent optical properties, quantum dots have become a promising fluorescent material used to encode the beads in bead-based array technology, which has further promoted the development of optically encoded bead-based multiplexed analysis technology.36,37 Because of the good thermal stability and remaining high PLQY after NC purification, the CdSe QDs synthesized in a paraffin liquid/OA mixture without further surface passivation were directly used to encode the PS beads via our route.17 Figure 8a shows the fluorescence photograph of different-sized CdSe QDs in n-hexane simultaneously excited by a hand-held UV lamp at a wavelength of 365 nm. Because of the high PLQY of the as-prepared CdSe QDs, the obvious emission colors could be observed. By using the as-prepared CdSe QDs, we prepared four different types of single-color-coded porous PS beads capped by tailored carboxyl groups, which could be used to couple with different target molecules, and the porous internal structure of the PS beads was favorable for the rapid and deep diffusion of the as-prepared CdSe QDs into the beads and provides more surface area for immobilization of QDs.37 The multicolor fluorescence images (Figure 8b) of the QD-encoded beads were obtained by using single-color quantum dots which were mixed and spread on a glass surface excited by a single light source. As shown, the remarkably bright fluorescence across the bead without a ring structure demonstrates that the CdSe QDs were able to penetrate deeply and uniformly into the porous beads. The absence of fluorescence quenching or PL spectral broadening confirmed that the embedded CdSe QDs were spatially separated and did not couple with each other by our procedure. The optically encoded beads showed no surface texture, which indicated that the embedding procedures did not damage the polymeric structure of the PS beads. Moreover, the PL intensity of the optically encoded beads showed little change after aging in the air for 50 days, as depicted in Figure 8c. The declining degree of the PL intensity was less than 10%, and the PL peak position and PL fwhm remained almost unchanged, indicating very limited escape of the CdSe QDs from the beads in the aqueous environment. As demonstrated above, the as-prepared CdSe QDencoded PS beads possessed properties similar to those of the CdSe/ZnS core/shell QD-encoded ones36,37 and had a promising application in multiplexed bioassays. 4. Conclusions In conclusion, we prepared high-quality CdSe QDs with good thermal stability (1-60 °C) in a mixture of paraffin liquid and oleic acid (OA) and detailed the effects of different reaction conditions on the properties of the as-prepared CdSe QDs. High growth temperatures, low precursor concentrations, and small quantities of oleic acid favor the presence of highly luminescent CdSe QDs in the blue to green emission window. High precursor concentrations and large quantities of oleic acid are found to be necessary to obtain large-sized CdSe QDs. After our NC purification procedure, the as-prepared CdSe QDs change little in their PL spectra (emission peak wavelength and PL fwhm) and maintain their high luminescence, which enables us to prepare optically encoded beads successfully by embedding various sized oil-soluble CdSe QDs into the carboxyl group-

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14323 capped porous PS beads, and the resultant highly luminescent QD-encoded beads with good stability show their promising application in multiplexed bioassays. Acknowledgment. We thank the Instrumental Analysis Center of Shanghai Jiao Tong University for the help with TEM and XRD measurements. We also thank Shanghai Sunny New Technology Development Co., Ltd., for their financial support. References and Notes (1) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628–17637. (2) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Chem. Phys. 1997, 82, 5837–5842. (3) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2015. (4) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016–2018. (5) Huynh, W. U.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923–927. (6) Artemyev, M. V.; Woggon, U.; Reinhold, W.; Jaschinski, H.; Langbein, W. Nano Lett. 2001, 1, 309–314. (7) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (8) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (9) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700–12706. (10) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183–184. (11) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389–1395. (12) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368–2371. (13) Asokan, S.; Krueger, K. M.; Alkhawaldeh, A.; Carreon, A. R.; Mu, Z.; Colvin, V. L.; Mantzaris, N. V.; Wong, M. S. Nanotechnology 2005, 16, 2000–2011. (14) Jasieniak, J.; Bullen, C.; van Embden, J.; Mulvaney, P. J. Phys. Chem. B 2005, 109, 20665–20668. (15) Sapra, S.; Rogach, A. L.; Feldmann, J. J. Mater. Chem. 2006, 16, 3391–3395. (16) Deng, Z.; Cao, L.; Tang, F.; Zou, B. J. Phys. Chem. B 2005, 109, 16671–16675. (17) Zhang, P.; Dou, H.; Li, W.; Tao, K.; Xing, B.; Sun, K. Chem. Lett. 2007, 36, 1458–1459. (18) Liu, T.; Huang, Z.; Wang, H.; Wang, J.; Li, X.; Zhao, Y.; Luo, Q. Anal. Chim. Acta 2006, 559, 120–123. (19) Efros, Al. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. G. Phys. ReV. B 1996, 54, 4843–4856. (20) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. J. Chem. Phys. 1997, 106, 9869–9882. (21) Nirmal, M.; Norris, D. J.; Bawendi, M. G. Phys. ReV. Lett. 1995, 15, 3728–3731. (22) Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333–337. (23) Qiong, L.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2049–2055. (24) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782–5190. (25) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343–5344. (26) Guo, J.; Yang, W.; Wang, C. J. Phys. Chem. B 2005, 109, 17467– 17473. (27) Dai, Q.; Li, D.; Chen, H.; Kan, S.; Li, H.; Gao, S.; Hou, Y.; Liu, B.; Zou, G. J. Phys. Chem. B 2006, 110, 16508–16513. (28) Chen, C. C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398–401. (29) Wickham, J. N.; Herhold, A. B.; Alivisatos, A. P Phys. ReV. Lett. 2000, 84, 923–926. (30) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854–2860. (31) Wuister, S. F.; Houselt, A.; Doneg, C. D. M.; Vanmaekelbergh, D.; Meijerink, A. Angew. Chem., Int. Ed. 2004, 43, 3029–3033. (32) Xing, B.; Li, W.; Dou, H.; Zhang, P.; Sun, K. Chem. Lett. 2007, 36, 1144–1145. (33) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivissatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019–7029. (34) Biju, V.; Makita, Y.; Sonoda, A.; Yokoyama, H.; Baba, Y.; Ishikawa, M. J. Phys. Chem. B 2005, 109, 13899–13905. (35) Kalyuzhny, G.; Murray, R. W. J. Phys. Chem. B 2005, 109, 7012– 7021. (36) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631–635. (37) Gao, X.; Nie, S. Anal. Chem. 2004, 76, 2406–2410.

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