Article Cite This: ACS Appl. Nano Mater. 2018, 1, 5460−5469
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Enhanced Exfoliation of Biocompatible MoS2 Nanosheets by Wool Keratin Xiangyu Xu,† Jianyang Wu,† Zhaohui Meng,† Yanran Li,† Qiaoling Huang,† Yue Qi,† Yufei Liu,§ Da Zhan,*,† and Xiang Yang Liu*,†,‡
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Research Institution for Biomimetics and Soft Matter, Fujian Key Provincial Laboratory for Soft Functional Materials Research, College of Materials and College of Physical Science and Technology, Xiamen University, Xiamen 361005, P. R. China ‡ Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore § Key Laboratory of Optoelectronic Technology and Systems, Chongqing University, Ministry of Education, Chongqing 400044, China S Supporting Information *
ABSTRACT: The unique properties of thin-layer transition metal dichalcogenides (TMDs) materials have been extensively investigated for both fundamental studies and practical applications in recent years. To develop the TMDs for medical engineering and bioelectronics in practical applications, the exfoliation and stabilization of TMDs using naturally biocompatible protein agents are in high demand. However, such strategies are far from being thoroughly studied. In this article, we present that wool keratin can assist exfoliation of MoS2 in aqueous solutions under sonication. And the wool keratin exfoliation of 2D material is systematically verified by molecular dynamics calculations and a series of experimental data observations. Moreover, compared with the traditionally used naturally biocompatible BSA under the same experimental conditions, wool keratin has been deemed to have an obvious better performance in exfoliating bulk MoS2. Relevant theoretical analysis indicates that high sulfur-containing functional groups on wool keratin molecules could provide sufficient possibility for the effective adsorption at the edges of MoS2 nanosheets. This discovery ensures that wool keratin biomolecules can provide superior colloidal stability for high concentration dispersion. The related investigations indicated that the biocompatibility of wool keratin (WK)-MoS2 films can also be demonstrated well on the basis of cell proliferation and viability assays. We also proved that wool keratin has a universal property for exfoliation of other types of two-dimensional materials. This discovery offers some new ways and opportunities for exfoliation of MoS2 and its biological applications. KEYWORDS: wool keratin, two-dimensional (2D) material, transition metal dichalcogenides (TMDs), liquid-phase exfoliation, nanosheets in the fields of field effect transistor,8,9 optoelectronic devices,10,11 biosensors,12,13 new energy,14,15 and bioimaging technologies16,17 in recent years. To date, the preparation methods of MoS2 nanosheets fall into two categories: one is bottom-up, the other is top-down.18 The former method, including hydrothermal synthesis19 and chemical vapor deposition,20 can produce large-area ultrathin film but usu ally requires harsh reaction conditions and complicated
1. INTRODUCTION Since the discovery of graphene in 2004,1,2 two-dimensional materials have become a focus for scientists in physics, chemistry, and biology. Although the two-dimensional (2D) materials possess the same chemical composition as their counterpart bulk forms, they show obvious different properties in physics and chemistry.3−7 Transition metal disulfides (TMDs) are a category of bulk layered materials, which can be exfoliated into 2D state with extraordinary electronic properties to be potentially applied practically in the next generation of flexible nanoelectronics. Among all TMDs, molybdenum disulfide (MoS2) is most representative and has been widely studied © 2018 American Chemical Society
Received: May 11, 2018 Accepted: September 24, 2018 Published: September 24, 2018 5460
DOI: 10.1021/acsanm.8b00788 ACS Appl. Nano Mater. 2018, 1, 5460−5469
Article
ACS Applied Nano Materials
2. EXPERIMENTAL SECTION
2.2. Preparation of Keratin Solution. WK was extracted from wool fibers via a classical reduction method.43,44 An amount of 10 g of wool fibers was immersed in 100 mL mixture solution of urea (8 M), sodium sulfide (0.2 M), and sodium dodecyl sulfate (0.1 M) and then stirred at 60 °C for 4 h. The impurities and undissolved fibers were removed by high speed centrifugation (9000 rpm, 20 min). Afterward, filtered keratin solution was dialyzed in deionized water with a cellulose tubing (MWCO 3500 Da, Solarbio, China) at room temperature for 3 days to remove excess small molecules for further experiments. 2.3. Exfoliation of MoS2. 1.25 g of MoS2 powder was added into 1 mg·mL−1, 2 mg·mL−1, 3 mg·mL−1, 5 mg·mL−1, and 10 mg·mL−1 of keratin solution, respectively. The mixed suspension was sonicated in a low-power sonic bath (KQ-250DE 250 W, Kun Shan Ultrasonic Instruments Co., Ltd., China) under controlled temperature at 25 °C, and the samples were collected once for every 3 h of sonication for the first 24 h and once for every 6 h for the next 48 h. After centrifugation at 5000 rpm for 45 min, all the supernatant was removed carefully, and then the collected precipitant was redispersed by sonicating in deionized water solvent with original volume for 10 min. The resulting solution was further centrifuged at 1500 rpm for 45 min, and the top two-thirds supernatant containing WK-MoS2 nanosheets was carefully collected for further measurements. 2.4. Determination of MoS2 Concentration. The precisely measured 100 mL WK-MoS2 dispersion was prefreezed in the ultralow temperature refrigerator, and then it was transferred into the freeze-dryer to freeze-dry (CS110-4, LaboGene, Denmark). The weight of WK-MoS2 was measured to confirm the solid content. Thermogravimetric analysis (TGA) was carried out by a TG209 F1 (Netzsch, Germany) with temperature increasing at a rate of 10 °C/min with a nitrogen flow at 75 mL/min to quantitatively analyze the proportions of WK and MoS2. 2.5. Characterization. UV−vis absorption spectra were recorded by a UV-1800 spectrophotometer (Xiamen Suo Yan Technology Co., LTD). ζ potential and size distribution of the exfoliated MoS2 were examined through a potential particle sizing analyzer (Mastersizer 2000 Malvern Instruments Ltd., U.K.). X-ray diffraction (XRD) of MoS2 nanosheets was done with an X-ray diffractometer (Bruker D8-A25). Atomic force microscopy (AFM) images were conducted by using a Multimode 8 (Bruker Nano Surfaces Business, USA). SEM images were observed by a field emission scanning electron microscope (SU-70, Hitachi High-tech Naka, JAN). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Quantum 2000 (Physical Electronics, USA) spectrometer. Raman spectra were recorded by a Horiba Labram HR Evolution instrument with a 532 nm laser, and the laser power is controlled at 0.5 mW to avoid laserinduced heating effect. 2.6. Cell Viability Texting. Cell viability was determined using the water-soluble tetrazolium salts (WST-1) cell proliferation and cytotoxicity assay kit (Beyotime, model C0036, China). On the first, fourth, and seventh day after seeding the osteoblast cells at a density of 104 cells/cm2 on the exfoliated MoS2 nanosheets, cells were washed with phosphate buffered saline (PBS) twice and incubated with medium containing 10% WST-1 cell proliferation reagent in a humidified atmosphere with 5% CO2 at 37 °C for 2 h. The solution was then retrieved from each well to a 96-well plate, and optical density (OD) was measured using a microplate reader (Molecular Devices, SpectraMax M2, USA) at 450 nm.
2.1. Materials. Molybdenum sulfide (MoS2) (99.5% metals basis,
