Theranostic 2D Tantalum Carbide (MXene) - Wiley Online Library

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Theranostic 2D Tantalum Carbide (MXene) Han Lin, Youwei Wang, Shanshan Gao, Yu Chen,* and Jianlin Shi* phosphorus (BP).[7] Very recently, MXenes, a new family of multifunctional 2D materials including a large group of carbides, nitrides, and carbonitrides with numbers of intriguing properties, have been developed by Gogotsi and co-workers.[8] MXenes were synthesized by the selective etching of the A-layer from layered ternary carbides of MAX phases, which have a general formula of Mn+1XnTx, where M stands for an early transition-metal carbide (Ti, Ta, Mo, Nb, V, and Zr), A is an A group (mostly groups 13 and 14) element, and X is either carbon or nitrogen, n = 1–3, T represents surface functional groups such as O, OH, and/or F.[9] MXenes exhibit both metallic conductivity and hydrophilicity, and together with good mechanical properties.[10] For these special characteristics, MXenes have been explored in a number of applications including electrodes for Li-ion/ Li-S batteries,[11] aqueous supercapacitors,[12] proton exchange membranes for fuel cells,[13] water purification,[14] and electromagnetic interference shielding.[15] In the respect of biomedical applications, despite very recent few reports on the biomedical applications of Ti3C2 MXenes on biosensors[16] and antibacterial activity,[17] the extensive and original investigations on the cellular and animal levels, including cytotocity, cellular uptake, in vivo toxicity, and other medical applications such as cancer theranostics, are urgently expected to be explored. Photothermal therapy (PTT) achieved by nanostructural agents with photothermal-conversion properties has attracted considerable attention due to its minimal or noninvasive therapeutic modality. Photothermal therapy employs agents to convert near-infrared (NIR) light energy into hyperthermia, leading to thermal ablation of tumor tissues. Various inorganic nanosystems have been developed for efficient PTT, including gold nanorods,[18] carbon-based nanomaterials,[19] and copper sulfide nanoparticles.[20] Recently, 2D nanomaterials, such as graphene or its analogues,[21] Mo2S,[22] WS2,[23] palladium (Pd) nanosheets (NSs),[24] BP,[25] and MXenes[26] have been also explored as novel photothermal agents for PTT of cancer. Especially, the success of the fabrication of new nanosystems with concurrent high PTT efficiency and diagnostic-imaging capability strongly relies on the composition, nanostructure, and physiochemical property of the nanosystems. A treatment strategy that combines diagnostic imaging with efficient PTT, aiming to monitor the response to treatments and increase therapy efficacy and safety,

The large-dimensional and rigid ceramic bulks fabricated by high-temperature solid-phase reaction and sintering have never been considered for possibly entering and circulating within the blood vessels for biomedical applications, especially on combating cancer. Here, it is reported for the first time that MAX ceramic biomaterials exhibit unique functionalities for dual-mode photoacoustic/computed tomography imaging and are highly effective for in vivo photothermal ablation of tumors upon being exfoliated into ultrathin nanosheets within atomic thickness (MXene). As a paradigm, 2D ultrathin tantalum carbide nanosheets (Ta4C3 MXenes) with nanosized lateral sizes are successfully synthesized based on a two-step liquid exfoliation strategy of MAX phase Ta4AlC3 by combined hydrofluoric acid (HF) etching and probe sonication. The structural, electronic, and surface characteristics of the asexfoliated nanosheets are revealed by various characterizations combined with first-principles calculations via density functional theory. Especially, the superior photothermal-conversion performance (efficiency η of 44.7%) and in vitro/in vivo photothermal ablation of tumor by biocompatible soybean phospholipid-modified Ta4C3 nanosheets are systematically revealed and demonstrated. Based on the large family members of MXenes, this work may offer a paradigm that MXenes can achieve the specific biomedical applications (here, theranostic) providing that their compositions and nanostructures are carefully tuned and optimized to meet the strict requirements of biomedicine.

2D nanomaterials have been generating great attention during the past decade due to their ultrathin structure and fascinating physiochemical property. Interest was further enhanced since the discovery of graphene’s unique electronic properties.[1] It thus seems natural to explore other ultrathin 2D nanomaterials that possess similar layered structure features but versatile properties, such as transition-metal dichalcogenides (e.g., MoS2, WS2),[2] hexagonal boron nitride,[3] layered metal oxides,[4] graphitic carbon nitride,[5] layered double hydroxides,[6] and black Dr. H. Lin, Dr. Y. Wang, Dr. S. Gao, Prof. Y. Chen, Prof. J. Shi State Key Laboratory of High Performance Ceramics and Superfine Microstructures Shanghai Institute of Ceramics Chinese Academy of Sciences Shanghai 200050, P. R. China E-mail: [email protected]; [email protected] Dr. H. Lin, Dr. S. Gao University of Chinese Academy of Sciences Beijing 100049, P.R. China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201703284.

DOI: 10.1002/adma.201703284

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combining hydrofluoric acid (HF) etching and probe sonication. The ultrathin, nanoscalelateral size Ta4C3 nanosheets exhibited an excellent NIR photothermal performance with a reasonable extinction coefficient and an extraordinarily high photothermal conversion efficiency, as well as good photothermal stability. Once etched and delaminated, the Ta4C3 nanosheets were surface modificated to produce Ta4C3-soybean phospholipid (SP) colloidal solutions, which substantially improves the biocompatibility and physiological stability of those nanosheets, exhibiting no noticeable toxicity as evaluated both in vitro and in vivo. The structure, electronic, and surface characteristics of the as-exfoliated nanosheets were revealed by several characterization techniques combined with first-principle calculations via density functional theory. Furthermore, in vivo enhanced CT and PA dual-mode imaging of tumors with obvious tumor contrasts after intravenous administration were achieved. Especially, highly effective in vivo photothermal ablation by Ta4C3SP nanosheets against tumor xenografts has been successfully demonstrated by both intra venous and intratumoral (i.t.) administrations. The Ta4AlC3 MAX phase (P63/mmc) was fabricated by a solid-phase sintering process according to the literature (Figure 2a).[10b] The highly dispersed suspension of ultrathin Ta4C3Tx nanosheets (T = F, O, OH) was synthesized by a two-step approach (Figure 2i). First, the Ta4AlC3 bulk was etched by 40% HF aqueous solution at room temperature to Figure 1. Schematic illustration of the synthesis of Ta4C3 nanosheets by delamination, surface remove the Al layer. A prolonged HF etching SP modification, and in vivo PA/CT dual-mode imaging combined with photothermal therapy. (3 d) could substantially reduce the planar dimensions. Second, the acquired Ta4C3 multilayers were delaminated by ultrasound probe sonication would be a key part of personalized medicine and acquire conin N-methyl pyrrolidone solutions to make ultrathin few-layer siderable advances in predictive medicine. or single-layer Ta4C3Tx nanosheets (noted as Ta4C3 NSs). The Very recently, we and other groups reported on 2D ultrathin Ti3C2 nanosheets for photothermal conversion.[26a,b,27] Howreported exfoliation procedure generally produces Ta4C3 layers ever, their low photothermal-conversion efficiency restricts with extremely large sheet sizes,[10b] which will not be suitable further broad applications. In addition, the common composifor biomedical applications like intravenous administration or tion (Ti and C) determines their only single photothermal funcintracellular uptake. Herein, we have succeeded in fabricating tionality, which means that they cannot exert the promising Ta4C3 nanosheets with nanoscale lateral size and atomic scale theranostic performance (concurrent diagnostic imaging and thickness, making the biomedical application of MXene possible. therapy). Herein, we report on the construction of a novel class Scanning electron microscopy (SEM) images show the microof multifunctional theranostic nanosystem based on new 2D structure of Ta4C3 multilayers after the HF etching (Figure 2b,c tantalum carbide (Ta4C3) MXenes for dual-mode photoacoustic/ and Figure S1, Supporting Information), confirming the successful exfoliation of individual grains along the basal planes computed tomography imaging (PA/CT) and their highly effiand the well-stacking of the uniform sheets. High-resolution cient in vivo photothermal ablation of mouse tumor xenografts transmission electron microscopy (HRTEM) images clearly (Figure 1). Compared to Ti3C2 MXenes, these Ta4C3 nanosheets show the crystalline lattice of multilayer Ta4C3 nanosheets possess biocompatible Ta element with high atomic number (Z = 73), which endows them with superior X-ray CT contrast for with hexagonal structure, and selected-area electron diffraction CT imaging. In addition, Ta4C3 MXenes show much higher photo (SAED) also confirms that the basal plane hexagonal symmetry structure of the parent MAX phase has been preserved during thermal-conversion efficiency as compared to that of traditional HF treatment (Figure 2d and inset). After further probe soniTi3C2 MXenes. The synthesis and delamination of 2D ultrathin cation, TEM images reveals thin, electron-transparent flakes Ta4C3 nanosheets were achieved by a liquid exfoliation method

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Figure 2. a) Photograph of Ta4AlC3 ceramic bulk (MAX phase). b,c) SEM images of layered Ta4C3 after HF etching. d) HRTEM image of multilayer Ta4C3 after HF etching. Inset shows the overall SAED pattern. e,f) TEM images of ultrathin Ta4C3 nanosheets after probe sonication. Middle inset shows lateral size distribution of the Ta4C3 nanosheets and right inset is a digital photo of Ta4C3 nanosheets dispersed in water. g) High-resolution transmission electron microscopy images of ultrathin Ta4C3 nanosheets with one layer and two layer. h) XRD patterns of Ta4AlC3 phase and Ta4C3 nanosheets. i) Scheme of Ta4C3 nanosheets exfoliation process based on a ball-and-stick model, including HF etching and probe sonication delamination. j) Raman spectra of Ta4AlC3 phase and Ta4C3 nanosheets. Inset shows the Raman-active modes by first-principle phonon calculation of Ta4AlC3 MAX phase. k) Schematic representation of the atomic displacement associated with the specific Raman-active modes (E2g) in Ta4AlC3 phase.

of delaminated Ta4C3 nanosheets (Figure 2e,f and Figure S2, Supporting Information), which exhibit the typical sheet-like morphology with an average size of ≈100 nm. Observing the basal planes in cross-section in TEM images (Figure 2g and Figure S3a–d, Supporting Information), it is clear that singlelayer Ta4C3 nanosheet (MXene) is ≈1 nm thick, which is slightly larger than theoretical thickness of single Ta4C3 layer (Figure S4, Supporting Information). The HRTEM images clearly show the crystalline lattice of few-layer or single-layer Ta4C3 nanosheets with hexagonal structure and corresponding SAED confirm the preserved hexagonal symmetry structure (Figure S3e, Supporting Information). The corresponding energy dispersive spectroscopic results confirm the presence of Ta, C, and finite O elements (Figure S5, Supporting Information). The electron energy loss spectrum confirms the existence of Ta, C, O and the absence of Al element, indicating its removal from the structure (Figure S6, Supporting Information). The X-ray diffraction (XRD) pattern shows the successful synthesis of Ta4AlC3 MAX phase (Figure 2h, blue curve). The peak intensities originating from the parent Ta4AlC3 bulk decreased

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substantially after HF treatment. Especially, the (002) peak broadened and downshifted significantly toward a lower 2θ angle of 5.86°, attributing to an enlarged c lattice parameter of 30.15 Å, from an initial value of 23.71 Å for pristine Ta4AlC3 (Figure 2h, green curve). The newly emerged low-angle (002) peak is typical for most reported HF etched MXenes, which implies that the entire sample has converted to MXenes.[28] Raman spectra of Ta4AlC3 phase and Ta4C3 nanosheets are shown in Figure 2j, with the former being generally similar in terms of the peak positions to the previous report.[29] Vibration modes ω3, ω6, and ω9 became suppressed or even disappeared after HF etching, indicating the elimination of Al layer or the exchange of Al atoms with lighter atoms. Modes ω4, ω7, and ω3 downshifted and shape changed, while modes ω5 and ω10 merged and weaken, which confirms the well-retained Ta4C3 layer and increased interlayer spacing of the MXene structure (Discussion S1, Supporting Information). X-ray photoelectron spectroscopy (XPS) measurements of Ta4AlC3 phase before and after HF etching treatment (Ta4C3 nanosheets) were performed to identify the elemental

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Figure 3. a) XPS spectra of Ta4AlC3 before and after HF etching treatment (Ta4C3 nanosheets) in the region containing all possible elements. b) XPS spectra of Ta4C3 nanosheets in Ta 4f region. c) XPS spectra of Ta4C3 nanosheets in O 1s region. Vertical lines represent the assigned species. d) Top and e) side views of the surface terminated Ta4C3Tx (T = O, F, and OH). f) The Brillouin zone of the 2D hexagonal lattice. g–i) Electronic energy bands of Ta4C3O2, Ta4C3F2, and Ta4C3(OH)2, respectively.(j–l) Spin-polarized partial density of states of Ta4C3O2, Ta4C3F2, and Ta4C3(OH)2, respectively. The Fermi levels are set to zero as indicated by the vertical red lines.

compositions and termination species. The XPS spectra of Ta4AlC3 phase and Ta4C3 nanosheets in the Al 2p region confirm the absence of Al-related peaks after etching (Figure 3a). After HF treatment, the spectrum of the Ta 4f region was fitted by following several components: Taδ+ (TaCx), Ta4+ (TaO2), and Ta5+ (Ta2O5).[30] As for the latter two species, Ta4+ (TaO2) and Ta5+ (Ta2O5), their quite weak peaks probably arise from the surface oxidation of Ta4C3 nanosheets. The binding energy of TaC species, including Ta 4f7/2 (TaCx) and Ta 4f5/2 (TaCx), matches the previous report on tantalum carbides (Figure 3b).[31] The spectrum in the O 1s region was fitted by following components:

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TaO2 and/or Ta2O5, TaCx, Ta4C3(OH)x, and Ta4C3(OH)x-H2Oads. The TaO2 and/or Ta2O5 species (530.5 eV) arise from surface partial oxidation. The peaks at 531, 531.8, and 533 eV were assigned, respectively, to TaCx (O terminated), Ta4C3(OH)x (OH terminated), and Ta4C3(OH)xH2Oads (OH terminated with strongly adsorbed water) (Figure 3c).[32] Consequently, the typical surface terminations of Ta4C3 MXenes were supposed to be O, OH, and/or F (Discussion S2, Supporting Information). As a starting point of geometry optimizations, the atomic model of the free-standing Ta4C3 nanosheet is constructed from the Ta4AlC3 phase. In analogy with Zr3C3T2, the stable

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atomic configurations of the 2D Ta4C3 nanosheets (a = 3.13 Å) were all determined. The top-view and side-view are given in Figure 3d,e, respectively. The terminated groups are on the top sites of the middle tantalum atoms. The Brillouin zone of the 2D hexagonal lattice is given in Figure 3f. Based on the first-principle calculations, both the valence and conduction band-edge states of the precursor Ta4AlC3 and surface terminated Ta4C3Tx (T = O, F, and OH) are mainly from the Ta–C interactions. We compared the electronic structures of Ta4C3 framework with/without interlayer Al atoms in Figure S8 (Supporting Information). It has been found that the interlayer atom Al has a great effect on the band-edge states by inducing new states. The different band structures mean that the materials may perform different carrier transport properties between the precursor Ta4AlC3 and Ta4C3 though the systems still present metallic properties. Compared with the significant change of band edge structures in Ta4AlC3 and Ta4C3, the bandedge structures in Ta4C3 terminated with O, F, and OH species are similar with that of layer-structured Ta4C3. Although the bandedge states are composed of Ta 5d and C 2p states (Figure 3g–l), the bonding states of these terminating O, F, and OH species deep lying in the valence band by around −5.0 eV from valence band maximum. As a result, the carrier transport properties will be negligibly affected by the terminating species. On the basis of above experimental and calculated results, the chemical reactions taking place during the etching and sonication delamination processes could be reasonably described by the following simplified equations: Ta 4 AlC3 + 3HF = AlF3 + Ta 4 C3 + 1.5H2

(1)

Ta 4 C3 + 2H2O = Ta 4 C3 (OH)2 + H2

(2)

Ta 4 C3 + 2HF = Ta 4 C3F2 + H2

(3)

Ta 4 C3 + O2 = Ta 4 C3O2

(4)

The reaction of (1–3) is similar to the reactions occurred during the HF etching process of Ti3AlC2 powders,[8a] and the reaction (4) arises from the surface partial oxidation.[28b] The as-synthesized Ta4C3 nanosheets at varied concentrations (200, 100, 50, 25 ppm, and pure water) were further exposed to an 808 nm laser at a power density of 1.0 (Figure S10, Supporting Information) and 1.5 W cm−2 (Figure 4a,b) to investigate their photothermal-conversion performance. At Ta4C3 concentration of 200 ppm, the solution temperature reached 56°C in 5 min of irradiation. In contrast, the temperature of pure water only increased by 0.5 °C, indicating that the presence of Ta4C3 nanosheets can efficiently and rapidly convert NIR light into thermal energy. The photothermal performance of the Ta4C3 nanosheets does not show any significant deterioration during five laser on/off cycles (Figure 4c), highlighting the high stability of Ta4C3 nanosheets as a durable photothermal agent for PTT cancer treatment. The photothermal performance of an agent used for photothermal conversion is based on two main parameters: the extinction coefficient (ε) and photo thermalconversion efficiency (η). The extinction coefficient reveals the light absorption ability while the photothermal-conversion

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efficiency indicates the performance of the agent in converting the light into heat. The optical absorption spectra acquired on Ta4C3 nanosheets show a board, strong absorption band, which is similar to those of other 2D nanomaterials, such as graphene [21b] and MoS2.[22] The normalized adsorption intensity over the length of the cell (A/L) at λ = 808 nm at varied concentrations (C) was determined (Figure 4d). Following the Lambert–Beer law (A/L = εC, where ε is the extinction coefficient), a linear dependence of A/L on the concentration was obtained, and the extinction coefficient at 808 nm was measured to be 4.06 Lg−1cm−1 (Figure 4d, inset), which was significantly higher than that of graphene oxide (GO) nanosheets (3.6 Lg−1cm−1),[19] implying a strong NIR laser absorption property of the Ta4C3 nanosheets. Furthermore, the photothermalconversion efficiency (η) of Ta4C3 nanosheets was calculated based on the results of time constant for heat transfer and the maximum steady-state temperature (Figure S11, Supporting Information), which gave the value of as high as 44.7%, markedly or significantly higher than those of Au nanorods (21%),[33] Ti3C2 nanosheets (30.6%),[34] Cu9S5 NCs (25.7%),[20] gold nanovesicles (37%),[35] and Prussian blue (41.4%).[36] In addition, the photothermal-conversion performance for various nanoagents including traditional photothermal agents and novel 2D nanomaterials has been summarized in Table S2 (Supporting Information), which shows the noticeable advantage of Ta4C3 nanosheets used as photothermal agent compared to reported traditional inorganic nanoagents. Although the as-synthesized Ta4C3 nanosheets can be well dispersed in water and ethanol for up to several weeks without apparent aggregation, the stability of Ta4C3 nanosheets in saline is rather poor. Therefore, the surface of Ta4C3 nanosheets was further modified with SP to improve their stability in physiological conditions (Figure 4e). The zeta potential of Ta4C3 nanosheets shows a decrease after surface SP modification (Figure S12, Supporting Information). Owing to the steric hindrance of polymer chains, the surface modification by SP chains endowed Ta4C3 nanosheets with excellent colloidal stability in physiological environments, including H2O, phosphate buffered saline (PBS), simulated body fluid (SBF), saline, and Dulbecco’s modified Eagle medium (DMEM) (Figure 4f and Figure S13, Supporting Information). In addition, the STEM images show that Ta4C3-SP nanosheets remain the 2D sheet-like morphology with a lateral size of ≈100 nm (Figure S14a–d, Supporting Information). The corresponding element mapping further confirms the successful surface modification of Ta4C3 nanosheets with SP (Figure S14e,i, Supporting Information). The atomic force microscopy data also show the change of the thickness before and after surface SP modification (Figure S15, Supporting Information). It has also been found that the photothermalconversion property and stability of Ta4C3-SP nanosheets show no obvious change compared to nonmodified Ta4C3 nanosheets, indicating that the SP modification has no significant influence on the photothermal property of Ta4C3 nanosheets (Figure S16, Supporting Information). The in vitro toxicities of Ta4C3-SP to cells were tested by a standard CCK-8 assay. Breast 4T1 cancer cells were incubated with Ta4C3-SP at varied concentrations (400, 200, 100, 50, 25, 12, 6, and 0 µg mL−1) for 24 and 48 h. Ta4C3-SP shows negligible effect on the survival of 4T1 cells, even at the concentration

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Figure 4. a) Photographs and infrared thermal images of aqueous solutions containing varied concentrations of Ta4C3 nanosheets (200, 100, 50, and 25 ppm, and pure water) taken before and after irradiation of 808 nm laser for 5 min (1.5 W cm−2). b) Photothermal heating curves of pure water and Ta4C3-SP nanosheets-dispersed aqueous suspension at varied concentrations (200, 100, 50, and 25 ppm, and pure water) under irradiation using an 808 nm laser (1.5 W cm−2). c) Recycling heating profile of Ta4C3 nanosheets suspension dispersed in water (200 ppm) with an 808 nm laser irradiation (1.5 W cm−2) for five laser on/off cycles. d) Absorbance spectra of Ta4C3-SP nanosheets dispersed in water at varied concentrations (20, 10, 5, 2.5, and 1.25 ppm). Inset: Normalized absorbance intensity at λ = 808 nm divided by the characteristic length of the cell (A/L) at corresponding concentrations. e) Schematic representation of surface modification of Ta4C3 nanosheets using SP (Ta4C3-SP). f) Dynamic light scattering size distribution profiles of Ta4C3 and Ta4C3-SP nanosheets dispersed in water. g) Relative viabilities of 4T1 cells after being incubated with varied concentrations (400, 200, 100, 50, 25, 12, 6 and 0 µg mL−1) of Ta4C3-SP nanosheets. Error bars were based on the standard deviations of six parallel samples. h) Relative viabilities of 4T1 cells after Ta4C3-SP (100 µg mL−1)-induced photothermal ablation at different laser power densities (2.0, 1.5, 1.0, 0.5, 0.25, and 0 W cm−2). i) Confocal fluorescence imaging of Ta4C3-SP induced photothermal ablation (0.5, 1.0, and 1.5 W cm−2) after various treatments (control, Ta4C3-SP only, NIR laser only and Ta4C3-SP + NIR laser group); scale bar: 100 μm.

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up to 400 µg mL−1 (Figure 4g). Then, Ta4C3-SP nanosheets as photothermal agents for in vitro cancer ablation under laser irradiation were evaluated. 4T1 cells were incubated with Ta4C3-SP nanosheets at 100 µg mL−1 for 4 h and then exposed to 808 nm laser of varied power densities (0, 0.25, 0.5, 1.0, 1.5, and 2.0 W cm−2). It is revealed that with the increase of laser power density, more cells incubated with Ta4C3-SP nanosheets are killed upon the NIR laser irradiation (Figure 4h). Additionally, the cell apoptosis after photothermal ablation is further demonstrated by confocal microscopic imaging. After NIR laser irradiation, the live and dead cells were differentiated by calcein-AM (green) and propidium iodide (PI) (red) costaining, respectively. The control groups including the 4T1 cells without any treatment, only NIR laser irradiation, and only Ta4C3-SP treatment are not significantly affected by the treatments. In contrast, the majority of 4T1 cells have been killed by the photothermal ablation after treated with Ta4C3-SP under NIR laser irradiation. These results clearly demonstrate the remarkable in vitro photothermal effect of the Ta4C3-SP nanosheets in promoting cancer cell ablation (Figure 4i). A detailed in vivo investigation of the biocompatibility of Ta4C3-SP was further conducted to explore its in vivo translation potential. Healthy Kunming mice were divided into the control group and three treatment groups at three different dosages (Ta4C3-SP dosages of 5, 10, and 20 mg kg−1). The mice after intravenous injections of Ta4C3-SP were euthanized at the 30th day. During a month period, the body weight of mice was recorded without abnormality, and no significant behavioral changes were observed in treatment groups compared to control group (Figure S17, Supporting Information). The hematoxylin and eosin (H&E) staining results of major organs (heart, liver, spleen, lung, and kidney) after one month feeding show no significant acute, chronic pathological toxicity, and adverse effects among the control group and the treatment groups (Figure S18, Supporting Information). The blood indexes, including key biochemistry parameters, in the treatment groups have no abnormity compared to the control group (Figure S19, Supporting Information). These results demonstrate that Ta4C3-SP is biocompatible for further in vivo photothermal therapy of cancer. The biodistribution of Ta4C3-SP in main organs and tumor was investigated at varied time points of intravenous injection on nude mice bearing 4T1 tumor xenograft, which showed that around 1.41% ID g−1 of Ta4C3-SP had accumulated into tumor via the typical enhanced permeability and retention (EPR) effect (Figure S20a, Supporting Information). The circulation of Ta4C3-SP in bloodstream was investigated and the blood circulation half-time of Ta4C3-SP was calculated to be 1.59 h (Figure S20b, Supporting Information). Indeed, the Ta4C3-SP nanosheets feature negligible cytotoxicity and satisfactory in vivo biocompatibility, while in light of the current results, further investigation and optimization of in vitro and in vivo toxicity evaluation such as genotoxicity[37] or reproductive toxicity[38] is highly needed to exploit the full potential of MXene-based theranostic nanoplatform as a novel photothermal nanoagents for tumor hyperthermia. Owing to the strong NIR-absorbance and photothermal conversion efficiency of Ta4C3 nanosheets in the NIR region, PA imaging was further carried out by using the Ta4C3-SP nanosheets as contrast agents (CAs) (Figure 5a). The PA

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images of a series solutions of Ta4C3-SP nanosheets at different concentrations clearly show their contrast-enhancement performances (Figure 5b,d). Based on these results, 4T1-tumor-bearing mice were intravenously (i.v.) administrated with Ta4C3-SP (20 mg kg−1), and PA images were acquired at varied time points of postinjection (Figure 5d). Compared to the precontrast image, it is clear that the intensity of PA signal increases from 0.39 to 1.0 a.u. (Figure 5c), and the tumor site is gradually enlightened, showing a maximum signal in around 24 h postinjection, largely due to the accumulation of nanosheets through the EPR effect.[39] Thereafter, the signal of the tumor site starts to decrease, suggesting that the nanosheets captured by tumor tissue are being gradually excreted out. Nanomaterials that comprise high atomic number elements have been demonstrated to be useful as CT CAs. Therefore, Ta-based nanosheets could also act as a CT CAs because of its high atomic number (Z = 73). The Hounsfield unit (HU) values and CT images of different concentrations of Ta4C3-SP in Xanthan gum show a sharp signal enhancements at the increased Ta4C3-SP concentrations. The slope of the HU value versus concentration plot for Ta4C3-SP was 155.28 HU L g−1, which appears to be much higher than that of iopromide (44.765 HU L g−1), a commercial iodine-based CT CAs used in the clinic (Figure 5g,i). 4T1-tumor-bearing mice were intravenously injected with Ta4C3-SP (20 mg kg−1) for in vivo CT imaging. CT images acquired 24 h postinjection reveals evident tumor contrast with the HU value increasing from 83 ± 8.8 HU before injection to 232.3 ± 25.2 HU after i.v. injection of Ta4C3-SP (Figure 5h,j). Consistent to PA imaging results (Figure 5e), remarkably enhanced CT contrast was observed in the tumor, suggesting high tumor accumulation of Ta4C3-SP nanosheets after intravenous administration. Therefore, our results provide a strong evidence that Ta4C3-SP nanosheets could act as promising CAs for CT imaging of tumors upon through i.v. administration. Encouraged by the high NIR laser absorbance and in vitro PTT effect of Ta4C3-SP nanosheets, we further carried out the in vivo PTT experiments (Figure 6a). After i.v. administration of Ta4C3-SP in PBS (20 mg kg−1) for 24 h or i.t. administration of Ta4C3-SP in PBS (4 mg kg−1) for 0.5 h, 4T1-tumor-bearing mice were anesthetized and exposed to an 808 nm laser at a power density of 1.5 W cm−2 (Figure 6b). For mice i.v. and i.t. injected with Ta4C3-SP nanosheets, the tumor site temperatures rapidly increased from ≈30 °C to ≈60 °C and from ≈30 °C to ≈68 °C, respectively, in 6 min of laser irradiation. In comparison, the tumor temperature on mice injected with PBS under the same irradiation condition only showed very slight change (Figure 6c). Further in vivo photothermal therapeutic experiments were performed after the tumor sizes reached around 100 mm3 (Figure S21, Supporting Information). The 4T1-tumor-bearing mice were divided into six groups: control, laser only, Ta4C3-SP (i.v.) only, Ta4C3-SP (i.t.) only, Ta4C3-SP (i.v.) + laser, and Ta4C3-SP (i.t.) + laser. Laser irradiations were carried out either in 24 h postinjection (i.v.) or in 0.5 h postinjection (i.t.) of Ta4C3-SP. In 2 d after photothermal therapy, tumors in two treated groups (Ta4C3-SP + laser, i.v. or i.t.) disappeared, leaving black scars at the initial tumor sites. The tumor volumes of six groups were measured every 2 d using a digital caliper (Figure 6d), and the digital photos of tumor regions were taken every 2 d during half

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Figure 5. a) Schematic representation of in vivo PA imaging. PA imaging is a newly emerged biomedical imaging modality based on the photoacoustic signal conversion effect of light absorbers and offers notably enhanced imaging depth and spatial resolution compared to traditional in vivo optical imaging. b) In vitro PA values and d) PA images of Ta4C3-SP nanosheet solutions as a function of concentration (0.024, 0.047, 0.094, 0.188, 0.375, 0.75, 1.5, 3, and 6 mg mL−1 with respect to Ta). c) In vivo PA value temporal evolution and e) PA images of the tumor site at different time intervals (0, 0.5, 1, 2, 4, 12, 24, and 48 h) postinjection. f) Schematic representation of in vivo CT imaging. g) In vitro CT contrasts and i) CT images of Ta4C3-SP nanosheet solutions (top) and iopromide solutions (bottom) at varied concentrations (0.078, 0.156, 0.312, 0.625, 1.25, 2.5, 5, and 10 mg mL−1 with respect to Ta and I, respectively). h) In vivo CT contrasts before and after i.v. injection. j) In vivo CT contrast (right) and 3D reconstruction CT (left) images of mice before and after i.v. administration (10 mg mL−1, 200 µL) for 24 h.

a mouth after the treatments (Figure S22, Supporting Information). Remarkably, mice in the groups of control, laser only, Ta4C3-SP (i.v.) only, and Ta4C3-SP (i.t.) only showed average life spans of 18–24 d with the volumes reached up to 1000 mm3. In comparison, tumors in two treated groups (Ta4C3-SP + laser, i.v. or i.t.) were tumor free after treatment and all survived over 60 d (Figure 6e,g). H&E and TUNEL staining results show the highly significant necrosis of tumor cells of (a) Ta4C3-SP (i.v. injection) + laser and (b) Ta4C3-SP (i.t. injection) + laser groups compared to the mice groups of control, laser only, Ta4C3-SP (i.v. injection) only, and Ta4C3-SP (i.t. injection) only. The in vivo proliferative activities were measured by Ki-67 antibody staining and the groups of: (a) Ta4C3-SP (i.v. injection) + laser and (b) Ta4C3-SP (i.t. injection) laser presented strong suppression effect on the cell proliferation, while the other four

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groups showed almost no adverse effects on the proliferative activity of cancer cells (Figure 6h and Figure S23, Supporting Information). Finally, all the mice demonstrate negligible weight fluctuations, thus confirming negligible adverse effects of these treatments on the health of mice (Figure 6f). A novel class of multifunctional nanosystem was constructed based on 2D tantalum carbide (Ta4C3 MXenes) for dual-mode photoacoustic/CT imaging and highly effective in vivo photo thermal ablation of tumors in mouse tumor xenografts. 2D ultrathin Ta4C3 nanosheets (MXenes) were initially synthesized using a liquid exfoliation method combining HF etching and probe sonication, and systematically investigated of their structural, electronic, and surface characteristics by several characterization techniques combined with first-principles calculations via density functional theory. With a lateral size of

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Figure 6. a) Schematic representation of in vivo photothermal therapy. b) IR thermal images at the tumor site of 4T1-tumor-bearing mice in groups of control, Ta4C3-SP (i.v.) + NIR laser, and Ta4C3-SP (i.t.) + NIR laser during laser irradiation at different time intervals. c) The corresponding temperature elevations at the tumor sites of 4T1-tumor-bearing mice in groups of control, Ta4C3-SP (i.v.) + NIR laser, and Ta4C3-SP (i.t.) + NIR laser during laser irradiation. d) Time-dependent tumor growth curves (n = 6, mean ± s.d.) after different treatments (control, laser only, Ta4C3-SP (i.v.) only, Ta4C3-SP (i.t.) only, Ta4C3-SP (i.v.) + laser, Ta4C3-SP (i.t.) + laser). The treatments were performed only once. e) Photographs of tumors harvested from mice in 16 d after the treatments. f) Time-dependent body weight curves of nude mice after different treatments. g) Survival curves of mice after various treatments as indicated in (d). h) Photographs of 4T1 tumor-bearing mice and its tumor regions in 16 d after different treatments. H&E staining for pathological changes in tumor tissues from each group to reveal the effectiveness of in vivo photothermal therapy by intravenous/intratumoral administrations of Ta4C3-SP (scale bar, 200 µm). Antigen Ki-67 immunofluorescence staining for cellular proliferation in tumor sections (scale bar, 200 µm).

≈100 nm, the ultrathin 2D Ta4C3 nanosheets exhibit an excellent NIR photothermal performance with reasonable extinction coefficient of 4.06 Lg−1cm−1 at 808 nm, an extraordinarily high

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photothermal conversion efficiency of 44.7%, as well as excellent photothermal stability. Importantly, the SP-modified Ta4C3 nanosheets show no noticeable toxicity as evaluated both in

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vitro and in vivo. Utilizing the strong X-ray attenuation ability and high NIR optical absorbance of Ta4C3-SP nanosheets, largely enhanced in vivo CT and PA dual-mode imaging of tumors have been achieved. Especially, highly effective in vivo photothermal ablation by Ta4C3-SP nanosheets of tumor xenografts has been successfully demonstrated. Therefore, this work highlights the promise of using 2D tantalum carbide (MXenes) as theranostic agent for biomedical applications, especially on the specific cancer diagnostics and therapeutics.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors greatly acknowledge the financial support from the National Key R&D Program of China (Grant No. 2016YFA0203700), National Nature Science Foundation of China (Grant Nos. 51672303, 51722211), Young Elite Scientist Sponsorship Program by CAST (Grant No. 2015QNRC001), Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2013169), and Development Fund for Shanghai Talents (2015). Animal experimental procedures were in agreement with the guidelines of the Regional Ethics Committee for Animal Experiments and the care regulations approved by the administrative committee of laboratory animals of Fudan University.

Conflict of Interest The authors declare no conflict of interest.

Keywords dual-mode imaging, nanosheets, photothermal therapy, tantalum carbide, theranostic Received: June 12, 2017 Revised: October 3, 2017 Published online: December 11, 2017

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