Photoinduced Morphology Switching of Polymer Nanoaggregates in

Aug 12, 2010 - ... coumarin end groups upon 365 nm irradiation and reform the micellar morphology ..... pH-induced outward movement of star centers wi...
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Photoinduced Morphology Switching of Polymer Nanoaggregates in Aqueous Solution Jinqiang Jiang,* Qiaozhen Shu, Xin Chen, Yiqun Yang, Chenglin Yi, Xiaoqing Song, Xiaoya Liu,* and Mingqing Chen School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu, China 214122 Received May 26, 2010. Revised Manuscript Received July 30, 2010 A novel photosensitive C-PNIPAAm comprising hydrophilic PNIPAAm conjugated with a relatively short but very hydrophobic coumarin part was designed and prepared using a coumarin-containing disulfide derivative (C-S-S-C) as transfer agent in the presence of Bu3P and water. It was found that C-PNIPAAm can form polymer micelles in aqueous solution. And the micellar morphology in aqueous solution can be photoswitched into hollow spheres according to the photodimerization of coumarin end groups upon 365 nm irradiation and reform the micellar morphology after the subsequent photoscission of dimers upon 254 nm. This instant morphology changing phenomenon was successfully monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. TEM observations showed the small spherical shape of micelles with diameter at 30-50 nm before photo-cross-linking, the big vesicles with diameter at 200-350 nm after photo-cross-linking, and the small micelles with diameter at 30-50 nm after the subsequent photo-de-cross-linking in the first irradiation cycle. The reason for this significant morphology switching can be attributed to the reversible photoinduced amphiphilic structure transformation between the telechelic “hydrophobic end-hydrophilic chain” structure and the ABA type of “hydrophilic chain-hydrophobic centerhydrophilic chain” one upon alternating irradiation.

1. Introduction Coumarin and its derivatives have been widely used in a variety of functional polymer materials including biochemicals, electrooptical materials, organic-inorganic hybrid materials, liquid crystalline materials, and light harvesting/energy transferring materials.1-9 Moreover, their reversible photo-cross-linking and photocleavage behavior has been widely investigated for the application of photoresponse materials.1-13 It is well-known that the direct irradiation (λ > 300 nm) of coumarin leads to photochemical dimerization resulting in cyclobutane-type dimers, and which can revert to the starting compound upon irradiation with light of shorter wavelength (λ = 254 nm).1-13 Therefore, the polymers containing coumarin units perform quick photoresponse and effective photoreversibility with alternative irradiation of UV light of different wavelength. Recently, applications of this reaction were explored where coumarin and its derivates have been used as phototriggers for light-sensitive polymers, namely, light-responsive polymer *To whom correspondence should be addressed. Telephone: 86-51085917763. Fax: 86-510-85917763. E-mail: [email protected] (J.J.); lxy@ jiangnan.edu.cn (X.L.). (1) Trenor, S. R.; Shultz, A. R.; Love, B. J.; Long, T. E. Chem. Rev. 2004, 104, 3059. (2) Graf, C.; Sch€artl, W.; Hugenberg, N. Adv. Mater. 2000, 12, 1353. (3) Mal, N. K; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350. (4) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (5) Brun, M. P.; Bischoff, L.; Garbay, C. Angew. Chem., Int. Ed. 2004, 43, 3432. (6) Zhao, L.; Loy, D. A.; Shea, K. J. J. Am. Chem. Soc. 2006, 128, 14250. (7) Jackson, P. O.; O’Neill, M. Chem. Mater. 2001, 13, 694. (8) Chen, M.; Ghiggino, K. P.; Launikonis, A.; Mau, A. W. H.; Rizzardo, E.; Sasse, W. H. F.; Thang, S. H.; Wilson, G. J. J. Mater. Chem. 2003, 13, 2696. (9) Kim, C.; Trajkovska, A.; Wallace, J. U.; Chen, S. H. Macromolecules 2006, 39, 3817. (10) Mustafa, A.; Kamel, M.; Aliam, M. A. J. Org. Chem. 1957, 22, 888. (11) Wells, P. P.; Morrison, H. J. Am. Chem. Soc. 1975, 97, 154. (12) Trenor, S. R.; Shultz, A. R.; Love, B. J.; Long, T. E. Chem. Rev. 2004, 104, 3059. (13) Zhao, D.; Ren, B.; Liu, S.; Liu, X.; Tong, Z. Chem. Commun. 2006, 779.

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micelles and nano/microgels.14-25 In general, the coumarin functional groups were introduced as hydrophobic pendants of the amphiphilic polymer chain. Zhao et al. studied polymer micelles with coumarin pendants for the stabilization of micelles by UVinduced cross-linking.16 However, the nanoaggregates selfassembled by this kind of polymer do not show a large or significant morphological or size change. This is because the photoreversible dimerization of coumarin may have small or no effect on the amphiphilic balance of polymer micelles since the coumarin moieties are in the already compact hydrophobic core. And since there are several polymeric structure parameters (such as the nature and the relative length and position of the blocks) which can make the actual change to the hydrophilic-to-hydrophobic balance of amphiphilic polymers in aqueous solution. It is therefore probable to develop an effective approach by changing the position of the hydrophobic unit that allows polymer nanoaggregates to be regulated by light. Herein, we report on such a new strategy and demonstrate the photoinduced morphology switching and size change of polymeric nanoaggregates in aqueous solution. Figure 1 schematically illustrates the design of such photoswitching polymer nanoaggregates in aqueous solution. The amphiphilic (14) Tian, Y.; Akiyama, E.; Nagase, Y.; Kanazawa, A.; Tsutsumib, O.; Ikeda, T. J. Mater. Chem. 2004, 14, 3524. (15) Barberis, V. P.; Mikroyannidis, J. A.; Cimrova, V. J. Polym. Sci., Part. A: Polym. Chem. 2006, 44, 5750. (16) Jiang, J. Q.; Qi, B.; Lepage, M.; Zhao, Y. Macromolecules 2007, 40, 790. (17) Qi, Fu.; Cheng, L. L.; Zhang, Y.; Shi, W. F. Polymer 2008, 49, 4981. (18) He, J.; Tong, X.; Tremblay, L.; Zhao, Y. Macromolecules 2009, 42, 7267. (19) Jiang, J. Q.; Feng, Y.; Wang, H. M.; Liu, X. Y.; Zhang, S. W.; Chen, M. Q. Acta Phys. Chim. Sin. 2008, 24, 2089. (20) He, J.; Zhao, Y.; Zhao, Y. Soft Matter 2009, 5, 308. (21) Zhao, Y. Chem. Rec. 2007, 7, 286. (22) Dahmane, S.; Lasia, A.; Zhao, Y. Macromol. Chem. Phys. 2008, 209, 1065. (23) Zhao, Y. J. Mater. Chem. 2009, 19, 4887. (24) Zhao, Y.; Bertrand, J.; Tong, X.; Zhao, Y. Langmuir 2009, 25, 13151. (25) He, J.; Tong, X.; Zhao, Y. Macromolecules 2009, 42, 4845.

Published on Web 08/12/2010

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Figure 1. Schematic illustration of photoreversible dimerization of coumarin end groups resulting in the photoinduced structure switching between the telechelic polymer and a type of ABA polymer upon alternating irradiation at 365 and 254 nm and the photoinduced morphology switching of polymer nanoaggregates in aqueous solution at room temperature.

telechelic C-PNIPAAm contains a reversible photo-cross-linking coumarin chromophore as a hydrophobic end. With 365 nm light irradiation, the neighboring hydrophobic coumarin end groups photodimerized to form the ABA polymer of PNIPAAm-C-CPNIPAAm with the amphiphilic structure transformation from “hydrophobic end-hydrophilic chain” structure to a “hydrophilic chain-hydrophobic center-hydrophilic chain” one, which would instantly break the original amphiphilic balance in aqueous solution to form large size of polymer vesicles. Subsequently, upon the photo-de-cross-linking irradiation of 254 nm, the dimerized hydrophobic coumarin center of PNIPAAm-C-C-PNIPAAm would photocleave to reform the telechelic C-PNIPAAm, resulting in the reversion of amphiphilic balance and the size decrease of polymer nanoaggregates.

2. Experimental Section 2.1. Materials. 7-Hydroxy-4-methylcoumarin, azodiisobutyronitrile (AIBN), epichlorohydrin (ECH), phosphotungstic acid (PTA), petroleum ether, triphenylphosphine, chloroform, anhydrous ethanol, ethyl ether, tetrahydrofuran (THF), methanol, and dioxane were purchased from Sinopharm Chemical Reagent ShangHai Co., Ltd. and used without further purification. N-Isopropyl acrylamide (NIPAAm), sodium sulfide, and sulfur were purchased from Shanghai Aladdin Reagent Co., Ltd. and used without further purification. Tributyl phosphine (Bu3P) was purchased from J&K China Chemical Ltd. 2.2. Characterization. 1H NMR spectra were obtained with an AVANCE III 400 MHz Digital NMR spectrometer, using CDCl3 as solvent. Gel permeation chromatography (GPC) measurement was performed using a Waters system equipped with a refractive index and a photodiode array detector, with dimethylformamide (DMF) being used as eluent (elution rate, 1.0 mL/min) and polystyrene (PS) standards used for calibration. LC-MS was performed on a WATERS MALDI SYNAPT Q-TOF MS system. UV-vis spectra were recorded on a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). Fluorescence emission spectra were recorded on a RF5301PC instrument (Shimadzu). The photo-cross-linking procedure was performed by applying a LED spot curing system (400 mW, λ = 365 nm) vertically to the quartz wall. And for photo-de-crosslinking, the irradiation light was obtained directly from a UV-C Air sterilizer lamp (λmax = 254 nm, 8.0 mW/cm2). Dynamic light scattering (DLS) measurements were carried out on an ALV5000/E dynamic light scattering instrument at 90° and Malvern Instruments Zetasizer Nano-ZS apparatus with temperature at 25 °C. Transmission electron microscopy (TEM) measurements 14248 DOI: 10.1021/la102771h

were carried on a JEOL JEM-2100 microscope operating at 200 KV.

2.3. Synthesis of Coumarin-Containing Disulfide Derivative (C-S-S-C). A mixture of 0.2 g of triphenylphosphine, 100 mL of ECH, and 50 g of 7-hydroxy-4-methylcoumarin was heated under reflux for 20 h. Then ECH was evaporated and the system was washed with petroleum ether and water. The residual solid was pure enough to use in the next step without further purification (90%). Na2S2 was prepared from Na2S 3 9H2O (37.5 g, 0.156 mol) and sulfur (5 g, 0.156 mol) after being stirred for 1 h in 20 mL of distilled water at 40 °C. Then half of the Na2S2 solution was added dropwise into a mixture of 60 g of coumarin compound prepared in the first step and a little PEG 400 (as phase transfer catalyst) in 100 mL of chloroform at 40 °C. The organic layer was collected after being washed with 200 mL of water for three times and evaporated to remove chloroform. The residual solid was resolved in THF and precipitated from an excess of petroleum ether, filtered, and dried at 40 °C under vacuum for 24 h (yield: 85%). 1 H NMR (400 MHz, CDCl3), δ (TMS): 7.45 (1 H, ArH), 6.87-6.75 (2 H, ArH), 6.10 (1 H, CO-CH=), 2.36 (3 H, CH3). MS: 531.1 (m/z). 2.4. Polymerization. In polymerization, a round-bottom flask was charged with C-S-S-C (0.265 g, 5  10-4 mol), NIPAAm (3.4 g, 3.0  10-2 mol), AIBN (0.1 g, 6  10-4 mol), Bu3P (0.12 g, 6  10-4 mol), 0.1 mL of deionized water, and 5 mL of dioxane. The reaction mixture was degassed and sealed under vacuum, and then the reaction bulb was placed in a preheated water bath (70 °C) for 24 h. Then the solution was diluted with THF and precipitated into ethyl ether. The telechelic C-PNIPAAm was collected by being precipitated twice into ethyl ether and dried under vacuum (yield: 85%). Mn = 3.21  103, Mw/Mn = 1.50 (GPC). And for the polymer after the photo-cross-linking reaction (the photodimerization degree of coumarin is about 63.41%), Mn = 5.72  103, Mw/Mn = 1.40 (GPC). From the 1H NMR spectrum (see the Supporting Information), comparing the integrals of CO-CHd (6.10 ppm) in the coumarin structure and N-CH (3.90 ppm) in the NIPAAm structure led to the estimation of 25 units of NIPAAm in the telechelic polymer structure, corresponding to a NMR-based Mn of 3000 g/mol.

2.5. Critical Micelle Concentration (CMC) Determination of C-PNIPAAm. In order to determine the CMC value of C-PNIPAAm in aqueous solution, the fluorescence techniques were applied using pyrene and the coumarin end groups as probe, respectively. Fluorescence characteristics of pyrene and coumarin were recorded on RF5301PC with 3 nm of EX and EM width, respectively. For the pyrene probe, calculated concentrations of polymer in previously pyrene saturated aqueous solution were Langmuir 2010, 26(17), 14247–14254

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Scheme 1. Synthetic Route to the Telechelic Polymer of C-PNIPAAm

excited at 334 nm; and for the coumarin probe, calculated concentrations of polymer in deionized water were excited at 320 nm.

2.6. Photoswitching of Polymeric Nanoaggregates in Aqueous Solution. First, the calculated amount of polymer

Figure 2. UV-vis spectra of C-PNIPAAm in different solvents with polymer concentration at 0.5 mg/mL.

was added into deionized water to a concentration of 0.6 mg/ mL. Then the polymer solution (0.6 mg/mL  2 mL) was filled into a quartz cell and photo-cross-linked by 365 nm irradiation from a LED spot curing system (400 mW) with mild stirring for 10 min; the distance between the quartz wall and the spot light was about 0.2 cm. And for the subsequent photo-de-cross-linking procedure, the above aqueous solution was irradiated by UV irradiation at 254 nm from a UV-C air sterilizer lamp (8.0 mW/ cm2) for 60 s; the distance between the quartz wall and the lamp was about 0.5 cm. The photo-cross-linking (365 nm, 10 min) and photocleavage (254 nm, 60 s) cycles were repeated for five times to evaluate their reversibility and monitored via UV-vis spectra. The photodimerization degree (PD) was calculated from the UV-vis spectra by comparing the peak absorption at 320 nm assigned to the coumarin group by the following equation:

thus often the “weak link” in many molecules and is susceptible to scission by polar reagents, both electrophiles and especially nucleophiles.27-29 For example, in the presence of Bu3P and water, disulfide groups can be degrade quantitatively into the corresponding thiols,29 which can be used as chain transfer agents in the free radical polymerization. Moreover, in terms of their physical properties, the introduced thioethers by transfer polymerization can afford polymers more hydrophobic ability. And as shown in Scheme 1, the telechelic C-PNIPAAm was prepared by mercapto chain-transfer polymerization using a coumarincontaining disulfide derivative (C-S-S-C) as transfer agent in the presence of Bu3P and water. The coumarin absorption part (∼322 nm, in Figure 2) of polymer in aqueous solution is clearly visible, confirming the successful transfer polymerization. As shown in Scheme 1, the amphiphilic telechelic C-PNIPAAm consists of hydrophilic PNIPAAm conjugated with a relatively short but very hydrophobic coumarin part. This type of amphiphilic polymer is similar to that of water-soluble polymers conjugated with hydrophobic lipids and can form micelles in aqueous solution if C-PNIPAAm exceeds the CMC value.30-33 C-PNIPAAm exhibits good solubility in organic solvents (for example, chloroform, methanol, and dioxane) and water. As shown in Figure 2, the polymer in dioxane gives characterized narrow absorption at 319 nm; when applied in methanol, it gives a broadened absorption spectrum with an obvious intensity decrease at 322 nm; and when applied in water, it gives an absorption spectrum very similar to that obtained in methanol with the absorption peak at 322 nm, suggesting the presence of both H- and J-aggregates of coumarin chromophore in aqueous solution; that is, the coumarin end groups in aqueous solution are stacking in a mixture of the end-to-end and face-to-face arrangement.34-40 The

PDð%Þ ¼ ½A0 - At =A0 Here, A0 and At are the peak absorptions centered at 320 nm assigned to the coumarin group. 0 and t represent before irradiation and after the t time of irradiation by alternating light of 365 and 254 nm, respectively. To investigate the photoinduced morphology switching, the polymer aqueous solution (0.6 mg/mL  30 mL) was kept on irradiation by alternating light of 365 and 254 nm to the desired photodimerization degree; then 1 mL of aqueous solution was taken out and diluted to 0.1 mg/mL for DLS measurements with the ALV-5000/E dynamic light scattering instrument. And 0.5 mL of such diluted micelle solution was mixed with PTA solution (1.5% in distilled water) for 2 h and diluted to 0.01 mg/mL.26 Then such micelle solutions were deposited on the copper grids with Formvar film and dried before TEM observation. Second, in order to avoid procedural errors in the DLS measurement by the ALV-5000/E dynamic light scattering instrument, the micellar solution (0.1 mg/mL  2 mL) was filled into a screw-cap cuvette and kept on irradiation by alternating light of 365 and 254 nm to the desired photodimerization degree, then the polymer solution in the enclosed cuvette was taken for a measurement of hydrodynamic size by using the Malvern Zetasizer NanoZS instrument at room temperature.

3. Results and Discussion 3.1. Preparation of Polymer Micelles in Aqueous Solution. In chemistry, a disulfide bond (S-S) is a covalent bond, usually derived by the coupling of two thiol groups. Being about 40% weaker than C-C and C-H bonds, the disulfide bond is (26) Harris, J. R.; Roos, C.; Djalali, R.; Rheingans, O.; Maskos, M.; Schmidt, M. Micron 1999, 30, 289.

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(27) Humphrey, R.; Hawkins, J. Anal. Chem. 1964, 36, 1812. (28) Humphrey, R.; Potter, J. Anal. Chem. 1965, 37, 164. (29) Tsarevsky, N.; Matyjaszewski, K. Macromolecules 2005, 38, 3087. (30) Hwang, M. L.; Prud’homme, R. K.; Kohn, L.; Thomas, J. L. Langmuir 2001, 17, 7713. (31) Lundquist, A.; Wessman, P.; Rennie, A. R.; Edwards, K. Biochim. Biophys. Acta 2008, 1778, 2210. (32) Lukyanov, A. N.; Torchilin, V. P. Adv. Drug Delivery Rev. 2004, 56, 1273. (33) Zalipsky, S.; Mullah, N.; Harding, J. A.; Gittelman, J.; Guo, L. K.; DeFrees, S. A. Bioconjugate Chem. 1997, 8, 111. (34) Ushiroda, S.; Ruzycki, N.; Lu, Y.; Spitler, M. T.; Parkinson, B. A. J. Am. Chem. Soc. 2005, 127, 5158. (35) Ogawa, M.; Kawai, R.; Kuroda, K. J. Phys. Chem. 1996, 100, 16218. (36) Al-Kaysi, R. O.; M€uller, A. M.; Ahn, T.-S.; Lee, S.; Bardeen, C. J. Langmuir 2005, 21, 7990. (37) Seki, T.; Ichimura, K. J. Phys. Chem. 1990, 94, 3769. (38) Alekseev, A. S.; Konforkina, T. V.; Savransky, V. V.; Kovalenko, M. F.; Jutila, A.; Lemmetyinen, H. Langmuir 1993, 9, 376.

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Figure 3. (a) fluorescence spectra excitated at 334 nm as a function of polymer concentration in previously pyrene saturated aqueous solution; (b) changes in the ratio I1/I3 of pyrene emission as a function of polymer concentration in previously pyrene saturated aqueous solution.

Figure 4. Fluorescence emission spectra of polymer in water and dioxane excitated at 320 nm, with the inset showing the CMC value based on the ratio of I385/I450 as a function of polymer concentration.

mixed aggregates of coumarin end groups can help explain the formation of the double-layered micelles shown in Figure 1.41-43 It was found that the polymer micelles can be obtained by directly adding C-PNIPAAm into water and the micellarization can be monitored by pyrene probe technique. As shown in Figure 3a, the fluorescence spectra excitated at 334 nm as a function of C-PNIPAAm polymer concentration in previously pyrene saturated aqueous solution exhibit a characterized vibrational fine five peaks emission of pyrene and a distinct coumarin emission at 450 nm. It has been shown that the ratio of the first (I1 at 373 nm) and third peaks (I3 at 384 nm) of pyrene emission is a sensitive parameter characterizing the polarity of the probe’s environ(39) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (40) Guo, Z. X.; Jiao, T. F.; Liu, M. H. Langmuir 2007, 23, 1824. (41) Funhoff, A. M.; Monge, S.; Teeuwen, R.; Koning, G. A.; SchuurmansNieuwenbroek, N. M. E.; Crommelin, D. J. A.; Haddleton, D. M.; Hennink, W. E.; van Nostrum, C. F. J. Controlled Release 2005, 102, 711. (42) Koulic, C.; Jer^ome, R. Macromolecules 2004, 37, 888. (43) Bao, H. Q.; Hu, J. H.; Gan, L. H.; Li, L. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 6682. (44) Ananthapadmanabhan, K. P.; Goddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, 1, 352. (45) Lin, Y.; Alexandridis, P. Langmuir 2002, 18(11), 4220. (46) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730.

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ment.44-46 As shown in Figure 3b, the dependence of the I1/I3 ratio on the concentration of C-PNIPAAm remains constant up to a certain polymer concentration and decreases sharply above it. This change reflects the onset of micelle formation and the partitioning of the pyrene between the aqueous and micellar phases. Therefore, the CMC value for C-PNIPAAm in aqueous solution is 8.4  10-4 mg/mL. It is interesting that the CMC value of C-PNIPAAm in aqueous solution can also be determined using the coumarin end as fluorescence probe. As shown in Figure 4, the fluorescence spectra of the polymer in water are distinctly different from that in dioxane. The emission spectra in aqueous solution are characterized by easily detected dual fluorescence emission covering the 350-600 nm interval. And the maximum emission peaks in aqueous solution show significant red shifts of 70 nm in comparison to that in dioxane, where the coumarin moieties are separated and the fluorescence spectrum resembles that of the isolated chromophore. Furthermore, as shown in the inset of Figure 4, the composite emission spectra of polymer in aqueous solution above a certain polymer concentration always show a nearly fixed intensity ratio of I385/I450: the shoulder band peaks at 385 nm (λsh) and the maximum band at 450 nm (λmax). And when below that polymer concentration, the I385/I450 ratio increases sharply with a decrease of polymer concentration. We assigned this change as the micellization of polymer in aqueous solution, and the CMC value (8.0  10-4 mg/mL) determined by this change is very consistent with that determined by the pyrene probe. 3.2. The Reversible Photodimerization of Polymer upon Alternating Irradiation. The reversible photodimerization of coumarin ends in polymer aqueous solution upon alternating wavelength irradiation was monitored by UV-vis measurement. As shown in Figure 5a, irradiation of polymer micelles (0.6 mg/ mL  2 mL) with UV light of 365 nm (400 mW from a spot LED curing system) induces the photodimerization of neighboring coumarin ends and thus results in a gradual decrease in the absorption band centered around 320 nm; the estimated photodimerization degree based on the absorbance change at 320 nm in the inset indicates that further irradiation beyond 10 min yields very few dimers. Subsequently, when the photo-cross-linked micelle solution was irradiated by the light of 254 nm (UV-C air sterilizer lamp, 8.0 mW/cm2, and placed at ∼0.5 cm from the solution), the scission of the photodimerized coumarin moieties occurred, which could be indicated by the recovery of the absorption at 320 nm (Figure 5b); however, only about 40% of Langmuir 2010, 26(17), 14247–14254

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Figure 5. UV-vis spectra of polymer nanoaggregates (0.6 mg/mL  2 mL) upon 365 nm irradiation (a) and the subsequent 254 nm irradiation (b); the insets are plots of the estimated photodimerization degree based on the absorbance change at 320 nm as a function of irradiation time.

Figure 6. UV-vis spectra of polymer nanoaggregates (0.6 mg/mL  2 mL) upon 254 nm irradiation (a) and the subsequent 365 nm irradiation (b), with the insets showing the changes of photodimerization degree as a function of the irradiation cycle by the different time and wavelength irradiations.

the dimers reverted back to the original structure within 60 s of irradiation. We contributed this to the dynamic equilibrium of photo-cross-linking and photo-de-cross-linking occurring at 254 nm.1,47 To elucidate this dynamic equilibrium upon 254 nm irradiation, the polymer aqueous solution (0.6 mg/mL  2 mL) without any irradiation was first irradiated with 254 nm light. As shown in Figure 6a, long time irradiation of 254 nm light can also result in a decrease in the absorption band centered around 320 nm, indicating the photodimerization of coumarin chromophore in the polymeric nanoaggregates. To obtain more details, this irradiation time of 254 nm was divided into two irradiation cycles according to the irradiation time. As shown in the inset of Figure 6a, after the first 20 min of 254 nm irradiation, the photodimerization degree of coumarin increased to 12.58%; and within the subsequent 60 s irradiation of 254 nm light, the photodimerization degree decreased only a little from 12.58% to 11.74% in 15 s and then it increased to 12.89% in the remaining 45 s; the second cycle irradiated by 254 nm shows similar features of photo-cross-linking and photo-de-cross-linking, which indicates that both the photo-cross-linking and photo-de-cross-linking can take place at 254 nm irradiation, and the photo-cross-linking of (47) Lewis, F. D.; Barancyk, S. V. J. Am. Chem. Soc. 1989, 111, 8653.

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coumarin is faster than the photo-de-cross-linking. This is why the photodimer of coumarin after the long time of 254 nm irradiation shows very weak photo-de-cross-linking ability. To evaluate the reversible photodimerization ability of the polymer solution after the long time of irradiation at 254 nm, the above polymer solution was irradiated for another two cycles at 365 and 254 nm. As shown in Figure 6b, upon the subsequent irradiation of 365 nm light, the absorption band centered around 320 nm of coumarin also shows a gradual decrease, indicating that the coumarin chromophore after long time of irradiation at 254 nm can still be photo-cross-linked by the 365 nm irradiation. Furthermore, as shown in the inset of Figure 6b, the photo-de-cross-linking ability of coumarin dimers yielded by the 365 nm irradiation is much higher than that of dimers yielded by the 254 nm irradiation. This is because that only the photo-cross-linking of coumarin takes place when irradiated by 365 nm light. Here, we chose 365 nm light for the photo-cross-linking procedure and 254 nm for the photo-de-cross-linking procedure to evaluate the photoreversibility of coumarin end groups in aqueous solution. As shown in Figure 7, when the polymer solution (0.6 mg/mL  2 mL) was exposed upon alternating irradiation (365 nm for 10 min, 254 nm for 60 s) for five cycles, the photodimerization of coumarin upon 365 nm irradiation would reach the same level in every cycle; and the photode-cross-linking DOI: 10.1021/la102771h

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degree decreases progressively along with the irradiation cycle. Thus, the reversible photodimerization of coumarin ending inside polymeric nanoaggregates could just be achieved to a certain degree upon alternating irradiation at 365 and 254 nm. However,

Figure 7. Plot of the photodimerization degree based on the absorbance change at 320 nm as a function of irradiation cycle.

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this partially reversible change in the dimerization degree can still be observed, indicating that alternating irradiation can be used as a trigger to predictably alter the physicochemical properties of these micelles. 3.3. The Photoswitching of Polymeric Nanoaggregates Detected by DLS and TEM Measurements. Under the used preparation conditions, the size of polymer nanoaggregates in aqueous solution after alternating irradiation within five cycles was monitored by using an ALV-5000/E dynamic light scattering instrument. As shown in Figure 8a, the average size of polymer nanoaggregates increases gradually during the first photo-crosslinking procedure, and the average Rh of the final nanoaggregates (the photodimerization degree is 63.41%) significantly triples against that of the initial. We hypothesize that these diameterchanging phenomena are caused by the intermolecular cycloaddition of neighboring coumarin end groups and the induced amphiphilic structure transformation from the telechelic “hydrophobic end-hydrophilic chain” structure to the ABA type of the “hydrophilic chain-hydrophobic center-hydrophilic chain” one. As stated above, before the photo-cross-linking, the coumarin end groups are stacking parallel in the combination of H- and J-aggregates. Therefore, at the beginning of the 365 nm irradiation, the neighboring coumarin groups in the J-aggregates yield

Figure 8. Size distributions of polymer micelles upon alternating irradiation at 365 and 254 nm detected by using an ALV-5000/E dynamic light scattering instrument at a concentration of 0.1 mg/mL: (a) the first photo-cross-linking procedure; (b) schematic illustration of three isomers resulting from the photodimerization of coumarin in the polymer nanoaggregates upon 365 nm irradiation; (c) the first photode-cross-linking procedure; (d) the size changes within five irradiation cycles, with the red and blue points from A to H referring to the samples used in TEM observations. 14252 DOI: 10.1021/la102771h

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Figure 9. Size distributions of polymer nanoaggregates upon irradiation at 365 nm (a) and the subsequent irradiation at 254 nm (b) detected via the Malvern Zetasizer Nano-ZS instrument at a concentration of 0.1 mg/mL.

anti head-to-tail dimers, and the H-aggregates prefer to form the syn head-to-head dimers because of the high concentration of coumarin in the hydrophobic region; then the concentration of coumarin decreases quickly along with the 365 nm irradiation, where it prefers to yield anti head-to-head dimers because of the inefficient intersystem crossing of the coumarin chromophore.48 As shown in Figure 8b, the flexible PNIPAAm chains linked to the syn head-to-head dimers may keep their conformation in water because of the remaining face-to-face stacking of the coumarin structure; however, for the anti head-to-head photodimers, the PNIPAAm chains would prefer a much more extended conformation in aqueous solution to the initial one before irradiation, resulting in the break of the initial amphiphilic balance and the morphology switching from polymer micelles to vesicles. Subsequently, upon 254 nm irradiation, the dimerized hydrophobic coumarin center of PNIPAAm-C-C-PNIPAAm would photocleave to reform the “hydrophobic end-hydrophilic chain” structure of telechelic C-PNIPAAm, resulting in the reversion of amphiphilic balance and the shrinking of polymer nanoaggregates. As shown in Figure 8c, the size of polymer micelles gradually decreases along with the photo-de-cross-linking irradiation. However, the photoswitching efficiency decreases gradually when the polymer solution is repeatedly exposed to alternating UV light within five cycles (Figure 8d). To avoid procedural errors in the DLS measurement via the ALV-5000/E dynamic light scattering instrument, the polymer solution (0.1 mg/mL  2 mL) was filled into a screw-cap cuvette and remained irradiated by alternating light of 365 and 254 nm to the desired photodimerization degree, then the hydrodynamic size of the polymer nanoaggregates in aqueous solution was determined by using a Malvern Zetasizer Nano-ZS instrument at room temperature. As shown in Figure 9a, the diameter of the polymer nanoaggregates increases gradually as a function of irradiation time and the diameter varies from 148 to 536 nm. Then, the photocross-linked polymer solution in the gastight cuvette was subsequently photo-de-cross-linked to the designed photodimerization degree by the irradiation of 254 nm light. As expected, the diameter of polymer nanoaggregates gradually decreases along with the 254 nm irradiation time from 536 to 194 nm. These reversible size changes upon alternating irradiation are very consist with the corresponding Rh distribution determined with the ALV-5000/E dynamic light scattering instrument, indicating the feasible sizeswitching of polymer nanoaggregates by the reversible photo(48) Chen, Y.; Wu, J. D. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1867.

Langmuir 2010, 26(17), 14247–14254

dimerization of coumarin end groups upon alternating irradiation at 365 and 254 nm. TEM images at high resolution have been the most powerful technique to study morphologies of polymeric nanoparticles. In this section, we report our TEM images of polymer nanoaggregates in the first irradiation cycle by means of negative staining techniques. The results of the morphological analysis performed by TEM are shown in Figure 10. It can be seen that the average diameter measured by TEM is a little smaller than that measured by light scattering at 25 °C. The discrepancy in size was attributed to the fact that DLS data directly reflect the dimension of polymer nanoaggregates in solution, where the hydrophilic chains are well dispersed in water, although one side of the hydrophilic chain is attached to the core of micelle. However, for TEM measurement, the micelle solution was deposited onto a copper grid coated with carbon film, where PNIPAAm chains shrank during the evaporation of water, which resulted in the smaller diameter.49-54 The shape of the polymer micelles before irradiation (Figure 10A) and at low photodimerization degree (17.50%, Figure 10B) was observed as approximately spherical shape, and the diameters of these micelles are in the range 30-50 nm as a dehydrated state. Further irradiation with 365 nm yielded hollow spheres. For example, when the photodimerization degree rose up to 44.24% (Figure 10C) and 52.98% (Figure 10D), their morphologies were observed as big vesicles with the small vesicles and micelles connected to the bilayer; and when the photodimerization degree rose to 63.41% (Figure 10E), all the polymer nanoaggregates fused into unilamellar vesicles. And the pictures of these vesicles show two types of domains: those appearing darker and those brighter. The dark domains correspond to the hydrophilic-rich bilayer domains of PNIPAAm chains stained with PTA; the brighter domains correspond to the inner pool not stained. Then upon the sequent 254 nm irradiation, the photoscission of coumarin dimers takes place to reform the telechelic structure of C-PNIPAAm, resulting in the reversion of amphiphilic balance and the morphology switching from vesicles to polymer micelles (49) Wei, H.; Zhang, X. Z.; Cheng, H.; Chen, W. Q.; Cheng, S. X.; Zhuo, R. X. J. Controlled Release 2006, 116, 266. (50) Soppimath, K. S.; Tan, D. C. W.; Yang, Y. Y. Adv. Mater. 2005, 17, 318. (51) Xue, Y. N.; Huang, Z. Z.; Zhang, J. T.; Liu, M.; Zhang, M.; Huang, S. W.; Zhuo, R. X. Polymer 2009, 50, 3706. (52) Zhu, J. L.; Zhang, X. Z.; Cheng, H.; Li, Y. Y.; Cheng, S. X.; Zhuo, R. X. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5354. (53) Ye, Q.; Zhang, Z. C.; Jia, H. T.; He, W. D.; Ge, X. W. J. Colloid Interface Sci. 2002, 253, 279. (54) Li, J. B.; Ren, J.; Cao, Y.; Yuan, W. Z. Polymer 2010, 51, 1301.

DOI: 10.1021/la102771h

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Figure 10. TEM images for the polymer nanoaggregates after being stained with PTA: Images of polymer aqueous solution irradiated by 365 nm. The photodimerization degrees of these samples were (A) 0%, (B) 17.50%, (C) 44.24%, (D) 52.98%, and (E) 63.41%. Then when the polymer solution was irradiated with 254 nm, the photodimerization degrees were (F) 58.10%, (G) 37.58% , and (H) 36.01%.

(Figure 10F-H). Furthermore, the radii of these micelles and the bilayer thickness of vesicles are around 25-30 nm, which doubles the length of the telechelic C-PNIPAAm, confirming the doubledlayered micelles in aqueous before irradiation.

4. Conclusions In conclusion, we have designed and synthesized a novel coumarin-ended telechelic C-PNIPAAm, whose micelle size can be reversibly photoswitched according to the reversible photo-cross-linking reaction of coumarin ends in hydrophobic cores upon alternating irradiation at 365 and 254 nm. This instant diameter-changing phenomenon was reproducible for at least five cycles, and the reason for this significant size change can be attributed to the induced amphiphilic structure transformation between the telechelic “hydrophobic end-hydrophilic chain” structure and the ABA type of “hydrophilic chain-hydrophobic center-hydrophilic chain” one. This reversible photoswitched mechanism of polymer micelles is of 14254 DOI: 10.1021/la102771h

fundamental interest not only for stimuli-controlled release of loaded guest molecules but also for other potential applications which require the reversible control of micelle morphologies. Acknowledgment. We acknowledge financial support from the National Nature Science Foundation of China (NSFC) (under Grant Nos. 20704017, 50973044, and 50673038), the Fundamental Research Funds for the Central Universities (JUSRP31003), the scientific & technological innovation team project of Jiangsu Province (2007-5), and the Qing Lan Project of Jiangsu Province. Supporting Information Available: MS spectrum of C-SS-C, 1H NMR and GPC spectra of C-PNIPAAm before and after photo-cross-linking upon 365 nm irradiation. UV-vis, fluorescence spectra, and DLS of polymer micelles in aqueous solution within five irradiation cycles. This material is available free of charge via the Internet at http://pubs.acs.org. Langmuir 2010, 26(17), 14247–14254