Structure and Ultrasonic Sensitivity of the Superparticles Formed by

Dec 17, 2013 - F.Z.: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA ... high concentration of the copolymer when the cross-linking ...
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Structure and Ultrasonic Sensitivity of the Superparticles Formed by Self-Assembly of Single Chain Janus Nanoparticles Feng Zhou,†,‡ Mingxiu Xie,‡ and Daoyong Chen* The State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan University, Handan Road 220, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Single chain Janus nanoparticles (SCJNPs) (tadpole-like Janus nanoparticles) with a PEO (poly(ethylene oxide)) chain as the “tail” and a cross-linked PCEMA (poly(2-cinnamoyloxyethyl methacrylate)) chain as the “head” were synthesized conveniently and efficiently by directly photo-cross-linking PCEMA block of PEO-b-PCEMA diblock copolymer in the common solvent DMF; intramolecular cross-linking occurred dominantly at a relatively high concentration of the copolymer when the cross-linking speed is relatively low, leading to SCJNPs. In selective solvent for the “tails”, the rigid “heads” aggregated into superparticles. It is significant that under a gentle ultrasonic treatment (40 kHz for 10 min) the spherical superparticles formed in DMF/ethanol (1/4, v/v) dissociated into individual SCJNPs. It is also demonstrated that even in pure water in which the superparticles have a more closely aggregated structure, there are still hydrophilic channels within the superparticles connecting the surrounding medium and the inside of the superparticles, which allows rapid transport of hydrophilic small molecules within the superparticles, as demonstrated by the fast acid quench of fluorescence of the encapsulated ANS (8-anilino-1naphthalenesulfonate). These features should make the superparticles promising in the applications as templates for biomimetic mineralization, highly efficient microreactors for interfacial chemical reactions, and ultrasound responsive nanovehicles for controlled drug release.

1. INTRODUCTION

efficiently, which is quite difficult in most cases. We note that tadpole-like single chain Janus nanoparticles (SCJNPs) are the anisotropic nanoparticles that can be prepared efficiently.7,15,16 Besides, SCJNPs composed of a collapsed and cross-linked polymer chain as the “head” connected with a linear polymer chain as the “tail” are the intermediate between solid Janus nanoparticles and flexible block copolymers; they should be capable of self-assembling efficiently into regular superstructures. However, studies on the self-assembly of SCJNPs have seldom been reported.17 Besides, no study on the structure and properties of the resultant superparticles has been reported. In the present study, we report a new method for preparing SCJNPs more efficiently, the self-assembly behavior of the

Self-assembly of nanoparticles has attracted much attention since it can generate superparticles that are entirely different from molecular assemblies in structures, morphologies, and properties. Usually, the nanoparticles capable of self-assembling into regular superstructures are anisotropic nanoparticles.1,2 So far, many kinds of anisotropic nanoparticles have been prepared,3−6 and their self-assembly led to regular superstructures including spherical supermicelles,7−9 multicompartment cylinders,10 nanoribbons,11 tubes,12 and sheets.12−14 However, compared with molecular assemblies that have been widely and deeply studied, nanoparticle assemblies (superparticles) have been much less studied. To the best of our knowledge, no study on the structure and property of superparticles has been reported yet. For studying structure and property of superparticles, the primary anisotropic nanoparticles that are capable of selfassembling into regular superparticles should be prepared © 2013 American Chemical Society

Received: July 29, 2013 Revised: December 8, 2013 Published: December 17, 2013 365

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filtered and added to excess diethyl ether to precipitate PEO-bPCEMA. The polymer was dried at room temperature under vacuum. Preparation of the Tadpole-like SCJNPs. PEO-b-PCEMA was dissolved in the common solvent DMF at a concentration from 0.5 to 20 mg/mL. After being filtered through a 450 nm pore size Teflon membrane filter, the solution was then exposed to UV light obtained by passing the light of a 500 W mercury lamp (CHF-XM-500 W) through a 254 nm cutoff filter. Considering that the dimerization rate depends on both the polymer concentration and the irradiation intensity, at each of the concentrations, the irradiation was conducted at different distances (0.2, 1, and 2 m) between the light source and the sample. UV−vis spectra and GPC measurements were used to monitor the photo-cross-linking kinetics. The CEMA double-bond conversion was calculated according to conversion = 1 − At/A0, where A0 and At are the absorbance at 274 nm before and after the irradiation for a certain time (t), respectively. At a certain concentration of the copolymer, the irradiation distance and irradiation time were adjusted to obtain a CEMA double-bond conversion as high as possible but without causing considerable interchain cross-linking. Self-Assembly of the Tadpole-like SCJNPs in Selective Solvent. 1. Preparation of the Superparticles in DMF/Ethanol (1/ 4, v/v). 1.0 mL of DMF solution of the SCJNPs (1 mg/mL) was added dropwise into 4.0 mL of ethanol using a microsyringe at a speed of 2.0 mL/h with mild stirring. 2. Preparation of Block Copolymer Micelles. Block copolymer micelles were prepared as control of the superparticles. For preparing micelles, PEO-b-PCEMA block copolymer was used in place of SCJNPs, and the micellization of the block copolymer was carried out in the same medium and under the same conditions as that for preparing the superparticles. Ultrasonic Response of the Superparticles. Ultrasonic response of the superparticles was characterized by transmission electron microscope (TEM) observation. The suspension of the superparticles was treated ultrasonically in an ultrasonic bath (DL360D, power 360 W) operating at 40 kHz at a power output of 60% for 10 min. Then, without withdrawing the ultrasonic treatment, a drop of the solution was rapidly transferred to a copper grid frozen in liquid nitrogen, and the copper grid with the sample was freeze-dried in a freeze-dryer at −50 °C and subsequently observed by TEM. Investigation of Microstructures of the Superparticles and the Micelles via a Fluorescence Probe Method. A fluorescence probe 8-anilino-1-naphthalenesulfonate (ANS) was used for probing the microstructures of the superparticles and the micelles. ANS was incorporated via a coassembly technique. Generally, 100 μL of the ANS stock solution (2.0 mM in ethanol) was added to 1 mL of DMF solution of either the SCJNPs or the block copolymer. Then the same process for preparing the superparticles (or the micelles as the control of the superparticles) was applied to prepare the superparticles (or the micelles) encapsulating ANS; in the final suspensions, the concentration of ANS is 40 μM. Then each of the suspensions was dialyzed against water containing ANS at the concentration of 40 μM; the ANS aqueous solution instead of pure water was used to avoid leaking of the dye during dialysis. All the samples were protected from light to avoid photobleaching. For the investigation of diffusion of protons into the microstructures of the suparparticles and the micelles, 1.0 M HCl solution was added into each of the suspensions to adjust the pH value to 2.0 in short time. Then the changes in fluorescence spectra of ANS were tracked. ANS was excited at 325 nm, and the slit width was set at 1.5 nm for the excitation and 0.9 nm for the emission. Characterization Methods and Instruments. 1H NMR measurements were recorded with a Bruker Advance 400 spectrometer and a Bruker DMX 500 spectrometer. Gel permeation chromatography−multiangle laser light scattering (GPC-MALLS) analysis was carried out with a Waters Breeze 1525 GPC analysis system with two PL mix-D columns, combined with a Wyatt Dawn Heleos II LS detector, using DMF with 0.5 M LiBr as eluent at the flow rate of 1 mL/min at 80 °C, and PEO calibration kit (purchased from TOSOH) as the calibration standard. Astra software (Wyatt Technology Corp.) was used to determine the molecular weight characteristics from injected mass and assuming 100% mass recovery. UV−vis spectra were

SCJNPs, and the structure and properties of the resultant superparticles. For the first time, the tadpole-like morphology of SCJNPs was observed by TEM. The SCJNPs prepared in the present study self-assembled in the selective solvent for the tail to form regular superstructures. Because the self-assembly is driven by aggregation of the spherical “heads” and the superstructures depend on their stacking, the self-assembly behavior of SCJNPs and the structure and properties of the resultant superparticles are quite unique compared with the behavior of block copolymers and the structure and property of polymeric micelles. Especially, the unique structure and properties would make the superparticles promising in addressing relative theoretical and practical problems.

2. EXPERIMENTAL SECTION Materials. Poly(ethylene glycol) monomethyl ether (PEO113-OH) (Mn = 5000), 2-bromoisobutyryl bromide (98%), 2,2′-bipyridine (bpy, ≥99%), cinnamoyl chloride (98%), and 8-anilino-1-naphthalenesulfonate (ANS, 98%) were purchased from Aldrich and used as received. 2-Hydroxyethyl methacrylate (HEMA, 98%, Aldrich) were purified by passing the monomer through a column filled with basic alumina twice to remove the inhibitor. Dichloromethane (CH2Cl2) and triethylamine (TEA) were purified by distilling over CaH2. 4-(Dimethylamino)pyridine (DMAP) was recrystallized from ethanol to remove impurities. CuCl was washed with acetic acid followed by methanol to remove impurities. Pyridine was dried over CaH2 overnight, distilled over CaH2, and stored in contact with 4 Å molecular sieves. All other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. PEO-Br Macroinitiator. PEO-Br macroinitiator was synthesized by esterification of PEO113-OH with 2-bromoisobutyryl bromide.18 In a three-neck round-bottom flask, DMAP (0.732 g, 6.0 mmol) in 16 mL of dry methylene dichloride was mixed with TEA (0.404 g, 4.0 mmol). The flask was cooled down to 0 °C in an ice−water bath. Then, to the flask, 2-bromoisobutyryl bromide (2.3 g, 10.0 mmol) in 16 mL of dry CH2Cl2 was added, followed by slow addition of PEO113OH (20 g, 4 mmol) in 80 mL of dry CH2Cl2 for 1 h under dry argon. The flask was then taken out from the ice−water bath, and the reaction was allowed to proceed for 18 h at room temperature. After filtration to remove the insoluble salts, the reaction mixture was washed with saturated NaCl solution, and then the water layer was extracted twice by CH2Cl2. The CH2Cl2 layers were combined and dried over anhydrous MgSO4 under stirring overnight followed by filtration and removing most of the solvent in vacuo. Subsequently, the macroinitiator was precipitated twice into cold diethyl ether and dried under vacuum to obtain the white powder (15.8 g, yield = 76.7%). 1H NMR (CDCl3, 400 MHz): 4.33 (t, CH2CH2OCO), 1.94 (s, COC(CH3)2), 3.38 (s, CH3O), 3.83−3.45 (m, (CH2CH2O)113). PEO-b-PHEMA. A clean and dry ampule charged with CuCl (10 mg, 0.101 mmol) was treated by three cycles of evacuation under vacuum and backfilled with argon. Then, the ampule filled with argon was immersed partly in liquid nitrogen, followed by sequential addition of HEMA (5 mL, 0.041 mol), PEO-Br macroinitiator (300 mg, 0.06 mmol) and bpy (26 mg, 0.167 mmol) in 3 mL of methanol, methanol (2 mL) into the ampule; all the additions were conducted by slow injections to ensure that the agents were frozen as soon as added. The polymerization mixture was deoxygenated by five freeze−pump−thaw cycles, flame-sealed under vacuum, and then heated in an oil bath at 40 °C for 10 h. The reaction was quenched via exposure to air and dilution with methanol. The resultant polymer solution was filtered through a column filled with neutral alumina to remove the copper complex. The polymer was precipitated twice in cold diethyl ether and dried under vacuum at room temperature for 2 days, giving 2.27 g of pure copolymer (HEMA conversion was about 37%). PEO-b-PCEMA. 100 mg of PEO-b-PHEMA containing 0.67 mmol of hydroxyl group was dissolved in 5 mL of dry pyridine. Cinnamoyl chloride (170 mg, 1.02 mmol) was then added to the solution. The mixture was stirred overnight at room temperature before it was 366

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HEMA appearing in the 1H NMR spectrum of the PEO-bPHEMA (Figure 2a) at 3.89 and 3.57 ppm disappear completely in the spectrum of the resultant PEO-b-PCEMA (Figure 2b), accompanied by appearance of the ethylene signal of CEMA at 4.22 ppm. This demonstrates complete cinnamoylation of PHEMA. According to the relative signal intensities of CEMA and EO (repeating units of the PCEMA and PEO blocks, respectively) in the spectrum (Figure 2b), CEMA/EO number ratio of the obtained PEO-b-PCEMA was calculated to be 2.53. Therefore, the polymerization degree of PCEMA in the PEO-b-PCEMA is 286 (2.53 × 113 (polymerization degree of the PEO block is 113)), and the numberaveraged molecular weight of the block copolymer (Mn,NMR) is 79 kg/mol. According to 1H NMR spectrum, the numberaveraged molecular weight (Mn,NMR) of the precursor PEO-bPHEMA is 44 kg/mol, based on which the theoretical molecular weight of the PEO-b-PCEMA produced by 100% cinnamoylation of the precursor PEO-b-PHEMA is 83 kg/mol, close to the molecular weight of the PEO-b-PCEMA determined by 1H NMR (79 kg/mol). 3.2. Preparation of SCJNPs. It is known that PCEMA can be cross-linked by UV irradiation due to photodimerization of CEMA groups.20 Liu et al. reported that cross-linking PCEMA block of PS-b-PCEMA in the selective solvent for PS at the concentration below the critical micellization concentration (cmc) of the block copolymer can lead to SCJNPs.21 The preparation efficiency of SCJNPs was remarkably improved by the same research group through slow addition of the diblock copolymer micelle solution into the selective solvent for the uncross-linkable block under constant UV irradiation.16 The combination of the slow addition and constant irradiation ensures that the newly added copolymer chains are immediately converted into SCJNPs and the copolymer concentration in the photoreactor remains low. Differently, in the present study, PEO113-b-PCEMA286 in its common solvent DMF at different concentrations was irradiated at different irradiation intensities; the irradiation intensity was controlled by changing the distance between the UV light source and the samples (section S3, Supporting Information). GPC-MALLS and dynamic light scattering characterizations confirmed that without using the slow addition method, UV irradiation (254 nm) of the block copolymer solution at 1.26 mg/mL (the concentration of CEMA groups is 4.6 × 10−3 M) and at an irradiation distance of 1 m for 3 h resulted in mainly intrachain cross-linking (Table S1, entry 8, Supporting Information) (the photo-cross-linking kinetics of PEO-b-PCEMA at different irradiation distances is given in section S4, Supporting Information); the irradiation led to a final dimerization conversion of 56% (the curve of the dimerization conversion versus the irradiation time under different reaction conditions is given in section S4, Supporting Information). As exhibited in Figure 3, after the cross-linking reaction, the retention time of the sample increases, its hydrodynamic size decreases, and the absolute molecular weight of the as-cross-linked sample (determined by MALLS) changes inconsiderably (87.7 kg/mol), indicating that there is no considerable interchain cross-linking. Also, 22.9% decrease in hydrodynamic radius, which can reflect the shrinkage of the single polymer chain after the cross-linking, occurs during the cross-linking; the hydrodynamic radii of the sample before and after the cross-linking were measured to be 6.1 and 4.7 nm, respectively. Therefore, the sample for self-assembly is mainly composed of tadpoles, although a small amount of multichain clusters may exist.

recorded in a conventional quartz cell (light path 10 mm) on a PerkinElmer Lambda 35 spectrophotometer. Transmission electron microscopy (TEM) experiments were carried out on a Philips CM120 microscope operated at an accelerating voltage of 80 kV. The SCJNPs were stained by RuO4 for 15 min. Fluorescence spectra were measured by using a FLS920 spectrophotometer. Critical micellization concentrations of the SCJNPs and the block copolymer precursor were determined based on light scattering intensity measurements of the respective solutions at different concentrations in DMF/ethanol (1/4, v/v), using an ALV-5000 laser light scattering spectrometer. The light scattering intensities were recorded at a fixed scattering angle of 90° after dust removal (procedures for purifying the samples for the DLS measurements are described in section S1, Supporting Information).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of the Block Copolymers. We synthesized the block copolymer poly(ethylene oxide)-block-poly(2-hydroxyethyl methacrylate) (PEO-b-PHEMA) by ATRP of HEMA in methanol using PEO-Br (Mn = 5000, Mw/Mn = 1.05) as the macroinitiator and CuCl/2,2′-bipyridine (bpy) as the catalyst, following the work by Matyjaszewski.19 The primary PEO-b-PHEMA block copolymer had a number-averaged molecular weight (Mn,GPC) of 44.4 kg/mol and a polydispersity index (Mw/Mn) of 1.30, determined by GPC. As demonstrated by a shoulder in the higher molecular weight end of the GPC curve (the blue curve in Figure 1), there are larger polymer chains (which should

Figure 1. GPC curves of the primary PEO-b-PHEMA (blue), PEO macroinitiator (cyan), and the third polymer fraction (red) obtained by gradual precipitation.

result from chain−chain coupling in the polymerization system), leading to the relatively wide molecular weight distribution (section S2, Supporting Information). To obtain block copolymer with a narrower molecular weight distribution, the primary PEO-b-PHEMA was fractionated via gradual precipitation (section S2, Supporting Information). As exhibited in Figure 1, after fractionation, the third polymer fraction has a molecular weight (Mn,GPC) of 51 kg/mol and a polydispersity index of 1.15. This block copolymer was used for further study. PEO-b-PCEMA was prepared by reacting the PEO-bPHEMA precursor with excess cinnamoyl chloride in dry pyridine.20 As exhibited in Figure 2, the ethylene signals of 367

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Figure 2. 1H NMR spectrum of PEO-b-PHEMA (a) in DMSO-d6 and PEO-b-PCEMA (b) in CDCl3.

conclusion should be inspiring to those interested in highly efficient preparation of single chain particles. It has been demonstrated in the literature that intrachain cross-linking of homopolymer chains in the common solvents will result in collapse of the polymer chains, leading to single chain polymer particles.24,25 In the present study, intrachain cross-linking of the PCEMA block of PEO-b-PCEMA in the common solvent should lead to tadpole-like single chain Janus nanoparticles (SCJNPs) with PEO as the “tail” and the crosslinked PCEMA block as the “head” (Scheme 1). The head of Scheme 1. Self-Assembly of SCJNPs in Selective Solvent for the “Tail” and Ultrasonic Response of the Resultant Superparticles

Figure 3. GPC traces of the PEO-b-PCEMA at different irradiation time. The concentration of PEO-b-PCEMA is 1.26 mg/mL. The irradiation distance is 1 m, and the irradiation time is 3 h.

In the method using ultradilute solution or the method based on slow and continuous addition, to avoid interchain crosslinking, concentrations of the cross-linkable groups in the reactors are below 10−5 M.22,23 However, the SCJNPs used in the present study were prepared at a block copolymer concentration of 1.26 mg/mL, at which the concentration of the cross-linkable groups (CEMA units) in the photoreactor at the initial stage of the cross-linking reaction is as high as 4.6 × 10−3 M. We also demonstrated that the highest concentration of the block copolymer in the photoreactor for preparing SCJNPs is 4 mg/mL (the CEMA concentration at the initial stage of the cross-linking reaction is 1.45 × 10−2 M), and no remarkable interchain cross-linking reaction was detected in the system (Table S1, entry 12, Supporting Information). As explained in section S5 of the Supporting Information, instead of decreasing the concentration of the cross-linkable groups in the reactors, slowing down the cross-linking reaction speed could also help to avoid interchain cross-linking. This

the as-prepared tadpoles may be less compact relative to that reported by Liu et al.,26 since in the present study the neighboring CEMA units would dimerize first, and then the distant CEMA units dimerize when the PCEMA chains collapsed with the increasing of CEMA conversion. The tadpole-like morphology of SCJNPs is confirmed by our TEM observations. As exhibited in Figure 4, the nanoparticles resulting from the intrachain cross-linking stained by RuO4 show a Janus-like morphology in the TEM images. Specifically, each of the nanoparticles has two parts: the darker part and the gray part. (This tadpole-like morphology of the nanoparticles is entirely different from the background of copper grid shown in section S6 of the Supporting Information.) The darker part is the “head” formed by the cross-linked PCEMA block; the 368

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Figure 4. TEM image of the tadpole-like SCJNPs prepared at the concentration of 1.26 mg/mL (a) and the image at a larger magnification (b). TEM samples were stained by RuO4 for 15 min.

unreacted carbon−carbon double bonds (∼40 mol % of carbon−carbon double bonds remain unreacted after the crosslinking reaction (section S4, Supporting Information)) within the “head” make it sensitive to the staining. The size of the head is about 6 nm, and thus the molecular weight of the head can be roughly calculated to be 68 kg/mol (supposing that the density of the polymer sphere is 1.0 g/cm3), which is close to the molecular weight of the PCEMA block (74 kg/mol). The gray part is the PEO tail. The PEO tail has a relatively low contrast due to its low molecular weight and relatively low sensitivity to the staining compared with the head containing unreacted carbon−carbon double bonds. It should be mentioned here that it is the first time that we witness the morphology of SCJNPs, in which we can see the interesting conformation of a single PEO chain grafted on the surface of a small sphere. It is known that in the liquid 1H NMR spectrum of block copolymer micelles, the core component in an aggregated state has no or weak signal due to the low proton mobility.27 This phenomenon was used in the present study for confirming the aggregated state of the cross-linked PCEMA block chains and the percentage of PEO chain segments that were wrapped within the “head”. (The wrapped PEO chain segments will lose their mobility as well as their signals in the 1H NMR spectrum.) CDCl3 solutions of the precursor PEO113-b-PCEMA286 and the resultant SCJNPs at the same concentration were measured by 1 H NMR. The two solutions contain the same concentration of CH2Cl2, which was used as the internal standard for quantitative analysis of the signals. As exhibited in Figure 5, relative intensities of the PCEMA signals in the spectrum of SCJNPs (spectrum b) are much weaker than those in the spectrum of the block copolymer (spectrum a) (as indicated by the blue circles), indicating the aggregated state of the crosslinked PCEMA block chains. Besides, quantitative analysis reveals that the relative intensity of PEO in spectrum b is lower than that in spectrum a: the intensity ratio of PEO signal at 3.65 ppm to CH2Cl2 signal at 5.3 ppm is 0.616 in spectrum a, but it decreases to 0.499 in spectrum b. Because the EO/CH2Cl2 molar ratios in the two solutions for spectra a and b are the same, the decrease in EO/CH2Cl2 intensity ratio in spectrum b is due to the wrapping of EO units within the heads; the wrapped EO units lose their mobility so that they cannot be detected by liquid 1H NMR. The percentage of the wrapped EO units can be estimated by (IR1 − IR2)/IR1 (IR1 and IR2 represent the EO/CH2Cl2 intensity ratios in spectra a and b, respectively) to be 19%. 3.3. Critical Micellization Concentration. Figure 6 shows the light scattering intensities at different concentrations of

Figure 5. 1H NMR spectrum of the precursor PEO113-b-PCEMA286 (a) and the SCJNPs (b) in CDCl3. In each of the solutions, the concentrations of the polymer (the block copolymer or the SCJNPs) and CH2Cl2 are 6.4 mg/mL and 2.5 μL/mL, respectively.

Figure 6. Light scattering intensities at different concentrations of SCJNPs (square) and the precursor PEO-b-PCEMA (triangle) in DMF/ethanol (1/4, v/v).

SCJNPs in DMF/ethanol (1/4, v/v) and those of the precursor copolymer PEO113-b-PCEMA286 in the same medium. In each of the two curves in Figure 6, the point at which the scattering intensity begins to increase remarkably was the starting point of aggregation of the block copolymer or the SCJNPs. The cmc value is determined by the intersection of the two tangential lines drawn through the low and high concentration data (the red lines in Figure 6). As shown in Figure 6, the cmc values of SCJNPs and the block copolymer are 0.0191 and 0.0115 mg/ mL, respectively, suggesting that the SCJNPs have a lower tendency to aggregate. (Considering that the chemical composition of the SCJNPs is very similar to that of the precursor block copolymer, the difference in cmcs should be attributed to the different morphologies.) Since the cross-linked PCEMA heads of SCJNPs are rigid and spherical, it is relatively difficult for the heads to fuse together to get sufficient adhesion energy to overcome the entropy loss caused by the selfassembly. It is noted that the scattering intensity of suspension of the block copolymer increases gradually with the 369

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the superparticles (Scheme 1). Theoretical simulation of the properties of the tadpoles by Lee et al.29 suggests that stretched chains of linear block copolymer can more readily segregate from each other than the spherical heads and tails of the tadpoles. Therefore, the tadpole heads and tails are more overlapped in the superparticles than the two blocks of the counterpart block copolymer in the micelles. This may explain the facts that the SCJNPs are largely embedded within the superparticles. Because the “heads” of the SCJNPs are spherical and rigid, the contact area between the heads may be small. Besides, due to the cross-linked structure, the heads within the superparticles have no entanglement with each other. Differently, in the core of traditional block copolymer micelles, the core-forming polymer chains are fused together and entangled. This is consistent with the conclusion obtained by computer simulation29 that the tadpole heads are less effective than chains to aggregate together. Therefore, the superparticles of the SCJNPs should be entirely different from block copolymer micelles in both structure and properties. It is interesting to find that after a gentle ultrasonic treatment (see the Experimental Section for details of sample preparation), the superparticles were largely dissociated into individual SCJNPs (Figure 7b). And, some spherical particles coexisted with the dissociated SCJNPs; there may be a part of the PEO-b-PCEMA chains that are less collapsed after the photoreaction, which form particles less sensitive to the ultrasonic treatment since they are similar in structure to traditional block copolymer micelles. It should be mentioned here that micelles formed by the PEO-b-PCEMA precursor in the same mixed solvent do not response to ultrasonic treatment even when the power output is largely increased and the treatment time is prolonged (section S7, Supporting Information). After the ultrasonic treatment, the suspension of the superparticles was set still under ambient condition for 4 h and then observed by TEM. It can be seen that the superparticles formed again (Figure 7c), with the morphology being slightly different from that before the ultrasonic treatment. It is known that ultrasonic is a long-range interaction that can be focused on a spot from a long distance.30,31 The high sensitivity to ultrasonic treatment makes the superparticles very promising in applications for long-range controlling of drug releases. 3.5. Study on the Microstructure of the Superparticles. It is known that aggregates formed by close packing of rigid spheres should have continuous stacking voids inside the aggregates. In the present study, the superparticles resulted from the aggregation of the rigid spherical “heads”. Besides, a large part of PEO “tails” are embedded within the superparticles. Therefore, there must be channels connecting the surrounding medium and the inside of the superparticles. To study the structure of the superparticles, a probe that responses promptly to the change of the environment is required. Considering that the probes used in aqueous medium were most thoroughly investigated and developed, the solvent was switched from DMF/ethanol (1/4, v/v) to pure water. After switching the solvent into pure water, the superparticles are with the size and morphology similar to those formed in the DMF/ethanol mixed solvent (Figure 7d). ANS (8-anilino-1naphthalenesulfonate, Scheme 2) was used as the environmentsensitive fluorescent probe.32−34 In neutral water, ANS has a quantum yield of 0.004 and a maximum of fluorescence emission at 515 nm; when bound to a nonpolar domain, its quantum yield increases remarkably to 0.98, and the emission

concentration, whereas that of the suspension of SCJNPs increases abruptly at the concentration of 0.03 mg/mL, below which the scattering intensity remains constant (Figure 6). It is significant that the sharp phase transition in the suspension of SCJNPs exhibited in Figure 6 is very similar to that in the systems of small molecular surfactants.28 Obviously, the aggregation behavior of the “heads” is much simpler than that of the linear PCEMA chains, since in the latter case, conformational changes and chain entanglements occur during the chain aggregation. In other words, in SCJNPs, compared with the block copolymer precursor, PCEMA block chains are preorganized, which makes the self-assembly behavior of SCJNPs simpler. 3.4. Self-Assembly of SCJNPs and Response of the Resultant Superparticles to Ultrasonic. The tadpole-like SCJNPs with a cross-linked PCEMA chain as the “head” and a linear PEO chain as the “tail” are an intermediate between solid Janus nanoparticles and flexible block copolymers. Therefore, SCJNPs should have good capability in self-assembling into regular superstructures. In the present study, self-assembly of SCJNPs in DMF/ethanol (1/4, v/v) was carried out. Control experiments confirmed that PEO chains are soluble while the cross-linked PCEMA chains are insoluble in the mixed solvent; the mixed solvent is selective for the tail. As exhibited in Figure 7a, the SCJNPs form spherical superparticles in DMF/ethanol (1/4, v/v) mixed solvent. Because SCJNPs are several nanometers large, judging from the size of the superparticles, a substantial proportion of the SCJNPs are embedded within

Figure 7. TEM images of the superparticles in DMF/ethanol (1/4, v/ v) before (a) and after (b) the ultrasonic treatment and the ultrasonically treated superparticles observed after standing still for 4 h (c). (d) TEM image of the superparticles in pure water formed by dialyzing the suspension of the superparticles in DMF/ethanol (1/4, v/v) against pure water. The inset in (a) is the image with a larger magnification; the scale bar is 100 nm. 370

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(Figure 8c). Obviously, in the case of the micelles, the closely condensed core of the micelles makes the transport of protons much more difficult, resulting in a much slower quench of the fluorescence (Scheme 3). Differently, in the case of the

Scheme 2. Chemical Structure of ANS

Scheme 3. Illustration To Demonstrate That the AcidQuench Behavior of ANS Encapsulated in the Superparticles Is Faster Than That Loaded in the Control Micelles maximum shifts to 454 nm.32 ANS was incorporated into the superparticles and the PEO-b-PCEMA micelles via a coassembly method in DMF/ethanol (1/4, v/v) mixed solvent followed by switching the solvent to neutral water (see the Experimental Section for the details); owing to the hydrophobicity, ANS will preferentially bind to hydrophobic regions of the superparticles and the micelles.32 It should be mentioned here that the micelles were prepared using PEO-b-PCEMA with a lower molecular weight (Mn,NMR = 53 kg/mol, PDI = 1.24) to obtain stable aqueous micelle suspension, since micelles assembled from the precursor block copolymer of SCJNPs precipitated during the solvent switching. The fluorescence spectra of ANS in the superparticles and the micelles were measured (Figure 8a). The fluorescence intensity of ANS in the superparticles or the micelles is much higher than that in pure water. Besides, the emission maximum (λmax) remarkably blue-shifts to 425 nm for the superparticles and 402 nm for the micelles (λmax of ANS in pure water is 520 nm). The fluorescence spectra of ANS strongly suggest that ANS in the superparticles and the micelles was surrounded by hydrophobic environments.35,36 It is noted that λmax for the superparticles is slightly larger than that for the micelles, indicating the slightly higher polarity of the superparticles relative to the micelles; it is imaginable that the PEO tails embedded within the superparticles increases the polarity. It is known that protons can quench the fluorescence of ANS by direct interaction with its excited state.37 How quickly can the encapsulated ANS be quenched reflects the accessibility of the ANS encapsulated within the superparticles and the micelles. We adjusted the pH of the suspensions to 2.0 by a very short injection of 50 μL of 1 M HCl solution into 3 mL suspensions of the superparticles and the micelles and then tracked the changes in the fluorescence spectra of ANS after the pH adjustment. For the ANS in the superparticles, the fluorescence intensity at 425 nm decreases immediately to nearly half of the original value before the pH adjustment and then decreases rather slowly within 24 h (Figure 8b). Opposed to the superparticles, the fluorescence intensity (402 nm) of the ANS in the micelles decreases gradually after pH changed to 2.0

superparticles, the prompt acid quench confirms that there are channels connecting the surrounding medium and the inside of the superparticles, enabling fast diffusion of protons to promptly interact with ANS within the superparticles. The channels within the superparticles should make them unique in the related applications.

4. CONCLUSION SCJNPs were efficiently prepared by photo-cross-linking the PCEMA block of PEO-b-PCEMA at concentration of the block copolymer as high as 4.0 mg/mL in the common solvent DMF; the possible interchain cross-linking at the relatively high concentrations can be avoided by slowing down the crosslinking reaction. In selective solvent of the “tail”, self-assembly behavior of SCJNPs is different from that of block copolymer but similar to that of small molecular surfactants, which should result from preorganization of the PCEMA block in SCJNPs.

Figure 8. Fluorescence spectrum of ANS incorporated in the superparticles and that in the micelles and the spectrum of free ANS in neutral water (a); the spectra of ANS in the superparticles (b) (or the micelles (c)) in neutral water or incubated in water at pH of 2.0 for 0 min, 15 min, 30 min, 60 min, 90 min, and 24 h. ANS concentration is 40 μM. 371

dx.doi.org/10.1021/ma401589z | Macromolecules 2014, 47, 365−372

Macromolecules

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The superparticles are stabilized by solvophobic aggregation among the rigid spherical “heads”. Since the adhesion energy between the “heads” is low and there is no entanglements between the “heads”, the superparticles dissociate into individual SCJNPs when subjected to ultrasonic treatment. Additionally, there are opened hydrophilic channels within the superstructures, which allow rapid mass transport of small functional species within the superparticles. Therefore, compared with block copolymer micelles, the superparticles are quite unique regarding the self-assembly behavior, structure, and properties. We believe that the results of the present study should be inspiring to the researchers in the fields of nanomaterials, self-assembly, and controlled drug release.

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ASSOCIATED CONTENT

S Supporting Information *

Purification of the samples for light scattering; procedure for gradual precipitation; photo-cross-linking results of PEO-bPCEMA under different irradiation conditions (Table S1); UV monitoring of the photo-cross-linking kinetics; explanation about slowing down the photoreaction speed to avoid interchain cross-linking; enlarged TEM image of the background grid; TEM images of the micelles before and after ultrasonic treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.C.). Present Address †

F.Z.: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260. Author Contributions ‡

F.Z. and M.X. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of NSFC (91127030, 21334001), the Ministry of Science and Technology of China (2011CB932503).

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REFERENCES

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dx.doi.org/10.1021/ma401589z | Macromolecules 2014, 47, 365−372