Significant Fluorescence Enhancement of Spiropyran in Colloidal

Article pubs.acs.org/JPCC

Significant Fluorescence Enhancement of Spiropyran in Colloidal Dispersion and Its Light-Induced Size Tunability for Release Control Yinan Xue,† Jintao Tian,*,† Weiguo Tian,‡,§ Peizhen Gong,† Jinhui Dai,† and Xin Wang† †

Institute of Materials Science and Engineering, Ocean University of China, Songling Road 238, Qingdao 266100, China Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100039, China ‡

Downloaded by GEORGETOWN UNIV on September 8, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.jpcc.5b06905

S Supporting Information *

ABSTRACT: The spiropyran-contained nanomicelles with excellent fluorescence emission in colloidal dispersion are fabricated by grafting spiropyran onto a PAA−PS diblock copolymer followed by self-assembling under ultraviolet irradiation in nonpolar solvent of toluene. The spiropyran species is embedded in interior high polar cores of the micelles that can promote the concentration of the merocyanine form of spiropyran. Meanwhile, the embedding of spiropyran inside the micelles produces effects of conformational constraint and uncontact to solvent. The synergy of the above three effects significantly enhances the fluorescence emission of the micelles in colloidal dispersion whereas in the case of the pure spiropyran it shows virtually no emission in toluene. With the aid of ultraviolet and daylight, the fluorescent micelles dramatically shows lightinduced size tunability and excellent recyclability, corresponding to the significant change in polarity of the copolymer chain that is caused by the reversible transformation between weak polar spiropyran and zwitterionic merocyanine. Loading and controlled release of rhodamine 6G as a typical polar drug substance is successfully achieved with the micelles. These experimental results may bring about more promising applications of spiropyran species beyond their photochromic properties in microscopic fields such as nanocarrier for drug delivery or micro detector for biosensor. on the fluorescence emission by 1 order of magnitude approximately. The proposed mechanism is the synergy of the following three different effects: the interior high-polar environment in the porous cellulose matrix that can promote the concentration of merocyanine form, the conformational constraint of cellulose cavities, and the elimination of solvent influence, which can sufficiently develop the quantum yield of MC. Although this research may bring about more promising applications of spiropyran species beyond their photochromic properties, the drawbacks of the material should not be ignored and need to be overcome. The material can only be used in a solid state for eliminating the solvent effect. Meanwhile, it is millimeter-sized and cannot be synthesized into nanoscale that is quite crucial for applications in microscopic fields. The reasons for bringing about these drawbacks are the use of cellulose, a natural macromolecule in millimeter-scale with large molecular weight, and the surface heterogeneous appending of the spiropyran on the cellulose molecular skeleton. To overcome these drawbacks, here we artificially synthesize block copolymers instead of cellulose to bear spiropyran and then self-assemble into micelles in nanoscale with enhanced fluorescence. Our purpose is not only to overcome the aforementioned drawbacks

1. INTRODUCTION Photochromic molecules undergo a color change on light irradiation that can reverse to the initial color by subsequent irradiation.1 As one of the most important organic photochromic compounds, weak-polar spiropyran (SP) can isomerize to zwitterionic merocyanine (MC) with strong polarity under ultraviolet (UV) light and revert to the SP form responding to visible light or darkness.2,3 With this reversible transformation, SP has been successfully used in several specific areas such as bioimaging,4−6 chemical sensor,7−9 controlled release,10−12 photocontrollable wettability,11,13,14 mechanic sensor,15 data storage,16 structure regulation of short peptides,17 DNA binding,18,19 etc. The polar MC has a molecular structure of conjugated system that is a typical structural characteristic for fluorescent material.20 However, in comparison with the blooming applications of its photochromic property, only a handful of researches have reported in the literature upon the fluorescence of SP due to its commonplace fluorescent performance.21−23 Thus, the fluorescence enhancement of SP is quite crucial for broadening its application beyond the photochromic property. In our previous study, an excellent fluorescent material was derived from spiropyran species.24 It was facilely fabricated by appending spiropyran onto a natural cellulose matrix via a covalent link of an ester carbonyl group. In contrast with other spiropyran materials, the obtained spiropyran/cellulose material showed significant enhancement © 2015 American Chemical Society

Received: July 17, 2015 Revised: August 18, 2015 Published: August 20, 2015 20762

DOI: 10.1021/acs.jpcc.5b06905 J. Phys. Chem. C 2015, 119, 20762−20772

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The Journal of Physical Chemistry C

Figure 1. Synthesis of the diblock copolymer of the PSPAA−PS (a) and its micellizaiton in toluene (b). The red core of the micelle in (b) is rather polar whereas the outer blue shell is nonpolar.

of PtBA-b-PS to get PAA−PS. Details for the synthesis of the PAA39-b-PS75 are given in the Supporting Information. 2.4. Synthesis of the PSPAA−PS Block Copolymers. SPOH (2.46 g, 7 mmol) and DMAP (0.576 g, 0.520 mmol) were dissolved in THF (35.00 mL). Then PAA39-b-PS75 (1.87 g, 0.1795 mmol) was added into the solution. The mixture was stirred until it was cooled to 0 °C by the ice-salt bath. DCC (1.260 g, 6.240 mmol) was dissolved in THF (15 mL), and the solution was then dropped into the above mixture slowly. The mixture was stirred for 2 h while the temperature remained below 0 °C. The mixture was stirred for 48 h at 25 °C. When the reaction finished, the mixture was filtered and the precipitate was washed by n-hexane, ethanol, and water in that order for removing the unreacted reagents. The concentration of the copolymer in solution was 0.3 g/L. 2.5. Characterization and Measurements. The synthesized products were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet iN10 IR microscope, Thermo Fisher) and nuclear magnetic resonance spectroscopy (1HNMR, Varian Mercury plus 600, 600 MHz) in CDCl 3 with tetramethylsilane (TMS) internal standard. The transmission electron microscopy (TEM) images of the self-assembly samples were obtained using a JEM-2100 (JEOL Ltd., Japan). The samples were prepared by dropping the micelle solution onto copper grids coated with a thin carbon film and then dried at room temperature for 24 h. The ultraviolet−visible (UV−vis) spectra were obtained via a UV-2550 spectrophotometer (Shimadzu, Japan). The solutions were equilibrated for 30 min before measurement, and the concentration of the polymer solution was 0.3 g/L. The fluorescence (FL) spectra were

but also to exploit possible applications of the new spiropyrancontained material in microscopic fields, for instance, lightinduced loading and controlled release of drug substance.

2. EXPERIMENTAL PROCEDURE 2.1. Raw Materials. MeCN (AR) was distilled over CaH2. 2,3,3-Trimethyl-3H-indole was synthesized on the basis of ref 25. Copper(I) chloride (CuCl) was purified on the basis of a ref 26. tert-Butyl acrylate (tBA, 99%) and styrene (St, 99%) were used after passing through an alumina column to remove inhibitor. Azobis(isobutyronitrile) (AIBN, AR) was recrystallized from methanol. 2-Bromoethanol (96%), carbon tetrabromide (CBr4, 98%), pentamethyldiethylenetriamine (PMDETA, 99%), trifluoroacetic acid (TFA, 99%), dicyclohexylcarbodiimide (DCC, 99%), 4-dimethylamiopryidine (DMAP, 99%), toluene (AR), dichloromethane (AR), methanol (AR), tetrahydrofuran (THF, AR), and n-hexane (AR) were used as received. 2.2. Synthesis of the 2-(3′,3′-Dimethyl-6-nitro-3′Hspiro[chromene-2,2′-indol]-1′-yl)ethanol. The SP compound containing a hydroxyl group (SPOH) was synthesized on the bais of ref 27. Detailed process as well as synthetic parameters can be found in the Supporting Information. 2.3. Synthesis of the Poly(acrylic acid-b-styrene) Amphiphilic Block Copolymers. The PAA39-b-PS75 block copolymers (hereafter referred to as PAA−PS) were synthesized via a three-step procedure28 as shown in Figure 1a: (i) radical telomerization of tBA with CBr4 to form PtBA-Br; (ii) atom transfer radical polymerization (ATRP) of St using PtBA-Br telomere as a macroinitiator to obtain PtBA-b-PS; (iii) hydrolysis 20763


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Figure 2. FTIR spectra of the PSPAA−PS copolymer. The spectra for the PAA−PS is also incorporated in this fiugre for comparison.

Figure 3. Dissolution of the PSPAA−PS diblock copolymer in toluene and its spectral features: (a) UV−vis spectra; (b) FL spectra. The spectra of the PAA−PS, the pure SP, and the PAA−PS+SP in toluene are also measured for comparison. The photographs from the colloidal samples in (a) and (b, under 365 nm UV) are also recorded with a digital camera and shown in this figure. The red laser is employed to check if there is a Tyndall effect or not. The PAA−PS+SP colloidal sample is prepared by directly dissolving a mechanical mixture of the PAA−PS and the pure SP into toluene. The SP concentration in all the colloidal samples is 1.134 × 10−4 M. All the measuments are conducted at room temperattue. The peaks around 480 nm in Figure 3b are from test machine.

delivered at a flow rate of 1 mL/min. The calibration curve was based on six narrow MW linear polystyrene standards ranging from 2930 to 22200 g/mol.

obtained using a Fluorolog 3-P spectrofluorometer (Jobin Yvon). Excitation light of 420 nm was applied to minimize its impact on photochromism during the measurements.29 The gel permeation chromatography (GPC) analysis for determination of the molecular weights (MWs) and the molecular weight distributions of the random copolymers were from a Waters 515 HPLC Pump (Waters, America). The mobile phase was THF,

3. RESULTS AND DISCUSSION 3.1. Synthesis of the PSPAA−PS and Its Dissolution in Nonpolar Solvent of Toluene. To achieve an enhanced 20764


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Figure 4. Significant fluorescence enhancement of PSPAA−PS in toluene after 365 nm UV irradiation for 3 min: (a) UV−vis spectra; (b) FL spectra.

fluorescence emission in colloidal dispersion, spiropyran molecules should be embedded within inner polar cores so that the synergetic effects of the polar environment, the conformational constraint, and the elimination of solvent come into being. Thus, the diblock copolymers of the PSPAA−PS are designed to contain both hydrophilic and hydrophobic segments so that they can self-assemble into micelles in nonpolar solvent, as shown in Figure 1b. The SP molecules are grafted onto the PAA segments via a covalent link of ester carbonyl group. Although the FTIR spectra in Figure 2a does not obviously distinguish SP from the PSPAA−PS, further analysis on the fine structure of the spectra gives a definite confirmation of SP having been successfully grafted onto the PAA skeleton by the following characteristic peaks of SP: (i) the CO ester carbonyl group at 1735 cm−1 in Figure 2b; (ii) the COC cyclic ether at 809 and 1270 cm−1 in Figure 2c; (iii) the CC at 1650 cm−1 and HCCH at 960 cm−1 in Figure 2d; (iv) the CN at 1339 cm−1 in Figure 2e. The 1H NMR spectrum (see data in Figure S9) reveals repeating unit numbers of 39 and 75 for the PAA and PS segments in the diblock copolymer, respectively. On average, five SP molecules are grafted onto the PAA segment in each PSPAA−PS molecule. Considering the steric hindrance of the SP molecule, the five SP molecules are spaced on the PAA segment by approximately seven polar carboxyl groups that can provide the SP molecules with a polar environment. The synthesized PSPAA−PS is dissolved into nonpolar solvent of toluene. To well understand its dissolution as well as the subsequent micellization under UV irradiation, not only the PAA−PS and the pure SP but also the PAA−PS+SP, a mechanical mixture of the PAA−PS and the pure SP, are dissolved in toluene to achieve a comparative study. In all these cases the concentration of the SP moclecules is kept at the same level so that they are comparable. The resultant spectral behaviors are shown in Figure 3. A red laser is employed to

check if there is a Tyndall effect or not that is a sign of micelle formation. After dissolving in the nonpolar solvent of toluene, the amphiphilic PAA−PS molecules form small micelles, as revealed by a visible red laser path in the photos in Figure 3a. Similar results are obtained for the PAA−PS+SP and the PSPAA−PS. The small absorption peak at 555 nm in Figure 3a comes from the MC form, suggesting that a small amount of SP in the PSPAA−PS has been transferred into MC even without UV irradiation. Such results are in good agreement with our previous study in which MC is thermodynamically more stable than SP in polar environment.30 No micelles are formed for SP in toluene due to its weak polarity. The PAA−PS solution does not show any fluorescence emission in Figure 3b due to the absence of the SP group. The pure SP solution shows very weak green emission at 530 nm (see the very small emission peak in Figure 3b), a sign of emission of the nonconjugated SP.29 This result suggests that a few SP molecules stay together in the solution, though its size is too small to detect out by the red laser. The PSPAA−PS solution shows not only a weak green emission of SP at 530 nm but also a red emission of MC at 605 nm, in good agreement with the result in Figure 3a where the MC peak is observed at 555 nm. The comparable green emission behavior of the PAA−PS+SP suggests that in this case some amount of the SP molecules have been enclosed into the inner polar cores of the PAA−PS micelles. Note that the PAA−PS+SP is a mechanical mixture and no chemical bond exists between the SP molecule and the PAA skeleton. So this result suggests that SP is neither polar nor nonpolar but its polarity stays in a lower middle level, as revealed by our calculation in which dipole moment values of 0.290, 4.995, and 32.694 D are obtained for toluene, SP, and MC (see calculation of dipole moment values in Supporting Information). 3.2. Fluorescence Enhancement of the PSPAA−PS in Colloidal Dispersion. Although fluorescence emission has 20765


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Figure 5. Comparative study of the fluorescent behavior of PSPAA−PS in nonpolar solvent of toluene and polar solvent of ethanol: (a) UV−vis spectra; (b) FL spectra.

Figure 6. TEM images of the PSPAA−PS micelles. Before loading onto a copper grid, the colloidal sample is irradiated with a 365 nm UV lamp for 3 min.

Scheme 1. Graphical Illustrations of the Dissolution and Micellization of PAA−PS, Pure SP, PAA−PS+SP, and PSPAA−PS in Toluene without and with 365 nm UV Irradiationa

a

The Tyndall result is summarized in words below each drawing. The emission wavelength and the measured FL intensity from Figured 3 and 4 are also given in parentheses below each drawing. They correspond to particle formation (gray particle, no SP inside; green particle, SP inside; red particle, MC inside). The molecules of the solvent toluene are omitted for simiplification.

irradiation is applied to the PSPAA−PS solution to get more fluorescent MC. The results are shown in Figure 4. As irradiated with 365 nm UV for 3 min, the PAA−PS shows no

been observed in Figure 3b for the PSPAA−PS in toluene, the emission intensity is rather weak due to the nonconjugation nature of SP whereas the conjugated MC is very little. Thus, UV 20766


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Figure 7. UV−vis absorption behaviors of PSPAA−PS in toluene: (a) Abs. for the coloration process; (b) derived Abs. and wavelength from the absorption peak in (a); (c) Abs. for the discoloration process; (d) derived Abs. and wavelength from the absorption peak in (c). The results for the pure SP dissolved in toluene are also incorporated in (b) and (d) for comparsion. For the coloration process the colloidal sample is irradiated with a 365 nm UV lamp whereas in the case of the discoloration process it is under a white LED light.

The successful fluorescent enhancement of the PSPAA−PS in colloidal dispersion after UV irradiation comes from the transformation of SP to MC, a process accompanied by a significant increase of polarity (dipole moment value from 4.995 to 32.694 D) that can affect the dissolution and the subsequent micellization of the PSPAA−PS in toluene. A graphical illustration is proposed as Scheme 1 to demonstrate those results in Figures 3 and 4. The amphiphilic PAA−PS forms micelles in nonpolar toluene with no emission, regardless of UV irradiation (Scheme 1a,b). The pure SP molecules almost distribute uniformly in toluene (Scheme 1c), and their photochromism to the MC under UV is hindered to some extent by the nonpolar toluene molecules, leading to negligible emission (Scheme 1d). The weak polarity of SP in the PAA−PS +SP tends to stay in the inner polar cores of the PAA−PS micelles and, consequently, a weak green emission at 530 nm (Scheme 1e). The UV irradiation promotes its photochromism to the MC and another weak red emission is observed at 625 nm (Scheme 1f). The polar environment in the PSPAA−PS micelles is preferable for the formation of the MC, leading to not only a weak green emission peak of SP at 530 nm but also a small shoulder of the MC at 605 nm even without UV irradiation (Scheme 1g). The subsequent UV irradiation transforms almost all the SP molecules into the MC and strong red emission is obtained at 660 nm (Scheme 1h). 3.3. Photochromic and Emissive Kinetics of the PSPAA−PS in Toluene. Because the strong emission of the PSPAA−PS in Figure 4b comes from the transformation of SP to

photochromism. The pure SP in nonpolar toluene has a faint photochromic effect with a small MC absorption peak at 550− 600 nm in Figure 4a due to the nonpolar surrounding. In the case of the PAA−PS+SP, the polar carboxyl groups on the PAA−PS skeleton contribute to some extent to the photochromic process. Other than those above, a very strong MC absorption peak is observed at 570 nm for the PSPAA−PS. The bright laser path in this case in Figure 4a reveals the formation of micelles. Consequently, significant enhancement on the fluorescent intensity is achieved for the PSPAA−PS in toluene whereas the pure SP in toluene shows virtually no emission, as shown in Figure 4b. Thus, we have successfully obtained spiropyrancontained fluorescent material in colloidal dispersion. As pointed out previously, SP should be embedded within inner polar cores so that fluorescence enhancement can be achieved. It is worthy to verify how it goes if the SP molecules stay outside. Thus, a polar solvent of ethanol is employed for a comparative study and the results are shown in Figure 5. Only very weak emission can be observed for the PSPAA−PS in ethanol even after 3 min irradiation with 365 nm UV. It is clear that in this case the polar environment is satisfied for the SP molecules due to the polar solvent but the conformational constraint and the elimination of solvent are unavailable. This result further confirms our proposal that the synergy of the three effects is quite crucial for the fluorescence enhancement of spiropyran. The TEM observation in Figure 6 shows a size of 50−80 nm for the PSPAA−PS micelles in toluene after 3 min UV irradiation. 20767


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Figure 8. Fluoresence behaviors of the PSPAA−PS in tolunene: (a), (c), (e) FL spectra for the coloration and discoloration processes; (b), (d), (f) plots of the derived FL intensity (■, □) and wavelength (●, ○) against time from the emission peak in (a), (c), and (e). The emissions from SP and MC are separately plotted in (b), (d), and (f). The treatment conditions of the collodial samples for the coloration and discoloration processes in this figure are the same as those in Figure 7.

shown in Figure 7c,d. The colored PSPAA−PS can be well reversed to colorless within 30 min under a white LED light. The discoloration can also be achieved in dark, though it takes more time. The emission kinetics of the PSPAA−PS is shown in Figure 8. The FL intensity of the PSPAA−PS in Figure 8b almost reaches its maximum value within 60 s, much different from the Abs. behavior in Figure 7b where an ongoing increase on the Abs. value is observed for the PSPAA−PS in the whole irradiation time up to 300 s. Such a difference might be due to the fact that the fluorescence emission of SP is related not only to the polar environment but also to the conformational constraint and the solvent elimination. The colored SP in the PSPAA−PS molecules after 60 s UV may not be able to well reach the

the MC under UV, it is worthy to clarify its kinetics of the process against UV irradiation time. The results are shown in Figure 7. The results of the pure SP are also shown in this figure for comparison. With a 365 nm UV irradiation, the coloration of the pure SP in toluene is seriously restricted by the nonpolar molecules of toluene, giving a very low Abs. even up to 5 min. On the contrary, the high polar surrounding provided by carboxyl groups greatly promotes its coloration of the PSPAA−PS with a very large absorbance in Figure 7b. After UV irradiation, in comparison with the pure SP, a notable blue shift (∼10 nm) of the peak position is observed for the PSPAA−PS. This result is in good agreement with our previous study in which a continuous blue shift was observed with the increase of the molecule polarity surrounding the spiropyran.30 The discoloration process is 20768


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UV and its good reverse under daylight mostly suggest a continuous size increase and decrease. In other words, the size of the PSPAA−PS micelles is light-induced tunable and this process is reversible. The above light-induced size tunableness of the PSPAA−PS micelles in toluene is believed to be related to a dynamic balance between freely distributed and aggregated PSPAA−PS molecules. A graphical illustration is proposed as Scheme 2. The dissolution and micellization behavior of the PSPAA−PS molecules in toluene is governed by its polarity of the copolymer chain. The polarity of SP is weak. The grafting of SP onto the PAA segment decreases its polarity to some extent. Thus, most of the PSPAA−PS molecules tend to distribute freely in toluene (Scheme 2a), compared to that of the PAA−PS in toluene. The UV irradiation transforms the weak polar SP to strong polar MC, leading to significant increase of the polarity of the PAA segment. The resultant amphiphilic PSPAA−PS molecules are aggregated together to form micelles (Scheme 2b). Further UV irradiation promotes this process, leading to formation of micelles with large size (Scheme 2c). Because the photochromism of SP to MC is reversible, the size decrease in daylight is feasible. The above research reveals a significant change of the PSPAA−PS not only on its UV−vis and FL spectra but also on the size of the micelles in toluene. A good reversibility has been observed from Figures 7−9. It is also worthy to investigate the recyclability of this change. The results are shown in Figure 10. As seen, both the UV−vis and FL intensity measurements reveal a good recyclability for the PSPAA−PS in toluene. It should be noted that the PSPAA−PS has a long molecule chain (totally chain units of 114), compared to pure SP. The fluorescence behavior of the PSPAA−PS in toluene is related to its aggregation and disaggregation that is achieved by molecule diffusion. Because molecules with long chain are not in favor of frequent aggregation and disaggregation, the slight decrease of the FL intensity in Figure 10b with the increase of cycle number seems to be reasonable. 3.5. Loading and Release Control of Drug Substance. As described above, the size tunability of the PSPAA−PS micelles in toluene is due to the significant change on polarity of the PAA segment in the PSPAA−PS molecule. The formed inner cores of the micelles are rather polar whereas the outer shells are nonpolar. It is easy to image that if a polar drug substance is available during the formation process of the micelles, it can be embedded into the polar inner cores of the micelles in the aid of UV. The loaded polar substance can also be released out to some extent by disassembly of the micelles in daylight or in darkness. Thus, a light-induced loading and release control of polar drug substance can be achieved with the PSPAA−PS micelles. In fact, such expectation has been proved to be feasible by the results shown in Figure 3 and 4. To that of the PSPAA−PS, the comparable green emission of the PAA−PS+SP in Figure 3b demonstrates a fact that some amount of the free distributed weak polar SP molecules in the mechanically mixed PAA−PS+SP have been enclosed into the inner polar cores of the PAA−PS micelles even without UV irradiation. The subsequent UV irradiation promotes this loading process, giving visible fluorescence emission (see the photo in Figure 4b). This loading process has been graphically illustrated in Scheme 1e,f. To further clarify this loading and release process with the PSPAA−PS micelles, rhodamine 6G (R6G) is selected as a typical polar drug substance and some experiments are conducted. The results are shown in Figure 11. Before dealing with these results, two facts should be fully aware in mind. One is

inner cores of the micelles and thus contributes less to the emission due to the absence of the conformational constraint and poor elimination of the solvent. The wavelength trend of the emission peak in Figure 8b is similar to that of the FL intensity. A notable red shift appears from 530 to 660 nm before 60 s, corresponding to the transformation of SP to the MC. Note that both SP and the MC can emit if they aggregate together to form particles, as illustrated in Scheme 1. The emission kinetics of SP in Figure 8b goes well as opposed to those of MC. This implies an occurrence of a process in which the PSPAA−PS particles dissolve and the PMCAA−PS particles form under UV irradiation. Panels c and e of Figure 8 reveal good reversibility on emission of the PSPAA−PS either under a white LED light or in dark. One interesting thing in Figure 8 is that a bimodal characteristic in peak shape has been definitely observed for some emission curves, for instance the coloration curves with UV irradiation time of 10, 20, 30, 40 s in Figure 8a and the discoloration curves in dark for 5, 7, 10, 15, 20, and 30 min in Figure 8c. Although a sound explanation on this reslult is unavaiable at present, an assumption may be reasonable in which the dissolution and reaggregation of the PSPAA−PS micelles are quite crucial. 3.4. Light-Induced Size Tunability of the PSPAA−PS Micelles. The photochromism of the PSPAA−PS is affected by the polar environment whereas in the case of the emission both the conformational constraint and the solvent should be involved in addition. The kinetic behaviors on both absorption and emission in Figures 7 and 8 imply a continuous change of the PSPAA−PS in toluene. This change can be, as expected, either the size of the PSPAA−PS micelles or its concentration. To clarify this expectation, a series of Tyndall experiments is conducted and the results are shown in Figure 9. With an

Figure 9. Photographs of the PSPAA−PS in toluene for its coloration and discoloration processes. The red laser is employed to check if there is a Tyndall effect or not.

extension of UV irradiation, the red laser path becoms brighter and brighter. The subsequent storage in daylight gives a good reverse by a weaker and weaker light path. Note that for a colloidal system the brightness of the light path in a Tyndall experiment is more sensitive to the size of particles than to its concentration (the intensity of the scattering light is proportional to the size of particles in six power whereas in the case of the concentration it is only in one power from the famous Rayleigh scattering equation). Thus, the change of the red laser path under 20769


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The Journal of Physical Chemistry C Scheme 2. Graphical Illustrations of the Light-Induced Size Tunableness of the PSPAA−PS Micelles in Toluenea


a

The molecules of the solvent toluene are omitted in this figure for simiplification.

Figure 10. Recyclability of the PSPAA−PS micelles in toluene. The colloidal sample is prepared and kept in the dark for 1 h before measurents. For the coloration process, the sample is irradiated with a 365 nm UV lamp for 3 min. In the case of the discoloration process, the sample is kept under a white LED light for 1 h.

remarkable discoloration within 48 h. The disassembly of the formed micelles in this case, however, does not work well (see the strong laser path for the 48 h sample in Figure 11). Most of R6G is yet to release. By applying another interval treatment with time as long as 8 days (12 h under LED light + 12 h in dark, totally 8 days), the formed micelles have been well disassembled (see the weak laser path for the 8 days sample in Figure 11), and the loaded R6G is fully released. It is clear that the release rate of R6G can be well controlled by applying different external lighting condition. In comparison to those disassembled within 30 min in Figure 9, long time release is quite meaningful when the micelles are used as a sustained-release system.

that R6G dissolves very little in toluene due to its strong polarity (dipole moment value of 16.748 D). The other is that the formed PSPAA−PS micelles are too soft to be isolated from the colloidal samples by centrifugation. Thus, the loading of R6G with the PSPAA−PS in toluene can only be performed via a solubilization process. With these two facts, the R6G saturated toluene solution shows a very small absorption peak at 511 nm in Figure 11. The addition of the PSPAA−PS diblock copolymer into this saturated solution would increase the solubility of R6G due to the presence of the polar PAA segments. The subsequent long time diffusion confirms this result, as revealed by the continuous increase of the absorption peak at 511 nm in Figure 11 (see curves labeled with red arrow). Meanwhile, conversely, the solubilized R6G promotes the transformation of the SP into the MC form to some extent with an increasing absorption peak at 536 nm, leading to the formation of small micelles (see the weak laser path in the photo for 16 h sample). Thus, the solubilized R6G molecules, as expected, stay close to the polar PAA segments or even a few of them are inside the small micelles. The subsequent continuous UV irradiation (10−30 min) or interval treatment (5 min under UV + 25 min in dark, totally 1−6 h) lead to significant coloration and micelle formation (see the curves and photos labeled with blue and green arrows in Figure 11). Here the interval treatment is applied not only to reduce the decomposition of organic molecules from long time UV irradiation but also to promote diffusion of R6G during the loading process. Note that no R6G precipitates from the colloidal samples during this process (the polarity of the MC form is much higher than the SP structure). Thus, the solubilized R6G, as expected, has been successfully loaded into the PSPAA−PS micelles. The recovery experiments in Figure 11 show

4. CONCLUSIONS The spiropyran-contained diblock copolymer of the PSPAA−PS is fabricated by grafting the SP molecules onto a PAA−PS copolymer chain. The dissolution of the PSPAA−PS in nonpolar solvent of toluene shows weak green fluorescence emission of SP. The subsequent UV irradiation leads to self-assembling of the PSPAA−PS molecules into nanomicelles in toluene, corresponding to the significant increase on polarity of the copolymer chain caused by the transformation from the weak polar SP to the zwitterionic MC. During this process SP was embedded in interior high polar cores that can promote the concentration of the merocyanine form. Meanwhile, the embedding of SP inside the micelles produces effects of conformational constraint and uncontact to solvent. The synergy of the above three effects significantly enhances the fluorescence emission ability of the micelles in colloidal dispersion whereas in the case of the pure spiropyran it shows virtually no emission in toluene. With the aid of UV and daylight, the PSPAA−PS shows reversible changes on 20770


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Figure 11. Loading and controlled release of R6G with the PSPAA−PS micelles in toluene. The details for the five experiments shown in this figure are as follows. (1) R6G: the R6G saturated toluene solution. R6G+PSPAA−PS: the above solution is dissolved with the PSPAA−PS. (2) The obtained solution is kept in dark for several hours (1−16 h) to ahcieve R6G solubilization in toluene. (3) Subsequent loading of solubilized R6G in the PSPAA− PS micelles under UV with different irradiation times (10−30 min). (4) Subsequent loading of solubilized R6G in the PSPAA−PS micelles with interval treatment (5 min under UV + 25 min in dark) for several hours (1−6 h). (5) Release of R6G from the PSPAA−PS micelles in dark after loading. For UV−vis measuremnts, the solution samples in (1) are directly measured. The samples in (2) are first centrifuged for 30 min (4000 rpm), and the supernatant is used for UV−vis measurements. In the cases of the samples in (3)−(5), the time for centrifugation is shortened to be 15 min. The concentration of the PSPAA−PS in all the smaples is the same as those given in other figures. Typcial photographs are taken from the colloidal samples in (1)−(5) to show coloration and micelles formation.

photochromism and fluorescence emission. The photochromism of the PSPAA−PS is affected by the polar environment whereas in the case of the emission both the conformational constraint and the solvent should be involved in addition. The formed PSPAA−PS micelles show light-induced size tunability and excellent recyclability due to the reversible transformation between SP and MC. One of the applications from the size tunability of the PSPAA−PS micelles is loading and release control of drug substance, as revealed by the results from rhodamine 6G. Thus, our results in this study may bring about more promising applications of spiropyran species beyond their photochromic properties especially in microscopic fields such as nanocarriers for drug delivery or microdetectors for biosensors.

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ACKNOWLEDGMENTS

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REFERENCES

This work is financially supported by the Natural Science Foundation of Shandong Province of China (ZR2013EMM017).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06905. Synthesis of SP containing a hydroxyl group, synthesis of the poly(acrylic acid-b-styrene) amphiphilic block copolymers, FTIR and 1H NMR spectra, dipole moment calculation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J. Tian. Tel: +86-532-66781690. Fax: +86-532-66781320. Email: [email protected]. Notes

The authors declare no competing financial interest. 20771


Article


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Significant Fluorescence Enhancement of Spiropyran in Colloidal

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