Cytotoxicity of Group 5 Transition Metal Ditellurides (MTe2; M=V, Nb, Ta)

Article abstract of DOI:10.1002/chem.201704316

Much research effort has been put in to study layered compounds with transition metal dichalcogenides (TMDs) being one of the most studied compounds. Due to their extraordinary properties such as excellent electrochemical properties, tuneable band gaps, a

Full text of DOI:10.1002/chem.201704316

A Journal of
Accepted Article
Title: Cytotoxicity of Group 5 Transition Metal Ditellurides (MTe2; M=V,
Nb, Ta)
Authors: Martin Pumera
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Cytotoxicity of Group 5 Transition Metal Ditellurides (MTe2; M=V,
Nb, Ta)
Hui Ling Chia,[a],[b] Naziah Mohamad Latiff [a], Zdenĕk Sofer[c], Martin Pumera*[a]
phosphorus and hexagonal boron nitride (h-BN).[2-8]Among 2D
nanomaterials, TMDs are deemed to be one of the most studied
Abstract: Much research effort has been put in to study layered
compounds with transition metal dichalcogenides (TMDs) being
layered compounds due to their extraordinary electrochemical
one of the most studied compounds. Due to their extraordinary
properties.[9-12]
properties such as excellent electrochemical properties, tuneable
band gaps and low shear resistance due to weak van der Waals
TMDs have a general chemical formula of MX2, whereby M
interactions between layers, TMDs have been found to have a
wide application such as electrocatalyst for hydrogen evolution
reactions, supercapacitors, biosensors, field-effect transistors
(FETs), photovoltaic and lubricant additives. In very recent years,
Group 5 transition metal ditellurides have received immense
amount of research attention. However to date, little has been
known of the potential toxicities posed by these materials. As
such, we conducted the cytotoxicity study by incubating various
concentrations of the Group 5 transition metal ditellurides (MTe2;
VTe2, NbTe2, TaTe2) with human lung carcinoma epithelial A549
cells for 24 hours and the remaining cell viabilities after
treatment was measured. Our findings indicate that VTe2 is
highly toxic while NbTe2 and TaTe2 are deemed to exhibit mild
toxicities. This study constitutes an exemplary first step towards
the understanding of the Group 5 transition metal ditellurides’
toxicity effects in preparation for their possible commercialization
in the future.
represents a transition metal (for instance Ti, V, Nb, Ta, Mo, W and
so on) of a +4 oxidation state and X is a chalcogen (S, Se or Te) with
a -2 oxidation state.[9,13,14] Different permutations of these elements
give rise to approximately 60 different TMDs, where two-thirds of
these compounds are reported to assume layered structures.
Generally, transition metals from Group 4–7 generate
compounds that are predominantly layered while some Group
8–10 transition metals give rise to three-dimensional crystal
compounds. [14-16] Many TMDs possess a layered structure akin to
graphite and within one layer of TMDs, the transition metal is
sandwiched between two chalcogen, hence resulting in the MX2
stoichiometry. It has been reported that the bonds within each layer
are covalent, whereas the bonds between two different MX2 layers
are typically held by van der Waals forces of interactions, thus
allowing exfoliation of TMDs down to single layers.[9,13-15,17-19]
The unique and advantageous properties of TMDs such as large
surface area, tunable band gaps, stability against photocorrosion and
low shear resistance due to weak van der Waals interactions
between layers have attracted much research attention and led
to numerous and diversified applications of TMDs.[13,14,20,21] Such
applications include electrocatalytic hydrogen evolution, high
performance electrochemical supercapacitor, biosensors, field-
effect transistors (FETs), photodetectors, heterostructure
junctions, photovoltaics and lubricant additives.[5,9,13,14,17- 32 ] In
recent years, enormous efforts have been devoted to explore
the different applications of Group 5 transition metal ditellurides
with VTe2 being deemed to be a promising HER electrocatalyst,[33]
NbTe2 being reported to be an excellent lubricant additive[20] and
TaTe2 having potential applications as a supercapacitor.[34] However
to date, little has been known of the potential toxicities posed by
Group 5 transition metal ditellurides. There is still much to
address for the understanding of the toxicological behaviour of
TMD materials especially for bulk Group 5 ditelluride TMDs,
which, to our knowledge has been undetermined. This gap in
literature has to be filled in order to inform users of the potential
health implications posed in view of their possible
implementation in industries.
Introduction
Ever since the first isolation of graphene from graphite
1
]
by Novoselov et al. in 2004,[
two-dimensional (2D)
nanomaterials have gained intensive academic and industrial
research interest due to their extraordinary and unique
properties. To date, a vast range of 2D nanomaterials have been
reported,
such
examples
include
transition
metal
dichalcogenides (TMDs), graphitic carbon nitride (g-C3N4), black
[a]
H. L. Chia, N. M. Latiff, Prof M. Pumera
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
21 Nanyang Link, Singapore 637371, (Singapore)
E-mail: pumera.research@gmail.com
H. L. Chia
NTU Institute for Health Technologies
Interdisciplinary Graduate School
Nanyang Technological University
50 Nanyang Drive, Singapore 637553
Prof. Z. Sofer
As such, we conducted the cytotoxicity study of the bulk Group 5
transition metal ditellurides (MTe2; VTe2, NbTe2, TaTe2) with
A549 human lung carcinoma epithelial cells. The cell line was
specifically chosen as the lungs is one of the main potential
target organ during the processing of nanomaterials[10,35-39] and it
is one of the most used cell types in inhalation toxicity studies
allowing easy comparison between our results obtained with
other reports.[10,35] The remaining cell viability of the A549 cells
after 24 h incubation with various MTe2 concentrations was
measured and analyzed using the water-soluble tetrazolium salt
(WST-8) assay. Furthermore, control experiments were also
conducted in the absence of cells to check for possible sources
of particle-induced interferences between MTe2 with cell viability
[b]
[c]
Department of Inorganic Chemistry
University of Chemistry and Technology Prague
Technickά 5, 166 28/ Prague 6 (Czech Republic)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201704316
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assay markers which could cause false positive or false negative
results to be produced. [39-41]
Results and Discussion
Material characterisation
It has been reported that physicochemical properties such as
size and chemical compositions could contribute to the
nanomaterials’ toxicities.[ 42 - 45 ] Hence prior to assessing the
cytotoxicity of the Group
5 transition metal ditellurides,
characterization of the materials were necessary and carried out
through scanning electron microscopy (SEM), scanning
transmission electron microscopy (STEM), energy-dispersive X-
ray spectroscopy (EDX), X-ray diffraction (XRD) and dynamic
light scattering (DLS).
As seen in Figure 1, through the SEM and STEM images it is
deduced that all samples (VTe2, NbTe2 and TaTe2) display
structural features typical of bulk materials. VTe2 and TaTe2 are
predominantly seen to be layered structures while NbTe2 manifest in
irregular shapes. Furthermore, the STEM images reveal a wide
range of particle sizes ranging from 80 nm to over 3 μm in lateral
dimension.
Figure 2. SEM-EDX images of bulk Group 5 ditellurides (VTe2,
NbTe2 and TaTe2). The scale bars represent 10 μm.
XRD data (see Supporting Information, Figure SI-1) shows
single phase purity for all ditellurides with a strong preferential
orientation along (00l) direction, which originates from the
layered character of the materials. It was also determined that all
phases are monoclinic structures with C2/m space group. The
particle size distribution measurement performed by DLS (see
Supporting Information, Fig SI-2) shows that particles from all
three materials exhibit multimodal character which corresponds
to the wide range of sizes determined from STEM. However, the
DLS size ranges differ from the STEM measurements, where
DLS data shows a dominant distribution peak with sizes in the
range of 500–1000 nm and two smaller fractions with size
around 100 and 200 nm. This narrower size range from DLS
measurements could be attributed to the faster settling of the
larger particles.
Cytotoxicity Assessment
Upon the successful characterization of the materials, we
proceeded to determine the toxicity effect of these Group 5
transition metal ditellurides towards A549 cells. To study the
materials’ toxicity, A549 cells were exposed to the test materials
and the remaining cell viabilities were measured using WST-8
assay. WST-8, a mitochondrial activity-based assay, works on
the principle that soluble formazan is produce due to cellular
reduction by dehydrogenase activities in viable cells. [10,46-48] By
normalising the absorbance intensities obtained with a control
setup, where cells are not treated with any nanomaterials, the
extent of toxicity exhibited by the Group 5 ditellurides TMDs
towards A549 cells could be determined. Data collected from the
WST-8 assay is illustrated in Figure 3.
Figure 1. SEM (a,c,e) and STEM (b,d,f) images of VTe2, NbTe2, TaTe2
respectively. The scale bars represent 1 μm.
Another characterization technique used was the EDX, which
provides an elemental mapping of the elements present and
their distribution throughout the material. The SEM-EDX images
in Figure 2 suggest that the Group 5 transition metals and
telluride are well distributed in the materials and that the primary
difference present is the transition metal element.
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niobium and tantalum compounds ranges from several hundreds
to thousands mg/kg body weight.[54-56]
Nanomaterial Induced Interference
Several studies have reported that nanomaterials could interact
with the viability markers in cell viability assays, thus causing
false readings to be produced.[39-41] As such, control experiments
were performed in the absence of cells to check for possible
sources of particle-induced interferences, to determine the
suitability of the assays. Therefore, we set out to determine
possible forms of particle-induced interferences for two
commonly used assays which has similar working mechanisms,
WST-8 and methyl thiazolyldiphenyl-tetrazoliumbromide (MTT)
assays.
Figure 3. Cell viability percentages measured using WST-8 assay upon 24h
incubation of different concentrations of Group 5 transition metal ditellurides
with A549 cells.
One possible interference induced by the nanomaterials on
WST-8 is that the nanomaterials have the ability to reduce the
active tetrazolium salt to form formazan, thereby generating
false positive results and cause an overestimation in the number
of viable cells.[57-59] In order to check for any interference, a
control experiment was performed by incubating varying
concentrations of nanomaterials with WST-8 assay for an hour
in a cell free condition to check for possible formazan production
through nanomaterials induced reductions. Figure 4 details the
data of the WST-8 control experiments.
As depicted in the data above, a dose-dependent trend for VTe2
was observed. Furthermore, VTe2 exhibited significant
cytotoxicity towards A549 cells, whereby even at low
concentrations of 25g/mL more than 50% of the cells were not
viable and at the highest concentration of 200 g/mL less than
10% of the cells remained viable. This result is in line with
previous toxicity studies conducted on exfoliated VTe2, which
demonstrated that VTe2 was highly toxic.[10] On the other hand,
the percentage cell viability for NbTe2 and TaTe2 remained
relatively high (≥ 60%) even at high nanomaterial concentrations
above 50g/mL, indicating that these two Group 5 transition
metal ditelluride induce low toxicological effects towards the
A549 cells. Therefore, in this study the degree of cytotoxicity
displayed by the bulk Group 5 transition metal ditellurides can be
deemed as VTe2 being the most toxic while NbTe2 and TaTe2
are of comparable toxicity.
One plausible reason that could account for VTe2 being more
toxic is that VTe2 might produce more reactive oxygen species
(ROS) than NbTe2 and TaTe2. Studies have suggested that solid
surfaces could directly interact with biological target molecules or
indirectly interact with H2O2 or O2, by transfering electron or H˙
to/from solid surfaces to produce ROS. These ROS may in turn
cause oxidative stress to the cells and damage cellular proteins,
Figure 4. WST-8 control experiment: Formazan generated upon an hour
of WST-8 incubation with varying concentrations of Group 5 ditelluride
materials.
,
50 ]
lipids and nucleic acids.[ 49
A study by Chia et al.,
demonstrated that the heterogeneous electron transfer (HET)
rate of VTe2 is significantly faster than NbTe2 and TaTe2. A
faster HET rate indicates that a lower overpotential is required
for an electrochemical reaction to occur,[33] hence more
oxidation/reduction reactions could have occurred on the VTe2 solid
surfaces to produce more ROS, causing VTe2 to be significantly
more toxic.
It can be observed that there was an insignificant nanomaterial
induced reduction of the WST-8 by the bulk Group 5 ditellurides,
where the amount of formazan produced after an hour of
incubation with the nanomaterials were within the 15% of the
quantity measured from the control containing only 10% WST-8.
Furthermore during the cell viability assay, a washing step was
introduced to minimize the nanomaterial induced interference
prior to the introduction of the assay reagent, hence we believe
that the interference caused by the Group 5 ditellurides is
insignificant.
From the EDX images in Figure 2, the main fundamental
difference between the materials is the transition metals present,
hence one could deduce that the different transition metal
present in the Group 5 transition metal ditellurides is the main
determining factor for the cytotoxicity properties. The trend
observed in this study coincides with the trend from toxicities
study of the Group 5 transition metal (IV) compounds, whereby it
was observed that in general vanadium compounds are the
most toxic among the three elements while niobium and
tantalum compounds are considered to be relatively non-toxic.[51]
Toxicity studies on rats have shown that the LD50 values for
vanadium compounds ranges from a few tens to hundreds
mg/kg body weight[ 52 , 53 ] while the reported LD50 values for
Apart from using WST-8 assay, another commonly used MTT
assay was also considered for use as a cell viability assay in this
study. MTT working principle is similar to WST-8 such that in the
presence of viable cells, MTT will be converted to purple colored
formazan crystals with an absorbance maximum near 570 nm.
There exist two possible interferences induced by the
nanomaterials on MTT. Firstly, similar to WST-8, the
nanomaterials could reduce the active tetrazolium salt to form
formazan on its own, thereby generating a false positive result.
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Secondly, the nanomaterials could interfere through binding with
need to investigate the potential toxicity effects posed in view of
their possible implementation in industries. This paper
investigated the cytotoxicity effects of the Group 5 ditellurides
TMDs towards A549 cells using WST-8 assay. The findings of
this study indicate that VTe2 is highly toxic and is affected by the
concentration of VTe2 while NbTe2 and TaTe2 are both mildly
toxic with comparable results. Control experiments of WST-8
and MTT were carried out to determine the interference posed
by the test materials and the suitability of the assays. Results
from the WST-8 suggest minimal nanomaterial induced
reduction of the WST-8 indicating that there was insignificant
interference posed. On the other hand, MTT data shows a high
degree of interference with VTe2, making MTT assay an
unreliable cell viability assay for fair comparison between the
Group 5 transition metal ditellurides and was omitted from this
study. In our view, these results constitute an excellent initial
step towards the understanding of the toxicity effects of the bulk
Group 5 transition metal ditelluride. However, more studies have
to be conducted to explore the mechanisms behind the
biological responses to nanomaterial exposures to have a full
understanding of the toxicological effect of Group 5 ditellurides
TMDs.
the formazan crystals formed thereby generating
a false
negative result and cause an underestimation in the number of
viable cells.[41,57-62] To investigate whether the nanomaterials
could induce a reduction of the active tetrazolium salt, different
concentrations of the Group 5 ditelluride was exposed with the
MTT assay in a cell free environment. To study the probable
binding effect, ascorbic acid was added to different
concentrations of nanomaterials to induce the formazan crystals
formation from the MTT assay. If the nanomaterial binds to the
formazan crystals, one would expect to see a decrease in the
percentage of formazan with increasing amount of the test
materials. The result from the MTT control experiments is
illustrated in Supporting Figure SI-3.
It was found that high amount of interference was observed
between VTe2 with the MTT assay whereby the amount of
formazan produced increase significantly to approximately 280%
at the highest concentration tested while nanomaterial induced
reduction of the MTT by NbTe2 and TaTe2 is insignificant. It was
also revealed that there is minimal binding of the test materials
with the formazan crystals. Due to significant amount of
tetrazolium salt reduction by the VTe2, it is deemed that the MTT
assay would not be a fair assay to compare the toxicity between
the Group 5 ditelluride materials and hence MTT cell viability
assay was omitted in this study.
Experimental Section
Full details of the experimental procedures can be found in the
Supporting Information.
Comparison with other TMDs
Apart from studying the cytotoxicity of Group 5 transition metal
ditellurides, the toxicological data obtained could also be used to
do
a comparison study with other TMDs for a better
understanding of their relative toxicities. By comparing our data
with those reported for the widely used Group 6 TMDs (namely
MoS2, WS2 and WSe2)[ 63 ] under similar conditions (such as
undergoing a 24 h treatment of materials with A549 cells), we
found that the Group 5 transition metal ditellurides are generally
more toxic than the Group 6 TMDs. However an exception was
observed in WSe2 where it is more toxic than the Group 5
transition metal ditellurides with only VTe2 being more toxic than
WSe2 (Table 1).
Acknowledgements
Z.S. was supported by Czech Science Foundation (GACR No.
17-11456S). This work was created with the financial support of
the Neuron Foundation for science support. M. P. thanks to Tier
1 (99/13) from Ministry of Education for funding.
Keywords: cytotoxicity • ditellurides • transition metal
dichalcogenides • layered compounds
Table 1. Normalized cell viability percentages measured using WST-8
assays, after 24h exposure with 200g/mL of TMDs. Data for Group 6
TMDs are obtained from reference 63.
Materials
Cell Viability (%)
MoS2
WS2
WSe2
VTe2
NbTe2
TaTe2
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