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Theranostics 2017; 7(19):4763-4776. doi:10.7150/thno.21694 This issue Cite

Research Paper

Copper Manganese Sulfide Nanoplates: A New Two-Dimensional Theranostic Nanoplatform for MRI/MSOT Dual-Modal Imaging-Guided Photothermal Therapy in the Second Near-Infrared Window

Kaimei Ke¹, Wen Yang¹, Xiaoli Xie², Rui Liu², Li-Li Wang³, Wei-Wen Lin³, Guoming Huang², Chun-Hua Lu¹ , Huang-Hao Yang¹

1. MOE Key Laboratory for Analytical Science of Food Safety and Biology, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China;
2. College of Biological Science and Engineering, Fuzhou University, Fuzhou 350116, P. R. China;
3. Department of Diagnostic Radiology, Union Hospital, Fujian Medical University, Fuzhou 350001, P. R. China.

✉ Corresponding authors: chunhualuedu.cn; hhyangedu.cn

Abstract

Multifunctional nanoplatforms with integrated diagnostic and therapeutic functions have attracted tremendous attention. Especially, the second near-infrared (NIR-II) light response-based nanoplatforms hold great potential in cancer theranostic applications, which is because the NIR-II window provides larger tissue penetration depth and higher maximum permissible exposure (MPE) than that of the well-studied first near-infrared (NIR-I) window. Herein, we for the first time present a two-dimensional (2D)-nanoplatform based on Cu₂MnS₂ nanoplates (NPs) for magnetic resonance imaging (MRI)/multispectral optoacoustic tomography (MSOT) dual-modal imaging-guided photothermal therapy (PTT) of cancer in the NIR-II window. Methods: Cu₂MnS₂ NPs were synthesized through a facile and environmentally friendly process. A series of experiments, including the characterization of Cu₂MnS₂ NPs, the long-term toxicity of Cu₂MnS₂ NPs in BALB/c nude mice, the applications of Cu₂MnS₂ NPs for in vitro and in vivo MRI/MSOT dual-modal imaging and NIR-II PTT of cancer were carried out. Results: The as-synthesized Cu₂MnS₂ NPs exhibit low cytotoxicity, excellent biocompatibility as well as high photothermal conversion efficiency (~49.38%) and outstanding photostability. Together with their good T₁-shortening effect and strong absorbance in the NIR-I and NIR-II region, the Cu₂MnS₂ NPs display high-contrast imaging performance both in MRI and MSOT (900 nm laser source). Moreover, the subsequent in vitro and in vivo results demonstrate that the Cu₂MnS₂ NPs possess excellent PTT efficacy under 1064 nm laser irradiation with a low power density (0.6 W cm^-2). In addition, the detailed long-term toxicity studies further confirming the safety of Cu₂MnS₂ NPs in vivo. Conclusion: We have developed a new 2D Cu₂MnS₂ NPs as multifunctional theranostic agents for MRI/MSOT dual-modal imaging-guided PTT of cancer in the NIR-II window. Such biocompatible Cu₂MnS₂ NPs might provide a new perspective for exploring new 2D-based nanoplatforms with improved properties for clinical applications in the future.

Keywords: two-dimensional nanomaterial, copper manganese sulfide nanoplates, MRI/MSOT dual-modal imaging, second near-infrared window, photothermal therapy.

Introduction

Photothermal therapy (PTT) is regarded as a promising cancer treatment strategy which utilizes photoabsorbing agents to convert laser light energy into heat to burn cancer cells [1-3]. Compared with traditional cancer therapies, such as radiotherapy and chemotherapy, PTT is a non-invasive technique for tumor ablation without damaging surrounding normal cells or organs in preclinical trials [4-6]. An enormous amount of studies have focused on pursuing new agents with higher photothermal conversion efficiency [7-9]. However, only a few of studies specialized on the excitation wavelength of the NIR-absorbing agents [10]. The laser wavelengths used in PTT are highly depended on the absorbent range of the NIR-absorbing agents. Because of the low scattering and energy absorption of tissue transmission, NIR light displays much deeper tissue penetration depths than that of ultraviolet-visible (UV-vis) light [11, 12]. In the NIR region, there are two so-called biological transparency windows: the first biological window (NIR-I, from 650 nm to 950 nm) and the second biological window (NIR-II, from 1000 nm to 1350 nm) [13]. NIR-II window exhibits two advantages compared with the well-researched NIR-I window: deeper penetration depths in tissue and higher maximum permissible exposure (MPE) of laser [14]. In particular, the penetration depth is expected to be maximal in the spectral ranges 1000-1100 nm [15]. Moreover, according to the “American National Standard for Safe Use of Lasers” (ANSI Z136.1-2007), the MPE for skin exposure is 1 W cm^-2 at 1064 nm, while the value is 0.33 W cm^-2 at 808 nm. To date, various nanomaterials or nanostructures have been used as PTT agents in NIR-I range [16-21]. Although the nanomaterials capabling of emitting NIR-II light have been received some attention in biological imaging [22-24], rare reports have demonstrated that these nanomaterials could act as PTT agents in the NIR-II region [25].

Recently, several groups have successfully developed nanoplatforms for NIR-II responsive PTT [14, 26-31]. However, there are still some issues need to be addressed. For instance, these nanomaterials are usually synthesized in oxygen-free atmosphere, typically coated with hydrophobic ligands during the synthesis process and hence are water insoluble [14, 26-28, 30]. Therefore, a further surface modification or phase transfer step is required to render products water-soluble for biomedical applications [31]. Facile synthesis of NIR-II responsive PTT agents with good biocompatibility remains challenging. Furthermore, the usage power of laser of current NIR-II agents is much higher than MPE [25, 27], which might hinder their applications in the PTT of cancer. In this regard, the development of PTT agents with strong absorption in the NIR-II window, which could generate enough heat irradiated by laser with low power density, thus becomes extremely important.

Although the PTT of cancer possess substantial advances, this technique cannot provide more information of tumor conditions. In order to evaluate the therapeutic effects of PTT, an alternative strategy is using imaging-guided combination therapy to monitor real-time visualization of PTT [32]. Magnetic resonance imaging (MRI), a facile, non-invasive and powerful nuclear magnetic resonance (NMR)-based imaging modality, has been widely applied in clinic for disease diagnosis since MRI is able to obtain three-dimensional (3D) tomographical information with high temporal-spatial resolution in whole tissue samples [33]. However, the poor sensitivity of MRI may hinder its application for accurate cancer diagnosis [34]. Multimodal imaging which combines two or more imaging techniques could integrates the superiorities of each imaging techniques while avoiding their defects, thus becoming a desirable demand [35]. For instance, multispectral optoacoustic tomography (MSOT), which combines the best features of optical- and ultrasonic-imaging, is an emerging and promising imaging technique. MSOT has distinct advantages, including high sensitivity, non-ionization and background-free detection [36, 37]. Therefore, combining MRI and MSOT for dual-modal imaging with the advantages of high resolution (10-100 μm) and deep penetration (up to 50 mm) as well as high sensitivity (∼10^-9 mol L^-1) is considered to be a promising approach for accurate cancer diagnosis [38-43]. Up to now, many efforts have been devoted to develop nanoplatforms with both imaging and therapy properties [44-46]. Among these, two-dimensional (2D) nanomaterials-based theranostic platforms have attracted great attention to reaserchers because of their special compositional, structural and physical/chemical properties [47-50]. To further enhance the performance of their application in imaging-guided PTT, these 2D nanomaterials are usually hybridized with other functional elements or structures [51, 52]. However, current studies are mainly focused on the PTT in NIR-I window, and no relevant reports regarding the theranostic effect of 2D nanomaterials in the NIR-II window.

Inspired by the great potential of the 2D nanomaterial for biomedical applications, we for the first time present a 2D-nanoplatform based on Cu₂MnS₂ nanoplates (NPs) for MRI/MSOT dual-modal imaging-guided PTT of cancer in the NIR-II window (Figure 1). Cu₂MnS₂ NPs were synthesized using a facile and effective one-pot solvothermal method by adjusting the proportion of Cu/Mn/S. The as-synthesized Cu₂MnS₂ NPs exhibit low cytotoxicity, high photothermal conversion efficiency and T₁-shortening effect, excellent photostability and strong absorbance in NIR-I and NIR-II region. Taking advantage of the above properties, the Cu₂MnS₂ NPs could be a promising contrast agent for MRI/MSOT dual-modal imaging-guided PTT of cancer. Furthermore, Cu₂MnS₂ NPs can effectively kill cancer cells both in vitro and in vivo under the 1064 nm laser irradiation with a low power density (0.6 W cm^-2). Therefore, the 2D-nanomaterial Cu₂MnS₂ NPs could be used as an attractive multifunctional platform for simultaneous MRI/MSOT imaging and NIR-II PTT, holding great promise in biomedical applications.

 Figure 1 

Schematic illustration for the syntheses and applications of Cu₂MnS₂ NPs as a theranostic platform.

Results and Discussion

Synthesis and Characterization of Cu₂MnS₂ NPs

The synthetic strategy is based on a facile one-pot solvothermal route involving ethylene glycol (EG), monomethoxycarboxyl polyethylene glycol (mPEG-COOH), CuCl₂·2H₂O, MnCl₂·4H₂O and Na₂S, where EG was the reactive solvent and mPEG-COOH was the capping agent as well as the stabilizing agent. As shown in Figure 2a and Figure S1, nanoplates (NPs) can be obtained by optimizing experimental conditions (See “Materials and Methods” section). The as-synthesized product showed well-defined uniform NPs with a diameter around 100 nm (Figure 2a). The observed lattice spacing in the high-resolution TEM (HRTEM) image is about 0.259 nm for the (111) plane of NPs (Figure 2a, Insert). The hydrodynamic size of the NPs was about 120 nm determined by dynamic light scattering (DLS) (Figure 2b). The atomic force microscopy (AFM) images also demonstrated that most NPs had a diameter of about 100 nm and a thickness of approximately 6 nm, confirming the layered structure of NPs (Figure 2c, d). The chemical composition of NPs was then investigated by energy dispersive X-ray spectroscopy (EDS). EDS analysis and elemental distribution mappings clearly indicated the existence of Cu, Mn and S elements in the NPs with the atomic ratio of Cu : Mn : S to be 2.03 : 1 : 1.99 (Figure 2e,f). The structure of the NPs was also confirmed by X-ray powder diffraction (XRD), which matches well with the standard powder diffraction pattern of Cu₂MnS₂ (JCPDS No. 50-0540) (Figure 2g). Furthermore, Cu, Mn and S were detected in the X-ray photoelectron spectroscopy (XPS) spectra (Figure 2h and Figure S2). Moreover, the mPEG-COOH could bind to the Cu₂MnS₂ NPs through Cu-carboxylate coordinative couplings [53]. The characteristic bond vibrations illustrated in Fourier-transform infrared spectroscopy (FTIR) (Figure S3a) of the Cu₂MnS₂ NPs showed the typical peaks at ~1110 cm^-1 (C-O-C), ~1660 cm^-1 (C=O), ~2875 cm^-1 (C-H) and ~3400 cm^-1 (O-H), confirming the successful surface grafting of mPEG-COOH [54]. A weight loss of 13.7%, which could be attributed to the grafted mPEG-COOH chains, was calculated from the thermos-gravimetric analysis (TGA) curve of Cu₂MnS₂ NPs (Figure S3b). The zeta-potential (ζ) result with a negatively charged of -15.5 ± 3.6 mV further confirming the surface grafting of mPEG-COOH on the Cu₂MnS₂ NPs (Figure S4). Furthermore, the as-synthesized Cu₂MnS₂ NPs exhibited excellent dispersibility and stability in water and physiological solutions (Figure S5) owing to the surface-grafted mPEG-COOH chains. The DLS analyses showed that the hydrodynamic diameters of Cu₂MnS₂ NPs in different solutions did not change over 30 days (Figure S5), indicating the excellent colloidal stability of Cu₂MnS₂ NPs. Notably, the UV-vis-NIR spectrum showed that the Cu₂MnS₂ NPs displayed strong absorption from 800 to 1300 nm with peaks at ~900 nm, ~1075 nm and ~1260 nm (Figure 2i). This result indicated that the as-synthesized Cu₂MnS₂ NPs could be as a candidate agent for PTT in the NIR-II biological window.

 Figure 2 

(a) TEM image of Cu₂MnS₂ NPs (Inset: HRTEM image of Cu₂MnS₂ NPs). (b) The diameter distribution of Cu₂MnS₂ NPs by DLS test. (c) Atomic force microscopy (AFM) image and (d) the corresponding height analysis of Cu₂MnS₂ NPs. (e) STEM-EDS elemental mapping, (f) EDS analysis spectrum, (g) XRD patterns, and (h) XPS spectrum of Cu₂MnS₂ NPs. (i) UV-vis-NIR absorption spectrum of Cu₂MnS₂ NPs in water. Inset: photograph of Cu₂MnS₂ NPs aqueous solution.

 Figure 3 

(a) Cell viabilities of MCF-7, HeLa and S180 cells after incubated with different concentrations of Cu₂MnS₂ NPs for 24 h. (b) The percentages of hemolysis by different concentrations of Cu₂MnS₂ NPs. PBS and water were used as negative and positive controls. Insert: photograph of Eppendorf-tubes containing the supernatant from RBCs exposed to different concentrations of Cu₂MnS₂ NPs. (c) Blood clearance of Cu₂MnS₂ NPs in mice (n = 5). (d) Biodistribution of Cu₂MnS₂ NPs in tumors-bearing mice at 6 h, 12 h, 24 h and 48 h post-injection, measured by ICP-OES analysis of Mn in different organs and tissues (n = 5).

In vitro cytotoxicity and in vivo biodistribution study of Cu₂MnS₂ NPs

In order to explore the applications of Cu₂MnS₂ NPs in biomedicine, we then tested their cytotoxicity on several types of cells by standard cell counting kit-8 (CCK-8) assays. The results revealed that Cu₂MnS₂ NPs show low cytotoxicity to these cells even at concentration up to 400 μg mL^-1 (Figure 3a), indicating good biocompatibility of the Cu₂MnS₂ NPs. Efficient cellular uptake and internalization of the Cu₂MnS₂ NPs are paramount for therapeutic effects. Inductively coupled plasma-optical emission spectrometer (ICP-OES) was used to quantitatively the uptake amount of Cu₂MnS₂ NPs after incubation with different cells. The results demonstrated that the Cu₂MnS₂ NPs could be effectively internalized by cancer cells, and the amount of cellular uptake was steadily increased with the incubation time (Figure S6). For the in vivo application of Cu₂MnS₂ NPs, the hemolysis assay which is a crucial toxicological factor should be investigated. As shown in Figure 3b, no hemolysis of red blood corpuscles (RBCs) was observed at tested concentrations from 0.05 to 2.0 mg mL^-1 of Cu₂MnS₂ NPs, suggesting the admirable blood compatibility of Cu₂MnS₂ NPs for their further in vivo diagnostic and therapeutic applications. Furthermore, knowledge of the clearance and biodistribution of a nanomaterial is distinctly important for its application in medical treatment. Herein, in order to study the clearance and biodistribution of Cu₂MnS₂ NPs in vivo, S180 tumor-bearing BALB/c nude mice were injected with Cu₂MnS₂ NPs through the tail vein at the dose of 20 mg kg^-1, then the blood of mice were collected at different time points (0.5, 1, 2, 6, 12, 24, 36 and 48 h) for blood clearance analysis. The mass of Mn was measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES). The clearance of Cu₂MnS₂ NPs in the blood was followed a simple exponential decay curve with a half-decay time (t_1/2) of ~15.6 h (Figure 3c). For biodistribution studies, the mice were sacrificed, and organs were dissected, weighed, and digested after 6, 12, 24 and 48 h. The data of organ distribution and tumor uptake of Cu₂MnS₂ NPs were shown in Figure 3d. It clearly showed that the liver, spleen and kidney are the main target organs. Excitingly, the tumor uptake of Cu₂MnS₂ NPs was measured to be ~3.03% ID g^-1 and ~6.12% ID g^-1, at 6 h and 12 h post-injection, respectively, and this value was increased to ~9.57% ID g^-1 at 24 h and up to 10.05% ID g^-1 at 48 h (Figure 3d). The efficient tumor accumulation of Cu₂MnS₂ NPs could be attributed to the well-known enhanced permeability and retention (EPR) effect in solid tumors with tortuous and leaky vasculatures [55]. Therefore, the prolonged blood circulation half-decay time and high tumor accumulation of Cu₂MnS₂ NPs are favorable for in vivo applications.

 Figure 4 

(a) Temperature elevation over a period of 10 min of exposure to a laser (1064 nm, 0.6 W cm^-2) at various Cu₂MnS₂ NPs concentrations. (b) Infrared thermal images of aqueous solutions containing different concentrations of Cu₂MnS₂ NPs, captured before and after irradiation of 1064 nm laser (0.6 W cm^-2) at different time scale (0, 2, 4, 6, 8 and 10 min). (c) Photothermal heating curves of the Cu₂MnS₂ NPs aqueous solution under 1064 nm laser irradiation at different power densities. (d) CLSM images of differently treated HeLa cells stained with PI. Top panels: PI fluorescence corresponding to dead cells, bottom panels: the overlay of PI fluorescence and the bright field image. (d₁) 5 min laser only; (d₂) Cu₂MnS₂ NPs only (40 μg mL^-1); (d₃) Cu₂MnS₂ NPs (40 μg mL^-1) plus 2 min laser; (d₄) Cu₂MnS₂ NPs (40 μg mL^-1) plus 5 min laser. Laser: 1064 nm, 0.6 W cm^-2. All the pictures share the same scale bars: 100 μm. (e) Cell viabilities of HeLa cells incubated to different concentrations of Cu₂MnS₂ NPs with or without laser irradiation (the results are means ± SD, n = 5, *p<0.05, **p<0.01).

Photothermal property of Cu₂MnS₂ NPs

The strong NIR-II absorption of Cu₂MnS₂ NPs motivated us to explore their NIR photothermal properties. The photothermal effect of Cu₂MnS₂ NPs was first studied by measuring the solution temperature changes of different concentrations of Cu₂MnS₂ NPs under NIR-II laser irradiation (1064 nm, 0.6 W cm^-2). As clearly shown in Figure 4a and Figure 4b, with the increase of Cu₂MnS₂ NPs concentration or irradiation time, the temperature of the solutions increased rapidly, while pure water or PBS showed negligible changes. Particularly, the temperature of the solution with 30 μg mL^-1 of Cu₂MnS₂ NPs had a 31.1 °C increase (from 23.8 to 54.9 °C) after 10 min laser irradiation at the power density of 0.6 W cm^-2. It has been reported that cancer cells can be killed after being kept at 42 °C for 15-60 min, and the duration can be shortened to 4-6 min when the temperature is over 50 °C [16, 18, 56, 57]. Thus, the temperature increase is well above the required temperature rise for efficient cancer PTT. Moreover, Cu₂MnS₂ NPs displayed an obvious laser-power-dependent photothermal effect (Figure 4c). Subsequently, the molar extinction coefficient (ε) at 1064 nm for Cu₂MnS₂ NPs was calculated according to Lambert-Beer law, and it was determined to be 1.03 × 10¹⁰ M^-1 cm^-1 (Figure S7), which was more than two orders of magnitude higher than that of Au-Cu₉S₅ nanostructures [14]. Furthermore, the photothermal transduction efficiency (η) of Cu₂MnS₂ NPs was measured according to previous method [28], and it was calculated to be ~49.38% (Figure S8), which was apparently higher than that of the classical photoabsorber gold nanorods (Figure S8, AuNRs, ~26.85%) and most of the other reported PTT agents working in NIR-II window [14, 25-31]. These results demonstrated that Cu₂MnS₂ NPs could convert the 1064 nm laser energy into thermal energy efficiently owing to their strong NIR-II photoabsorbance. In addition, the absorbance and photothermal performance of Cu₂MnS₂ NPs solution remained almost identical even after four cycles of laser on/off (Figure S9 and S10), whereas AuNRs showed poor photothermal stability after cyclic laser irradiation (Figure S9 and Figure S10). Therefore, the demonstrated low cytotoxicity, large molar extinction coefficient, high photothermal transduction efficiency and excellent photostability of Cu₂MnS₂ NPs allow them to be the promising PTT agent working in the NIR-II window.

In vitro NIR-II photothermal therapy

To further verify the PTT effect, we then evaluated in vitro photothermal capacity of Cu₂MnS₂ NPs on human cervical cancer cells (HeLa). As shown in Figure 4d, the Cu₂MnS₂ NPs incubation or laser irradiation alone was unable to kill the HeLa cells. The red fluorescence (corresponding to dead cells) images demonstrated that almost all the cells were dead after incubated with 40 μg mL^-1 of Cu₂MnS₂ NPs for 4 h and treated with 1064 nm laser irradiation (0.6 W cm^-2) for 5 min. The standard CCK-8 assays were also performed to quantitatively evaluate the photothermal cytotoxicity of Cu₂MnS₂ NPs. Figure 4e showed that the viabilities of HeLa cells significantly decreased when the cells were treated with Cu₂MnS₂ NPs and laser exposure. In contrast, cells treated with the Cu₂MnS₂ NPs without laser irradiation remained more than ~95% viable even at a concentration of 80 μg mL^-1. These in vitro results indicated that the combination of Cu₂MnS₂ NPs and laser irradiation could kill the cancer cells effectively, thus Cu₂MnS₂ NPs could act as effective NIR-II photoabsorbers for cancer PTT.

In vitro and in vivo MRI/MSOT dual-modal imaging

Currently, multimodal imaging technology, especially the combination of MRI and MSOT, has attracted substantial attentions in biomedical applications towards the development of more sensitive and accurate biological imaging systems [58] and imaging-guided therapy. We herein hypothesized that the as-synthesized Cu₂MnS₂ NPs might be a new suitable agent for MRI/MSOT dual-modal imaging-guided PTT owing to their strong NIR absorbance, excellent photothermal conversion effects and the presence of paramagnetic Mn ions. To investigate the capability of Cu₂MnS₂ NPs as a MRI contrast agent. We first assessed the MRI performance in aqueous solutions with various concentrations of Cu₂MnS₂ NPs. As shown in Figure 5a, T₁-weighted phantom images became observably brighter as the concentration of Cu₂MnS₂ NPs was increased, suggesting the excellent T₁ contrast ability of Cu₂MnS₂ NPs. The longitudinal relaxivity (r₁) value of Cu₂MnS₂ NPs was measured to be 4.85 ± 0.31 mM^-1 s^-1 (Figure 5b), which was comparable to that of commercial contrast agent Gd-DTPA (3.22 mM^-1 s^-1) [39]. We further examined Cu₂MnS₂ NPs as a T₁-MRI contrast agent for cancer cell imaging. Compared with the cancer cells without Cu₂MnS₂ NPs incubation, brighter images were observed in Cu₂MnS₂ NPs-incubated cancer cells (Figure S11). Similarly, the concentration-dependent photoacoustic (PA) signal enhancement were observed not only in solution (Figure 5c) but also in cancer cell imaging (Figure 5d) after treated with Cu₂MnS₂ NPs, suggesting the potential application of Cu₂MnS₂ NPs as MSOT contrast agents. Encouraged by the excellent in vitro MRI/MSOT performance of the Cu₂MnS₂ NPs, we then evaluated the capacity of the Cu₂MnS₂ NPs for in vivo MRI/MSOT dual-modal tumor imaging. The contrast enhancing effect in vivo were evaluated in S180 tumors-bearing BALB/c nude mice. The mice were anaesthetized and scanned with MRI or MSOT system before (0 h) and after (24 h) intravenous injection of Cu₂MnS₂ NPs (20 mg kg^-1) solution. Firstly, the in vivo MRI contrast effects of Cu₂MnS₂ NPs were investigated on a 3 T MRI scanner. The tumor region showed marked positive enhancement (brighter signal) after injection, indicating that Cu₂MnS₂ NPs can produce strong T₁ contrast enhancements in the tumor (Figure 5e). To quantify the contrast enhancement, the signal-to-noise ratio (SNR) for MRI was calculated [52]. Compared to before injection, the SNR value of tumor region after injection had a 6.43-fold enhancement (Figure 5f), further demonstrating that Cu₂MnS₂ NPs can shorten T₁ relaxation time of tumor. We then examined the in vivo PA contrast effects of Cu₂MnS₂ NPs using a MSOT system. The PA signals in the tumor region were remarkably enhanced after injection with Cu₂MnS₂ NPs (Figure 5g), with the PA intensity having a 6.85-fold increase (Figure 5h) at 24 h of post injection under irradiation of a 900 nm laser source, confirming the feasibility of Cu₂MnS₂ NPs for in vivo MSOT contrast enhancement. The excellent in vitro and in vivo MRI/MSOT dual-modal imaging capabilities of Cu₂MnS₂ NPs made them as promising nanomaterials for cancer diagnosis, which could be further applied in imaging-guided PTT.

 Figure 5 

(a) T₁-weighted phantom images of Cu₂MnS₂ NPs with different [Mn] concentrations at 0.5 T. (b) The linear fitting of longitudinal relaxation rates at 0.5 T versus [Mn] concentrations for Cu₂MnS₂ NPs. The r₁ of Cu₂MnS₂ NPs was determined to be 4.85 ± 0.31 mM^-1 s^-1 (n = 3). (c) Photoacoustic (PA) images and (d) corresponding PA intensities of Cu₂MnS₂ NPs solutions with different concentrations. (e) In vivo T₁-weighted MRI images and (f) corresponding quantification of the signal-to-noise ratios (SNR), and (g) in vivo MSOT images (Scale bars: 3 mm) and (h) corresponding quantification of intensities of mice tumor region before and after the injection of Cu₂MnS₂ NPs (20 mg kg^-1), respectively.

Deep tissue photothermal therapy in NIR-II window

Because of its low scattering and energy absorption of tissue transmission, NIR-II light displays more deeply tissue penetration depths than NIR-I light [15]. Deep tissue photothermal effect of the as-synthesized Cu₂MnS₂ NPs in the NIR-II window were investigated by using different thicknesses of pork tissues as model biological tissues (Figure S12a). During the laser irradiation process (1064 or 808 nm, 1 W cm^-2), a thermal imaging camera was used to record the temperature changes of Cu₂MnS₂ NPs solution with or without cover of pork tissues (Figure S12b, c). Moreover, the obtained temperature changes under different thicknesses of pork tissues were normalized to the maximum temperature increase which measured the Cu₂MnS₂ NPs solution without tissue cover. The results were plotted as functions of tissue thicknesses and fitted with exponential decay curves. As shown in Figure S13, the relative rate of temperature changes under different pork tissue thicknesses exposed to 808 nm laser was obviously faster than that of under 1064 nm laser irradiation, confirming that the as-synthesized Cu₂MnS₂ NPs have great potential for deep tissue PTT in the NIR-II window.

In vivo photothermal therapy in NIR-II window and long-term toxicity study of Cu₂MnS₂ NPs

The feasibility of using Cu₂MnS₂ NPs for in vivo imaging-guided NIR-II PTT was investigated. The in vivo photothermal effect of Cu₂MnS₂ NPs was first measured by using an IR thermal camera to monitor the temperature changes of the tumor areas. IR thermographic images showed that the temperature at the tumor region can rapidly increase from ~33 °C to ~55 °C after intravenous injection of Cu₂MnS₂ NPs (20 mg kg^-1) under 1064 nm laser irradiation (0.6 W cm^-2) within 3 min (Figure 6a,b), which was capable of inducing sufficient local hyperthermia to kill tumor cells. In the control test, the temperature of the tumors region that injected with PBS increased only ~3 °C during the whole irradiation process (Figure 6a,b). To further verify the photothermal therapeutic effect of Cu₂MnS₂ NPs, the S180 tumor-bearing mice were randomly divided into four groups (n = 5 for each group) for different treatment processes: without any treatment (control group), intravenous injected with 200 μL of PBS and then irradiated with 1064 nm laser (0.6 W cm^-2) for 10 min (PBS + laser group), intravenous injected with 200 μL of 20 mg kg^-1 Cu₂MnS₂ NPs only (Cu₂MnS₂ NPs group), and intravenous injected with 200 μL of 20 mg kg^-1 Cu₂MnS₂ NPs and then irradiated with 1064 nm laser (0.6 W cm^-2) for 10 min (Cu₂MnS₂ NPs + laser group). The tumor volume and weight of mice were measured every other day after the treatments described above. As shown in Figure 6c and Figure 6d, in Cu₂MnS₂ NPs + laser group, the tumors were successfully eliminated without any reoccurrence. In striking contrast, the mice in the other three groups showed rapid tumors growth within 28 days (Figure 6c,d), suggesting that neither laser irradiation alone nor Cu₂MnS₂ NPs injection alone could suppresses the tumors growth. Meanwhile, the body weight of mice in Cu₂MnS₂ NPs + laser group was gradually increased, indicating the recovery of the mice after the PTT treatment, while the mice in other three groups were in poor health with weight loss because of the rapid growth of malignant tumors (Figure S14). Furthermore, the mice in Cu₂MnS₂ NPs + laser group could survive over 57 days and only one mouse dead at day of 57, while mice in the other three groups showed an average life time of less than 37 days (Figure 6e). Moreover, hematoxylin and eosin (H&E) staining of tumor slices revealed that the tumor cells in Cu₂MnS₂ NPs + laser group were severely destroyed (necrosis withcells deformation) due to the PTT treatment. However, tumor cells in the other three groups retained regular cell morphology with intact cell nucleus (Figure 6f). In addition, no obvious noticeable organ damage or adverse effect associated with the administration of the Cu₂MnS₂ NPs was observed at the tested dose (20 mg kg^-1) (Figure S15), indicating the safety of Cu₂MnS₂ NPs in vivo. Furthermore, to investigate the detailed long-term toxicity of Cu₂MnS₂ NPs, hematology analysis and blood biochemical assays were carried out for healthy mice after intravenous injection of Cu₂MnS₂ NPs (20 mg kg^-1), at the 1st, 7th, 14th, and 28th day post-injection (Figure S16 and Figure S17). Neither deaths nor serious body weight loss was observed in all test groups during the study period. Encouragingly, all of measured parameters (including routine blood parameters and serum biochemistry parameters) in Cu₂MnS₂ NPs injected groups were in normal reference ranges [59], and showed no significant physiological difference compared to the control groups (Figure S16 and Figure S17). In addition, the long-term in vivo biodistribution of Mn amounts in different organs (including the heart, liver, spleen, lung, kidney, tumor, intestine, stomach, brain and muscle) were also carried out. As shown in Figure S18, the amounts of Mn in all organs were decreased at 7th day, and were reached to very low levels at 28th day, implying that the Cu₂MnS₂ NPs could be excreted out of these organs, thus avoiding the potential adverse or side effects of the organs. These results clearly demonstrated that Cu₂MnS₂ NPs could be a promising agent for tumor NIR-II PTT in vivo.

 Figure 6 

(a) IR thermal images of tumor-bearing mice with intravenous injection of PBS or Cu₂MnS₂ NPs under 1064 nm laser irradiation (24 h of post-injection) (0.6 W cm^-2) for different times. (b) Tumor temperatures of mice monitored by the IR thermal camera as a function of the irradiation time. (c) Representative photos of mice in different groups before and after treatment. (d) Tumor growth curves of tumor-bearing mice in different groups after treatment (the results are means ± SD, n = 5 for each group, *p<0.05, **p<0.01). (e) Surviving proportion of four different groups after various treatments (n = 5 for each group). (f) H&E-stained images of tumor sections collected from mice of different groups after various treatments. Scale bars are 200 μm.

Conclusions

In summary, we have, for the first time, developed a new 2D nanoplatform based on Cu₂MnS₂ NPs for MRI/MSOT dual-modal imaging-guided PTT of cancer in the NIR-II window. The Cu₂MnS₂ NPs contain a variety of remarkable properties. First, the synthesis of Cu₂MnS₂ NPs is easy to achieve via a one-pot and facile solvothermal method. Second, the Cu₂MnS₂ NPs possesses excellent colloidal stability and biocompatibility. Third, with strong NIR-I and NIR-II absorption, large molar extinction coefficient (ε), high photothermal conversion efficiency (η) and outstanding photostability as well as enhanced T₁-shortening effect, the Cu₂MnS₂ NPs can serve as effective agents for MRI/MSOT (900 nm laser source) dual-modal imaging-guided cancer PTT in vitro and in vivo in the NIR-II window. Overall, taking advantage of the inspiring features of the Cu₂MnS₂ NPs, we might provide a new perspective for exploring new 2D-based nanoplatforms with inproved properties for the biomedical application.

Materials and Methods

Chemicals and Materials

Copper (II) chloride dihydrate (CuCl₂·2H₂O, ≥99%), Manganese (II) chloride tetrahydrate (MnCl₂·4H₂O, ≥98%), anhydrous sodium sulfide (Na₂S, ≥98%) and monomethoxycarboxyl polyethylene glycol (mPEG-COOH, MW=2000, AR) were purchased from Sigma-Aldrich. Cell Counting Kit-8 (CCK-8) was purchased from Dojindo (Japan). All other reagents with analytical reagent grade were purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and were used without further purification. The ultrapure water used in all experiments was obtained from a Millipore water purification system (resistivity 18.2 MΩ·cm).

Animal Models

All the animal experiments were executed according to the protocol approved by the Institutional Animal Care and Use Committee of Fujian Medical University.

Preparation of Cu₂MnS₂ NPs

In order to synthesis the desired Cu₂MnS₂ NPs, a series of experiments were carried out. The synthetic strategy is based on a one-pot solvothermal method involving mPEG-COOH , CuCl₂·2H₂O, MnCl₂·4H₂O and Na₂S (different proportion of Cu:Mn:S, molar ratio): (1) The reactions were under the same conditions but with different proportion of Cu:Mn:S (100 mg of mPEG-COOH, reaction temperature: 120 °C, reaction time: 6 h); (2) The reactions were under the same conditions but with different amounts of mPEG-COOH (Cu:Mn:S = 1:1:3, reaction temperature: 120 °C, reaction time: 6 h); (3) The reactions were under the same conditions but with different reaction time (Cu:Mn:S = 1:1:3, 100 mg of mPEG-COOH, reaction temperature: 120 °C); and (4) The reactions were under the same conditions but with different reaction temperature (Cu:Mn:S = 1:1:3, 100 mg of mPEG-COOH, reaction time: 6 h). Plate-like products with well-defined uniform morphology can be obtained under certain reaction conditions: Cu:Mn:S = 1:1:3, 100 mg of mPEG-COOH, reaction temperature: 120 °C, reaction time: 6 h. The detail procedures are as follows: CuCl₂·2H₂O (0.1705 g, 1 mmol), MnCl₂·4H₂O (0.1258 g, 1mmol) and mPEG-COOH (0.1000 g) was first dissolved in 20 mL ethylene glycol (EG) sufficiently. Then, another 20 mL of EG containing Na₂S (0.2340 g, 3 mmol) was slowly added into the above solution with vigorous magnetic stirring. After stirring for 2 h to form a dark brown turbid solution, the resulting solution was transferred to a stainless steel autoclave (50 mL), and heated at 120 °C for 6 h. After being cooled to room temperature, the green-dark precipitate was collected by centrifugation and washed with ethanol and deionized water two times, respectively, and finally redispersed in water. The obtained solution was stored in darkness at 4 °C for further use.

UV-vis-NIR Absorption Spectrum Measurements

To investigate the absorption of Cu₂MnS₂ NPs, 1 mL of Cu₂MnS₂ NPs aqueous solution with different concentrations were first prepared. Then, the UV-vis-NIR absorption spectra of Cu₂MnS₂ NPs were collected by an UH4150 Spectrophotometer (Hitachi Co., Ltd., Japan) in the range of 500-1500 nm.

Measurement of the Photothermal Performance

The photothermal performance of Cu₂MnS₂ NPs was investigated by recording the temperature changes of the solution under continuous laser irradiation. In a typical experiment, 1 mL aqueous solutions of Cu₂MnS₂ NPs with different concentrations were irradiated by 1064 nm (0.6 W cm^-2) lasers for 10 min. The temperature changes were monitored by a digital thermometer during the irradiation period.

In Vitro Photothermal Therapy

HeLa cells were incubated with or without Cu₂MnS₂ NPs (40 μg mL^-1) for 4 h and then exposed to a 1064 nm (0.6 W cm^-2) laser for 5 min. After staining with propidium iodide (PI) for 30 min, the cells were imaged by a confocal laser scanning microscope (Nikon C2, Japan) to capture the red fluorescence of dead cells.

To quantitatively evaluate the photothermal cytotoxicity of Cu₂MnS₂ NPs, HeLa cells were incubated in 96-well plates (100 μL, 1×10⁴ cells per well) at 37 °C in a humidified atmosphere containing 5% CO₂ for 24 h. The Cu₂MnS₂ NPs with different concentrations were added and the cells were further incubated for 4 h. After with or without irradiated by a 1064 nm (0.6 W cm^-2) laser for 5 min, the cells were allowed to incubate for another 24 h. The standard CCK-8 assays were carried out to evaluate the cell viabilities.

Measurement of MRI Relaxation Properties and Cellular MRI

To measure the T₁ relaxation properties, Cu₂MnS₂ NPs with different [Mn] concentrations were prepared and measured using a 0.5 T NMI20-Analyst NMR system (Niumag Corporation, Shanghai, China). The longitudinal relaxation times (T₁) were measured using an inversion recovery (IR) sequence. The longitudinal relaxivity (r₁) was determined from the slope of 1/T₁ values as the function of [Mn] concentrations. For cellular MRI, S180 cells with a density of 1×10⁵ cells/well were seeded in a 6-well tissue culture plate and incubated for 24 h. Then, the medium was replaced with fresh RPMI-1640 medium containing different concentrations of Cu₂MnS₂ NPs and cultured for different time. After removing the cell culture medium, cells were immobilized and gently washed with PBS buffer for five times, then the cells were resuspended and MRI scanning were carried out to collect the signals. T₁-weighted MR images were acquired using a 2D multi-slice spin-echo (MSE) sequence with the following parameters: TR/TE = 100/2 ms, 512 × 512 matrices, slices = 1, thickness = 1 mm, and NS = 4.

In Vivo MRI

Tumor-bearing mice were prepared by subcutaneously injecting a suspension of 2×10⁶ S180 cells into the back of the hind leg. MRI scanning was first performed before Cu₂MnS₂ NPs injection as the control. Next, the mice were intravenously injected with Cu₂MnS₂ NPs (20 mg kg^-1). T₁-weighted images of the mice were collected on a 3 T MRI scanner after 24 h of post-injection (GE Discovery MR750). All the images were obtained using an FSE sequence under the following parameters: TR/TE = 513/14.5 ms, FOV = 8 × 8 cm, 320 × 224 matrices, thickness = 1 mm. To quantify the contrast enhancement, the signal-to-noise ratio (SNR) was measured by finely analyzing regions of interest (ROIs) of the images of tumor sites [52].

In Vitro and In Vivo MSOT

Different concentrations of Cu₂MnS₂ NPs solutions (0, 20, 40, 60, 80, 100 μg mL^-1) were loaded into agar gel cylinders. The in vitro MSOT was carried out on the InVision 128 MSOT system (iThera Medical, Germany) to collect the optoacoustic signals (laser wavelengths: 680-980 nm, the main experimental parameters were 5 nm wavelengths for each slice). After the reconstruction (900 nm) of the obtained MSOT images, MSOT signals were calculated by finely analyzing regions of interest (ROIs) of the images of tumor sites. The cellular MSOT experiment processes were similar with the cellular MRI (see cellular MRI).

For in vivo MSOT, MSOT scanning was first performed before Cu₂MnS₂ NPs injection as controls. Next, the mice were intravenously injected with Cu₂MnS₂ NPs (20 mg kg^-1). After 24 h of post-injection, the mice were scanned from 680 nm to 980 nm with the MSOT (MSOT InVision 128, iThera medical, Germany) to collect signals. The main experimental parameters were 5 nm wavelengths for each slice, with the region of interest being 25 mm. A water heating system was used to maintain the body temperature of the mice to ~37 °C during the scanning processes.

In Vivo PTT

The mice were divided into four groups with five mice per group when the tumor size reached about 80 mm³. In one group, the mice were intravenously injected with Cu₂MnS₂ NPs (20 mg kg^-1). After the 24 h of post-injections, tumors were irradiated with or without NIR-II laser 1064 nm (0.6 W cm^-2) for 10 min. Thee other three groups are: mice without any treatment (control group), mice were only intravenously injected with Cu₂MnS₂ NPs (Cu₂MnS₂ NPs only group), and mice intravenous injected with PBS and then irradiated with laser (PBS + laser group). Infrared thermal imaging system was performed for studying the in vivo photothermal effect of Cu₂MnS₂ NPs. IR thermographs was recorded by a FLIR Ax5 infrared camera. The tumor sizes were measured by a digital caliper every other day. The tumor volumes were calculated through the equation: volume = ab²/2, in which a is the maximum diameter and b is the minimum diameter of tumor, respectively. Relative tumor volumes were calculated as V/V₀, where V₀ is the tumor volume when the treatment was initiated.

Hematology Analysis, Blood Biochemical Assay and Histology Analysis

For hematology and biochemical analysis, the blood was collected from mice and separated by centrifugation into cellular and serum fractions. The hematology data and serum biochemistry data were recorded on MINDRAY BC-5500 automated hematology analyzer (MINDRAY, China) and HITACHI 7020 automatic biochemical analyzer (HITACHI, Japan), respectively. For histology analysis, mice were sacrificed and major organs from those mice were harvested, dipped in 10% neutral buffered formalin, then embedded in paraffin, stained with H&E and pathology were examined by a digital microscope. All the statistics were based on standard deviations of 5 mice per group.

Abbreviations

2D: two-dimensional; MRI: magnetic resonance imaging; MSOT: multispectral optoacoustic tomography; PTT: photothermal therapy; Cu₂MnS₂ NPs: Cu₂MnS₂ nanoplates; MPE: maximum permissible exposure; NIR: near-infrared; NIR-I: the first biological window; NIR-II: the second biological window; EG: ethylene glycol; mPEG-COOH: monomethoxycarboxyl polyethylene glycol; TEM: transmission electron microscope; DLS: dynamic light scattering; AFM: atomic force microscopy; EDS: energy dispersive X-ray spectroscopy; XRD: X-ray powder diffraction; XPS: X-ray photoelectron spectroscopy; FTIR: fourier-transform infrared spectroscopy; TGA: thermos-gravimetric analysis; ICP-OES: inductively coupled plasma-optical emission spectrometer; UV-vis: ultraviolet-visible; CCK-8: cell counting kit-8; MCF-7: humanbreast-cancer cells; HeLa: human cervical cancer cells; S180: murine sarcoma cancer cells; AuNRs: gold nanorods; PI: pyridinium iodide; PA: photoacoustic; T₁: longitudinal relaxation times; r₁: longitudinal relaxivity; i.v. injection: intravenous injection.

Supplementary Material

Additional detailed experimental section, characterization and results: Cell Culture, Cellular Uptake of Cu₂MnS₂ NPs and Cytotoxicity Assay; Hemolysis Assay; Biodistribution; Calculation of the Photothermal Conversion Efficiency; Supplementary Figures S1-S18.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Nos. 21475026, U1505221, 21635002, 81501461, 21775025), Natural Science Foundation of Fujian Province of China (Nos. 2015H6011, 2017J05131, 2017J01199), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT15R11), and the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (No. 2014B02).

Competing Interests

The authors have declared that no competing interest exists.

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Author contact

Corresponding authors: chunhualuedu.cn; hhyangedu.cn

Received 2017-6-29
Accepted 2017-8-27
Published 2017-10-17

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