Dual Stimuli-Responsive Nucleobase-Functionalized Polymeric

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Dual Stimuli-Responsive Nucleobase-Functionalized Polymeric Systems as Efficient Tools for Manipulating Micellar Self-Assembly Behavior Belete Tewabe Gebeyehu,† Shan-You Huang,† Ai-Wei Lee,∥ Jem-Kun Chen,‡ Juin-Yih Lai,†,# Duu-Jong Lee,§,⊥,# and Chih-Chia Cheng*,† †

Graduate Institute of Applied Science and Technology, ‡Department of Materials Science and Engineering, and §Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ∥ Department of Anatomy and Cell Biology, School of Medicine, College of Medicine, Taipei Medical University, Taipei 11031, Taiwan ⊥ Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan # R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan 32043, Taiwan S Supporting Information *

ABSTRACT: Environmental stimuli-responsive nucleobase-functionalized supramolecular polymers, a combination of oligomeric polypropylene glycol segments as a thermosensitive element and hydrogen-bonded uracil as a photosensitive moiety, were successfully developed and undergo spontaneous self-assembly to form uniform nanosized micelles via self-complementary double hydrogen bonding interactions between the uracil moieties in an aqueous environment. These micelles exhibit unique properties such as dual thermo- and photoresponsiveness, controllable lower critical concentration solution temperature (LCST), photoreactivity, and morphological transformation, making them highly attractive for various applications. More importantly, phase transitions and morphological studies confirmed the LCST behavior, size, and shape of the micelles can be easily tuned by adjusting the concentration and duration of ultraviolet irradiation of samples in aqueous solution, indicating introduction of uracil molecules into a water-soluble polymer matrix may represent a promising approach toward development of multiple stimuliresponsive polymeric micelles whose self-assembly behavior can be manipulated. In view of the ease of fabrication, high biocompatibility, multifunctionality, and tailorable micellar properties, this newly developed supramolecular micelle may be a promising candidate nanocarrier for controlled drug delivery and bioimaging systems.

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nanocarriers but is synthetically challenging.10,11 In recent years, advances in nanotechnology have enabled the design of nanoscale polymeric micelles, providing powerful tools to enhance specificity, increase drug circulation times, minimize drug degradation, improve efficiency, and reduce side effects in biomedical applications.4,6 Despite the challenges related to synthesis, mimicking natural features is considered a very important future direction in the design of multiresponsive micelles with precisely tunable characteristics.5,12,13 Dual- or multiresponsive micelles can be designed by incorporating two or more types of stimuli-reactive moieties into the polymer structure. Recently, the syntheses of dualresponsive polymeric micelles that respond to thermal and light

INTRODUCTION

Stimuli-responsive polymeric micelles have attracted significant interest due to their potential applications in a variety of areas including nanotechnology, drug delivery, and gene transport.1−3 Over the past few decades, numerous stimuli-sensitive polymeric micelles with the ability to respond to external or internal triggers, such as temperature, light, pH, ultrasound, and redox potential, have been reported.4−7 Most existing stimuliresponsive polymeric micelles respond to a single stimulus;3,8,9 however, changes in the behavior of natural polymeric micelles (such as protein, hyaluronic acid, pullulan, and dextran) are often the result of a combination of multiple environmental changes.4 Therefore, multistimuli-responsive polymers could provide an ideal technology to mimic biological processes by adapting to multiple environmental changes. The synthesis of multistimuli-responsive polymers that respond to two or more stimuli is an emerging area of interest in the field of smart © XXXX American Chemical Society

Received: December 13, 2017 Revised: January 20, 2018

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DOI: 10.1021/acs.macromol.7b02637 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Chemical Structure of Uracil-Functionalized BU-PPG; (b) Structural Representation of Self-Complementary Hydrogen-Bonding Dimers within the BU-PPG System; (c) Schematic Illustration of the Formation of Spherical and Photoirradiated Micelles by the Temperature- and Light-Responsive BU-PPG Polymer in Aqueous Solution

stimuli have been reported by several groups.14−16 The design of dual-responsive polymeric micelles emphasizes tuning the lower critical concentration solution temperature (LCST) of the polymer by attaching photosensitive moieties; micelles incorporating coumarin,17 spiropyran,18 thymine,19 azobenzene, or salicylideneaniline14,15 have been investigated for a broad field of applications. However, despite the fact significant progress has been made in developing temperature- and lightsensitive polymeric micelles, several limitations remain including poor solubility in water, precise control of size, and a lack of sensitivity to rapidly changing stimuli.4,16 More efficient strategies are needed to address these issues. In this context, the emergence of stimuli-responsive supramolecular polymers has opened the door to the creation of a new generation of polymeric micelles that are more sensitive and effective than traditional micelles.2,5,20 Owing to the presence of strong noncovalent interactions within the polymer matrix, functional supramolecular polymers can spontaneously self-assemble into well-defined uniform micelles of the desired sizes with unique physical properties in aqueous solution.21 Crucially, this dynamic supramolecular micelle behavior may confer the ability to respond and dynamically react to the environment and could provide a strategy to mimic the responsiveness of living systems. We previously reported a number of stimuli-responsive supramolecular micelles synthesized from difunctional oligomers and nucleobases.22−25 These materials easily selfassembled into nanosized micelles in aqueous solution and possess a number of unique properties, such as temperature and/or pH responsiveness, high drug-loading capacity, and controlled drug/dye release kinetics. For example, we designed a dual pH/temperature-sensitive micelle incorporating selfcomplementary sextuple hydrogen bonds; this micelle was based on water-soluble thermosensitive polymers that enabled

controlled-release delivery of anticancer drugs. The drug-loaded micelles were stable under normal physiological conditions and could be efficiently triggered to release their cargoes in a slightly lower pH and higher temperature environment.26 These findings shed some light on the development of stimuliresponsive supramolecular micelles by functionalizing watersoluble polymers through incorporation of nucleobases. However, this area of research is still in its infancy and issues related to multiresponsiveness, morphological transformations, and general applicability remain to be solved.27,28 Our previous studies motivated us to design a new dualresponsive supramolecular polymer (BU-PPG) based on uracil and oligomeric polypropylene glycol (PPG) via a simple onestep Michael addition reaction. BU-PPG could easily selfassemble to form spherical-like, photoresponsive micelles in aqueous solution, in which the uracil moieties play a significant role owing to the presence of multiple self-complementary hydrogen-bonding interactions, as illustrated in Scheme 1. This newly developed material exhibits unique properties including dual responsiveness, controllable LCST, photoreactivity, and the ability to undergo morphological transformation, which make these micelles highly attractive as efficient multifunctional materials for various practical applications. In addition, the size, shape, and LCST values of the micelles could be easily tuned by controlling the concentration of the solution and/or the degree of photodimerization. To the best of our knowledge, incorporation of uracil as a photosensitive moiety into thermosensitive polymers with the aim of generating a novel family of dual temperature- and light-responsive supramolecular micelles has not previously been reported. Thus, the approach described in this study represents a new development in the application of supramolecular chemistry to create smart multifunctional supramolecular micelles for biomedical applications. B


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Figure 1. UV−vis spectra of (a) uracil and (b) BU-PPG (0.03 mg/mL in aqueous solution). Plots of photodimerization (c) efficiency and (d) kinetics for uracil and BU-PPG in aqueous solution.

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EXPERIMENTAL SECTION

respectively, and disappearance of the absorbance bands at 1640 cm−1 due to complete reaction of PPG diacrylate and uracil. Furthermore, the presence of the broad peak above 3100−3400 cm−1 for the amide group of uracil due to hydrogen bonding proved successful functionalization of PPG diacrylate with uracil. The number-average molecular weight (Mn) determined by GPC (Figure S4) was approximately 1400 g/mol, with a polydispersity index of 1.18, indicating a narrow and monomodal molecular weight distribution. Taken together, 1 H NMR, 13C NMR, FTIR, and GPC proved BU-PPG was successfully synthesized at high yield via a highly efficient Michael addition reaction. Next, we measured the critical micelle concentration (CMC) in aqueous solution by a fluorescence probe technique using pyrene as the fluorescent dye.24,25 Micelles formed from BU-PPG in aqueous solution had a CMC of 1.9 × 10−3 mg/mL (Figures S5 and S6), suggesting the introduction of uracil moieties into PPG altered the self-assembly behavior and structural conformation of the PPG backbone in aqueous solution. In addition, we further evaluated the cytotoxicity of BU-PPG toward Meng-1 oral epidermoid carcinoma cells (OECM-1). The MTT assay confirmed the supramolecular polymer was not cytotoxic (Figure S7). Therefore, these results indicate BU-PPG may represent a promising nanocarrier for controlled drug delivery applications. Photoreactive Supramolecular Assembly: Photodimerization of Uracil-Functionalized Supramolecular Micelles in Aqueous Solution. Uracil, a RNA nucleobase, is highly sensitive to light and can undergo [2π + 2π] cycloaddition reaction upon exposure to ultraviolet (UV) light, leading to formation of cyclobutane pyrimidine dimers (Scheme 1 and Scheme S1b).29 Since uracil is photosensitive,

BU-PPG was synthesized via a one-step Michael addition reaction between uracil and PPG diacrylate under base-catalyzed conditions. Chemicals, instrumentation, characterization, and detailed synthetic procedures are described in detail in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis of Dual Temperature- and Light-Responsive Supramolecular Micelles. As illustrated in Scheme S1a, the strategy for synthesizing the uracil end-capped difunctional telechelic supramolecular polymer (BU-PPG) was one-step Michael addition of oligomeric PPG diacrylate (average molecular weight, 800 g/mol; degree of polymerization, 14) with uracil to produce high yields (up to 96%) of BU-PPG, which endows self-complementary double hydrogen bonding motifs at end groups of the polymer chains (Schemes 1a and 1b). Interestingly, the resulting BU-PPG behaves as a yellow viscous liquid that dissolves easily in water, even at a concentration up to 20 mg/mL. The chemical structure of BU-PPG was confirmed by proton-1 and carbon-13 nuclear magnetic resonance spectroscopy (1H NMR and 13C NMR; see Supporting Information, Figures S1 and S2), Fourier-transform infrared spectroscopy (FTIR; Figure S3), and gel permeation chromatography (GPC; Figure S4). Figure S1 shows the 1H NMR spectrum of BU-PPG in deuterated oxide. The appearance of characteristic signals at 7.4 and 5.7 ppm due to the C5C6 protons of the uracil ring confirmed successful incorporation of uracil into PPG diacrylate. Furthermore, the 13 C NMR signals at 101.1, 143.2, 148.1, and 152.0 ppm proved the existence of the uracil terminus (Figure S2). In addition, the FTIR spectrum of BU-PPG (Figure S3) revealed two new carbonyl peaks at 1683 and 1727 cm−1, corresponding to the carbonyl groups of the urea moiety and ester linkage, C


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Figure 2. (a) Transmittance change as a function of temperature for different concentrations of BU-PPG in aqueous solution. (b) Plot of LCST values versus concentration of aqueous BU-PPG solution. (c) Reversibility of the temperature-dependent optical transmittance of 4 mg/mL BU-PPG solution at 500 nm during heating (black line) and cooling (red line). (d) Thermoreversible LCST behavior: photographs of vials containing BUPPG solutions below (left) and above the LCST (right).

segment within BU-PPG micelles significantly enhances the rate of photoreaction between the photosensitive uracil units in aqueous solution. In order to understand the photoreactivity kinetics of BUPPG and free uracil in aqueous solution, photodimerization of uracil moieties was further investigated (Figure 1c,d) and calculated as the change in absorbance at maximum wavelength using12 conversion (%) = (1 − At/A0) × 100, where A0 is absorbance before irradiation and At is absorbance after UV irradiation for different periods of time. Surprisingly, after 30 min of UV exposure, as much as 88% of BU-PPG was photochemically converted at 25 °C; this value was significantly higher than that of free uracil (55%) under the same conditions (Figure 1c). Further increasing the irradiation time from 30 to 60 min gradually increased the conversion values from 88 to 96%, suggesting the reduction in absorbance induced by formation of large amounts of cyclobutane uracil dimers may be attributed to the presence of the micellar structure in aqueous solution, which strongly confines the uracil moieties within the micellar matrix and gradually reduces the intensity of absorbance, resulting in a significantly enhanced photoreaction rate. Furthermore, the rate constants for the photodimerization of BU-PPG and free uracil were 0.049 and 0.034 min−1, respectively, clearly indicating the rate of photodimerization for BU-PPG was approximately 1.5 times faster than that of free uracil (Figure 1d). This finding may be attributed to the enhanced aqueous solubility of BU-PPG due to the presence of the PPG spacer in its structure.32,35 In addition, these results further demonstrate the critical role of uracil in regulating the extent of photopolymerization of BU-PPG; thus, it can be more

we reasonably speculated that introducing uracil photochromic moieties into the thermosensitive segment may generate dual temperature- and light-responsive materials. Our initial ultraviolet−visible (UV−vis) experiments indicated the maximum absorbance peak of BU-PPG in aqueous solution shifted to slightly higher wavelengths (from 260 to 265 nm) compared to free uracil as a reference compound (Figure S8). Inspired by this result, we compared the photoreactivity of BU-PPG micelles and uracil. The well-characterized [2π + 2π]cycloaddition reactions can be controlled by UV−vis spectroscopy.30,31 This method has been used to monitor the characteristics of the absorbance of the π−π* transitions of the CC double bond in uracil and the corresponding cyclobutane photoproduct upon exposure to irradiation with UV light.30,32 As a result, we used UV−vis spectroscopy to monitor the photoreactivity of BU-PPG and uracil in aqueous solution. After UV irradiation (254 nm, 50−70 mW/cm2), the intensity of the characteristic absorption peaks of BU-PPG at 265 nm gradually decreased and then leveled off as the duration of UV irradiation increased, suggesting near completion of the [2π + 2π] cycloaddition photoreaction31,33 within 45 min (Figure 1b). Uracil exhibited a similar trend, although a longer period of irradiation was required to decrease absorbance at 260 nm (Figure 1a), and a significant peak was still observed after 60 min irradiation. Clearly, as shown in Figures 1a and 1b, the decrease in absorbance is an indication of the photodimerization reaction due to the pyrimidine ring, while reaction of the 5,6-double bond in the dimer gives rise to a cyclobutane unit, which significantly decreases absorption.31,34 In addition, these observations further suggest the presence of the PPG D


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Figure 3. (a) Graphic representation of structural transformation due to photochemical cycloaddition of BU-PPG. (b) Plot of photodimerization efficiency for 4 mg/mL BU-PPG in aqueous in solution. (c) Temperature-dependent photodimerization (indicated by transmittance at 500 nm) for BU-PPG micelles (at 4 mg/mL). (d) Effect of duration of photoirradiation on the LCST of BU-PPG micelles.

enhanced interactions between water molecules and polar uracil groups.37 In addition, Figure 2b illustrates a negative linear relationship between concentration and LCST for BU-PPG micelles, similar to the results reported in our previous studies.22−25 Thus, this unique feature suggests that adjustment of the LCST values could be easily achieved in a highly controlled manner by only altering the concentration of BUPPG in aqueous solution.8 To further explore the reversibility of the structural phase transition, aqueous 4.0 mg/mL BU-PPG solution was cycled between 10 and 70 °C in a UV−vis spectrometer. As shown in Figures 2c and 2d, BU-PPG maintained a reversible temperature-dependent response and transmittance reverted back perfectly to the original values as temperature decreased, i.e., a macroscopic phenomenon reflecting the connection between hydrophilic/hydrophobic properties. Therefore, this newly developed micelle could represent a well-controlled multifunctional nanocarrier with desirable photoresponsibility, tunable LCST behavior, and stable thermoreversibility. Since the aim of this study was to manipulate the LCST behavior of photosensitive BU-PPG in aqueous solution via photoirradiation, we assessed the cloud point of various concentrations of aqueous BU-PPG solution. We chose 4 mg/mL to further determine the cloud point after different durations of UV irradiation; this was the minimum concentration with a moderate LCST value that underwent complete phase transition from 100% transparent at the lower temperature to opaque phase (nearly 0% transparent) above body temperature (see Figures 2a and 2c). The cloud points were assessed before and after different durations of UV irradiation (λ = 254 nm). The LCST values gradually decreased

easily photodimerized upon UV exposure, and the degree of photodimerization can be easily altered by monitoring the duration of irradiation. Temperature-Induced Switchable/Reversible Phase Transitions of Supramolecular Micelles in Aqueous Solution. Oligomeric PPG is a water-soluble thermoresponsive polymer that undergoes a phase transition from a hydrophilic to hydrophobic state when the solution temperature increases above the LCST.36 To further discuss the effects of the interactions between uracil moieties on the LCST transition of PPG, we monitored the optical transmission of different concentrations of BU-PPG (1.0−5.0 mg/mL) in aqueous solutions at 500 nm. Aqueous BU-PPG solution exhibited a transparent−opaque phase transition during heating and cooling (Figure 2). The solution became cloudy above the LCST, indicating dehydration and structural collapse of micelles. At the low concentration of 1.0 mg/mL, BU-PPG exhibited a fairly broad LCST at 66 °C. As the concentration of BU-PPG gradually increased from 1.0 to 5.0 mg/mL, the LCST values decreased progressively to 37 °C at 5.0 mg/mL (Figures 2a and 2b). Notably, the LCST of aqueous 4.0 mg/mL BUPPG solution was found to be 42 °C, somewhat higher than body temperature and a promising feature for tumor-targetable drug carriers.4 Furthermore, the LCST phase transitions at higher concentrations (3.0−5.0 mg/mL) were sharper than those at lower concentrations (1 and 2 mg/mL; Figures 2a and 2b). This suggests the dehydrated polymer chains aggregate more easily at higher concentration, which manifests as a lower cloud point.37 The concentration dependence of the sharpness of the LCST also suggests the phase transition originates from the E


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Figure 4. Typical SEM and AFM images of 4.0 mg/mL aqueous solutions of BU-PPG micelles at (a, c) 25 °C (below the LCST) and (b, d) 45 °C (above the LCST).

from 42 to 24 °C as the duration of UV irradiation increased, and a diminishing slope of the curves was observed (Figures 3b−d). The transmittance of BU-PPG in aqueous solution decreased from 100% to 90% after 60 min UV irradiation, suggesting the increased hydrophobicity of irradiated BU-PPG micelles substantially reduced the LCST. The reduction in cloud point could be explained by light-induced covalent photoreaction of uracil moieties, resulting in formation of nonpolar cyclobutane within the BU-PPG micelles (Scheme 1c and Figure 3a). In fact, as the cyclobutane ring replaces two double bonds (loss of conjugation) and leads to an asymmetric structure, irradiated BU-PPG micelles should have lower polarity than pristine BU-PPG micelles. Thus, the emergence of cyclobutane in the polymer restricted the conformational freedom of BU-PPG, weakening the self-complementary hydrogen bonding between uracil moieties and also the hydrogen bonding between water molecules and the polymer backbone. Moreover, after a long duration of UV irradiation, the hydrophilic BU-PPG micelles became more hydrophobic due

to formation of cyclobutane uracil photodimers, resulting in significant changes in the hydrophilic to hydrophobic balance and water solubility. Therefore, dimerization of uracil moieties should promote dissociation of the self-complementary hydrogen bonding and dehydration of the water molecules that contribute to decrease the LCST. Since the thermoresponsive behavior depends on the interaction of the polymer with the solvent and the hydrophilic/hydrophobic balance within the polymer molecules,17 these results further demonstrate that formation of cyclobutane within the BU-PPG system profoundly influences its phase transition behavior in aqueous solution. Interestingly, when the LCST of BU-PPG micelles in solution was plotted against duration of irradiation (Figure 3d), an almost negative linear trend was observed, indicating the opportunity to program the LCST of photoresponsive BU-PPG micelles by only adjusting the duration of irradiation. Therefore, in addition to their concentration-dependent LCST behavior (Figure 2b), the phase transition of BU-PPG micelles can be also controlled by adjusting the degree of photodimerization over the temperature region from 24 to 42 F


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Macromolecules °C for a given concentration, which may be an advantageous feature of nanocarriers for controllable drug delivery application.38 To further confirm the effect of temperature and light on the phase transition, hydrodynamic diameter was measured by dynamic light scattering (DLS). The mean diameter of aqueous 4 mg/mL BU-PPG solution was determined while increasing the solution temperature from 20 to 45 °C before and after irradiation, and was in good agreement with the transmittance data (Figures 2c and 2d). As expected, the average hydrodynamic diameter of micelles increased as the temperature approached and exceeded the LCST (Figure S9). The particle sizes before irradiation ranged from 148 to 370 nm (Figure S9b), while larger particle sizes were observed after irradiation (167−565 nm; Figure S9c). For example, the average particle size in nonirradiated solution at 25 °C (below LCST) was 148 nm (polydispersity index, PDI = 0.14), whereas the particle diameter and particle size distribution above the LCST at 45 °C increased to 370 nm and 0.22, respectively, making the solution cloudy completely. After 60 min of UV irradiation to achieve 96% photodimerization, the particle sizes of the micelles at 25 and 45 °C slightly increased to 167 nm (PDI = 0.089) and 565 nm (PDI = 0.20), respectively (Figure S9c). This indicates that the differences in the particle size before and after irradiation may be attributed to the loss of self-complementary hydrogen bonding between uracil moieties and the increased hydrophobicity of irradiated BU-PPG compared to pristine BU-PPG and the presence of cyclobutane linkages in the main chain (i.e., the formation of longer polymer chains) leading to formation of more dehydrated and more densely aggregated micelles upon heating.15,39 These observations strongly support our interpretation of the transmittance data (Figure 3) that irradiation increases the hydrophobic characteristics of BUPPG micelles, inducing formation of more stable covalent structures due to the presence of photodimerized uracil groups formed by photoreaction of the uracil units within the micelles in aqueous solution. Temperature and Light-Induced Morphological Changes in Uracil-Functionalized Supramolecular Micelles. In addition to phase transition behavior, the temperature- and light-induced morphological changes due to conformational transition of the PPG backbone and uracil moieties in aqueous solution were observed via scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM and AFM specimens were prepared by spincoating. At temperatures below the LCST, the SEM and AFM images suggested pristine BU-PPG micelles formed uniform spherical micelles with a diameter of 80−110 nm (Figures 4a and 4c). This implies the self-complementary hydrogenbonding interactions between the uracil moieties are a key component required for control over chain packing and polymer conformation and enable the formation of lowdimensional self-assembled nanosized micelles in aqueous solution.21−24 It is worth pointing out that the shape, size, and distributions prove the nanoparticles should be biologically compatible (Figure S7).40,41 Further increasing the temperature of the aqueous BU-PPG solution above the LCST (45 °C) caused the spherical micelles to adopt a shrunken globule conformation and form larger aggregates, as evidenced by the large spherical aggregates ranging from 200 to 350 nm on SEM and AFM (Figures 4b and 4d). This observation is consistent with the DLS (Figure S9b). The change in morphology and size below and above the

LCST indicates BU-PPG micelles are hydrophilic and readily disperse in water below the LCST. In contrast, at temperatures above the LCSTdue to dissociation of the self-complementary hydrogen bonding and dehydration of water molecules (Figure S10)the micelles shrink and then compact together to form large clusters of aggregates driven by the intermicellar hydrophobic interactions between PPG segments. Thus, the transition from a chain-relaxation process to a chainaggregation process can be easily observed microscopically, since particle size is significantly different below and above the LCST (Figure S9b). In addition to observing a thermal-induced phase transition, we also investigated light-induced morphological behavior. Upon UV irradiation, BU-PPG micelles in solution exhibited interesting, different morphologies to unirradiated micelles. After 60 min UV irradiation, the spherical micelle morphology transformed into irregular aggregates with a broad size distribution in the range of 60−150 nm (Figure S11), suggesting that light-induced structural changes facilitate the dissociation of hydrogen bonds and increase the hydrophobicity of the micellar structure. This further implies that UV light alters the polarization of the micelles by shifting the hydrophilic−hydrophobic balance,42 thus significantly changing the size and shape distributions. DLS analysis (Figure S9c) also showed the average hydrodynamic diameters of irradiated BUPPG micelles were slightly larger (167 nm). The discrepancy in micelle size obtained using AFM and DLS was attributed to methodological changes during sample preparation; DLS measured the hydrodynamic diameter of micelles swollen in aqueous solution, whereas AFM measured the morphology of dried particles.43 Nevertheless, both measurements revealed the same trend in micelle particle size. Similar morphological transitions upon heating and light irradiation have also been reported for other dual light-responsive micelles.44 In addition, our intriguing findings further suggest these efficient temperature/light-responsive BU-PPG micelles may provide a potential route toward achieving multiple-triggered drug release, as the physical properties of the micelles can be tightly controlled to endow excellent temperature/light-responsive properties in aqueous biological environments. Overall, this study clearly demonstrates this newly developed micelle not only possesses multifunctional responsiveness to manipulate the micellar assemblies but also provides a future direction toward the development of new multiresponsive smart micellular nanocarriers based on uracil nucleobases.

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CONCLUSIONS In summary, we provide the first report of simple, efficient development of a dual temperature- and light-responsive supramolecular BU-PPG polymer based on the presence of oligomeric PPG and hydrogen-bonded uracil moieties. BUPPG can spontaneously self-assemble into well-defined spherical micelles in aqueous solution with a diameter of around 150 nm. The micelles exhibit unique physical properties in aqueous environment, including a low CMC and low cytotoxicity against OECM-1 cells, as well as excellent selfassembly ability. Studies of the phase transitions in aqueous solution suggested the LCST values can be readily controlled by adjusting the concentration and duration of UV irradiation of the aqueous solutions. The LCST for BU-PPG micelles decreased over a wide range of temperature (from 66 to 37 °C) as the concentration of the solution increased. Increasing the degree of photodimerization decreased the LCST values of the G


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Macromolecules BU-PPG micelles over the temperature range from 42 to 24 °C, with a negative linear relationship between LCST and the duration of irradiation. In addition, the morphological transformation of irradiated BU-PPG micelles as temperature increased was markedly different to that of unirradiated micelles. Thus, we believe this study provides a new route toward the development of novel multistimuli-responsive supramolecular nanocarriers with a wide range of biomedical applications. Detailed investigations of the potential of this promising supramolecular drug delivery platform for cancer chemotherapy are underway by our research team.

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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.macromol.7b02637. Detailed synthetic procedures and characterization, general materials, and instrumentation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-C.C.). ORCID

Jem-Kun Chen: 0000-0003-3670-6710 Chih-Chia Cheng: 0000-0002-1605-6338 Author Contributions

B.T.G. performed all experiments and wrote the paper. S.-Y.H. helped with the in vitro cytotoxicity evaluation. C.-C.C. conceived the project, designed the research, and edited the paper. All authors discussed the results and commented on all or parts of this paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported financially by the Ministry of Science and Technology, Taiwan, under Contracts MOST 104-2221-E011-153 and MOST 105-2628-E-011-006-MY2.

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Dual Stimuli-Responsive Nucleobase-Functionalized Polymeric

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