Journal of Colloid and Interface Science 343 (2010) 155–161

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Journal of Colloid and Interface Science

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Optically switchable organic hollow nanocapsules

Ying Wu a,b, Xiaozhong Qu b, Liyan Huang a, Dong Qiu b, Chengliang Zhang b, Zhengping Liu a,*,Zhenzhong Yang b,*, Lin Feng c,*

a Institute of Polymer Chemistry and Physics of College of Chemistry, BNU Key Laboratory of Environmentally Friendly and Functional Polymer Materials,Beijing Normal University, Beijing 100875, Chinab State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, Chinac Department of Chemistry, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:Received 21 September 2009Accepted 18 November 2009Available online 23 November 2009

Keywords:NanocapsulesSpiropyranPhotoswitchableFluorescent

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.11.041

* Corresponding authors. Fax: +86 10 58806896 (Z.PYang).

E-mail addresses: lzp@bnu.edu.cn (Z.P. Liu), yanfl@mail.tsinghua.edu.cn (L. Fen).

a b s t r a c t

Hollow nanocapsules with both photoswitchable-fluorescent and reversible-photochromic properties aresynthesized via a one-pot non-templating microemulsion copolymerization using methyl methacrylateand methacrylated spiropyran as co-monomers. The strong photoswitchable fluorescence of the nano-capsules are switched between ‘‘on” and ‘‘off” by alternating irradiation of ultraviolet and visible light,which causes the reversible photoisomerization between spiropyran and merocyanine in the nanocap-sules. The distribution of spiropyran/merocyanine in a nanocapsule is mostly incorporated inside the wallof the nanocapsule, with only about 17.7% on the surface of the wall. This confinement is the reason of theunusually strong fluorescence of the merocyanine form yielded by the UV radiation. For the same reason,the photochemical stability of the chromophores is increased compared to those in the solution of water/DMF mixture.

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1. Introduction

Photochromic materials have gained extensive interests be-cause of their potential applications in optical and electricalswitches [1], optically rewritable data storage [2] and chemicalsensors [3], etc. Among various photochromic materials, spiropy-ran has attracted the most attention because it undergoes a revers-ibly structural transformation in response to external stimuli, i.e.light [4]. This special property of spiropyran has been exploitedto build various optically-controlled devices, such as molecularswitches [1,5–7] and molecular sensors [8,9]. In addition, spiropy-ran also has potential applications in optical memory [5,6,10,11],optical data storage [12,13], etc.

Organic optical switches with photoreversible fluorescence arerather scarce. Up to present, although efficient fluorescence switchhas been realized using photochromic spiropyran, the fluorescencehas been either due to a fluorophore linked to [14–16] or mixedwith it [1,17], etc.

In most cases, some fluorophores need to be bonded covalentlyto or mixed with spiropyran molecules in order to achieve a photo-reversible fluorescence modulation [18–22], and self-fluorescencesystems are relatively spare. Very recently, Hurst et al. reported

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. Liu), +86 10 62559373 (Z.Z.

gzz@iccas.ac.cn (Z.Z. Yang),

the incorporation of photochromic spiropyran dye into polymernanoparticles to generate optically addressable fluorescent sys-tems. By confining the spiropyran dye into hydrophobic cavitiesof the nanoparticles, the merocyanine form of the encapsulateddyes becomes highly fluorescent and its photostability is much im-proved [7]. They also integrated photoswitchable spiropyran dyeswith fluorescent perylene diimide into the hydrophobic core ofhydrophilic polymer nanoparticles to generate optically address-able two-color fluorescent systems that exhibit high photolumi-nescence and superior resistance to photobleaching [23]. Zengand co-workers used photochromic spiropyran to form fluorescentnanoparticles that exhibited on/off fluorescence switching proper-ties upon UV–visible irradiations [24]. However, the mechanism onthe enhancement of the fluorescence intensity remains unclear.

There are a few studies addressing the fluorescence behavior ofspiropyran derivatives. Devens Gust et al. found that there existedseveral geometrical isomers of the merocyanine due to the slowrotation about the carbon–carbon bond in the linkage joiningtwo rings of the derivatives on the time scale of observation, whichhave different fluorescence quantum yields and lifetimes, andapparently the inter-conversion between isomers is on the sametime scale as fluorescence decay. This makes spiropyran virtuallynot fluorescent [25]. Only one isomer is responsible for most ofthe fluorescence emission whereas the other components emit rel-atively weakly [25]. Accordingly, we presume that once spiropyranmolecules are incorporated into a rigid substrate, such as the casein Hurst’s work, the rotation of the carbon–carbon bonds of the

156 Y. Wu et al. / Journal of Colloid and Interface Science 343 (2010) 155–161

merocyanine would be restricted, keeping the molecule at a con-formation with high fluorescence quantum yield.

To our best knowledge, there has been no report on the prepa-ration of photoresponsive polymeric hollow nanocapsules. In addi-tion, the hollow particles may posses a number of obviousadvantages over the solid particles, which are versatile for manyfurther developments. Besides potential applications such asswitching, optical data storage and biological fluorescent labeling.[7,23], fluorescent hollow spheres can be used for encapsulationand controlled release of certain types of chemicals [26–29]. Inaddition, encapsulation of fluorophores into particles allows inte-grating multiple fluorophores within a single particle. This not onlysignificantly increases per-particle brightness, but also avoids thephotoinstability caused by the interactions between the fluoro-phore molecules [30]. Moreover, dual-color fluorescent particles,especially those that can be reversibly photoswitched are particu-larly desirable because they can distinguish sites of interest fromfalse positive signals generated by adventitious fluorescent bio-molecules [31].

In this report, we will demonstrate one-pot non-templatingsynthesis of photoswitchable-fluorescent poly(methyl methacry-late-methacrylated spiropyran) nanocapsules by microemulsionpolymerization. And the distribution and the fluorescence behaviorof spiropyran in the nanocapsule were systematically studied. Dueto the entrapment of spiropyran in the polymer matrix, the nano-capsules are indeed highly fluorescent. And the fluorescence can beoptically switched ‘‘on” and ‘‘off” with specific wavelengths oflight, i.e. UV and visible light. Also the photostability of spiropyranhas been significantly improved.

2. Materials and methods

2.1. Materials

Methyl methacrylate (MMA) and ethylene glycol dimethacry-late (EGDMA) were purchased from Beijing Chemical Reagent Co.and Alfa, respectively, and passed through an activated Al2O3

column to remove the inhibitors and stored at low temperaturebefore use. 10-(2-Hydroxyethyl)-30,30-dimethyl-6-nitrospiro(2H-1-benzopyran-2,20-indoline) (SPOH) was obtained from NankaiUniversity Fine Chemical Lab. Methacryloyl chloride was distilledunder vacuum and stored at 4 �C. Dichloromethane and triethyl-amine were dried over CaH2 and distilled before use. Sodium dode-cyl sulfate (SDS) was purchased from Beijing Xingli Fine ChemicalsLtd. Azobisisbutyronitrile (AIBN) was obtained from Beijing JinlongChemicals Ltd. Other reagents were used without furtherpurification.

2.2. Synthesis of 10-(2-methacryloxyethyl)-30,30-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,20-indo-line) (methacrylated spiropyran, SP)

SP was synthesized according to lecture with slight modifica-tion [32]. SPOH (3 g, 8.5 mmol) and triethylamine (1.03 g,10.23 mmol) were dissolved in dichloromethane (20 mL) and stir-red at 0 �C for 10 min. To this methacryloyl chloride (1.08 g,10.23 mmol) in dichloromethane (10 mL) was added dropwise.The mixture was then stirred at room temperature for 24 h andthe solvent was evaporated using a rotary evaporator. The residualproduct was dissolved in benzene and passed through a silica gelcolumn and then dried under vacuum to gain SP like yellow pow-der with 15.7% yield. 1H NMR (400 MHz CDCl3) d: 1.17 (s, 3H), 1.28(s, 3H), 1.92 (s, 3H), 3.40–3.60 (m, 2H), 4.30 (t, J = 5.6 Hz, 2H), 5.57(s, 1H), 5.88 (d, J = 10.2 Hz, 1H), 6.07 (s, 1H), 6.71 (d, J = 7.6 Hz, 1H),6.75 (d, J = 8.6 Hz, 1H), 6.89–6.92 (m, 2H), 7.09 (d, J = 7.0 Hz, 1H),7.21 (d, J = 7.4 Hz, 1H), 8.00–8.03 (m, 2H).

2.3. Preparation of poly(MMA-SP) nanocapsules

SDS (3 g) was dissolved in 40 mL of water by sonication for10 min. Then a mixture of 6 g of hexadecane (HD), 4.8 g of MMA,0.6 g of EGDMA and 1.2 g of SP was added dropwise into the SDSsolution within 3.5 h duration at ambient temperature. HD andmonomers are expected to penetrate into the micelles in aqueousmedia for another 2.5 h. Subsequently, 0.04 g of the initiator AIBNwas added into the mixed solution. After the dispersion had beendegassed with nitrogen for 30 min, the system was heated to70 �C under mechanical stirring for 4 h under N2 atmosphere tocomplete the polymerization. The residual surfactants were re-moved by washing with excess ethanol.

2.4. 1H NMR analysis

1H NMR spectra were recorded on a Bruker DMX400 spectrom-eter with CDCl3 as solvent and tetramethylsilane as an internalstandard at room temperature.

2.5. Transmission electron microscopy (TEM) measurement

TEM image was obtained using a JEOL 100CX instrument oper-ated at an accelerating voltage of 100 kV. The sample was dilutedwith water and dropped onto a carbon coated copper grid. Afterwater was evaporated, the dried nanocapsules were stained with1 wt.% phospho-tungstic acid (PTA) aqueous solution.

2.6. Size measurement

Particle size of the poly(MMA-SP) nanocapsules at different sur-factant or SP concentrations were measured using Zetasizer 3000HS (DLS) (Malvern Instruments, UK). All samples had passedthrough a membrane filter (450 nm pore size, Millipore) beforedetection. The concentration of the poly(MMA-SP) nanocapsuleswas fixed at 5 mg/mL for size measurement.

2.7. UV–visible absorption measurement

UV–visible absorption spectra of SP and the poly(MMA-SP)nanocapsules were recorded on a UV–visible spectrophotometer(UV-1601PC, Shimadzu, Japan). The original emulsion was centri-fuged and washed with ethanol to remove residual surfactantand monomers until no UV–visible absorption was observed inthe eluent phase (ethanol).

2.8. Fluorescence measurement

Fluorescence emission spectra were recorded using a CaryEclipse fluorescence spectrometer (Varian, USA). The excitationand emission slits were 5 nm and 10 nm, respectively. And theexcitation wavelength was 420 nm.

2.9. Spiropyran distribution study

Pyrene/acetone solution (0.05 mL, 5 � 10�4 M) was put into aflask. After acetone had been evaporated, 2.5 mL of the poly(-MMA-SP) nanocapsules dispersed in water (0.4 wt.%) was added.After sonicated for 10 min, the flask wrapped with aluminum foilwas allowed to equilibrate at 60 �C for 10 h. Then the dispersionwas irradiated by UV light (365 nm) for a desired spell before fluo-rescence emission spectra measurement. Emission spectra wereobtained by exciting at 335 nm and the excitation and emissionslits were 5 nm and 2.5 nm, respectively. The fluorescence spectraof pyrene in pure H2O and in SP monomer aqueous solution weremeasured following the same procedure as mentioned above.

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Before measurements, the nanocapsule dispersion was exposedto visible light (>450 nm) for 1 h to promote the ring-closed SPgroups. When the sample was simultaneously irradiated by UVand visible light, the two light sources were held at the same dis-tance from the sample.

Fig. 1. TEM image of the poly(MMA-SP) nanocapsules prepared using HD ashydrophobic oil by microemulsion polymerization.

3. Results and discussion

In this work, photoreversibly-modulated fluorescence nanocap-sules were synthesized using SP as co-monomer in a microemul-sion polymerization. The SP co-monomer was obtained byesterification of SPOH with methacryloyl chloride. The chemicalstructure of SP was confirmed by 1H NMR (Fig. S1, Supportinginformation). The NMR results indicate that SP was successfullysynthesized. The photochromic reaction of SP is illustrated inScheme 1. Upon UV irradiation, the neutral and colorless SP under-goes photochemical cleavage of the C�O bond to form a zwitter-ionic blue/purple merocyanine (ME). This isomerization processis reversible, where the ME isomer converts back to the SP struc-ture either under visible light irradiation at room temperature orby heat, and this process can also occur spontaneously at roomtemperature although it is slow [33]. This inter-conversion canbe confirmed by the different UV–visible absorption of SP and ME.

3.1. Preparation of the photochromic nanocapsules via microemulsioncopolymerization of MMA and SP

The reversible photoresponsive nanocapsules were fabricatedusing a modified microemulsion polymerization method [34,35],where MMA was used as the primary monomer with a minoramount of EGDMA as the cross-linker. The microemulsion was ob-tained by mixing the MMA, EGDMA, SP and HD with an aqueoussolution of SDS. Once HD and the monomers had penetrated intothe micelles, the reaction was initiated by an oil-soluble initiator,AIBN.

At the beginning of the reaction, the oil soluble AIBN initiatedthe polymerization of MMA and SP at both the oil/H2O interfaceand inside the oil phase. With increasing polymerization time,the oligomer of MMA and SP deposited at the interface due tothe immiscibility between alkyl and the oligomer of MMA andSP, and served as locuses for the further polymerization. About85% of SP co-monomer has been incorporated into the nanocap-sules based on UV-absorption analysis as described in the experi-mental section.

The hollow structure of the nanocapsules was confirmed byTEM (Fig. 1); the size of the nanocapsules with 2 g of surfactantin each reaction was found to be in the range of 50–60 nm in diam-eter and the wall thickness was about 5 nm. On the contrary, onlysolid nanoparticles would be obtained if MMA instead of themixture of MMA and HD was used as the dispersed phase to formmicroemulsion.

Scheme 1. Light-induced isomerization between the SP form and the ME for

In this microemulsion polymerization system, the size of thenanocapsules could range from ca. 49 nm to ca. 93 nm, dependingon the amounts of SDS and SP used in the reaction. The averageparticle diameters of the samples determined by DLS are shownin Table S1 (Supporting information).

3.2. The distribution of SP in the nanocapsules

The distribution of the photophore units in the nanocapsules,i.e. the surface portion and the portion in the depth of the nanocap-sules, is important for the properties of the resulting products. Inthis study, we used pyrene (PY) as a fluorescence probe to studythe distribution of SP molecules in the nanocapsules. It is knownthat the pyrene emission can be quenched by either spiropyranor its ring-open form merocyanine which has higher efficiency be-cause of intermolecular photoinduced electron transfer (PET) [36–38]. Fig. 2 plots the fluorescence emission intensity of PY in water,in the SP solution, and in the nanocapsule dispersion with the sameconcentration of SP as the SP solution, as function of UV irradiationtime. As indicated in Fig. 2, independent of the UV irradiation time,the PY emission intensity in pure water keeps constant at ca. 120while it is nearly zero for the SP aqueous solution (0.0023 mM).However, the PY fluorescence intensity in the nanoscapsule disper-sion containing the same amount of SP is much higher than that ofthe free SP solution, but is lower than that of pure water. In the lat-ter case, the PY fluorescence intensity drops with UV irradiationtime and reaches a plateau after 6 min. This is attributed to the dis-tribution of the SP molecule both on surface and inside the shell ofthe hollow spheres. As a quencher, it is presumed that only theportion of SP or ME on the nanocapsule surface takes effect.

m by alternating irradiation of UV (365 nm) and visible light (>450 nm).

Fig. 2. The fluorescence intensities of PY in H2O, in the poly(MMA-SP) nanocapsulesdispersed in H2O, and in the SP aqueous solution based on the emission value at373 nm from each curve for different UV irradiation time (d: PY in H2O; j: PY inthe poly(MMA-SP) nanocapsules dispersed in H2O; N: PY in the SP aqueoussolution.). The fluorescence emission spectra were obtained by exciting at 335 nmand measured at 373 nm. The concentration of PY in each case: 1 � 10�5 M; and theconcentration of SP in each case: 2.3 � 10�6 M.

158 Y. Wu et al. / Journal of Colloid and Interface Science 343 (2010) 155–161

The fluorescence intensity of PY in the SP solution with differentconcentrations was measured and the results are shown in Fig. 3,which serves as a calibration curve. The apparent molar concentra-tion of the SP molecules distributed on the nanocapsule surfacecould be evaluated with the calibration curve. It was found thatca. 17.7% of the SP molecules are on the outer surface of the nano-capsules (50–60 nm).

3.3. Photochromic properties of nanocapsules

The photochemical isomerization of the poly(MMA-SP) nano-capsules was observable to naked eyes when they were irradiatedwith either UV-light or visible light. The photographs displayed inFig. 4A show distinct color changes of the nanocapsule dispersionupon irradiation with a given wavelength. The nanocapsule disper-sion was yellow in the SP form and turned into pink within severalminutes under 365 nm UV irradiation, which confirms the conver-sion from the SP form to the ME form. However, on the irradiationwith visible light, the color changed from pink to faintly pink, thatis, the inverse isomerization process occurred. Compared to theleftmost picture, with the same visible light irradiation time asthe UV light, the residuary pink showed in the last picture ofFig. 4A shows that the residual ME dye molecules in the nanocap-sules are still in place. This indicates that, in the present system, itis slower to perform the ring closure reaction towards the SP formas compared to the ring opening process. Furthermore, this macro-scopic photochemical isomerization of the poly(MMA-SP) nano-

Fig. 3. Variation of the fluorescence emission intensities of PY in the absence and inthe presence of different amounts of SP excited at 335 nm. The fluorescenceintensities were measured at 373 nm.

capsules can be repeated reversibly for several cycles. And thephotographs in Fig. 4A show that the microemulsion is fairly trans-parent, simultaneously. UV spectra in Fig. 4B also show that themicroemulsion has good transparency in visible light region, witha sharp absorption peak around 575 nm.

3.3.1. UV absorption propertyThe photochromic reactions of SP in the poly(MMA-SP) nano-

capsules can be further demonstrated by UV–visible absorptionspectroscopy. The temporal evolution of the absorption spectra ofthe poly(MMA-SP) nanocapsule dispersion upon UV radiation isshown in Fig. 5A. Fig. 5B illustrates that the temporal evolutionof the absorption properties of the dispersion upon radiation withthe visible light that is applied to the dispersion after the completeconversion of SP by the UV radiation. It is apparent from Fig. 5Athat a strong absorption band at 550 nm appears after the UV irra-diation, which can be ascribed to the isomerization of the SP to theopen ME form [39]. At the initial stage, the absorption strength in-creases remarkably. With irradiation, the increase becomes slower(Fig. 5A). After the equilibrium system was irradiated under visiblelight, the ME form will be isomerized back to the SP form, which isconfirmed by the absorption peak weakening with time (Fig. 5B).Similar responds of the absorption spectra have also been acquiredfor the free monomer SP in aqueous solution (Fig. S2, Supportinginformation). Compared to the absorption spectra of the SP mono-mer aqueous solution, the visible absorption maxima of the photo-formed ME in the nanocapsules possess a 40 nm red-shift. Thisimplies that the SP moiety in the nanocapsules is situated in a rel-atively less polar microenvironment and that the complexation ofME units with the polar solvent molecules seems to be inhibited inthe present system because the ME moieties are not directly ex-posed to the solvent phase [40].

The ME isomer can also be converted back to the SP structureunder heat. Fig. 5C depicts the decoloring of ME in poly(MMA-SP) nanocapsules under various temperatures and shows that thedecoloring reaction accelerates with the temperature.

Fig. 6A shows gradual intensity changes of the visible absorp-tion at 550 nm with the UV irradiation, and Fig. 6B illustrates thereversible process upon the visible light irradiation. It needs about9 min of UV irradiation (4 w) to complete the transformation fromSP to ME in the nanocapsule dispersion (Fig. 6A), about 90% of theirequilibrium value. Both the initial isomerization and approachingthe equilibrium are significantly slowed down than that for theisomerization of the free SP molecules in the mixed solution ofH2O and DMF (the molar concentration of SP in the two systemswas equal) under the same irradiation intensity. To the monomerSP, the isomerization is faster at the initial stage, and reaches theequilibrium absorption in a shorter time, only 5 min. Similarly,the reversible process, from ME back to SP, is also slower for thenanocapsule dispersion (17 min) than that for the free SP co-mono-mer in the solution (13 min) with the irradiation of the visible light(4 w) (Fig. 6B).

The reversible isomerization can be further illustrated using thecorresponding kinetic constants (Table S2, Supporting informa-tion). The slower ring opening and closing rate in the nanocapsulesis ascribed to the geometric restriction of the solid-state microen-vironment of the nanocapsules, which is consistent with the re-sults discussed above.

3.3.2. Steady-state fluorescence propertyAs compared to the virtually non-fluorescent character of the SP

form and the ME form in most environments, such as water or po-lar organic solvents, [25], the nanocapsules with the fluorophore ofthe ME form in our studies are highly fluorescent (Fig. 7). This of-fers us another means to study the reversible photochromicprocess.

Fig. 4. (A) Digital photographs of the poly(MMA-SP) nanocapsules dispersed in H2O. a: before UV irradiation; b and c: after UV irradiation (365 nm) for 1 min and 5 min,respectively; d and e: subsequently, following visible light (>450 nm) irradiation for 1 min and 5 min, respectively. (B) UV–visible spectra of the poly(MMA-SP) nanocapsulesdispersed in H2O on UV (365 nm) irradiation. The broken line represents data before UV irradiation; the solid lines correspond to spectra acquired after UV irradiation at thetimes indicated.

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UV light (365 nm) induces the SP form to convert to the MEform, and the visible light (>450 nm) reverses the process. In con-trast, no photoconversion between SP and ME occurs upon excita-tion at 420 nm [7]. Consequently, the excitation wavelength of420 nm was chosen for fluorescence measurement.

Upon conversion the SP form to the ME form by UV radiation,Fig. 7 shows that the resulting ME in the nanocapsules fluorescestrongly in the red region (640–650 nm) in water because of theirsolid-state encapsulation. The fluorescence intensity increasesgradually with UV irradiation (Fig. 7A) and oppositely, weakenswith the visible light irradiation time (Fig. 7B).

The fluorescence intensities of the free SP co-monomer, SP inthe solid poly(MMA-SP) particles, and SP in the poly(MMA-SP)nanocapsules are plotted in Fig. 8A as a function of the UV irra-diation time (the molar concentrations of SP in the three systemswere equal). With the same SP content, very weak fluorescence isobserved from the free SP dissolved in solvents, which is in wellagreement with the previous studies [7]. However, in our presentsystem, the poly(MMA-SP) nanocapsules fluoresce more stronglythan the solid poly(MMA-SP) particles do. The excitation spectraof 640 nm emission of the free SP after UV irradiation show amaximum absorption at 535 nm (Fig. 8B). On the other hand,the peaks of the excitation spectra occur at 390 nm and575 nm for both the nanocapsule dispersion and nanoparticledispersion (Fig. 8B). These results infer that the fluorophore, i.e.the ME, inside the nanocapsules and the solid nanoparticles isdifferent from that of the free ME in solution [7]. Previous studyhas shown that upon UV radiation SP converts to several geomet-rical isomers with different fluorescence quantum yields and life-times ascribed to the level of rotation of the carbon–carbon bondjoining the two rings (Scheme 1), and the inter-conversionamong those isomers is on the same time scale as the fluores-cence decay. By incorporation of the SP in the nanocapsules,the steric barrier of the polymer matrix could restrict the rotationof the carbon–carbon bond of ME during the ring opening pro-cess to a certain level that results in a specialized conformationof the ME with high fluorescence quantum yield, and retard therate of inter-conversion of the excited geometrical isomers aswell as the decolorization process [41]. Contrarily for the free

ME, converted from SP by the UV radiation, in solution, the bar-rier imposed by fluidic solvent molecules should be much lessthan that in the solid phase [7]. Consequently, nonradiative relax-ation through internal motions of the excited molecules mayminimize because solid-state environment of the nanocapsuleshave restricted fluorophore molecules conformational flexibility.Additionally, they may isolate from nonradiative decay pathwaysor electron-transfer pathways generated by collisions with solu-tion components.

3.4. Photochemical stability of the nanocapsules

So far, the application of spiropyran has been hindered by theshort life time of the colored merocyanine-form which revertsthermally to the ring-closed colorless spiropyran and by toleratinga limited number of switching cycles due to the irreversible photo-degradation behavior.

To study the stability of the nanocapsules, we have investigatedthe photo-physical cycling of the photoswitchable nanocapsules byalternating illumination with UV (365 nm) and visible light(>450 nm) at room temperature. The absorption maximum of eachspectrum after each irradiation was determined. The photoswitch-able nanocapsules show a good fatigue resistance (Fig. 9A), theabsorption intensity of the opened form can be almost completelyrecovered to the original level in repetitive cycles. Moreover, com-pared Fig. 9A with Fig. S3 (Supporting information), it is clear thatcyclic photoswitching of the poly(MMA-SP) nanocapsules dis-persed in water (Fig. 9A) exhibits better reversibility than that ofSP in the mixed solution of H2O and DMF (Fig. S3, Supporting infor-mation). This may attribute to copolymerization which minimizesbimolecular degradation reactions.

Photochemical stability of the SP-containing nanocapsules canalso be studied using the fluorescence switching. From Fig. 9B,one can see that the fluorescence switching of the nanocapsulesdisplays good photoreversibility and can be repeated for severaltimes without any apparent ‘‘fatigue” effects or photobleaching.

For spiropyran moieties, the ‘‘fatigue” effects caused by photo-degradation of spiropyran have been discussed elsewhere[14,42]. However, we can see from Fig. 9 that the incorporation

Fig. 5. UV–visible spectra of the poly(MMA-SP) nanocapsules dispersed in H2O on:(A) UV (365 nm) and (B) visible light (>450 nm) irradiation. The dotted linesrepresent data before UV irradiation; the solid lines correspond to spectra acquiredafter (A) UV irradiation and (B) visible light irradiation at the times indicated. (C)The effect of temperature on the decoloring of poly(MMA-SP) nanocapsules. Theconcentration of the nanocapsules dispersed in H2O is 0.4 wt.%.

Fig. 6. The intensities based on the absorption maximum from each curve in theUV–visible spectra for the different irradiation time: (A) SP to ME form irradiated onUV (365 nm) and (B) ME to SP form irradiated on visible light (>450 nm) (d:monomer SP molecules; j: poly(MMA-SP) nanocapsules). The concentration of theSP in each case is 2.3 � 10�6 M.

160 Y. Wu et al. / Journal of Colloid and Interface Science 343 (2010) 155–161

of spiropyran in the nanocapsules can make the nanocapsules dis-play good reversibility. This is attributed to the protective nano-scale environment which may impede those UV irradiation-involved photodegradation reactions.

Fig. 7. Fluorescent emission spectra (excited at 420 nm) of the poly(MMA-SP)nanocapsules dispersed in H2O after: (A) UV (365 nm) and (B) visible light(>450 nm) irradiation. The dotted lines represent data before UV irradiation; thesolid lines correspond to spectra acquired after (A) UV irradiation and (B) visibleirradiation at the times indicated. The concentration of the nanocapsules dispersedin H2O is 0.4 wt.%.

4. Summary

In summary, a one-pot non-templating approach is developedfor preparing photo-responsive hollow poly(MMA-SP) nanocap-sules by microemulsion polymerization. SP is found to be mostlyexcluded from the surface of the nanocapsules using PY moleculesas a fluorescence probe. In contrast to the virtually non-fluorescentcharacter in most environments, the nanocapsules with the fluoro-phore of the ME form are highly fluorescent. The reversible SP-MEphotoisomerization in the nanocapsules is photo-chemicallycontrolled. Such reversible, stable, photochromic, and emissivenanocapsules may be potentially interesting for a variety of appli-cations as discussed above.

Fig. 8. (A) The intensities based on the maximum from each curve in thefluorescence emission spectra for the different UV (365 nm) irradiation time. j:poly(MMA-SP) nanocapsules dispersed in H2O; �: poly(MMA-SP) nanoparticlesdispersed in H2O; N: SP molecules dissolved in DMF; d: SP molecules dissolved inH2O. (B) Fluorescence excitation spectra monitored at 640 nm after UV (365 nm)irradiation: solid line: poly(MMA-SP) nanoparticles dispersed in H2O; dash line:poly(MMA-SP) nanocapsules dispersed in H2O; dash dot line: SP moleculesdissolved in H2O. The concentration of SP in each case is 2.3 � 10�6 M.

Fig. 9. (A) UV absorption intensities of the poly(MMA-SP) nanocapsules dispersedin H2O upon UV (365 nm) illumination and visible light (>450 nm) irradiationcycles. (B) Fluorescence switching of the poly(MMA-SP) nanocapsules irradiatedwith alternating UV (365 nm) and visible light (>450 nm) and then excited at420 nm, respectively. The concentration of the nanocapsules dispersed in H2O is0.4 wt.%.

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Acknowledgments

We thank financial supports by the NSF of China (50573083,50325313, 50373004 and 50521302), Chinese Academy of Sci-ences, China Ministry of Science and Technology (2004-01-09and KJCX2-SW-H07), and Beijing Municipal Commission of Educa-tion. We also thank Prof. Liusheng Chen at Institute of Chemistry,the Chinese Academy of Sciences for valuable discussion.

Appendix A. Supplementary material

Supplementary material associated with this article can befound, in the online version, at doi:10.1016/j.jcis.2009.11.041.

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