Synthesis of Yolk–Shell Polymeric Nanocapsules Encapsulated with

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Synthesis of Yolk−Shell Polymeric Nanocapsules Encapsulated with Monodispersed Upconversion Nanoparticle for Dual-Responsive Controlled Drug Release Xiaotao Wang,† Xiaoping Liu,† Li Wang,† Chak-Yin Tang,*,‡ Wing-Cheung Law,*,‡ Gaowen Zhang,† Yonggui Liao,§ Chuang Liu,† and Zuifang Liu*,†

Macromolecules Downloaded from pubs.acs.org by COLUMBIA UNIV on 12/05/18. For personal use only.

†

Hubei Provincial Key Laboratory of Green Materials for Light Industry, Collaborative Innovation Center for Green Light-weight Materials and Processing, School of Materials Science and Engineering, Hubei University of Technology, Wuhan, Hubei Province 430068, P. R. China ‡ Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, P. R. China § Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China S Supporting Information *

ABSTRACT: Dual-responsive (light and pH) yolk−shell structured drug delivery nanocapsules, each consisting of a movable upconversion nanoparticle (UCNP) core and a shrinkable poly(methacrylic acid) (PMAA) shell, were prepared by distillation precipitation polymerization. Monodispersed NaYF4: Yb3+/Tm3+ UCNPs were synthesized and encapsulated in silica templates, followed by coating to form PMAA shells. Subsequently, the silica templates were dissolved to form nanocavities for drug loading. The PMAA shell contains pH and ultraviolet (UV) light sensing moieties, enabling a control release upon the exposure of nanocapsules to these stimuli. The near-infrared (NIR)-to-UV feature of UCNPs allows azobenzene isomerization to be light triggered remotely to control contraction and swelling of PMAA shells. The loading efficiency of the anticancer drug doxorubicin (DXR) was up to 17 wt % due to the unique nanoporous structure of PMAA shells. The values of the diffusion coefficient under different release conditions were determined using the Baker−Lonsdale model to facilitate the design of dual-responsive drug release devices or systems.

1. INTRODUCTION Responsive polymers exhibit physical or chemical property changes under external stimulus, such as pH, temperature, ionic strength, or electric/magnetic field.1−4 These intelligent features can be applied to drug delivery systems. In particular, hollow nanostructures (nanocapsules) have generated intensive interest because of the large loading capability and easy configurable features by controlling the permeability of a thin layer of the shell.5,6 Many strategies have been developed to fabricate smart nanocapsules as drug carriers, which are responsive to a wide variety of external stimuli.7−9 Among them, light offers a higher spatial and temporal degree of freedom for remote activation of a wide range of materials at a specific time and location with relatively high precision.10 Therefore, a large number of nanocapsules with lightresponsive groups, such as azobenzene, spiropyran, o-nitrobenzyl, and coumarin,11 have been designed and applied in polymeric drug release systems. Because of the inhibition effect of photoisomeric moieties,12 the self-assembling method through block copolymer was usually used to construct © XXXX American Chemical Society

photoresponsive micelles or microcapsules, in which the release of payloads was accomplished by UV light.13 Typically, most copolymers containing photoresponsive groups are sensitive to UV light.14 However, biomedical applications prefer a longer wavelength (e.g., NIR) light source that has less absorption by tissues and causes less damage to healthy cells. UCNPs exploit the physical properties of the lanthanide dopants to sequentially absorb several photons of NIR light and emit a single photon in the UV or visible range.15 Based on this upconversion process, diverse UV−vis photochemistry processes can be controlled through NIR light. Incorporating UCNPs could potentially bridge the gap between the available photoresponsive polymers and noninvasive, low-energy NIR light ideal for biomedical applications. Received: August 16, 2018 Revised: November 21, 2018

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DOI: 10.1021/acs.macromol.8b01770 Macromolecules XXXX, XXX, XXX−XXX

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min until a homogeneous transparent yellow solution was obtained. After removing the heating mantle and allowing the mixture to cool slowly to room temperature, we pipetted 5 mL of a NaOH−methanol stock solution and 8 mL of a NH4F−methanol stock solution into a 15 mL centrifuge tube. Then the mixture was added into a reaction flask and stirred at room temperature for 1 h. After the methanol was evaporated, the solution was heated to 300 °C under nitrogen protection as soon as possible and maintained for 90 min before it was cooled to room temperature. The OA-NaYF4: Yb, Tm nanoparticles were collected by centrifugation at 10000 rpm for 10 min and washed with ethanol/cyclohexane three times prior to use. 2.2. Synthesis of OA-NaYF4: Yb/Tm@NaYF4(UCNPs) Core− Shell Nanoparticles. One millimole of OA-NaYF4: Yb/Tm@NaYF4 (UCNPs) nanoparticles was first prepared using similar procedures to those described above and was added to a 100 mL flask. Then 1 mmol of YCl3 in aqueous solution was added into the flask containing 7 mL of oleic acid and 15 mL of 1-octadencene. The flask was placed in a thermometer and slowly heated to 120 °C to eliminate water and oxygen under a nitrogen atmosphere and maintained at 150 °C for about 50 min until a homogeneous transparent yellow solution was obtained. After removal of the heating mantle, the reaction mixture was allowed to cool slowly to room temperature while stirring. At room temperature, 3.5 mL of a NaOH−methanol stock solution and 2 mL of a NH4F−methanol stock solution were pipetted into a 15 mL centrifuge tube. Then the mixture was quickly added into the reaction flask and stirred at room temperature for 1 h. After the methanol evaporated at 50 °C, the solution was heated to 300 °C as soon as possible and kept for 90 min before it was cooled to room temperature. The same washing procedure was followed, and sample was redispersed in 10 mL of cyclohexane (UCNPs stock solution). 2.3. Coating of Silica on the UCNPs. UCNPs@SiO2 core/shell nanoparticles were prepared by a reverse microemulsion method. Typically, 1.3 mL of CO-520, 20 mL of cyclohexane, and 1.1 mL of UCNPs stock solution were mixed and stirred for 10 min. Then 0.16 mL of ammonium hydroxide (30 wt %) was added, and the container was sealed and sonicated for 30 min until a transparent emulsion was formed. Finally, 0.25 mL of TEOS was added into the solution and stirred for 24 h at a speed of 600 rpm. To functionalize the surface of the UCNPs@SiO2 nanospheres, 2.5 mL of MPS was added to the dispersion and stirred for another 24 h at the same speed. UCNPs@ SiO2 core/shell nanoparticles were precipitated by adding acetone, and the nanoparticles were washed with ethanol three times and then stored in ethanol. 2.4. Synthesis of Bis(methacryloylamino)azobenzene (BMAAB). The synthesis of 4,4′-diaminoazobenzene (DAAB) was performed in accordance with ref 24. A mild dehydration reaction was used to prepare BMAAB. In detail, 2 g of DAAB was dissolved in 800 mL of chloroform. Ten grams of methacrylic acid and 3.6 g of EDC· HCl were added into the flask. The reaction was kept at ambient temperature for 3 days. The organic solution was washed twice with water and evaporated under reduced pressure. The crude product was recrystallized with ethanol and dried at 50 °C. The FTIR spectrum of BMAAB is illustrated in Figure S1. 2.5. Synthesis of UCNPs@SiO2@PAzo/MAA. UCNPs@SiO2@ PAzo/MAA nanospheres were prepared by distillation precipitation polymerization. Typically, 0.2 g of vacuum-dried UCNPs@SiO2 nanospheres was dispersed in a 250 mL flask with 160 mL of acetonitrile for 30 min of ultrasonication bath; then a mixture of BMAAB (0.8673 g), MAA (162 mL), DVB (42 μL), and AIBN (0.0226 g, 2 wt % relative to the comonomer) was added into the flask, which was equipped with an oil−water separator, a condenser, a receiver, and nitrogen protection device. The flask was submerged in a heating mantle, and the reaction mixture was heated from ambient temperature to 80 °C within 30 min and kept for 15 min. The reaction was ended after 80 mL of acetonitrile was distilled from the reaction system within 4.5 h. The resultant UCNPs@SiO2@PAzo/ MAA nanospheres were purified by three cycles of centrifugation and redispersed in ethanol and THF. 2.6. Synthesis of Dual-Responsive Nanocapsules (UCNPs@ PAzo/MAA NCs). Nanocapsuleswere prepared by removal of the

Recently, self-assembled copolymer micelles with encapsulated UCNPs have been widely studied. Zhao and co-workers utilized an NIR laser to disassembly block copolymer micelles by the encapsulated UCNPs.16 Wang’s group17 fabricated UCNPs@polymer composites through self-assembly of the amphiphilic spiropyran/N-isopropylacrylamide block copolymers for controlled release by NIR irradiation. Wu et al. also constructed self-assembly of NIR-responsive nanoclusters with an azobenzene-containing amphiphilic copolymer.18 In addition, the photochromic-modified mesoporous silica systems based on UCNPs have attracted much attention. Liu first reported the precise control of in vitro drug delivery using an azobenzene-modified mesoporous silica integrated with UCNPs.19−21 Photoresponsive moieties were constructed in an immobilized mesoporous silica matrix encapsulated with UCNPs system to obtain an on-demand NIR release pattern.21 However, some challenges, such as the micelle’s stability in a biological environment, burst release after the dissociation under light, and the limitation of the structure design in a silica matrix, still exist in either the self-assembled or mesoporous silica/UCNPs composite systems.18,22 Research on fabrication of monodispersed robust polymeric nanocapsules with repeatable “ON/OFF” switching features and high drug loading capability has seldom been reported. Photoresponsive polymers do not easily form particles. Our group pioneered the use of the distillation precipitation polymerization process to prepare robust monodispersed photoresponsive microcapsules to solve the problems.23 In this contribution, we prepared yolk−shell stable nanocapsules consisting of UCNPs as movable cores and PMAA shells crosslinked by bis(mathacryloylamino)azobenzene (BMAAB) through silica hard templates and the distillation precipitation method. UCNPs in the yolk transformed the NIR light to UV/ vis light to isomerize BMAAB, which could tune the permeability of the shell through the photomechanical effect. PMAA would realize pH-responsive release. The yolk−shell structure can act as a large loading space for a drug (DXR) and a protective layer to prevent them from interacting with other species, such that release is triggered upon receiving the pH/ NIR light stimulus.

2. EXPERIMENT Yuelong New Material Co., Shanghai, China, supplied rare-earth oxides (Y2O3, Yb2O3, and Tm2O3). Oleic acid (OA; 90%), 1octadecene (ODE; 95%), divinylbenzene (DVB), and azobis(isobutyronitrile) (AIBN) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Sodium hydroxide, ammonium fluoride, hydrochloric acid (HCl; 36.7%), ethanol, chloroform, cyclohexane, ammonia aqueous solution (33 wt %), tetraethyl orthosilicate (TEOS), and methacrylic acid were obtained from Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China. Igepal CO520 was purchased from Sigma-Aldrich Trading Co., Ltd., Shanghai, China. Organic Silicon Company at Wuhan University, China, supplied 3-methacryloxypropyltrimethoxysilane (MPS), while highpurity azobis(isobutyronitrile) (Huacheng Industrial Co., Ltd., Shanghai, China) was recrystallized by ethanol. The other chemicals were purchased from Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China. 2.1. Synthesis of Oleic Acid (OA)-Coated NaYF4: Yb3+/Tm3+ Nanoparticles (OA-NaYF4: Yb/Tm). Typically, a solution of YCl3 (0.795 mmol), YbCl3 (0.200 mmol), and TmCl3 (5 μmol) was added into a mixture of 7 mL of oleic acid and 15 mL of 1-octadecene in a 100 mL flask. The flask was then placed in a heating mantle, and the mixture was slowly heated to 120 °C to eliminate water and oxygen under a nitrogen atmosphere and maintained at 150 °C for about 50 B

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Macromolecules SiO2 shell from the UCNPs@SiO2@PAzo/MAA nanospheres. The above nanospheres were immersed in 40% HF solution for 48 h. The excess HF and SiF4 were expelled from UCNPs@ PAzo/MAA nanospheres in three centrifugation−redispersion cycles in THF. Then the mixture was put into a dialytic bag with ethanol. 2.7. Loading of Doxorubicin Hydrochloride (DXR). DXR (12 mg) was dispersed in deionized water (20 mL). UCNPs@ PAzo/ MAA (60 mg) was added to 12 mL of the solution, and the suspension was stirred at room temperature for 24 h. The as-prepared DXR-UCNPs@PAzo/MAA was collected by centrifugation at 14000 rpm. The precipitates were washed three times with ethanol to completely remove free DXR molecules adsorbed on the outer surface. The loading efficiency is calculated from the weight of DXR in the UCNPs@PAzo/MAA nanocapsules divided by the weight of UCNPs@PAzo/MAA: loading efficiency =

Scheme 1. Route of UCNPs@PAzo/MAA NCs and Loading and Release of DXR

Wadministrated − Wresidual Wnanocapsules

The weight of DXR was determined from the standard curve as shown in Figure S9. 2.8. Photocontrolled Releasing Behavior of the UCNPs@ PAzo/MAA NCs. DXR-loaded UCNPs@ PAzo/MAA NCs (20 mg) were redispersed in 20 mL of water and then poured into the dialysis tubes with a molecular weight cutoff of 8000−14000 (Spectra/Por, Spectrum Laboratories, Inc., USA). The dialysis tubes were placed in reservoirs with 20 mL of water under NIR irradiation. The solution (3 mL) was periodically taken out to determine the concentration of DXR released by UV−vis spectrometer analysis at a wavelength of 480 nm, and then the test solution was returned to the reservoir immediately. 2.9. pH-Controlled Release Behavior of the UCNPs@PAzo/ MAA NCs. The pH-controlled release behaviors of DXR from theUCNPs@PAzo/MAA nanocapsules were studied as follows. DXRloaded UCNPs@PAzo/MAA nanocapsules (20 mg) were redispersed in 20 mL of water, and the resulting dispersion was poured into the dialysis tubes with a molecular weight cutoff of 8000−14000 (Spectra/Por, Spectrum Laboratories, Inc., USA). The dialysis tube was placed in a reservoir with 20 mL of aqueous solution at pH 4.5, 7.0, and 9.0. Subsequently, the solution (3 mL) was periodically taken out to determine the concentration of DXR released. 2.10. Characterizations. Powder X-ray diffraction (XRD) measurements were taken with a Bruker D8 Advance X-ray powder diffractometer. The morphologies of UCNPs were observed on a Tecnai G20 (FEI Co.) transmission electron microscope (TEM) at 200 kV. Samples were prepared by placing a drop of the dilute dispersion in cyclohexane on the surface of a copper grid. Fourier transform infrared (FT-IR) spectra were recorded from samples in a KBr slice using a Nicolet iS10 spectrometer (Thermo Scientific). Upconversion luminescence (UCL) spectra were recorded on a Hitachi F-500 fluorescence spectrophotometer equipped with a nearinfrared (NIR) laser (980 nm, Hi-Tech Optoelectronics Co., Ltd., China, 2.5 W) as the excitation source instead of the xenon source in the spectrophotometer.

photographs are shown in Figures S2−S4. The XRD result of β-NaYF4 is given in Figure S5. The as-prepared monodispersed NaYF4: Yb/Tm@NaYF4 (UCNPs) nanoparticles were coated with a thin layer of oleic acid molecules to prevent them from aggregation and assist dispersing them well in nonpolar solvents, as characterized by FTIR (Figure S6). Because of the hydrophobic nature of OA, the OA-capped UCNPs are insoluble in water or biological buffers. To disperse the UCNPs in aqueous solutions, silica was chosen as an intermediate layer and a solid template which could respectively confer the hydrophilicity and allow us to crosslink the photoresponsive polymers. When the silica layer is etched, the hollow core can be used for carrying drugs. The loading capacity is determined by the thickness of silica layer. A modified water-in-oil reverse microemulsion technique was utilized to prepare UCNPs@SiO2. First, oleic acid-coated UCNPs nanoparticles and CO-520 dispersed in cyclohexane and microemulsion were formed by the CO-520 after adding water. Then, UCNPs nanoparticles and TEOS transformed into microemulsion and formed UCNPs@SiO2 nanoparticles through the hydrolysis of TEOS. There are similar examples on the silica deposited on nanoparticles through the reversible microemulsion method,25,26 which proved a rapid ligand exchange took place at the quantum dots surface. It was found that hydrolyzed TEOS has a high affinity for the nanoparticles surface and replaces the hydrophobic ligands, which enables the transfer of the nanoparticles to the interior of the micelles where silica growth takes place. In this work, the silica shell thickness and silica@UCNPs nanoparticles dispersibility were systematically investigated. In detail, the concentrations of tetraethyl orthosilicate (TEOS) and UCNPs core−shell nanoparticles were varied to investigate their influence on the morphology of silica. The influence of the amount of UCNPs on the UCNPs@ SiO2 nanoparticles content was studied within the range 0.5− 2.0 mL, whereas all other reagent amounts were kept constant (250 μL of TEOS) as described above. The TEM images of the UCNPs@SiO2 particles prepared in UCNPs amounts of 2.0, 1.1, 0.7, and 0.5 mL are shown in Figure 2a−d, respectively. As shown in Figure 2a−d, when the ratio of TEOS and UCNPs increases, the thickness of the SiO2 layer increases. With UCNPs amount of 0.7 mL (Figure 2c), the thickness of the SiO2 shell was about 5 times thicker than the

3. RESULTS AND DISCUSSION Scheme 1 describes the route of the monodispersed yolk−shell UCNPs@PAzo/MAA NCs. First, the lanthanide-doped UCNPs nanoparticles (core = NaYF4: 20 mol % Yb3+/0.5 mol % Tm3+; shell = NaYF4) were synthesized by a solvothermal method.17 From the transmission electron microscope (TEM) images shown in Figure 1a,b, it can be seen that the as-prepared UCNPs are spherical with core diameter of 29 ± 1 nm (Figure 1a). With the NaYF4 shell, the UCNPs became rod shaped and the size increased to 32 ± 2 nm (Figure 1b). The control of the reaction conditions including amount of OA, temperature, and reaction time of NaYF4:20 mol % Yb3+/0.5 mol % Tm3+ are discussed in detail in the Supporting Information. The corresponding TEM C

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Figure 1. TEM images of (a) NaYF4: Yb3+/Tm3+ and (b) NaYF4: Yb3+/Tm3+@NaYF4.

number of azo chromophores.12 The distillation precipitation polymerization method can be used to fabricate photoresponsive polyazobenzene particles through its novel feature. A lower monomer concentration in the beginning of reaction can prevent the particles from aggregation. By distilling solvent at a constant speed, we can achieve a high monomer conversion, which is the precondition of forming polyazobenzene particles.28 In the present work, UCNPs@SiO2@ PAzo/MAA NCs were prepared by distillation precipitation polymerization using UCNPs@SiO2 as a template, pHresponsive MAA as a monomer, and photoresponsive BMAAB as a cross-linker. The morphologies of the UCNPs@SiO2@PAzo/MAA nanospheres are shown in Figure 3. The content of these reagents was adjusted to yield thicker

Figure 2. TEM micrographs of core/shell/shell structured NaYF4: Yb3+/Tm3+@NaYF4@SiO2 particles prepared under different initial amounts of NaYF4: Yb3+/Tm3+@NaYF4: (a) 2.0, (b) 1.4, (c) 0.7, and (d) 0.5 mL. Figure 3. TEM micrographs of (a) UCNPs@SiO2@PAzo/MAA particles and (b) UCNPs@PAzo/MAA NCs.

one with UCNPs amount of 2.5 mL and TEOS amount of 250 μL, suggesting that the silica thickness can be tuned by the amount of UCNPs. Similar phenomena of tuning the silica thickness by adjusting the amount of nanoparticles were also reported on coating quantum dots.25,26 Therefore, to obtain a thicker SiO2 shell for allowing sufficient space for DXR loading, the requisite amount of UCNPs under 250 μL of TEOS was determined at 0.7 mL. Li et al. reported that the core/shell-structured pure-hexagonal-phase UCNPs had very thin and uniform silica coatings.27 Our work demonstrated a reverse microemulsion method to control the thickness of silica shell, with high monodispersity, by simply tuning the TEOS:UCNPs ratio. UCNP@SiO2 particles were prepared under different initial amounts of TEOS: (a) 160, (b) 200, and (c) 250 μL (Figure S7). Because of the inhibition of azo groups to the free-radical chain reaction, it is hard to form particles containing a large

shell and monodispersed UCNPs@SiO2@PAzo/MAA nanospheres. The amount of solvent (i.e., acetonitrile) affects significantly the morphologies and dispersity of nanospheres.29,30 When 70 mL of acetonitrile, 85 mg of BMAAB, and 143 μL of MAA were used, monodispersed nanospheres with thinner shell nanocapsules were formed (Figure 3a). UCNPs@PAzo/MAA NCs were obtained by etching the SiO2 layer with hydrofluoric acid (Figure 3b). The clear yolk−shell structure was obtained with UCNPs as the movable core and PAzo/MAA as the shell, in which the valuable silica space could be used for drug loading. Synthesis of UCNPs@SiO2@ PAzo/PMAA at different reaction conditions is discussed in Figure S8. D

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Macromolecules 3.1. Loading of DXR on UCNPs@PAzo/MAA Nanocapsules. A commonly used DXR was chosen as the drug model. Hydrophilic DXR in water was easily loaded into the inner cavities of UCNPs@PAzo/MAA nanocapsules through diffusion precipitation polymerization. It was found that the loading efficiency of DXR is up to 17 wt %. Compared with other reported upconversion-based phototriggered release systems, the loading efficiency was increased.31−33 The comparative loading values for the polymer coating UCNPs system and the porous silica coating were respectively around 8%31,32 and 5%,33 while the yolk−shell polymer system had better loading performance. Therefore, it was more favorable for sustainable release application. 3.2. pH-Controlled Release Behavior of DXR-UCNPs@ PAzo/MAA Nanocapsules. The shells of UCNPs@PAzo/ MAA NCs were composed of a photoresponsive cross-linked network and pH-responsive MAA monomer. Figure 4 shows

DXR. On the other hand, the hydrogen bonds and charge attraction would affect the release of DXR. DXR (zeta potential: +2.2 mV) would have charge interactions with ionized PMAA (pKa = 4.8) in pH 7.0, while there were no obvious charge attractions at pH 4.5. As for the influence of the hydrogen bonds effects, two kinds of hydrogen-bonding interaction between the host UCNPs@PAzo/MAA NCs and DXR molecules may be formed, i.e., a carboxylic acid group of PMAA segment with the hydroxyl group of DXR and a carboxylic acid group of PMAA segment with the amino group of DXR. However, under acidic conditions, the protonated −NH3+ cannot play an active role for the hydrogen-bonding interaction, which causes only one kind of hydrogen-bonding interaction between them, i.e., the carboxylic acid group with the hydroxyl group. These three factors (weak hydrogen bonding, weak charge attraction, and shrinkage shell) lead to faster release at pH 4.5. It is well-known that cancerous tissue lysosomes have lower pH (pH 4.5−5.0)34 as compared to physiological pH 7.0. Our UCNPs@PAzo/MAA NCs could both achieve controlled release in cancerous tissues lysosomes and prevent the undesirable side effects on normal tissue by minimizing the release of drug at pH 7.0, which is of crucial importance in the cure of cancer. The release in an alkaline condition indicated instability of DXR at pH 9 in Figure S11.

Figure 4. Hydrodynamic diameters of UCNPs@PAzo/MAA NCs at different pH values.

the size of UCNPs@PAzo/MAA nanocapsules at different pH values. By varying the pH value, the size of UCNPs@PAzo/ MAA nanocapsules was gradually increased from 55.8 nm at pH 5 to 80 nm at pH 7 and further to 115.8 nm at pH 9. This change arises from the deionization/ionization of −COOH groups (pKa = 4.8) in PMAA. At low pH, the PMAA polymer can maintain a contracted state due to its hydrophobicity under acidic conditions. At high pH, the PAzo/MAA networks are in a swollen state because most of the carboxylic acid groups of PMAA are ionized as −COO− anions, leading to an electrostatic repulsion force between the PMAA polymers. Fluorescence microscopy images of UCNPs@PAzo/MAA NCs loaded with DXR (a) at pH 4.5, (b) after the release of DXR for 24 h, and (c) the solution outside dialysis tube are shown in Figure S10. After DXR was successfully loaded in the nanocapsules, the release behavior was studied through UV absorption of DXR outside the dialysis bag. UV−vis spectra of DXR release from UCNPs@PAzo/MAA NCs under pH 4.5, 7.0, and 9.0 were recorded and are shown in Figure S11. Clearly, the absorption of released DXR at 480 nm increases with time until it reaches a maximal release amount. Interestingly, it took 460 min to achieve only a 3.88% release amount to be in a balanced state with pH 7.0. The release amount reached 47.34% under pH 4.5 during 223 min, much faster than that for pH 7.0. On the one hand, the shrinkage under pH 4.5 would accelerate the release of the encapsulated

Figure 5. Cumulative release of DXR from UCNPs@PAzo/MAA NCs as a function of time under pH 4.5 (red) and pH 7.0 (green). Inset: surface structure of UCNPs@PAzo/MAA NCs and chemical structure of the DXR molecule.

The upconversion luminescence (UCL) of rare-earth ions has been investigated extensively because of these unique optical properties, which allow minimization of autofluorescence and photodamage as well as high penetration depth in tissue under 980 nm NIR light irradiation.35 The UCL mechanism of NaYF4: Yb3+/Tm3+ is detailed as shown in Figure S12. The near-ultraviolet to visible emission bands of the UCL spectra of the Yb3+/Tm3+ transitions include ∼365 nm (1D2−3H6), ∼450 nm (1D2−3H4), and ∼475 nm ( 1 G 4 − 3 H 6 ), which are mostly and partially absorbed immediately by the photoresponsive azobenzene. Simultaneous UV and visible light emitted by the core UCNPs could produce reversible photoisomerization of azo in the shell network. Figure 6a is the upconversion luminescent spectra of UCNPs (red) and UCNPs@PAzo/MAA (black). UCNPs@ PAzo/MAA has no luminescence spectrum at 365 nm as compared with the luminescent spectra of UCNPs. It can be proven that the UV light and visible light (450 nm) emitted by E

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Figure 6. (a) Upconversion luminescent spectra of UCNPs (red) and UCNPs@PAzo/MAA (black). (b) Hydrodynamic diameters of UCNPs@ PAzo/MAA NC after 45 min of NIR irradiation and then after 60 min of visible light. (c) UV−Vis spectra of UCNPs@PAzo/MAA NC under NIR irradiation. (d) Cumulative release of DXR from UCNPs@PAzo/MAA NCs under different irradiation modes: vis at pH 7 (black curve); NIR at pH 7 (green curve); alternate vis/NIR at pH 7 (cyan curve); vis at pH 4.5 (red curve) and alternate vis/NIR at pH 4.5 (blue curve).

Figure 7. Linear regression analysis using the Baker−Lonsdale model profile of DXR from UCNPs@PAzo/MAA NCs as a function of time under different conditions.

to the cis-isomer, this led to shrinkage of the whole shell (red curve in Figure 6b). After visible light irradiation for 60 min (green curve in Figure 6b), the cis-isomer can transform to the trans-isomer with the structure returning in Figure 6b (black curve). This interesting photoinduced deformation behavior, arising from the isomerization of azobenzene moieties, has been intensively studied and is called mechanical effects. Baczkowski37 reported azobenzene-functionalized polyimides materials which showed large deformation with a new bis(azobenzene− diamine) monomer incorporated. Kulawardana reported a photoresponsive oil sorber with bis(methacrylamino)azobenzene as the photoresponsive cross-linker. The higher cross-linking caused more contraction resulting from trans−cis

the UCNPs under NIR excitation are absorbed by the azobenzene. Both UCNPs and UCNPs@PAzo/MAA had luminescence spectra at 650 nm before and after the azobenzene coating. The difference of luminescent spectra of UCNPs and UCNPs@PAzo/MAA is also proven by the photograph of them under 980 nm irradiation, transformed from blue-violet light for UCNPs to red light for UCNPs@ PAzo/MAA. Figure 6c illustrates 45 min of NIR light irradiation triggered the transformation of the trans-isomer to the cis-structure, and eventually equilibrium can be reached. The distance between 4- and 4′-carbons and its corresponding dipole moment are 9.0 Å and 0 D for the trans-form and 5.5 Å and 3 D for the cis-one, respectively.36 With the distance of cross-linking sites decreasing when the trans-isomer transforms F

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delivery system with pH- and NIR-responsive features. The system has great potential for improving drug efficacy to prevent undesirable side effects in normal tissue.

isomerization of the azobenzene functionality, which leaded to faster photoinduced desorption of oil.38 In our work, cumulative release curves of DXR under different conditions are shown in Figure 6d. Figure 6d shows that it takes 459 min to reach only a 4.04% release amount and 120 min to reach 18.3% maximal release amount under NIR light. It is attributed to the photomechanical effects in which the shrinkage of PMAA shell under NIR irradiation would extrude the DXR out of the nanocapsules. Interestingly, when alternate visible light (60 min)/NIR (45 min) (vis/NIR) irradiation was used, a faster release rate could be obtained. The movement of the shell network under alternate vis/NIR promoted the release of DXR until a balanced state was reached. At pH 7.0, the final release amount could only reach 36.6%, even under alternate vis/NIR. It can be explained by the strong electrostatic and hydrogen-bonding interactions at pH 7.0. The results indicate that 67% release could be reached under vis/NIR at pH 4.5. This release profile can be used as a controlled release mechanism in cancerous tissues lysosomes (pH 4.5). Our work also first demonstrated a NIR controlled mechanical effects on the robust polymeric nanocapsules. The UV spectra of DXR under alternating vis/NIR and pure NIR light irradiation are shown in Figure S11. There are various models to describe the release of drugs from different matrices. The Baker−Lonsdale model, widely used for a spherical matrix, is applied for modeling the release under different conditions.23,39−44 ÄÅ É 2/3Ñ Å Ñ 3DC M t yzz ÑÑÑ Mt 3 ÅÅÅÅ jij = 2 st zzz ÑÑÑ − ÅÅ1 − jjj1 − Å Ñ 2 ÅÅ M∞ { ÑÑ M∞ r0 C0 k ÅÇ ÑÖ

4. CONCLUSIONS In this study, yolk−shell structured UCNPs@PAzo/MAA nanocapsules (NCs) were prepared as a model drug carrier by distillation precipitation polymerization. Unique dual-responsive (pH and NIR) controlled release profiles with high loading efficiency were obtained. UCNPs absorb 980 nm NIR and convert it into UV for the isomerization of azobenzene at the shell cross-link, conferring photomechanical shrinkage and swelling capability and controlled release feature to UCNPs@ PAzo/MAA NCs upon the exposure of NIR light and alternate vis/NIR. Interestingly, a more rapid release was found at pH 4.5 (cancerous lysosomes tissues) than that at pH 7.0, which could achieve the controlled endogenous release. This work demonstrates the fabrication of polymer drug nanocarriers which respond to pH and light stimuli and provides a potential solution to the problem of low tissue penetration depth and low loading capacity. Moreover, the Baker−Lonsdale model was employed to calculate the values of the diffusion coefficient under different conditions for providing the information to assist the design of nanocarriers based on these nanocapsules.

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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.8b01770.

where Mt and M∞ are the amounts of the DXR at time t and at infinite time, respectively, the ratio of Mt/M∞ is the cumulative release of DXR, D is the diffusion coefficient of the molecules from the system, r0 is the initial radius of the nanocapsule, Cs is the solubility of the DXR in the system, and C0 is the initial concentration of DXR. The results in Figure 7 indicate that the Baker−Lonsdale model fits the release profile of DXR under different pH and illumination values. The values of the release rate constants (3DCs/r02C0) and the correlation coefficients R2 are 5.3 × 10−4 and 0.97 (pH: 4.5, vis/NIR), 2.1 × 10−4 (pH: 4.5, visible light), 5.4 × 10−7 and 0.99 (pH: 7.0, visible light) 7.3 × 10−5 and 0.99 (pH: 7.0, NIR light) and 1.17 × 10−4 and 0.99 (pH: 7.0, Vis/NIR), providing an independent NIR light and pH controlled release. In the same system, Cs is equal to C0, and the diffusion factor D under NIR irradiation is 2 orders of magnitude larger than that in visible light, demonstrating a photomechanically driven release. The diffusion coefficient under alternate vis/NIR irradiation is 60% larger than for pure NIR light according to their shrinkage and swelling cycles in pH 7.0. The diffusion coefficient at pH 4.5 (visible light) is large, which is due to the weaker electrostatic and hydrogenbonding interaction. The alternate light can accelerate release in cancerous tissues lysosomes to achieve D = 5.3 × 10−4, 2.5 times that in pH 4.5. This controllable DXR release feature is useful to improve the drug delivery efficiency. Similar phenomena in which the external conditions affected the diffusion coefficient were observed.23,45 Furthermore, the accuracy can be further improved by conjugating targeted biomolecules (e.g., folic acid, transferrin, and monoclonal antibodies) on the nanocapsule surface through the free carboxyl groups (i.e., carbodiimide chemistry). This work successfully demonstrated a yolk−shell structured drug

Preparation conditions and corresponding structure characterization of UCNPs under different reactions; synthesis of UCNPs@SiO2 and UCNPs@SiO2@PAzo/ MAA at different reaction conditions; fluorescence microscopy images of UCNPs@PAzo/MAA nanocapsules loaded with DXR (a), after the release of DXR for 24 h at pH 9 (b), and the solution outside dialysis tube (c); release behavior under pH 4.5, 7.0, and 9; and the UC luminance mechanism of NaYF4: Yb3+/ Tm3+ (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. *E-mail [email protected]. ORCID

Wing-Cheung Law: 0000-0003-3855-6170 Yonggui Liao: 0000-0003-2943-1501 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural National Science Foundation of China (51303049 and 51603065) and the Research Committee of The Hong Kong Polytechnic University (G-YBMY). G

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DOI: 10.1021/acs.macromol.8b01770 Macromolecules XXXX, XXX, XXX−XXX