Silicene: Wet-Chemical Exfoliation Synthesis and Biodegradable Tumor Nanomedicine

Article abstract of DOI:10.1002/adma.201903013

Silicon-based biomaterials play an indispensable role in biomedical engineering; however, due to the lack of intrinsic functionalities of silicon, the applications of silicon-based nanomaterials are largely limited to only serving as carriers for drug delivery systems. Meanwhile, the intrinsically poor biodegradation nature for silicon-based biomaterials as typical inorganic materials also impedes their further in vivo biomedical use and clinical translation. Herein, by the rational design and wet chemical exfoliation synthesis of the 2D silicene nanosheets, traditional 0D nanoparticulate nanosystems are transformed into 2D material systems, silicene nanosheets (SNSs), which feature an intriguing physiochemical nature for photo-triggered therapeutics and diagnostic imaging and greatly favorable biological effects of biocompatibility and biodegradation. In combination with DFT-based molecular dynamics (MD) calculations, the underlying mechanism of silicene interactions with bio-milieu and its degradation behavior are probed under specific simulated physiological conditions. This work introduces a new form of silicon-based biomaterials with 2D structure featuring biodegradability, biocompatibility, and multifunctionality for theranostic nanomedicine, which is expected to promise high clinical potentials.

Full text of DOI:10.1002/adma.201903013

CommuniCation
Silicene
www.advmat.de
Silicene: Wet-Chemical Exfoliation Synthesis
and Biodegradable Tumor Nanomedicine
Han Lin, Wujie Qiu, Jianjun Liu, Luodan Yu, Shanshan Gao, Heliang Yao, Yu Chen,*
and Jianlin Shi*
or skin.[2] Porous silicon (pSi) is a specific
Silicon-based biomaterials play an indispensable role in biomedical
engineering; however, due to the lack of intrinsic functionalities of silicon,
the applications of silicon-based nanomaterials are largely limited to only
serving as carriers for drug delivery systems. Meanwhile, the intrinsically
poor biodegradation nature for silicon-based biomaterials as typical inorganic
materials also impedes their further in vivo biomedical use and clinical
translation. Herein, by the rational design and wet chemical exfoliation
synthesis of the 2D silicene nanosheets, traditional 0D nanoparticulate
nanosystems are transformed into 2D material systems, silicene nanosheets
and vitally important form of the chemical
element silicon, featuring intrinsic bioac-
tivity, superhydrophobicity, and lumines-
cence properties coupled with the large
surface to volume ratio rendered by pore
microstructure, which has provided con-
siderable potentials for optoelectronics,
sensors, biomedicine, and even a battery
[3]
anode. Silica is the most popular oxidized
form of chemical element silicon, among
which, especially, mesoporous silica shows
broad application potentials in medicine,
industrial catalysis, energy storage, and
(SNSs), which feature an intriguing physiochemical nature for photo-triggered
therapeutics and diagnostic imaging and greatly favorable biological effects
of biocompatibility and biodegradation. In combination with DFT-based
molecular dynamics (MD) calculations, the underlying mechanism of silicene
interactions with bio-milieu and its degradation behavior are probed under
specific simulated physiological conditions. This work introduces a new form
of silicon-based biomaterials with 2D structure featuring biodegradability,
biocompatibility, and multifunctionality for theranostic nanomedicine, which
is expected to promise high clinical potentials.
[4]
imaging. Nonetheless, the diversity of sil-
icon topology brings no increment for the
single-component silicon on physiochem-
ical properties, which impedes interdisci-
plinary research based on silicon materials.
As the third topology of silicon mate-
rials, 2D silicene differs strikingly from
other silicon materials such as pSi or
[
5]
silica, featuring unique physiochemical
[
6]
virtues like Quantum spin Hall effect,
[8]
Silicon, the most abundant element of the Earth’s crust after
oxygen, is an essential, nontoxic, environmentally friendly, bio-
compatible, and ubiquitous chemical element in living system
and industrial applications. Silicon has not only been shown to
elevate plant-cell wall strength and structural integrity in plant,
giant magnetoresistance,[7] and chiral superconductivity by its
unique low-buckled topography. To date, silicene material has
been fabricated by physical vapor deposition (PVD) method on a
[
9]
variety of substrates. The first synthesis of monolayer silicene
[
10]
was achieved on Ag(111) surface. Besides, other studies have
[1]
[5c]
[11]
[12]
but also plays an indispensable role in human health. Silicon
is necessary for the synthesis of collagen and elastin, which are
the main components of aorta in the human body, is also an
essential constituent in organs and tissues such as hair, bone,
achieved silicene growth on Ir(111),
MoS2,
ZrC(111),
[
13]
and ZrB(001)
substrates under ultrahigh vacuum (UHV)
ambience. However, these as-synthesized silicene derived from
epitaxial growth methods encounter three major drawbacks:
scalable synthesis limitation, nonuniformity in vertical scale,
and great difficulties in the transfer of layered materials from
Dr. H. Lin, Dr. W. Qiu, Dr. J. Liu, Dr. L. Yu, Dr. S. Gao, H. Yao,
Prof. Y. Chen, Prof. J. Shi
[
14]
substrates. Given that the same dilemma has impeded the
research advances and practical application of 2D materials
State Key Laboratory of High Performance Ceramics and Superfine
Microstructures
Shanghai Institute of Ceramics
Chinese Academy of Sciences
Shanghai 200050, P. R. China
[15]
such as transition metal carbides nanosheets (MXene), the
chemical vapor deposition (CVD) approach could only be uti-
lized to grow α-Mo C MXene, but exhibited the limitations of
2
[
16]
E-mail: chenyu@mail.sic.ac.cn; jlshi@mail.sic.ac.cn
low-yield production and substrate dependence. Fortunately,
these issues have been overcome by using selective extrac-
Dr. H. Lin, Dr. S. Gao
Center of Materials Science and Optoelectronics Engineering
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
tion method upon converting MAX-phase layered compounds
[17]
to versatile 2D MXene-phase nanosheets.
By analogy, 2D
silicene, a hexagonal honeycomb lattice featuring non-planar
buckled configuration, could be supposedly synthesized from
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201903013.
the precursor Zintl-phase silicide (e.g., CaSi ) via a topochem-
2
DOI: 10.1002/adma.201903013
ical deintercalation process of selective Ca-removal.
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So far, the current experimental efforts in the 2D silicene
synthesis by chemical manipulation are still suffering 2D
silicene applications from the free-standing morphology, large-
scale production, and high-quality flakes.[18] Despite the recent
progress in the exploration of electronic properties of silicene,
there has been no reports that could be found to date on the
applications of 2D silicene nanomaterials in biomedical field.
Herein, we report the wet-chemical synthesis and biomedical
performances of 2D silicene, as the next-generation silicon-
based biodegradable phototherapeutic agent, together with
temporal/thermo-dependent degradation behavior attributing
to its high photothermal-conversion performance, desirable
photoacoustic effect, and intriguing ambience-triggered biodeg-
radation. The high photothermal therapeutic capacity, intrinsic
high biocompatibility of silicon component, and the desirable
biodegradative nature of 2D silicene give great promises for
broadening the biomedical use of silicon-based nanoplatforms
and their further clinical translation.
silicide bulks with the by-product CaI being well dissolved in
2
CH CN. The secondary electron SEM imaging gives the pre-
3
cise message about morphology and surface topography, which
exhibit the typical layered morphology of silicene structure after
Ca-layer removal, proving the efficient deintercalation of indi-
vidual grains and the uniform stack-up of the silicene sheets
(Figure 1h,i).
TEM images further depict the electron-transparent mor-
phology and a well-stacked microstructure of ML silicene
(Figure 1j). HRTEM image and the corresponding SAED pat-
tern clearly exhibit crystalline lattice of ML silicene nanosheets
with unchanged hexagonal structure (Figure 1k,l). After several
cycles of rapid quenching and gas-exfoliation process, the
bright-field and dark-field TEM images corroborate that the pre-
pared few- or single-layered silicene nanosheets are ultrathin
and almost electron-transparent (Figure 1m,n), which feature
the typical 2D stand-alone morphology. The elemental mapping
of silicene nanosheets exfoliated from Zintl phase precursor
clearly suggests the efficient removal of Ca layer (Figure 1o).
The HRTEM image reveals the well-crystallized feature of
silicene nanosheets with hexagonal structure, and the corre-
sponding original SAED pattern also indicates the preserved
well-defined hexagonal structure (Figure 1p,q), revealing the
disappearance of the most intense peaks of Nb2AlC at 2θ ≈ 39°
because of the exfoliation. The energy dispersive spectro-
The 2D silicene nanosheets were synthesized via a mild
oxidation and delamination process based on wet chemistry
mechanism, which was an analogue strategy reported by our
and other groups for 2D MXene fabrication.[
17a,18c]
In brief,
the precursor Zintl phase CaSi crystals were subject to a mild
2
oxidation process in anhydrous acetonitrile (CH CN), together
3
with a weak oxidant, iodine (I ). Based on the layered binary
2
silicide CaSi , alternatingly consisting of planar Ca layers and
scopic analyses for CaSi and final product silicene reveal the
2
2
buckled Si layers, the Si layers were oxidized by I through
an outer to inner diffusion process, and the by-product CaI2
disappearance of intense peaks of Ca element because of the
exfoliation, demonstrating the complete removal of Ca atoms
in the product of silicene and the inexistence of Ca-related by-
products like CaI or CaSi residuals (Figure S2, Supporting
Information). The small amount of oxygen measured by EDS is
due to the partial surface oxidation during the characterization
process, which would be hard to remove completely.
2
was well dissolved in solvent CH CN simultaneously. These
3
as-prepared multilayered silicene (ML silicene) nanosheets
exhibit accordion-like microstructure. In order to thoroughly
exfoliate the freestanding silicene nanosheets, the ML silicene
nanosheet precipitate was thermo-expanded at 380 °C, and
subsequently immersed into the liquid N (L-N ) (quenching
2
2
To further determine the composition and microstructure
of 2D silicene nanosheets, Raman spectroscopy was adopted
(Figure S3, Supporting Information). Generally, the Raman
frequencies of 2D sheets produced from Zintl phase precursor
contain the out-of-plane vibrational mode (A ) and in-plane
2
2
to −196 °C) until the complete evaporation of L-N in the con-
2
tainer. The freestanding silicene nanosheets, few- or single-
layered silicene (FL silicene), were peeled off by the L-N2
gasification (Figure 1a). The few- or single-layered silicene fab-
ricated by this efficient strategy features high-quality sheet and
freestanding morphology with scalable production potentials,
probably meeting the ever-strict needs in, such as, biomedical
application or electronic device fabrications (Figure 1b).
1
g
[
19]
vibrational mode (E ). The vibrational mode A , a vertical
2g
1g
buckling in the nonplanar corrugated configuration of silicene,
responds to the asymmetric shoulder in the Raman shift
−
1
ranging from 450 to 500 cm (Mode A ), which well matches
1g
Scanning electron microscopy (SEM) images exhibit that the
the typical silicene vibrations via epitaxial synthesis, and is, in
3
[5a]
Zintl phase CaSi features the specific structure of a layered
contrast, absent in bulk sp silicon and pristine CaSi . In
2
2
−
1
binary compound, promising the potential to extract Si layers
addition, the sharp and intense peak (515−522 cm ), a finger-
print Raman vibrational mode of silicene, dominated by the
symmetric stretching of Si–Si interactions in planar hexag-
from the layered parent CaSi (Figure 1c and Figure S1, Sup-
2
porting Information). High-resolution transmission electron
microscopy (HRTEM) imaging clearly demonstrates that the
onal rings (E2g vibrational mode), suggests the integrity of the
+
[10c]
pristine CaSi consists of alternating [Ca] layers and buckled
silicene nanosheets.
The measured Raman spectrum con-
2
n
−
honeycomb [Si]
n
layers in atomic resolution (Figure 1d).
firms the pseudo-planar flatland in silicene and the integrity of
basal plane hexagonal symmetry in silicene layered microstruc-
ture. The X-ray photoelectron spectroscopic (XPS) analyses of
Indeed, these silicene sheets feature a lower symmetry and
pseudo-planar flatland structure differing strikingly from that
of graphene of planar hexagonal rings. The corresponding
element mapping further confirms that these alternating 2D
silicene buckled sheets are separated from each other by planar
monolayer of Ca (Figure 1e,f).[ The selected area electron dif-
Zintl phase CaSi and exfoliated silicene nanosheets in the Ca
2
2p region verify the absence of Ca-related peaks after silicene
delamination (Figure S4a,c, Supporting Information), also
confirming the complete removal of Ca atoms in the product
of exfoliated silicene nanosheets and inexistence of Ca-related
by-products like CaI or residual CaSi precursor. The Si 2p
9]
fraction (SAED) pattern shows that the pristine CaSi possesses
2
a rhombohedral (R3m) structure (Figure 1g). After the “gentle”
2
2
oxidation by I , the Ca layers were extracted out of calcium
peak at a binding energy around 99.4 eV is consistent with
2
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Figure 1. Synthesis and characterization of 2D silicene nanosheets. a) Scheme for the synthesis of 2D silicene nanosheets. b) Schematic diagram of
silicene nanosheet exfoliation process including mild oxidation and ultrasound delamination. c) SEM images of pristine CaSi . d) HR-STEM image,
2
−
1
e) elemental mapping, f) corresponding element-linear scanning profiles and g) SAED pattern of CaSi . Scale bars, 5 µm (c); 1 nm (d); 1 nm (e); 10 nm
2
(
(
g). h,i) SEM images, j) TEM image, k) HRTEM image and l) corresponding SAED pattern of multilayer silicene. Scale bars, 2 µm (h); 1 µm (i); 50 nm
−
1
j); 1 nm (k); 5 nm (l). m) Bright-field and n) dark-field TEM image of few- or single-layered silicene nanosheets. o) Elemental mapping, p) HR-STEM
−
1
image and q) the corresponding SAED pattern of as-synthesized silicene nanosheets. Scale bars, 0.5 µm (m,n); 100 nm (o); 1 nm (p); 5 nm (q).
0
the reported Si 2p peaks in silicene (Figure S4b,d, Supporting
ambient conditions is of great significance in exploiting its future
biomedical applications. To evaluate the biodegradability of
silicene nanosheets (noted as SNSs), SNSs dispersions were dis-
persed in water (exposed to the air) at room temperature (RT) for
14 days and then their optical/spectroscopic properties, chemical
structure, and microscopic morphology changes were analyzed
at varied time intervals (0, 3, 7, and 14 days). The color of the
SNSs suspension became weaker over time, and the suspension
changed to a nearly translucent solution in 14 days (Figure 2a).
The corresponding visible–near-infrared (vis–NIR) absorption
spectra of SNSs suspension exhibit the significantly decreased
absorbance intensity with the increasing time duration of dis-
Information).[
18c]
Atomic force microscopy (AFM) measure-
ment provides further evidence for the typical 2D sheet mor-
phology of freestanding silicene nanosheets (Figure S5a,b,
Supporting Information). The statistical lateral size is statisti-
cally distributed from 50 to 350 nm, indicating the consistency
with the TEM characterizations (Figure S5c, Supporting Infor-
mation). In addition, Figure S5d in the Supporting Information
clearly exhibits a planar structure of silicene nanosheets with a
monolayer thickness of 0.6 nm similar to graphene. In fact, the
as-obtained silicene nanosheets contain monolayer, two-layer
and three-layer structures with the typical thickness ranging
from 0.3 to 1.8 nm.
[
20]
persion (Figure 2b and Figure S8, Supporting Information),
Degradation performance of inorganic nanoagents is still one
of the major obstacles in their clinical-translations. Thus, the
investigation of oxidation/degradation nature of silicene under
and the extinction coefficient (ε) of the biodegrading SNSs
suspension was plotted as a function of time, demonstrating
an exponential decay behavior, i.e., marked degradation,
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Figure 2. Composition and in vitro biodegradability of 2D silicene nanosheets. a) Photographs, b) absorbance spectra, c) mass extinction coefficient,
−
1
d) Raman intensity analysis, and e) TEM images of silicene nanosheets (80 µg mL ) in varied degradation durations (0, 3, 7, and 14 days). Images
share the same scale bar of 300 nm. f) XPS analysis of silicene Si 2p region in varied degradation durations (3 and 14 days). g) Quantitative analysis
of bonding groups in silicene in degradation durations of 3 and 14 days. h) Representative reaction–degradation path of monolayer silicene in ambient
system containing O and H O. Minimum energy pathways (MEP) for ambient biodegradation behavior of monolayer silicene obtained by using DFT
2
2
calculations. Silicene nanosheets in reaction with aqueous environment (H O and O ) at temperature of 310 K. i) Oxidization and disintegration
2
2
process of monolayer silicene degradation. j) Extinction coefficient residual and k) Raman intensity residual of silicene nanosheets and traditional 2D
materials families (MnO , MoS , BN, GO, BP, MXene, C N ) after storing in water for different periods of time. l) Comparisons of Raman intensity
2
2
3
4
residual versus extinction coefficient residual of different 2D nanomaterials. Each symbol indicates a set of material category.
in the time course of dispersion (Figure 2c and Figure S9,
Supporting Information). From the Raman scattering analysis,
the degradation was found to strongly affect the fingerprint
the visible small nanoparticles could be found attached on the
surface of SNSs in 3 days, which was attributed to the surface
oxidation of silicene by oxygen dissolved in water. It could
be found that the degradation was evident as indicated by the
substantial shape changes in 7 days, and the original mor-
phology of SNSs was completely disrupted and only very few
−1
Raman peak of SNSs (located around 515–522 cm ), resulting
in gradually reduced intensity upon increasing dispersion time
interval from the beginning to the end of 14th day (Figure 2d),
which might be due to the breakage of chemical bonds of
[
22]
sheet-like objects could be observed in 14 days (Figure 2e).
[
21]
SNSs in the environment containing water and oxygen.
XPS characterization was carried out to measure the oxidized
silicon species after treating the SNSs suspension in 3 and
14 days (Figure 2f). Compared to the original Si 2p region of
silicene (0 day) dominatingly featuring a binding energy of Si–Si
Microscopic technique was further employed to acquire more
insights into the degradation of SNSs. The nanosheets of the
original silicene were flat, clear, and transparent. Comparatively,
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atoms around 99.0 eV (Figure S4d, Supporting Information), the
peaks of Si oxidation states at 102.3, 103.54, and 105.8 eV were
The optical absorption spectra acquired on SNSs exhibit an
intense and broad absorption ranging from NIR-I to NIR-II
biowindow (750−1350 nm), which was unique in the inorganic
materials employed as a photothermal agent (Figure 3a). Com-
pared to the well-explored NIR-I biowindow, working in the
NIR-II biowindow features two merits, the rational maximum
permissible exposure (MPE) and desirable laser penetration
assigned, respectively, to Si–OH (–OH terminated), SiO (–O
x
terminated), and SiO–OH (–OOH terminated). These intense
x
and broadened oxidation peaks of silicene occurred in 3 and
14 days by exposing to oxygen dissolved in water, suggesting a
strong oxidation on SNSs framework (Figure 2f). The surface
zeta potentials of degradation products gradually declined with
the increasing time, indicating that the silicene did degrade over
time (Figure S10, Supporting Information). Such a massive con-
[
25]
depth. Normalized adsorption intensities over the length of
the sample cell (A/L) at λ = 1064 nm with different concen-
trations (C) were calculated. According to the Lambert–Beer law
(A/L = εC, where ε is the extinction coefficient), a linear func-
tion of A/L versus the concentration was obtained, and the
extinction coefficient at λ = 1064 nm was determined to be
version of oxidation species from SiO (54% to 21%) to Si–OH
x
(17% to 52%) suggests a main degradation byproduct of Si(OH)x
(
Figure 2g), for example, orthosilicic acid (Si(OH) ), which is a
4
−
1
−1
predominant absorbable form in human body and naturally
found in diverse organs or tissues, especially featuring a high
bioavailability in maintaining bone health.[ More importantly,
silicic acid administered to human being would be efficiently
16.0 L g cm (Figure 3a, inset), which is higher than that of
representative photoabsortion nanoagents such as graphene
23]
−1
−1 [26]
−1
−1 [27]
oxides (3.6 L g cm ), Au nanorods (13.9 L g cm ), and
−1
−1 [28]
black phosphorus QDs (14.8 L g cm ), suggesting/depicting
the strong NIR-II laser absorption potential of silicene.
Furthermore, the photothermal-conversion efficiency (η),
another essential parameter used to estimate the photothermal
performance of silicene, was calculated according to the results
of time constant regarding heat conversion and the extreme
steady-state temperature (Figure 3b), showing that the η value
reaches 36.09% in the NIR-II biowindow, remarkably higher
[3d,24]
metabolized from the living body by the urine or feces.
To understand ambient degradation behavior of few-layered
silicene nanosheet interacting with O and H O, DFT-based mole-
2
2
cular dynamic (MD) simulations were performed to reveal reac-
tion mechanisms of silicene+O , silicene+H O, silicene+O +H O
2
2
2
2
systems. The comparison for these MD simulations is used to
reveal the different role of O or H O in degradation reactions
2
2
[
27]
(Figure S11, Supporting Information). In order to speed up this
than that of typical Au nanorods (21%), Cu S nanocrystals
(25.7%), and Ti C nanosheets (30.6%).
3 2
9 5
[
29]
[17b]
process, monolayer silicene was employed for simulating degra-
dation mechanism of FL silicene interacting with different mole-
cules at 310 K (Figure 2h). Some important MD snapshots for
silicene+O +H O, silicene+O , and silicene+H O are presented
To evaluate the photothermal-conversion performance, the
aqueous suspensions of dispersed SNSs upon exposure to a
1064 nm laser were utilized to study its thermal effect. Under
2
2
2
2
−
2
in Figures S12–S14 in the Supporting Information, respectively.
exposure to 1.0 W cm of 1064 nm laser, the MPE standard
for skin exposure (following the American National Standard
The MD simulation shows that H O alone cannot disrupt the
2
[
30]
structure integrity of silicene sheets. Our comparison on MD
for Safe Use of Lasers, ANSI Z136.1-2007), a relatively low
−
1
simulations of silicene+O +H O and silicene+O suggests that the
SNSs concentration of 40 µg mL shows temperature elevation
up to 47 °C in 5 min of irradation (Figure 3c). In contrast, the
temperature of water exhibits no remarkable temperature rise,
elucidating that the presence of SNSs can rapidly and effec-
tively convert NIR-II irradiation to thermal energy (Figure 3d).
More detailed experiments were performed to evaluate the
photothermal-conversion potential of SNSs at varied concentra-
2
2
2
existence of H O accelerates chemical bond formation between
2
silicene and O . The calculation results are also in agreement with
2
the experimental outcomes by TEM observations (Figure S15,
Supporting Information). In order to clearly reveal ambient
degradation mechanism of silicene interacting with O +H O,
2
2
we exhibit several representative snapshots from MD simula-
tions. A three-step reaction mechanism is revealed according
−
1
tions (40, 20, 10, 5 µg mL , and water) under NIR-II irradiation
−
2
to MD simulation: i) association of O with three-coordinated
(1.0 W cm ) (Figure 3d).
2

Si– to form a stronger Si–O bond, which is thermodynamically
To better understand the photothermal-promoted biodegra-
driven reaction. ii) A low-barrier hydrogen transfer from H O
dation behavior of silicene in the interactions with H O and
2
2
to Si–O–O–Si to cause O–O bond-breaking. iii) Si–Si bond-
O , we performed microcanonical molecular dynamic simu-
2
breaking to form Si–O and Si–OH. Therefore, the whole deg-
lations on different temperatures (310, 333, 373, and 410 K)
(Figures S19–S22, Supporting Information). On one hand, the
monolayer silicene model gradually decomposes along with the
Si–Si bond cleavage, as verified by internal reaction coordinate
analysis. On the other hand, the simulation results at varied
temperatures upon certain reaction coordinate, suggest acceler-
ated disruption of Si–Si bonds, implying that the photothermal
effect will promote the degradation of silicene topologic frame-
work (Figure 3e).
Furthermore, the detailed experimental evaluations were per-
formed to confirm such photothermal-induced biodegradation
behavior. The SNS-dispersed aqueous suspensions exposed to
NIR-II laser irradiation (10 min) with varied power densities,
were employed to guarantee different temperature elevations (25,
37, 60, and 90 °C) (Figure 3f). The absorbance intensity of the
radation process can be described as the reaction mechanism of
−
3
+
(Si–) + O + H O → –SiO– + –Si–OH + Si– + OH → –SiO–
2
2
2(–SiOH–) (Figure 2i).
Given that inorganic nanoparticles, unlike other traditional
organic materials, generally show poor biodegradability and
long-term body retention, degradation behavior of the main 2D
materials were measured and then compared with silicene. The
Raman intensities and extinction coefficient values of silicene
and other classic inorganic nanoagents (graphene oxides (GO),
black phosphorus (BP), MXenes, C N , MnO , MoS , and
3
4
2
2
hBN) in different time points have been measured and ana-
lyzed in Figure 2j–l, which illustrate the noticeable advantages
of silicene on degradation nature over most of the inorganic
photothermal agents (Figures S16,17, Supporting Information).
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Figure 3. Photothermal-conversion performance and photothermally accelerated degradation of 2D biodegradable silicene nanosheets. a) Absorbance
−
1
spectra of silicene dispersed in water at varied concentrations of silicene (from top to bottom: 56, 28, 14, and 7 µg mL ). Inset in a): Mass extinction
coefficient of silicene at 1064 nm. b) Calculation of the photothermal-conversion efficiency at 1064 nm (NIR-II) laser irradiation. Black line: photo-
thermal performance of an aqueous dispersion of silicene with NIR-II laser irradiation on (heating period) and off (cooling period). Blue line: time
constant (τ ) for the heat conversion from the system determined by linear-fitting the time-dependent data in the cooling periods. c) Photothermal-
s
−
1
conversion heating curves of aqueous suspensions of dispersed silicene (40 µg mL ) under the irradiation of a 1064 nm (NIR-II) laser at varied
power densities (0.5, 0.75, 1.0, and 1.5 W cm ). d) Photothermal-conversion heating curves of silicene-dispersed aqueous suspension under NIR-II
laser irradiation (1064 nm, 1.0 W cm ) at varied concentrations (0, 5, 10, 20, and 40 µg mL ). e) Snapshots of atomic layer-structures matching the
minimum energy pathways for photothermally accelerated biodegradation behavior of silicene nanosheets in the aqueous environment (H O and O ) at
−
2
−
2
−1
2
2
varied temperatures (310, 333, 373, and 410 K). f) Photothermal-conversion heating curves (10 min) and the corresponding digital photographs and
−
1
infrared thermal images of silicene aqueous solutions (132 µg mL ) showing varied photothermal-conversion temperatures (25, 37, 60, and 90 °C).
g) Absorbance spectra, h) mass extinction coefficient, i) TEM images with different magnifications, j) the corresponding schematic diagram for silicene
−
1
degradation, and k) hydrodynamic size and monodispersity analysis of silicene aqueous solutions (132 µg mL ) at the corresponding temperatures
by photothermal treatment (25, 37, 60, and 90 °C). Images in (i) share the same scale bar of 200 nm.
around 16 µg mL− (Figure 3h and Figures S24,S25, Supporting
Information). TEM observation results also demonstrate such a
photothermal-augmented biodegradation process. The silicene
nanosheets show significant change in morphology with the
1
SNSs in water decreases with temperature elevation (Figure 3g
and Figure S23, Supporting Information). The corresponding
mass extinction coefficient of 90 °C-treated sample declines to
−1
1
4.3 µg mL compared with the that of 25 °C-treated sample at
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increasing of treatment temperature. At the treatment tempera-
ture of 37 °C, the flat-layered morphology became porous. Then,
when the temperature was raised to 60 °C, a further evolution in
the morphology occurred, which presents a gradual cleavage of
silicene configuration along with a certain number of nanoscale
fragments, which is attributed to the high porosity and drastic
oxidation on silicene sheets. Finally at 90 °C, the majority of
silicene nanosheets were converted into highly dispersed frag-
ments with amorphous structure, where the 2D planar motif of
SNSs was completely damaged (Figure 3i,j). This regular varia-
tion was further proven by dynamic light scattering (DLS) meas-
urements. The average lateral size of SNSs samples declines
with the temperature elevating, while the average PDI values
drastically decreases in the course of temperature elevation
quantitatively determined by a flow cytometry apoptosis assay
based on the typical AnnexinV-FITC and propidium iodide (PI)
stained protocol (Figure S28g,h, Supporting Information). The
quantitative result matches well the conclusions from CCK assay
and CLSM observation, as evidenced by the apoptosis of majority
cells in sample of SNSs-BSA + NIR-II group, suggesting the
highly effective photothermal ablation of cancer by employing
SNSs-BSA as a novel silicon-based biodegradable nanoagent.
A precise study of in vivo biosafety of SNSs-BSA was intro-
duced to investigate its clinical and translational potential. The
SPF-level mice were raised and divided into groups on the basis of
different experimental treatments: i) control, ii) mice with SNSs-
BSA intravenously treated, and iii) mice with SNSs-BSA intrave-
nously treated under artificial daylight (Figure 4a). In a four-week
duration, the mice was recorded free of abnormality by body
weight, and no significant behavioral alterations were observed in
treated groups compared to control group (Figure 4b).
(Figure 3k). These results also emphasize a thermo-dependent
degradation behavior of the synthesized silicene.
Owing to the enhanced steric hindrance of organic chain
molecules, the surface modification of bovine serum albumin
The histological observations of major organs (heart, liver,
spleen, lung, and kidney) were collected for hematoxylin and
eosin (H&E) staining (Figure 4c). No significant acute patho-
logical toxicity and side-effects can be observed during the
treatment duration for control or treated groups, depicting
no marked histological abnormalities in the treated groups.
In addition, the regular hematology indexes were examined
(Figure 4d). It could be found that meaningless changes can
be observed in the SNSs-BSA-treated groups at varied time
intervals compared with the control mice. These results indi-
cate that the SNSs-BSA have caused insignificant inflammation
or infection in the treated groups. The normal blood biochem-
ical parameters were conducted and various biomarkers were
detected. All the parameters in the SNSs-BSA-treated groups
at varied time intervals present no abnormity compared with
the control group and such changes are of no statistical sig-
nificance. Thus, the SNSs-BSA treatment does exert no passive
influence on the blood biochemistry. Furthermore, since ALT,
AST, ALP, UREA, CREA, and UA featuring key parameters for
the kidney and liver function, the normal level of these indexes
also indicates that the SNSs-BSA induce negligiblerenal and
hepatic toxicity in mice model (Figure 4e).
(
BSA) on as-synthesized silicene nanosheets (SNSs-BSA) guar-
antees a superb colloidal stability in physiological medium
Figure S26a,b, Supporting Information). The mass percentage
(
of BSA in SNSs-BSA after surface modification was measured
by thermogravimetric analysis (TGA) (Figure S26c, Supporting
Information). It could be found that the coating amount of BSA
on the SNSs-BSA is 5.76%. SNSs-BSA exhibits excellent colloidal
dispersity and stability in a variety of simulated physiological
media, including 0.9% aqueous NaCl (saline), phosphate buffered
saline (PBS), fetal bovine serum (FBS), and Dulbecco’s modified
Eagle medium (DMEM) (Figure S27, Supporting Information).
Confocal laser scanning microscopy (CLSM) images present
the effective cellular uptake of FITC-labeled SNSs-BSA after the
co-incubation for 8 h, which is quantitatively supported by the
flow cytometry analysis for determining the effective endocy-
tosis amount of SNSs-BSA (Figure S28a,b, Supporting Infor-
mation). An evident elevation in fluorescence intensity of FITC-
labeled SNSs-BSA was measured from 0% to 78.5% (Figure S29,
Supporting Information). In vitro toxicities of SNSs-BSA to
cells were determined via the standard cell counting kit-8
(CCK-8) assay. Breast 4T1 cancer cells and glioma U87 cancer
cells were co-incubated with SNSs-BSA at varied concentrations
Moreover, the excretion pathways containing urine and feces
exhibit that around 12% of Si element was excreted out of mice
in 48-hour intravenous injection, which demonstrates the possible
mode of SNSs-BSA excreted out via the urine and feces (Figure 4f),
proving the desirable in vivo biosafety for clinical translation.
On the basis of the above practices, even though the SNSs-
BSA-treated mice are of exposure to artificial daylight, no signif-
icant negative impacts could be observed, which demonstrates
that the SNSs-BSA induces no apparent phototoxicity. These
results exhibit that SNSs-BSA are of high biocompatibility for
further safe in vivo tumor nanomedicine.
−1
(
0, 12.5, 25, 50, 100, and 200 µg mL ) for 24 h. SNSs-BSA
exhibit insignificant effect on cell viability of either 4T1 or
−
1
U87 cancer cells, even at a high concentration of 200 µg mL
Figure S28c, Supporting Information). Furthermore, SNSs-
(
BSA as photothermal-conversion nanoagent for in vitro cancer-
cell hyperthermia upon exposure to NIR-II laser irradiation was
investigated. 4T1 cell lines were incubated with the SNSs-BSA-
−
1
dispersed culture medium at the concentration of 50 µg mL
for 8 h, then exposed to NIR-II laser of varied power densities (0,
−2
0
.25, 0.5, 0.75, 1.0, and 1.5 W cm ). It could be found that with
the increase of input laser energy, more cancer cells were killed
by the efficient photothermal-conversion ablation (Figure S28d,
Supporting Information). In order to distinctly characterize the
silicene-based photo-induced cancer-cell ablation, the potent
cell apoptosis/necrosis after photothermal ablation was further
demonstrated by CLSM observation (Figure S28e,f, Figure S30,
and Discussion S2, Supporting Information).
Besides, fluorescence imaging was performed to analyze
the distribution of SNSs-BSA in living body of mice. Figure 6g
shows the ex vivo fluorescence photographs of main organs
obtained at varied time intervals, where the accumulation of
SNSs-BSA at tumor increased up to the maximum in 12 h. For
quantitative analysis, tumors and organs were solubilized by
lysis buffer to quantify the amount of accumulated SNSs-BSA.
As shown in Figure S31 in the Supporting Information,
time-dependent biodistribution of SNSs-BSA at tumor and
To uncover the underlying mechanism of in vitro PTT
utilizing SNS-NSA to kill cancer cells, the therapeutic effect was
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Figure 4. In vivo toxicity evaluation and pharmacokinetic and biodistribution analysis. a) Schematic diagram of in vivo toxicity evaluation model. b) Time-
dependent body-weight curves of CD-1 mice within 28 days after various treatments (group I: control; group II: SNSs-BSA only; group III: SNSs-BSA +
daylight). c) Histological analysis based on colleting H&E staining images of the major organs (heart, liver, spleen, lung, and kidney) of CD-1 mice in 1, 7,
and 28 days (stage 1, stage 2, and stage 3) postinjection after various treatments (group I: control; group II: SNSs-BSA only; group III: SNSs-BSA + daylight).
Optical images share the same scale bar (100 µm). d) Blood biochemistry and hematological parameters of CD-1 mice in 1, 7, and 28 days postinjection
after various treatments (group I: control; group II: SNSs-BSA only; group III: SNSs-BSA + daylight). Optical images share the same scale bar (100 µm).
e) Core hematological, hepatic, and renal biochemical parameters of CD-1 mice in 28 days post-injection under after various treatments. f) Accumulated
Si (in feces and urine) excretion out of the CD-1 mice after the postinjection of SNSs-BSA for varied durations (0.5, 2, 6, 12, 24, and 48 h). g) Fluorescence
images of major organs and tumor harvested from 4T1-tumor-bearing mice after post-injection of Cy5.5-labeled silicene-BSA at varied time points (0.5, 2, 6,
12, 24, and 48 h). h) The blood circulation curve of SNSs-BSA after intravenous injection into mice (n = 3). The half-time (T1/2) was calculated to be ≈1.82 h.
major organs was measured and high signal of SNSs-BSA
was observed in the tumor site, as well as reticuloendothelial
systems (RES) including liver, lung, and kidney, which are com-
patible with the ex vivo fluorescence photographs (Figure 4g).
The blood circulation half-time of SNSs-BSA in blood stream
was measured to be 1.82 h (Figure 4h). Thus, the drug–light
interval was determined to be 12 h between post-injection and
light irradiation for the PTT treatment.
performed on xenograft-tumor-bearing mice (Figure 5a). After
−1
intravenous (IV) administration of SNSs-BSA/PBS (20 mg kg ),
4T1-tumor-bearing mice were exposed to 1064 nm NIR-II laser
−2
(1.0 W cm ), and the tumor-site temperatures rapidly increased
from ≈34 to ≈48 °C in 10 min of therapeutic irradiation. In con-
trast, the tumor temperature under the NIR-II laser irradiation
only exhibited slight fluctuation (Figure 5b,c).
For further in vivo phototherapeutic evaluation, the 4T1 tumor-
bearing mice were randomly divided into four groups: control,
SNSs-BSA (IV) only, NIR-II only, and SNSs-BSA (IV) + NIR-II.
NIR laser irradiations were carried out in 12 h post-injection
Encouraged by the in vitro high NIR-II laser absorbances, desir-
able photothermal-transduction effect and in vivo accumulation
efficiency of SNSs-BSA, the in vivo PTT experiments were further
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Figure 5. In vivo photothermal cancer therapy and photoacoustic imaging. a) In vivo treatment schedule of SNSs-BSA therapeutic evaluation.
b) Infrared thermal images and c) the corresponding temperature elevations at the tumor regions of 4T1-tumor-bearing mice in groups of NIR-II and
SNSs-BSA + NIR-II during laser irradiation. d) Time-dependent body weight curves of nude mice (n = 5, mean ± SD) after different treatments (control,
NIR-II only, SNSs-BSA only, and SNSs-BSA + NIR-II). The treatments were performed only once. e) In vivo 4T1 tumor proliferation curves of nude mice
(
n = 5, mean ± SD) after various treatments. f) Tumor weights of mice 21 days after the treatments. g) Survival curves of mice after different treatments.
h) H&E staining and TUNEL staining for pathological changes, and antigen Ki-67 staining for cellular proliferation in tumor sections from each group.
Images share the same scale bar of 50 µm. i) Digital photographs of 4T1-tumor-bearing mice and their tumor sites in 16 days after treatments. j) In
−
1
vitro PA images and k) PA values of SNSs-BSA solutions as a function of concentration (0, 0.025, 0.05, 0.1, and 0.2 mg mL with respect to Si). l) In
vivo PA value temporal evolution and m) PA images of the tumor region at different time intervals (0, 2, 6, 12, 24, and 48 h) after tail-vein injection.
(
IV) of SNSs-BSA. In three days after photothermal ablation,
tumor volumes for each group were measured every two days
(Figure 5e), and the digital photographs of tumor sites were
captured every three days in a half-month duration after the
tumors in treated group (SNSs-BSA (IV) + NIR-II) were elimi-
nated, leaving black scars at the initial tumor regions. The
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treatments (Figure 5i). Remarkably, mice in the groups of con-
trol, SNSs-BSA (IV) only, NIR-II only exhibited average life-span
of 14–22 days. In contrast, all treated mice in SNSs-BSA (IV) +
NIR-II group with tumors exhibited a complete tumor eradica-
tion and survived over 40 days without the tumor reoccurence
Figure S32, Supporting Information) under NIR irradiation. In
comparison to the precontrast image, it is clear that the intensity
of PA signal increased from 0.06 to 0.6 a.u. (Figure 5m), and the
tumor region was gradually enlightened, exhibiting a maximum
enhance in around 12 to 24 h post-injection, largely owing to the
accumulation of SNSs-BSA through the enhanced permeability
and retention (EPR) effect. Thereafter, the signal of the tumor
region started to decline, presenting that the SNSs-BSA accumu-
lated by tumor tissue were being gradually excreted out.
(Figure 5f,g). All the mice feature negligible weight fluctuations,
thus confirming negligible negative impacts of these treatments
on the health of mice (Figure 5d). H&E and TdTmediated dUTP
Nick-End Labeling (TUNEL) staining results show the signifi-
cant necrosis of tumor tissues of SNSs-BSA (IV) + NIR-II group
compared to the mice groups of control, SNSs-BSA (IV) only,
and NIR-II only. The in vivo proliferative effect was investigated
by Ki-67 antibody staining, and the SNSs-BSA (IV) + NIR-II
group presented strong suppression capacity on the cell prolif-
eration, while the other three groups exhibited nearly no passive
influence on the proliferative capacity of cancer cells (Figure 5h).
Apart being a novel photothermal-transduction nanoagent to
achieve high efficiency of in vivo tumor eradication under NIR
irradiation, SNSs-BSA can also act as a robust contrast agent
To elucidate the underlying mechanism of tumor death trig-
gered by SNSs-BSA-based photothermal therapy, the tumors
were collected to isolate primary tumor cells for flow cytometric
apoptosis assay and related quantitative analysis. As expected,
the proportion of apoptotic cells in certain SNSs-BSA + NIR-II
−
2
laser (1.0 W cm ) samples was dramatically higher than those
of the groups of no treatment, SNSs-SBA only, and NIR-II laser
−
2
(1.0 W cm ) only (44.6% vs 5.7%, 8.1%, 5.7%, respectively),
proving SNSs-BSA an efficient photothermal therapeutic
agent. Furthermore, despite the similar cell apoptosis tendency
showed in three core experimental group (SNSs-BSA + NIR-II
(CA) for photoacoutic (PA) imaging. The PA signals of a series
−
2
of SNSs-BSA at varied concentrations clearly demonstrate their
contrast-enhancement performances (Figure 5j,k). On the basis
of these results, 4T1-tumor-bearing mice were intravenously
laser of 0.75, 1.0, and 1.25 W cm , respectively), the cells show
a significantly increased apoptosis at the elevated intensities
of NIR-II laser, further verifying the capacity of SNSs-BSA-
based PTT modality to eliminate the malignant tumor in vivo
(Figure 6a,b). In addition, tumor tissues were collected from
−1
administrated with SNSs-BSA (20 mg kg ), and PA images were
obtained at varied time intervals post-injection (Figure 5l and
Figure 6. In vivo mechanism of cell death triggered by photothermal ablation based on 2D silicene as a biodegradable bio-nanoagent. a) In vivo flow
cytometric apoptosis assay and b) the corresponding quantitative analysis of cells isolated from tumor tissues of 4T1-tumor-bearing mice under
−
2
−2
various treatments. Groups: control, SNSs-SBA only, NIR-II laser only, SNSs-SBA + NIR-II laser (0.75 W cm ), SNSs-SBA + NIR-II laser (1.0 W cm )
−
2
and SNSs-SBA + NIR-II laser (1.25 W cm ). c) mRNA levels of Bax, Bcl-2, and β-Actin in the tumor tissues across groups (Group I: control, Group II:
SNSs-BSA only, Group III: NIR-II of 1.0 W cm only, Group IV: SNSs-BSA + NIR-II of 0.1 W cm , Group V: SNSs-BSA + NIR-II of 0.25 W cm , Group
VI: SNSs-BSA + NIR-II of 0.5 W cm , Group VII: SNSs-BSA + NIR-II of 0.75 W cm , and Group VIII: SNSs-BSA + NIR-II of 1.25 W cm ) were analyzed
by qPCR (mean ± SD, n = 3 for each group). d) Heatmap showing mRNA expression levels of differentially expressed genes in various groups. All data
are presented as mean ± SD (n = 3 for each group) with *P < 0.05, **P < 0.01, ***P < 0.001 (two-tailed Student’s t-test). e) Protein levels of Bax, Bcl-2,
and β-Actin as detected by Western blot after lysis of tumor tissues in Groups I–VIII.
−
2
−2
−2
−
2
−2
−2
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mice in all groups after various treatments and the expression Keywords
levels of mRNA and proteins (Bax and Bcl-2) were detected by
2
D topology, biodegradation, nanomedicine, silicene, wet-chemical
qPCR and Western blot (Figure 6c–e). Increased expression of
Bax and reduced expression of Bcl-2 at both the mRNA and
protein levels were observed in SNSs-BSA + NIR-II laser group
but not in the other groups, which implies that photothermal-
induced downregulation of Bcl-2, by activating the mitochon-
drial apoptotic pathway, plays a significant role in PTT-mediated
cancer-cell apoptosis.[
exfoliation
Received: May 11, 2019
Revised: June 18, 2019
Published online:
31]
The contemporary uses of silicon-based nanomedicines for
biomedical engineering has been largely limited to only serving
as carriers in drug delivery systems. Though featuring intriguing
physiochemical natures, the bio-related degradability and clear-
ance of siliceous nanomaterials have been questioned worldwide,
since these issues are the crucial prerequisites for the successful
translation in clinics. Here, we successfully fabricated a novel
kind of 2D silicene nanosheets (SNSs) with a marked degradable
nature, and demonstrated their in vivo highly efficient cancer
phototherapy of mouse tumor xenografts in NIR-II biowindow.
Such a 2D silicene was synthesized by a facile and scalable two-
step liquid/gas exfoliation method combining stepwise mild
oxidation-triggered delamination and gasification-induced exfoli-
ation procedures. The ultrathin, lateral-nanosized silicene exhib-
ited extraordinarily high photothermal conversion efficiency
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