Amphiphilic block copolymer terminated with pyrene group: from switchable CO₂-temperature dual responses to tunable fluorescence

Abstract

An amphiphilic block copolymer poly(ε-caprolactone)-block-poly(N-isopropylacrylamide-co-N,N-dimethylaminoethyl methacrylate) terminated with a pyrene group (Py-PCL-b-P(NIPAM-co-DMAEMA)) was synthesised by the combination of ring-opening polymerisation (ROP), DCC reaction and reversible addition-fragmentation chain transfer polymerisation (RAFT). The micelles self-assembled from the copolymer and showed switchable CO₂-temperature dual responses. The copolymerisation incorporating DMAEMA was used as a CO₂-responsive trigger into NIPAM, and the lower critical solution temperature (LCST) value could be switched by the gas. The fluorescence intensities and the controlled drug release properties could be adjusted and achieved by altering the temperature of the micelle solution and bubbling CO₂/Ar into the micelle solution.

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Recently, stimuli-responsive polymers have attracted considerable interest owing to their potential applications in biomedical (e.g., drug and gene delivery) and nano-smart technology (e.g., nano-sensor and nano-reactor) fields.^1–10 Many stimuli-responsive polymers have been extensively investigated and reported, such as temperature-, pH-, light- and ionic strength-responsive polymers.^11–14 Furthermore, novel types of stimuli-responsive polymers such as carbon dioxide (CO₂)-responsive polymers have attracted great attention due to the availability, nontoxicity, biocompatibility, low cost and abundance of CO₂. CO₂-responsive polymeric materials including polymeric organogels, supramolecular polymers, polymeric vesicles and organic–inorganic hybrid nanomaterials have been investigated.^15–22 In general, CO₂-responsive polymers are molecules containing amidine functional groups that can react with CO₂ and water to form charged amidinium bicarbonates and be recovered upon CO₂ removal. Taton et al. reported the reaction of poly(N-heterocyclic-carbene)s with CO₂ and its use in organocatalysis.²³ Feng et al. investigated an amidine-based CO₂-responsive polymer with a suitable hydrophobic backbone and amidine pendants derived from polystyrene prepared via a facile route by combination of reversible addition-fragmentation chain transfer polymerisation (RAFT) and “click” reaction.²⁴ Yuan et al. prepared an amphiphilic diblock copolymer composed of poly(ethylene oxide) (PEO) and a polyacrylamide bearing an amidine side group and found that its self-assembled vesicle could undergo a reversible expansion and contraction upon exposure to CO₂ and Ar.²⁵ Incorporating the amidine functional group into a polymer represents a general means to render the polymer CO₂-response, but the synthesis is demanding, and the amidine-containing polymers may be hydrolytically unstable.²⁶ As a result, it is very important to explore more general, robust, and efficient approaches to enable the use of CO₂ as a trigger for a broad range of polymers and materials.^27–30 Zhao et al. discovered that poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) can react directly with CO₂ in water without functionalisation with amidine, which can drastically increase its lower critical solution temperature (LCST), characterizing the transition from a soluble (hydrated) to an insoluble (dehydrated) state.³¹ The change in LCST is reversible upon removal of CO₂ using Ar. They also found that incorporating DMAEMA or similar amine-containing monomer units into non-CO₂-responsive, thermoresponsive polymers (e.g., poly(N-isopropylacrylamide), PNIPAM) could endow them with CO₂-responsive properties.³¹ Because LCST is a key property in thermo-switchable polymers, it is significant to impart a CO₂-switchable LCST to polymers and make it possible to use CO₂ to trigger reversible structural changes of smart materials. The LCST can be changed and recovered by bubbling CO₂ or Ar. Namely, this kind of polymer can achieve the connection of temperature and CO₂ responses. Based on this, the switchable CO₂-temperature dual responsive system deserves further investigation.

Herein, we designed and synthesised an amphiphilic block copolymer, poly(ε-caprolactone)-block-poly(N-isopropylacrylamide-co-N,N-dimethylaminoethyl methacrylate) terminated with a pyrene group (Py-PCL-b-P(NIPAM-co-DMAEMA)) by the combination of ring-opening polymerisation (ROP), DCC reaction and RAFT (Scheme 1). The P(NIPAM-co-DMAEMA) block is hydrophilic and endowed with dual thermo-CO₂ responses; the PCL block is a hydrophobic, biocompatible and biodegradable segment. The presence of the pyrene group provides the fluorescence property for the block copolymer.^32–35 The amphiphilic block copolymer can self-assemble into micelles with CO₂-temperature dual responses. The fluorescence of the micelle solution can be adjusted through the alteration of temperature and the bubbling of CO₂ or Ar. The controlled drug release of the dual responsive micelles was also investigated.

Scheme 1 Synthesis of amphiphilic block copolymer Py-PCL-b-P(NIPAM-co-DMAEMA).

Experimental

Materials

ε-Caprolactone (CL, Acros Organic, 99%) was purified with CaH₂ by vacuum distillation. Tin 2-ethylhexanoate (Sn(Oct)2, Aldrich) was distilled under reduced pressure. 1-Pyrenemethanol, dicyclohexylcarbodiimide (DCC; GL Biochem, Shanghai) and 4-dimethylaminopyridine (DMAP; Fluka, USA) were used as received. S-Dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate (DDMAT) was prepared according to the literature.³⁶ 2,2′-Azobisisobutyronitrile (AIBN, Aldrich, 98%) was recrystallised from ethanol. 2-(N,N-Dimethylamino)ethyl methacrylate (DMAEMA; Acros Organic) was dried over CaH₂ and distilled under reduced pressure. N-Isopropyl acrylamide (NIPAM) was purified by recrystallisation from a toluene–hexane mixture (1 : 3).

Characterisation

Nuclear magnetic resonance (NMR). ¹H NMR spectra of samples were obtained using a Bruker DMX 500 NMR spectrometer with CDCl₃ as solvent. The chemical shifts were relative to tetramethylsilane.

Gel permeation chromatography (GPC). The molecular weight and molecular weight distribution were measured on a Waters GPC at 30 °C. THF was used as the eluent, and narrow-distributed polystyrene standards were used for calibrations.

Optical transmittances. The optical transmittances of the aqueous solution of copolymer micelles (30 mg mL⁻¹, deionised water was used as the solvent) at various temperatures were measured at a wavelength of 500 nm on a UV-visible spectrophotometer (Lambda 35, PerkinElmer). The temperature of the sample cell was thermostatically controlled using an external super-constant temperature bath. The solutions were equilibrated for 10 min at each measuring temperature. The LCST value of each copolymer micelle solution was defined as the temperature producing a 50% decrease in optical transmittance.

Dynamic light scattering (DLS). The hydrodynamic radius (R_h) of the copolymer micelles was investigated using DLS techniques. The experiments were performed on a Malven Autosizer 4700 DLS spectrometer. DLS was performed at a scattering angle 90°. The R_h was obtained by a cumulant analysis.

Transmission electron microscopy (TEM). The morphologies of copolymer micelles were observed by TEM (JEOL JEM-2010) at an accelerating voltage of 120 kV. The samples for TEM observation were prepared by placing 10 μL of copolymer micelle solution on copper grids coated with thin films and carbon.

Fluorescence. Fluorescence spectra were collected on a Fluorolog-2 spectrofluorometer (Spex Industries, Edison, NJ) under the control of the dedicated SPEX DM3000F software. The temperatures of the micelle solutions were adjusted using a water bath, and the solutions were equilibrated for 10 min at each temperature. Fluorescence scans were performed in the range of 350–800 nm with increments of 1 nm. The excitation wavelength was 340 nm, which corresponds to the maximum absorption wavelength of pyrene. The micelle concentration was 2.3 mg mL⁻¹.

Synthesis of Py-PCL

Py-PCL was synthesised by the ROP of CL with 1-pyrenemethanol as the initiator. The detailed procedure is described as follows. CL (10.00 g, 87.6 mmol), 1-pyrenemethanol (0.50 g, 2.2 mmol of hydroxyl groups), Sn(Oct)₂ catalyst (87.7 μmmol) and a magnetic stirrer were added into a flame-dried polymerisation tube. The tube then connected to a Schlenkline, and an exhausting-refilling process was repeated three times. The polymerisation tube was sealed in argon atmosphere. Under stirring, the bulk polymerisation was carried out at 115 °C for 24 h. The crude polymer was dissolved in methylene chloride and precipitated three times in methanol. The purified Py-PCL was dried under vacuum until a constant weight was reached (yield: 91%).

M_n,NMR = 4792 g mol⁻¹, M_n,GPC = 4170 g mol⁻¹, M_w/M_n = 1.27. ¹H NMR (CDCl₃, δ, ppm): 8.00–8.30 (protons in pyrene group), 5.84 (pyrene–CH₂O), 4.06 (OOCCH₂CH₂CH₂CH₂CH₂O), 3.64 (OOCCH₂CH₂CH₂CH₂CH₂OH), 2.30 (OOCCH₂CH₂CH₂CH₂CH₂O), 1.65 (OOCCH₂CH₂CH₂CH₂CH₂O), 1.39 (OOCCH₂CH₂CH₂CH₂CH₂O).

Synthesis of Py-PCL–DDMAT

Py-PCL (7.0 g, 1.46 mmol), DDMAT (5.1 g, 14.1 mmol), DCC (2.9 g, 14.1 mmol) and DMAP (0.94 g, 7.69 mmol) were dissolved in 40 mL of anhydrous dichloromethane, and the reaction was performed at room temperature for 48 h under argon atmosphere. The reaction byproduct dicyclohexylcarbodiurea was removed by filtration. The purified product was obtained after removing of solvent and precipitating in methanol (yield: 70%).

M_n,NMR = 5150 g mol⁻¹, M_n,GPC = 4450 g mol⁻¹, M_w/M_n = 1.29. ¹H NMR (CDCl₃, δ, ppm): 8.00–8.30 (protons in pyrene group), 5.86 (pyrene–CH₂O), 4.06 (OOCCH₂CH₂CH₂CH₂CH₂O), 3.50 (OOCCH₂CH₂CH₂CH₂CH₂OOCC(CH₃)₂), 3.26 (SCH₂(CH₂)₁₀CH₃), 2.32 (OOCCH₂CH₂CH₂CH₂CH₂O), 1.65 (OOCCH₂CH₂CH₂CH₂CH₂O, (CH₃)₂CS), 1.40 (OOCCH₂CH₂CH₂CH₂CH₂O, SCH₂(CH₂)₁₀CH₃), 0.88 (SCH₂(CH₂)₁₀CH₃).

Synthesis of Py-PCL-b-P(NIPAM-co-DMAEMA)

Py-PCL-b-P(NIPAM-co-DMAEMA) was synthesised by RAFT with Py-PCL–DDMAT as the macro-RAFT agent. NIPAM (2.716 g, 24 mmol), DMAEMA (0.164 g, 1 mmol) and Py-PCL-DDMA (1.0 g, 0.194 mmol) were dissolved in 6 mL of dioxane, and AIBN (8.2 mg, 49 μmol) was then added. The flask was degassed with three freeze–evacuate–thaw cycles. The polymerisation reaction was performed at 70 °C for 7 h. Py-PCL-b-P(NIPAM-co-DMAEMA) copolymer was obtained after precipitation in diethyl ether twice (yield: 82%).

M_n,NMR = 16 658 g mol⁻¹, M_n,GPC = 11 320 g mol⁻¹, M_w/M_n = 1.26. ¹H NMR (CDCl₃, δ, ppm): 8.00–8.30 (protons in pyrene group), 5.92–6.87 (CONH), 4.07 (OOCCH₂CH₂CH₂CH₂CH₂O, NHCH, COOCH₂CH₂N), 2.59 (COOCH₂CH₂N), 2.31 (OOCCH₂CH₂CH₂CH₂CH₂O, N(CH₃)₂), 2.13 (CH₂CH(CO)), 1.66 (OOCCH₂CH₂CH₂CH₂CH₂O, CH₂CH(CO), CH₂C(CH₃)), 1.40 (OOCCH₂CH₂CH₂CH₂CH₂O), 1.16 (NHCH(CH₃)₂, CH₂C(CH₃)).

Preparation of Py-PCL-b-P(NIPAM-co-DMAEMA) micelles and doxorubicin (DOX)-loaded micelles

Py-PCL-b-P(NIPAM-co-DMAEMA) micelles were prepared by self-assembly of the block copolymer in water. The block copolymer (50 mg) was dissolved in DMF (10 mL). The solution was then dialysed against deionised water in a dialysis tube (molecular weight cut-off: 3500 Da) for 3 days at 25 °C.

The anti-cancer drug DOX was chosen as the model drug. The mixture of doxorubicin hydrochloride (DOX·HCl; 15 mg), Et₃N (3 mL) and DMF (10 mL) was stirred overnight at room temperature to obtain the DOX/DMF solution was obtained. Fifty milligrams of Py-PCL-b-P(NIPAM-co-DMAEMA) copolymer was added. After stirring for 3 h, the mixture was dialysed against phosphate buffered saline (PBS) solution with a dialysis tube (molecular weight cut-off: 3500 Da) at 25 °C for 24 h. The solution was replaced with fresh PBS at 6 h intervals.

Results and discussion

Synthesis of Py-PCL-b-P(NIPAM-co-DMAEMA) amphiphilic block copolymer

Py-PCL-b-P(NIPAM-co-DMAEMA) amphiphilic block copolymer was synthesised in three steps. First, Py-PCL was synthesised by the ROP of CL with 1-pyrenemethanol as the initiator. Sn(Oct)₂ was used as the catalyst. The structure of Py-PCL was characterised by ¹H NMR spectroscopy (Fig. 1(a)). In addition to the proton signals of the PCL chain and pyrene group, the signal at 3.64 ppm, which is assigned to the methylene protons next to the terminal hydroxyl groups, can be detected. The average-number molecular weight (M_n,NMR) of Py-PCL was calculated by eqn (1):

M_n,NMR (Py-PCL) = (I_e/I_e′ + 1) × 114.14 + 232.29 (1)

Fig. 1 ¹H NMR spectra of (a) Py-PCL, (b) Py-PCL–DDMAT and (c) Py-PCL-b-P(NIPAM-co-DMAEMA).

Here, 114.14 is the molecular weight of the CL monomer, and 232.29 is the molecular weight of 1-pyrenemethanol. The M_n,NMR of Py-PCL was 4792 g mol⁻¹. According to the GPC trace (Fig. 2), the average-number molecular weight of Py-PCL was 4170 g mol⁻¹.

Fig. 2 GPC traces of Py-PCL, Py-PCL–DDMAT and Py-PCL-b-P(NIPAM-co-DMAEMA).

Second, Py-PCL–DDMAT macroinitiator was prepared by DCC reaction of the terminal hydroxyl group of Py-PCL with the carboxyl group of DDMAT. Excess DDMAT was used to ensure the complete transformation of the hydroxyl group of Py-PCL. Fig. 1(b) shows the ¹H NMR spectrum of Py-PCL–DDMAT. The M_n,NMR and M_n,GPC of Py-PCL–DDMAT were 5150 g mol⁻¹ and 4450 g mol⁻¹, respectively.

Finally, Py-PCL-b-P(NIPAM-co-DMAEMA) amphiphilic block copolymer was synthesised by RAFT of NIPAM and DMAEMA with Py-PCL–DDMAT as the macroinitiator. The feed ratio of NIPAM to DMAEMA was 96 : 4 (mol% : mol%). As shown in the ¹H NMR spectrum (Fig. 1(c)), all the protons in Py-PCL-b-P(NIPAM-co-DMAEMA) can be found. The M_n,NMR and M_n,GPC of Py-PCL-b-P(NIPAM-co-DMAEMA) were 16 658 g mol⁻¹ and 11 320 g mol⁻¹, respectively. The fact that the M_n,GPC was lower than the M_n,NMR should be attributed to the possible absorption of P(NIPAM-co-DMAEMA) onto the GPC column, resulting in an increase in retention time and a lower molecular weight detected by GPC.

Self-assembly and thermo-CO₂ dual responses of Py-PCL-b-P(NIPAM-co-DMAEMA) amphiphilic block copolymer

As an amphiphilic copolymer, Py-PCL-b-P(NIPAM-co-DMAEMA) can self-assemble into micelles in aqueous solution with hydrophilic P(NIPAM-co-DMAEMA) shells and hydrophobic Py-PCL cores. The morphologies of the copolymer micelles at 25 °C and 42 °C and after passing CO₂ (5 min) and Ar (5 min) are shown in Fig. 3. At 25 °C, the spherical micelles can be self-assembled from Py-PCL-b-P(NIPAM-co-DMAEMA) in water (Fig. 3(a)). When the temperature increased to 42 °C, the micelles tended to aggregate into aggregates due to the hydrophilic–hydrophobic transition of the P(NIPAM-co-DMAEMA) shell (Fig. 3(b)). After bubbling CO₂ for 5 min, the micelle aggregation reduced because the P(NIPAM-co-DMAEMA) segment became hydrophilic due to the reaction of DMAEMA units with CO₂ and water and the formation of bicarbonate salts of protonated amine groups. Therefore, the micelle aggregation was relieved, as shown in Fig. 3(c). However, serious micelle aggregation again occurred after the micelle solution was bubbled with Ar (Fig. 3(d)), which should be attributed to the formation of deprotonated amine groups. The micelle morphology can be reversibly adjusted through the alteration of temperature and CO₂/Ar bubbling.

Fig. 3 TEM images of Py-PCL-b-P(NIPAM-co-DMAEMA) micelles at (a) 25 °C and (b) 42 °C and after passing (c) CO₂ (5 min) and (d) Ar (5 min).

Fig. 4(a) shows the transmittance curves of the Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solutions. It can be seen that the transmittance curve shows a sharp transition during the heating process of the initial solution. The LCST value of the copolymer was 36.0 °C. After bubbling CO₂ through the solution for 5 min, the transmittance increased to above 60%, and no obvious transition occurred during the heating process. After bubbling Ar for 5 min, the sharp transition during heating process appeared again, and the LCST value was 37.1 °C. The inset photograph shows the reversible transparency–turbidity transition of the copolymer micelle solutions after heating–cooling and CO₂/Ar cycles. The expanded inset images can be found in Fig. S1 (ESI).† The reversible transmittance curves of Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solutions at 25 °C and 50 °C are found in Fig. S2 (ESI).†

Fig. 4 (a) Transmittance curves of Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solutions at different conditions. Inset photograph: the reversible transparency–turbidity transition of the copolymer micelle solution after heating–cooling and CO₂–Ar bubbling cycles (micelle concentration: 2.3 mg mL⁻¹). (b) Temperature dependence of hydrodynamic radius (R_h) for Py-PCL-b-P(NIPAM-co-DMAEMA) micelles at different conditions (micelle concentration: 2.3 mg mL⁻¹).

Fig. 4(b) shows the plot of the hydrodynamic radius (R_h) of Py-PCL-b-P(NIPAM-co-DMAEMA) micelles in water as a function of temperature. In the lower temperature ranges, the R_h values were relatively small and changed slightly. However, the values increased in the higher temperature ranges. At low temperatures, the P(NIPAM-co-DMAEMA) chains in the micelles existed in random coil conformations due to the hydrogen-bonding interactions between the polymers and water molecules. When the temperature increased to a critical value, the polymer chains shrank to a globular structure because the hydrogen bonds between the polymers and water collapsed and became hydrophobic. Therefore, the micelles tended to aggregate into aggregates with large sizes. After bubbling CO₂ for 5 min, the R_h values were relatively small and changed slightly with increasing temperature because the protonated amine groups made the P(NIPAM-co-DMAEMA) chains remain hydrophilic at high temperature. As a result, aggregation did not occur in the high temperature range. If Ar was bubbled into the solution, CO₂ would be expelled from the solution by Ar, and amine groups were deprotonated at high temperature. Therefore, the hydrophobic P(NIPAM-co-DMAEMA) chains collapsed at high temperature, and the micelles aggregated to form aggregates. Due to the presence of partially protonated amine groups, the aggregation of micelles was weaker than in the initial solution.

The schematic self-assembly process of Py-PCL-b-P(NIPAM-co-DMAEMA) and the CO₂-temperature dual responses are shown in Fig. 5. The reversible hydrophilic–hydrophobic change of P(NIPAM-co-DMAEMA) chains during the heating–cooling processes led to the reversible aggregation/disaggregation behaviour. After bubbling CO₂, the micelles still remained stable at high temperature. However, bubbling Ar into the solution would lead to the aggregation of micelles due to the hydrophilicity–hydrophobicity transition of P(NIPAM-co-DMAEMA) chains after bubbling CO₂/Ar at high temperature.

Fig. 5 The schematic self-assembly process of Py-PCL-b-P(NIPAM-co-DMAEMA) and the CO₂-temperature dual responses.

In order to confirm the temperature and CO₂ responses of Py-PCL-b-P(NIPAM-co-DMAEMA), the ¹H NMR spectra of the polymer in D₂O at different temperatures and bubbled with CO₂ and Ar were recorded (Fig. 6). Compared to the proton peaks in the spectra of N-isopropylacrylamide and dimethylaminoethyl at 25 °C, the intensities of the proton peaks decreased at 45 °C. In the high temperature range, the hydrophilicity of P(NIPAM-co-DMAEMA) was obviously weakened. After bubbling CO₂ into the solution for 5 min, the intensities of the proton peaks of N-isopropylacrylamide and dimethylaminoethyl increased to some degree. Because of the reaction of DMAEMA units with CO₂ and water and the formation of bicarbonate salts of protonated amine groups, the P(NIPAM-co-DMAEMA) chains can maintain their hydrophilic state. However, after bubbling Ar into the solution for 5 min, CO₂ was removed and replaced by Ar. As a result, the protonated amine groups were deprotonated. At high temperature (42 °C), the P(NIPAM-co-DMAEMA) chains changed from hydrophilic to hydrophobic, and the intensities of the proton peaks of P(NIPAM-co-DMAEMA) obviously decreased.

Fig. 6 ¹H NMR spectra of Py-PCL-b-P(NIPAM-co-DMAEMA) in D₂O conducted at 25 °C and 42 °C and before and after passing CO₂ (5 min) and Ar (5 min) through the solution (42 °C).

Tunable fluorescence of Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solution

Due to the fluorescence of the pyrene groups, the Py-PCL-b-P(NIPAM-co-DMAEMA) micelles would also present fluorescent properties. Moreover, in the micelles, the pyrene groups were at the core part, and the fluorescence intensity was influenced by the spatial conformation variation in P(NIPAM-co-DMAEMA), which could be adjusted by the changing the temperature or bubbling with CO₂/Ar.

As shown in Fig. 7(a), the fluorescence intensity of the Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solution decreased with the increase of the temperature, especially when the temperature was higher than the LCST value of the solution. This can be explained primarily by the change in the conformation of P(NIPAM-co-DMAEMA) in aqueous solution with the change in temperature, resulting in the change in the light transmittance of the system. After CO₂ was bubbled into the solution, the decrease in the fluorescence intensity of the micelle solution was much less than that of the solution without CO₂ bubbling in the temperature range above the LCST value (Fig. 7(b)). After bubbling with CO₂, the P(NIPAM-co-DMAEMA) chains still remained hydrophilic at high temperature. As shown in Fig. 8(c), when Ar was bubbled into the solution for 5 min, CO₂ was removed, and the P(NIPAM-co-DMAEMA) chains became hydrophobic. At high temperatures, the fluorescence intensity of the micelle solution rapidly decreased. Therefore, the tunable fluorescence properties of the Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solution could be achieved by altering the temperature of the solution or bubbling CO₂/Ar into the micelle solution.

Fig. 7 The fluorescence spectra of Py-PCL-b-P(NIPAM-co-DMAEMA) micelle solutions (a) at different temperatures, (b) at different temperatures after bubbling with CO₂ for 5 min and (c) at different temperatures after bubbling with Ar for 5 min.

Fig. 8 Controlled release of DOX at 25 °C, 40 °C and 40 °C with alternative bubbling with CO₂/Ar.

Controlled drug-release of Py-PCL-b-P(NIPAM-co-DMAEMA) micelle

The controlled drug-release behaviours of the Py-PCL-b-P(NIPAM-co-DMAEMA) micelles at 25 °C, 40 °C and 40 °C (with alternative CO₂/Ar) were investigated. Model drug (DOX) loading content (DLC) and drug loading efficiency (DLE) were calculated according to the following formulae:

DLC (wt%) = (weight of loading DOX/weight of polymer) × 100%

DLE (wt%) = (weight of loading DOX/weight of DOX in feed) × 100%

The final DLC and DLE of the supramolecular block copolymer micelles were 6.1% and 21.7%, respectively.

As shown in Fig. 8, the drug release profile from micelles shows greater changes with temperature alternation around the LCST of the copolymer. Under the LCST (25 °C), the highly hydrated P(NIPAM-co-DMAEMA) segments stabilised the hydrophobic–hydrophilic core–shell structure of the micelles, and only small amount of drug could diffuse out from the micelles. As a result, the drug release was slow, and about 79% of the drug still remained in the core of the micelles after 48 h. However, when the temperature was increased above the LCST (40 °C), the drug release was accelerated due to the temperature-induced structural changes of the micelles. Namely, the P(NIPAM-co-DMAEMA) shell became hydrophobic, and the micellar core–shell structure was deformed. Therefore, the hydrophobic DOX incorporated in the core diffused out quickly, and about 59% of the drug was released from the micelles after 48 h. Obviously, the release rate of the model drug DOX from the micelles could be effectively controlled by changing the solution temperature. Moreover, the release curve of DOX with alternating CO₂/Ar bubbling presented a fast–slow alternation feature, which should be attributed to the extension–retraction motion of the micelles under the alternative stimulation of CO₂/Ar. As a result, about 80% of the DOX was released from the micelles after 48 h.

Conclusions

A novel amphiphilic copolymer with a terminal pyrene group (Py-PCL-b-P(NIPAM-co-DMAEMA)) was synthesised successfully by the combination of ROP, DCC reaction and RAFT. The copolymer could self-assemble into micelles with Py-PCL cores and P(NIPAM-co-DMAEMA) shells. The micelles show switchable CO₂-temperature dual responses. Without bubbling CO₂, the copolymer presented a LCST value of 36 °C and exhibited thermo-responsivity. After bubbling with CO₂, the P(NIPAM-co-DMAEMA) chains remained hydrophilic at high temperature. Removing CO₂ by Ar led to the appearance of a similar LCST value (37.1 °C). Investigation shows that the copolymer micelle solution could present fluorescence, and the fluorescence intensities could be adjusted by altering the solution temperature or bubbling CO₂/Ar into the solution. Moreover, as a drug delivery system, the DOX-loaded micelles accomplished the controlled release of DOX by changing the temperature and alternatively bubbling CO₂/Ar into the solution.

Acknowledgements

The authors are thankful for the financial support from the National Key Technology R&D Program (no. 2012BAI15B061), the National Basic Research Program of China (973 Program: 2011CB013805) and the National High Technology Research and Development Program (no. 2013AA032202).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13118e

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