Fabrication of monodispersed Au@Ag bimetallic nanorod-loaded nanofibrous membrane with fast thermo-responsiveness and its use as a smart free-standing SERS substrate

Article abstract of DOI:10.1039/c6ra04247c

A novel type of metal nanoparticle-loaded smart nanofibrous membrane with fast thermo-responsiveness was fabricated by electrospinning an aqueous solution containing poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide) and monodispersed Au@Ag bimetallic nanorods with a core-shell structure, followed by heat treatment. The results obtained by electron microscopy show that the anisotropic nanoparticles are oriented along the axes of its constituent nanofibers. The membrane produced has high stability in aqueous media and remarkable thermo-responsiveness. It takes less than 10 seconds to reach its deswelling or swelling equilibrium state with the temperature alternating between 25 °C and 50 °C. The smart nanofibrous membrane with macroscopic mechanical strength can be used as a free-standing surface-enhanced Raman scattering (SERS) substrate with high Raman signal reproducibility for quantitative analysis, and its SERS efficiency can be readily elevated by raising the detection temperature across its phase transition temperature due to its fast thermo-response rate. Particularly, since the composite nanofibrous membrane possesses catalytic properties toward the reduction of 4-nitrothiophenol to 4-aminothiophenol by NaBH₄, it has the ability to act as an in situ SERS monitor for the reaction, and it was deduced from the detected intermediate that the reaction proceeds via a condensation reaction route.

Full text of DOI:10.1039/c6ra04247c

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: S. Chen, L. Wang,
X. Dong, X. Liu, J. Zhou, J. Yang and L. Zha, RSC Adv., 2016, DOI: 10.1039/C6RA04247C.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Please do not adjust margins
RSC Advances
Page 1 of 10
View Article Online
DOI: 10.1039/C6RA04247C
RSC Advances
PAPER
Fabrication of monodisperse Au@Ag bimetallic nanorods loaded
nanofibrous membrane with fast thermo-responsiveness and its
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
use as a smart free-standing SERS substrate
†
Siyuan Chen,a Liying Wang,a Xu Dong,b Xiaoyun Liu,c* Jianfeng Zhou,c Jianmao Yangc and Liusheng
Zhaa,c
*
A novel type of metal nanoparticles loaded smart nanofibrous membrane with fast thermo-responsiveness was fabricated
by electrospinning the aqueous solution containing poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide) and
monodisperse Au@Ag bimetallic nanorods with core-shell structure, followed by heat treatment. The results obtained by
electron microscopy show that the anisotropic nanoparticles are oriented along the axes of its constituent nanofibers. The
produced membrane has high stability in aqueous medium and remarkable thermo-responsiveness. It take less than 10
seconds to reach its deswelling or swelling equilibrium state with the temperature jumping or plunging between 25 ℃ and
50 ℃. The smart nanofibrous membrane with macroscopic mechanical strength is able to be used as a free-standing
surface enhanced Raman scattering (SERS) substrate with high Raman signal reproducibility for quantitative analysis, and its
SERS efficiency can be readily elevated by raising detection temperauture across its phase transition temperature due to its
fast thermo-response rate. Particularly, since the composite nanofibrous membrane possesses catalysis for the reduction of
4-nitrothiophenol to 4-aminothiophenol by NaBH4, it has the ability to in-situ SERS monitor the reaction, and it was deduced
from the detected intermediate that the reaction proceeds via a condensation reaction route.
stimulus responsive bulky hydrogels and microgels (including
nanogels) with nanometer to micrometer size.4,5,7 It is known
that smart bulky hydrogels have much slower stimulus-response
Introduction
In recent years, metal nanoparticles loaded stimulus responsive
composite materials have attracted more and more attention
due to their prominent characteristics, i.e., the properties of the
loaded nanoparticles can be tuned by external stimulus or the
stimulus can be sensed by detecting their properties change.1-4
Therefore, the smart composite materials have fascinating
potential applications in many fields, such as sensor, actuator,
biomedicine, catalysis, etc..1,4-6 Up to date, the smart polymer
materials used for loading metal nanoparticles have been
than smart microgels due to the estimation that the response
rate is inversely correlated to their size.8 Since fast
stimulus-response of smart materials is important to their
applications, such as tunable catalysis, sensing or detection,
actuation, drug delivery,1,9 smart microgels are preferentially
used for loading metal nanoparticles.4,5,10,11 For example, in our
group, pH and/or temperature responsive microgels have been
utilized for loading Ag or Au nanoparticles, and the produced
composite microgels were used as smart catalysts or surface
enhanced Raman scattering (SERS) substrates with
temperature-controllable activity.12-15 However, smart microgels
usually exist as colloidal particles in aqueous solution, so they
suffer from poor colloidal stability and no macroscopic
morphology and mechanical strength for use in some device like
sensor or actuator. Moreover, when used as catalysts, they are
difficult to be recycled by facile process. Hence, developing new
type of smart polymer material for loading metal nanoparticles,
which has both macroscopic mechanical strength and fast
stimulus-response, is of important practical significance.
a.State Key Laboratory for Modification of Chemical Fibers and Polymer Materials &
College of Materials Science and Engineering, Donghua University, Shanghai
201620, P. R. China. E-mail: lszha@dhu.edu.cn
b.State Key Laboratory of Molecular Engineering of Polymers and Department of
Macromolecule Science, Fudan University, Shanghai 200433, P. R. China.
c. Research Center for Analysis and Measurement, Donghua University, Shanghai
201620, P. R. China. E-mail:xyliu@dhu.edu.cn
Stimulus responsive nanofibrous membrane is the smart
polymer material which consists of orderly or randomly aligned
nanofibers of tens to hundreds of nanometers diameters.16
†Electronic
Supplementary
Informaꢀon
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2016
RSC Advances, 2016, 00, 1-3 | 1
Please do not adjust margins

RSC Advances
Page 2 of 10
View Article Online
DOI: 10.1039/C6RA04247C
Compared to smart bulky hydrogel, it has much faster
stimulus-response owing to their constituent ultra thin fibers
and porous structure helpful for the transfer of the stimulus to
the whole material. Its reported applications have demonstrated
that the smart material allows for convenient use in a device
because of their macroscopic mechanical strength and flexible
morphology.17,18 If used as the carrier for nanocatalysts, it can be
readily recovered from the reaction system19,20 The nanofibrous
membrane is usually produced by electrospinning technique,
which features simplicity, flexible morphology tuning and wide
variety in usable polymers.19,21,22 Therefore, it could be expected
that stimulus responsive electrospun nanofibrous membrane is
the substitute of smart bulky hydrogel and microgel for loading
metal nanoparticles.
has the ability to in-situ SERS monitor its catalyzed reaction,
which is beneficial to understanding the reaction mechanism.
Experimental
Materials
Hexadecyltrimethylammonium
bromide
(CTAB,
99%),
chloroauric acid trihydrate (HAuCl4·3H2O, 99.5%), sodium
borohydride (NaBH4, 95%), silver nitrate (AgNO3, 99.8%), glycine
(99.5%),
hydrochloric
acid
(HCl,
37%),
N-hydroxymethylacrylamide (NMA, 98%), 4-nitrothiophenol
(4-NTP, 99%) , 2,2’-azobis(isobutyronitrile) (AIBN, 99%) and
sodium hydroxide (NaOH, analytical grade) were all purchased
from Shanghai Chemical Reagent Co. and used as received.
L(+)-Ascorbic acid (AA, 99%) was supplied by Acros Organics.
Sodium oleate (NaOL, 97%), N-isopropylacrylamide (NIPAM,
95%) were purchased from TCI America. NIPAM was purified by
recrystallization from a mixture of toluene and hexane (60:40,
V/V). Deionized water used in the synthesis and characterization
was made using a Millipore Direct-Q system.
Within the field of metal nanoparticles-based nanoscience and
nanotechnology, the bimetallic nanoparticles with core-shell
structure have evolved increasing interest due to their superior
physicochemical, optical and catalytic properties as compared to
their respective monometallic analogues.23,24 Among them, the
bimetallic nanorod based on gold nanorod (AuNR) core and
silver shell (Au@AgNR) is particularly attractive because of its
potential applications in surface enhanced Raman scattering
(SERS) substrate, catalyst, etc..23,25,26 Recently, in our group,
monodisperse Au@AgNR prepared through seed-mediated
growth process by reduction of AgNO3 using AuNR with narrow
size and shape distribution as seed was used as SERS substrate
with high Raman signal reproducibility for quantitative
analysis.26 Its SERS efficiency is remarkably increased with its Ag
shell thickness thanks to the enhancing surface electric field and
the chemical enhancement associated with electronic ligand
effect. Furthermore, it has been reported that, compared to
monometallic Ag nanoparticles, the bimetallic Au@Ag
nanoparticles with core-shell structure has much higher
chemical stability owing to free electron transfer from the Au
core to Ag shell.27 This can avoid weakening Au@AgNRs’ SERS
efficiency during usage and storage, otherwise their oxidized Ag
surfaces hinder the charge transfer to the analyte molecule
according to chemical enhancement mechanism.28 In addition to
SERS substrates, the nanorods can be used as the nanocatalysts
with high catalytic reactivity and selectivity for some chemical
reactions owing to the synergistic catalytic effect of their
constituent gold and silver components.29,30 For example, the
results obtained by Monga et al. demonstrated that Au@AgNRs
exhibit the selective formation of 3-nitroaniline for the reduction
of 1,3-dinitrobenzene as compared to 1,3-phenylenediamine in
case of Au and Ag nanorods.31
Synthesis of monodisperse Au@AgNRs
Monodisperse AuNRs were synthesized by binary surfactant
method reported by Ye et al. 32 Then, the AuNRs were used as
seeds for the synthesis of monodisperse Au@AgNRs. 26 Briefly, to
5mL of 0.4 M glycine buffer solution at pH = 8.9 (adjusted with
1.0 M NaOH solution), 10 mL of the AuNRs solution and certain
volume of 0.01mM AgNO3 aqueous solution were added.
Subsequently, 400 µL of 0.1 M ascorbic acid aqueous solution
was added under gentle magnetic stirring. After 12 h, the
solution was centrifuged for 15 min at 6000 rpm, and the
supernatant was removed and the precipitate was redispersed in
deionized water.
Fabrication of Au@AgNRs loading thermo-responsive
composite nanofibrous membrane
Firstly,
poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide)
(PNN) was synthesized by free radical polymerization in ethanol,
as reported by Chen et al.33 The molar ratio of
N-isopropylacrylamide to N-hydroxymethylacrylamide in the
feed recipe is 5:1. Then, certain amount of purified Au@AgNRs
dispersed in 1 mL of deionized water were added into 3 mL of
PNN aqueous solution of 30 wt% concentration, followed by
vigorous stirring at room temperature for 24 h to produce the
electrospinning solution of Au@AgNRs concentration of 25 nM,
50 nM or 100 nM. Subsequently, the electrospinning solution
was loaded into a 10 mL plastic syringe and electrospun at
positive voltage of 13 kV, 15 cm working distance (distance
between the needle tip and the grounded metal plate), and 0.9
mL/h flow rate for 4 h at room temperature. Finally, the formed
nanofibrous membrane was heated at 110 ℃ for 5 h. Keeping
constant the polymer concentration in the spinning solution,
three kinds of Au@AgNRs/PNN composite nanofibrous
membranes were fabricated by changing the concentration of
Au@AgNRs (from 25 nM, 50 nM to 100 nM), which are labeled
as HNF-1, HNF-2 and HNF-3, respectively.
Herein, we report the fabrication of monodisperse Au@AgNRs
loaded thermo-responsive composite nanofibrous membrane by
electrospinning
the
aqueous
solution
containing
poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide) and
the nanorods, followed by heat treatment. The observations by
electron microscopy confirm that Au@AgNRs are oriented along
the axes of its constituent nanofibers. The produced composite
nanofibrous membrane with fast thermo-response and stable
macroscopic morphology in aqueous medium is able to be used
as
a
smart
free-standing
SERS
substrate
with
temperature-tunable efficiency. Specially, it was found that it

Page 3 of 10
RSC Advances
View Article Online
DOI: 10.1039/C6RA04247C
Fabrication and characterization of Au@AgNRs/PNN composite
nanofibrous membranes
Characterization
1 H NMR spectrum of PNN copolymer was measured by a Bruker
AV 400 nuclear magnetic resonance spectrometer based on the
residual proton resonance of deuterated dimethylsulfoxide
(DMSO-d6). Its number-average molecular weight was obtained
The fabrication route of Au@AgNRs loading thermo-responsive
composite nanofibrous membrane (Au@AgNRs/PNN composite
nanofibrous membrane) is schematically illustrated in Fig. 1(a).
Firstly, the fiber forming thermo-responsive polymer, i.e.
poly((N-isopropylacrylamide)-co-(N-hydroxymethylacrylamide))
(PNN), was firstly synthesized via azobisisobutyronitrile initiated
free radical polymerization in ethanol solution.33
by
a Brookhaven BI-MwA gel permeation chromatography
equipped with light scattering detector using HPLC-grade
tetrahydrofuran as eluent.
TEM images of Au@AgNRs/PNN composite nanofibrous
membranes were taken with a JOEL 2100F transmission electron
microscope at a voltage of 200 kV. The samples were collected
on copper grids during electrospinning, followed by heating at
110 ℃ for 5 h. Their SEM images were obtained with a Hitachi
S-4800 field emission scanning electron microscopy at a voltage
of 5 kV. The samples were sputtered with gold.
UV-vis adsorption spectra of diluted Au@AgNRs/PNN
composite nanofiber aqueous dispersion at various
temperatures were measured from a Perkin Elmer Lambda 35
UV-vis spectrometer with a temperature controller. A quartz cell
with 1 cm optical path length was used. Fourier transformation
infrared (FTIR) spectra of uncrosslinked PNN nanofibrous
membrane and Au@AgNRs/PNN composite nanofibrous
membrane were measured with a Thermo Fisher Nicolet 6700
FTIR spectrometer using an ATR attachment with a horizontal Ge
crystal.
(a)
SERS measurements were performed using
a Renishaw
inVia-Reflex micro-Raman system equipped with a multi-channel
charge-coupled device detector and a DM2500M Leica confocal
microscope upon excitation by 532, 633 or 785 nm laser source.
All the SERS spectra were the results of 10 s accumulations. In
order to assess the SERS efficiency of Au@AgNRs/PNN
composite nanofibrous membrane, it (its size: 0.7 cm×0.7 cm)
was put in the quartz sample cell, then 200 μL of 4-NTP aqueous
solution of different concentrations was injected into the cell,
followed by recording SERS spectra using Raman microscope
with 50X objective. Temperature dependent SERS spectra were
measured on a piece of membrane (HNF-3, 0.7 cm×0.7 cm) that
was immersed in 10-9 M 4-NTP aqueous solution in a Linkam
THMS600 temperature-controllable cell, which was equilibrated
thermally for at least 5 min before SERS measurement.
(b)
Fig. 1 The fabrication route of thermo-responsive Au@AgNRs/PNN composite
nanofibrous membrane (a) and its use in in-situ SERS monitoring catalytic
reaction (b)
Its NIPAM/NMA composition determined from its 1H NMR
spectrum (see Fig. S1(a) in the Electronic supplementary
information (ESI)†) is 100:20.4 (molar ratio), very close to the
feeding ratio of 100:20, and its number-average molecular
weight estimated by gel permeation chromatography using
tetrahydrofuran as eluent is 1.27×105 g/mol. Fig. S1(b) in the
ESI† shows temperature dependent absorbance (at 500 nm) of
In situ SERS monitoring catalytic reaction
To test the ability of Au@AgNRs/PNN composite nanofibrous
membrane to monitor the catalytic reaction by SERS, the
membrane (HNF-2) was immersed in 100 μL of 10-4 M 4-NTP
aqueous solution, and then 100 μL of 0.1 M NaBH4 aqueous
solution was injected into the sample cell, as schematically
illustrated in Fig. 1(b). The reaction progress was monitored by
the continuous collection of SERS signals (each scan from 300
cm-1 to 1800 cm-1 took ca. 5 min) from the membrane surface
during the reaction. The reaction temperature in the cell can be
controlled as above.
the diluted PNN aqueous solution, which exhibits
a
thermo-responsive hydrophilic-to-hydrophobic phase transition
in aqueous medium. Its lower critical solution temperature (LCST)
is 41.8 ℃ , which is remarkably higher than that of
poly(N-isopropylacrylamide) (32 ℃), due to the hydrophilicity of
its constituting NMA moiety.34 Concurrently, monodisperse
Au@AgNRs were prepared and purified by our recently reported
method.26 The TEM image of the Au@AgNRs is shown in Fig. S2
(a) in the ESI†, and we can note that they have excellent
monodispersity and well-defined core-shell structure. Their
average length and diameter obtained by randomly measuring
Results and discussion

RSC Advances
Page 4 of 10
View Article Online
DOI: 10.1039/C6RA04247C
50 particles are 81±4 nm and 25±2 nm, with the calculated
aspect ratio being 3.2, and the average silver shell thicknesses of
their lateral facets and two tips are all 2.7±0.2 nm. The
ultraviolet visible (UV-vis) spectrum of the diluted Au@AgNRs
aqueous dispersion and its optical image are shown in Fig. S2(b)
in the ESI†. According to the previous report,26 the two bands at
708 nm and 505 nm are attributed to the longitudinal and
transverse dipolar plasmon mode of Au@AgNRs, respectively,
and the band at 385 nm is ascribed to their octupolar plasmon
mode. It can also be found from Fig. S2(b) in the ESI† that the
longitudinal plasmon band of Au@AgNRs are relatively narrow,
further proving their monodispersity. In electrospinning stage,
PNN was mixed with Au@AgNRs aqueous dispersion to generate
spinning solution after stirring for 24 h. Subsequently, the
solution was electrospun at positive voltage of 13 kV, 15 cm
working distance (distance between the needle tip and the
grounded metal plate), and 0.9 mL/h flow rate at room
spinning solution, the whole solution was polarized. Further, if
the repulsive force within the charged solution was larger than
its surface tension, a jet would erupt from the tip of the
spinneret, producing the flow field. Shortly, the jet entered a
bending instability stage, and the nanofibers were formed by the
stretch and acceleration of the fluid filament in the instability
region as the solvent evaporated. During the formation of
nanofibers, the sink-flow effect and strong shear forces were
exerted on the Au@AgNRs, leading to their oriented alignment
along the flow direction, which is the axial direction of the fibers.
Even when a high concentration of the Au@AgNRs (100 nM) was
present in the spinning solution, the nanorods were all
directionally aligned, which demonstrates that electrospinning is
a powerful technique to form oriented alignment of anisotropic
nanorods on a large scale.
temperature for
4 h. Finally, the produced nanofibrous
membrane was heated at 110 ℃ for 5 h to form crosslinking
structure between the polymer chains within its constituent
fibers, as illustrated in Scheme S1 in the Supporting Information.
The optical images of Au@AgNRs/PNN composite nanofibrous
membranes and PNN nanofibrous membrane as control sample
are shown in Fig. S3 in the ESI†, and it can be seen that the
fabricated products are nonwoven membranes with the area of
over 240 cm2 and the thickness of ca. 60 μm, and their colors at
dry state change from white to light green with their Au@AgNRs
(a)
(c)
(e)
(g)
(b)
(d)
(f)
content increasing. Fig.
2 exhibits their scanning electron
microscopy (SEM) and transmission electron microscopy (TEM)
images of low and high magnification. From their SEM images
we can note that these membranes have three-dimensional
network structure consisting of lots of randomly deposited fibers,
and the fibers are homogeneous in their diameters, without
beaded morphology. Their average diameters determined by
randomly measuring 50 fibers within each membrane are
806±112 nm, 682±104 nm, 635±34 nm and 607±84 nm,
respectively, decreasing with their Au@AgNRs content
increasing. This behavior is presumably related to the increase in
solution
conductivity
concomitant
to
Au@AgNRs
incorporation.35 If observing the amplified SEM images of the
nanofibers, the bright spots on their surfaces are the Au@AgNRs
orienting along their axes, whose amounts are increased with
Au@AgNRs concentration in their spinning solution. From their
TEM images we can clearly see that the Au@AgNRs are
embedded within the fibers and directionally aligned along their
axis direction. With the increase in Au@AgNRs content within
the nanofibers, the average end-to-end distance between the
adjacent nanorods decrease from 146 nm to 89 nm to 41 nm.
The low-magnification TEM images show that the oriented
alignments are extremely uniform over long distances
throughout the whole sample. In contrast to the spinning
solution where the Au@AgNRs are randomly dispersed, we can
conclude that their oriented alignments inside the nanofibers
result from the electrospinning process.36 There are two external
fields that trigger such an alignment of the nanorods during the
electrospinning process. One is the electric field supplied by the
needle, and the other is the flow field formed during the
formation of the fibers. When a high voltage was applied to the
(h)
Fig. 2 SEM (left column) and TEM (right column) images of Au@AgNRs/PNN

Page 5 of 10
RSC Advances
View Article Online
DOI: 10.1039/C6RA04247C
composite nanofibrous membranes with different Au@AgNRs contents
(insets are their magnified images, respectively.) (from top to bottom: control
sample ((a) and (b)), HNF-1((c) and (d)), HNF-2 ((e) and (f)) and HNF-3 (g) and
(h))
immersed alternately into the water of 25 or 50 ℃ after the
interval of 2 min, and the results are shown in Fig. 3(c). It is
well-known that deswelling of poly(N-isopropylacrylamide) bulky
membrane requires a few days to reach the equilibrium state,
which is ascribed to the formation of a dense skin layer on the
gel surface hindering diffusion of the water.38 In comparison,
Au@AgNRs/PNN composite nanofibrous membranes shrunk to
deswelling equilibrium state within 2 seconds after a jump in
temperature from 25 to 50 ℃ (see the attached video). On the
other hand, they reversibly swelled up to 95% of their initial
areas within 10 seconds and reached swelling equilibrium state
after the temperature plumped down from 50 to 25 ℃. The
reversible deswelling-swelling cycle was repeated at least 3
times. With respect to fast stimulus-response rate of smart
nanofibrous membrane, it is generally thought to be associated
with very small diameters of the constituting nanofibers and
porous structure of the membrane, which facilitates the transfer
of the stimulus to the whole material.9,16 To clarify the reason of
their fast and remarkable response to temperature change, we
tried to visualize the microstructures of Au@AgNRs/PNN
composite nanofibrous membranes at swelling or deswelling
equilibrium state comparatively. For this purpose, the
equilibrium-swollen nanofibrous membrane in 25 ℃ or 50 ℃
water was rapidly transferred into liquid nitrogen for freezing
and then freeze dried for SEM observation. The rapid freezing by
liquid nitrogen should immobilize the microstructure of the
membrane in water and the sizes of its constituent nanofibers.
From the SEM images of two freeze dried samples, as shown in
Fig. 3(a), we can find that the nanofibers are highly separated
with very large mesh structure within the membrane obtained
from its equilibrium-swelling state in 25 ℃ water, whereas
they adhere with each other to form bundles of fibers, leading to
dense mesh structure within the one from its
Stability of Au@AgNRs/PNN composite nanofibrous
membranes
Obviously, the stability of Au@AgNRs/PNN composite
nanofibrous membranes in aqueous medium is important to
their application, so it is necessary to evaluate it. For this
purpose, about 40 mg of the membrane were immersed in 20
mL water and the solution was oscillated at 100 rpm for 2 h. Fig.
S4 in the ESI† shows the optical images of control sample and
Au@AgNRs/PNN composite nanofibrous membranes in water
before and after oscillation. It can be seen that, after the harsh
oscillation, control sample was slightly damaged, whereas the
composite membranes were left intact, indicating that they have
high stability in water. Hence, it could be speculated that the
interactions between Au@AgNRs and PNN polymer are present
within the composite nanofibers, which acts as physical
crosslinking points. The speculation could be confirmed by the
FTIR results, as illustrated in Fig. S5 in the ESI†, which shows the
FTIR spectra of uncrosslinked PNN nanofibrous membrane and
HNF-2. It can be found by comparing the two spectra that the
intensity of 3650 cm-1 peak assigned to the O–H stretching mode
of NMA units within PNN is significantly decreased for HNF-2,
indicating the occurrence of the crosslinking reaction between
the polymer chains after heat treatment, as shown in Scheme S1
in the Supporting Information. Moreover, it is worthwhile noting
that a small blue shift of amide I and II bands (1650 cm−1 and
1530 cm−1) attributed to the amide groups of PNN appears in the
spectrum of HNF-2, which may stem from the interactions
between the nitrogen atoms in the groups and the Ag atoms on
the loaded Au@AgNRs.12
equilibrium-deswelling state in 50
℃
water. If carefully
visualizing their amplified SEM images, it can be noted that the
nanofibers within the shrunken membrane are slightly thinner
than the ones inside the swollen membrane. Thus, we can
suppose that the shrinkage of the composite nanofibrous
membrane in 50 ℃ water should mostly arise from adhesion
between the nanofibers and formation of fibrous bundles,
leading to steep decrease of the free space inside its interior.
With regard to what make the nanofibers adhere with each
other, we think that it is the hydrophobic interaction between
Fast thermo-responsive property of Au@AgNRs/PNN
composite nanofibrous membranes
Typical optical images of Au@AgNRs/PNN composite
nanofibrous membrane immersed in 25 or 50 ℃ water are
exhibited in Fig. 3(a), and it can be seen that its area at 50 ℃ is
much smaller than the one at 25 ℃, implying that it possesses
remarkable thermo-responsive property. To identify their phase
transition temperature, temperature dependent areas of
Au@AgNRs/PNN composite nanofibrous membrane in water
were measured at a heating rate of 0.1 ℃·min-1, and the results
are depicted in Fig. 3(b). We can note that all the composite
nanofibrous membranes notably shrink with temperature
increasing from 25 ℃ to 50 ℃, and their final area shrinking
percentages are lowered from 82% to 52% with their Au@AgNRs
content rising. They all steeply shrink around 41 ℃, i.e. their
phase transition temperature, slightly lower than the LCST of
PNN. Moreover, the temperature range for the membrane area
variation is wider than that for the hydrophilic-to-hydrophobic
phase transition of PNN, presumably attributed to
inhomogeneous distribution of the crosslinking points inside its
nanofibers.37 Further investigation on their thermo-responsive
property is to assess their thermo-response rates by measuring
the time-dependent areas of the membrane membranes
the
nanofibers
after
their
polymers
undergo
phase
temperature-induced
hydrophilic-to-hydrophobic
transition. If too many Au@AgNRs are located on their surfaces,
the hydrophobic interactions will be weakened. This is why the
area shrinking percentage of Au@AgNRs/PNN composite
nanofibrous membranes is lowered with their Au@AgNRs
content rising, as depicted in Fig. 3(b).
Subsequently, it is essential to investigate the effect of
temperature change on the plasmonic property of the
Au@AgNRs embedded within the composite nanofibrous
membrane. To this end, a nanofibers aqueous dispersion was
prepared by adding small amount of the broken membrane
(HNF-2) to water, followed by sonicating the solution for 5 h, and
its UV-vis spectra in the range of 300-1100 nm at different
temperatures were obtained, as shown in Fig. 3(d). Firstly, the

RSC Advances
Page 6 of 10
View Article Online
DOI: 10.1039/C6RA04247C
longitudinal plasmon band of the Au@AgNRs embedded within
(a)
the membrane becomes broader compared to the one before
electrospinning (Fig. S1(b) in the ESI†), which may arise from
their plasmonic coupling due to their approaching inside its
constituent nanofibers. Secondly, as the temperature rises from
39 ℃ to 47 ℃, close to the temperature range of the area
change of the membrane, both their transverse and longitudinal
plasmon bands are abruptly enhanced, thanks to the refractive
index increase of the dispersed nanofibers after temperature
induced phase transition. Thirdly, their maximum wavelengths
are both red-shifted within the same temperature range, which
could be due to two reasons. One is that the collapsed polymer
network around the Au@AgNRs could increase their surrounding
refractive index.39 However, as investigated by Schmidt et al.,40
this contribution to the wavelengths red-shift should be limited
owing to slight variation of the refractive index before and after
phase transition. Another is that temperature triggering phase
transition of the composite nanofibers could reduce the
distances between the loaded Au@AgNRs. As reported by
Sánchez-Iglesias, in the gold-nanorod-pair system, side-to-side
interactions lead to a red-shift of the transverse plasmon band
(b)
and
a blue-shift of the longitudinal plasmon band, while
tip-to-tip ones would red-shift both bands.41 In our system, both
the tip-to-tip and side-to-side distances between the Au@AgNRs
should be shortened after temperature driven phase transition
of their lodged nanofibers because of their directional alignment
along the fibers axes, as above evidenced. However, it can be
deduced from the red-shift of their longitudinal plasmon band
that their tip-to-tip interactions is dominant over their
side-to-side ones, due to the possibility that the longitudinal
shrinkage percentage of the nanofibers is larger than the
transverse one.
(c)
25℃
50℃
(d)
Fig. 3 (a) Optical images of HNF-2 immersed in 25 and 50 ℃ water and
SEM imges of the membranes prepared by liquid nitrogen freezing and then
freeze drying. (b) Temperature-dependent area change of Au@AgNRs/PNN
composite nanofibrous membranes (3.6 cm×3.6 cm) in pure water measured
at
a
heating rate of 0.1 ℃·min-1. (c) Reversibility of time-dependent
deswelling and swelling of Au@AgNRs/PNN composite nanofibrous
membranes (3.6 cm×3.6 cm) immersed alternately in 25 ℃ or 50 ℃ water
after 2 min. (d) UV-visible spectra of the nanofibers aqueous dispersion at

Page 7 of 10
RSC Advances
View Article Online
DOI: 10.1039/C6RA04247C
different temperatur
used as the SERS substrate for quantitative analysis.
Use of Au@AgNRs/PNN composite nanofibrous membrane as
SERS substrate and its temperature-tunable SERS efficiency
Compared to previously reported smart composite
microgels,13,14
Au@AgNRs/PNN
composite
nanofibrous
membrane should be more suitable for the use as SERS
substrate because of its free-standing and flexible features.36
Here, its SERS efficiency was evaluated using 4-NTP as a standard
Raman active probe molecule. Fig. S6 (a) in the ESI† shows the
SERS spectra of 4-NTP aqueous solution with the same
concentration, obtained with three excitation wavelengths, i.e.
532 nm, 633 nm and 785 nm. The typical Raman peaks of 4-NTP
are reflected in the spectra, including peaks at 1080 cm-1, 1334
cm-1 and 1571 cm-1, attributed to C–S stretching, O–N=O
symmetric stretching and the phenyl ring stretching modes,
respectively.42 From Fig. S6 (a) in the ESI†, we can see that the
membrane as substrate exhibits the strongest SERS efficiency
with the excitation wavelength of 785 nm, since it is the closest
to the longitudinal plasmon resonance wavelength of its
entrapped Au@AgNRs, resulting in the strongest surface electric
field enhancement effect. Au@AgNRs/PNN composite
nanofibrous membranes with three different Au@AgNRs
contents were used as the substrates to measure the SERS
spectra of 10-7 M 4-NTP aqueous solution at 25 ℃, respectively,
and the results are given in Fig. S6 (b) in the ESI†. The plot of
their 1334 cm-1 peak intensities against the Au@AgNRs
concentrations in their spinning solutions is depicted in Fig. S6 (c)
in the ESI†. It can be viewed that their SERS efficiency is nearly
proportionally increased with the Au@AgNRs concentration,
revealing that their SERS efficiency comes from the bimetallic
nanorods. Then, the composite nanofibrous membranes were
used as the substrates to measure the SERS spectra of different
concentrations of 4-NTP solution from 10-13 M to 10-7 M at
25 ℃, respectively, and the result for HNF-2 is given in Fig. 4(a)
and the others in Fig. S7 in the ESI†. With the increase of 4-NTP
concentration, its SERS intensity is progressively raised. Even on
HNF-1 substrate, the SERS signal of 10-12 M 4-NTP can be
obviously observed (see Fig. S7(b) in the ESI†. If the membrane
HNF-3 was used as a SERS substrate, 10-13 M 4-NTP in aqueous
solution was able to be detected (Fig. S7 (e) in the ESI†). The plot
of 1334 cm-1 peak intensities in Fig. 4(a) versus the logarithmic
concentrations (log (c)) of 4-NTP is illustrated in Fig. 4(b), which
can be fitted into a line with the correlation coefficient of 0.993
(n=9). This result shows that there is a good linear relationship
between the SERS intensity and the logarithmic concentration of
the analyte, indicating that the SERS signal based on
Au@AgNRs/PNN composite nanofibrous membrane as substrate
is highly reproducible. The high SERS signal reproducibility
should arise from monodispersity and homogeneous distribution
of the Au@AgNRs within its constituent nanofibers. In addition,
fifteen times of measurements of 10-6 M 4-NTP aqueous solution
were carried out on the same membrane (HNF-2), and the
obtained SERS spectra are exhibited in Fig. 4(c). The estimated
relative standard deviation (RSD) of the intensities of their 1334
cm-1 peaks is 3.87%, further confirming high reproducibility of
the SERS signals from the membrane. These results mean that
Au@AgNRs/PNN composite nanofibrous membrane could be
(a)
(b)
(c)
Fig. 4 (a) SERS spectra of different concentrations of 4-NTP in 25℃ aqueous
solution based on HNF-2 as substrate with 785 nm laser excitation; (b) Plot of
the SERS intensity of the 1334 cm-1 peak in (a) vs. log (c) of 4-NTP; (c) SERS
spectra of 10-6 M 4-NTP acquired from 15 different spots on the membrane
HNF-2.
The effect of temperature on the SERS efficiency of
Au@AgNRs/PNN composite nanofibrous membrane was
investigated using 10-9 M 4-NTP aqueous solution as the analyte,
as illustrated in Fig. 5(a). One can observe that all peaks in its
SERS spectrum are remarkably enhanced as the temperature is
increased from 20 ℃ to 50 ℃. The plot of the 1334 cm-1 peak
intensity as a function of temperature is exhibited in Fig. 5(b). Its
marked change occurs within the temperature range from 35 ℃
to 45 ℃, in agreement with the one for the area change of the
composite nanofibrous membrane with temperature. It can be
inferred from the agreement that the temperature driven SERS
signal enhancement possibly results from the plasmonic coupling

RSC Advances
Page 8 of 10
View Article Online
DOI: 10.1039/C6RA04247C
between the approached Au@AgNRs after temperature
triggered adhesion between its constituent nanofibers, as
previously discussed.13 This result implies that the detection
sensitivity based on Au@AgNRs/PNN composite nanofibrous
membrane as SERS substrate can be improved by elevating
detection temperature. The estimation has been confirmed
through SERS detecting 10-13 M 4-NTP in 50 aqueous solution
using HNF-2 as substrate, as shown in Fig. 5(c) and (d). By
comparing the results obtained at 25 ℃ and 50 ℃, the low
detection limit of 4-NTP at the high detection temperature is
lowered one order of magnitude. Because of its fast
thermo-response rate, swelling or deswelling equilibrium of the
membrane as SERS substrate at each detection temperature can
be shortly reached, which is helpful to shortening measurement
time.
(c)
(a)
(d)
Fig. 5 (a) SERS spectra of 10-9 M 4-NTP aqueous solution using the membrane
HNF-3 as substrate at different temperatures; (b) The 1334 cm-1 peak intensity
in (a) as
a function of temperature; (c) SERS spectra of different
concentrations of 4-NTP in 50 ℃ aqueous solution based on the membrane
HNF-2 as substrate; (d) Plot of the intensity of the 1334 cm-1 peak in (c) vs. log
(c) of 4-NTP
Application of Au@AgNRs/PNN composite nanofibrous
membrane for in-situ SERS monitoring its catalyzed reaction
As reported recently,31 Au@AgNRs possess high catalytic activity
and selectivity for some reactions, so it is expected that flexible
free-standing
Au@AgNRs/PNN
composite
nanofibrous
membrane with macroscopic strength can be used for label-free
in-situ SERS monitoring its catalyzed reaction. To validate the
expectation, the reduction of 4-NTP into 4-aminothiophenol
(4-ATP) by NaBH4 in the presence of the membrane was selected
as a model reaction, as schematically illustrated in Fig. 1 (b). The
reaction can be considered as a two-step consecutive one (see
Scheme S2 in the Supporting Information), either by a direct
(b)
route
(4-thiolphenylhydroxylamine) or a condensation route through
the formation of azobenzene intermediate
via
a
hydroxylamine
intermediate
(4,4’-dimercaptoazobenzene (4,4’-DMAB )).43,44 In the direct
route, the nitro group of 4-NTP is reduced into nitroso,
hydroxylamine, and amino groups successively. In the

Page 9 of 10
RSC Advances
View Article Online
DOI: 10.1039/C6RA04247C
condensation route, an azobenzene compound is generated as
an intermediate from the condensation of a nitroso compound
and a hydroxylamine compound, and it is finally reduced into
two amino compounds. Therefore, detection of the intermediate
is crucial to making clear the reaction mechanism. Fig. 6 displays
the time dependent SERS spectra, which were recorded every 5
min interval after adding the NaBH4 solution to 10-4 M 4-NTP
solution containing the membrane (HNF-2) at 25 ℃ (from top
to bottom). In the SERS spectrum at its top, only the Raman
peaks of 4-NTP are present. After addition of NaBH4, its peaks
are gradually weakened and three new peaks at 1140, 1385 and
1427 cm-1 appear accordingly, which can be assigned to the C–N
symmetric stretching, N=N stretching, and C–H in-plane bending
modes of 4,4’-DMAB, respectively.42 Their intensities initially
increase with reaction time (from black to green spectra) and
then gradually decrease up to near-zero (from green to yellow
ones), corroborating its character as an intermediate. In addition
to the peaks of 4,4’-DMAB, a new peak at 1590 cm-1 shows up
nearly simultaneously and gradually dominates the spectrum,
which is attributed to the amino vibrational mode of the product
4-ATP. The results indicate that the reduction of 4-NTP into
4-ATP by NaBH4, catalyzed by Au@AgNRs/PNN composite
nanofibrous membrane, proceeds via a condensation route. The
mechanism can be explained by the speculation that the
transformation of 4-NTP to 4,4’-DMAB may be caused by
plasmon-induced charge transfer under the laser irradiation.45
Concretely, the reaction could be catalyzed via electron transfer
between laser irradiation induced plasmon resonance surface of
Au@AgNR and the nitro group of 4-NTP through the molecule,
because of its chemisorption on the Ag surface by its thiol group.
along the axes of its constituent fibers, so their tip-to-tip and
side-to-side interactions can be tuned by temperature, resulting
in their temperature-tunable plasmonic property. Compared to
the previously reported thermo-responsive composite materials
based on thermo-responsive bulky hydrogels or microgels, the
composite
nanofibrous
membrane
has
both
fast
thermo-response and macroscopic mechanical strength. It can
be used as a smart free-standing SERS substrate with high signal
reproducibility for quantitative analysis, and its SERS efficiency
can be increased by elevating temperature. Moreover, the
composite nanofibrous membrane has the ability to in-situ SERS
monitor its catalyzed reaction, which is beneficial to
understanding the reaction mechanism.
Acknowledgements
The authors are grateful to the National Natural Science
Foundation of China (51373030, 51503033) and the
Fundamental Research Funds for the Central Universities
(2232014D3-43). They gratefully acknowledge Prof. S. Agarwal of
Universität Bayreuth for her valuable suggestions.
Notes and references
1 M. A. C. Stuart, W. T. S. Huck, J. Genzer, M. Mueller, C. Ober, M.
Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk, M. Urban, F.
Winnik, S. Zauscher, I. Luzinov and S. Minko, Nat. Mater., 2010,
9, 101-113.
2 H. Zhang, J. S. Han and B. Yang, Adv. Funct. Mater., 2010, 20,
1533-1550.
3 X. H. Li, G. Y. Chen, L. B. Yang, Z. Jin and J. H. Liu, Adv. Funct.
Mater., 2010, 20, 2815-2824.
4 M. Molina, M. Asadian-Birjand, J. Balach, J. Bergueiro, E. Miceli
and M. Calderon, Chem. Soc. Rev., 2015, 44, 6161-6186.
5 M. Karg, Colloid. Polym. Sci., 2012, 290, 673-688.
6 I. Tokarev and S. Minko, Soft Matter, 2012, 8, 5980-5987.
7 R. Messing and A. M. Schmidt, Polym. Chem., 2011, 2, 18-32.
8 S. G. Cao, B. H. Hu and H. Q. Liu, Polym. Int., 2009, 58,
545-551.
9 S. H. Jiang, F. Y. Liu, A. Lerch, L. Ionov and S. Agarwal, Adv.
Mater., 2015, 27, 4865-4870.
10 J. Perez-Juste, I. Pastoriza-Santos and L. M. Liz-Marzan, J.
Mater. Chem. A, 2013, 1, 20-26.
11 M. Karg and T. Hellweg, J. Mater. Chem., 2009, 19, 8714-8727.
12 Y. Y. Liu, X. Y. Liu, J. M. Yang, D. L. Lin, X. Chen and L. S. Zha,
Colloid Surf. A: Physicochem. Eng. Asp., 2012, 393, 105-110.
13 X. Y. Liu, X. Q. Wang, L. S. Zha, D. L. Lin, J. M. Yang, J. F. Zhou
and L. Zhang, J. Mater. Chem. C, 2014, 2, 7326-7335.
14 X. Y. Liu, C. Zhang, J. M. Yang, D. L. Lin, L. Zhang, X. Chen and L.
S. Zha, RSC Adv., 2013, 3, 3384-3390.
Fig. 6 SERS spectra for in-situ monitoring the reduction of 4-NTP catalyzed by
15 X. Dong, X. B. Zou, X. Y. Liu, P. Lu, J. M. Yang, D. L. Lin, L. Zhang
and L. S. Zha, Colloid Surf. A: Physicochem. Eng. Asp., 2014,
452, 46-50.
the membrane HNF-2
16 C. B. Huang, S. J. Soenen, J. Rejman, B. Lucas, K. Braeckmans, J.
Demeester and S. C. De Smedt, Chem. Soc. Rev., 2011, 40,
2417-2434.
Conclusion
In summary, monodisperse Au@AgNRs were successfully loaded
17 S. Ramakrishna, K. Fujihara, W. E. Teo, T. Yong, Z. W. Ma and R.
Ramaseshan, Mater. Today, 2006, 9, 40-50.
18 I. D. Kim, A. Rothschild, B. H. Lee, D. Y. Kim, S. M. Jo and H. L.
Tuller, Nano Lett., 2006, 6, 2009-2013.
into
thermo-responsive
nanofibrous
membrane
via
electrospinning process, followed by heat treatment. The
prominent structural feature of the produced composite
nanofibrous membrane is oriented alignment of the nanorods

RSC Advances
Page 10 of 10
View Article Online
DOI: 10.1039/C6RA04247C
19 C. L. Zhang and S. H. Yu, Chem. Soc. Rev., 2014, 43, 4423-4448.
20 F. Mitschang, H. Schmalz, S. Agarwal and A. Greiner, Angew.
Chem. Int. Edit., 2014, 53, 4972-4975.
21 W. E. Teo and S. Ramakrishna, Nanotechnology, 2006, 17,
R89-R106.
22 C. Huang, S. J. Soenen, J. Rejman, B. Lucas, K. Braeckmans, J.
Demeester and S. C. De Smedt, Chem. Soc. Rev., 2011, 40,
2417-2434.
23 R. B. Jiang, H. J. Chen, L. Shao, Q. Li and J. F. Wang, Adv. Mater.,
2012, 24, OP200-OP207.
24 Z. Y. Bao, D. Y. Lei, R. B. Jiang, X. Liu, J. Y. Dai, J. F. Wang, H. L.
W. Chan and Y. H. Tsang, Nanoscale, 2014, 6, 9063-9070.
25 E. C. Cho, P. H. C. Camargo and Y. N. Xia, Adv. Mater., 2010, 22,
744-748.
26 X. Dong, J. F. Zhou, X. Y. Liu, D. L. Lin and L. S. Zha, J. Raman
Spectrosc., 2014, 45, 431-437.
27 T. N. A. Dao, P. Singh, C. Shankar, D. Mott and S. Maenosono,
Appl. Phys. Lett., 2011, 99, 073107.
28 M. Erol, Y. Han, S. K. Stanley, C. M. Stafford, H. Du and S.
Sukhishvili, J. Am. Chem. Soc., 2009, 131, 7480-7481.
29 X. Guo, Q. Zhang, Y. H. Sun, Q. Zhao and J. Yang, ACS Nano,
2012, 6, 1165-1175.
30 S. Jayabal and R. Ramaraj, Appl. Catal. A-Gen., 2014, 470,
369-375.
31 A. Monga and B. Pal, Colloid Surf. A: Physicochem. Eng. Asp.,
2015, 481, 158-166.
32 X. Ye, C. Zheng, J. Chen, Y. Gao, C. B. Murray, Nano Lett. 2013,
13, 765-771.
33 L. N. Chen, Y. C. Chiu, J. J. Hung, C. C. Kuo and W. C. Chen,
Macromol. Chem. Phys., 2014, 215, 286-294.
34 W. D. Zhang, W. Zhang, N. C. Zhou, Z. P. Cheng, J. Zhu and X. L.
Zhu, Polymer, 2008, 49, 4569-4575.
35 C. D. Saquing, J. L. Manasco and S. A. Khan, Small, 2009, 5,
944-951.
36 C. L. Zhang, K. P. Lv, H. P. Cong and S. H. Yu, Small, 2012, 8,
648-653.
37 C. Wu, S. Q. Zhou, S. C. F. Auyeung and S. H. Jiang, Angew.
Makromol. Chem., 1996, 240, 123-136.
38 R. Yoshida, K. Uchida, Y. Kaneko, K. Sakai, A. Kikuchi, Y. Sakurai
and T. Okano, Nature, 1995, 374, 240-242.
39 H. X. Xu, J. Xu, Z. Y. Zhu, H. W. Liu and S. Y. Liu,
Macromolecules, 2006, 39, 8451-8455.
40 S. Schmidt, H. Motschmann, T. Hellweg and R. von Klitzing,
Polymer, 2008, 49, 749-756.
41 A. Sanchez-Iglesias, M. Grzelczak, J. Perez-Juste and L. M.
Liz-Marzan, Angew. Chem. Int. Edit., 2010, 49, 9985-9989.
42 G. C. Zheng, L. Polavarapu, L. M. Liz-Marzan, I.
Pastoriza-Santos and J. Perez-Juste, Chem. Commun., 2015, 51,
4572-4575.
43 A. Grirrane, A. Corma and H. Garcia, Science, 2008, 322,
1661-1664.
44 A. Corma, P. Concepcion and P. Serna, Angew. Chem. Int. Edit.,
2007, 46, 7266-7269.
45 W. Xie, B. Walkenfort and S. Schluecker, J. Am. Chem. Soc.,
2013, 135, 1657-1660.