Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-enhanced Raman scattering with high accessibility

Article abstract of DOI:10.1016/j.jallcom.2019.153107

Noble metal particles (Au and Ag) with porous structure are promising surface-enhanced Raman scattering (SERS) substrates, but their preparation processes are generally complex. Here, we report a remarkably facile, rapid and inexpensive method to synthesize two kinds of porous Ag submicrocubes, cage- and sponge-like submicrocubes with highly clean and accessible surfaces. These particles were synthesized in a completely green solvent (water) by using Ag₃PO₄ submicrocubes as templates at room temperature. Merely by optimizing the concentration of a reducing agent (NaBH₄), three-dimensional cage- and sponge-like Ag submicrocubes were fabricated. As a result, as SERS substrates, the cage- and sponge-like Ag submicrocubes can achieve near-infrared (NIR)-SERS activity because they have broad absorption throughout the visible and NIR regions. Moreover, compared with the sponge-like Ag submicrocubes, cage-like submicrocubes with a highly rough surface had higher SERS enhancement due to the presence of narrow and deep nanoslits on their shells. Due to the high-density of ‘‘hot spots’’ produced by the narrow and deep nanoslits on the shell, cage-like Ag submicrocubes have a low SERS detection limit, 1.479 × 10^?11 M for 4-aminothiophenol and 1.50 × 10^?9 M for thiram. Thus, a new SERS substrate based on highly rough cage-like submicrocubes was obtained by an easy and green method.

Full text of DOI:10.1016/j.jallcom.2019.153107

Journal of Alloys and Compounds xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4
templates for near-infrared surface-enhanced Raman scattering with
high accessibility
,
Lifeng Hang a d, Yingyi Wu b, Honghua Zhang b, Junhuai Xiang b, Yiqiang Sun c,
,
, *
Tao Zhang c **, Dandan Men b
a The Department of Medical Imaging Guangdong Second Provincial General Hospital, Guangzhou, 518037, PR China
b Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang, 330013, PR China
c University of Science and Technology of China, Hefei, 230031, PR China
d Department of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen, 518060, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Noble metal particles (Au and Ag) with porous structure are promising surface-enhanced Raman scat-
tering (SERS) substrates, but their preparation processes are generally complex. Here, we report a
remarkably facile, rapid and inexpensive method to synthesize two kinds of porous Ag submicrocubes,
cage- and sponge-like submicrocubes with highly clean and accessible surfaces. These particles were
synthesized in a completely green solvent (water) by using Ag3PO4 submicrocubes as templates at room
temperature. Merely by optimizing the concentration of a reducing agent (NaBH4), three-dimensional
cage- and sponge-like Ag submicrocubes were fabricated. As a result, as SERS substrates, the cage-
and sponge-like Ag submicrocubes can achieve near-infrared (NIR)-SERS activity because they have
broad absorption throughout the visible and NIR regions. Moreover, compared with the sponge-like Ag
submicrocubes, cage-like submicrocubes with a highly rough surface had higher SERS enhancement due
to the presence of narrow and deep nanoslits on their shells. Due to the high-density of ‘‘hot spots’’
produced by the narrow and deep nanoslits on the shell, cage-like Ag submicrocubes have a low SERS
detection limit, 1.479 ꢀ 10ꢁ11 M for 4-aminothiophenol and 1.50 ꢀ 10ꢁ9 M for thiram. Thus, a new SERS
substrate based on highly rough cage-like submicrocubes was obtained by an easy and green method.
© 2019 Elsevier B.V. All rights reserved.
Received 12 July 2019
Received in revised form
16 November 2019
Accepted 19 November 2019
Available online xxx
Keywords:
Ag3PO4 submicrocubes
Cage- and sponge-like Ag submicrocubes
Near-infrared surface-enhanced Raman
scattering
4-aminothiophenol
Thiram
1. Introduction
absorption in the NIR region makes these porous noble metal NPs
promising materials for NIR-SERS [8,9,18] and thermal therapy
Porous nanoparticles (NPs) have been widely applied in opto-
electronic nanodevices [1], catalysis [2,3], sensors [4,5], electro-
chemistry [6,7], surface-enhanced Raman spectroscopy (SERS)
[8e13], and thermal therapy [14,15] by virtue of their unique
properties, such as their high surface area, high gas permeability
and low density [1]. In particular, porous and hollow noble metal
NPs have attracted increasing attention in recent years because
their surface plasmon resonance (SPR) can be tuned from the
visible region to the near-infrared (NIR) region [8,9,16,17]. Their
[19,20] applications.
To the best of our knowledge, combining physical and chemical
methods is an effective route to prepare porous noble metal NPs by
physically depositing thin metal films on templates to form alloyed
NPs, followed by annealing under a high temperature and subse-
quent selective etching of less-stable metal from fully alloyed NPs.
Then, the remaining metal-rich species form a spongy nano-
structure framework with a lot of nanoligaments and nanopores.
For example, Li and co-workers prepared two-dimensional (2D)
sponge-like AueAg alloyed nanosphere arrays using monolayer
polystyrene (PS) colloidal crystals as the initial template, followed
by physical deposition of metal layer, annealing and subsequent
chemical etching of Ag element from AueAg alloyed nanospheres
[21]. Although providing precise control over the size of the spongy
nanospheres and clean surface (without surfactants), this method
* Corresponding author.
** Corresponding author.
E-mail addresses: nano_zhangt@163.com (T. Zhang), mendandan1999@126.com
(D. Men).
https://doi.org/10.1016/j.jallcom.2019.153107
0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107

2
L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
suffers from a remarkably low yield and requires a high tempera-
ture during the annealing process, which hampers its practical
application [22]. To date, wet-chemical syntheses, such as galvanic
replacement [15,23e25], template method [26e28], and dealloying
method [29e31], which have the advantage of large-scale pro-
duction, have been verified for the preparation of porous or spongy
NPs. Among these methods, the galvanic replacement reaction is a
typical wet-chemical method that can be used to synthesize porous
nanostructures [15]. Nevertheless, in this method, the reduced
atoms cannot be uniformly distributed throughout the entire re-
gion (reduced atoms only randomly occupy the outermost region)
owing to the nature of the displacement reaction mechanism
[32,33]. Compared with the random structure synthesized by
general galvanic replacement strategy, template method could
produce highly controllable three-dimensional (3D) porous net-
works with a more regular nanopore distribution [34]. However,
the surface area of samples fabricated by the template method is
relatively lower [1]. In addition, the dealloying method has also
been widely employed to fabricate porous noble metal nano-
structures by synthesizing alloyed NPs first, followed by the se-
lective etching of less-stable metal from fully alloyed NPs. In this
method, alloyed NPs are usually synthesized though a coreduction
[33] or calcinations process [6,14]. If the alloyed NPs are synthe-
sized through simultaneous reduction of HAuCl4 and AgNO3, sur-
factants are indispensable and remain on the surfaces of the porous
NPs. The residual surfactants can prevent the surfaces of the NPs
from being accessed by target analytes and interference with the
SERS signal when these NPs are applied as a SERS substrate [8,35].
Therefore, surfactants adsorbed on the surfaces of NPs should be
removed before SERS measurement [36]. In comparison, calcina-
tion can produce porous NPs with clean surfaces, whereas a high-
temperature is needed to converted Au@Ag@Cu2O or Au@ Cu2O
core/shell NPs into the fully alloyed form [6,14]. Moreover, all of the
above processes are multi-step and complicated for the preparation
of the template or wet-chemical coating. Recently, the reduction of
preformed metal oxide NPs into network structures has been
investigated. For example, Fang’s group has prepared cubic porous
Ag NPs and cubic Ag mesocages using mesocrystal Ag2O particles as
sacrificial templates through reduction with H2O2 [11]. This method
can be used to easily obtain porous Ag mesocubes and mesocages,
but it requires relatively expensive electrochemical equipment to
fabricate mesocrystal Ag2O particles. Therefore, it is still difficult to
develop new synthetic strategies to prepare porous noble metal
particles with highly clean and accessible surfaces through a facile
and inexpensive method with broad applicability.
Scheme 1, the manufacturing process includes the synthesis of
Ag3PO4 submicrocubes and a subsequent reduction step using
NaBH4. It is worth noting that both steps are mildly performed in
water at room temperature, so that the entire process can be
conveniently and efficiently carried out. Additionally, in this design,
these porous Ag particles have a clean surface and high accessibility
for target analytes due to these particles being free of any
surfactant.
2. Materials and methods
2.1. Materials
Ammonium hydroxide (NH$3H2O, 25.0e28.0%), sodium phos-
phate dibasic (Na2HPO4, ꢂ99.0%), silver nitrate (AgNO3, ꢂ99.8%),
sodium borohydride (NaBH4, 98%) and 4-aminothiophenol (4-ATP,
97.0%), thiram (ꢂ97.0%) were purchased from Sinopharm Chemical
Reagent Corporation (Shanghai, China). All chemicals were used as
received without further purification. Deionized water
(18.2 MU
.cm) was obtained from an ultrafilter system (Milli-Q,
Millipore, Marlborough, MA).
2.2. Preparation of Ag3PO4 submicrocubes
A silver-ammonia complex was used as a silver-ion source for
preparing Ag3PO4 submicrocubes [37,38], as shown in Scheme 1. In
a typical synthesis, an ammonia aqueous solution (0.45 M) was
added drop by drop to 2 mL of a AgNO3 solution (0.45 M) to form a
transparent solution silver-ammonia ([Ag(NH3)2]þ) aqueous solu-
tion under vigorous stirring. Then, the Na2HPO4 aqueous solution
(0.15 M) was introduced into the above [Ag(NH3)2]þ aqueous so-
lution with vigorous stirring, and Ag3PO4 submicrocubes with a
matching color were synthesized. The final products were collected
by centrifugation, washed with deionized water, and redispersed
into 10 mL deionized water for further use.
2.3. Preparation of sponge-like Ag submicrocubes
Sponge-like Ag submicrocubes were obtained from Ag3PO4
submicrocubes by reduction with NaBH4. Briefly, 1 mL of an Ag3PO4
submicrocube aqueous solution was dispersed into 9 mL deionized
water, and then, a specific amount (40
mL) of a NaBH4 solution
(0.5 M) was added under stirring at room temperature for
approximately 2 min. When the reaction completed, the produced
black solids were floated on the reaction medium. The floating
solids were collected by centrifugation and dried under vacuum at
room temperature.
In this work, we report a remarkably facile, rapid and inex-
pensive method for the fabrication of cage- and sponge-like Ag
submicrocubes with highly accessible surface. Briefly, as shown in
Scheme 1. Schematic diagram of the preparation process for cage- and sponge-like Ag submicrocubes. (a) Silver-ammonia complex; (b) Ag3PO4 submicrocubes, (c) Cage- and
sponge-like Ag submicrocube.
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107

L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
3
2.4. Preparation of cage-like Ag submicrocubes
keeping the other conditions unchanged. As shown in Fig. 2a, when
the concentration of NaBH4 is 0.25 mM, only a few nonuniform Ag
NPs anchored on the surface of the Ag3PO4 particles are observed.
Meanwhile, the morphology of the residual Ag3PO4 submicrocubes
becomes irregular. By increasing concentration of NaBH4 to
0.50 mM, the number of Ag NPs with different sizes increases, as
shown in Fig. 2b. As the concentration of NaBH4 is increased to
1.00 mM, the number of produced Ag NPs is further increased, but
Ag3PO4 still exists, as shown in Fig. 2c. A magnified SEM image
(Fig. 2d) shows that the produced Ag NPs undergo self-assembly to
form a 3D architecture with a highly rough surface. Additionally,
when the concentration of NaBH4 is gradually increased to
2.00 mM, micrometer-sized ligament networks are formed at large
quantities and no obvious Ag3PO4 particles are observed in the SEM
image (Fig. 2e). The sponge-like submicrocubes inherit the shape
and size of the templates well, and are composed of a nanoporous
and 3D network of interconnected nanoligaments. Moreover, these
nanoligaments are not uniform in size and are created by many
nanosections and nanobranches (Fig. 2f). This phenomenon is
coincident with Eswaramoorthy’s results [1], which can be ascribed
to the formation of nanoligaments through the fusion of small NPs.
Furthermore, as the concentration of NaBH4 further increases to
4.00 mM, the products are also sponge-like submicrocubes, similar
to the products prepared at a NaBH4 concentration of 2.00 mM
(Fig. S3), indicating that sponge-like submicrocubes with a rela-
tively monotonous surface topography can be obtained by using
Ag3PO4 submicrocubes as sacrificial templates when the concen-
tration of NaBH4 exceeds 2.00 mM.
Cage-like Ag submicrocubes were also obtained from Ag3PO4
submicrocubes by reducing with NaBH4. Briefly, 1 mL of an Ag3PO4
submicrocube aqueous solution was dispersed into 9 mL deionized
water, and then, 20
mL of a NaBH4 solution (0.5 M) was added
dropwise into the above solution under stirring at room tempera-
ture for approximately 2 min. When the reaction was completed,
the produced black solids were found to be floating on the reaction
medium. The floating solids were collected by centrifugation and
washed with deionized water. Then, the solid samples were
redispersed into 3 mL deionized water, followed by the addition of
12 mL of a NaBH4 aqueous solution (0.5 M). The final product was
separated by centrifugation and dried at room temperature in
vacuum.
2.5. Instrumentation and characterization
The morphologies of the obtained samples were observed
through a field-emission scanning electron microscope (FE-SEM,
Sirion 200) at an accelerating voltage of 10 kV. The composition of
the synthesized products was analyzed by Energy-dispersive X-ray
spectroscopy spectra (EDS) using an Oxford INCA analysis system.
The X-ray diffraction (XRD) patterns were measured by a Philips
X’pert Pro X-ray diffractometer with Cu
Ka
radiation
(
l
¼ 0.15419 nm). The XRD patterns were measured from 20ꢃ to 90ꢃ
with a scanning rate of 5ꢃ/s. The UVeViseNIR spectra were
measured using a UV/Vis spectrophotometer (Cary 500, Varian).
Raman spectra were obtained through a confocal microprobe
Raman spectrometer (Renishaw inVia Reflex) with a laser wave-
length of 785 nm, a power of 0.5 mW and an integration time of 5 s.
For the SERS intensity mapping investigation, the laser power and
integration time were set as 0.5 mW and 1 s, respectively.
To further confirm the evolution of the constituents, the XRD
patterns of the as-prepared products obtained with different con-
centrations of NaBH4 are presented in Fig. 3. When the concen-
tration of NaBH4 is 0.25 mM, the XRD pattern (curve i) confirms
that the samples are composed of two components: Ag and Ag3PO4.
The peaks at 2
q
¼ 38.14ꢃ, 44.42ꢃ, 64.54ꢃ and 77.42ꢃ belong to the
3. Results and discussion
(111), (200), (220) and (311) lattice planes of the fcc structure of Ag,
respectively. Compared to the standard spectrum of Ag3PO4 in the
3.1. Morphology control of porous Ag submicrocubes
range from 2
q
¼ 30ꢃe70ꢃ, the remaining main peaks can be
matched to the structure of Ag3PO4. When the concentration of
NaBH4 is increased to 0.50 mM, its XRD pattern is also consistent
with that for a mixed phase of Ag3PO4 and Ag, as shown by curve ii
in Fig. 3. Furthermore, when the concentration of NaBH4 is
increased to 1.00 mM, the intensities of the peaks due to Ag3PO4
significantly decrease and the characteristic peaks due to Ag
remarkably increase in the XRD patterns, as shown by curve iii from
Fig. 3. It is worth noting that all the peaks of the sample can be
attributed to Ag with a fcc structure (JCPDS 01e1164) upon
increasing the concentration of NaBH4 to 2.00 mM, as shown by the
curve iv in Fig. 3. Meanwhile, no characteristic diffraction peaks due
to the Ag3PO4 crystal or other impurities are observed, indicating
that the final products consist of a pure fcc structure for Ag. This
result demonstrates that the synthesized products are composed of
a pure phase Ag at a NaBH4 concentration of 2.00 mM. Further-
more, the EDS result shows that only Ag is detected (Fig. S4),
indicating that Ag3PO4 is completely reduced to Ag, which is
consistent with the XRD results.
In this work, porous Ag submicrocubes were synthesized using
Ag3PO4 submicrocubes as sacrificial templates, which were pre-
pared at room temperature by a simple reaction of the [Ag(NH3)2]þ
complex with Na2HPO4 aqueous solutions, as described in previous
reports [37,38]. Fig. 1a shows a low-magnification SEM image of the
Ag3PO4 submicrocubes. It can be clearly seen that submicrocubes
were prepared with smooth surfaces and sharp corners and edges,
as shown in Fig. 1b. And the average size of the edge of the as-
prepared submicrocubes is approximately 860 nm (Fig. 1c). More-
over, the EDS result (Fig. S1) shows that only O, P, and Ag are
detected in the submicrocubes. Furthermore, all the diffraction
peaks observed in the XRD pattern of the submicrocubes (Fig. 1d)
correspond to an Ag3PO4 crystal with a body-centered cubic
structure (JCPDS 06e0505) [37]; these diffraction peaks are
assigned to the (200), (210), (211), (220), (310), (222), (320), (321),
(400) and (330) planes, indicating that Ag3PO4 submicrocubes were
successfully prepared with high yield.
The as-prepared Ag3PO4 microcubes were used as templates to
synthesize porous Ag particles by using NaBH4. In the preparation
process, it is noteworthy that when the NaBH4 aqueous solution
was added into the Ag3PO4 suspension, the color of the solution
immediately changed from tawny to dark gray, as shown in Fig. S2.
Subsequently, we found that black solids were produced that
floated on the reaction solution. To study the effect of the con-
centration of NaBH4 on the formation of the porous Ag particles, a
series of experiments with different NaBH4 concentrations
(0.25 mM, 0.50 mM, 1.00 mM and 2.00 mM) were carried out,
Next, we found that cage-like Ag submicrocubes can also be
prepared from the Ag3PO4 templates by a secondary reduction. In
brief, the as-prepared samples composed of Ag and Ag3PO4 with a
3D architecture (Fig. 2c and d) prepared via reduction with 1.00 mM
NaBH4 were further reduced by using 2.00 mM NaBH4. The SEM
images (Fig. 4aec) show that cage-like Ag submicrocubes with a
rough surface are obtained by a secondary reduction. The obtained
hollow Ag submicrocubes also maintain the shape of the Ag3PO4
templates. Compared with the sample obtained by a single reduc-
tion at a NaBH4 concentration of 1.00 mM (Fig. 2c), no obvious
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
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4
L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 1. Characterizations of the samples. (a) SEM image of the as-obtained sample synthesized by reacting the [Ag(NH3)2]þ complex with Na2HPO4, (b) high-magnification SEM
image of (a), (c) size distribution of the Ag3PO4 submicrocubes, (d) XRD pattern of the Ag3PO4 submicrocubes.
Fig. 3. XRD patterns of the as-prepared products obtained for NaBH4 concentrations of
0.25 mM (i), 0.50 mM (ii), 1.00 mM (iii) and 2.00 mM (iv), respectively.
unlike those for the sponge-like Ag submicrocubes (Fig. 2e).
Importantly, compared with the sample obtained via one-time of
reduction, no diffraction peaks due to Ag3PO4 are detected in the
XRD pattern of the cage-like submicrocubes synthesized by a sec-
Fig. 2. SEM images of the as-obtained samples prepared by using Ag3PO4 sub-
ondary reduction (Fig. 4d), demonstrating that the obtained prod-
microcubes as sacrificial templates with varying concentrations of NaBH4: (a)
ucts are composed of a pure fcc Ag structure by a secondary
reduction.
0.25 mM; (b) 0.50 mM; (c) 1.00 mM, (d) high magnification SEM image of the simple
obtained at NaBH4 concentration of 1.00 mM, (e) SEM image of the simple obtained at
NaBH4 concentration of 2.00 mM, (f) high magnification SEM image of sponge-like
submicrocubes. The inset of (e) is a SEM image of a sponge-like submicrocube.
3.2. Growth mechanism of the cage- and sponge-like Ag
submicrocubes
Ag3PO4 particles are observed, indicating that the residual Ag3PO4
is reduced to Ag. The magnified SEM images shown in Fig. 4b and c
shows that the walls of the mesocages are composed of small NPs,
Our studies clearly show that the final structure and
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107

L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
5
Fig. 4. (aec) SEM images and (d) XRD pattern of the cage-like Ag submicrocubes synthesized by secondary reduction.
morphology of the porous Ag submicrocubes depend on the initial
concentration of NaBH4. According to the experimental results, a
growth mechanism is proposed, as shown by the schematic dia-
gram in Fig. 5. Ag3PO4 can be reduced to generate Ag nuclei by
adding NaBH4 to an Ag3PO4 particle solution. These produced Ag
nuclei can act as nucleation centers for further growth to form Ag
NPs [1]. The number of nucleation centers produced is directly
proportional to the concentration of NaBH4. As a result, if the
concentration of NaBH4 is too low (0.25 mM), only the surfaces of
Ag3PO4 submicrocubes are reduced to nonuniform Ag NPs, as
shown in scheme (a) in Fig. 5. With the increasing concentration of
NaBH4 (0.50 mM), the number of Ag NPs was increased due to
increasing number of nucleation centers, as illustrated in scheme
(b) in Fig. 5. The formation process for the cage-like Ag sub-
microcubes is illustrated in schemes (c1) and (c2). When the
concentration of NaBH4 is further increased to 1.00 mM, more Ag
NPs are generated, and they self-assemble to form a porous shell on
the surface of the Ag3PO4 particles. In this case, the formed shell has
an ordered superstructure composed of individual Ag NPs. At the
secondary reduction stage, the residual Ag3PO4 particles are
reduced to individual Ag NPs. In the further ongoing reduction,
individual Ag NPs are formed with a highly rough outer shell on the
surface of the submicrocube by slightly welding with each other.
Therefore, an abundance of narrow and deep nanoslits is formed
between the Ag NPs on the surface of the cage-like Ag sub-
microcubes. To enhance the SERS signals, this topography may be
an ideal high-enhanced SERS substrate. Moreover, as the concen-
tration of NaBH4 is further increased to 2.00 mM, sponge-like Ag
submicrocubes composed of a nanoporous and 3D network of
interconnected nanoligaments are formed in scheme (d). In this
process, the Ag NPs were formed firstly, and then the newly formed
Ag NPs are rapidly weld to one another to fuse into Ag nanoliga-
ments to construct a 3D interconnected network structure.
3.3. SERS properties of the cage- and sponge-like Ag submicrocubes
Cage- and sponge-like Ag submicrocubes were selected for
investigation according to their UVeViseNIR spectra and further
SERS measurements. Their corresponding optical properties were
recorded by UVeViseNIR spectra, as shown in Fig. S5. One can see
that the absorption bands of the cage- and sponge-like Ag sub-
microcubes are broad (approximately 400e1200 nm), containing
the overall visible region and extending to the NIR region. This
phenomenon indicated that the LSPR resonance produced by the
cage- and sponge-like Ag submicrocubes can cover the entire
visible region as well as overlap in the NIR region because the
network and cage nanostructures are composed of the nanoliga-
ments and welded NPs that have suitable length scales due to the
presence of abundant nanopores and nanoslits in the Ag sub-
microcubes with a larger size [1]. This is the reason why a near
black color was observed during the preparation. Because of these
broad absorption bands in the entire visible region as well as in the
Fig. 5. Schematic showing the formation of Ag particles with different morphologies
through reducing Ag3PO4 submicrocubes by NaBH4 with various concentrations.
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107

6
L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
NIR region, a laser excitation wavelength of 785 nm can be selected
for Raman measurement. Generally, laser sources with visible
excitation are applied in SERS experiments. However, for many
applications, visible laser irradiation will cause photobleaching and
plasmonic heating as well as fluorescence from the detected mol-
ecules, which can reduce the SERS signals in the presence of large
and broad background features [39e43]. The use of a laser source in
the NIR spectral range is an effective way to avoid photochemical
reactions and reduce analyte fluorescence because biological ma-
terials and common semiconductors are transparent in this range
[44]. Thus, the use of NIR wavelengths with a low power density is
highly desirable. Here, due to the broad absorption bands in the
entire visible region as well as in the NIR region, a laser with an
excitation wavelength of 785 nm was selected to carry out further
Raman measurement.
In this work, the SERS sensitivity of the cage- and sponge-like Ag
submicrocubes was evaluated by using 4-ATP molecules as target
molecules under a laser excitation wavelength of 785 nm, as
illustrated in Fig. 6. Compared to sponge-like Ag submicrocubes,
cage-like Ag submicrocubes with a highly rough surface had a
higher SERS signal. The intensity of the Raman signal (1077 cmꢁ1
for 4-ATP) from the cage-like Ag submicrocubes is comparatively
approximately two times higher than that found for the sponge-
like Ag submicrocubes, which can be attributed to the deeper and
narrower slits present on the cage-like Ag submicrocubes surfaces.
To further explain the relationship among the SERS activity and
structure of the Ag submicrocube, the distributions of the local
electric field for cage- and sponge-like Ag submicrocubes were
theoretically simulated by the finite-difference time-domain
(FDTD) method (Fig. 7). In the simulation process, cage- and
sponge-like Ag submicrocubes were constructed as simplified
artificial models surrounded by air and irradiated with incident
light at a wavelength of 785 nm (Fig. 7a and c). The models for cage-
and sponge-like Ag submicrocubes with a size of 600 nm were built
from spherical NPs (diameter: 100 nm) and nanoligaments (col-
umn, height: 100 nm, diameter: 100 nm), respectively. Then, inci-
dent light with a wavelength of 785 nm was irradiated along the z-
axis; the incident light was polarized along the direction of the x-
axis. As shown in Fig. 7b, the “hot spot” is found to be distributed in
the junctions between the NP units on the cage shell due to strong
plasmon coupling between Ag NPs. Compared with sponge-like Ag
submicrocubes, the deeper and narrower slits produced in the
junctions on the surface of the cage-like Ag submicrocube can more
effectively confine light in the slit cavity, thereby resulting in a
stronger localized electric field to greatly improve the SERS
enhancement. Additionally, the Raman intensity is generally
accepted to increase by a factor |E|4 with respect to the local electric
field on the surface of the SERS substrate [11]. The maximal |E/E0|2
enhancement is found to be 104.2 and 104.0 for cage- and sponge-
like Ag submicrocubes, respectively. Regarding the factor |E|4, the
value used for the model A for cage-like Ag submicrocube is about
two times higher than that used for the model C for the cage-like Ag
submicrocube, which agrees well with the experiment result
(Fig. 6, approximately 2.3 times).
Additionally, as a control, the SERS sensitivity for Ag3PO4 sub-
microcubes and a mixture of Ag and Ag3PO4 particles used as SERS
substrates were evaluated by using 4-ATP molecules (10ꢁ8 M) as
the target molecule. As shown in Fig. S6, in the SERS spectra ob-
tained for the Ag3PO4 submicrocubes and mixtures of Ag3PO4 and
Ag particles used as SERS substrates, the obvious peaks at 910 cmꢁ1
and 1000 cmꢁ1 are assigned to the characteristic peaks of Ag3PO4
[45]. In addition, there were no obvious SERS signals due to the 4-
ATP molecules, revealing that the existence of Ag3PO4 particles can
cause interference to the SERS signal for the target molecule.
Moreover, the enhancement factors for cage- and sponge-like Ag
submicrocubes were estimated by a same method based on pre-
vious works [46,47]. The enhancement factors for the cage- and
sponge-like Ag submicrocubes were determined to be ~3.43 ꢀ 107
and 2.04 ꢀ 107, respectively, as shown in Fig. S7. Compared to other
literatures, i.e., studies of P(AAm-co-AA) hydrogel microsphere@Au
nanospheres (EF: 109) [36] and porous AueAg alloy nanocubes (EF:
108) [9], this value is not so high.
Furthermore, the detection limits of the cage- and sponge-like
Ag submicrocubes used as SERS substrates were studied. Fig. 8
presents the SERS spectra and linear calibration plots for 4-ATP
for SERS substrates containing cage- and sponge-like Ag sub-
microcubes. In the SERS spectra (Fig. 8a and b), there are two
distinct peaks at 1077 cmꢁ1 and 1587 cmꢁ1, which are assigned to
the CeC stretching vibration of the benzene rings and C (benzene
ring)-S stretching vibration of the 4-ATP molecule [48], respec-
tively. Therefore, these two peaks can be used as fingerprint peaks
to detect 4-ATP molecules through SERS enhancement. As shown in
Fig. 8a and b, it can be observed that the intensity of the SERS signal
gradually decreases with decreasing 4-ATP concentration. As
shown in Fig. 8a, when the concentration of 4-ATP is as low as
10ꢁ11 M, characteristic peaks are obviously observed. However, at a
concentration of 10ꢁ11 M, the SERS signal peaks could not be
identified by using the sponge-like Ag submicrocubes as a SERS
substrate. Additionally, Fig. 8c and d presents the linear calibration
plots between the Raman intensity of the standard peaks and the
concentration of 4-ATP. The y value presents the intensity of the
Raman peak at the standard peak, and the x value represents the
concentration of 4-ATP. The linear regression equations and
detection limits are presented. As shown in Fig. 8c and d, the
detection limits for 4-ATP of the cage- and sponge-like Ag sub-
microcubes as SERS substrates are 1.479
ꢀ
10ꢁ11
M and
1.62 ꢀ 10ꢁ10 M, respectively, indicating that the cage-like substrates
have a lower detection limit compared to that of the sponge-like
substrates. Additionally, the detection limits of the cage-like Ag
submicrocubes (10ꢁ11 M) can be compared to the other reported
SERS substrates in the literature, i.e., porous alloyed AueAg nano-
sphere arrays (10ꢁ10 M) [21], sponge-like AueAg alloy nanocubes
(10ꢁ10 M) [9], and the Ag@Cu@ cicada wing (10ꢁ11 M) substrate
[49]. Moreover, the stability of the cage- and sponge-like Ag sub-
microcubes as SERS substrates was studied, as shown in Fig. S8.
After storage in air for three months, the Raman signal intensity
Fig. 6. SERS spectra measured for 4-ATP (10ꢁ8 M) using SERS substrates containing
cage- and sponge-like Ag submicrocubes under
a laser excitation wavelength of
785 nm (integral time: 5 s and laser power: 0.5 mW).
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107

L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
7
Fig. 7. Two models of a cage- (a) and sponge-like (b) Ag submicrocube used for FDTD simulations, Calculated distributions of the local electric field intensity for a cage- (c) and
sponge-like (d) Ag submicrocube under irradiation with a wavelength of 785 nm.
Fig. 8. SERS spectra for 4-ATP with different concentrations from the SERS substrates containing the cage-like (a) and sponge-like (b) Ag submicrocubes (integral time: 5 s and laser
power: 0.5 mW), Linear calibration plot of the SERS intensity versus 4-ATP concentration for SERS substrates containing cage-like (c) and sponge-like (d) Ag submicrocubes.
showed an obvious decrease due to the oxidation of Ag [50].
Additionally, the cage-like Ag submicrocubes as SERS substrates
can be applied to detect thiram. Fig. 9a shows the Raman spectral
intensity measured for thiram at various concentrations,
10ꢁ6e10ꢁ10 M, of the cage-like Ag submicrocube. The Raman peak
at 1381 cmꢁ1 is related to the symmetrical CH3 strain mode, which
is weakly coupled to the CN stretching mode of the thiram molecule
[51]. It can be observed that as the concentration of thiram
decreases, the intensity of the SERS signal gradually decreases.
When the concentration of thiram decreases to 10ꢁ10 M, the char-
acteristic peak (1381 cmꢁ1) could not be identified. Fig. 9b presents
a linear calibration plot between the Raman intensity measured at
the standard peaks and the concentrations of thiram. As shown in
Fig. 9b, the detection limit of the cage-like Ag submicrocubes as
SERS substrates for thiram is 1.50 ꢀ 10ꢁ9 M. Moreover, the SERS
mapping image of thiram on the substrate of cage-like Ag
Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107

8
L. Hang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 9. (a) SERS spectra obtained for different concentrations of thiram using cage-like Ag submicrocube SERS substrates (integral time: 5 s and laser power: 0.5 mW), (b) Linear
calibration plot between the SERS intensity and thiram concentration for cage-like Ag submicrocube substrates, (c) SERS intensity mapping of thiram (10ꢁ6 M) measured for the
peak at 1381 cmꢁ1 across an area of 20 ꢀ 20
m mm (integral time: 1 s and laser power: 0.5 mW), (d) Intensity
m2 for a cage-like Ag submicrocube substrate with a step size of 2
distribution at 1381 cmꢁ1 corresponding to part c.
submicrocubes was used to estimate the spot-to-spot reproduc-
ibility. The Raman intensity mapping (Fig. 9c) for thiram (10ꢁ6 M) at
temperature could be extended to prepare other NPs.
1381 cmꢁ1 was obtained using a square area (20
m
m ꢀ 20
m
m; step
Author contribution section
length: 2
mm), which confirmed the reproducibility of the SERS
substrate. Through statistical calculations, the relative standard
deviation (RSD) value of the Raman intensity obtained from more
than 100 points for the cage-like Ag submicrocubes substrate,
corresponding to Fig. 9c, is 18.7% at 1381 cmꢁ1 (Fig. 9d). For a SERS-
active substrate with good reproducibility, the maximal RSD value
of the signal intensities for a major SERS peak should be below 20%
[52,53], which demonstrates good reproducibility.
Lifeng Hang: Writing- Original draft preparation, Data curation,
Software
Yingyi Wu: Investigation, Software
Honghua Zhang: Investigation
Junhuai Xiang: Data curation
Yiqiang Sun: Software, Validation
Tao Zhang: Conceptualization, Methodology, Software
Dandan Men: Writing- Reviewing and Editing, Supervision
4. Conclusions
Declaration of competing interest
The authors declared that there is no conflict of interest.
Acknowledgements
In this work, Ag3PO4 particles were used as a sacrificial template
to fabricate Ag submicrocubes through a remarkably facile, rapid,
inexpensive and green method in a completely green water solvent.
In the reduction process, cage- and sponge-like Ag submicrocubes
with highly accessible surfaces were obtained by changing the
initial concentration of NaBH4 without the aid of any surfactant.
The cage- and sponge-like Ag submicrocubes exhibited broad ab-
sorption bands, containing the entire visible and NIR region, which
make them potential candidates for used as NIR-SERS substrates. In
addition, compared with sponge-like Ag submicrocubes, highly
rough cage-like Ag submicrocubes had increased SERS enhance-
ment (approximately two times higher than that of the sponge-like
Ag submicrocubes). This finding was attributed to the presence of
narrow and deep nanoslits on the outer shell of the cage-like Ag
submicrocubes with highly rough surfaces. Moreover, the detection
limit of the cage-like Ag submicrocubes as SERS substrates for 4-
ATP molecules is 1.479 ꢀ 10ꢁ11 M. Thus, a new SERS substrate
based on highly rough cage-like Ag submicrocubes was obtained by
an easy and green method. Moreover, this preparation strategy for
porous Ag submicrocubes with different surface topographies and
inner structures by reducing metal salts with NaBH4 at room
This work is supported by the National Natural Science Foun-
dation of China, China (Grant Nos: 51701054, 51861008 and
51903162), the Jiangxi Provincial Natural Science Foundation, China
(GJJ170684, 20181BAB216004, 20181BAB206004).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2019.153107.
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Please cite this article as: L. Hang et al., Green and rapid synthesis of porous Ag submicrocubes via Ag3PO4 templates for near-infrared surface-
enhanced Raman scattering with high accessibility, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153107