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Environmental Transformations and Algal Toxicity of SingleLayer Molybdenum Disulfide Regulated by Humic Acid Wei Zou, Xingli Zhang, Qixing Zhou, and Xiangang Hu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04397 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018
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Environmental Transformations and Algal Toxicity of Single-Layer
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Molybdenum Disulfide Regulated by Humic Acid
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Wei Zou, Xingli Zhang, Qixing Zhou, Xiangang Hu*
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Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education)/Tianjin
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Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental
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Science and Engineering, Nankai University, Tianjin 300071, China.
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Corresponding authors:
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Xiangang Hu,
[email protected] 9
Fax, 0086–022–85358121;
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Tel, 0086–022–85358121.
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ABSTRACT
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The environmental transformations of nanomaterials are correlated with their behaviors and
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ecological risks. The applications of single-layer molybdenum disulfide (SLMoS2) have rapidly
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developed in environmental fields, but the potential transformations and biological effects of
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SLMoS2 remain largely unknown. This study revealed that humic acid (HA, over 10 mg/L) induced
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the scrolling of SLMoS2 with light irradiation over a 56-day incubation. The colloidal stability of
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SLMoS2 increased and the aggregation ratio decreased from 0.59±0.07 to 0.08±0.01 nm/min after
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HA hybridization. Besides, compared with pristine SLMoS2, the chemical dissolution rate of
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SLMoS2 was up to 4.6-fold faster with HA exposure. These results demonstrate that HA affects the
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environmental fate and transformations of SLMoS2. SLMoS2–HA possessed a significantly widened
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direct band gap (2.06 eV) compared with that of SLMoS2 (1.8 eV). SLMoS2 acted as an electronic
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acceptor from HA, resulting in the separation of electron-hole pairs. Consequently, SLMoS2–HA
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exhibited stronger peroxidase-like catalytic activity, which was approximately 2-fold higher than 1
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that of SLMoS2. Moreover, the morphology and layered structure of SLMoS2 changed, and the
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damage SLMoS2 inflicted on microalgae was significantly reduced. This work provides insights
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into the behaviors and related biological risks of SLMoS2 in aqueous environments.
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INTRODUCTION
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Recently, single-layer molybdenum disulfide (SLMoS2) has been applied in various fields, such as
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electronics and optoelectronics,1 energy utilization,2 biomedicine,3, 4 and environmental
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applications.5–7 During the use and disposal of products that contain SLMoS2, e.g. tactile sensors,8
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desalination membranes,9 and antibacterial agents,6 the release of SLMoS2 into the environment is
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inevitable. A previous study found that SLMoS2 (over 10 mg/L) could induce proinflammatory and
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profibrogenic responses in vitro in THP–1 and BEAS–2B cell lines.10 Appel et al.11 proposed that
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SLMoS2 (10~100 mg/L) triggered toxicity to human epithelial kidney cells (HEK293f) and the
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bacterial strain S. typhimurium TA100 but did not induce mutation or malformation. Environmental
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transformations of nanomaterials are correlated with their environmental behaviors and ecological
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risks,12, 13 but the potential transformations of SLMoS2 in the environment remain largely unknown.
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The transformations of nanomaterials in environmental media, including changes in the physical
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structure, chemical dissolution (ion release), and sulfidation, are factors that determine the
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biocompatibility and risk assessment of nanomaterials.14–16 The cytotoxicity of Ag and CuO has
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been shown to be attenuated with partial physical release and chemical transformations in
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environmental media.17–19 Therefore, whether environmental or biological transformations are
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beneficial for the biological safety of SLMoS2-based nanomaterials is worth investigation. A deep
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understanding of the environmental transformations of SLMoS2 nanosheets must be gained prior to
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their large-scale applications. 2
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Natural organic matter (NOM) is an important component in aquatic environments, and the
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interactions of nanoparticles with NOM have been shown to be major determinants of the
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environmental fates and properties of nanoparticles in environmental media.20 As a widespread
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NOM, humic acid (HA) with abundant hydrophilic groups could change the surface properties of
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nanomaterials in aqueous environments, thus regulating the biological toxicities of nanoparticles.
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HA adsorption has been shown to reduce the uptake of fullerene (C60) and graphene oxide in
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Daphnia and zebrafish due to size effects and surface charge alteration.21, 22 The HA-improved
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suspension stability of metal oxide (e.g., CeO2 and TiO2) nanoparticles also induced mitigation of
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their toxicities in aquatic organisms.23, 24 Noncovalent interactions with HA affect the chemical
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properties and biological effects of nanomaterials; however, these relevant mechanisms remain
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unresolved. Considering the low energy level of HA,25, 26 its ability to act as an electronic source
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and influence the environmental fates and biological toxicity of SLMoS2 should be explored.
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In the present study, SLMoS2 was incubated with HA for approximately two months to evaluate
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the effect of HA on the physicochemical transformations of the materials. The corresponding
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mechanisms were thoroughly analyzed, and the mitigation of algae toxicity of SLMoS2 by HA was
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determined. This work aims to highlight the advancement and effectiveness of NOM in regulating
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the environmental transformations and biological risks of SLMoS2 to widen the applications of
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SLMoS2.
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MATERIALS AND METHODS
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Materials and reagents
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Pristine SLMoS2 nanosheets (XF137, purity>90%) were purchased from the Nanjing XFNANO
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Materials Tech Co., Ltd., China. HA (extracted from lignite) was obtained from the Shanghai Hui 3
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Cheng Biological Technology Co., Ltd., China. 3,3′,5,5′–Tetramethylbenzidine (TMB) and a
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polyethersulfone (PES) ultrafiltration membrane (0.1 µm) were obtained from Sigma-Aldrich
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(USA). Amicon Ultra-15 (3kD) was purchased from Millipore (USA). The molybdenum plasma
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standard was purchased from AccuStandard®, Inc. (USA). In addition, Chlorella vulgaris (C.
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vulgaris) was obtained from the Freshwater Algae Culture Collection at the Institute of
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Hydrobiology, Wuhan, China. Other chemical reagents were of chromatography or analytical grade.
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Hybridization of SLMoS2 with HA
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A HA solution (500 mg/L) was initially prepared in an alkaline environment (pH = 11.0, 1 M NaOH)
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and then magnetically stirred at 1500 rpm for 2 h. After filtering the solution through a 0.22-µm
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polytetrafluoroethylene membrane, the pH value was adjusted to 7.0 with 1 M HCl. Surface water
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contains NOM in the form of HA at a concentration of approximately 20 mg/L.27 To ensure
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sufficient hybridization between HA and SLMoS2, the concentrations of the utilized HA
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suspensions were 100 mg/L, 50 mg/L and 10 mg/L. The prepared HA solutions (100 mL) were then
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gently mixed with 5 mg of SLMoS2 in a 250-mL glass conical flask. Meanwhile, 100 mL of
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ultrapure water (18.2 MΩ cm, pH=7.0) without HA was also mixed with 5 mg of SLMoS2
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nanosheets as a negative control (named cSLMoS2). After ultrasonication (40 kHz, 30 min) in ice
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water, all the samples were simultaneously irradiated (light/dark=14:10) by a 300 W Xenon arc
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lamp (CEL–HXF300, Ceaulight, Beijing, China) with a UV cutoff (λ< 420 nm). The light intensity
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of the liquid level in all treatments was approximately 600 mW/cm2. The irradiation experiment
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was conducted for eight weeks with gentle shaking (150 rpm). The spectra of the Xenon arc lamps
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are shown in Figure S1. The used HA was characterized in our recent study, and the particle sizes of
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HA ranged from approximately 1 to 20 nm.22 Ultrapure water was used to wash the free HA through
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a 0.1-µm PES membrane. The cSLMoS2 in the negative control groups and SLMoS2-HA in the 4
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HA-exposed groups were both collected and freeze dried for further characterizations, as described
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in the Supporting Information (SI). The content of absorbed HA by SLMoS2 was determined using
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a total organic carbon (TOC) analyzer (multi N/C3100, Analytikjena, Germany). Each experiment
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for the material characterizations was performed in triplicate or more (the specific repetitions are
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described in the methods or figure legends).
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Dissolution of the SLMoS2 nanosheets
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The incubated cSLMoS2 and SLMoS2-HA suspensions were filtered through 0.1-µm PES
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membranes, and the filtrates were collected at the 7th, 14th, 21th, 28th, and 56th days. Afterwards, the
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filtrates were concentrated to approximately 5 mL, followed by digestion with 4 mL of a
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HNO3/H2O2 (volume ratio, 3:1) solution. The total Mo and S contents were determined by
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inductively coupled plasma mass spectrometry (ICP–MS, Agilent 7700, Santa Clara, CA, USA) and
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inductively coupled plasma-atomic emission spectrometry (ICP–AES, IRIS Intrepid II XSP, Thermo
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Elemental, USA). The total Mo and S contents in the filtrates collected through the 0.1-µm
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membranes consisted of secondary SLMoS2 particles (
