Article pubs.acs.org/Langmuir
Nanoparticle Loading Induced Morphological Transitions and Size Fractionation of Coassemblies from PS‑b‑PAA with Quantum Dots Wei Liu,†,‡ Jun Mao,† Yanhu Xue,† Ziliang Zhao,† Haishan Zhang,*,§ and Xiangling Ji*,† †
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Colorectal and Anal Surgery, China-Japan Union Hospital, Jilin University, Changchun 130033, People’s Republic of China S Supporting Information *
ABSTRACT: Inorganic nanoparticles play a very important role in the fabrication and regulation of desirable hybrid structures with block copolymers. In this study, polystyrene-bpoly(acrylic acid) (PS48-b-PAA67) and oleic acid-capped CdSe/ CdS core/shell quantum dots (QDs) are coassembled in tetrahydrofuran (THF) through gradual water addition. QDs are incorporated into the hydrophilic PAA blocks because of the strong coordination between PAA blocks and the surface of QDs. Increasing the weight fraction of QDs (ω = 0−0.44) leads to morphological transitions from hybrid spherical micelles to large compound micelles (LCMs) and then to bowl-shaped structures. The coassembly process is monitored using transmission electron microscopy (TEM). Formation mechanism of different morphologies is further proposed in which the PAA blocks bridging QDs manipulates the polymer chain mobility and the resulting morphology. Furthermore, the size and size distribution of assemblies serving as drug carriers will influence the circulation time, organ distribution and cell entry pathway of assemblies. Therefore, it is important to prepare or isolate assemblies with monodisperse or narrow size distribution for biomedical applications. Here, the centrifugation and membrane filtration techniques are applied to fractionate polydisperse coassemblies, and the results indicate that both techniques provide effective size fractionation.
■
INTRODUCTION Coassembly1−9 of block copolymers (BCPs) and inorganic nanoparticles (NPs) in solution is very effective for preparing hybrid materials with synergetic functions for potential applications in bioimaging, drug delivery, and functional materials.10−12 The diverse hybrid nanostructures and their comprehensive performance have attracted much attention in coassembly fields. On the one hand, the primary principles of BCPs self-assembly13−15 can be used to manipulate morphology of coassemblies; on the other hand, how the NPs influence the self-assembly process of BCPs remains a challenge. Recently, several studies3,4,16 have revealed that the incorporation of NPs has a drastic effect on assembly structure. Hickey et al.4 demonstrated that encapsulation of hydrophobic NPs could alter the relative volume ratio of hydrophobic to hydrophilic blocks and consequently make the morphology change. As a result, increasing the amount of magnetic NPs produced magneto-polymersomes instead of magneto-micelles for micelle-forming polystyrene-b-poly(acrylic acid) (PS-bPAA). Cai et al.16 investigated the coassembly of poly(γbenzyl-L-glutamate)-b-poly(ethylene glycol) (PBLG-b-PEG) with Au NPs. The Au NPs destroyed the ordered packing of © XXXX American Chemical Society
PBLG rods in the micelle core, resulting in the morphological transitions from long cylindrical micelles to short cylinders and then to spherical micelles. The common feature of these results is that the NPs interact with the micelle core-forming blocks and are consequently encapsulated into the core of the coassemblies. Inorganic NPs can also be incorporated into the corona of assemblies through coordination or electrostatic interaction with the corona-forming block.17−20 Winnik and co-workers7,17,19 prepared hybrid micelles or vesicles with multiple NPs being incorporated into the corona using BCPs of polystyreneb-poly(4-vinylpyridine) (PS-b-P4VP) or PS-b-PAA. The coronal location of NPs was triggered by the favorable interaction between surface of NP and the P4VP or PAA blocks. Particularly, in the PS-b-P4VP system, the hybrid micelles could transform to worm-like networks under vigorous magnetic stirring.17 NPs play a very important role in this morphological transition, which is evidenced by the observation Received: June 12, 2016 Revised: July 15, 2016
A
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Furthermore, both centrifugation and membrane filtration are applied to fractionate the resultant coassemblies according to size.
of substantial NPs at the junction between micelles. NPs were inferred to bridge the P4VP corona of two adjacent micelles and thus promote the aggregation and fusion of micelles. Cui et al.20 reported micelle structure transformation of triblock copolymer poly(acrylic acid)-block-poly(methyl acrylate)block-polystyrene (PAA-b-PMA-b-PS) from spheres to onedimensional segmented cylinders by adding amine functionalized gold NPs. The positive ligands on gold NP surface will complex with the PAA corona of spherical micelles through electrostatic interaction, which gives rise to the one-dimensional growth of collapsed spherical micelles. The active role of NPs in the coassembly process leads to more unpredictable but attractive final hybrid structures. Getting insight into the formation mechanism of hybrid structure is very critical for regulating and designing coassemblies. Therefore, one aim of this study is to investigate the formation and transition mechanism of hybrid structures. Self-assembly of BCPs commonly produces assemblies with polydispersity, multi morphologies, or mixed compositions due to thermodynamic and kinetic aspects.15,21 The size and structure variations of coassemblies may be more complicated for the addition of NPs. Fractionation of polymer micelles or hybrid micelles based on size, morphology, or compositions becomes very important for fundamental research and applications. Grubišc-́ Gallot et al.22 used size exclusion chromatography (SEC) to characterize the polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) in the mixed solvent 1,4-dioxane/cyclohexane and achieved good separation of unimer and micelle. However, strong perturbation in the unimer-micelle equilibrium may occur during the SEC experiments.23 Multidetector thermal field-flow fractionation (ThFFF) was employed to analyze the size and morphology changes of PS-b-PMMA micelles with temperature by Greyling and Pasch.24 The results showed a decreased thermal diffusion coefficient of micelles with increasing temperature which was independent of the tacticity of the corona. They further demonstrated that ThFFF enabled separation of the polymer micelles based solely on the tacticity of the corona. Fractionation of casein micelles, which are highly polydisperse both in size and composition in milk has been investigated a lot recently. Consecutive ultracentrifugations25 regarding different centrifugal forces, asymmetrical flow field-flow fractionation (AsFlFFF)26 and controlled pore glass chromatography27 coupled with several techniques, such as small-angle X-ray scattering, dynamic light scattering, cryo-transmission electron microscopy, atomic force microscopy, and so on, have been applied to fractionate the casein micelles and further investigate the relationship between size and structure or proteic composition. AsFlFFF was also applied to separate drug-loaded polymeric core/shell assemblies from void polymeric micelles based on size by Kang et al.,28 which was proven to be a useful tool for size characterization of assemblies with multimodal size distributions. So, another aim of current work is to fractionate the resultant hybrid structures based on size. Herein, PS48-b-PAA67 is coassembled with oleic acid-capped CdSe/CdS core/shell quantum dots (QDs) which interact with the hydrophilic PAA blocks. The core/shell QDs exhibit higher quantum yield (47%) than that of CdSe QDs (5%) because the outer CdS shell can decrease the trap emission.29 The morphological transitions of coassemblies induced only by increasing the QD loading and the effect of QDs on the coassembly process are investigated. Mechanisms of the formation and transitions of hybrid structures are proposed.
■
EXPERIMENTAL SECTION
Materials. Cadmium oxide (CdO, 99.5%), myristic acid (MA, 99%), oleic acid (OA, 90%), thiourea (99%) and selenourea (98%) were purchased from Sigma-Aldrich. PS48-b-PAA67 (Mn = 5000 for PS block and Mn = 4800 for PAA block, PDI = 1.4, where the subscripts indicate the number of average degree of polymerization) was purchased from Polymer Source, Inc., Canada. Ethylenediaminetetraacetic acid (EDTA, 99.5%) was purchased from Tianjin East China Factory of Reagents, China. All of the materials were used as received. Synthesis of CdSe/CdS Core/Shell QDs. OA-capped CdSe/CdS core/shell QDs were prepared at 40−70 °C via a two-phase approach at an environmentally friendly n-heptane-water interface.29 The precursor cadmium myristate (Cd-MA) was prepared through a reaction of CdO (1.926 g) and MA (7.500 g) at 210 °C. The crude product was recrystallized twice using toluene.30 In typical experiments, Cd-MA (0.1134 g), OA (1 mL), and n-heptane (10 mL) were mixed in a flask and heated to 90 °C to dissolve the precursor. The precursor solution was then cooled to room temperature. Selenourea (0.0125 g) was dissolved in nitrogen-saturated water (10 mL) at room temperature and then the solution was injected into the flask. The flask was maintained at 40 °C for 20 min to prepare CdSe QDs. The crude solution of CdSe QDs was precipitated with acetone and further isolated by centrifugation and decantation. Then, Cd-MA (0.1134 g), OA (1 mL), and n-heptane (10 mL) were mixed in a flask and heated to 90 °C to dissolve the precursor, and the solution was cooled to room temperature. As-prepared CdSe QDs were dispersed in the above precursor solution. Thiourea (0.0418 g) was dissolved in water (10 mL) at room temperature, and the solution was injected into the flask. The flask was maintained at 70 °C for 180 min, and CdSe/CdS core/shell QDs were formed. The crude solution was precipitated with acetone and further isolated by centrifugation and decantation. The purified QDs were dried in a vacuum oven at 60 °C until constant weight was reached. Coassembly of PS48-b-PAA67 and CdSe/CdS Core/Shell QDs. In a typical experiment, 1 mL aliquot of a PS48-b-PAA67 THF (tetrahydrofuran) solution (1.0 mg/mL) was mixed with 500 μL of CdSe/CdS core/shell QDs THF solution (concentration from 0.1 to 1.6 mg/mL). The mixture was stirred for 15 min at room temperature. Then, water (300 μL) was added to the mixture of QDs and BCPs at a rate of 20 μL/min. The solution was stirred for 18 h, and then 1.5 mL water was added at a rate of 100 μL/min. The above solution was dialyzed against water for 48 h. For all experiments, the BCP concentration was kept at a constant and the weight fraction of QDs was adjusted. The weight fraction (ω) of CdSe/CdS core/shell QDs was defined by the weight of the dried QDs over the total weight of dried QDs and BCPs. Removal of QDs from Surface of Coassemblies Using Ethylenediaminetetraacetic Acid (EDTA). The QDs adsorbed on the surface of coassemblies were removed by EDTA according to the method reported in literature19 with modification. An aqueous solution of EDTA (pH = 9.3, c = 6.8 × 10−2 M) was prepared by using NaOH solution (1.0 M) to adjust the pH. Then aqueous solution (1 mL) of coassemblies was added to EDTA solution (500 μL) and the mixture was stirred for 12 h. The solution was centrifugated at 12 000 rpm for 60 min and then discarded 80 percent of the supernatant solution. The retained solution was diluted by adding water and centrifugated. The purified process was repeated twice. Transmission Electron Microscope (TEM). TEM measurements were performed on a JEOL JEM-1011 transmission electron microscopy operating at an accelerating voltage of 100 kV. The samples were prepared by depositing one drop of the coassembly solution onto a 300 mesh copper grid coated with a carbon film and drying in air. B
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir For the observation of the coassembly morphologies at different water contents, freeze-drying was used to preserve the original morphology as in the solution. In typical experiments, 6 μL of the coassembly solution was deposited onto a 400 mesh carbon coated copper grid which was placed on an iron block that had been cooled to liquid-nitrogen temperature and then freeze-dried under vacuum at room temperature for 24 h. Scanning Electron Microscope (SEM). SEM measurements were performed on a XL-30 ESEM FEG (Micrion FEI PHILIPS) operating at an acceleration voltage of 20 kV. Ten microliters of the dilute coassembly solution after dialysis was dropped onto the silicon (Si) wafers and then dried in air at room temperature. The Si wafers were coated with a thin layer of gold (Au) before measurements. Dynamic Light Scattering (DLS). The hydrodynamic diameters were measured using Zeta Sizer (Nano ZS, Malvern Instruments, UK). The laser wavelength was 633 nm. Data were collected at a scattering angle of 90° at 25 °C. Ultraviolet−visible (UV−vis) and Photoluminescence (PL) Spectra. UV−vis measurement was performed on a Shimadzu UV2450 PC spectrometer over the wavelength range of 300−800 nm. PL spectra were recorded on a Shimadzu RF-5301 PC fluorometer. The excitation wavelength was 400 nm and the scanning wavelength range was 410−700 nm. The excitation slit was set at 1.5 nm and emission slit was 3 nm. Centrifugation. For size fractionation of coassemblies, the sample was first subjected to centrifugation at a relative centrifugal force (RCF) of 1467 × g for 10 min (Sigma 3K15, Germany). The supernatant was replaced by same volume of ultrapure water and then the solution was centrifuged again. The procedure was repeated three times to obtain the first fraction (fraction A). All the supernatant of fraction A was collected and centrifuged at a RCF of 5867 × g for 20 min to obtain fraction B. The fraction C was separated from the supernatant of the fraction B by centrifugation at a RCF of 20627 × g for 60 min. The concentrated supernatant of fraction C was fraction D. Thus, the coassemblies at ω = 0.44 were fractionated into four fractions (fraction A, B, C and D) by repeated cycles of centrifugation. Each of the obtained fractions was characterized by DLS and TEM, respectively. Membrane Filtration. The coassemblies were fractionated into different fractions by successively extruding the solution through a polycarbonate membrane with three decreased pore sizes of 1, 0.4, and 0.1 μm via a mini-extruder (Avanti Polar Lipids Inc.). A syringe pump (Baoding Longer Precision Pump Co., Ltd.) was connected to the mini-extruder to provide constant extrusion rate. The former three fractions (fraction a, b and c) were collected by placing the membranes into water via ultrasonication at room temperature, and the fourth one (fraction d) was the solution filtered from the 100 nm membrane. All fractions were further analyzed by DLS and TEM, respectively.
Figure 1. (a) TEM image and (b) size distribution of oleic acidcapped CdSe/CdS core/shell QDs.
PS48-b-PAA67 with long PAA block self-assembles into starlike micelles, i.e., spherical micelles (Figure 2a). The average diameter measured from TEM is 18 ± 2 nm, indicating a relatively uniform size. When the weight fraction (ω) of QDs is fixed at 0.05, the coassemblies show hybrid spherical micelles (Figure 2b) with average size of 19 nm similar to that of the blank spherical micelles of PS48-b-PAA67. Several black spots locate on the surface of hybrid spherical micelles. These black spots disappear (Figure 2c) after treatment with EDTA, which is a strong metal chelator.19 This phenomenon indicates that the QDs are embedded into the corona of micelles at ω = 0.05. This is because PAA blocks replace the original OA ligands on the QD surface through a ligand exchange process.31−33 The binding of PAA block to the QD surface is much stronger than that of small-molecule ligands because of the multiple binding sites on each PAA chain.31 In our case, PAA blocks with ∼67 repeating units have strong coordination with QD surface leading to the incorporation of QDs into the corona. The size and shape of resultant micelles are less influenced by the presence of QDs, which suggests that PS48-b-PAA67 micelles serve as templates for arranging QDs at low ω. OA-capped QDs could also be encapsulated into the core of PS-b-PAA micelles through hydrophobic interaction between PS blocks and OA ligands of QDs.34 However, our results indicate that long PAA blocks bind to the QD surface prior to the microphase separation induced by water addition. This process leads to QDs solely incorporated into the corona of hybrid spherical micelles. The QD loading shows obvious influence on the size and morphology of the coassemblies (Figure 3 and 4). With the increase of ω to 0.09, larger coassemblies than the hybrid spherical micelles were formed as indicated by red arrows in Figure 3a. The average diameter is 27 nm. The coassembly size increases to 51 nm for ω = 0.17 (Figure 3b) and 74 nm for ω = 0.33 (Figure 3c), respectively. The coassembly morphology was mainly solid spheres at ω of 0.09 and 0.17. Spheres with a void inside, resembling the morphology of a vesicle, start to form with ω increasing to 0.33. However, some of the voids are decentered with the wall thickness varying from 8 to 40 nm, which indicates that the hollow spheres are not vesicles. The SEM image (Figure 3d) confirmed that the spheres with a void inside were bowl-shaped structures.35 After treated with EDTA to remove QDs from the surface, the solid spheres show a 13 nm thick shell at the exterior as presented in Figure 3e. According to the large size and existence of the shell, the solid spheres are speculated to be large compound micelles (LCMs) consisting of reverse micelles inside and a monolayer of polymers outside, similar to the structure reported by Zhang and Eisenberg.36
■
RESULTS AND DISCUSSION Characterizations of the Oleic Acid-Capped CdSe/CdS Core/Shell QDs. The OA-capped CdSe/CdS core/shell QDs were synthesized through a two-phase approach at low temperature.29 The average diameter is 2.5 ± 0.5 nm measured from TEM images as shown in Figure 1a,b. The UV−vis and PL spectra are shown in Figure S1 (see Supporting Information). The absorption peak is at 488 nm, and an emission peak locates at 510 nm. QD Loading Induced Morphological Transitions of Coassemblies. The preparation of coassemblies involves two main steps. First, PS48-b-PAA67 and OA-capped CdSe/CdS core/shell QDs were mixed in a common solvent, i.e., THF. Second, upon addition of a selective solvent, i.e., water, the two components start to self-assemble simultaneously to form hybrid structures. We focus on the morphological transitions of the coassemblies induced only by varying the weight fraction of QDs (ω). C
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 2. TEM images of (a) spherical micelles of PS48-b-PAA67 (stained with a solution of 0.4 wt % phosphotungstic acid in water, the PS domain corresponds to brighter area), and hybrid spherical micelles at ω = 0.05 (b) before and (c) after treatment with EDTA. The black spots on the surface of micelles in panel b are QDs. The blue line indicates PAA block, and the gray line indicates PS block. The red spheres indicate QDs.
Figure 3. TEM images of coassemblies obtained at different weight fractions of QDs: (a) ω = 0.09 (the red arrows indicate larger coassemblies than hybrid spherical micelles), (b) ω = 0.17, (c) ω = 0.33 (the blue arrows indicate decentered voids inside the coassemblies), (d) SEM image of coassemblies obtained at ω = 0.33, and (e) sample in c treated with EDTA.
PAA blocks are long, so the addition of QDs is inferred to be essential in the formation of LCMs. LCMs, bowl-shaped structures and porous spheres belong to a morphological family, which are aggregates containing an assembly of reverse micelles. The bowl-shaped structure was first observed by Riegel and Eisenberg in the self-assembly of poly[5-(N, N, Ndiethylmethylammonium) isoprene]-b-polystyrene-b-poly[5(N, N, N-diethylmethylammonium) isoprene] (PAI-b-PS-bPAI) triblock copolymers.35 The authors demonstrated that the mechanism of bowl-shaped structure involved formation of LCMs and release of trapped bubbles, where the internal viscosity and polymer chain mobility played a very important role. The internal viscosity should be high enough to form a hardened “skin” which leads to inhomogeneous shrinkage of the aggregates and formation of solvent bubbles during water addition, but also low enough to allow coalescence of the bubbles. Therefore, the internal viscosity window must be very narrow and some additional viscosity-control mechanisms, such as hydrogen bonding37 or polymer rigidity,38 are needed to form bowl-shaped structures. If the internal viscosity is very high and prevents the coalescence of the solvent bubbles, then porous spheres are formed.
Interestingly, typical LCMs, bowl-shaped structures, and several porous spheres (Figure 4a, 4c, 4e and 4f) were formed with increasing ω to 0.44. The bowl-shaped structure and porous sphere are special large compound micelles.35 The bowl-shaped structure contains a single hole on the surface (Figure 4b and 4e), whereas the porous sphere contains some trapped bubbles as indicated by red arrows in Figure 4f. It is clear that each LCM or bowl-shaped structure or porous sphere contains many small spheres distributed in the core which is surrounded by a polymer shell. These small spheres are reverse micelles composed of PAA core and PS corona. Highresolution TEM image shows that most QDs are located inside the cores of the reverse micelles (blue circles in Figure 4d), which is still due to the strong coordination between PAA blocks and QD surface as described above. The diameter of coassemblies at ω = 0.44 widely ranges from 28 nm to 2 μm, which agrees with the high polydispersity of LCMs previously reported.36 Formation Mechanism of the LCM and Bowl-Shaped Structure. Eisenberg and co-workers36 reported LCMs from the self-assembly of BCPs such as PS-b-PAA with short hydrophilic PAA block. However, in our case, the hydrophilic D
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 4. (a) TEM and (b) SEM image of coassemblies obtained at ω = 0.44, (c) low- and (d) high-resolution TEM image of LCMs obtained at ω = 0.44, TEM images of (e) bowl-shaped structures and (f) porous spheres obtained at ω = 0.44. The inset in (c) shows the magnified image of reverse micelles within the LCMs. The blue circles in (d) indicate that the QDs are incorporated into the PAA cores of reverse micelles. The red arrows in (f) indicate the trapped bubbles inside the porous sphere.
QDs were formed, as shown in Figure 5a. At this high ω, PAA blocks bridging QDs is speculated to occur. This phenomenon suggests that one PAA block may coordinate with more than one QD (Scheme 1), leading to the formation of these highly swollen and homogeneous aggregates. At this stage, numerous PAA blocks with QDs adsorbed on them were buried inside the aggregates. After the CWC, microphase separation between the hydrophilic PAA block and the hydrophobic PS block starts, resulting in the association of adjacent PAA blocks inside the aggregates. One large aggregate is divided into several swollen spheres (Figure 5b). The spheres are stabilized by a monolayer of PS-b-PAA chains on the surface. These spheres would transform into LCMs and be kinetically “frozen” at high water content as shown in Figure 5c and 5d. However, PAA blocks bridging QDs would restrict the mobility of polymer chains. Moreover, as the water addition continues, the extraction of THF or the phase separation of PAA blocks would further decrease the mobility of polymer chains inside the spheres. Since THF was extracted more rapidly from the surface region than the interior, the high viscosity and low chain mobility at the surface region caused the formation of a hardened “skin”.35 The hardened “skin” would prevent the homogeneous shrinkage of the whole spheres during water addition, and then bubbles filled with solvents but devoid of polymer are formed. Alternatively, solvent bubbles may result from a liquid−liquid phase separation due to the low chain mobility.35 As shown in Figure 5c, some trapped bubbles have been captured in the TEM image. If the bubbles have the chance to coalesce and break through the surface, bowl-shaped structures are thus formed, otherwise, porous spheres are produced. The formation of the LCM, bowl-shaped structure and porous sphere is depicted in Scheme 1. In our case, the additional viscosity control is speculated to be provided by the PAA blocks bridging QDs, which makes the internal viscosity located within the window suitable for forming LCMs or bowl-shaped structures. The final morphol-
To investigate the formation mechanism of LCM and bowlshaped structure in our case, the coassembly process at ω = 0.44 with the addition of water was monitored by TEM. The coassembly structures with the addition of water were preserved through freeze-drying process and then visualized by TEM, as presented in Figure 5. At low water content before the critical water content (CWC, which is 11.9 wt % as shown in Figure S2), many large irregular aggregates of PS-b-PAA and
Figure 5. Changes in morphology of coassemblies (ω = 0.44) with the addition of water: (a) 7.0 wt %, (b) 23.1 wt %, (c) 37.5 wt % (the black arrow indicates the trapped bubbles within the spheres captured by TEM.), and (d) 57.4 wt %. E
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Scheme 1. Schematic Illustration of the Formation of LCM, Bowl-Shaped Structure, and Porous Sphere
ogy is evidently dependent on the QD loading. The morphology changes from hybrid spherical micelles to LCMs and then to bowl-shaped structures with the increase of QD loading. The chain mobility decreases with the increased QD loading because more PAA blocks will participate in bridging QDs. At low QD loading, the high chain mobility causes homogeneous shrinkage of the swollen spheres. Thus, no voids are produced during water addition. This condition leads to the exclusive formation of LCMs. With the increase of QD loading, the decreased chain mobility generates solvent bubbles within the swollen spheres during water addition. If the bubbles get the chance to coalesce and release, bowl-shaped structures are produced. However, at the highest QD loading, the trapped solvent bubbles may have less chance to coalesce due to the very low chain mobility, and then porous spheres are formed in this way. The polymer molecular weight, concentration, solvent and temperature are critical factors in determining the chain mobility and the assembly structure. Keeping ω constant at 0.33, the influences of these factors on the morphology of coassemblies were examined. As shown in Figure 6, LCMs and bowl-shaped structures coexist over the wide polymer concentration range of 0.3−3.0 mg/mL. At the highest polymer concentration of 3.0 mg/mL, the low chain mobility prevents
the coalescence of solvent bubbles, resulting in the formation of some porous spheres as shown in Figure 6d. Bowl-shaped structures can also be prepared at high temperature of 60 °C as presented in Figure 7a. The chain mobility increases with the
Figure 7. Coassemblies prepared (a) through increasing temperature to 60 °C, (b) using different block copolymer of PS159-b-PAA63 with longer PS block compared to PS48-b-PAA67 (the inset indicates porous sphere).
increase of temperature but is still suitable for forming bowlshaped structures. This will make the trapped solvent bubbles easy to coalesce to form large holes (from 6 to 165 nm) on the surface. Figure 7b shows the coassemblies from QDs with PS159-b-PAA63 which has a nearly identical PAA block length but much longer PS block as compared to PS48-b-PAA67. LCMs, bowl-shaped structures, and some porous spheres were obtained. The size distribution is also broad over a range of 67−444 nm and the outer shell thickness is about 20 nm. This shell thickness is larger than that of coassemblies from PS48-bPAA67 with QDs due to the longer hydrophobic PS block. These observations indicate that the PS block length influence slightly on the morphology as the PAA block long enough to bridge QDs. The formation of porous spheres was caused by the low mobility of polymer chains with high molecular weight. All these results confirm that PAA blocks bridging QDs dominates the chain mobility and the resulting morphology. The chain mobility is so limited by bridging QDs that adjusting the factors of polymer concentration, temperature or polymer molecular weight only results in the morphological transitions among LCMs, bowl-shaped structures and porous spheres with no formation of other morphologies. Yusuf et al.39 have also obtained LCMs of PS-b-PAA and QDs. They first prepared PS-stabilized CdS QDs through in situ synthesizing QDs in the core of PS-b-PAA reverse micelles, and then coassembled the PS-CdS with PS-b-PAA into LCMs. By contrast, our approach of preparing LCMs by adding water to the cosolvent of polymer and NPs is simpler and easier. In addition, some simulation results reported in the literature could support our experimental results. Zhang et al.40 have investigated the coassembly behavior of BCPs with NPs in
Figure 6. Coassemblies obtained at different initial polymer concentrations: (a) 0.3 mg/mL, (b) 0.6 mg/mL, (c) 2.0 mg/mL, and (d) 3.0 mg/mL. The weight fraction of QDs (ω) keeps constant at 0.33. F
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 8. (a) DLS curves of stock solution at ω = 0.44 and four fractions separated by centrifugation. TEM images of (b) stock solution, (c) fraction A, (d) fraction B, (e) fraction C, and (f) fraction D.
Table 1. Data from DLS and TEM Measurements of Fractions Fractionated by Centrifugation DLS
a
TEM
sample
Z-average size (d·nm)
SDa (d·nm)
RSDa
average size (d·nm)
SD(d· nm)
RSD
morphology
stock solution fraction A fraction B fraction C fraction D
889.3 1234.1 789.0 481.5 202.6
399.6 245.7 180.3 127.0 52.3
44.9% 19.9% 22.9% 26.4% 25.8%
383.3 703.3 383.9 149.0 76.3
295.4 247.4 155.9 52.2 48.3
77.1% 35.1% 40.6% 35.0% 63.3%
LCMs/bowl-shaped structures LCMs/bowl-shaped structures LCMs/bowl-shaped structures hybrid spherical micelles/bowl-shaped structures
SD: standard deviation; RSD: relative standard deviation.
common methods, i.e. centrifugation and membrane filtration, are applied to fractionate the resultant coassemblies. Centrifugation or ultracentrifugation has been widely used to purify or fractionate micelles,2,25 which is simple to operate and implement in a laboratory. Herein, the coassemblies undergo a sequence of centrifugations at different RCFs (relative centrifugal forces) to get four size fractions with decreased average sizes. The DLS and TEM results are displayed in Figure 8, and related data are listed in Table 1. In Figure 8a, the intensity-weighted particle size distributions determined by DLS are all unimodal. The stock solution shows a Z-average size of 889.3 nm, and its RSD (relative standard deviation) is 44.9%. The corresponding TEM image in Figure 8b shows mixed morphologies and very broad size distribution. The diameters of dried sample vary from 15 to 1942 nm, and the average size is 383 nm with a RSD of 77.1%. Both results confirm that the coassemblies at ω = 0.44 are considerably polydisperse. The four fractions in Figure 8a display relatively narrower distribution width with Z-average sizes at 1234.1 nm (fraction A), 789.0 nm (fraction B), 481.5 nm (fraction C), and 202.6 nm (fraction D) by increasing RCF. Compared with stock solution, the RSD of these fractions is in the range of 19.9−26.4%, which reveals that fractions have relative narrower size distribution. However, some of the size curve of fraction D seems to exceed the size range of stock solution. It is wellknown that the scattering intensity is roughly proportional to
dilute solution through combination of self-consistent-field theory (SCFT) and density functional theory (DFT). Their simulation results showed that the addition of large amounts of NPs induced the formation of LCMs in the solution when NPs were not selective to the hydrophobic and hydrophilic blocks, which was consistent with the formation of LCMs in our system. BCPs containing long P4VP or PAA blocks are usually used as structural motif to organize preformed inorganic NPs7,17−19,41 or to combine with metal salt followed by the reduction of salt to synthesize NPs in situ.42,43 However, the hybrid structures cannot bear too many NPs and flocculate in aqueous solution,7 because incorporation of NPs into the hydrophilic corona will affect the solubility and stability of hybrid micelles. In our case, the excessive QDs are confined in the core of each reverse micelle within the LCMs or bowlshaped structures at high QD loading. The hybrid structures could remain stable for at least six months. The NP loading is remarkably enhanced. Using our approach, LCMs and bowlshaped structures containing large amounts of Fe2O3 NPs (ω = 0.5) can also be achieved as shown in Figure S3. We anticipate these structures encapsulating fluorescent, magnetic, or gold NPs can be served as bioimaging agent or drug carrier. Size Fractionation of Coassemblies Using Centrifugation and Membrane Filtration. A remarkable feature of the coassemblies obtained at ω = 0.44 is their polydispersity. Two G
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 9. (a) DLS curves of stock solution at ω = 0.44 and four fractions separated by membrane filtration. TEM images of (b) fraction a, (c) fraction b, (d) fraction c, and (e) fraction d.
Table 2. Data from DLS and TEM Measurements of Fractions Fractionated by Membrane Filtration DLS
TEM
samples
Z-average size (d.nm)
SD (d· nm)
RSD
average size (d.nm)
SD (d.nm)
RSD
stock solution fraction a fraction b fraction c fraction d
889.3 673.3 336.2 207.1 134.3
399.6 207.0 97.9 53.3 21.8
44.9% 30.7% 29.1% 25.7% 16.2%
383.3 580.0 328.1 163.6 49.3
295.4 151.6 113.2 58.6 26.6
77.1% 26.1% 34.5% 35.8% 54.0%
morphology LCMs/bowl-shaped structures LCMs/bowl-shaped structures LCMs/bowl-shaped structures hybrid spherical micelles/LCMs/bowl-shaped structures
large bowl-shaped structures may precipitate at large RCF, which will increase the size range. Therefore, fraction B, C and D have broader size distributions than fraction A. Relationship between RCF and the sizes of fractions is displayed in Figure S4a. The fraction separated by centrifugation is still fluorescent as shown in Figure S5, which indicates that size fractionation has no influence on the fluorescence property of coassemblies. Membrane filtration has also been applied to the separation and purification of colloidal dispersions.45,46 The assemblies are sequentially filtered through membranes with decreased pore size. First of all, it should be stated that fractionation by membrane filtration using mini-extruder is limited by the pore size of membrane. Only six membranes with fixed pore sizes are supplied, in which three membranes (1.0, 0.4, and 0.1 μm) are chosen in our case. Therefore, the size range of each fraction is fixed before the fractionation experiment, which is larger than 1.0 μm for fraction a, 1.0−0.4 μm for fraction b, 0.4−0.1 μm for fraction c and smaller than 0.1 μm for fraction d. The DLS and TEM results are shown in Figure 9 and related data are listed in Table 2. The four fractions in Figure 9a also display a narrower distribution width with Z-average sizes at 673.3 nm (fraction a), 336.2 nm (fraction b), 207.1 nm (fraction c) and 134.3 nm (fraction d) by decreasing pore size of membrane and their RSDs are all smaller than stock solution. The RSD decreases from fraction a to d, which indicates that the smallest fraction is the least polydispersed one. The corresponding TEM images
the sixth power of size, so large particles contribute more to the total scattering intensity than the small particles. The small particles with very low concentration cannot be detected by DLS, thus the curve of stock solution is lack of contribution from small particles. The fraction D was concentrated under vacuum for 2 h at room temperature and then was applied to characterization. The fraction A presented in Figure 8c contains huge LCMs and bowl-shaped structures with average diameter of 703 nm, and it owns the smallest RSD regardless of the characterization methods, which means that it has the narrowest size distribution among four fractions. Fraction B in Figure 8d consists of large LCMs and bowl-shaped structures with average diameter of 384 nm. The fraction C (Figure 8e) has a smaller size of 149 nm and lower internal density, in which individual reverse micelle is distinct. Fraction D is the supernatant of fraction C, it contains mainly bowl-shaped structures and hybrid spherical micelles with the average size of 76 nm (Figure 8f). The bowl-shaped structures are much larger than hybrid spherical micelles, which leads to a very large RSD (63.3%) from TEM statistic measurement. The RSD (25.8%) of fraction D from DLS is much smaller than TEM result. It may be because that DLS gives the hydrodynamic diameter of equivalent sphere, which experiences during its Brownian motion in solution a friction identical in magnitude to that of bowl-shaped structures.44 It should be mentioned that fractionation by centrifugation is actually based on mass, so H
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Langmuir
■
are shown in Figure 9b−e. The fraction a and b consist of both large LCMs and bowl-shaped structures with average sizes of 580 nm for fraction a and 328 nm for fraction b, which are smaller than the expected size ranges for fraction a and b. The size difference between predicted and experimental fractions may be attributed to the blocking of pores by large coassemblies or adsorption of coassemblies on the membrane, both of which will increase the proportion of small particles in the fractions. Fraction c contains LCMs and bowl-shaped structures with average size of 164 nm. Fraction d contains LCMs, bowl-shaped structures and hybrid spherical micelles, and the average size is 49 nm. Both fraction c and d match the predicted size ranges. It can be seen that fraction d with the smallest size and size distribution is separated only by membrane filtration, which suggests that the membrane filtration may be effective to isolate monodisperse small particles by using appropriate pore size. Relationship between pore sizes of membranes and the sizes of fractions is depicted in Figure S4b.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.Z.). Fax: +86-431-8987-6788. Tel: +86-431-8987-6788. *E-mail:
[email protected] (X.J.). Fax: +86-431-8526-2075. Tel: +86-431-8526-2876. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS We are grateful to the financial support from National Natural Science Foundation of China (21274145). REFERENCES
(1) Kim, B.-S.; Qiu, J.-M.; Wang, J.-P.; Taton, T. A. Magnetomicelles: Composite Nanostructures from Magnetic Nanoparticles and CrossLinked Amphiphilic Block Copolymers. Nano Lett. 2005, 5, 1987− 1991. (2) Kang, Y.; Taton, T. A. Core/Shell Gold Nanoparticles by SelfAssembly and Crosslinking of Micellar, Block-Copolymer Shells. Angew. Chem., Int. Ed. 2005, 44, 409−412. (3) Sanchez-Gaytan, B. L.; Cui, W.; Kim, Y.; Mendez-Polanco, M. A.; Duncan, T. V.; Fryd, M.; Wayland, B. B.; Park, S.-J. Interfacial Assembly of Nanoparticles in Discrete Block-Copolymer Aggregates. Angew. Chem., Int. Ed. 2007, 46, 9235−9238. (4) Hickey, R. J.; Haynes, A. S.; Kikkawa, J. M.; Park, S.-J. Controlling the Self-Assembly Structure of Magnetic Nanoparticles and Amphiphilic Block-Copolymers: From Micelles to Vesicles. J. Am. Chem. Soc. 2011, 133, 1517−1525. (5) Mai, Y.; Eisenberg, A. Controlled Incorporation of Particles into the Central Portion of Block Copolymer Rods and Micelles. Macromolecules 2011, 44, 3179−3183. (6) Mai, Y.; Eisenberg, A. Selective Localization of Preformed Nanoparticles in Morphologically Controllable Block Copolymer Aggregates in Solution. Acc. Chem. Res. 2012, 45, 1657−1666. (7) Wang, M.; Kumar, S.; Lee, A.; Felorzabihi, N.; Shen, L.; Zhao, F.; Froimowicz, P.; Scholes, G. D.; Winnik, M. A. Nanoscale Coorganization of Quantum Dots and Conjugated Polymers Using Polymeric Micelles As Templates. J. Am. Chem. Soc. 2008, 130, 9481− 9491. (8) Li, W.; Liu, S.; Deng, R.; Zhu, J. Encapsulation of Nanoparticles in Block Copolymer Micellar Aggregates by Directed Supramolecular Assembly. Angew. Chem., Int. Ed. 2011, 50, 5865−5868. (9) Wang, H.; Chen, L.; Shen, X.; Zhu, L.; He, J.; Chen, H. Unconventional Chain-Growth Mode in the Assembly of Colloidal Gold Nanoparticles. Angew. Chem., Int. Ed. 2012, 51, 8021−8025. (10) Nasongkla, N.; Bey, E.; Ren, J.; Ai, H.; Khemtong, C.; Guthi, J. S.; Chin, S.-F.; Sherry, A. D.; Boothman, D. A.; Gao, J. Multifunctional Polymeric Micelles as Cancer-Targeted, MRI-Ultrasensitive Drug Delivery Systems. Nano Lett. 2006, 6, 2427−2430. (11) Yang, X.; Grailer, J. J.; Rowland, I. J.; Javadi, A.; Hurley, S. A.; Steeber, D. A.; Gong, S. Multifunctional SPIO/DOX-loaded Wormlike Polymer Vesicles for Cancer Therapy and MR Imaging. Biomaterials 2010, 31, 9065−9073. (12) Jain, T. K.; Richey, J.; Strand, M.; Leslie-Pelecky, D. L.; Flask, C. A.; Labhasetwar, V. Magnetic Nanoparticles with Dual Functional Properties: Drug Delivery and Magnetic Resonance Imaging. Biomaterials 2008, 29, 4012−4021. (13) Mai, Y.; Eisenberg, A. Self-assembly of Block Copolymers. Chem. Soc. Rev. 2012, 41, 5969−5985. (14) Yu, Y.; Zhang, L.; Eisenberg, A. Morphogenic Effect of Solvent on Crew-Cut Aggregates of Apmphiphilic Diblock Copolymers. Macromolecules 1998, 31, 1144−1154. (15) Zhang, L.; Eisenberg, A. Thermodynamic vs Kinetic Aspects in the Formation and Morphological Transitions of Crew-Cut Aggregates Produced by Self-Assembly of Polystyrene-b-poly(acrylic acid) Block Copolymers in Dilute Solution. Macromolecules 1999, 32, 2239−2249.
■
CONCLUSIONS Coassemblies of PS48-b-PAA67 and OA-capped CdSe/CdS core/shell QDs are prepared by gradually adding water into the THF solution. QDs are incorporated into the hydrophilic PAA phase through strong coordination between PAA blocks and QD surface. The main results are summarized as follows. (a) Coassemblies can change from hybrid spherical micelles to LCMs and then to bowl-shaped structures by increasing the weight fraction of QDs. The formation mechanism of LCMs and bowl-shaped structures is proposed based on the TEM results during water addition. PAA blocks bridging QDs is so essential to the formation of LCMs and bowl-shaped structures that other factors, such as polymer concentration, temperature or polymer molecular weight, influence little on the morphology of coassemblies. (b) Centrifugation and membrane filtration techniques are applied to fractionate coassemblies into fractions successfully, and the fractions have narrower size distribution than stock solution. Definitely, this study provides effective methods for fractionating self-assemblies or coassemblies according to size difference.
■
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02202. Additional figures: UV−vis absorption and fluorescence spectra of oleic acid-capped CdSe/CdS QDs, turbidity measurement of the coassembly solution with the addition of water, transmission electron micrographs of coassemblies prepared by PS48-b-PAA67 and 4.5 nm oleic acid-capped Fe2O3 nanoparticles, relationship between relative centrifugal forces or pore sizes of membranes with the sizes of fractions, fluorescence spectra of coassemblies before and after fractionated by centrifugation, as well as influence of centrifugations on the hydrodynamic diameters of coassemblies (PDF) I
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
PDMAEMA-Capped II−VI Semiconductor Quantum Dots Nanocomposites. Chin. J. Polym. Sci. 2013, 31, 1233−1241. (34) Kim, B.-S.; Taton, T. A. Multicomponent Nanoparticles via SelfAssembly with Cross-Linked Block Copolymer Surfactants. Langmuir 2007, 23, 2198−2202. (35) Riegel, I. C.; Eisenberg, A.; Petzhold, C. L.; Samios, D. Novel Bowl-Shaped Morphology of Crew-Cut Aggregates from Amphiphilic Block Copolymers of Styrene and 5-(N,N-Diethylamino)isoprene. Langmuir 2002, 18, 3358−3363. (36) Zhang, L.; Eisenberg, A. Multiple Morphologies and Characteristics of “Crew-Cut” Micelle-like Aggregates of Polystyrene-b-poly(acrylic acid) Diblock Copolymers in Aqueous Solutions. J. Am. Chem. Soc. 1996, 118, 3168−3181. (37) Liu, X.; Kim, J.-S.; Wu, J.; Eisenberg, A. Bowl-Shaped Aggregates from the Self-Assembly of an Amphiphilic Random Copolymer of Poly(styrene-co-methacrylic acid). Macromolecules 2005, 38, 6749− 6751. (38) Wang, J.; Kuang, M.; Duan, H.; Chen, D.; Jiang, M. pHDependent Multiple Morphologies of Novel Aggregates of CarboxylTerminated Polymide in Water. Eur. Phys. J. E: Soft Matter Biol. Phys. 2004, 15, 211−215. (39) Yusuf, H.; Kim, W.-G.; Lee, D. H.; Guo, Y.; Moffitt, M. G. Size Control of Mesoscale Aqueous Assemblies of Quantum Dots and Block Copolymers. Langmuir 2007, 23, 868−878. (40) Zhang, L.; Lin, J.; Lin, S. Self-Assembly Behavior of Amphiphilic Block Copolymer/Nanoparticle Mixture in Dilute Solution Studied by Self-Consistent-Field Theory/Density Functional Theory. Macromolecules 2007, 40, 5582−5592. (41) Wang, H.; Lin, W.; Fritz, K. P.; Scholes, G. D.; Winnik, M. A.; Manners, I. Cylindrical Block Co-Micelles with Spatially Selective Functionalization by Nanoparticles. J. Am. Chem. Soc. 2007, 129, 12924−12925. (42) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Preparation of CdS Nanoparticles in Salt-Induced Block Copolymer Micelles. Langmuir 2001, 17, 8428−8433. (43) Jin, J.; Wang, J.; Sun, P.; Zhao, H. Hydrophilic InterfaceCrosslinked Polymer Micelles: A Platform for Nanoreactors and Nanocarriers. Polym. Chem. 2013, 4, 4499−4505. (44) Schärtl, W. Light Scattering from Polymer Solutions and Nanoparticle Dispersions; Springer-Verlag: Berlin, 2007. (45) Ghosh, R.; Cui, Z. F. Protein Purification by Ultrafiltration with Pre-Treated Membrane. J. Membr. Sci. 2000, 167, 47−53. (46) Pan, S.; Tzoc Torres, J. M. G.; Hoare, T.; Ghosh, R. Transmission Behavior of pNIPAM Microgel Particles through Porous Membranes. J. Membr. Sci. 2015, 479, 141−147.
(16) Cai, C.; Wang, L.; Lin, J.; Zhang, X. Morphology Transformation of Hybrid Micelles Self-Assembled from Rod−Coil Block Copolymer and Nanoparticles. Langmuir 2012, 28, 4515−4524. (17) Zhang, M.; Wang, M.; He, S.; Qian, J.; Saffari, A.; Lee, A.; Kumar, S.; Hassan, Y.; Guenther, A.; Scholes, G.; Winnik, M. A. Sphere-to-Wormlike Network Transition of Block Copolymer Micelles Containing CdSe Quantum Dots in the Corona. Macromolecules 2010, 43, 5066−5074. (18) Zhang, M.; Hu, Y.; Hassan, Y.; Zhou, H.; Moozeh, K.; Scholes, G. D.; Winnik, M. A. Slow Morphology Evolution of Block Copolymer-Quantum Dot Hybrid Networks in Solution. Soft Matter 2013, 9, 8887−8896. (19) Wang, M.; Zhang, M.; Siegers, C.; Scholes, G. D.; Winnik, M. A. Polymer Vesicles as Robust Scaffolds for the Directed Assembly of Highly Crystalline Nanocrystals. Langmuir 2009, 25, 13703−13711. (20) Cui, H.; Chen, Z.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Block Copolymer Assembly via Kinetic Control. Science 2007, 317, 647−650. (21) Terreau, O.; Bartels, C.; Eisenberg, A. Effect of Poly(acrylic acid) Block Length Distribution on Polystyrene-b-poly(acrylic acid) Block Copolymer Aggregates in Solution. 2. A Partial Phase Diagram. Langmuir 2004, 20, 637−645. (22) Grubišc-́ Gallot, Z.; Gallot, Y.; Sedlácě k, J. Study of Polystyreneblock-poly(methyl methacrylate) Micelles by Size Exclusion Chromatography/Low-Angle Laser Light Scattering, 1. Influence of Copolymer Concentration and Flow Rate. Macromol. Chem. Phys. 1994, 195, 781−791. (23) Gohy, J.-F. Block Copolymer Micelles. Block Copolymers II; Springer: Berlin, Heidelberg, 2005; Vol. 190, pp 65−136. (24) Greyling, G.; Pasch, H. Multidetector Thermal Field-Flow Fractionation as A Unique Tool for the Tacticity-Based Separation of Poly(methyl methacrylate)-polystyrene Block Copolymer Micelles. J. Chromatogr., A 2015, 1414, 163−172. (25) Marchin, S.; Putaux, J.-L.; Pignon, F.; Léonil, J. Effects of the Environmental Factors on the Casein Micelle Structure Studied by Cryo Transmission Electron Microscopy and Small-Angle X-Ray Scattering/Ultrasmall-Angle X-Ray Scattering. J. Chem. Phys. 2007, 126, 045101. (26) Glantz, M.; Håkansson, A.; Lindmark Månsson, H.; Paulsson, M.; Nilsson, L. Revealing the Size, Conformation, and Shape of Casein Micelles and Aggregates with Asymmetrical Flow Field-Flow Fractionation and Multiangle Light Scattering. Langmuir 2010, 26, 12585−12591. (27) Risso, P.; Relling, V.; Armesto, M.; Pires, M.; Gatti, C. Effect of Size, Proteic Composition, and Heat Treatment on the Colloidal Stability of Proteolyzed Bovine Casein Micelles. Colloid Polym. Sci. 2007, 285, 809−817. (28) Kang, D.; Kim, M.; Kim, S.; Oh, K.; Yuk, S.; Lee, S. Size Characterization of Drug-Loaded Polymeric Core/Shell Nanoparticles Using Asymmetrical Flow Field-Flow Fractionation. Anal. Bioanal. Chem. 2008, 390, 2183−2188. (29) Ma, X.; Nie, W.; Pan, D.; Ji, X.; Wang, C.; Zhang, W. Low Temperature and Low Toxicity Synthesis of Highly Luminescent CdSe/CdS Core-Shell Nanocrystals in A Two-Phase System. CrystEngComm 2011, 13, 5243−5249. (30) Pan, D.; Jiang, S.; An, L.; Jiang, B. Controllable Synthesis of Highly Luminescent and Monodisperse CdS Nanocrystals by a TwoPhase Approach under Mild Conditions. Adv. Mater. 2004, 16, 982− 985. (31) Zhang, T.; Ge, J.; Hu, Y.; Yin, Y. A General Approach for Transferring Hydrophobic Nanocrystals into Water. Nano Lett. 2007, 7, 3203−3207. (32) Lin, W.; Fritz, K.; Guerin, G.; Bardajee, G. R.; Hinds, S.; Sukhovatkin, V.; Sargent, E. H.; Scholes, G. D.; Winnik, M. A. Highly Luminescent Lead Sulfide Nanocrystals in Organic Solvents and Water through Ligand Exchange with Poly(acrylic acid). Langmuir 2008, 24, 8215−8219. (33) Zhao, N.; Nie, W.; Mao, J.; Wang, W.; Ji, X. Synthesis and Properties of Bifunctional Magnetic-Optical Nanomaterials: Fe2O3@ J
DOI: 10.1021/acs.langmuir.6b02202 Langmuir XXXX, XXX, XXX−XXX
