Colorimetric Thermometer from Graphene Oxide Platform Integrated

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Colorimetric Thermometer from Graphene Oxide Platform Integrated with Red, Green, and Blue Emitting, Responsive Block Copolymers Junhyuk Lee, Hyunseung Yang, Chan Ho Park, Han-Hee Cho, Hongseok Yun, and Bumjoon J. Kim* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea S Supporting Information *

ABSTRACT: We developed an efficient colorimetric thermometer that detects temperature ranging from 25 to 45 °C using a novel platform of block copolymers (BCPs) integrated onto graphene oxide (GO). In order to achieve colorimetric sensing over a wide range of temperatures, GO was functionalized with three different BCPs: each BCP is composed of a temperatureresponsive block and a fluorescent block. Each temperature responsive block had a different lower critical solution temperature to monitor different but complementary temperature ranges. The fluorescent blocks of the three BCPs emitted red, green, or blue light to provide a colorimetric response to temperature changes. The fluorescent intensities of the three BCPs on GO were independently switched on and off with changes in temperature, producing a colorimetric behavior from white to yellow to red and then to green in the range of 25−45 °C with excellent reversibility and stability. Importantly, we successfully demonstrated the use of our sensors for detecting the temperature response within microsized domains, suggesting that our system is a promising platform for practical applications, for example, in clinical and environmental monitoring.

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and physical interactions, allowing facile sensor fabrication.38−45 Also, the GO platform could provide good dispersion behavior in water and alcohol,46,47 which is particularly useful for biological and environmentally friendly sensing systems. Recently, we demonstrated the potential of the GO-based optical sensors functionalized with pH-responsive polymer and quantum dot hybrids that exhibited pH-sensitive behavior and a colorimetric output.48 However, to the best of our knowledge, the colorimetric temperature sensors with a wide detection range based on GO have not yet been reported. Furthermore, the potential of the GO-based optical sensors for detecting local signals within microsized domains remains unexplored. Herein, we developed a colorimetric thermometer based on block copolymer (BCP)-functionalized GO (BCP-GO) that detects temperature changes from 24 to 42 °C (Scheme 1). To generate a thermoresponsive behavior over a wide temperature range, three BCPs with different LCSTs of 27, 32, and 39 °C were attached to GO. Each BCP emits red, green, or blue light, which provides distinct color-displaying responses of the GO thermometer from white to yellow to red and then to green as the temperature was increased. The fluorescent intensities of the three BCPs on the GO substrate were independently switched on and off by changing the conformation of each BCP chain in response to different but complementary ranges of temperatures

INTRODUCTION Detecting temperatures in the range of 25−45 °C is critical for clinical,1 biomedical,2−4 and environmental5,6 studies. Fluorescent thermometers based on temperature-responsive polymers have recently been in the spotlight due to their remote sensing ability and thermal sensitivity within small confined spaces, where conventional thermometers can not be used.7−15 However, such sensors are typically limited to the detection of single transition temperature, because temperature-responsive polymers usually have a single lower critical solution temperature (LCST) where their chain conformations are sharply changed.16−18 In addition, temperature sensors consisting of stimuli-responsive polymers often suffer from poor dispersion stability in aqueous media, which limits their use in biological applications.19,20 The detection of external stimuli using graphene oxide (GO) has attracted great attention because of their electrochemical properties21−24 and great sensitivity to Förster resonance energy transfer (FRET)25−27 that provides a high signal-to-noise ratio. For example, graphene and GO have been used for sensing applications by monitoring a change in conductivity28−31 or optical properties.32−35 However, conductivity measurements often require specific detectors, limiting their use only in gaseous environments.36,37 Alternatively, integrating fluorophores onto GO allowed easy detection of signals by direct visual observation.32−34 The surface modification of GO with various stimuliresponsive polymers could provide a simple approach to achieve effective detection of multiple external stimuli, and the GO can be functionalized by organic and polymeric molecules via chemical © XXXX American Chemical Society

Received: March 4, 2016 Revised: April 18, 2016

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DOI: 10.1021/acs.chemmater.6b00913 Chem. Mater. XXXX, XXX, XXX−XXX

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Scheme 1. (a) Schematic Illustration of the BCP-GO Thermometer and (b) the Color Changes of BCP-GO as a Function of Temperature

Figure 1. Synthesis of pyrene-functionalized P27-B, P32-G, and P39-R.

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RESULTS AND DISCUSSION Scheme 1 illustrates our design of a colorimetric BCP-GO thermometer, and Figure 1 shows the synthetic route and the chemical structure of BCPs. We designed a GO platform integrated with three different BCPs consisting of temperature-responsive blocks and light emitting blocks (blue, green, and red). The temperatureresponsive block of each BCP exhibited distinct LCST values

and, thus, controlling the efficiencies of FRET from the BCPs to the GO. The detailed mechanism of the temperature-dependent response of the BCP-GO was elucidated by UV−vis spectroscopy, photoluminescence (PL), and time-resolved fluorescence (TRF) spectroscopy. Lastly, we used our BCP-GO sensors for temperature detection within microsized capsules, demonstrating their potential for bioimaging and environmental monitoring applications. B

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Chemistry of Materials of 27, 32, and 39 °C, respectively, tuning the interspacing between the light emitting block of BCPs and the GO substrate. Thus, the FRET efficiency between each BCP and GO was independently controlled for detecting different but complementary ranges of temperature at and around the human body temperature. We used poly(N-isopropylacrylamide) (PNIPAM) for P32, which is well-known to have LCST of 32 °C due to dehydration at higher temperatures.49 To adjust the LCST, we synthesized P(NIPAM 95%-r-butyl methacrylate (BMA) 5%) and P(NIPAM 90%-r-dimethylaminopropylacrylamide (DMAPAM) 10%) for P27 and P39, respectively. Copolymerization of NIPAM with hydrophobic BMA is known to decrease the LCST, while the incorporation of hydrophilic DMAPAM monomers into the PNIPAM backbone increases the LCST.50 We adjusted the amount of BMA or DMAPAM in the polymers to tailor the LCSTs to 27 and 39 °C. Figure S1 shows the temperaturedependent optical transmittances of P27, P32, and P39 in water. The polymers were prepared by reversible addition−fragmentation transfer (RAFT) polymerization to yield similar number-average molecular weights (Mn) with narrow polydispersity index (PDI) (Table 1). It is worth noting that a specially designed

concentration (Figure S2). Ungrafted polymers were completely removed by repeated centrifugation (13 500 rpm, 5 min) and decantation more than three times. The conjugation of the P27-B, P32-G, and P39-R to the surface of the GO was confirmed by attenuated total-reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements. As shown in Figure S3, the peaks at 2960 and 3300 cm−1 of the BCP-GO corresponded to C−H and N−H stretching in the P27-B, P32-G, and P39-R,55 indicating the successful anchoring of the BCPs onto the GO. Thermogravimetric analysis (TGA) was performed to determine the amount of polymers grafted onto the GO surface (Figure S4). TGA curves of BCP-GO showed a two-step decomposition process in Figure S4a, where decomposition of the oxygen-containing functional groups and the grafted polymers on the GO occurred at around 200 °C and from 300 to 500 °C, respectively. The grafting densities of the polymer chains on the GO surface can be estimated on the basis of the weight fractions of the polymers in the BCP-GO. Assuming that the grafted polymer chains were completely decomposed at 500 °C (Figure S4b), the weight percentage of the anchored polymer chains in the BCP-GO was found to be 41%, which corresponded to a grafting density of 0.044 chain nm−2. This is comparable to the previously reported values for PNIPAM brushes on GO.16,19,56 A distinct advantage of this BCP-GO composite is its thermal sensitivity at around the human body temperature with colorimetric output. The thermal response of the BCP-GO was examined by monitoring the change in the PL intensity at various temperatures from 24 to 42 °C (Figure 2a). Due to the different LCSTs of the BCPs, their relative PL intensities changed dramatically with temperature as can be observed in Figure 2b. At 24 °C, the PL spectra of the BCP-GO showed strong three emission peaks at 393, 518, and 585 nm from P27-B, P32-G, and P39-R, producing near white luminescence. Above 27 °C, the LCST of P27-B, the intensity of the blue emission peak at 393 nm was significantly decreased, resulting in a yellow-green color. When the temperature was higher than 32 °C, the LCST of P32-G, the intensity of the green emission peak at 518 nm was quenched, resulting in a pale orange luminescence. Finally, when the temperature exceeded 39 °C (LCST of P39-R), the intensity of the red emission peak at 585 nm was also drastically reduced and the color of the solution changed to green. Figure 2c shows the relative intensity changes of the blue, green, and red emissions of the BCP-GO as a function of temperature. Interestingly, the temperatures, where each of the three emission peaks sequentially dropped, coincided with the LCSTs of P27-B, P32-G, and P39-R. The results described above can be attributed to the coil-to-globule conformational transition of the polymers upon the increase of temperature, which brought the dye block of each BCP into the more proximate vicinity to the GO surface. At temperatures below the LCST, the distance between the dye blocks and the GO should be sufficient to suppress FRET between them. In contrast, at temperatures above the LCST, the polymer chains collapse, leading to the stronger FRET from the dyes to the GO and the consequent reduction of PL intensity. As a result, the intensities of PL were 37% for blue emission (393 nm), 45% for green emission (518 nm), and 38% for red emission (585 nm) compared to their initial intensities before the chains collapsed. It is worth noting that the PL emission of the BCP-GO was not completely quenched above 42 °C because even the polymers in globule states provided some distance between the dye blocks and the GO, limiting complete FRET between them. For example, the lengths of the collapsed P27,

Table 1. Characteristics of P27-B, P32-G, and P39-R Block Copolymers Mn of temperature responsive blocka (kg/mol) P27-B P32-G P39-R

16.7 16.8 18.2

Mn of emissive blocka PDIa LCSTb (kg/mol) (Mw/Mn) (°C) 3.2 4.4 4.3

1.30 1.19 1.18

27 32 39

a

Determined by GPC using THF as the eluent calibrated by standard PS. bDetermined by the optical transmittance measured at 500 nm by UV−vis spectroscopy.

pyrene-functionalized RAFT chain transfer was used for the polymerization to introduce the pyrene group as the terminal unit of each polymer. This pyrene end-functional group promotes the grafting of polymers to the GO surface by a strong π−π stacking interaction.51,52 Next, blue (coumarin), green (fluorescein), and red (rhodamine) color emitting blocks were added to P27, P32, and P39, respectively, using sequential RAFT polymerization (Figure 1). For convenience, we denoted BCPs as P27-B, P32-G, and P39-R according to their LCSTs and emissive colors. The Mn of the emissive blocks in the P27-B, P32-G, and P39-R was carefully controlled to have similar but small values of 3−4 kg mol−1, which ensured sufficient PL intensity without interfering with the solubility of the BCPs in water (Table 1).16,53 The successful preparation of three different thermoresponsive, light emitting BCPs was confirmed by their PL spectra. The PL spectra of the P27-B, P32-G, and P39-R showed strong emission peaks at 393, 518, and 585 nm, respectively, from each emissive block of the polymers. The weak emission peaks at 377 and 395 nm were from the pyrene terminal groups (Figure S2).54 The P27-B, P32-G, and P39-R-anchored GO (BCP-GO) was synthesized by simultaneous functionalization of a single GO sheet with three BCPs, which was simply achieved by π−π interactions between the pyrene end groups of the polymers and the basal plane of the GO surface. To produce a similar PL intensity from each emissive block at room temperature, the feed weight ratio of P27-B, P32-G, and P39-R was adjusted to 2:1:2 because the PL intensity of P32-G is approximately two times higher than that of P27-B and P39-R at the same C

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Figure 2. (a) PL spectra of BCP-GO at various temperatures; (b) photographs of BCP-GO under irradiation at 365 nm; (c) temperature-dependent intensities of blue (393 nm), green (518 nm), and red (585 nm) emissions of BCP-GO.

Figure 3. τave values of the (a) P27-B, (b) P32-G, and (c) P39-R in BCP-GO as a function of temperature, which were obtained by fitting the TRF curves (Figure S5) with a double exponential decay model.

indicating faster and more efficient energy transfer from P27-B to the GO. Similarly, the τave of P32-G was 3.50 ns below 32 °C, with two decay components of 1.99 ns (45.41%) and 4.76 ns (54.59%), and then, the τave decreased significantly to 2.62 ns with two decay components of 1.47 ns (64.13%) and 4.66 ns (35.87%) at 34 °C. The τave curve reached a plateau with a further increase of temperature. We also observed the same trend of the τave value for P39-R with a different transition temperature of the τave value (39 °C). However, the transition was not as sharp as those of the other two polymers probably due to the temperature-dependent emission properties of the rhodamine B in P39-R.61−63 The drastic decrease of the τave value was induced by an increase in the nonradiative decay rate, suggesting more efficient FRET from the dyes to the GO.64−68 Additionally, the trends in the τave values in the BCP-GO were remarkably consistent with those in the conformational changes of the BCPs as a function of temperature. High reversibility and stability are crucial for the successful applications of optical sensors.8,9 Therefore, we monitored the PL intensity over multiple cycles to examine the reversibility of the thermal response of the BCP-GO (Figure 4a). For the measurement, the BCP-GO in water was heated up to 42 °C and cooled down to 24 °C, and then, this cycle was repeated. During the heating process (red shadow), the PL intensities of the dyes decreased, showing the same trend as in Figure 2c. At above the LCSTs of P27-B, P32-G, and P39-R, significant intensity drops of the blue, green, and red emission were observed. In contrast, during the cooling cycle (blue shadow), an opposite trend was observed such that the PL intensities were enhanced as the temperature decreased. After cooling, the intensities were fully restored back to their original values. After 10 cycles, a similar trend was obtained again, and also, the color was recovered completely (Figure 4b). This high reversibility of the PL indicates the excellent stability of the BCP-GO during heating and cooling.

P32, and P39 chains were estimated to be 3.17, 3.18, and 3.26 nm on the basis of the assumption that the collapsed chain formed a dense sphere with a radius scaling with N1/3 (N: degree of polymerization).57,58 The lengths of the expanded P27, P32, and P39 chains were also calculated to be 7.00, 7.05, and 7.62 nm from L = Na(a/D)2/3 (a: the size of the monomer; D: the average distance between neighboring chains).59,60 The PL quenching efficiency (Q) between the 2D GO plate and the dyes is Q = 1/[1 + (d/d0)4], where d is the fluorescent emitters-to-GO distance and d0 is the characteristic distance that yields 50% PL quenching efficiency.16,27 With the assumption that the d0 value of coumarin is 5.5 nm,16 we calculated the ratios of the PL intensities (Q 1/Q 2) between the globule (Q 1) and the expanded states (Q 2) for P27-B. The Q 1/Q 2 ratio of P27-B was estimated to be 31%, which was in a good agreement with the experimental observations (37%). A deeper insight into the FRET efficiency can be acquired by measuring the average fluorescence lifetimes (τave) of each fluorophore in the BCP-GO using time-resolved fluorescence (TRF) spectroscopy. The TRF curves of the BCP-GO are shown in Figure S5. The fluorescent decays of the P27-B, P32-G, and P39-R in the BCP-GO were examined as a function of temperature by TRF measurements with filtering at 393 nm for P27-B, at 518 nm for P32-G, and at 585 nm for P39-R under irradiation at 370 nm. The TRF spectra were fitted by a double exponential decay model to obtain the τave values (Table S1). Figure 3 presents the τave values of the P27-B, P32-G, and P39-R in the BCP-GO as a function of temperature. The τave values were temperature-dependent due to the dynamic quenching between the dyes and GO.27,48 Below 27 °C, the τave of the P27-B was 1.42 ns with two decay components of τ1 = 1.29 ns (population A1 = 95.93%) and τ2 = 4.38 ns (population A2 = 4.07%). As the temperature increased to above 27 °C, the LCST of P27B, the τave dropped to 0.54 ns with two decay components of τ1 = 0.42 ns (A1 = 97.33%) and τ2 = 4.18 ns (A2 = 2.67%), D

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Figure 5. Fluorescence microscopy images of BCP-GO confined within microsized capsules with the indicated temperature values. The scale bars are 50 μm.

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CONCLUSIONS In this work, we described our development of a new platform for an effective colorimetric sensor with a wide operating temperature range using fluorescent and temperature responsive BCP-functionalized GO. Three different light emitting BCPs with distinct LCSTs were grafted onto the GO substrate. The facile tuning of LCSTs from 27 to 39 °C was successfully achieved by the copolymerization of BMA or DMAPAM in PNIPAM. The fluorescence of each BCP on the GO platform was independently switched on and off via the coil-to-globule transition of the temperature-responsive BCPs, producing a distinct colorimetric response of the sensor in the range of 24−42 °C with excellent reversibility and stability. The PL intensities and the τave values of the BCP-GO as a function of temperature clearly support that the colorimetric temperature response was originated from the changes in FRET efficiency between the dyes and GO due to the coil-to-globule conformational transition of each BCP at above the LCST. Importantly, we successfully demonstrated the use of our sensors in detecting the temperature change within local spaces, suggesting that our system is a promising platform for practical applications including clinical and environmental ones.

Figure 4. (a) Reversibility of the PL behavior of BCP-GO. PL intensities at 393, 518, and 585 nm were monitored as a function of temperature. (b) Photographs of the BCP-GO solution were obtained before and after 10 cycles of the heating and cooling process at 365 nm irradiation using a UV lamp.

The remote sensing ability within small confined spaces or at many locations simultaneously is highly beneficial for the clinical and biological applications such as intracellular thermometry.69,70 In order to demonstrate the use of BCP-GO for the colorimetric sensing of the local temperature change, we developed a model system with BCP-GO encapsulated within micron-sized droplets and examined their emission properties using fluorescence microscopy (Figure 5). Water-in-toluene emulsions having BCP-GO were prepared from BCP-GO/water droplets stabilized by 0.5 wt % sorbitan monostearate in toluene solution (see the Supporting Information for details).71 The emulsion capsules containing the BCP-GO were successfully prepared with the average diameters of 12 ± 5 μm (Figure S6). The fluorescence of the BCP-GO in the capsules was measured at different temperatures of 24, 30, 36, and 42 °C. All of the capsules exhibited fluorescent emission, whereas the continuous phase did not. Also, the capsules were homogeneously emissive, indicating that the BCP-GO was well dispersed and stable in the water-intoluene emulsion. Heating the emulsion solution from 24 to 42 °C led to a distinct colorimetric change from white to yellow to red to green without interfering with the stability of the BCP-GO in the capsules. These results demonstrated that our BCP-GO system can detect a specific temperature within microsized spaces.

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METHODS

Materials. Anhydrous K2CO3 (99.5%) and MgSO4 (99%) were purchased from Daejung Chemical & Metal Co. Azobis(isobutyronitrile) (AIBN, 98%) was purchased from Junsei Chemical Co. and purified by recrystallization from ethanol. NIPAM (99%), butyl methacrylate (BMA, 99%), N-[3-(dimethylamino)propyl]methacrylamide (DMAPAM, 99%), tert-butyl acrylate (tBA, 98%), N,N′-dicyclohexylcarbodiimide (DCC, 99%), 4-(dimethylamino)pyridine (DMAP, 99%), 1,4-dibromobutane (99%), ethanolamine (99%), ethylene glycol (99.8%), rhodamine B (95%), fluorescein isothiocyanate isomer I (FITC, 90%), 7-hydroxycoumarin (99%), N,N-dimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.8%), and other materials were purchased from Sigma-Aldrich. NIPAM was purified by recrystallization in hexane, and BMA and tBA were purified by passing through an alumina column. DMAPAM was purified through vacuum distillation before polymerization. Detailed synthetic schemes for polymers were shown in Figure 1. Synthesis of Pyrene-Functionalized RAFT Chain Transfer Agent (1). The trithiocarbonate RAFT chain transfer agent was synthesized according to methods reported in the literature.72 The pyrene-functionalized E

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Chemistry of Materials RAFT agent was synthesized as in a previously published procedure.48 1 H NMR (CDCl3, 500 MHz): δ 8.0−8.32 (m, 9H), 4.21 (t, 2H), 3.38 (t, 2H), 3.27 (t, 2H), 1.72 (s, 6H), 1.26−1.71 (m, 20H), 0.89 (t, 3H). Synthesis of 7-(4-(Acryloyloxy)butoxy)coumarin (7AC) (5). The 7AC monomer was synthesized as previously reported.39,73 1 H NMR (CDCl3): δ 7.62 (d, 1H), 7.34 (d, 1H), 6.80 (m, 2H), 6.41 (d, 1H), 6.24 (m, 1H), 6.11 (m, 1H), 5.81 (m, 1H), 4.23 (s, 2H), 4.04 (s, 2H), 1.90 (s, 2H), 1.55 (s, 2H). Synthesis of N-(Fluorescein-5-yl)-N′-(2-hydroxyethyl)thiourea (6). 0.3 mmol of FITC was added into the solution of 0.6 mmol of ethanolamine in 5 mL of water, and the mixture was stirred for 2 h. The reaction mixture was diluted with acetic acid/acetate buffer and extracted with ethyl acetate. The separated organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure resulting in a yellow solid. 1H NMR (DMSO-d6, 500 MHz): δ 10.08 (br, 3H), 8.31 (br, 1H), 7.77 (br, 1H), 7.74 (d, 1H), 7.17 (d, 1H), 6.67 (d, 2H), 6.61 (d, 2H), 6.65 (dd, 2H), 4.89 (br, 1H), 3.58 (br, 4H). Synthesis of Rhodamine B Ethanol Ester (7). N-(6-(Diethylamino)9-(2-((2-hydroxyethoxy)carbonyl)phenyl)-8a,10a-dihydro-3H-xanthen3-ylidene)-N-ethylethanaminium: 10 mmol of ethylene glycol, 2.5 mmol of DCC, and 0.25 mmol of DMAP were dissolved in 20 mL of dichloromethane (DCM), followed by the dropwise addition of 2 mmol of rhodamine B in DCM. The mixture was stirred for 12 h. The resulting product was purified with diluted hydrochloric acid, dried over MgSO4, and concentrated under reduced pressure. 1H NMR (CDCl3, 500 MHz): δ 8.41−8.46 (br, 1H), 7.7−7.8 (br, 2H), 7.48 (br, 1H), 7.10 (dd, 2H), 6.87 (d, 2H), 6.85 (d, 2H), 4.10 (q, 2H), 3.45−3.66 (br, 10H), 0.99−1.33 (br, 12H). Synthesis of Pyrene-Functionalized P(NIPAM-r-BMA)-b-P(coumarin) (2-1). For P(NIPAM-r-BMA) (2), a solution of NIPAM, BMA (NIPAM/BMA molar ratio of 95:5), pyrene-functionalized RAFT chain transfer agent (1), and AIBN in DMSO was added into a glass ampule for polymerization under vacuum. After polymerization for 12 h at 70 °C, P(NIPAM-r-BMA) was precipitated into cold diethyl ether twice and vacuum-dried. A solution of P(NIPAM-r-BMA) (2), 7AC (5), and AIBN in DMF was added into a glass ampule for sequential polymerization during 12 h at 70 °C under vacuum. The product was precipitated in cold diethyl ether twice and vacuum-dried, obtaining the white powder of P(NIPAM-r-BMA)-b-P(coumarin) (2-1). Synthesis of Pyrene-Functionalized PNIPAM-b-P(fluorescein) (3-3). For PNIPAM (3), a solution of NIPAM, pyrene-functionalized RAFT chain transfer agent (1), and AIBN in DMSO was added into a glass ampule for polymerization under vacuum. After polymerization for 12 h at 70 °C, PNIPAM was precipitated into cold diethyl ether twice and vacuum-dried. A solution of PNIPAM (3), tBA monomer, and AIBN in DMF was added into a glass ampule for sequential polymerization under vacuum. After reaction for 12 h at 70 °C, the product was precipitated in a methanol/water mixture twice and vacuum-dried, obtaining the white powder of PNIPAM-b-PtBA (3-1). The tert-butyl ester groups on PtBA were hydrolyzed by using excess trifluoroacetic acid, and then, PNIPAM-b-poly(acrylic acid) (PAA) (3-2) was precipitated using diethyl ether. PNIPAM-b-P(fluorescein) (3-3) was synthesized by a DCC coupling reaction. Briefly, PNIPAM-b-PAA (3-2), N-(fluorescein5-yl)-N′-(2-hydroxyethyl)thiourea (6), and DMAP were mixed in DCM. After 15 min, DCC in DCM was added to the mixture. After 36 h, the mixture was filtered, and the crude products were precipitated in diethyl ether, obtaining the orange powder of PNIPAM-b-P(fluorescein) (3-3). Synthesis of Pyrene-Functionalized P(NIPAM-r-DMAPAM)-bP(rhodamine) (4-3). For P(NIPAM-r-DMAPAM) (4), a solution of NIPAM and DMAPAM (mole ratio of NIPAM/DMAPAM = 90:10), pyrene-functionalized RAFT chain transfer agent (1), and AIBN in DMSO was added into a glass ampule for polymerization during 12 h at 70 °C under vacuum. P(NIPAM-r-DMAPAM) was precipitated into cold diethyl ether twice and vacuum-dried. A solution of P(NIPAM-rDMAPAM) (4), tBA monomer, and AIBN in DMF was added into a glass ampule for sequential polymerization under vacuum. After reaction for 12 h at 70 °C, the product was precipitated in a methanol/water mixture twice and vacuum-dried, obtaining the white powder of P(NIPAM-r-DMAPAM)-b-PtBA (4-1). The tert-butyl ester groups on PtBA were hydrolyzed by using excess trifluoroacetic acid, and then,

P(NIPAM-r-DMAPAM)-b-PAA (4-2) was precipitated using diethyl ether. P(NIPAM-r-DMAPAM)-b-P(rhodamine) (4-3) was synthesized via a DCC coupling reaction. Briefly, P(NIPAM-r-DMAPAM)-b-PAA (4-2), rhodamine B ethanol ester (7), and DMAP were mixed in DCM. After 15 min, DCC in DCM was added to the mixture. After 36 h, the mixture was filtered, and the crude products were precipitated in diethyl ether, obtaining the red powder of P(NIPAM-r-DMAPAM)-bP(rhodamine) (4-3). Preparation of BCP-GO. GO was prepared by using the improved Hummers’ method.74 To prepare BCP-GO, 10 mg of GO, 50 mg of P(NIPAM-r-BMA)-b-P(coumarin) (2-1), 25 mg of PNIPAM-bP(fluorescein) (3-3), and 50 mg of P(NIPAM-r-DMAPAM)-bP(rhodamine) (4-3) were stirred in DMF for 2 days. The mixture was purified by repeated centrifugation in DMF, deionized water (DI water), and methanol at 13 500 rpm for 5 min to remove ungrafted polymers. Characterization of BCP-GO. The structure of the materials used in this study was analyzed by 1H-nuclear magnetic resonance (NMR, Bruker AMX 500) and size exclusion chromatography (SEC, Waters 1515) with a differential refractometer (Waters 2414) with THF as the eluent at a flow rate of 1 mL min−1; the column was calibrated using polystyrene standards. ATR-FTIR spectra were acquired using a Bruker ALPHA spectrometer. Fluorescence images were aquired through a fluorescence optical microscope (Nikon Eclipse Ti−U). PL spectra was aquired by Horiba Jobin Yvon NanoLog spectrophotometer, using a 10 mm quartz cuvette. Fluorescence lifetimes were analyzed using a NanoLED laser light source at 370 nm for the excitation, and the emissions were collected at 393, 518, and 585 nm. PL measurements were performed by dissolving BCP-GO in DI water to exclude the effect of ionic strength on the conformation of PNIPAM, and all samples for PL measurement were prepared at very low concentrations (1.0 mg mL−1).

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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.chemmater.6b00913. Additional characterization data (SEC, PL, UV−vis, TGA, and TRF data) and synthetic scheme of the synthesized materials. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301-07. We thank Rachel Letteri for helpful discussions.

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REFERENCES

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DOI: 10.1021/acs.chemmater.6b00913 Chem. Mater. XXXX, XXX, XXX−XXX