TiO2âx Based Nanoplatform for Bimodal Cancer ... - ACS Publications

Article Cite This: Chem. Mater. 2017, 29, 9262-9274

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TiO2−x Based Nanoplatform for Bimodal Cancer Imaging and NIRTriggered Chem/Photodynamic/Photothermal Combination Therapy Wei Guo,†,‡ Fei Wang,†,‡ Dandan Ding,‡ Chuanqi Song,‡ Chongshen Guo,*,†,‡ and Shaoqin Liu*,†,‡ †

School of Life Science and Technology and ‡Key Lab of Microsystem and Microstructure (Ministry of Education), Academy of Fundmental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China S Supporting Information *

ABSTRACT: Integration of cancer diagnosis and treatment, namely theranostics, is an important issue in the biomedical field. Benefiting from an excellent photothermal effect, ROS generation ability, and the desired mesoporous structure of the TiO2−x matrix, we strategically designed and fabricated a TiO2−x based theranostic system for realizing fluorescence/ photoacoustic tomography (PAT) bimodal imaging guided triple therapy for photothemal/photodynamic/chemotherapy in this work. Nonstoichiometric TiO2−x nanospheres are excellent near-infrared absorptive material, which takes on both photosensitizer and photothermal agent roles in implementing PDT/PTT combination therapy and PAT imaging. Moreover, the mesoporous structure of TiO2−x also allowed drug loading, and the polydopamine sealing layer enabled it to induce NIR/pH-triggered drug controlled release. Resultantly, both the in vitro and in vivo experiment manifested the remarkable tumor inhibition and tumor imaging effects by the TiO2−x based theranostic system. The antitumor mechanism was attributable to a synergistic therapeutic effect (combination index = 0.318) of DOX-induced DNA damage, and PDT/PTT caused mitochondrial dysfunction and a change in the cell membrane permeability. Innovatively, the B-mode ultrasonography was adopted to monitor the rehabilitation process at the solid tumor site after treatment, which observed a liquefaction necrosis process.

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tumor.14−16 While for PTT, the highlighted performance is exhibited in the late stages of treatment because it requires a procedure to reach a critical temperature for killing cancer cells.17 However, local hyperthermia can also cause heat shock response and lower the PTT efficiency.18 What is exciting is that the combination therapy involved in the two or more therapeutic approaches above engenders a significant improvement. For example, collaborative methods of PTT/PDT or PTT/drug controlled releasing have been tentatively investigated and demonstrated the synergetic therapeutic actions in the previous works.19−22 Another approach to promote therapeutical accuracy and reduce damage to healthy tissue is to introduce image surveillance in the process of cancer treatment. Theranostics of imaging-guided tumor treatment can realize real-time regulation.23−26 Among various imaging modalities, photoacoustic tomography imaging (PAT) depends on a lighttriggered photothermal effect and subsequent ultrasonic waves,27,28 which could be synchronously implemented with PDT/PTT under NIR irradiation and is more compatible with NIR-triggered chem/photodynamic/photothermal combination therapy.

INTRODUCTION Significant side effects, inevitable secondary actions, and poor therapeutic efficacies from traditional antitumor therapies (like radiotherapy or chemotherapy) in clinical practice motivate medical scientists to seek new ways that are expected to achieve better treatment outcome, high accuracy of therapy, and great controllability.1−5 The first representative strategy is controlled drug delivery and the releasing process which requires fewer dosages of drugs in virtue of on-site and real-time releasing, hence, it shows greater superiority than traditional chemotherapy and results in fewer side effects.6−8 Meanwhile, nearinfrared-mediated phototherapy is also regarded as a promising alternative to radiotherapy on the merits of low body damage.9−11 Phototherapy relies on light-triggered local hyperthermia (photothermal therapy, PTT) or reactive oxygen species (ROS, photodynamic therapy, PDT) to realize high spatiotemporal accuracy since the antitumor effect only takes place at a site where both incident light and optical active substance are available.12,13 Despite all this, the three emerging therapeutic methods above are still in the developing stage, and any of them alone can hardly generate a desired tumor treatment effect. Even by controlled drug delivery, prolonged administration of an antitumor drug still causes drug resistance and leads to deterioration in treatment outcomes. PDT mainly takes effect in the early stages of treatment, and its efficiency will remarkably decrease with the oxygen depletion around the © 2017 American Chemical Society

Received: July 31, 2017 Revised: October 13, 2017 Published: October 19, 2017 9262

DOI: 10.1021/acs.chemmater.7b03241 Chem. Mater. 2017, 29, 9262−9274

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Chemistry of Materials Scheme 1. Schematic Illustration of the Synthetic Method and Bioimaging-Guided Triple Therapy

drying at 60 °C. The TiO2−x nanosphere was synthesized by reducing the above TiO2 powder at 500 °C for 1 h under a hydrogen atmosphere. DOX Loading and Cy 5.5 Modification. First, 5 mg of TiO2−x was dissolved in the 5 mL Tris-HCl solution (1 M, pH = 8.5) under ultrasonic dispersion. Then, 1 mL of the DOX solution (1 mg/mL in Tris-HCl) was added into the above solution. The mixture was stirred at room temperature for 12 h. After that, 5 mL of dopamine in TrisHCl solution (2 mg/mL) was added. After having been stirred for 2 h, the product was centrifuged and washed with DI water. Then, the content of DOX in the supernatant was measured using a fluorescence spectrometer (Ex: 495 nm, Em: 595 nm). The experiment was repeated three times. The loading efficiency (LE) of DOX was calculated according to the following equation

The hydrogenated black titanium dioxide nanoparticles have drawn much attention from researchers of different areas since Chen et al. reported them in 2011.29 However, very few efforts worked on employing TiO2−x for cancer therapy until now.30−32 In this work, we established a TiO2−x based nanoplatform for fluorescence/PAT bimodal cancer imaging and NIR-triggered chem/photodynamic/photothermal combination therapy. The TiO2−x sphere matrix possesses strong optical absorbance in the NIR region and can produce both hyperthermia and ROS upon NIR irradiation, thus producing triple functions of PTT, PDT, and PAT imaging. What is more, the ordered mesoporous channels inside the TiO2−x sphere matrix enable it to be a drug carrier after sealing by the polydopamine layer, which is pH sensitive and has a photothermally sensitive shell for DOX controlled releasing. Finally, fluorescence cancer imaging could be achieved as a fluorescent molecule of Cy5.5 labeling the outer layer of polydopamine (Scheme 1). Herein, the TiO2−x matrix behaves as both photosensitizer (PS) for PDT and photothermal agent (PTA) for PTT, which could be exempted from the problem of using different ingredients for PS and PTA in others work, such as a mismatch of the absorption band and mutual interference between PS and PTA, as well as operation complexity.

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LE (%) =

Mt − Mu × 100% Mt

where Mt is the total mass of DOX for drug loading, and Mu is the mass of unencapsulated DOX. The drug loaded solid powder of DOX@TiO2−x@PDA was obtained by freeze-drying. Afterward, 12 mg of DOX@TiO2−x@PDA was dissolved in 20 mL of DI water under ultrasonic dispersion. Then, 8 mg of EDC and 12 mg of NHS were added into the above solution with magnetic stirring for 0.5 h. 1.6 mg of Cy 5.5 was subsequently introduced into the mixed solution with magnetic stirring for another 12 h. The final product of DOX@ TiO2−[email protected] was collected after freeze-drying. DOX Releasing Test. The drug release profiles of DOX were studied at 37.5 °C, pH 5.5 or 7.4, in phosphate buffer solution. Briefly, 10 mg of DOX@TiO2−[email protected] was dispersed in 5 mL of PBS solution and sealed in a dialysis bag (molecular weight cutoff 8000), which was submerged in 20 mL of PBS solution later. At selected intervals, the solution was taken out to determine the release amount by a fluorescence spectrometer. The drug release of DOX from the DOX@TiO2−[email protected] under NIR irradiation (1.0 W/cm2) was performed in a similar way. Characterization. Transmission electron microscopy (TEM) images were obtained on a JEM-1400 microscope at an acceleration voltage of 100 kV. The phase composition of the sample was determined by X-ray diffraction analysis (XRD, Shimadzu XD-D1). The chemical valence of Ti ions was measured by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5600). The optical properties were measured using a spectrophotometer (U-4100, Hitachi). The thermogravimetric analysis (TG, Rigaku, TG8101D) was performed from room temperature to 800 °C with a heating rate of 10 °C/min in

EXPERIMENTAL SECTION

Materials. Titanium(IV) isopropoxide, Calcein-AM, propidium iodide (PI), dopamine, 3-[4,5-dimethylthialzol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), 1,3-diphenylisobenzofuran (DPBF), and 2′,7′-dichlorodihydrofluoresceindiacetate (H2DCFDA) were obtained from Sigma-Aldrich. Doxorubicin hydrochloride (DOX), 4′,6diamidino-2-phenylindole (DAPI), ethanol, and JC-1 were purchased from Aladdin. Cy5.5 was obtained from Shanghai Jun Sheng Biotechnology Co., Ltd. Synthesis of TiO2 and TiO2−x Nanospheres. TiO2 nanospheres were prepared by a solvothermal method. First, 0.1 g of titanium isopropoxide was dissolved in 45 mL of ethanol with magnetic stirring, forming a clear solution. Then, 5 mL of acetic acid was added into the above solution. The resulting solution was transferred into a Teflonlined autoclave of 100 mL internal volume and heated at 200 °C for 4 h. After the reaction, the white powder was centrifuged and washed with water and ethanol. Then, the sample was collected after vacuum 9263


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Chemistry of Materials

Figure 1. Characterization of samples. (a−c) TEM images of TiO2, TiO2−x, and DOX@TiO2−[email protected], respectively. (d, e) Nitrogen adsorption−desorption isotherms and pore size distribution, (f) XRD patterns, (g) fitted Ti 2p XPS spectra of TiO2−x, (h) FT-IR spectra, (i) thermogravimetric curve (TG) the air. MTT experiments were measured using a microplate reader (Infinite M200, Tecan). B-mode ultrasonography experiments were performed on a HI VISION Ascendus system. Cell Culture. MDA-MB-231 cells were grown in a monolayer in DMEM. They were all supplemented with 10% (v/v) fetal bovine serum (FBS, Gibbico) and penicillin/streptomycin (100 U mL−1 and 100 mg mL−1, respectively, Gibco) in a humidified 5% CO2 atmosphere at 37 °C. Detection of ROS. The extracellular ROS generation was divided into two groups, that are pure water with NIR 808 nm irradiation and TiO2−x solution (500 μg/mL) with NIR 808 nm irradiation. The DPBF was employed to detect the ROS generation. Briefly, 20 μL of N,N-dimethylformamide solution containing DPBF (1 mg/mL) was added to 3 mL of the above two solutions. After that, they were irradiated by an NIR laser for varied durations. After centrifugation, the collected supernatant was tested by a spectrophotometer. MDAMB-231 cell lines were chosen to detect the intracellular ROS level by staining with the H2DCFDA probe. 5 × 103 cells per well were seeded into a 6-well plate and incubated at 37 °C in a humidified atmosphere with 5% CO2 for 24 h. MDA-MB-231 cells were incubated with 200 μL of TiO2−x@PDA (250 μg/mL) at 37 °C for 24 h. The positive control cells were prepared with 200 μL of 50 mM H2O2 at 37 °C for 30 min. Cells were incubated with 50 μL of H2DCFDA (10 mM in DMSO) for another 1 h at 37 °C. After that, cells were washed with PBS twice and irradiated under the 808 nm laser (1.0 W/cm2) for 10

min. The ROS level was immediately measured by an Olympus BX53 fluorescence microscope using an excitation of 488 nm and an emission of 515−540 nm. In Vitro and in Vivo PA Imaging. The in vitro and in vivo PA imaging experiments were performed on a MOST invision 128 system. Briefly, 1 mL of the DOX@TiO2−[email protected] aqueous at various concentrations (0, 0.03125, 0.0625, 0.125, 0.25, and 0.5 mg/mL) was added into the agar-phantom container and placed in the testing system for signal detection in vitro. The in vivo PAT imaging of DOX@ TiO2−[email protected] was carried out on MDA-MB-231 tumor-bearing mice. The PAT imaging data of the tumor site were collected before and after the intratumoral injection of the DOX@TiO2−[email protected] solution (100 μL, 2 mg/mL). Cytotoxity Assay (MTT). The viability of cells was investigated using an MTT assay. For the in vitro PDT experiment, MDA-MB-231 cells seeded in a 96-well plate were incubated with TiO2−x@PDA (250 μg/mL) for 24 h and then irradiated by the 808 nm laser at a power density of 1.0 W/cm2 on an ice box for 10 min. Whereas for in vitro PTT experiments, 50 μL of sodium azide (10 μM) was added into the cells which had been incubated with TiO2−x@PDA (250 μg/mL) for 24 h and then irradiated by the 808 nm laser at a power density of 1.0 W/cm2 for 10 min. An in vitro combination of the PDT/PTT experiment was carried out in the same way as above, but it is free of an ice bath and addition of sodium azide. For triple therapy of PDT/ PTT/chemotherapy, DOX@TiO2−x@PDA and 10 min 808 nm laser 9264


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TiO2 nanospheres under hydrogen atmosphere at 500 °C for 1 h. As can be seen in Figures 1b and S2, the TiO2−x NPs remain in a spherical shape and good dispersion compared with the TiO2 after a hydrogen reduced process. In addition, they possess nearly the same hydrodynamic diameter (TiO2: 599.9 nm, TiO2−x: 606.0 nm). Therefore, the TiO2−x crystal well inherits the size and morphological characteristics of TiO2 nanospheres after reduction. Then, a typical anticancer drug of doxorubicin hydrochloride (DOX) was encapsulated in the mesoporous pores of TiO2−x (named as DOX@TiO2−x) and followed by PDA modification onto the surface of DOX@ TiO2−x by a well-established method to form a DOX@ TiO2−x@PDA structure,37 which could avoid premature drug release and achieve a controlled drug release. The final procedure of binding the Cy5.5 fluorescent molecule to the PDA layer was realized by a standard EDC/NHS method (Scheme 1). From Figure 1c, we can see that the thickness of the PDA/Cy5.5 layer is about 31 nm in the final sample of DOX@TiO2−[email protected]. The N2 adsorption−desorption isotherm shown in Figure 1d reveals that both of the TiO2 and TiO2−x isotherms are of typical IV curve with hysteresis loops located at a relative pressure (P/P0) of 0.4−0.7, indicating that the capillary condensation happened in the mesoporous pores. After the reduction process, the specific surface area decreased from 142.27 m2/g for TiO2 to 84.43 m2/g for TiO2−x, but the area of hysteresis loop became larger. In addition, there is no difference in the pore size for TiO2 and TiO2−x, both of which are around 3.5 nm. For the DOX@TiO2−[email protected], it has a very low specific surface area (5.06 m2/g) and no porous structure characteristics, suggesting the successful DOX loading and PDA modification (Figure 1d,e). The crystal phase of the obtained TiO2, TiO2−x, and DOX@TiO2−[email protected] could be well indexed to anatase (JCPDS no. 84-1285), illustrating that the reduction process, drug loading, and surface modification processes have limited influence on the crystallographic structure (Figure 1f). However, we suspected that the surface oxygen defects may have emerged after the reduction process since we observed an obvious color change from white to dark gray. Thus, an X-ray photoelectron spectroscopy (XPS) survey was employed to confirm the chemical valence of Ti ions in the reduced sample. As expected, there are two spin−orbit doublets at Ti2P 3/2 458.9 and Ti2P 1/2 464.6 eV and at Ti2P 3/ 2 457.8 and Ti2P 1/2 463.5 eV in Figure 1g, corresponding to Ti4+ and Ti3+ ions, respectively. Hence, the XPS result verified the composition of TiO2−x, and this is consistent with previous reports.38,39 Then, the FT-IR spectroscopy was used to confirm the success in establishing the DOX@TiO2−[email protected] structure. As illustrated in Figure 1h, the TiO2−x only shows a broad band at 587 cm−1 related to the Ti−O mode. The absorption peaks at 2956, 2925, and 1386 cm−1 are assigned to the C−H stretching and symmetric bending modes of the PDA layer. In addition, it also gives the characteristic absorption peaks at 1597 and 1386 cm−1 due to the polymeric structure. Besides characteristic peaks of TiO2−x and PDA, the DOX@ TiO2−[email protected] also possesses phenolic hydroxyl (1215 cm−1), alcohol hydroxyl (1115 cm−1), and CN (1457 cm−1) from DOX and Cy5.5, thus confirming the achievement of drug loading, PDA coating, and Cy5.5 modification. Finally, thermogravimetric (TG) measurement was used to determine the contents of the PDA layer and DOX roughly. As shown in Figure 1i, the weight loss difference between TiO2−x and TiO2−x@PDA originates from the PDA modification, which indicates that the PDA layer accounts for 6.4 wt % in TiO2−x@

irradiation were applied in a similar way to the above. Finally, a typical MTT assay was used to determine the relative cell viabilities. The assay was performed out in triplicate. In brief, resultant cells were incubated in media containing 20 μL of MTT (5 mg/mL) for 4 h. Then, the media with MTT were removed, and 150 μL of DMSO was added to dissolve formazan crystal at room temperature for 30 min. The absorbance was measured at 490 nm by a multidetection microplate reader (Synergy TMHT, BioTek Instruments Inc., USA). In Vitro Living-Dead Staining. MDA-MB-231 cells were incubated in a 35 mm quartz cuvette at a density of 3 × 105 cells per dish and were allowed to grow for 24 h at 37 °C. Then, 2.0 mL of medium containing TiO2−x@PDA or DOX@TiO2−x@PDA (250 μg/ mL) was added into the cuvette to replace the culture medium. After incubating for another 2 h, the cells were rinsed three times with PBS. After that, the cells were irradiated with an 808 nm laser for different time intervals (2, 6, and 10 min, 1.0 W/cm2). Finally, the irradiated cells were rinsed with PBS and stained with calcein-AM and PI, respectively. The stained cells were immediately measured by an Olympus BX53 fluorescence microscope. Mitochondrial Membrane Potential Change. MDA-MB-231 cells were cultured with TiO2−x@PDA solution (250 μg/mL) in DMEM culture medium for 2 h. Then, the cells were rinsed three times with PBS. Afterward, the cells were irradiated under an 808 nm laser (1.0 W/cm2) for 10 min. Cells treated with the TiO2−x@PDA solution without irradiation were used as control. Subsequently, the medium was removed, and the JC-1 staining solution was added according to the manufacturer protocol. After staining for 25 min, cells were washed and imaged by an Olympus BX53 fluorescence microscope. In Vivo Antitumor Effect. Female BALB/C nude mice (5 weeks old) were obtained from Beijing Vital River Laboratory Animal Technology Co., Ltd., and all the in vivo experiments were implemented according to the criterions of the National Regulation of China for Care and Use of Laboratory Animals. First, the tumors were inoculated by subcutaneous injection of 1 × 107 MDA-MB-231 cells in the left flank of each BALB/c mouse using 50% matrigel. After growing for about 2 weeks, the tumor size reached about 200 mm3. The tumor-bearing nude mice were randomized into five groups (n = 5, each group): (1) control group received 100 μL of PBS injection only; (2) group with 100 μL (1 mg/mL) of TiO2−x@PDA injection; (3) group with NIR irradiation; (4) group with both NIR irradiation and TiO2−x@PDA; (5) group with both an NIR laser and DOX@ TiO2−[email protected]. The power density of an NIR laser was 1.0 W/ cm2, and the treatment began at 2 h postintratumorally injection. The irradiation duration for groups with NIR irradiation is 10 min. The tumor inhibition effects were evaluated by measuring the tumor volumes, which is calculated as tumor volumes (V) = length × width2/ 2. Relative tumor volume was calculated as V/V0 (V0 was the tumor volume before treatment). Relative body weight was calculated as W/ W0 (W0 was the corresponding mouse’s weight when the treatment was initiated). The combination of chemotherapy, PDT, and PTT was evaluated by Chou and Martin’s CompuSyn software for drug combinations and for general dose effect analysis and the user’s guide (ComboSyn, Inc. Paramus, NJ 2007). The CI value quantitatively defines synergism (CI < 1), additive effect (CI = 1), and antagonism (CI > 1).33−35 Histology Analyses. Histology analysis was carried out on the 14th day after different treatments. The major organs and tumor tissues were isolated, fixed in a 4% paraformaldehyde solution, and embedded in paraffin. The sliced tumor tissues and organs were stained with hematoxylin and eosin (H&E) and analyzed by an Olympus BX53 fluorescence microscope (Olympus, Tokyo, Japan).

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RESULTS AND DISCUSSION First, we synthesized the mesoporous TiO2 nanospheres by a solvothermal method according to our previous work.36 The diameter of these uniform TiO2 spheres is in the range of 300− 500 nm (Figure 1a and Figures S1 and S2). Then, the nonstoichiometric TiO2−x was obtained by reducing the above 9265


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Figure 2. Photoabsorption and photothermal effect. (a) UV-vis-NIR spectra of powder TiO2, TiO2−x, TiO2−x@PDA, and DOX@TiO2−[email protected]. (The top shows the color of the samples.) (b) UV-vis-NIR spectra of the DOX@TiO2−[email protected] solution with different concentrations. (c) The photothermal heating curves of the DOX@TiO2−[email protected] solution with different concentrations under NIR irradiation at a power density of 1.0 W/cm2. (d) Plot of temperature change (ΔT) over a period of 10 min irradiation versus DOX@TiO2−[email protected] concentration.

Figure 3. ROS generation tests. (a) Absorption spectra of the DPBF probe under different irradiation times. (b−e) Fluorescence microscope images of ROS generation in MDA-MB-231 cells after receiving different treatments: (b) MDA-MB-231 cells control, (c) TiO2−x@PDA treated MDA-MB231 cells, (d) NIR laser treated MDA-MB-231 cells, (e) H2O2 treated MDA-MB-231 cells as a positive control, (f) NIR laser + TiO2−x@PDA treated MDA-MB-231 cells (NIR laser: 1.0 W/cm2, t = 10 min, scale bar = 200 μm).

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Figure 4. Fluorescence and PAT imaging. (a) In vivo fluorescence imaging of MDA-MB-231 tumor-bearing mice before and after intratumoral injection of DOX@TiO2−[email protected]. (The upper group did not receive NIR laser irradiation, while the lower group received laser irradiation). (b) The correlative fluorescence signal ratio of the red area (St/S0) versus time. (S0 is the area of red region preinjection, and St is the area of the red region after t min postinjection). (c) The PAT signal of DOX@TiO2−[email protected] solution at different concentrations. (d) In vitro PAT images of DOX@TiO2−[email protected] dispersed in water with different concentrations. (e) The PAT signal of DOX@TiO2−[email protected] as a function of concentration under 808 nm excitation. (f) In vivo PAT images of MDA-MB-231 tumor-bearing mice before and after intratumoral injection of DOX@TiO2−[email protected] NPs for different post times. (Tumor sites are marked with white dashed circles.)

absorption of DOX and Cy5.5 in DOX@TiO2−[email protected], suggesting successful drug loading and fluorescence labeling again. In addition, integration of PDA, DOX, and Cy5.5 into the TiO2−x matrix also enhances the optical absorption in the NIR range slightly, which is desirable for NIR-triggered phototherapy. The optical response of DOX@ TiO2−[email protected] solutions with varied concentrations was also investigated. As can be seen in Figure 2b, the absorbance of the DOX@TiO2−[email protected] solution was remarkably enhanced with its concentration increment. To evaluate the photothermal effect of DOX@TiO2−[email protected], we next measured the temperature rise of the DOX@TiO2−x@PDA-

PDA. Considering the trace amount of Cy5.5, further weight loss of DOX@TiO2−x@PDA-Cy 5.5 is mainly because of DOX decomposition. In this way, about 12.04 wt % DOX is determined to be loaded in the TiO2−x mesopore. Then, we checked the photoabsorption of powder samples using a UV-vis-NIR spectrophotometer. Compared with white TiO2 that shows limited optical absorption in the biological window of 800−1300 nm, the nonstoichiometric TiO2−x displays higher absorption in the NIR region of 800−1500 nm, and the absorbance at 808 nm is about 4-fold of TiO2, making it a splendid NIR absorbing material (Figure 2a). The absorption bands at 500 and 678 nm are characteristic 9267


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Figure 5. (a) Relative cell viability of MDA-MB-231 and (b) in vitro living-dead staining studies after different treatments. (Scale bar = 500 μm.)

Cy5.5 solution under 808 nm NIR laser (1.0 W/cm2) irradiation. In Figure 2c,d, the temperature of the solutions increased with the concentration of DOX@TiO2−[email protected] and NIR irradiation duration, proving the role of the DOX@TiO2−[email protected] nanoplatform as a photothermal agent. The effective ROS generation is a necessary requirement of a photosensitizer for the PDT implement. We next investigated the photosensitive ability of TiO2−x to form ROS. The 1,3-diphenylisobenzofuran (DPBF) and 2′,7′dichlorofluorescin diacetate (H2DCFDA) probes were employed to detect the extracellular and intracellular ROS production, respectively.40 The DPBF probe for detecting extracellular ROS production lies in its decomposition by ROS. Resultantly, a decrement of DPBF characteristic absorption could be observed accordingly. As displayed in Figure 3a, the absorbance of the DPBF solution with TiO2−x decreased significantly as irradiation time went on, suggesting the sufficient ROS level was induced by TiO2−x. In contrast, the pure water led to very limited ROS generation with the same laser irradiation. The ROS generation was further confirmed by ESR spectra. First, the 5,5′-dimethylpyrroline-1-oxide (DMPO) was used as a spin trap agent for the detection of hydroxyl radicals. After irradiation (808 nm laser, 1.0 W/cm2) for 10 min, the appearance of 1:2:2:1 multiplicity in the ESR spectrum of the TiO2−x solution (0.5 mg/mL) can be assigned to the characteristics of the DMPO−OH adduct, indicating the generation of hydroxyl radicals30 (Figure S3a). Moreover, the generation of singlet oxygen was also detected by the ESR method with a trapping agent of 2,2,6,6-tetramethyl-4piperidone (TEMP). As can be seen in Figure S3b, when the mixture of the TiO2−x solution (0.5 mg/mL) and the TEMP probe is irradiated by an 808 nm NIR laser (1.0 W/cm2) for 10 min, a typical three-line ESR signal of the TEMPO adduct with

an equal intensity is obtained, thus suggesting the formation of singlet oxygen.28 Then, intracellular ROS production inside MDA-MB-231 cells was examined by the H2DCFDA probe, which would convert to green fluorescent molecules after being oxidized by ROS. The bare MDA-MB-231 cells were used as a negative control, and the H2O2 (50 mM) treated MDA-MB-231 cells were used as a positive control. As shown in Figure 3b−f, there is no observable green fluorescence from the negative control and TiO2−x@PDA treated cells, while a very weak signal is observed from the NIR irradiated group. The strong green fluorescence was emitted from both H2O2 and TiO2−x@PDA + NIR laser irradiation treated groups, indicating that TiO2−x@ PDA can produce adequate ROS within cells under NIR irradiation, and it is a promising candidate for PDT. To confirm whether H2 reduction treatment and DOX/PDA modification have an influence on the photoactivity of the sample, the ROS generation ability and photothermal effect of TiO2, TiO2−x, DOX@TiO2−x@PDA, and DOX@TiO2−[email protected] were investigated. As can be seen in Figure S4, the DOX/PDA/ Cy5.5 modification can obviously enhance the photothermal performance of TiO2−x. Meanwhile, the ROS level determined by the DPBF probe reveals that there is only slight decrement on ROS generation of TiO2−x. Moreover, the DOX and the Cy5.5 can give rise to a chemotherapeutic efficacy and a fluorescence imaging for the multifunctional theranostic system, respectively. The PDA layer can not only achieve the dual pH/ NIR laser irradiation-triggered drug release but also enhance biocompatibility of TiO2−x. Therefore, the PDA layer is a ligament for this cancer multifunctional theranostic platform. We then evaluated the bioimaging behaviors of the DOX@ TiO2−[email protected] nanoplatform using MDA-MB-231tumor-bearing nude mice as the model animal. The fluorescent 9268


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Figure 6. In vitro drug release. (a) DOX release from DOX@TiO2−[email protected] at pH 5.5 and 7.4. (b) NIR irradiation-triggered DOX release from DOX@TiO2−[email protected]. (c) Intracellular DOX release from DOX@TiO2−[email protected] after incubation for 2 h with and without NIR laser irradiation (1.0 W/cm2, scale bar = 50 μm). The control group is free of DOX@TiO2−[email protected]. The blue color of DAPI is used to label the cell nucleus, while the DOX shows a red color.

obvious PAT signal, suggesting that the tumor itself is not an endogenous contrast and needs photothernal material for image formation. In contrast, the signals from the tumor location enhanced remarkably after introducing DOX@ TiO2−[email protected]. The PAT signals at the tumor site reached a maximum after 12 h postinjection, and it lasted 24 h, indicating a long-term imaging effect of DOX@TiO2−[email protected] via intratumoral injection. Taken together, the DOX@ TiO2−[email protected] could be potentially applied to bimodal fluorescence/PAT imaging to sketch tumors. Next, we investigated the in vitro therapeutic effect of a composited nanoplatform. Considering that PTT, PDT, and chemotherapy all may induce an antitumor effect, it is very essential to distinguish the contributions from each one. We designed the following methods to realize this. First, TiO2−x@ PDA enriched MDA-MB-231 cells were irradiated by an NIR laser for 10 min in an ice bath so that the temperature of the irradiated area never exceeded 10 °C. In this way, the PTT effect was removed, and the cancer cell death was mainly induced by a PDT effect (noted as PDT in Figure 5a). Second, instead of an ice bath, a free radical quencher of Na3N was added into the above system to eliminate ROS generated by TiO2−x@PDA under NIR irradiation. Thus, it only reflects PTT efficiency (noted as PTT in Figure 5a). Third, a combination therapy of PTT and PDT could be realized by the above method but free of an ice bath and Na3N addition, where both the photothermal effect and ROS could cause cancer cell death (noted as PTT + PDT). Fourthly, equivalent free DOX to DOX@TiO2−[email protected] was administrated to MDA-MB231 cells to determine the contribution of chemotherapy (noted as DOX). Finally, DOX@TiO2−x@PDA was used to simultaneously trigger PTT, PDT, and chemotherapy effects on cancer cells under NIR irradiation. MTT assay was carried out for all of the above experiments to evaluate the cytotoxicity.

molecules of Cy5.5 in the nanoplatform allow us to monitor biodistribution of the DOX@TiO2−[email protected] in the tumor site using in vivo NIR florescence microscopy (Figure 4a). After in intratumoral injection, fluorescence signals from the tumor site are clearly observed. The high fluorescent region with a red color indicates the real-time location of DOX@TiO2−[email protected]. Without NIR irradiation, the area of the red region (S) is nearly unchanged within 10 min (Figure 4b). However, we speculated that the NIR irradiation during the phototherapy might enhance the diffusion and expansion of DOX@TiO2−x@ PDA-Cy5.5 at the tumor site, thus improving the antitumor effect. As expected, the spread of DOX@TiO2−[email protected] happened upon NIR irradiation as evidenced by about a 4-fold increment on the red area after 10 min of treatment (Figure 4a,b), which is closely related to the thermal diffusion of the particles under the NIR irradiation. Given this, it is also expected that such a process can promote the distribution of DOX in the solid tumor as well and further enhance the therapeutic effect. PAT imaging is an effective diagnostic technique with high spatial resolution, noninvasiveness, and deep penetration. Usually, photothermal materials are potential exogenous contrast agents for PAT imaging, especially in the case of that tissue having a weak PAT signal. Inspired by the aforementioned photothermal test, DOX@TiO2−[email protected] was tentatively used as PAT contrast. In Figure 4c−e, an in vitro test reveals that the photoacoustic signal increases with the increment of sample concentration (Figure 4c,d), and a linear relationship is found between signal intensity and concentration (Figure 4e). Then, in vivo PAT imaging was carried out by intratumoral injection of DOX@TiO2−[email protected] and monitoring imaging changes at different time intervals postinjection. As shown in Figure 4f, the control group without injection of DOX@TiO2−[email protected] displays no 9269


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Figure 7. Detection for mitochondrial potential changes of different groups by JC-1 staining. (Scale bar = 20 μm.)

multifunctional agent with PDT, PTT, and chemotherapy effects. The pH-sensitive and NIR irradiation-triggered drug release behavior of the DOX@TiO2−[email protected] were investigated. The encapsulation efficiency of DOX was 83.5% ± 0.7% according to the fluorescence spectra results (Figure S5). The release profiles of DOX@TiO2−[email protected] in PBS buffers (pH = 7.4 and pH = 5.5) are shown in Figure 6a. It can be seen that the DOX@TiO2−[email protected] displayed very low release cumulative amounts (below 25%) until 48 h in pH = 7.4 PBS buffer. However, the release behavior became easier and faster when the pH of the PBS buffer was changed to 5.5, and the release cumulative amounts reached nearly 86%. In this case, the pH sensitive properties of the DOX@TiO2−[email protected] can be attributed to the detachment of the PDA layer from the surface of the TiO2−x in the acidic condition.41 To verify its NIR-triggered release feature, the release profiles of DOX@ TiO2−[email protected] were studied under different pH conditions (pH = 7.4 and 5.5) with NIR laser irradiation occasionally. In Figure 6b, both of the two release curves show the “on−off” drug release properties responding to NIR irradiation. The intervention of NIR irradiation accelerated the drug release prominently, especially in pH = 7.4 PBS buffer. The NIR-triggered “on−off” drug release was attributed to the enhanced photothermal efficiency of TiO2−[email protected] Moreover, the TEM and dynamic light scattering (DLS) measurement results of DOX@TiO2−[email protected] were examined after the NIR irradiation-triggered DOX release test shown in Figure 6b. From TEM images, we can see the PDA layer detachment from the surface of the TiO2−x for both of the above two groups. (Figure S6) In addition, the hydrodynamic diameters of DOX@TiO2−[email protected] decreased from 719.3

As shown in Figure 5a, for a single function of PDT, PTT, or chemotherapy group, the inhibition on MDA-MB-231 cancer cells is just 27.5%, 44.7%, and 59.7%, respectively. The inhibition rate on cancer cells could be remarkably enhanced by a combination of PDT and PTT (69.7%), which makes a “dual-punch” effect in killing cancer cells. Obviously, the triple therapy was realized as the best treatment effect as expected, and it can inhibit 94.8% of MDA-MB-231 cancer cells. On the basis of the above results, the combination index is determined to be 0.318. Hence, we can conclude that multiple treatments established in this work would be mutual promotion and achieve a synergistic antitumoral effect. Then, we evaluated antitumor performance by a living-dead staining. Calcein AM with green fluorescent and propidium iodide (PI) with red fluorescent were used to distinguish the living and dead cells, respectively. In Figure 5b, the untreated cells, the NIR laser treated group, and the TiO2−x@PDA treated group showed strong green fluorescence but no red fluorescence, indicating that either TiO2−x@PDA or a NIR laser alone caused almost no cell death. For the DOX treated group, there is no apparent red color in the fluorography, but we do find the detachment of death cells from the culture dish during the observation. Hence, cell density in the DOX treated group is lower than others due to the departure of DOX-treated death cells. Synergistic action of PTT and PDT realized by applying TiO2−x@PDA and NIR irradiation can give rise to distinct apoptosis, and the area of the red circle enlarged with irradiation duration. If DOX is loaded in TiO2−x@PDA, additional more massive cells shedding from the boundary of the light spot to outside were found (marked by blue arrows), suggesting that the DOX@TiO2−x@PDA is an efficient 9270


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Figure 8. In vivo antitumor studies. (a) Infrared thermal images of MDA-MB-231 tumor-bearing mice and (b) corresponding temperature profiles. (c) Quantitative measurement of tumor volume in mice. (d) Representative photographs of mice and tumors. (e) Body weight changes of mice during the treatment. (f) H&E staining of tumor slices collected from different groups of mice after 14 days of treatment.

situates at the cytoplasm and gives a green fluorescence, while it aggregates within the mitochondria in healthy cells to emit a red fluorescence. Thus, the ratio of red to green fluorescence reflects the degree of MMP changes. As illustrated in Figure 7, the control groups of untreated cells, TiO2−x@PDA treated cells, and NIR treated cells show red fluorescence but no green color, suggesting that nearly no MMP changes occurred in these cells. In sharp contrast, strong green fluorescence emerged from the cells that was mediated by both TiO2−x@ PDA and NIR irradiation, and it indicates that hyperthermia and ROS during the phototherapy process damaged cancer cells by mitochondria dysfunction to a certain degree. Second, the influence of phototherapy on membrane permeability was studied by ethidium bromide (EB) staining, which is a fluorescent molecule and impermeable to normal cells. As membrane permeability changed, it can stain the dead/ apoptosis cells by binding to the cell nucleus to emit red fluorescence. From Figure S7, TiO2−x@PDA mediated phototherapy can greatly change the cell membrane permeability and make it even penetrable to EB. However, the control group and TiO2−x@PDA and NIR treated groups are not in this case as no ethidium bromide (EB) fluorescence can be seen. Consequently, the antitumor mechanism of our nanoplatform may involve DOX-induced DNA damage, and PDT/PTT caused mitochondrial dysfunction/change of cell membrane permeability, as well as synergistic therapeutic effects between them.

to 644.7 nm for NIR laser/pH 5.5 and 654.4 nm for NIR laser/ pH 7.4, respectively (Figure S6). Therefore, we concluded that both the pH and photothermal effect could accelerate the detachment of the PDA layer from the surface of the TiO2−x. NIR-triggered DOX release was evaluated further at the cellular level by confocal laser scanning microscopy. The blue color of DAPI was used to label the cell nucleus, while the DOX showed a red color. As can be seen from Figure 6c, very weak red fluorescence is seen from MDA-MB-231 cells after incubation with DOX@TiO2−[email protected] for 2 h, indicating that the PDA layer has a good sealing effect on DOX. Upon the NIR irradiation, the intracellular red fluorescence enhanced significantly due to the DOX releasing, which may be attributed to the thermally driven diffusion process and thermodestruction on the PDA layer. We next studied the antitumor mechanism of the TiO2−x based nanoplatform under NIR laser irradiation. As to the chemotherapy drug of DOX, the antitumor mechanism is intensively reported as via intercalating into DNA and halting transcription.43 The PTT and PDT induced cells apoptosis by the TiO2−x matrix may lead to detrimental biochemical changes, such as dysfunction of mitochondria and membrane permeability.44 First, we checked the phototherapeutic disturbance on mitochondria membrane potential (MMP) by JC-1 staining. JC-1 is a green-fluorescent monomer at low membrane potential, while it forms red-fluorescent JC-1 aggregates at higher potentials. In the apoptotic cells, JC-1 9271


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Figure 9. B-mode ultrasonography for monitoring evolution of tumors after treatment. The dashed circles indicate the tumor area, and the arrows mark the original tumor site.

The above in vitro antitumor studies disclosed a significantly enhanced cancer cell inhabitation efficacy by our synergistic treatment system, which encourages us to go a further step with in vivo cancer treatment. We first monitored the temperature elevation in tumors under NIR laser irradiation. The in vivo thermographic images and relevant temperature profiles of tumor-bearing mice were achieved by an IR thermal camera. As depicted in Figure 8a,b, the surface temperature of the tumor site only increases about 4 °C after 10 min NIR irradiation. However, it could be greatly improved by cooperating with photothermal agents, such as TiO2−x@PDA or DOX@ TiO2−[email protected]. The temperature of NIR mediated TiO2−x@PDA and DOX@TiO2−[email protected] groups reached up to 53.5 and 58.5 °C, respectively, which could lead to tumor cell apoptosis even within a short time exposure of a few minutes. The in vivo therapeutic effects of DOX@TiO2−x@ PDA-Cy5.5 were then evaluated on tumor bearing mice by measuring the inhibitory rate of the tumors. In Figure 8c,d, the tumor volumes of mice receiving only TiO2−x@PDA or NIR irradiation treatment show no conspicuous difference in that of the control group without any treatment, all of which present rapid growth of the solid tumors, indicating that neither the material itself nor NIR irradiation could give rise to an antitumor effect. It seems to be effective for the NIR mediated TiO2−x@PDA group, which triggered PDT and PTT effects, during the first 10 days. However, the volume of the tumors increases again after that time, suggesting that the hyperthermia and ROS produced during the phototherapy are not effective enough for completely damaging the whole tumor tissue, especially for the inside part of the tumor. When the DOX was introduced into the system, the NIR mediated DOX@TiO2−x@ PDA-Cy5.5 displayed a remarkable tumor inhibition effect, and some tumors even vanished after the treatment. The on-site release of DOX during the phototherapy made an additional antitumor effect. In addition, hyperthermia driven DOX diffusion can permeate the whole tumor and remedy the weaknesses of PDT/PTT that has a poor therapeutic effect on the tumor in-depth. As shown in Figure 8e, the mice in the five groups show no obvious weight loss, implying that therapeutic method and materials in this work have no acute toxicity toward the mice. After the treatment, the histological sections of tumor and

major organs were studied by hematoxylin and eosin (H&E) staining. In Figure 8f, there is nearly no obvious tumor cell necrosis for control, TiO2−x@PDA, or NIR treated ones. While for the NIR mediated TiO2−x@PDA and DOX@TiO2−x@ PDA-Cy5.5 groups, sliced tumors exhibited massive cell necrosis and apoptosis. In addition, no remarkable lesions (e.g., necrosis or inflammatory) were found in major organs after the treatment, highlighting the in vivo biosafety of using DOX@TiO2−[email protected] for cancer treatment (Figure S8). Finally, we employed B-mode ultrasonography (US) to monitor the evolution of tumors after treatment. As shown in the B-mode sonogram, the tumor volume gradually decreases from the first day to the seventh day and eventually completely vanished after 14 days of treatment (Figure 9), which is consistent with previous in vivo antitumor observation. Moreover, the color Doppler flow images (CDFI) of different days show low vascularity, which is accorded with the characteristics of the MDA-MB-231 tumor model. In the USelastogram, the stiffness of tumor tissue is high as evidenced by a blue color on the first day post-treatment. However, the bluegreen-red pattern artifact appeared around the tumor on the third day, suggesting that the tumor began liquefaction necrosis. On the seventh day, the tumor softened further with the bigger swelling area because of the more serious liquefaction necrosis. After 14 days of treatment, the tumor disappeared, and the stiffness of the previous tumor site (red arrow) was nearly the same as the peripheral tissue, indicating the outstanding tumor ablation ability of the DOX@TiO2−[email protected] tripletherapy system.

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CONCLUSION In this work, we established a multifunctional theranostic nanoplatform based on the mesoporous TiO2−x matrix, which is an excellent NIR-harvesting material to simultaneously produce ROS, hyperthermia, and the PAT signal under NIR excitation. Moreover, the mesoporous structure of the TiO2−x matrix also endowed it to be an adequate drug carrier with controllable capability of drug release. As a result, the composited DOX@ TiO2−[email protected] system realized fluorescence/PAT dualmode bioimaging and triple combination therapy of PDT/ PTT/chemotherapy. Both in vitro and in vivo antitumor studies 9272


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revealed the superiority of triple combination therapy, which can greatly inhibit tumor growth and even thoroughly ablate the solid tumor. The antitumor mechanism was confirmed as a synergistic action of DOX-induced DNA damage, and PDT/ PTT caused mitochondrial dysfunction/change of membrane. B-mode ultrasonography certified the elimination of solid tumors via a liquefaction necrosis process.

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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.7b03241. Membrane permeability study of MDA-MB-231 cells of different groups and histology staining of organ (liver, kidney, heart, lung, and spleen) tumor slices collected from different groups of mice after 14 days of treatment (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.S.G.). *E-mail: [email protected] (S.Q.L.). ORCID

Chongshen Guo: 0000-0002-8000-7434 Shaoqin Liu: 0000-0001-6990-8728 Author Contributions

W.G. and F.W. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51572059).

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REFERENCES

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