crystals
Article
Silver Nanoprism-Loaded Eggshell Membrane:
A Facile Platform for In Situ SERS Monitoring of
Catalytic Reactions
Yaling Li 1 , Yong Ye 1 , Yunde Fan 1 , Ji Zhou 1, *, Li Jia 1 , Bin Tang 2,3, * and Xungai Wang 2,3
1
2
3
*
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Key Laboratory for the
Synthesis and Application of Organic Functional Molecules, Ministry of Education & College of Chemistry
& Chemical Engineering, Hubei University, Wuhan 430062, China; [email protected] (Y.L.);
[email protected] (Y.Y.); [email protected] (Y.F.); [email protected] (L.J.)
National Engineering Laboratory for Advanced Yarn and Fabric Formation and Clean Production,
Wuhan Textile University, Wuhan 430073, China; [email protected]
Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia
Correspondence: [email protected] (J.Z.); [email protected] (B.T.); Tel.: +86-27-88662747;
Fax: +86-27-88663043
Academic Editor: Helmut Cölfen
Received: 27 December 2016; Accepted: 3 February 2017; Published: 18 February 2017
Abstract: We reported the fabrication of an in situ surface-enhanced Raman scattering (SERS)
monitoring platform, comprised of a porous eggshell membrane (ESM) bioscaffold loaded with
Ag nanoprism via an electrostatic self-assembly approach. The localized surface plasmon resonance
(LSPR) property of silver nanoprism leads to the blue color of the treated ESMs. UV-vis diffuse
reflectance spectroscopy, scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray
photoelectron spectroscopy (XPS) measurements were employed to observe the microstructure
and surface property of Ag nanoprisms on the ESMs. The silver nanoprism-loaded eggshell
membrane (AgNP@ESM) exhibited strong catalytic activity for the reduction of 4-nitrophenol
by sodium borohydride (NaBH4 ) and it can be easily recovered and reused for more than six
cycles. Significantly, the composites also display excellent SERS efficiency, allowing the in situ
SERS monitoring of molecular transformation in heterogeneous catalysis. The results indicate that
the AgNP@ESM biocomposite can achieve both SERS and catalytic functionalities simultaneously in
a single entity with high performance, which promotes the potential applications of ESM modified
with functional materials.
Keywords: eggshell membrane; silver nanoprism; catalysis; SERS
1. Introduction
Noble metal nanoparticles, including gold and silver nanoparticles, are currently of great
interest for their distinctive plasmonic properties and extensive research attention in near-field
related applications, such as surface-enhanced spectroscopy [1,2], biomedicine [3,4], photonics [5,6],
and biosensing [7,8]. Recently, metal nanoparticle-based catalysis has become an increasing area of
research [9,10]. Lots of catalytic systems for various reactions are being explored. In order to pursue
more efficient catalysis, gaining insights into the reaction paths and kinetics of reacting systems is still
an ongoing topic of great interest.
UV-vis absorption spectroscopy and surface-enhanced Raman scattering (SERS) spectroscopy are
the commonly used techniques for in situ investigation of metal-catalyzed reactions [11–14]. UV-vis
absorption spectroscopy can provide the reactant transformation information of the reacting systems
conveniently and facilely. SERS possesses inherent advantages of high specificity, sensitivity, and
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selectivity, which can obtain detailed fingerprint vibrational information of the reactant, and the final
product as well as the unstable transient reaction intermediate. Therefore, SERS has become a versatile
and powerful tool for real-time monitoring of metal-catalyzed reactions, even at the single-nanoparticle
level [15].
Among noble metal nanoparticles, silver nanoparticles have attracted intensive attention due to
their high SERS/catalytic performance, easy preparation and control over size and shape. Typically,
silver nanoparticles with diameters less than 10 nm possess a large surface area to volume ratio which
contributes to the strong catalytic activity. However, it is almost impossible to acquire high Raman
enhancement of adsorbed molecules on these small nanoparticles. It seems that the single type of silver
nanoparticles can hardly exhibit both excellent catalytic and SERS activity. Therefore, combining
catalytically-responsive and SERS-active functionalities into an entire platform through suitable
methods of core–shell [16], hybrid [17,18], alloy [19] or assembly [20] is the most popular strategy.
However, it is difficult to achieve a balance between high catalytic efficiency and SERS enhancement,
as the plasmonic near-field enhancement is distance-dependent and the surface coverage of catalytic
nanoparticles (NPs) may also remarkably reduce the SERS enhancement. It is well known that the shape
and size control of silver nanoparticles is significant because it determines their optical properties and
thus their related application field. Silver nanoprisms possess numerous low coordination atoms on
their edges and corners, which may provide abundant activation sites for breaking the chemical bonds
in catalytic reactions [21,22]. Meanwhile, the near infrared (NIR) LSPR bands of silver nanoprisms do
not notably overlap with the absorption bands of water and most fluorescent molecules, leading to
real-time SERS monitoring in an aqueous environment.
In addition, nanocatalyst colloid suspensions are commonly undesirable for practical applications
because of easy aggregation and difficulties in product separation and catalysts recycling. Immobilizing
metal nanoparticles on/into a porous matrix is an effective route to maintaining their activity and
stability, as well as their reusability. Many supports including filter paper [23], cellulosic fibers [24],
and polymer hydrogel [25] for the immobilization of nanoparticles have been already explored.
Recently, natural biomass materials such as eggshell membrane [26–28], plant tissue [29] and
mushroom [30] as effective catalyst supports provide “green” platforms to the catalytic reaction.
Generally, ESMs, together with eggshells, are disposed of as waste materials. Recently, ESMs have
been widely investigated because they are a rich resource with fascinating structures. Utilizations
based on ESMs have been developed in various fields, such as heavy metal ions adsorption [31],
high-performance electrode materials [32], as a separator in supercapacitor [33,34], biosensing [35] and
catalysis [26–28].
Herein, a functional ESM modified with silver nanoprisms was fabricated through a self-assembly
process. The interwoven fibrous structure of natural ESMs offers a large surface area for immobilization
of silver nanoprisms. In this biomass-based platform, integration of a sensitivity SERS activity with an
excellent catalytic property was achieved. The catalytic feature and recyclability were investigated
through studying the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) by UV-vis
spectroscopy. Furthermore, the obtained AgNP@ESM composites were then utilized for real-time
monitoring of the catalytic reaction process of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP)
by observing the fingerprint signals of the reactants and products using SERS technology.
2. Results and Discussion
2.1. Optical Properties of AgNPs
Figure 1 shows the UV-vis extinction spectrum of Ag nanoprism colloid. Three extinction bands
centered at 330, 504 and 700 nm were seen in the extinction spectrum. They are characteristic LSPR
bands of Ag nanoprisms, ascribed to out-of-plane quadrupole, in-plane quadrupole and in-plane
dipole resonance modes, respectively, consistent with the results in our previous report [36].
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Figure1.1. UV-vis
UV-vis extinction
extinction spectrum
nanoprisms
synthesized
by the
Figure
spectrumrecorded
recordedfor
forAg
Agtriangular
triangular
nanoprisms
synthesized
by the
photoinducedconversion
conversion approach.
approach.
photoinduced
Figure 1. UV-vis extinction spectrum recorded for Ag triangular nanoprisms synthesized by the
conversion approach.
2.2. photoinduced
Characterization
of AgNP@ESM Composites
2.2. Characterization of AgNP@ESM Composites
ESM, as a natural
semi-permeable
membrane, is mainly composed of interwoven and coalescing
2.2. Characterization
of AgNP@ESM
Composites
ESM, as a natural semi-permeable membrane, is mainly composed of interwoven and coalescing
nanofibers, and thus intrinsically possesses a large surface area. Thus, ESMs can be utilized as threeESM, asand
a natural
membrane,
is mainly
composed
interwoven
nanofibers,
thus semi-permeable
intrinsically possesses
a large
surface
area. of
Thus,
ESMs and
can coalescing
be utilized as
dimensional frameworks to load high density nanoparticles. In this study, PDDA was used to modify
nanofibers,
and
thus
intrinsically
possesses
a
large
surface
area.
Thus,
ESMs
can
be
utilized
threethree-dimensional
frameworks
to to
load
high density
nanoparticles.
this study,
PDDAas
was
the surface property
of ESM prior
treatment
with AgNPs
and then In
AgNPs
were assembled
onused
the to
dimensional
frameworks
to
load
high
density
nanoparticles.
In
this
study,
PDDA
was
used
to
modify
modify
the
surface
property
of
ESM
prior
to
treatment
with
AgNPs
and
then
AgNPs
were
assembled
ESM through electrostatic interaction between negatively charged AgNPs and positively charged on
the ESM
surface
property
of ESM prior
to treatment
with AgNPs
andcharged
then AgNPs
were
assembled
on charged
the
the
through
electrostatic
interaction
between
negatively
AgNPs
and
positively
PDDA modified
ESM. SEM characterization
was used
to observe
the morphologies
of ESMs before
ESM
through
electrostatic
interaction
between
negatively
charged
AgNPs
and
positively
charged
PDDA
modified
ESM.(Figure
SEM characterization
to observe
the morphologies
of ESMs
before and
and after
treatment
2). The pristine was
ESMused
exhibited
a three-dimensional
grid-like
structure,
PDDA
modified(Figure
ESM. SEM
characterization
was
used toa observe
the morphologies
of ESMs
before
after
treatment
2).
The
pristine
ESM
exhibited
three-dimensional
grid-like
structure,
with a
with a fibrous skeleton diameter ranging from 0.5 to 2.0 µm (Figure 2A,B). The hierarchically porous
and after treatment (Figure 2). The pristine ESM exhibited a three-dimensional grid-like structure,
fibrous
skeleton
diameter
ranging
from 0.5
to 2.0 µm (Figure
2A,B).
The hierarchically
structure
structure
can endow
ESM
with good
permeability,
allowing
reactants
to contact theporous
inner fibers
with a fibrous skeleton diameter ranging from 0.5 to 2.0 µm (Figure 2A,B). The hierarchically porous
can
endow ESM
good permeability,
allowing
reactants
to contact
the structure
inner fibers
sufficiently.
sufficiently.
Afterwith
the assembly
of AgNPs, the
inherent
interconnected
fibrous
of ESM
was
structure can endow ESM with good permeability, allowing reactants to contact the inner fibers
still the
preserved
(Figure
2C). As
be seen
from the SEMfibrous
imagestructure
with highofmagnification,
lots of
After
assembly
of AgNPs,
thecan
inherent
interconnected
ESM was still preserved
sufficiently. After the assembly of AgNPs, the inherent interconnected fibrous structure of ESM was
nanoparticles
immobilized
on SEM
the surface
thehigh
fibrous
structures oflots
ESMs,
dominated by
(Figure
2C). Aswere
can be
seen from the
image of
with
magnification,
of nanoparticles
were
still preserved (Figure 2C). As can be seen from the SEM image with high magnification, lots of
triangular silver
nanoplates
(Figure
2D). The
average
sizes
of dominated
AgNPs on the
ESMs weresilver
measured
as
immobilized
on
the
surface
of
the
fibrous
structures
of
ESMs,
by
triangular
nanoplates
nanoparticles were immobilized on the surface of the fibrous structures of ESMs, dominated by
58.5 ± 2D).
8.5 nm
edge length.
The
successful
assembly
of AgNPs
on the
can in
be edge
visually
(Figure
Theinnanoplates
average
sizes
of AgNPs
on average
the
ESMs
were
measured
as ESMs
58.5
±also
8.5 nm
length.
triangular
silver
(Figure
2D). The
sizes
of AgNPs
on the
ESMs
were
measured
as
witnessed
by
the
color
change
of
ESM
from
white
to
blue.
It
is
suggested
that
the
color
of
the
treated
The
of AgNPs
on the ESMs
also of
can
be visually
change
58.5 successful
± 8.5 nm inassembly
edge length.
The successful
assembly
AgNPs
on thewitnessed
ESMs alsoby
canthe
becolor
visually
ESMs
isfrom
generated
from
the ItLSPR
optical feature
of AgNPs.
The
result indicates
that
the assembly
of
ESM
white
to
blue.
is
suggested
that
the
color
of
the
treated
ESMs
is
generated
from
witnessed by the color change of ESM from white to blue. It is suggested that the color of the treated the
method based
on electrostatic
interaction
isindicates
an effective
to realize amethod
combination
of ESMs
and
LSPR
feature
of AgNPs.
result
thatroute
theThe
assembly
based
electrostatic
ESMs optical
is generated
from
the LSPRThe
optical
feature
of AgNPs.
result indicates that
theonassembly
nanoparticles.
interaction
is an
route
to realizeisa an
combination
of ESMs
and nanoparticles.
method based
oneffective
electrostatic
interaction
effective route
to realize
a combination of ESMs and
nanoparticles.
Figure 2. Scanning electron micrographs of pristine ESM (A,B) and the as-prepared AgNP@ESM
composites (C,D) at different magnifications.
Figure 2.
of of
pristine
ESM
(A,B)
andand
the the
as-prepared
AgNP@ESM
Figure
2. Scanning
Scanningelectron
electronmicrographs
micrographs
pristine
ESM
(A,B)
as-prepared
AgNP@ESM
composites
(C,D)
at
different
magnifications.
composites (C,D) at different magnifications.
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ToTo
further
observe
the
optical
properties
of of
treated
ESMs,
UV-vis
diffuse
reflectance
spectra
further
observe
the
optical
of
treated
ESMs,
UV-vis
diffuse
reflectance
spectra
were
further
observe
the
optical
properties
treated
ESMs,
UV-vis
diffuse
reflectance
spectra
were
measured
(Figure
3A).
A
small
peak
at
330
nm
and
a
broad
peak
around
724
nm
were
observed,
measured
(Figure
3A).
A
small
peak
at
330
nm
and
broad
peak
around
724
nm
were
observed,
were measured (Figure 3A). A small peak at 330 nm and a broad peak around 724 nm
observed,
attributed
to to
out-of-plane
quadrupole
and
in-plane
dipole
LSPR
bands
ofof
AgNPs.
Compared
with
attributed
out-of-plane
quadrupole
and
in-plane
AgNPs.
attributed
out-of-plane
quadrupole
and
in-plane
dipole
LSPR
bands
Compared
with
LSPR
bands
of of
AgNPs
in in
solution,
thethe
in-plane
dipole
band
of of
AgNPs
onon
ESM
red-shifted
to to
724
nm
LSPR
bands
AgNPs
solution,
in-plane
dipole
band
AgNPs
ESM
red-shifted
724
nm
from
700
nm,
which
is is
due
toto
the
changes
inin
the
NPs’
surroundings.
Also,
X-ray
diffraction
(XRD)
from
700
nm,
which
due
the
changes
the
NPs’
surroundings.
Also,
X-ray
diffraction
(XRD)
Also, X-ray diffraction (XRD)
measurement
was
performed
to to
explore
thethe
crystal
information
of of
samples.
The
XRD
of of
ESM
measurement
was
performed
toexplore
explore
the
crystal
information
of samples.
Thepatterns
XRD
patterns
of
measurement
was
performed
crystal
information
samples.
The
XRD
patterns
ESM
and
AgNP@ESM
areare
shown
in in
Figure
3B.3B.
A3B.
visible
peak
corresponding
to to
Ag
(111)
lattice
plane
ESM
and AgNP@ESM
are shown
in Figure
A visible
peak
corresponding
toAg
Ag
(111)
lattice
and
AgNP@ESM
shown
Figure
A
visible
peak
corresponding
(111)
lattice
plane
confirms
thethe
presence
of of
AgAg
(JCPDS
7440-22-4)
onon
ESM.
Moreover,
thethe
characteristic
XRD
peaks
of of
7440-22-4)
characteristic
confirms
presence
(JCPDS
7440-22-4)
ESM.
Moreover,
XRD
peaks
ESM
did
not
visibly
change
after
thethe
assembly
of of
AgNPs,
implying
that
crystal
structures
of of
ESM
structures
ESM
did
not
visibly
change
after
assembly
AgNPs,
implying
that
crystal
structures
ESM
remained
unchanged
during
treatment
with
silver
nanoprisms.
remained
unchanged
during
treatment
with
silver
nanoprisms.
Figure
3. UV-vis
diffuse
reflectance
spectra
(A)(A)
and
XRD
pattern
(B)(B)
of of
pristine
ESM
and
AgNP@ESM.
3. UV-vis
UV-vis
diffuse
pristine
AgNP@ESM.
Figure
3.
diffuse
reflectance
spectra
and
XRD
pattern
pristine
ESM
and
AgNP@ESM.
Moreover,
X-ray
photoelectron
spectroscopy
(XPS)
measurement
was
carried
out
toout
analyze
thethe
Moreover,
X-ray
photoelectron
spectroscopy
(XPS)
measurement
was
carried
out
to
analyze
Moreover,
X-ray
photoelectron
spectroscopy
(XPS)
measurement
was
carried
to analyze
surface
elements
of
ESMs
after
treatment
with
AgNPs.
As
shown
in
Figure
4,
the
survey
scan
surface
elements
of ESMs
after
treatment
with
AgNPs.
AsAs
shown
the surface
elements
of ESMs
after
treatment
with
AgNPs.
shownininFigure
Figure4,4,the
the survey
survey scan
scan
spectrum
of
AgNP@ESM
exhibits
the
presence
of
C
1s,
N
1s,
O
1s,
Ag
3p,
and
Ag
3d
core
levels
spectrum
of
AgNP@ESM
exhibits
the
presence
of
C
1s,
N
1s,
O
1s,
Ag
3p,
and
Ag
3d
core
levels
spectrum of AgNP@ESM exhibits the presence of C 1s, N 1s, O 1s, Ag 3p, and Ag 3d core levels without
without
significant
impurities.
InIna ahigh-resolution
XPS
(Figure
4B),
were
without
significant
impurities.
high-resolution
XPSspectrum
spectrum
(Figure
4B),two
twopeaks
peaks
were
significant
impurities.
In a high-resolution
XPS spectrum
(Figure
4B),
two peaks
were
observed
at
observed
at
367.9
and
373.9
eV
which
are
attributed
to
Ag
3d
5/2 5/2
and
Ag
3d
3/2 3/2
of
elemental
silver.
The
observed
at
367.9
and
373.9
eV
which
are
attributed
to
Ag
3d
and
Ag
3d
of
elemental
silver.
The
367.9 and 373.9 eV which are attributed to Ag 3d5/2 and Ag 3d3/2 of elemental silver. The splitting of
splitting
of of
thethe
3dof
doublet
AgAg
is is
6.06.0
eV,
indicating
presence
of of
metallic
Ag,
which
provides
splitting
3d
doublet
of
eV,
indicating
the
presence
metallic
Ag,
which
provides
the 3d doublet
Ag
is 6.0ofeV,
indicating
the
presencethe
of
metallic
Ag,
which
provides
solid
evidence
solid
evidence
for
the
assembly
of
AgNPs
on
the
fiber
surface
[27].
solid
evidence
forofthe
assembly
of fiber
AgNPs
on the
fiber surface [27].
for the
assembly
AgNPs
on the
surface
[27].
Figure
4. XPS
spectra
of of
AgNP@ESM
composite:
(A)(A)
survey;
(B)(B)
AgAg
3d.3d.
(A)
survey;
(B)
Ag
3d.
Figure
4. XPS
spectra
AgNP@ESM
composite:
survey;
2.3.
Monitoring
of of
thethe
Catalytic
Reaction
with
UV-vis
2.3.
Monitoring
Catalytic
Reaction
with
UV-vis
Monitoring
AgAgnanoparticles
have
often
been
to tocatalyze
thethereduction
reactions
involving
nanoparticleshave
have
often
been
used
catalyze
reductioninvolving
reactions
involving
nanoparticles
often
been
usedused
to
catalyze
the reduction
reactions
nitrophenols,
nitrophenols,
nitroanilines,
and
dyes
[37,38].
In
the
present
research,
the
interconnected
nitrophenols,
nitroanilines,
and
dyes
[37,38].
In
the
present
research,
the
interconnected
porous
nitroanilines, and dyes [37,38]. In the present research, the interconnected porous structure of porous
the
ESMs
structure
of
the
ESMs
is
beneficial
to
the
catalytic
effect
of
AgNPs.
Besides,
the
monolith
piece
of of
structure
of to
thethe
ESMs
is beneficial
the catalytic
effect
of AgNPs.piece
Besides,
the monolith
is beneficial
catalytic
effect ofto
AgNPs.
Besides,
the monolith
of functional
ESM piece
is easily
functional
system.
ESMs
a alow-cost
functional
ESMisthe
iseasily
easilyseparated
separated
fromthe
thereaction
reaction
system.
ESMsnot
notonly
onlyprovide
provide
low-cost
separatedESM
from
reaction
system.from
ESMs
not
only
provide
a low-cost
platform
for
immobilization
of
platform
for
immobilization
of
AgNPs,
but
also
facilitate
the
separation
and
reuse
of
AgNPs
after
the
platform
for
immobilization
of
AgNPs,
but
also
facilitate
the
separation
and
reuse
of
AgNPs
after
the
AgNPs, but also facilitate the separation and reuse of AgNPs after the catalytic reaction. The reduction
catalytic
The
reduction
4-NP
byby
NaBH
4 was
chosen
as
a model
reaction
evaluate
thethe
catalytic
reaction.
reduction
NaBH
4 to
was
chosen
as
a model
reaction
to
evaluate
of 4-NPreaction.
by
NaBH
was
chosenof
asof
a 4-NP
model
reaction
evaluate
the
catalytic
activitytoof
AgNP@ESM.
4The
catalytic
activity
AgNP@ESM.
Generally,
thethe
original
4-NP
solution
anan
absorbance
catalytic
activity
of
AgNP@ESM.
Generally,
original
4-NP
solution
absorbance
Generally,
the of
original
4-NP solution
displays
an
absorbance
peak displays
at displays
317 nm,
which
shiftspeak
topeak
400
at at
317
nm,
which
shifts
to
400
nm
after
adding
NaBH
4
due
to
the
formation
of
4-nitrophenolate
ions
nm,adding
which NaBH
shifts 4todue
400to
nm
after
adding of
NaBH
4 due to the formation
of 4-nitrophenolate
ions
nm317
after
the
formation
4-nitrophenolate
ions via deprotonation
[39], with
viavia
deprotonation
[39],
with
color
changes
to to
yellow-green
from
light
yellow.
The
UV-vis
absorption
deprotonation
[39],
with
color
changes
yellow-green
from
light
yellow.
The
UV-vis
absorption
spectra
of of
4-NP
solution
changed
slightly
in in
thethe
presence
of of
pristine
ESM
after
6 h
(Figure
5A),
spectra
4-NP
solution
changed
slightly
presence
pristine
ESM
after
6 h
(Figure
5A),
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color changes to yellow-green from light yellow. The UV-vis absorption spectra of 4-NP solution
changed slightly in the presence of pristine ESM after 6 h (Figure 5A), revealing that the pristine ESM
does Crystals
not show
activity for the reduction of 4-NP. However, the addition of AgNP@ESM
2017, catalytic
7, 45
5 of 11 led
to the fading of the yellow-green color of the reaction mixture of 4-NP and NaBH4 within 15 min.
revealing that
the reaction
pristine ESM
doescontaining
not show catalytic
activity
for4 the
reduction
of 4-NP.
However, was
The absorption
of the
solution
4-NP and
NaBH
in the
presence
of AgNP@ESM
the
addition
of
AgNP@ESM
led
to
the
fading
of
the
yellow-green
color
of
the
reaction
mixture
of 4monitored with respect to time using UV-vis spectroscopy, and the resultant time-dependent evolution
NP and NaBH4 within 15 min. The absorption of the reaction solution containing 4-NP and NaBH4 in
of the spectra is presented in Figure 5B. The intensity of the absorption peak at 400 nm decreased
the presence of AgNP@ESM was monitored with respect to time using UV-vis spectroscopy, and the
dramatically with reaction time, indicating the decrease of the initial reactant (4-NP). Meanwhile,
resultant time-dependent evolution of the spectra is presented in Figure 5B. The intensity of the
a new
absorption
approximately
300 nmwith
appeared
this process,
suggesting
absorption
peakpeak
at 400atnm
decreased dramatically
reactionduring
time, indicating
the decrease
of the the
generation
of
reduction
product
4-AP
[27,40].
Figure
5C
displays
the
plot
of
the
absorption
intensity
initial reactant (4-NP). Meanwhile, a new absorption peak at approximately 300 nm appeared during at
400 nm
a function
of time
to the treated
However,
thisas
process,
suggesting
thecorresponding
generation of reduction
productESMs.
4-AP [27,40].
Figurethe
5Cabsorption
displays the intensity
plot
corresponding
to AgNP@ESM
for aofcertain
time and thentodecreased
dramatically,
of the absorption
intensity remained
at 400 nm constant
as a function
time corresponding
the treated
ESMs.
However,
theAgNP@ESM
absorption intensity
corresponding
tocatalytic
AgNP@ESM
remained
constant
for aof
certain
time
implying
that the
exhibited
remarkable
activity
for the
reduction
4-nitrophenol
and
then
decreased
dramatically,
implying
that
the
AgNP@ESM
exhibited
remarkable
catalytic
after an induction time [41]. In the presence of excess NaBH4 , the reduction of 4-NP is generally treated
activity for the reduction
of reaction
4-nitrophenol
afterFigure
an induction
timethe
[41].
In of
theln(A
presence
of excess
as a pseudo-first-order
kinetic
[41,42].
5D shows
plot
t /A0 ) versus time.
NaBH4, the reduction of 4-NP is generally treated as a pseudo-first-order kinetic reaction [41,42].
At and A0 represent the absorption intensity at 400 nm at the time of t and the initial stage, respectively.
Figure 5D shows the plot of ln(At/A0) versus time. At and A0 represent the absorption intensity at 400
It was observed that the reaction over this composite catalyst was almost complete within 800 s in
nm at the time of t and the initial stage, respectively. It was observed that the reaction over this
the presence
ofcatalyst
NaBH4was
. The
linear
correlation
ln(A
time,
as seen from Figure 5D,
t /A0 ) and
composite
almost
complete
withinbetween
800 s in the
presence
of NaBH
4. The linear correlation
supports
the pseudo-first-order
assumption.
The apparent
ratethe
constant
(kapp ) of the
catalytic reactions
between
ln(At/A0) and time, as
seen from Figure
5D, supports
pseudo-first-order
assumption.
The
can beapparent
obtained
from
the linear
slope
of ln(At /A
)
versus
time.
The
k
value
of
the
reduction
rate
constant
(kapp) of
the catalytic
reactions
can
be
obtained
from
the
linear
slope
of
ln(A
treaction
/A0)
app
0
−
3
−
1
−3
−1
was estimated
toThe
be 4.90
× 10 of the
s reduction
. The kapp
valueswas
obtained
from
AgNP@ESM
compared
versus time.
kapp value
reaction
estimated
to be
4.90 × 10 s are
. The
kapp valuesto the
obtained
AgNP@ESM
are compared
to the relatedAgNPs
results in
literature Itfor
matrixrelated
resultsfrom
in literature
for various
matrix-supported
[24,26–28].
is various
believed
that good
supported
[24,26–28].
It isto
believed
that goodproperty
permeability
of ESM can contribute
good to
permeability
ofAgNPs
ESM can
contribute
good catalytic
of AgNP@ESM,
allowing to
reactants
catalytic
property
of
AgNP@ESM,
allowing
reactants
to
contact
with
the
active
surface
of
Ag
contact with the active surface of Ag nanoprism sufficiently.
nanoprism sufficiently.
Figure
5. The
(A) The
UV-vis
absorption
spectraof
of4-NP
4-NP solution
solution before
after
adding
NaBH
4 6 h with
Figure
5. (A)
UV-vis
absorption
spectra
beforeand
and
after
adding
NaBH
4 6 h with
pristine
ESM.
(B)
Evolution
of
the
UV-vis
absorption
spectra
of
4-NP
solution
with
AgNP@ESM
after after
pristine ESM. (B) Evolution of the UV-vis absorption spectra of 4-NP solution with AgNP@ESM
NaBH4 solution was added. (C) Plot of absorption intensity of 4-NP (400 nm) as a function of reaction
NaBH4 solution was added. (C) Plot of absorption intensity of 4-NP (400 nm) as a function of reaction
time corresponding to AgNP@ESM. (D) Plot of ln(At/A0) of the absorption peak at 400 nm versus time
time corresponding to AgNP@ESM. (D) Plot of ln(At /A0 ) of the absorption peak at 400 nm versus
in the presence of AgNP@ESM. The ln(At/A0) are averages of five measurements from different
time in
the presence of AgNP@ESM. The ln(A /A0 ) are error
averages
of five measurements from different
AgNP@ESMs and the error bars represent thet standard
of these measurements.
AgNP@ESMs and the error bars represent the standard error of these measurements.
Crystals 2017, 7, 45
6 of 11
Crystals 2017, 7, 45
6 of 11
In order to evaluate the reusability of the catalyst, the AgNP@ESM was recovered and reused in
repeated reduction
At/Aof0 the
versus
reaction
time for was
each
complete
was
In orderreactions
to evaluateof
the4-NP.
reusability
catalyst,
the AgNP@ESM
recovered
andconversion
reused in
reactions
4-NP.
At /A0 versusstill
reaction
time good
for each
completeactivity
conversion
wasafter six
plotted inrepeated
Figure reduction
6. It is found
thatofthe
AgNP@ESM
exhibit
catalytic
even
plotted
in Figure
6. It is found
thatthe
theAgNP@ESM
AgNP@ESM still
exhibit good catalytic
activity
even
after sixactivity
cycles. These
results
demonstrate
that
biocomposites
possess
strong
catalytic
cycles. These results demonstrate that the AgNP@ESM biocomposites possess strong catalytic activity
and good durability.
and good durability.
Figure 6. Recycling and reuse of AgNP@ESM for the reduction of 4-NP to 4-AP.
Figure 6. Recycling and reuse of AgNP@ESM for the reduction of 4-NP to 4-AP.
2.4. In Situ Monitoring of Catalytic Reaction with SERS
2.4. In Situ Monitoring of Catalytic Reaction with SERS
In addition to the catalytic properties of AgNP@ESM, we further explored its performance in
In addition
the catalytic
properties as
of aAgNP@ESM,
further
explored
itsmonitoring
performance in
SERS. Wetointended
to use AgNP@ESM
catalytic SERS we
platform
for the
real-time
chemical reactions
that occur on their
TheSERS
conversion
of 4-nitrothiophenol
(4-NTP)
into
SERS. Weofintended
to use AgNP@ESM
as asurface.
catalytic
platform
for the real-time
monitoring
of
4-aminothiophenol
(4-ATP)
was
selected
as
a
model
reaction,
in
which
the
thiol
groups
drive
the
chemical reactions that occur on their surface. The conversion of 4-nitrothiophenol (4-NTP) into 4chemisorption of the molecule on the surface of AgNPs through a strong Ag–S bond (see Figure 7A).
aminothiophenol (4-ATP) was selected as a model reaction, in which the thiol groups drive the
Firstly, we put the treated ESMs into the solution of 4-NTP and left them for a certain period of
chemisorption
the molecule
on themonolayer
surface of
a strong
Ag–S
bond (see
7A).
time toof
allow
a self-assembled
of AgNPs
4-NTP tothrough
adsorb on
the surface
of AgNPs,
thenFigure
the
Firstly, we
put the was
treated
the solution
of 4-NTPwere
andwashed
left them
a certain
period
membrane
takenESMs
out andinto
the residual
4-NTP molecules
out. for
Finally,
the reduction
of of time
into 4-ATP was
followed by
SERS
after dropping
a NaBH4of
solution
on the
membrane.
to allow a4-NTP
self-assembled
monolayer
ofreal-time
4-NTP to
adsorb
on the surface
AgNPs,
then
the membrane
Figure
7B
shows
the
time-dependent
SERS
spectra
of
the
4-NTP-functionalized
AgNP@ESM,
which
was taken out and the residual 4-NTP molecules were washed out. Finally, the reduction
of 4-NTP
was recorded continuously for 270 s. The SERS spectra of 4-NTP exhibit four main characteristic peaks
into 4-ATP was followed by real-time
SERS after dropping a NaBH4 solution on the membrane.
at 853, 1079, 1331 and 1568 cm–1 , attributed to C-H wagging, C-S stretching, O-N-O stretching, and
Figure 7Bphenyl
showsring
thestretching
time-dependent
SERS spectra
of the
thereaction
4-NTP-functionalized
AgNP@ESM,
modes, respectively
[19]. As
progresses, new vibration
Raman which
was recorded
continuously
for
270
s. The30SERS
of 44-NTP
four
main characteristic
bands of
an intermediate
were
observed
s later spectra
upon NaBH
addition,exhibit
which can
be assigned
to the
0 -dimercaptoazobenzene
0 -DMAB) at 1138, 1384, and 1428 cm–1 , ascribed to
–1
spectral
features
of
4,4
(4,4
peaks at 853, 1079, 1331 and 1568 cm , attributed to C-H wagging, C-S stretching, O-N-O stretching,
C-N
symmetric
stretching,
N=N
stretching and
CHAs
in-plane
bendingprogresses,
modes, respectively
[16,19,43].Raman
and phenyl
ring
stretching
modes,
respectively
[19].
the reaction
new vibration
Meanwhile, the strong peaks at 853, 1331 and 1568 cm–1 , which correspond to R-NO2 of 4-NTP,
bands of an intermediate were observed 30 s later upon NaBH4 addition, which can be assigned to
gradually decrease. After 120 s, the intensity of the 4-NTP and 4,4’-DMAB bands decreased significantly
–1, ascribed
the spectral
of 4,4′-dimercaptoazobenzene
(4,4′-DMAB)
atof
1138,
1384,
and
cmthat
andfeatures
a new band
at 1590 cm−1 emerged, confirming
the formation
4-ATP.
Thus,
we1428
propose
to C-N symmetric
N=N
stretching
andbut
CH
in-plane
bending
modes,
respectively
[16,19,43].
4,4’-DMABstretching,
is generated in
the catalytic
reaction
was
immediately
reduced
to 4-ATP
by the reducing
–1
agent
NaBH
a sequential
hydride
of 4-NTP
to 4,4’-DMAB
finally
to 4-NTP,
Meanwhile,
the
strong
peaksthere
at is
853,
1331 and
1568reduction
cm , which
correspond
toand
R-NO
2 of
4 . In short,
4-ATP.
During
the
reaction
time,
it
should
be
noted
that
the
slight
decrease
of
the
SERS
signals
may
be
gradually decrease. After 120 s, the intensity of the 4-NTP and 4,4’-DMAB bands decreased significantly
due to the loss of the catalysts, but the vibrational SERS bands produced by the interfacial reaction can
and a new band at 1590 cm−1 emerged, confirming the formation of 4-ATP. Thus, we propose that 4,4’still be obtained. It should be re-emphasized that the AgNP@ESM can achieve both SERS and catalytic
DMAB isfunctionalities
generated ininthe
catalytic
was immediately reduced to 4-ATP by the reducing
a single
entity reaction
with high but
performance.
agent NaBH4. In short, there is a sequential hydride reduction of 4-NTP to 4,4’-DMAB and finally to
4-ATP. During the reaction time, it should be noted that the slight decrease of the SERS signals may
be due to the loss of the catalysts, but the vibrational SERS bands produced by the interfacial reaction
can still be obtained. It should be re-emphasized that the AgNP@ESM can achieve both SERS and
catalytic functionalities in a single entity with high performance.
Crystals
Crystals2017,
2017,7,7,4545
77ofof1111
Figure
Figure7.7.(A)
(A)Schematic
Schematicillustration
illustrationofof4-NTP
4-NTPconversion
conversiontoto4-ATP
4-ATPon
onthe
theAgNP@ESM
AgNP@ESMsurface
surfaceand
and
probing
of
the
progress
by
SERS.
(B)
Time-dependent
SERS
spectra
of
4-NTP
conversion.
probing of the progress by SERS. (B) Time-dependent SERS spectra of 4-NTP conversion.
3.3.Materials
Materialsand
andMethods
Methods
3.1.
3.1.Materials
Materials
Silver
nitrate (AgNO
(AgNO
, >99%), trisodium citrate (Na C6 H2O,
≥99.0%),
sodium
5 O≥99.0%),
7 ·2H2 O, sodium
Silver nitrate
3, 3>99%), trisodium citrate (Na3C6H5O37·2H
borohydride
borohydride
(NaBH
,
>98%),
4-nitrophenol
(4-NP,
99%),
and
4-nitrothiophenol
(4-NTP,
99%)
4
(NaBH4, >98%), 4-nitrophenol
(4-NP, 99%), and 4-nitrothiophenol (4-NTP, 99%) were purchasedwere
from
purchased
from
Aladdin
Reagent
Company
(Shanghai,
China).
Poly(diallyldimethylammonium
Aladdin Reagent Company (Shanghai, China). Poly(diallyldimethylammonium chloride) (PDDA, 20
chloride)
(PDDA,
20 wt from
%) was
purchased from
were and
of analytic
grade,
wt %) was
purchased
Sigma-Aldrich.
AllSigma-Aldrich.
chemicals wereAll
of chemicals
analytic grade,
used without
and
used
without
further
purification.
Deionized
water
was
obtained
from
Hangzhou
Wahaha
Co.
further purification. Deionized water was obtained from Hangzhou Wahaha Co.
3.2. Characterization
3.2. Characterization
Scanning electron microscopy (SEM) measurements were performed with a Hitachi S-4800 field
Scanning electron microscopy (SEM) measurements were performed with a Hitachi S-4800 field
emission SEM (Hitachi, Chiyoda, Japan). X-ray diffraction (XRD) patterns were recorded using
emission SEM (Hitachi, Chiyoda, Japan). X-ray diffraction (XRD) patterns were recorded using a
a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation
Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (2θ =
(2θ = 10◦ –90◦ ). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos
10°–90°). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos
XSAM800 XPS system (Thermo Fisher, Waltham, MA, USA) with a Kα source and a charge neutralizer.
XSAM800 XPS system (Thermo Fisher, Waltham, MA, USA) with a Kα source and a charge
The ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) of the as-prepared AgNP@ESM
neutralizer. The ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) of the as-prepared
composites were obtained with an Ocean Optics USB4000 Spectrometer (Ocean Optics, Dunedin,
AgNP@ESM composites were obtained with an Ocean Optics USB4000 Spectrometer (Ocean Optics,
FL, USA) and recorded with a reflection and backscattering probe. The UV-vis absorption spectra were
Dunedin, FL, USA) and recorded with a reflection and backscattering probe. The UV-vis absorption
obtained with an Ocean Optics USB4000 Spectrometer and recorded using Ocean Optics SpectraSuite
spectra were obtained with an Ocean Optics USB4000 Spectrometer and recorded using Ocean Optics
software. The SERS analysis was performed on a Renishaw inVia Raman microscope system (Renishaw
SpectraSuite software. The SERS analysis was performed on a Renishaw inVia Raman microscope
plc, Wotton-under-Edge, UK). A 50 × /N.A. 0.75 objective and a 785 nm high power diode laser
system (Renishaw plc, Wotton-under-Edge, UK). A 50 × /N.A. 0.75 objective and a 785 nm high power
diode laser excitation source (>280 mW) were used in all measurements. The spectra within a Raman
Crystals 2017, 7, 45
8 of 11
excitation source (>280 mW) were used in all measurements. The spectra within a Raman shift window
between 800 and 1800 cm−1 were recorded using a mounted CCD camera with an integration time of
3 s through single scan.
3.3. Synthesis of Silver Nanoparticles
The silver nanoprisms were synthesized according to a photoinduced method. Briefly, an aqueous
solution of AgNO3 (0.1 mM, 100 mL) and trisodium citrate (100 mM, 1 mL) was mixed and vigorously
stirred under ambient condition. NaBH4 solution (8 mM, 1 mL) was then added dropwise into the
mixing solutions. Yellow seed solutions were obtained and then irradiated under a sodium lamp
(NAV-T 70 model from Osram China Lighting Co., Ltd.) for about 12 h. Finally, blue silver colloids
were produced and kept in the dark at room temperature.
3.4. Fabrication of AgNP@ESM
Fresh eggs were obtained from a local supermarket. Raw eggshells were cleaned carefully with
deionized water and broken gently. The inner yolk was removed and the ESM was carefully stripped
from the raw egg and cleaned with deionized water. The clean white semipermeable ESMs were
dried in air at ambient conditions, cut into small pieces (3 × 3 cm2 ), and immersed in PDDA aqueous
solution (2 wt %) for 2 h. Subsequently, the PDDA-ESMs were rinsed with abundant deionized water
and then immersed in the silver nanoprism colloid. The weight ratio of colloid to ESM was 400:1.
The solutions with ESMs were shaken for 3 h at 40 ◦ C in an oscillating water bath. The ESM turned to
blue from white, due to the assembly of Ag nanoprisms. Then, the AgNP@ESMs were rinsed with
running deionized water and placed in glass Petri dishes and left for 24 h at room temperature in dark
for drying.
3.5. Catalysis and SERS
To investigate the catalytic efficiency and reusability of the as-prepared AgNP@ESM, the catalytic
reduction of 4-NP by NaBH4 was performed according to our previously reported procedure.
In a typical experiment, 2.0 mL of 4-NP aqueous solution (1.0 × 10−5 M) was put into a quartz
cuvette with a path length of 1 cm. An amount of 3 mg of the AgNP@ESM was added to the 4-NP
solution. Subsequently, 50 µL of freshly prepared NaBH4 solution (0.6 M) was added to the mixed
solution of 4-NP and AgNP@ESM under stirring. Meanwhile, the UV-vis absorption spectra were
recorded. The parameters of the UV-vis absorption spectra were set as follows: integration time, 8 ms;
scans to average, 10; boxcar width, 10; and interval, 8 s. The same composite membrane was used as a
catalyst for at least six cycles.
To achieve in situ SERS monitoring, the conversion of 4-nitrothiophenol (4-NTP) into
4-aminothiophenol (4-ATP) was selected as another model reaction. The AgNP@ESM was immersed
in 1.0 × 10−5 M 4-NTP ethanol solution for 1 h, and the 4-NTP molecules were arranged on silver
surface as a result of surface crowding and strong Ag–S binding. Then, the membrane with 4-NTP
was took out, rinsed with ethanol and placed on a clean slide. An amount of 3 µL of NaBH4 solution
(0.3 M) was then dropped onto the composite membrane to initiate the reaction. The whole process was
monitored by SERS spectroscopy, and each curve was recorded with an interval of 30 s. We collected
the SERS spectra at different time points at an excitation of 785 nm and 1% of the laser output power.
4. Conclusions
In summary, we reported the preparation of a facile platform for the in situ SERS monitoring of
catalytic reactions. In this low-cost platform, AgNPs were immobilized onto the eggshell membranes
via electrostatic interaction. The pristine ESMs with an interwoven fibrous structure not only offer
a large surface area for immobilization of AgNPs, but also feature good permeability that allows
reactants to contact with the active surface of Ag nanoprism sufficiently. Moreover, the as-prepared
AgNP@ESM also facilitates the separation and reuse of AgNPs catalysts. The treated ESMs exhibited
Crystals 2017, 7, 45
9 of 11
good catalytic activity and high reusability for the reduction of 4-NP. Furthermore, the application of
the in situ SERS monitoring of the catalytic reduction of 4-NTP has been successfully carried out.
Acknowledgments: This research was supported by the National Natural Science Foundation of China
(No. 51403162, 51273153) and the Educational Commission of Hubei Province of China (No. T201101).
We also acknowledge support from the MoE Innovation Team Project in Biological Fibers Advanced Textile
Processing and Clean Production (No. IRT13086).
Author Contributions: Ji Zhou and Bin Tang conceived and designed the experiments; Yaling Li and Yunde Fan
performed the experiments; Yaling Li and Yong Ye analyzed the data; Li Jia contributed analysis tools; Yaling Li,
Ji Zhou, Bin Tang and Xungai Wang wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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