Encapsulation of Quantum Dot Clusters in Stimuli-Responsive

Jun 26, 2014 - Novel fluorescence-labeled spherical polyelectrolyte brushes consisting of a fluorescent polystyrene (PS) nanocomposite core and a ...
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Encapsulation of Quantum Dot Clusters in Stimuli-Responsive Spherical Polyelectrolyte Brushes Xiaochi Liu,†,# Yisheng Xu,*,†,‡,# Shijian Ma,§ Yunfei Ma,§ Ayyaz Ahmad,† Yuchuan Tian,† Xinhua Zhong,*,§ and Xuhong Guo*,† †

State-Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Zhejiang Provincial Key Laboratory for Chemical & Biochemical Processing Technology of Farm Products, School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, 318 Liuhe Road, Hangzhou, 310023, China § Shanghai-Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, Shanghai 200237, China ‡

S Supporting Information *

ABSTRACT: Novel fluorescence-labeled spherical polyelectrolyte brushes consisting of a fluorescent polystyrene (PS) nanocomposite core and a poly(acylic acid) (PAA) brush shell were successfully prepared. Quantum dots (QDs) were well confined in the PS core through hybrid emulsion polymerization. PAA chains were then grafted onto the surface of the fluorescent PS core to form a brush structure through photoemulsion polymerization. The obtained fluorescent spherical polyelectrolyte brushes are highly pH sensitive in addition to their excellent dispersibility in water. Fluorescent nanoclusters were introduced into spherical polyelectrolyte brushes to acquire high sensitive detection in the applications of spherical polyelectrolyte brushes as catalyst tracer, as biosensor, and in protein coding.

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INTRODUCTION Quantum dots (QDs), also known as semiconductor nanocrystals, are a new type of inorganic nanomaterials which have been widely investigated in the field of optics, biology, and medical imaging owing to their special luminescence properties.1,2 QDs exhibit more significant advantages over conventional organic fluorescent dyes such as broad excitation spectra, narrow emission spectra and extreme photostability. Furthermore, the emission wavelength can be easily and precisely modulated by varying their sizes and compositions.3−5 Recently, great interest has been focused on the combination of QDs with organic polymer structures for the desire of biological encoding.6,7 Some researchers were able to use a swelling method to prepare microspheres containing QDs by mixing QDs and synthesized polymer beads in a solvent/ nonsolvent system.8−12 Layer-by-layer (LbL) strategy was also employed to implement self-assembly of QDs and polymer.13,14 However, these composite particles mentioned above are prepared or assembled by physical interactions, the leakage of incorporated QDs tends to happen. Because of the weak interactions, the content of QDs is also limited and restricted only on the surface of the substrates. To resolve this issue, it is highly desirable to incorporate QDs into microbeads covalently.15−23 Suspension polymerization was utilized to incorporate QDs into polymer beads where QDs were captured during the polymerization of monomers followed by the hard beads formation.18,19 Miniemulsion polymerization was also reported to prepare QDs-incorporated nanoparticles by the addition of surfactant to disperse droplets into nanoscale sizes.21 In another case, RAFT (reversible addition−fragmentation chain transfer polymerization) emulsion polymerization was applied to synthesize QD-embedded polymeric nano© 2014 American Chemical Society

spheres, in which single core−shell nanoparticles were initially obtained followed by aggregation of these nanoparticle moieties to form nanocomposites containing multiple QD−polymer nanoparticles.22,23 However, these particles are limited by their poor stability and simple functionalities. To overcome these issues, the surface of NPs can be modified. For instance, polymer chains can be grafted onto the surface of nanospheres to generate spherical polymer brushes (SPBs).24 SPBs consist of a solid polymer core and a polyelectrolyte shell covalently attached on the core surface at one end. Such a structure shows good stability and dispersibility due to electrostatic repulsion and excludedvolume effect of brush layer.25 As smart nanoparticles, it has been reported that SPBs were used as ideal carriers for proteins and nanometal catalysts,26−31 thus the introduction of QDs into the SPB core certainly extends the applications of SPBs to such uses as protein encoding and catalyst tracer. For example, catalysts can be loaded on these fluorescent particles so the distribution of catalysts in the reactor can be monitored. Furthermore, such NPs can also be applied to test the residence time distribution for reactors. In this work, we demonstrated a method to synthesize monodispersed fluorescent spherical polyelectrolyte brushes (FSPBs) with narrow size distribution in which QD clusters were successfully embedded into polystyrene (PS) cores with well-controlled size by hybrid emulsion polymerization. Poly(acrylic acid) (PAA) chains were grafted from the surface of Received: Revised: Accepted: Published: 11326

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fluorophore-embedded PS cores by the aid of photoinitiator 2[p-(2-hydroxy-2-methylpropiophenone)]-ethylene glycol methacrylate (HMEM) to form well-defined core−shell structures. The length of PAA chains are regulated by the dose of acrylic acid (AA). As smart nanoparticles, the brush thickness is tunable by pH and ionic strength.

Table 1. Reaction Parameters of Core/Shell QDs Particles Formation 1 2 3 4 5 6

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EXPERIMENTAL SECTION Materials. All solvents were analytical grade and used without further purification. Cadmium oxide (99.99%), selenium powder (99.999%, 100 mesh), sulfur powder (99.5%), 1-octadecene (ODE, 90%), oleic acid (OA, 98%), oleylamine (OAm, 97%), trioctylphosphine (TOP, 99%), and tributylphosphine oxide (TOPO, 90%) were purchased from Aldrich and zinc acetate (Zn(OAc)2·2H2O, 99%), sodium dodecyl sulfate (SDS, 99%), K2S2O8 (KPS, 99%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (SCRC) and used as received. Styrene and acrylic acid (AA) from Lingfeng Chemical Reagent Co. Ltd. were used after distillation under reduced pressure to remove the inhibitor and were stored in the refrigerator. Photoinitiator HMEM was synthesized and characterized as reported previously.25 Pyridine, 2-hydroxy-4′-hydroxyethoxy-2-methylpropiophenone (HMP), and methacryloyl chloride (MC) were purchased from J&K Chemical Co. Ltd., Technical Choices, Inc., and Ciba Specialty Chemicals Inc., respectively. DI water was used in all experiments. Synthesis of Quantum Dots. CdSe/CdS/ZnS QDs were prepared by using methods reported previously.32 In a typical reaction, a 50 mL three-necked flask containing a mixture of 25.6 mg (0.2 mmol) CdO, 1.0 mL OA, 3.0 mL ODE, and 1.3 g TOPO (6 mmol) was vacuumed for 0.5 h and then heated to 320 °C until the solution became transparent. An 80 mg sample of Se powder (1.01 mmol) was dissolved in a mixture of 2.0 mL of TOP and 3.0 mL of ODE; 1.5 mL of such a solution was injected into the reaction flask. The heat source was then removed, and the reaction continued until the temperature dropped to 70 °C. The obtained CdSe core nanoparticles were precipitated by the addition of methanol. The precipitate was centrifugated and washed by methanol and acetone, respectively. Finally, the CdSe core nanoparticles were dispersed in hexane for further processing. To grow the shell, the cadmium precursor solution was prepared by dissolving 256.8 mg (2 mmol) of CdO in 4 mL (12 mmol) of OA and 16 mL of ODE; the zinc precursor was prepared by dissolving 440 mg (2 mmol) of Zn(OAC) in 1.6 mL of OA and 18.4 mL of ODE; the sulfur precursor solution was prepared by dissolving 64 mg (2 mmol) of sulfur powder in 20 mL of ODE. All the steps were performed at 90 °C under vacuum, and then heated at 160 °C under nitrogen. Presynthesized CdSe nanoparticles were loaded into a 50 mL reaction vessel with 1.0 mL of OAm and 2.0 mL of ODE. Hexane residue was removed under vacuum at 90 °C for 30 min and then the temperature was adjusted to 220 °C during which the cadmium precursor solution was added at 150 °C to ensure the surface of the QD cores to have saturated Cd-atoms. Different precursor solutions were subsequently injected in sequence to form shells. All pertinent parameters are shown in Table 1. After the enhancement of stability and fluorescence by the cladding process, the products continued to be heated for 2 h at 120 °C and finally were cooled down to room temperature. The prepared CdSe/CdS/CdS/Zn0.5Cd0.5S/ZnS QDs were

Ta (°C)

tb (min)

150 250→220c 220 220 200 200

5 10 10 10 20 20

dosage of precursor solutions (mL) Cd (0.25) S (0.4) S (0.45) S (0.5) S (0.6) S (0.7)

Cd (0.4) Cd (0.45) Cd (0.3) + Zn (0.2)d Zn (0.6) Zn (0.7)

a Reaction temperature. bReaction time. cThe actual temperature exceeded set point during temperature rise. Use overheats for reaction. d Transition layer between CdS and ZnS shells.

redispersed in hexane after the same purification as CdSe core nanoparticles. Synthesis of Fluorescent Polystyrene Core (FPC). FPC was synthesized in two steps including the preparation of the QDs colloidal solution and synthesis of the polystyrene core containing fluorescent nanoparticles. In the first step, a solution of the QDs redispersed in octane was dropwisely added to 40 mL of 0.83 mg/mL (2.88 × 10−3 mmol/mL) SDS solution. After the mixture was stirred (300 rpm) and sonicated for 210 min, an optical clear QDs colloidal solution (∼5 mg/mL) was obtained. 53 kHz and 350 W were chosen as the sonication parameters by employing a sonicator (SK7210LHC from KUDOS Co.) in all of the experiments. The incorporation of QDs into PS core was achieved by using hybrid emulsion polymerization as described before.33 A 0.5 g (48 mmol) aliquot of styrene was emulsified in 20 mL of 0.83 mg/mL SDS solution followed by 30 min sonication; 0.013 g (48.1 × 10−3 mmol) of KPS was dissolved in a QD colloidal solution, then the styrene emulsion was added into the QD solution at a slow addition rate of 6 s per drop (6 s/d) at 80 °C under 300 rpm stirring. The reaction was continued for 20 h under nitrogen. At the end of polymerization, 1.5 g of acetone solution containing 0.15 g (0.514 mmol) of photoinitiator HMEM was added at a rate of 6 s/d. The reaction was allowed to run for an extra 2.5 h to form a thin layer of photoinitiator around the PS core. Finally the products were purified through dialysis against DI water to remove unreacted small molecule residues. Preparation of Fluorescent Spherical Polyelectrolyte Brushes (FSPBs). FPC was diluted to 0.2 wt % with water and mixed with a different amount of AA. The entire system was vacuumed and charged with nitrogen respectively for three times. Photoemulsion polymerization was accomplished by UV radiation at room temperature for 2.5 h under vigorous stirring. The obtained FSPB with different AA content (AA content is the ratio of m(AA)/m(PS) in photoemulsion polymerization) was purified by dialysis against water to remove undesired small molecules. Characterizations. Dynamic light scattering (DLS) was carried out using a particle sizing system, NICOMP 380 ZLS equipped with a scattering angle of 90°. X-ray diffractometer (XRD) measurements were made by a Rigaku D/max-2550v Xray diffractometer; the scattering patterns were recorded over the range of angles corresponding to 2θ from 10° to 80°. Thermogravimetric analyses (TGA) were employed to determine the QDs contents in fluorescent PS composites by a SDT Q600 simultaneous DSC−TGA instrument, in which measurements were made under nitrogen with a heating rate of 10 °C/min. Transmission electron microscopy (TEM) was 11327

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increases from 10 to 50 °C: (1) the droplet sizes dramatically drop in the first 50 min suggesting that large n-octane droplets containing QDs were disrupted; (2) between 50 and 90 min, the size of the n-octane droplets slightly decreases as the droplets with different sizes start to become uniform through dynamic exchange of QDs; (3) after 100 min, all droplets become smaller again due to the evaporation of n-octane at higher temperature and droplets are stable. (4) after 180 min, the size of QD clusters becomes constant. The above four stages were further supported by our observations in UV measurement as shown in Figure 1b (full UV spectra were shown in Supporting Information Figure S3). The UV absorbance follows the same trend including a sharp decrease, a semistable stage, and a decrease to a steady state as seen from the change of hydrodynamic size. While as the temperature increases, the fluorescence intensity in Figure 1b exhibits an exact opposite change to the size and UV absorbance. We believe two factors account for the increase of the fluorescence intensity. In the first stage, as large n-octane droplets break into small droplets, fluorescent surface area of QD clusters increase. In the last stage, the evaporation of solvent leads to a higher quantum efficiency of QDs in air. Nevertheless, the four stages were clearly distinguished. The aforementioned data indicated that the optimal sonication time was 210 min (located in the fourth stage) when Dh, UV absorption, and PL intensity hardly changed, and a mere thin layer of octane covered the QD clusters for protection. The sonication time cannot last too long, or it will reduce the compatibility of QDs with styrene leading to failure of encapsulation. The whole process of the formation of QDcluster colloids was schematically described in Figure 2a.

performed by a JEOL JME-1400 transmission electron microscope at an acceleration voltage of 100 kV. TEM samples were prepared by adding one drop of solution/emulsion to a copper grid (300 mesh) that was then air-dried completely. The optical properties of QDs, FPC, and the FSPBs were characterized by UV−vis spectrophotometer, UV-3250 (SHIMADZU), and fluorometer, RF-5301PC (SHIMADZU).

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RESULTS AND DISCUSSION Characterizations of Quantum Dots. Oil-dispersible QDs modified by oleylamine (OAm) were synthesized by following the method reported by Xie et al.32 The crystal structure of the QD-nanocrystals was measured by XRD (Supporting Information Figure S1). UV adsorption and photoluminescence (PL) spectra of the QDs in hexane were also measured (Supporting Information Figure S2). The maximum adsorption peak appeared and a maximum emission peak was noticed with a full width at half-maximum (fwhm) of less than 30 nm, suggesting a narrow size distribution of QDs. QDs-Cluster Colloids, Fluorescent Polystyrene Core (FPC) and FSPBs. Oil-soluble QDs were emulsified in water to form homogeneous cluster colloids through sonication as described in other regular miniemulsion processes.21,34,35 In our experiment, hydrodynamic diameter (Dh) was monitored by DLS (Figure 1a) during the stirring (300 rpm) and sonication. Generally, the size of QDs decreases with time during which four stages can possibly be involved as the temperature

Figure 1. (a) Hydrodynamic diameter (○) and temperature (□) as a function of sonication time during the formation of QD-cluster colloids, (b) UV absorbance (○) and PL intensity (□) at peak maximum as a function of sonication time during the formation of QD-cluster colloids.

Figure 2. (a) Schematic of the process to obtain QD-cluster colloids. (b) Schematic representation of the synthetic procedure for FSPBs. 11328

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Figure 3. (a) TEM image of isolated QDs dispersed in hexane; (b) TEM image of QDs-cluster colloids, (c) FSPBs with a high QDs content of 31.04 wt % determined by TGA, scale bar = 50 nm. (d) Size and size distribution of FPC (black), FSPBs with AA content of 300% at pH 4 (red), and pH 10 (blue) determined by DLS, and (e) effect of pH on the brush thickness of FSPBs with different AA contents: (○) 300% AA; (□) 200% AA; and (△) 100% AA. QDs content = 19.3%.

The synthesis of fluorescent spherical polyelectrolyte brushes was schematically presented in Figure 2b. Conventionally, emulsion polymerization was tentatively tried to prepare FPC, but the encapsulation efficiency was quite limited leading to low fluorescence intensity. Miniemulsion polymerization was also employed to incorporate QDs into the PS core, but the addition of styrene monomer at a high concentration inhibits the formation of QD clusters under sonication environment, therefore a large amount of QDs may distribute on the surface of the PS core which results in the failure of copolymerization of HMEM in the next step; hence to increase QD content in the PS core, and maintain defined morphology of the QD clusters, a modification of the hybrid emulsion polymerization was applied.33 This strategy was originally used to increase the magnetic content (Fe3O4) of polystyrene beads. In our experiments, emulsifier-free styrene emulsion via sonication was added in a mixture of a KPS and QD-cluster mini-emulsion under 80 °C in nitrogen atmosphere. In the first 30 min, a styrene emulsion was added at a starving rate of 10 s/d. This low rate is favorable for a diffusion controlled surface nucleation process in which the styrene emulsion is functionalized as a “monomer warehouse”. Styrene monomers diffuse from styrene droplets to QD clusters due to effective pressure difference. During the polymerization of styrene, QD clusters are successfully encapsulated into PS beads. At the end of the polymerization, photoinitiator HMEM was added in a starving condition to ensure a thin layer of HMEM to attach covalently to the styrene monomers residing on the surface of the PS core. PAA chains were then grafted onto the PS core to generate a brush-like structure through photoemulsion polymerization.25 The size of the free QDs is around 5 nm (Figure 3a), but the aggregated QD clusters apparently have a well-defined spherical structure with a diameter of 100 nm as seen from the TEM images presented in Figure 3b. Such aggregates were formed after 210 min stirring and sonication in the presence of surfactant SDS. The TEM image of FSPBs is shown in Figure 3c. It is noticed that QDs were successfully embedded in the PS beads and mainly located in the core area. The PAA brush layer

can hardly be observed since PAA chains shrink onto the PS core on the dry TEM carbon film. Brush Structure and Properties. As polyelectrolyte chains germinate from spherical PS substrate, the hydrodynamic diameter (Dh) increases. Figure 3d shows the size distribution before (152 nm) and after (205 nm) PAA polymerization at acidic polymerization conditions (pH ≈ 4). Similar to the SPBs without QDs embedded as reported previously,36−38 the FSPB is pH responsive as well. When the pH was adjusted to 10, the Dh of the FSPB increases to 240 nm due to the deprotonation of carboxylic groups of PAA. This is because a significant amount of H-bonds were formed among carboxylic groups at low pH, while at high pH, PAA chains stretch against the QDs-PS core due to the strong electrostatic repulsion between negatively charged AA moieties. However, the size distribution (PDI) of FSPBs at pH 10 is slightly increased than those for FSPBs at low pH. This is probably due to some extent of cross-linking among highly extended PAA chains. We then carefully studied the pH effects in 1 mM NaCl (Figure 3e) since ionic strength has a strong effect on the thickness of polyelectrolyte brush layer. As seen in Figure 3e, the brush thickness (L) increases with pH. A remarkable stretching of PAA chains appears from pH 3 to 6, and the chain length reaches maximum at pH 8 suggesting carboxylic groups are nearly deprotonated. The brush thickness of the FSPB can also be modulated by the dose of AA during the synthesis.25 The effects of AA content on thickness (L) of PAA brushes are shown in Figure 3e. Upon increasing the AA dose, the brush thickness at the same pH increases monotonically. One has to be noted that at low pH ≤ 4.5, some aggregates presented in the dispersion since PAA chains are too short to have adequate Coulombic repulsion force for good dispersion stability. The effect of salt concentration on the swelling behavior of pure SPB has been reported.31,39,40 The brush layer shrinks as the salt concentration is increased. This is because at low ionic strength, the counterions are confined in a polyelectrolyte brush layer, resulting in the osmotic pressure caused by the 11329

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Donnan potential difference inside and outside the brush, which accounts for the extension of the polyelectrolyte chains, whereas at high salt concentration, additional salt screens the electrostatic interaction of the charged groups, leading to the shrinkage of brush thickness. The current results for QD embedded brushes (Table 2) were found to be consistent to Table 2. Effect of Salt Concentration on the Size of FSPBs (AA content = 300%) I (mM) Dh (nm) a

10−2 233

10−1 231

1 222

10 205

100 NAa

Aggregation occurred in high salt concentration.

what was previously observed. The brush thickness exclusively decreases with ionic strength regardless of the AA content. Moreover, the FSPB starts to aggregate at high salt concentration since the PAA chains shrink onto the PS surface and the repulsion force is minimized. To increase the PL intensity of FSPBs, more QDs being encapsulated in the PS core is indispensible. However, high QD content will significantly affect the PAA chain length and dispersion of FSPBs. As shown in Table 3, upon increasing the Table 3. Effect of QDs Content on Brush Thickness and PDI FSPB

QDs/ Sta

AA content (wt %)

Lb (nm)

1 2 3 4 5

0 0.05 0.1 0.25 0.5

100 100 100 100 100

170 96 59 25 15

a c

PDI

QD content (wt %)

theoretical content (wt %)c

0.012 0.054 0.085 0.106 0.198

0 2.87 6.35 19.3 31.04

0 4.76 9.09 20 33.33

Feeding ratio of QDs to styrene monomer. Theoretical content = m(QDs)/m(QDs + St).

b

Figure 4. Effect of encapsulation and grafting on fluorescence properties: (a) normalized integrated fluorescence spectra of QDs in hexane and FPC; (b) fluorescence spectra of FPC and FSPBs with different AA content. From top to bottom: FPC, 100% AA content, 200% AA content, 300% AA content.

Brush thickness.

the surface defects of the QD-nanocrystals are relieved by polystyrene.42 The effect of PAA grafting on photoluminescence properties was then investigated, which is shown in Figure 4b. Different AA contents have no observable impact on the maximum emission wavelength (PL maximum at 609 nm). Nevertheless, PL intensity declines with the increase of AA content, which is most likely due to the screening of UV irradiation as the brush thickness is increased. Durability of the Brushes. FPC is stable for about 7 days. After storage at room temperature for a month, a small amount of orange precipitate can be observed. FPC particles tend to self-aggregate because of their large specific surface area and surface energy. As shown in Figure 5a, FPC nanoparticles demonstrate some stickiness and form clusters after a long time, and it is difficult to redisperse them after aggregation. From the TEM images (Figure 5b,c) we can notice that even after standing for several months, the morphology and good dispersibility of the FSPBs are still retained in contrast to the instability of FPC nanoparticles. After lyophilization, FSPBs with long AA chains in the dry state can be readily redispersed in water simply by sonication. The graft copolymerization of PAA chains significantly enhance the stability and redispersity, which is attributed to steric hindrance of neighboring particles and most critically due to the electrostatic repulsion among particles, especially at high pH. The stability of the FSPBs is also affected by QD content since the QD content significantly influences the brush

ratio of QDs to styrene, the QD content in FSPBs can be significantly enhanced. We can obtain QD content up to 31.04 wt % measured by TGA. Such content is close to theoretical value. Accordingly, the fluorescence intensity of the FSPBs increases with QD content and the QDs/styrene ratio does not affect the maximum emission wavelength. However, the brush thickness reduces dramatically along with a broader size distribution (PDI) indicating less stability of these particles. When the QD/styrene is as high as 0.5, the thickness of PAA brushes barely increase even though AA content is greatly increased, and the brush does not show any regular pH and ionic strength response. This is probably due to the inhibition of covalent attachment of photoinitiator on the PS core surface leading to little PAA formation. Fluorescence Properties. Since QDs are encapsulated into polymer beads, both FPC and FSPB nanoparticles exhibit orange light under UV irradiation. The fluorescence spectra of QDs incorporated in the PS core and QD nanocrystals dispersed in hexane are shown in Figure 4a. A minor 2 nm shift of the maximum emission peak can be observed in the QDs embedded PS core (PL maximum at 609 nm) in comparison to QD nanocrystals (PL maximum at 607 nm). This shift may result from possible fluorescence resonance energy transfer (FRET) since the distance between QDs particles is reduced in the formation of QD−cluster colloids.41 Interestingly, peak width becomes narrower after QDs are encapsulated in the PS core, which can be interpreted to be that 11330

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions #

X.L. and Y.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Natural Science Foundation of China (21306049 and 51273063), the Fundamental Research Funds for the Central Universities (222201314029), and the higher school specialized research fund for the doctoral program to this work (20110074110003).

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Figure 5. TEM images of (a) FPC after standing for 1 month, scale bar = 50 nm; (b) FSPBs, QDs content = 19.3% and (c) FSPBs, QDs content = 6.35%, after standing for three months, scale bar = 50 nm.

(1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 1994, 370, 354−357. (2) Dabbosusi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Electroluminescence from CdSe quantum-dot/polymer composites. Appl. Phys. Lett. 1995, 66, 1316−1318. (3) Murry, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706− 8715. (4) Alivisatos, A. P. Semiconductor cluster, nanocrystals, and quantum dots. Science 1996, 271, 933−937. (5) Li, J. J.; Wang, A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. Large-scale synthesis of nearly monodisperse CdSe/ CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction. J. Am. Chem. Soc. 2003, 125, 12567− 12575. (6) Finkel, N.; Lou, X.; Wang, C.; He, L. Barcoding the Microworld. Anal. Chem. 2004, 76, 352A−359A. (7) Wang, F.; Tan, W. B.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Luminescent nanomaterials for biological labeling. Nanotechnology 2006, 17, R1−R13. (8) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules. Nat. Biotechnol. 2001, 19, 631−635. (9) Gao, X.; Nie, S. QD-encoded mesporous beads with high brightness and uniformity: Rapid readout by flow cytometry. Anal. Chem. 2004, 76, 2406−2410. (10) Li, Y.; Song, T.; Liu, J.; Zhu, S.; Chang, J. An efficient method for preparing high-performance multifunctional polymer beads simultaneously incorporated with magnetic nanoparticles and quantum dots. J. Mater. Chem. 2011, 21, 12520−12528. (11) Song, T.; Zhang, Q.; Lu, C.; Gong, X.; Yang, Q.; Li, Y.; Liu, J.; Chang, J. Structural design and preparation of high-performance QDencoded polymer beads for suspension arrays. J. Mater. Chem. 2011, 21, 2169−2177. (12) Salcher, A.; Nikoli, M. S.; Juarez, H. J. CdSe/CdS nanoparticles immobilized on pNIPAm-based microspheres. J. Mater. Chem. 2010, 20, 1367−1374. (13) Wang, D.; Rogach, A. L.; Caruso, F. Semiconductor quantum dot-labeled microsphere bioconjugates prepared by stepwise selfassembly. Nano Lett. 2002, 2, 857−861. (14) Allen, C. N.; Lequeux, N.; Chassenieux, C.; Tessier, G.; Dubertret, B. optical analysis of beads encoded with quantum dots coated with a cationic polymer. Adv. Mater. 2007, 19, 4420−4425.

thickness of the FSPBs. When more QDs were incorporated, the electrostatic repulsion among the FSPBs reduced with the decrease of brush thickness, leading to less durability of the nanoparticles. However, FSPBs with high QD content (31.04%) are also more stable in ambient conditions (stable for about one month) compared with FPC (stable for about 7 days).

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CONCLUSIONS A new approach of developing fluorescent spherical polyelectrolyte brushes containing quantum dots clusters in the core has been presented. Fluorescent properties were introduced into the core of conventional SPBs. FSPBs were prepared by photoemulsion polymerization using PAA chains grafted onto fluorescent PS nanoparticles after the surface was covered with a thin shell of photoinitiator HMEM. Fluorescent polystyrene cores with embedded fluorophores were synthesized by QD aggregation via sonication followed by hybrid emulsion polymerization. The obtained FSPBs integrate the advantages of fluorescence labeling and the properties of polyelectrolyte brushes. Transmission electron microscopy shows QDs were introduced and mainly located in the PS core, contributing to greater stability of the fluorophore. Photoluminescence studies prove that FSPB has similar optical properties to QDs dispersed in hexane. The brush layer of FSPBs is also pH and ionic strength responsive. The length of PAA chains is controllable by varying the AA dose. We believe such work facilitates the development of new applications of SPB in the area of bioengineering and chemical engineering such as in the field of catalyst tracer, labeled drug delivery, disease diagnosis, and protein coding.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

XRD pattern of QD nanoparticles, UV absorbance, and fluorescence spectra of synthesized QDs in hexane, and variations of UV absorption spectra over ultrasonication time. 11331

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dx.doi.org/10.1021/ie501035s | Ind. Eng. Chem. Res. 2014, 53, 11326−11332