Hybrid Core Shell Microspheres from Coassembly of ......2017/11/23 · cross-linked structure and good stability. This was further conﬁrmed by SEM images. The hybrid CSM-1 can keep

Hybrid Core−Shell Microspheres from Coassembly of Anthracene-Containing POSS (POSS-AN) and Anthracene-Ended HyperbranchedPoly(ether amine) (hPEA-AN) and Their Responsive Polymeric HollowMicrospheresZhilong Su, Bing Yu, Xuesong Jiang,* and Jie Yin

School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao TongUniversity, Shanghai 200240, People’s Republic of China

*S Supporting Information

ABSTRACT: We demonstrated a novel core−shell micro-sphere (CSM) fabricated from coassembly of anthracene-ended hyperbranched poly(ether amine) (hPEA-AN) andanthracene containing polyhedral oligomer silsesquioxane(POSS-AN). The obtained CSMs are cross-linked throughphotodimerization of anthracene and possess the well-definedcore−shell structure according to the images of SEM, TEM,and AFM. The shell of the obtained CSM is comprised ofhPEA-AN, while POSS-AN prefers the ordered crystallizedaggregation in the core. The size of the obtained CSMs is uniform and tunable. With the increasing content of hPEA-AN in thecoassembly, the diameter of CSMs decreased from 930 to 616 nm, while the thickness of shell increased from 95 to 170 nm.Moreover, polymeric hollow microsphere (PHM) was prepared by removing the POSS-AN core of CSM in hydrofluoric acid(HF). The obtained PHM is amphiphilic and fluorescent, and its size is responsive to environmental stimulus such astemperature and pH. PHM can be used in the encapsulation and controlled release of guest molecules. Moreover, the controlledrelease of guest molecules from PHM can be monitored by itself fluorescence change.

■ INTRODUCTIONMicrospheres with core−shell structures comprise an expand-ing area of research during the past few decades. Considerableefforts have been devoted to the preparation of core−shellmicrospheres (CSM) because of their interesting functionssuch as optical, electrical, thermal, mechanical, magnetic, andcatalytic properties, which come from different functionalcomponents in both core and shell.1−8 An important approachto create new materials with fancy properties is to preparehybrid systems composed of inorganic and organic compo-nents.9,10The unique physical properties of inorganic compo-nents and the excellent process ability of polymers can becombined in hybrid materials.11−13Therefore, various facileprocedures have been developed to prepare the functionalhybrid CSM, including template-directed polymerization,14,15

template directed self-assembly,16,17 and coassembly ofinorganic and polymeric components.18,19

Among these established methodologies to fabricate thehybrid CSM, coassembly of block copolymer and inorganiccomponents is widely used to fabricate the hybrid nano-structured materials.20−23 By controlling the arrangement ofinorganic components especially nanoparticles (NPs) inpolymer matrix, micelles of rod and sphere with inorganicNPs in their cores or vesicles with inorganic NPs in their wallshave been obtained. Taton, Chen, and Eisenberg’s groupsincorporated gold NPs into the micelles of block copolymers to

obtain core−shell nanostructures, respectively.24,25 By cross-linking the assembled shell, a permanent core−shell structurewas further demonstrated by Taton’s group.26 Hickey alsoreported for the formation of micelles and vesicles through thecoassembly of magnetic NPs and amphiphilic block copolymerpoly(acrylic acid)-b-polystyrene (PAA-b-PS).27 In these studies,the driving force for coassembly of inorganic NPs and polymermatrix is hydrophobic−hydrophilic interaction, and amphiphilicpolymers are used as polymer components.Responsive polymers with response to multistimulus such as

pH and temperature are widely studied for potentialapplications in drug delivery, targeting, and sensors.28−30

Such fancy proprieties make them as the suitable polymercomponents to fabricate hybrid materials.31,32 Instead of thewell-defined block copolymers, however, hyperbranchedpolymers especially with response to environmental stimuluswere rarely reported in the fabrication of hybrid nanostructuredmaterials through coassembly.33 This might be caused by therelatively less defined structure of hyperbranched polymer,consequently resulting in difficulties in controlling the locationof inorganic NPs in polymer matrix. In comparison to blockcopolymers, however, hyperbranched polymers have some

Received: January 19, 2013Revised: March 11, 2013Published: April 16, 2013

Article

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© 2013 American Chemical Society 3519 dx.doi.org/10.1021/ma400129e | Macromolecules 2013, 46, 3519−3528

pubs.acs.org/Macromolecules

advantages such as facile preparation with low cost and easymodification due to the peripheral functional groups.34

In this text, we reported for microspheres with hybrid coreand responsive shell fabricated by the coassembly of hPEA-ANand POSS-AN. Hyperbranched poly(ether amine)s developedby our groups recently, are responsive to pH, temperature, andionic strength.35 Also, it can be easily modified with functionalgroups such as anthracene through the efficient chemicalreactions between amino and epoxy groups.36 As the smallestprecisely defined cubic silica nanoparticle, POSS is of ourinterest because it is widely used in preparation of the novelhybrid materials and can enhance the performance such asmechanical strength and thermal stability. In the design of thecoassembly of hPEA-AN and POSS-AN, anthracene groupshave two functions: (1) anthracene possesses the strong π−πinteraction with each other and (2) part of the anthracenegroups can undergo photodimerization to make the coassem-blies cross-linked. The strategy for the coassembly of POSS-ANand hPEA-AN is illustrated in Scheme 1, and the obtainedhybrid microsphere of POSS-AN@hPEA-AN can take theuniform-sized core−shell structure. The size of hybrid core indiameter and thickness of shell are tunable with feed ratiobetween POSS-AN and hPEA-AN in coassembly. Upon theexposure of UV-light, the obtained coassemblies were furthercross-linked through the photodimerization of anthracenegroups, and can keep their shape in organic solvents.Furthermore, we removed the core from the hybrid micro-spheres to obtain the responsive polymeric hollow spheres,which find potential application in the encapsulation andcontrolled release of guest molecules.

■ RESULTS AND DISCUSSIONPreparation and Characterization of Hybrid Core−

Shell Microspheres (CSM). As one of the components forcoassembly, hPEA-AN is comprised of the hydrophilicbackbone of hPEA and hydrophobic ended groups ofanthracene. Because of the presence of poly(ethylene oxide)(PEO) chains and amino groups in hPEA backbone,amphiphilic hPEA-AN exhibits multiresponse to temperature,pH and ionic strength,37 which makes it as an ideal component

for coassemblies. The hydrophobic component POSS-AN wassynthesized by introducing anthracene moieties to theperiphery of octaamino-POSS through the reaction betweenamino and epoxy groups. The detailed characterization ofPOSS-AN is shown in Figures S1 and S2 (SupportingInformation). One molecule of POSS-AN is comprised ofabout 10 anthracene moieties. The amphiphilic hPEA-AN andhydrophobic POSS-AN are expected to interact with each otherthrough π−π stacking of anthracene moieties and hydrophilic−hydrophobic interaction, resulting in the formation of thesupramolecular coassemblies in aqueous solution.The whole process of coassembly of hPEA-AN and POSS-

AN to fabricate CSM is shown in Scheme 1. The coassemblywas carried out through gradually adding water to thehomogeneous dioxane solution of hPEA-AN and POSS-AN.Briefly, amphiphilic hPEA-AN and hydrophobic POSS-AN witha certain ratio were first mixed in dioxane. Then, water wasadded slowly to the solution to induce the coassembly of hPEA-AN and POSS-AN. As backbone of hPEA-AN is composed ofthe hydrophilic PEG chains and amino groups, water is a goodsolvent for the hydrophilic hPEA but a precipitant for thehydrophobic POSS and anthracene moieties. Along theaddition of water, the solvent became progressively worse forPOSS and anthracene moieties. When the water contentreached a critical value, the solution would undergo themicrophase separation, resulting in the formation of coassem-ble. The POSS-AN aggregated to form the hydrophobic core,while the amphiphilic hPEA-AN formed the outer layer tocontact with water. After aging for 24 h, the resultingcoassemblies of hPEA-AN@POSS-AN were further cross-linked through photodimerization of anthracene moieties by365 nm UV-light. The process of anthracene photodimerizationwas traced by UV−vis spectra (Figure S3, SupportingInformation). Upon exposure with 365 nm UV-light for 5min, the dimerization degree of anthracene is around 90%,which can lead to the tightly cross-linked coassemblies.The resulting coassemblies of hPEA-AN@POSS-AN were

fully characterized. Taking the coassemble of CSM-1 (the feedratio between hPEA-AN and POSS-AN is 1/1) as example,scanning electron microscopy (SEM) and atomic force

Scheme 1. (a) Structure and Model of hPEA-AN and POSS-AN and (b) Strategy for Preparation of the Core−Shell Microsphere(CSM) through Co-Assembly of hPEA-AN and POSS-AN and the Procedure for the Preparation of Polymeric HollowMicrosphere (PHM)

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microscopy (AFM) images revealed the morphology of thehighly uniform-sized microsphere (Figure 1, parts a, b, and c).As shown in parts e and f of Figure 1, the size in diameter ofCSM-1 determined by SEM and dynamic light scattering(DLS) is 930 and 850 nm with the low polydispersity index(PdI) of 0.10 and 0.06, respectively. The size of CSM-1determined by SEM analysis is a little higher than that of DLSanalysis, which might be ascribed that the flexible shell of CSM-1 was somewhat flattened under the vacuum condition of SEMexperiments. As shown in Figure 1d, the obvious core−shellstructured morphology of CSM-1 was revealed by transmissionelectron microscopy (TEM) image (Figure 1d). The dark coreof the POSS-AN aggregation is about 740 nm in diameter, andthe shell of hPEA-AN is about 95 nm in thickness. Taking thethickness of shell into consideration, the hydrophilic shellshould be comprised of the multilayered aggregates of hPEA-AN. DLS results revealed that the obtained hybrid CSM-1 doesnot even disassemble in good organic solvents such astetrahydrofuran (THF) (Figure 1f), indicating the tightlycross-linked structure and good stability. This was furtherconfirmed by SEM images. The hybrid CSM-1 can keep itsmorphology of sphere in THF (Figure S4, SupportingInformation). It should be noted that the z-average diameterof CSM-1 in THF determined by DLS is larger than that inwater, which might be ascribed to the swelling of CSM-1 in thegood solvent. The component of the obtained hybrid CSM-1was further analyzed. Both energy-dispersive X-ray spectrosco-py (EDS, Figure 1g) and FT-IR spectra of CSM-1 (Figure S5,Supporting Information) revealed the presence of the inorganicPOSS, indicating that POSS-AN participated into thecoassembly. Additionally, the Si intensity line scan of CSM-1showed the higher intensity of Si at the core of coassemble(Figure S6, Supporting Information), which is well inagreement with the core−shell structure revealed by TEM.Furthermore, TGA of CSM-1 was conducted and the residue at700 °C is 7.04%, which is close to the theoretical value 7.18%(Figure S7, Supporting Information). In other words, thecontent of hPEA-AN and POSS-AN in CSM-1 meets well withthe feed ratio.The aggregation behavior of POSS-AN and hPEA-AN in the

coassembly of CSM-1 was investigated by differential scanningcalorimetry (DSC). Figure 2 shows DSC thermograms of theuncross-linked coassemble of hPEA-AN@POSS-AN, as well ashPEA-AN, POSS-AN and the hPEA-AN/POSS-AN compositefor reference. hPEA-AN takes a glass transition at temperaturearound −26.3 °C (Tg), which should be ascribed to thepresence of PEO chains in the backbone of hPEA-AN. The

temperature for melting point of the crystallized POSS-AN isabout 62−75 °C. The composite of hPEA-AN/POSS-ANexhibites only one glass transition at 0.9 °C, indicating that thecomposite is homogeneous, and hPEA-AN and POSS-AN canmix together in a molecular scale. The excellent compatibilitybetween hPEA-AN and POSS-AN might be caused by theinteraction of anthracene moieties of two components. Themuch higher Tg of hPEA-AN/POSS-AN composite than that ofpure hPEA-AN should be ascribed to the introduction of POSS,which can enhance Tg of polymer matrix obviously.

38 In thecontrast, the uncross-linked coassemblies of hPEA-AN@POSS-AN exhibit an obvious glass transition of −22.8 °C and amelting peak around 60 °C, suggesting the microphaseseparation. From DSC results, we can conclude that twoindependent microphases of hPEA-AN and POSS-AN exist incoassemble of hPEA-AN@POSS-AN, while there is one phasein composite. This can be well explained by the model of thecore−shell structured microsphere revealed by TEM. Themelting peak at 60 °C should be ascribed to the high orderedcrystallized aggregation of POSS-AN in the core, and the glasstransition of −22.8 °C should be related to the shell of hPEA-AN. In comparison to the pure POSS-AN, the melting peak forthe coassemblies of hPEA-AN@POSS-AN shifted to lowertemperature and became more narrow, which might be due tothe crystallization of POSS-AN in the uniform core ofmicroscale. The interfacial interaction between the core ofPOSS-AN and shell of hPEA-AN in the coassemble of hPEA-

Figure 1. (a) SEM image of CSM-1 in large scale; (b) close view of CSM-1; (c) AFM image of CSM-1; (d) TEM image of CSM-1; (e) sizedistribution of CSM-1 determined by SEM; (f) size distribution of CSM-1 in water and THF obtained by DLS; (g) EDS spectra of CSM-1.

Figure 2. DSC thermograms of hPEA-AN, POSS-AN, un-cross-linkedCSM-1 and hPEA-AN/POSS-AN composite. The scans were run at aheating rate of 10 °C/min.



AN@POSS-AN might lead to the higher Tg of hPEA-AN shell(−22.8 °C) than that of the pure hPEA-AN (−26.3 °C).39To understand the mechanism of coassembly of hPEA-AN

and POSS-AN, we also traced the aggregation process of hPEA-AN and POSS-AN in dioxane with addition of water. Thetransparent solution of hPEA-AN and POSS-AN in dioxanebecame completely turbid and scattering intensity of DLSincreased sharply after addition of 20 wt % of water, suggestingthe formation of coassemble of hPEA-AN and POSS-AN at thisstage. At the same time, the fluorescence intensity of thesolution of hPEA-AN and POSS-AN increased obviously withthe addition of water. Especially, a new fluorescence peak at481 nm appeared and became the highest when the water

content is around 90 wt %, and the color of fluorescenceemission changed from purple-blue to cyan (Figure 3, parts aand b). Interestingly, this system has both characters ofaggregation-induced emission enhancement (AIEE)40,41 andaggregation-induced emission (AIE).42,43 When water contentfalls below 20 wt %, the fluorescence emission of anthracene isenhanced by the aggregation, while a new emissive peak around481 nm appears with the further increasing content of water.Because of the well-known concentration-quenching effect ofaromatic compounds, the fluorescence of anthracene at thehigh concentration in good solvent is weak.44 The fluorescencequenching in solution can be understood as the dominantnonradiative relaxation, resulted from the free molecular

Figure 3. (a) Fluorescence intensity@424 nm and transmittance of solutions at different water fraction. (b) Steady-state fluorescence spectra ofhPEA-AN@POSS-AN = 1/1 in mixed solvents with different volume ratio of H2O/dioxane, excitation wavelength is 381 nm. (c) Steady-statefluorescence spectra of uncross-linked coassemble of hPEA-AN@POSS-AN = 1/1 at different aging time. (d) Photograph of hPEA-AN@POSS-AN= 1/1 in dioxane, mixture of dioxane/water (10/90) and aging for 24 h under 365 nm UV-light illumination in the dark.

Figure 4. Morphology and size distribution of CSM: (a) SEM images; (b) TEM images; (c) AFM images; (d) size-distribution of CSM determinedby SEM and DLS. Key: line 1, CSM-2; line 2, CSM-3; line 3, CSM-4.



torsional motion and intermolecular fluorescence-quenchinginteraction. With the addition of water, the torsional motion ofanthracene was greatly restricted and the molecular quenchinginteraction was suppressed to some extent due to theaggregation of hPEA-AN and POSS-AN during the processof coassembly, resulting in the AIEE. It should be also notedthat the emission peak at 481 nm become very obvious whenwater content above 20%.This peak should be attributed to theexcimer formed by the π−π overlapped anthracene.45 Thisindicates that π−π interaction of anthracene exists in theformation of coassemble of hPEA-AN@POSS-AN. However,such aggregates of anthracene are neither close nor well-ordered.46 After aging for 24 h, the fluorescent peak ofcoassemblies of hPEA-AN@POSS-AN at 481 nm decreasedsignificantly, and the color of fluorescence emission changedfrom cyan to purple (Figure 3, parts c and d). The decreasingfluorescent intensity at 481 nm implied that the excimer incoassemble decreased. This might due to the formation of thehighly ordered crystallized aggregation of POSS-AN in the coreof coassemble during process of aging, which was revealed byDSC analysis (Figure 2). Generally, the excimer is alwaysdifficult to be observed in the crystal state of organicluminophors.47,48 To understand the effect of the molecularinteraction between anthracene moieties, a control experimentof coassembly of POSS-AN and hPEA (without anthracenegroups) was carried out. The obtained assemble can not bedispersed stably in water, and the morphology is not uniform(Figure S8, Supporting Information). This control experimentalso indicated that π−π interaction of anthracene andhydrophilic−hydrophobic interaction between two componentshPEA-AN and POSS-AN play the key roles in the formation ofthe uniform-sized coassemble with the core−shell structure.To verify the feasibility of our strategy to fabricate the core−

shell microsphere, we changed the feed ratio between twocomponents of hPEA-AN and POSS-AN in coassembly. Aseries of microspheres with the core−shell structure were

obtained with the decreasing content of POSS-AN (Figure 4).SEM and AFM images also revealed the uniform size of theobtained coassemblies of CSM-2, CSM-3, and CSM-4 whosePOSS-AN content is 33.3%, 25.0%, and 11.1%, respectively(Figure 4, parts a and c). According to SEM analysis, the size ofCSM-2, CSM-3, and CSM-4 are 860, 772, and 616 nm indiameter with the low PdI of 0.10, 0.13 and 0.16, respectively.DLS results indicated that the Z-average diameter of CSM-2,CSM-3, and CSM-4 are 820, 637, and 542 nm with low PdI of0.21, 0.22, and 0.24, respectively. These data of the obtainedcoassemblies are summarized in Table 1. The size ofcoassemblies determined by SEM is always a little larger thanthat of DLS (Figure 4d), which might due to the somewhatflattened morphology in SEM measurements. The well-definedcore−shell structure for CSM-2 and CSM-3 was revealed byTEM images (Figure 4c). Although there is no obviousboundary between core and shell for CSM-4 in TEM image, thecore−shell structure of CSM-4 was confirmed by the followingetching experiment. It might be the relatively thick shell andsmall core of CSM-4 to make the boundary not easy to beobserved by TEM. The thicknesses of shell for CSM-2 andCSM-3 determined by TEM are 150 and 170 nm, respectively.The diameter of CSM decreased from CSM-1 to CSM-4 withthe decreasing content of hydrophobic POSS-AN, while thethickness of shell increased with the increasing content ofamphiphilic hPEA-AN. As for the coassembly of twocomponents, the decreasing content of the hydrophobiccomponent POSS-AN can make the system of coassemblymore hydrophilic, resulting in the smaller microsphere ofcoassemble. On the basis of these results, we can conclude thatthe size and the thickness of shell for the obtained CSM aretunable.

Responsive Polymeric Hollow Microspheres (PHM).The core−shell structure of hPEA-AN@POSS-AN coassemblecan be further confirmed by the etching experiments. Becausethe cross-linked coassemblies of hPEA-AN@POSS-AN are

Table 1. Data of Formulation, Diameter, Shell, and Core of CSMs

no. feed ratio diametera (nm) PdIa Z-avb (nm) PdIb thickness of shellc (nm) diameter of corec (nm)

CSM-1 1/1 930 0.10 848 0.06 95 740CSM-2 1/2 860 0.13 820 0.21 150 560CSM-3 1/4 772 0.10 637 0.22 170 432CSM-4 1/8 616 0.16 542 0.24 − −

aThe diameter and PdI of CSM determined by SEM. bThe diameter and PdI of CSM determined by DLS. cThe shell thickness and core sizedetermined by TEM.

Figure 5. (a) Photograph of CSM-1 aqueous solution and PHM-1 aqueous solution; (b) AFM images of obtained PHM-1.



comprised of the hPEA-AN shell and POSS-AN core, it can beexpected that polymeric hollow microsphere (PHM) with shellof hPEA-AN will be obtained if the core of POSS-AN can beremoved. PHM is also of great interest for its potentialapplications in the encapsulation and controlled release of guestmolecules.49−51 To verify this assumption, CSM-1 was put intohydrofluoric acid (HF) THF solution, which can dissolve thecore of POSS-AN aggregation. After treated by HF corrosion,the color of CSM-1 aqueous solution turned from light yellowto white (Figure 5a). AFM images revealed the typicalmorphology of PHM (Figure 5b; Figure S9, SupportingInformation). Compared to the AFM morphology of CSM-1(Figure 4c-1), the obvious collapse was observed in AFM imageafter etching, suggesting the removal of the POSS-AN core andthe formation of polymeric hollow microsphere (PHM-1). Theobtained PHM cannot disassemble in THF solvent, whichshould be ascribed to the cross-linked hPEA-AN shell of CSM-1. The removal of the POSS-AN core was further confirmed byFT-IR (Figure S5, Supporting Information) and TGA analysis(Figure S7, Supporting Information). The characteristic peaks

related to POSS-AN disappeared in FT-IR of PHM-1, and theweight retention of PHM-1 at 700 °C is almost the same to thatof hPEA-AN in TGA. The etching experiment also proved thatCSM-4 preferred the core−shell structure (Figure S9,Supporting Information).Because of the presence of PEO short chains and amino

groups in the hydrophilic shell of hPEA-AN, the obtainedPHM-1 in aqueous solution are expected to be responsive totemperature and pH (Figure 6a). To investigate the responsivebehavior, we checked the size-distribution of PHM-1 atdifferent temperatures and pH by DLS. As shown in Figure6b, the diameter of PHM-1 decreased with the increasingtemperature. This phenomenon should be caused by theshrinkage of the hPEA-AN shell which is comprised of PEOchains. With the increasing temperature, hydrogen bondsbetween the PEO chain and water molecule are destroyed,which makes the shell of PHM-1 less hydrophilic and shrink inwater. Figure 6c shows size-distribution of the PHM-1 atdifferent pH, indicating that the peak of the PHM-1 increasefrom 950 to 1488 nm with the decrease of pH value from 7.0 to

Figure 6. (a) Proposed mechanism of the responsive behavior of PHM-1 to temperature and pH. (b) Size distribution of PHM-1 in aqueoussolution determined by DLS at different temperature. (c) Size distribution of PHM-1 in aqueous solution determined by DLS at different pH. (d)Reversible change of size of PHM-1 in aqueous solution at different temperatures and pH. Key: red line, between 25 and 80 °C; black line, betweenpH = 7.0 and 4.0.

Figure 7. Photograph of the encapsulation of dyes: (a) the hydrophobic NR in water before and after addition of PHM-1 at room temperature; (b)the hydrophilic MO in ethyl acetate before and after addition of PHM-1 at room temperature. The concentrations of dyes and PHM are 0.1 and 1mg/mL, respectively. The pictures were taken after the samples stayed for 15 min.



4.0. This can be explained by the fact that protonation of aminogroups at the lower pH increases the hydrophilicity of PHM-1,leading to the swellin of PHM-1. It should be noted that theresponsive behavior of PHM-1 to temperature and pH isreversible. When external environment backs to the initial state,the size of PHM-1 can recover, which was confirmed by severalcycles (Figure 6d).Encapsulation and Controlled Release of Dyes. Our

previous studies proved that micro and nano- hydrogel ofhyperbranched poly(ether amine) (hPEA) can encapsulate thehydrophilic guest molecules such as dyes even in water.52 As acommon hydrophobic dye, Nile Red (NR) could be dispersedhomogeneously in water in the presence of PHM-1 (Figure 7a).Similarly, the water-soluble Methyl Orange (MO), which isinsoluble in ethyl acetate, could be dispersed homogeneously inethyl acetate with the help of PHM-1 (Figure 7b). Thesephenomena can also be reflected by change of the solution’scolor in the presence of PHM-1(Figure7). The dispersionbehaviors can be explained by the encapsulation properties ofpolymeric hollow microsphere. Hydrophobic NR can beencapsulated in hydrophobic part of the PHM-1, whilehydrophilic MO can be encapsulated in hydrophilic part ofthe PHM-. Thus, NR and MO can be dispersed well in waterand ethyl acetate in the presence of PHM-1, respectively.The fluorescence of PHM-1 solution resulting from the

residue of unphotodimerized anthracene moieties wasweakened after encapsulation of MO. As shown in Figure 8,

the fluorescent intensity of PHM-1 aqueous solution decreasedobviously with the increasing concentration of MO. This mightbe explained by the fact that the encapsulated guest molecule ofMO can quench the emission of anthracene moieties. Becauseof the residue of un-cross-linked anthracene moieties, PHM-1can be excited to emit blue fluorescence around 400−500 nm,where MO molecules possess a relatively strong absorption.When MO is encapsulated in PHM-1, the distance betweenMO molecules and the anthracene moieties becomes so close,resulting in the quenching of fluorescence of PHM-1 by MO.Because of the novel characteristics of PHM-1 such as

amphiphilicity, responsive behavior, and encapsulation of guestmolecules with changes of fluorescence, we speculated that therelease of guest molecules from PHM-1 can be controlled by

the environmental stimulus and monitored simultaneously byfluorescence change. To verify this idea, the release experimentsin buffer solution of pH 3.0 and 7.0 at 25 °C were carried out.The detailed process is listed in the Experimental Section. Thecontent of MO released from PHM-1 is determined by UV−visspectra. As shown in Figure 9a, the rate of MO release is muchfaster in pH 3.0 than that in pH 7.0. About 75% of MO isreleased within 120 h at pH 3.0, while only about 32% of MO isreleased over the same period at pH 7.0. These results may beascribed to the fact that PHM-1 swells at the lower pH,resulting in the faster release rate of MO molecules.Therefore,environmental stimuli can control the release of modelmolecule MO from PHM-1. Meanwhile, the fluorescence ofPHM-1 changes during the release process, which is shown inFigure 9b. The fluorescence of PHM-1 was very weak after theencapsulation of MO molecules, but enhanced after the MOmolecules were released over 120 h. This process was alsoconfirmed by the laser scanning confocal microscope (LSCM)images in Figures 9c−e. After loading of MO, the bluefluorescent PHM-1 image in Figure 9a became completely dark,suggesting that the fluorescence of PHM-1 was quenched. Afterthe release of part of the loaded MO, the fluorescent PHM-1can be observed by LSCM again. These results from bothfluorescent spectra and LSCM images implied that thefluorescence emission of PHM-1 can be quenched by theencapsulated MOmolecules and recovered after the release ofMO. Therefore, the controlled release of the guestmoleculesfrom PHM-1 can be monitored by itself fluorescence changes.

■ CONCLUSIONIn summary, a series of novel core−shell microspheres (CSMs)were fabricated through the coassembly of anthracene endedhyperbranched poly(ether amine) (hPEA-AN) and anthracene-containing polyhedral oligomer silsesquioxane (POSS-AN),followed by cross-linking through the photodimerization ofanthracene. The obtained CSMs were comprised of hPEA-ANas shell and POSS-AN as core, where POSS-AN preferred thehigh ordered crystallized aggregation. During the coassembly,the π−π interaction of anthracene plays an important role inthe formation of the well-defined structured coassemblies. Thesize of the obtained CSMs is uniform and tunable. Thediameter of CSMs decreased, while the thickness of shellincreased with the increasing content of hPEA-AN in thecoassembly. Furthermore, by removing the POSS-AN core ofCSMs, we obtained polymeric hollow microsphere (PHM),which is amphiphilic, fluorescent, and responsive to temper-ature and pH. PHM can be used in the encapsulation andcontrolled release of guest molecules, which can be monitoredby the change of itself fluorescence. The coassembly ofamphiphilic hyperbranched poly(ether amine) and POSSprovides a robust and thorough way to prepare the functionalmicro- and nanomaterials.

■ EXPERIMENTAL SECTIONMaterials. Anthracene-ended hyperbranched poly(ether mine)

(hPEA-AN) and 9-anthracenemethoxyl glycidyl ether (E-AN) weresynthesized according to previous work of our group.37 Octaamino-polyhedral oligomer silsesquioxane (POSS-NH2) was synthesizedaccording to the literature.53−55 Anthracene-containing polyhedraloligomer silsesquioxane (POSS-AN) was synthesized by modifyingPOSS-NH2 with E-AN. The amount of anthracene moieties wasquantified by both UV−vis and 1H NMR spectra and the average ratioof anthracene moieties and POSS-NH2 is about 10. Nile Red (NR,

Figure 8. Fluorescence emission spectra of PHM-1 aqueous solutionwith different MO concentrations. Inset: plot of the fluorescenceemission intensity of PHM-1 aqueous solution at 416 nm at differentconcentration of MO. The concentration of PHM-1 is 1 mg/mL.



TCI) and Methyl Orange (MO, Sinopharm Chemical Reagent) wereall used as received without further purification.Preparation of Core−Shell Microspheres (CSM). Typically, a

certain amount of hPEA-AN and POSS-AN were first dissolved into 1mL of dioxane, which is a good solvent for both hPEA-AN and POSS-AN. The solution was left to equilibrate at 25 °C for 1 h before 9 mLof ultrapure water was added slowly to the solution (5 mL of water perhour). After addtion of water, the solution was stirred for another 2 hunder N2 atmosphere to remove oxygen. Then the solution wasexposed to a 365 nm ultraviolet LED lamp (Uvata, China) withintensity of about 8.4 mW/cm2 to make the anthracene moietiesphotodimerized. The irradiation time is 10 min unless specificallymentioned. The above-obtained CSM was centrifuged and redispersedin water for three times. Finally, certain amount of water was addedand thus cross-linked CSM aqueous solution with a concentrationabout 1 mg/mL was obtained.Preparation of Polymeric Hollow Microspheres (PHM).

Typically, 5 mg of CSM-1 was centrifuged and dispersed in 5 mL ofTHF. Then 100 uL of 10% HF aqueous solution (Caution! extremelycorrosive) was added and reacted for 1 h to remove the corecompletely. Then the treated particles were dialyzed against THF for24 h and water for another 24 h (cellulose ester dialysis membrane:MWCO is 14000). Finally, a certain amount of water was added andthus a PHM-1 aqueous solution with a concentration about 1 mg/mLwas obtained.Drug Release Experiment. First, 5 mg of MO was added to 5 mL

of PHM-1 solution with a concentration of 1 mg/mL. A UV−visspectrum of MO with PHM in the initial aqueous solution wasrecorded immediately (the peak absorptionat 464 nm was recorded asAa). After equilibrated at 25 °C for 24 h, the solution was centrifugedat 4000 rpm. The supernatant after centrifugation were then recordedby UV−vis again (the absorption at 464 nm was recorded as Ab).Meanwhile, the precipitate was dispersed into 5 mL of 0.1 M citrate

buffered aqueous solution with pH = 3.0, and transferred into a dialysisbag (MWCO 3500). Then the solution dialyzed against 95 mL of thecorresponding buffer solution with gently stirring at 25 °C for 120 h.Three mL of dialysate was taken out for UV−vis study (the absorptionat 502 nm was recorded as At, and t stands for the release time) andfilled backward right after UV−vis study.The percentage of content released was calculated as follows:

εε

=

=+

−

= ×−

×

CC C

AA A

Released content (%)Released

uptake(0.095 0.005)

( )0.005

20( )

100%

t

a b

t

a b

1

2

Here ε1 and ε2 are the molar extinction coefficient of MO for purifiedwater and pH = 3.0 buffer solution according to yhr UV−vis standardcurve, respectively. The drug release experiment at pH = 7.0 wascarried out in the same way.

Measurements. Transmission Electron Microscopy (TEM). TheTEM images of coassemblies were obtained using a JEM-2100 (JEOLLtd., Japan) transmission electron microscope operated at anacceleration voltage of 200 kV. The sample was prepared by droppingthe solution of coassemblies onto copper grids coated with a thincarbon film, and then dried at 25 °C for 48 h. No staining treatmentwas performed for the measurement.

Scanning Electron Microscopy (SEM). The SEM images ofassemblies were obtained using a JSM-7401F (JEOL Ltd., Japan)field emission scanning electron microscope operated at anacceleration voltage of 5 kV. The samples were prepared by droppingthe solution of coassemblies onto silica wafers or aluminum foil, anddried at 25 °C for 48 h. Then the samples were sputter coated withgold to minimize charging.

Atomic Force Microscopy (AFM). The AFM images of assemblieswere obtained using a Nanonavi E-Sweep (SII, Japan). The surfacemorphologies of samples were acquired in tapping mode. The sampleswere prepared by dropping the solution of coassemblies onto micasheet, and dried at 25 °C for 48 h.

UV−Vis Spectra. The UV−vis spectra of assemblies were carriedout with a UV-2550 spectrophotometer (Shimadzu, Japan). Thesolutions were equilibrated for 10 min before measurement, and theconcentration of the CSM or PHM is 0.1 mg/mL.

Fluorescence Spectra. The fluorescence spectra of assemblies wererecorded using a LS-55B fluorescence meter (Perkin-Elmer, Inc.,USA). The solutions were equilibrated for 10 min before measure-

Figure 9. (a) Percentage of MO released from PHM-1 in pH 3.0 and pH 7.0 buffer solution at different time, at 25 °C. (b) Fluorescence emissionspectra of PHM-1 at different time during the release of MO in buffer solution at pH 7.0; LSCM images of PHM-1, before loading of MO (c), afterloading of MO (d), and after release of MO for 120 h at 7.0 (e), respectively.



ment, and the concentration of the CSMs for fluorescence spectra is0.1 mg/mL. The excitation wavelength is 381 nm.TGA analyze. TGA analyze of assemblies was carried out on a

Q5000IR Thermogravimetric Analyzer (TA, USA), the samples wererun under flowing airand at a heating rate of 20 °C/min. The sampleswere prepared by freeze-drying of the solution of CSM or PHM.Laser Scanning Confocal Microscope (LSCM). Fluorescence

images of assemblies were viewed with a Leica TCS-SP5 laserscanning confocal microscope (LSCM, Leica, Germany) equippedwith UV laser. The sample was prepared by dropping the solution ofCSM or PHM onto coverslip, and then dried at 25 °C for 24 h.Dynamic Light Scattering (DLS). DLS measurements were

performed on the CSM aqueous solutions using a ZS90 ZetasizerNano ZS instrument (Malvern Instruments Ltd., U.K.) equipped witha multi-s digital time correlation and a 4 mW He−Ne laser (λ = 633nm) at an angle of 90°.

■ ASSOCIATED CONTENT*S Supporting InformationSynthesis and characterization of POSS-AN, kinetics ofphotodimerization of anthracene for CSM-1, and character-ization of CSM and PHM. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*(X.J.) Telephone: +86-21-54743268. Fax: +86-21-54747445.E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the National Nature Science Foundation of China(21174085, 21274088), Science & Technology and EducationCommiss ion of Shanghai Municipal Government(11QA1403100, 12ZZ020), and the Shanghai LeadingAcademic Discipline Project (B202) for their financial support.X.J. is supported by the NCET-12-3050 Project.

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