Polyphenol-grafted collagen fiber as reductant and stabilizer for one-step synthesis of size-controlled gold nanoparticles and their catalytic application to 4-nitrophenol reduction

Article abstract of DOI:10.1039/c0gc00843e

A facile method for one-step synthesis of size-controlled gold nanoparticles (AuNPs) supported on collagen fiber (CF) at room temperature was proposed. Epigallocatechin-3-gallate (EGCG), a typical plant polyphenol, was grafted onto CF surface to serve as reducing/stabilizing agent, so that the AuNPs were generated on CF surface without introduction of extra chemical reagents or physical treatments. The prepared AuNPs were fully characterized, and the results showed that the dispersed AuNPs were successfully produced and the mean particle size of AuNPs could be effectively controlled in range of 18 to 5 nm simply by varying the grafting degree of EGCG on CF surface. These stabilized AuNPs were found to be active heterogeneous catalysts for the reduction of 4-nitrophenol to 4-aminophenol in aqueous phase. The catalytic behaviors of AuNPs depended on the particle size and the grafting degree of EGCG. A distinct advantage of these catalysts is that they can be easily recovered and reused at least twenty times, because of the high stability of the AuNPs supported by EGCG-grafted CF. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00843e

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PAPER
Polyphenol-grafted collagen fiber as reductant and stabilizer for one-step
synthesis of size-controlled gold nanoparticles and their catalytic
application to 4-nitrophenol reduction†
Hao Wu,a Xin Huang,b Mingming Gao,b Xuepin Liao*b and Bi Shi*a,b
Received 23rd November 2010, Accepted 20th December 2010
DOI: 10.1039/c0gc00843e
A facile method for one-step synthesis of size-controlled gold nanoparticles (AuNPs) supported
on collagen fiber (CF) at room temperature was proposed. Epigallocatechin-3-gallate (EGCG), a
typical plant polyphenol, was grafted onto CF surface to serve as reducing/stabilizing agent, so
that the AuNPs were generated on CF surface without introduction of extra chemical reagents or
physical treatments. The prepared AuNPs were fully characterized, and the results showed that the
dispersed AuNPs were successfully produced and the mean particle size of AuNPs could be
effectively controlled in range of 18 to 5 nm simply by varying the grafting degree of EGCG on
CF surface. These stabilized AuNPs were found to be active heterogeneous catalysts for the
reduction of 4-nitrophenol to 4-aminophenol in aqueous phase. The catalytic behaviors of AuNPs
depended on the particle size and the grafting degree of EGCG. A distinct advantage of these
catalysts is that they can be easily recovered and reused at least twenty times, because of the high
stability of the AuNPs supported by EGCG-grafted CF.
from failure in obtaining highly dispersed and stable gold
Introduction
nanoparticle catalysts. For example, agglomeration of AuNPs
Gold was long regarded as chemically inert and catalytically
supported by SiO2 occurs easily because the interaction between
inactive due to its completely filled d-band. However, since
Haruta’s discovery that supported gold nanoparticles (AuNPs)
are exceptionally active for low temperature oxidation of CO,1
the interest of exploring gold-based catalysts has been sub-
stantially booming. Highly dispersed AuNPs have been found
to be very active for a number of chemical reactions such
as oxidation2 and reduction.3 Although a high dispersion of
AuNPs is basically important to present high catalytic activity,
the associated tendency of AuNPs to aggregate would lower
their catalytic activity and reuse life-time. Therefore, how to
design and prepare AuNPs with long-term dispersion stability
and high catalytic efficiency is a primary challenge. During
the past few decades, solid inorganic materials like activated
carbon (AC)4 and metal oxides5 have been widely used as
supports to protect AuNPs against aggregating and facilitate
their recovery. However, use of these supports often suffers
gold and SiO2 is inherently weak.5c Conventional deposition-
precipitation method is unlikely to produce highly dispersed
AuNPs supported by AC, due to the acidic nature of AC.4b
Organic polymers have been recently recognized as a new class
of supports for stabilizing AuNPs because in addition to stabi-
lizing and protecting these particles, polymers can offer unique
possibilities for modifying both the environment around AuNPs
and access to the catalytic sites.6 Polyvinyl alcohol (PVA),6b,6c
polyvinyl pyrrolidone (PVP)2a,6d and polystyrene (PS)6e are the
commonly used polymeric supports in the synthesis of size-
controlled AuNPs, in which the stabilized AuNPs displayed
distinct catalytic activities. Ishida’s work showed that the
catalytic activity for oxidation of glucose over AuNPs deposited
on ion-exchange resins was more influenced by the nature of
the polymer supports than the size of the AuNPs.2c However,
the synthesis steps of these polymer supported AuNPs often
involve addition of chemical reducing agents such as sodium
borohydride, sodium citrate, hydrazine, or organic solvents
like N,N-dimethylformamide (DMF) and glycol. All these
chemicals are highly reactive and have potential environmental
and biological risks, which could be a problem for large-scale
production of AuNPs and their subsequent applications.
aNational Engineering Laboratory for Clean Technology of Leather
Manufacture, Sichuan University, Chengdu, P. R. China.
E-mail: sibitannin@vip.163.com
bDepartment of Biomass Chemistry and Engineering, Sichuan University,
Chengdu, P. R. China. E-mail: xpliao@scu.edu.cn;
Fax: +86-28-85400356; Tel: +86-28-85400382
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c0gc00843e
As an alternative to these hazardous chemicals, a variety
of naturally-occurring macromolecules have attracted growing
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interest in the synthesis of gold and silver nanoparticles over the
last decade.7 Huang et al. first reported the use of chitosan for
reduction of Au3+ ions and stabilization of AuNPs without any
additional reducing and stabilizing agents.7a Cellulose has been
utilized as both reducing agent and stabilizer for the controlled
“green” synthesis of gold and silver nanoparticles.7d–7f Singh
et al. reported one-step in situ generation of AuNPs on spider-
silk fiber which is accomplished by a simple reaction of the silk
with aqueous chloroauric acid.7g These hydroxyl- and amine-
containing biomacromolecules have shown potential as support
with dual roles of reductant and stabilizer for AuNP synthesis,
in which additional reducing agent is not needed. Among the
biomacromolecules, collagen that has been widely studied in
biomedical fields and tissue engineering is recognized to be an
ideal candidate for synthesis of AuNPs.
Collagen, coming from skin, tendon, and other tissues of
animals, is one of the most abundant renewable biomass in
nature. A wealth of merits of collagen such as biological origin,
specific molecular structure, excellent biocompatibility and
biodegradability make the fabrication of collagen-stabilized
AuNPs more fascinating. In recent years, there has been
several literatures referring to collagen-mediated AuNPs
assembly and synthesis,8 but use of collagen for the reduction
of Au3+ to Au0 is often time-consuming, and almost all
the present research still required an introduction of extra
chemical reductants (sodium borohydride and citrate) or
physical approaches (heat treatment and UV irradiation) in
order to achieve rapid and efficient reduction. In addition,
although collagen itself contains some stabilizing groups for
AuNPs, our previous research showed that collagen-stabilized
AuNPs as heterogeneous catalysts lacked sufficient stability
for reuse in practical catalysis applications.8c The same
problem is sometimes faced by other biomacromolecule-
stabilized AuNPs used in catalytic reactions, e.g.
chitosan.7b
Quite recently, we found that grafting of epigallocatechin-3-
gallate (EGCG) onto the surface of collagen fiber (CF) could
significantly improve the dispersion and stabilization of Pd0
nanoparticles on CF support,9 which prompted us to investigate
the capability of the modified collagen serving as an efficient sup-
port for preparing stable AuNPs. We describe herein a one-step,
reductant-free, and size-controlled synthesis of stable AuNPs
by using EGCG-grafted CF (EGCG-CF) as the support that
possesses efficient reducing/stabilizing ability for the formation
AuNPs. EGCG, a natural plant polyphenol extracted from tea,
contains a large number of phenolic hydroxyls which endows
it with the capability of reducing Au3+ to Au0.10 Moreover, due
to the high affinity of phenolic hydroxyls with AuNPs, EGCG
may provide AuNPs robust shielding to prevent them from
aggregation. The synthesis processes were carried out in aqueous
solution at room temperature without any extra reagents or
treatments, which is compatible with green chemistry principles.
The main physical and chemical properties of the prepared
AuNPs were fully characterized. The reduction of 4-nitrophenol
(4-NP) to 4-aminophenol (4-AP) by NaBH4 in aqueous phase
was chosen as a model reaction to investigate the catalytic
behaviors of the AuNPs. The effects of EGCG-grafting degree
on the particle size and catalytic activity of AuNPs were studied,
and the reuse stability of AuNPs was also investigated.
Experimental
Chemicals
Collagen fiber was prepared from cattle skin according to our
previous work.11 Epigallocatechin-3-gallate (99%) was provided
by the Department of Tea Science, Sichuan Agricultural Univer-
sity. HAuCl4 (99%) was purchased from Sigma-Aldrich. Sodium
borohydride (NaBH4), glutaraldehyde (50%), 4-nitrophenol
(99%) and other chemicals were all analytical reagents.
Preparation of EGCG-grafted CF (EGCG-CF)
The procedure of preparing EGCG-grafted CF was the same
as that in our previous work.9 In brief, 0.5 g of EGCG was
dissolved in 100 mL of distilled water and then mixed with 5 g
of CF. The mixture was stirred at 298 K for 2 h. Then, 50 mL of
glutaraldehyde solution (2.0 wt%) at pH 6.5 was added into the
mixture and stirred at 318 K for 6 h. Subsequently, the product
was filtrated, fully washed with distilled water and dried in
vacuum at 308 K, and then EGCG-grafted CF (EGCG-CF) was
obtained. The concentrations of EGCG in the reaction solution
before and after grafting reaction were determined by High-
performance Liquid Chromatography (HPLC, Agilent 1100),12
and the actual amount of EGCG grafted on CF was determined
by the mass balance calculation.
Preparation of AuNPs supported by EGCG-CF (Au-EGCG-CF)
In a typical preparation of Au-EGCG-CF, 1 g of EGCG-CF
was suspended in 100 mL of 0.5 ¥ 10-3 M HAuCl4 aqueous
solution. The mixture was stirred at room temperature for 6 h,
and then filtrated, fully washed with distilled water and dried in
vacuum at 308 K. Finally, a wine red product of Au-EGCG-
CF was obtained. As control, Au-CF was also prepared in
the same procedure as described above, where Au was directly
loaded onto CF. To ascertain the actual loading amount of
Au, the catalyst was digested to obtain Au solution. Then,
the Au amount on catalyst was determined by Inductively
Coupled Plasma Atomic Emission Spectrometry (ICP-AES,
Perkin-Elmer Optima 2100DV, Germany).
Characterization
Ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) were
recorded by means of UV-vis-NIR spectrophotometer (UV-
3600, Shimadzu, Japan) equipped with an integrating sphere
and using BaSO4 as reference. X-Ray diffraction (XRD, Philips
X’ Pert Pro-MPD) studies were performed by using Cu-Ka X-
radiation (l = 0.154 nm). The morphology of Au-EGCG-CF
was observed by Scanning Electron Microscopy (SEM, JEOL
LTD JSM-5900LV). The size and distribution of AuNPs on
EGCG-CF were determined using Field Emission Transmission
Electron Microscopy (FE-TEM, 200kV, Tecnai G2 F20, FEI,
Netherlands) equipped with an energy-dispersive X-ray analysis
(EDAX) attachment. Fourier Transform-infrared Spectroscopy
(FT-IR, Perkin-Elmer, USA) analyses were carried out by using
compressed films of KBr pellets and sample powders. X-Ray
Photoelectron Spectroscopy (XPS, Kratos XSAM-800, UK)
analyses were conducted by employing Mg-Ka X-radiation (hv =
1253.6 eV) and a pass energy of 31.5 eV. All of the binding energy
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peaks of XPS spectra were calibrated by placing the principal
C 1s binding energy peak at 284.8 eV. Peaks from all the high-
resolution core spectra were fitted with XPSPEAK 4.1 software,
using mixed Gaussian-Lorentzian functions.
supramolecular cages provide steric hindrance to the AuNPs
formed, and subsequently prevent individual particles from
coagulating.13a,15
The typical preparation procedure of Au-EGCG0.1-CF (sub-
script number stands for initial mass ratio of EGCG to CF)
is illustrated in Fig. 1. When the synthesized EGCG0.1-CF was
added into the aqueous solution of HAuCl4, Au3+ ions were
chelated by the orthophenolic hydroxyls of EGCG and then
were spontaneously reduced to Au0 in situ at room temperature
without any other chemical reducing agents. The generation of
AuNPs on EGCG0.1-CF matrix can be visually witnessed by the
color change of EGCG0.1-CF from pale brown to wine red.
Catalytic reduction of 4-nitrophenol
In a typical run, 1 mL of 4-NP aqueous solution (1 mM) was
mixed with 25 mL of distilled water containing a certain amount
of catalyst (Au: 1.0 ¥ 10-3 mmol). The suspension was then
purged with N2 for 30 min to remove the dissolved O2. Freshly
prepared 4 mL of NaBH4 aqueous solution (0.33 M) was then
added to start the reduction reaction. The solution mixture was
stirred during the reaction. A sample of 1.5 mL was withdrawn at
a regular interval and measured by UV-vis spectrophotometer in
the range of 200–600 nm. The reduction process was monitored
through measuring the change of absorbance at 400 nm as a
function of time.
Results and discussion
Preparation of AuNPs supported by EGCG-CF
In the present study, EGCG can be easily grafted onto CF
surface via a Mannich reaction.9,13b As shown in Scheme 1,
EGCG is covalently bonded with amino groups of collagen
molecules by the crosslinking of glutaraldehyde, which results in
an increase of anchoring sites on CF surface for gold precursors.
As a typical plant polyphenol, EGCG consists of multiple
orthophenolic hydroxyls, and it has been proven to be excellent
bidentate ligand to bond with Au3+ ions by forming a stable
five-member chelating ring.13 Due to the high redox potential
of Au3+, the chelated Au3+ ions could be reduced into Au0
atoms in situ, while a part of phenolic hydroxyls of EGCG
are simultaneously oxidized to corresponding carbonyl groups,
quinone.10c,13a,14 According to Tripathy’s research,14a both the
formed carbonyls and free hydroxyls are able to stabilize AuNPs
by the interaction between the surface Au atoms of AuNPs
and oxygen atoms of EGCG molecules. More importantly,
high density of hydroxyl groups in EGCG could lead to
extensive inter- and intramolecular hydrogen bonding, which
favors formation of unique supramolecular assemblies.15b These
Fig. 1 Preparation of gold nanoparticles supported on EGCG-grafted
CF. The inset shows the UV-vis DRS spectra of CF, Au-CF, EGCG0.1-CF
and Au-EGCG0.1-CF.
UV-vis DRS measurement was performed to further confirm
the formation of AuNPs. The inset in Fig. 1 presents the UV-vis
DRS spectra of CF, Au-CF, EGCG0.1-CF and Au-EGCG0.1-CF.
Compared with the spectrum of EGCG0.1-CF, the UV-vis DRS
spectrum of Au-EGCG0.1-CF displays an intense absorption
band with the maximum at 530 nm, which is attributed to
the characteristic surface plasmon resonance (SPR) of metallic
gold nanoparticles.2d The SPR in metal nanoparticles arises
from the collective oscillation of the free conduction band
electrons induced by incident electromagnetic radiation, and
it is sensitive to particle size, shape, distribution, surrounding
medium, etc. Moreover, the time-dependent variation of the
UV-vis DRS spectrum for the reduction of Au3+ is presented
in Fig. S1 (see the ESI† for details), in which the intensity
of SPR band at 530 nm increases systematically with the
increase of reaction time. About 6 h later, the intensity reaches a
maximum and almost keeps constant. This observation indicates
the formation of AuNPs on EGCG-CF. Furthermore, as can
be seen in Fig. 1, the unmodified CF only has two relatively
weak absorption peaks, located at 210 nm and 280 nm, mainly
ascribed to the polypeptide chains and the benzene rings in side
chains, respectively. However, after grafting reaction, EGCG-
CF exhibits enhanced absorbance in the range of 200–800
nm, which is consistent with the color change of CF from
white to pale brown. For comparison, CF was directly used
Scheme 1 Schematic plot illustrating the formation and stabilization
of AuNPs with EGCG-grafted CF as support.
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as support for preparing gold catalyst (Au-CF) without adding
any reductant, and its UV-vis DRS spectrum is also presented
in Fig. 1. Clearly, Au-CF does not show any adsorption peak
around 530 nm, indicating that almost no metallic Au particles
are generated in Au-CF. The color of CF after reaction with
-
AuCl4 changed from white to faint yellow (image not shown),
implying that the gold loaded on CF are mainly in the form of
ionic state. This observation demonstrates that CF itself has
no ability to reduce Au3+ to form Au0 nanoparticles under
present preparation conditions, even though it can combine
with Au3+ through its plenty of functional groups, such as –
COOH and –NH2, in side chains. Additionally, as reported in
our previous work,9 the grafting degree of EGCG on CF can
be easily tunable depending on the initial mass ratio of EGCG
to CF. As summarized in Table S1 (see ESI†), a series of Au-
EGCGx-CF (x = 0.01, 0.05, 0.1, 0.3, 0.5, and 1.0) with varying
grafting degree of EGCG on CF were prepared in this study.
It is found that the actual grafting quantity of EGCG on CF
increases with the increase of initial amount of EGCG, which
simultaneously enhances the interaction of CF with Au ions.
Fig. 2 TEM images and the corresponding particle size distribution of
Au-EGCGx-CF with various EGCG/CF initial mass ratio: x = (a) 0.01,
(b) 0.1, (c) 0.3 and (d) 1.0. The bar 100 nm.
Characterization of Au-EGCG-CF
Fig. S2 shows the typical X-ray diffraction patterns of EGCG0.1-
CF, Au-EGCG0.1-CF, and Au-CF (see ESI†). The XRD patterns
of all these samples exhibit a broad signal around 23◦, which is
attributed to the amorphous polymer phase of collagen fiber. As
expected, we can note the absence of crystal diffraction peaks in
the EGCG0.1-CF. In contrast, the XRD pattern of Au-EGCG0.1-
CF shows four diffraction peaks at 2q = 38.1, 44.2, 64.6, and
77.5◦ corresponding to the (111), (200), (220), and (311) crystal
planes, respectively, which are the characteristic of face-centered
cubic (fcc) gold (JCPDS-4748). As for Au-CF, however, no ob-
vious crystal diffraction peaks are observed in its XRD pattern,
implying no gold nanocrystals were formed in Au-CF, which is
consistent with the result of UV-vis DRS characterization. Fig.
S3 presents the SEM images of Au-EGCG0.1-CF (see ESI†), in
which the sample is in a highly ordered fibrous state, indicating
that the inherent fibrous morphology of natural collagen fiber is
still preserved well.
Fig. 3 HRTEM images of (a) Au-EGCG0.01-CF, (b) Au-EGCG0.1-CF
and (d) Au-EGCG0.01-CF after twenty cycles in reduction reaction. (c)
EDAX pattern of Au-EGCG0.1-CF. The inset in Fig. 3a shows the FFT
image of an individual Au nanoparticle.
The morphology and size of AuNPs in Au-EGCGx-CF (x =
0.01, 0.05, 0.1, 0.3, 0.5 and 1.0) were further determined
by TEM observation, and the results of size distribution are
summarized in Table S1 (see ESI†). It is noted that the particle
size distribution of AuNPs supported on EGCG-CF shows a
remarkable dependency on the grafting degree of EGCG on
CF. The representative TEM images of Au-EGCGx-CF (x =
0.01, 0.1, 0.3, and 1.0) and their corresponding histogram of
particle size distribution are presented in Fig. 2. As shown in
Fig. 2a, the AuNPs with nearly spherical shape are presented
in Au-EGCG0.01-CF, and their mean diameter is estimated to be
18.6 7.4 nm. The high-resolution TEM micrograph (HRTEM)
of an individual gold nanoparticle in Au-EGCG0.01-CF and its
fast Fourier transform (FFT) image (inset) are presented in Fig.
3a. The multiple lattice fringes with an interplanar spacing of
0.238 nm were observed, which is consistent with the interplanar
distance of gold (111) planes.16 Evidently, the size distribution
of AuNPs on EGCG0.01-CF is in a wide range, which suggests
that a relatively low grafting degree of EGCG on CF is not
enough to efficiently control the dispersion of AuNPs. The
AuNPs with small size and narrow distribution can be obtained
if the initial mass ratio of EGCG to CF was increased from
0.01 to 1.0, as shown in Fig. 2b–2d. The HRTEM observation
of Au-EGCG0.1-CF in Fig. 3b also reveals that the small and
well-crystallized AuNPs were produced. The EDAX analysis
(Fig. 3c) confirms that the nanoparticles observed in the TEM
images consist of pure gold atoms. In addition, no AuNPs were
observed in Au-CF by TEM (image not shown), which is in
agreement with the results of UV-vis and XRD analyses. All
these observations suggest that the grafting degree of EGCG on
CF plays an important role in the formation of AuNPs as well
as their distribution.
Accordingly, we propose a kinetic-controlled process for the
formation of AuNPs stabilized by EGCG-CF. At a low dosage
of EGCG, as in Au-EGCG0.01-CF, the auto-reduction rate of
Au3+ is relatively faster than the coating speed of AuNPs with
EGCG molecules, which easily leads to the assembly and growth
of the reduced Au0 species into large-sized AuNPs. However,
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as increasing dosage of EGCG from 0.01 to 1.0, the hydrogen
bond interactions among EGCG molecules are strengthened,
which consequently construct dense supermolecular cages. In
these cages, the coordination affinities of EGCG to Au ions are
improved and therefore, the coating speed is increased to that
being comparable to the reduction of Au3+. The strong coating
effect could sterically prevent or slow individual particles from
coalescing with each other, and thus give a small size and narrow
distribution of AuNPs.15
be attributed to the oxidation of phenolic hydroxyls and the
formation of quinones.17e It should be noted that a new peak
arising at a relatively lower binding energy of 530.6 eV implies the
presence of the Au→O electron charge transfer, which indicates
the nature of the interaction between the AuNPs and EGCG to
stabilize AuNPs on CF.14a,19e
Fig. 5 presents the FT-IR spectra of CF, EGCG0.1-CF and
Au-EGCG0.1-CF. In the IR spectrum of CF (Fig. 5a), the
amide I adsorption band around 1660 cm-1 arises predominantly
from protein amide C O stretching vibration, while amide
II adsorption band at 1548 cm-1 arises from amide N–H
bending vibration and C–N stretching vibration. The amide
III adsorption band at 1238 cm-1 consists of the components
from C–N stretching vibration and N–H in-plane bending
vibration from amide linkages.18a For EGCG0.1-CF (Fig. 5b), the
adsorption band around 3385 cm-1 appears to be broadened,
mainly due to the strong hydrogen bond interaction between
the phenolic hydroxyls of EGCG and the amino/amide groups
of CF.18b The appearance of a new adsorption peak at 1120
cm-1 is ascribed to the C–O–H stretching vibration of phenolic
hydroxyls in EGCG, while the two strengthened adsorption
peaks at 1045 and 1337 cm-1 belong to the C–O–C stretching
vibration and O–H in-plane bending vibration, respectively.18c
As for Au-EGCG0.1-CF (Fig. 5c), however, the adsorption band
around 3385 cm-1 appears to be narrowed and shifted to long
wavelength probably due to the partial destruction of hydrogen
bonds between EGCG and CF, which implies the involvement
of the O–H groups in the reduction of gold.9,14d In addition,
the damping of the peak at 1120 cm-1 further indicates that the
The chemical state of gold in Au-EGCG-CF was determined
by XPS analysis, and the typical XPS spectra are presented in
Fig. 4. As seen in Fig. 4a, the survey scan spectrum of Au-
EGCG0.1-CF shows the presence of C 1s, N 1s, O 1s, Au 4d, and
Au 4f core-levels with no evidence of impurities. It is worthwhile
to mention that almost no chlorine ions were detected in the Au-
EGCG-CF, because of the absence of Cl 2p peak at the 197 eV
binding energy region, as shown in the inset of Fig. 4a. This fact
suggests that the Cl- ions were completely washed out and no
-
AuCl4 precursor was reserved on the surface of AuNPs.17 Fig.
4b shows the Au 4f core-level spectra of Au-EGCG0.1-CF. The
Au 4f core-level is curve-fitted with two pairs of doublets from
spin-orbital splitting of 4f7/2 and 4f5/2. According to Tripathy’s
research,14a the most intense doublets observed at 84.0 and 87.8
eV arise from the inner Au atoms of AuNPs, being consistent
with zero valent Au0, while the second set of doublets located at
85.3 and 89.1 eV originate from the outer surface Au atoms
bonded with EGCG molecules. These surface Au atoms of
AuNPs serve as the main interaction sites with EGCG-CF to
stabilize AuNPs. Negishi and Tanaka have also pointed out that
the Au 4f peaks located at relatively higher binding energy should
be attributed to the surface Au atoms of AuNPs bonded to
surface surrounding stabilizer or passive molecules, suggesting
that a substantial electron donation from AuNPs to stabilizer
molecules is present.17a,17b
Fig. 4 XPS spectra of Au-EGCG0.1-CF. (a) Survey scan spectrum. (b)
Au 4f core-level spectrum. The inset in Fig. 4a shows Cl 2p core-level
spectrum.
Fig. 5 FT-IR spectra of (a) CF, (b) EGCG0.1-CF and (c) Au-EGCG0.1
CF.
-
Moreover, the O 1s XPS spectra of CF, EGCG0.1-CF and Au-
EGCG0.1-CF are also presented in Fig. S4 (see ESI†). There is
only one peak at 531.6 eV in the O 1s signal of CF, which is
mainly attributed to O C–N in peptidic carbonly groups (Fig.
S4a†).17c However, as for EGCG0.1-CF, the O 1s signal peak
changes, where a new peak with a higher intensity clearly appears
at 532.8 eV (Fig. S4b†), which is assigned to the oxygen atoms
in phenolic hydroxyl (HO–C).17d This fact indicates that EGCG
was successfully grafted onto CF. After reaction with Au3+,
the O 1s peak corresponding to the HO–C group dramatically
decreases in intensity, while the O 1s signal around 531.8 eV
is greatly strengthened (Fig. S4c†). This observation should
Fig. 6 (a) Successive UV-vis adsorption spectra taken after adding
NaBH4 into 4-NP solution in the presence of Au-EGCG0.1-CF as
catalyst. (b) Plot of ln(C4-NP) versus time corresponding to the reduction
of 4-NP catalyzed by Au-EGCG0.1-CF at 298 K.
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reduction of gold ions could be coupled to the oxidation of
phenolic hydroxyls of EGCG.14d
Catalytic properties of Au-EGCG-CF
To study the catalytic characteristics of Au-EGCG-CF, the
reduction of 4-nitrophenol to 4-aminophenol by sodium boro-
hydride in aqueous phase was chosen as a model reaction. This
reaction has been extensively used as a benchmark system to
evaluate the catalytic activity of noble metal nanoparticles.3d,19
Metal nanoparticles can effectively catalyze the reduction of
nitro compounds by acting as an electronic relay system,
Fig. 7 (a) Effect of reaction temperature on reduction rate. (b)
Arrhenius plots of ln(k) versus 1/T in the temperature range of 298–
329 K.
-
wherein the electron transfer takes place from donor BH4 to
acceptor nitro groups. As shown in Fig. 6a, the adsorption
peak of 4-NP was red-shifted from 317 to 400 nm immediately
upon the addition of NaBH4 solution, which corresponds to
a color change of 4-NP solution from light yellow to yellow-
green due to the formation of 4-nitrophenolate ion in alkaline
condition. In absence of catalyst, the adsorption peak at 400
nm remained unaltered even for a couple of days. In contrast,
addition and mixture of a proper amount of Au-EGCG0.1-
CF into the solution caused a decolorization of the yellow-
green 4-nitrophenolate solution (inset of Fig. 6a), while the
adsorption peak height at 400 nm successively decreased with
a concomitant appearance of two new adsorption peaks at 235
and 295 nm due to the formation of 4-AP. Moreover, two points
are visible in the UV-vis spectra, where all the spectra intersect
each other, indicating that the catalytic reduction of 4-NP to
4-AP proceeded without formation of by-products.19a The blank
experiment using EGCG0.1-CF did not show any change in color
or adsorption peak of 4-nitrophenoloate for more than 24 h,
clearly demonstrating that the reduction of 4-NP by NaBH4 is
solely activated by AuNPs stabilized on EGCG-CF.
As the initial concentration of NaBH4 largely exceeds that
of 4-NP, the reduction rate can be assumed to be independent
of NaBH4. Therefore, the pseudo-first-order rate kinetics with
respect to the 4-NP concentration could be used to evaluate
the catalytic rate.19 The reaction kinetics can be described as
-ln(Ct/C0) = kt, where k is the rate constant at a given temper-
ature and t is the reaction time. C0 and Ct are the concentration
of 4-NP at beginning and at time t, respectively. As expected, a
good liner correlation of lnCt versus reaction time t was obtained
(shown in Fig. 6b), whereby a kinetic rate constant k was
estimated to be 14.16 ¥ 10-2 min-1. Furthermore, Fig. 7a shows
that the rate of the reduction reaction is increased when reaction
temperature rises from 298 to 328 K. According to the principle
of the Arrhenius equation, the activation energy (Ea) of the
reduction of 4-NP catalyzed by Au-EGCG0.1-CF is determined
by plotting lnk vs 1/T (Fig. 7b). The Ea obtained is 37.3 1.8
kJ mol-1. This value is relatively small, and is comparable to
those of other nanoparticle catalysts in the reduction of 4-NP
such as Au/poly(methyl methacrylate) (38 kJ mol-1),19b Au/ion-
exchange resin (31 kJ mol-1),19a Au/polyelectrolyte brushes (43
kJ mol-1).19c
Fig. 8 Reduction of 4-NP catalyzed by Au-CF and Au-EGCG-CF with
varying grafting degree of EGCG.
attributed to the size effect of AuNPs in catalysts. As confirmed
by TEM above, the gold particle size in Au-EGCG0.1-CF is much
smaller than that in Au-EGCG0.01-CF, which would significantly
promote the accessibility of reactants to catalyst active centers
and thus enhance the catalytic reduction rate. However, when
the particle size of gold further decreased from 18.6 7.4 nm in
Au-EGCG0.1-CF to 5.2 1.6 nm in Au-EGCG1.0-CF, a tendency
of decreasing reduction rate was observed, as shown in Fig. 8.
In general, a smaller size of metal particle implies that a larger
fraction of metal atoms are exposed at surfaces and available for
catalysis. The unusual phenomenon in our experiment should
be ascribed to the fact that a high grafting degree of EGCG
may introduce a high density of phenolic hydroxyls around
Au nanoparticle, which can sterically hinder the diffusion of
reactant toward the active sites of catalyst and eventually the
catalytic activity is depressed.9 Accordingly, it is remarkable that
the grafting degree of EGCG on CF surface is an important
factor in controlling the formation of AuNPs and affecting their
catalytic activity.
An attention should be paid to the fact that a delay time
at the beginning of the catalytic reduction was found in the
cases of Au-EGCG0.01-CF, Au-EGCG0.3-CF, Au-EGCG0.5-CF,
and Au-EGCG1.0-CF. This is in accordance with other studies
of this reduction reaction catalyzed by metal nanoparticles.19
According to the literature, two factors including gold particle
size and support (EGCG-CF) structure should be responsible
for the appearance of the induction period in this study.
In the case of Au-EGCG0.01-CF, since the size of AuNPs is
Fig. 8 presents the dependency of reduction rate on the
grafting degree of EGCG on CF. It was found that Au-EGCG0.01
-
CF exhibited a relatively lower catalytic activity, whereas the
rate of the reduction reaction was greatly increased by using Au-
EGCG0.1-CF. This increased reduction rate should be mainly
656 | Green Chem., 2011, 13, 651–658
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relatively larger, and they are mainly composed of low-index
and high-coordinative saturated surface atoms related with
lower surface roughness,19a which depresses the chemisorption
of 4-nitrophenolate ions and thereby hampers the reaction.
Panigrahi et al. has also reported that the induction period
decreased and subsequently vanished with decreasing particle
size of AuNPs.19a As for Au-EGCG0.3-CF, Au-EGCG0.5-CF
and Au-EGCG1.0-CF, however, the induction period may be
resulted from the diffusion process of reactant on the catalysts.
When the grafting degree of EGCG on CF was increased to
some extent, the interactions among EGCG molecules were
largely strengthened, which constructs dense supermolecular
cages around Au0 nanoparticles. As a result, the reactants and
reduced products will take time to diffuse into and out of these
cages, once they reach and leave the catalyst active centers.
Kuroda et al. has concluded in detail the dependency of the
induction period in 4-NP reduction on the type and structure of
various polymer supports.19b
Additionally, one phenomenon beyond our expectation
should be noted in Fig. 8, in which a maximum catalytic
reduction rate was displayed by Au-CF as compared with that
of other six catalysts despite the absence of gold nanocrystals
in Au-CF proved by TEM above. To uncover the underlying
reason accountable for this unexpected result, an additional
experiment was employed. We performed a filtration to remove
the Au-CF catalyst from the 4-NP solution when the reduction
reaction proceeded for 180 s. Following that, the filtrated
solution was continuatively stirred and monitored by UV-vis
spectrophotometer at a regular interval, and the spectrum is
presented in Fig. S5a (see ESI†). It is clearly seen that although
the solid Au-CF catalyst was removed from the reaction system,
the reduction of 4-NP was still proceeded and completed within
1500 s, as indicated by the decrease of adsorption peak height
at 400 nm with time. This phenomenon implies that Au-CF
suffered a leaching of active Au species during the initial 180 s,
which was further confirmed by the fact that a certain amount of
Au species remained in the filtrated solution detected by element
analysis (ICP-AES). Due to the relatively weak stabilizing action
of CF, some unstable Au ions loaded on CF could be leached
into the reaction solution and further reduced into Au0 in the
presence of NaBH4. Accordingly, catalytic reduction of the 4-
NP solution containing these naked active Au0 species can be
considered as a quasi-homogeneous catalysis system, in which
the reduction of nitrophenol could be extremely promoted.
For comparison, a similar filtration experiment in the case
of Au-EGCG0.1-CF was also carried out. As shown in Fig.
S5b†, the reaction almost stopped as soon as the Au-EGCG0.1-
CF was removed, indicating the heterogeneous nature of the
reduction of 4-NP catalyzed by AuNPs supported by EGCG-
CF. Such a difference in catalytic behaviors between naked Au0
and supported Au0 species was also reported by Xiong and co-
workers in their research.19g
catalyst Au-EGCG0.1-CF was recovered from the reaction
mixture simply by filtration and washing with distilled water,
and reused under the same conditions as for the initial cycle. As
shown in Fig. 9, Au-EGCG0.1-CF can be successfully recycled
and reused in twenty successive reactions with a conversion of
>98%. The catalyst was digested and analyzed by ICP-AES
after twenty cycles. Compared to the Au amount on the initial
catalyst (1.0 ¥ 10-3 mmol), no significant Au loss on the used
catalyst (0.98 ¥ 10-3 mmol) was determined. Further TEM
observation confirms that almost no aggregation of AuNPs
occured in the used catalyst (Fig. 3d), indicating the high stability
of AuNPs supported by EGCG-CF. For comparison, the results
of recycling uses of Au-CF are also presented in Fig. 9. Au-CF
suffered a sharp decrease in catalytic activity after twenty cycles,
due to the leaching of Au species from Au-CF, proved by the
fact that a Au loss of 69% was detected by ICP-AES analysis. It
should be noted that the catalytic activity of Au-CF decreased
step by step after its first cycle, which could be attributed to the
fact that collagen itself possesses certain ability of stabilizing
Au because of its functional groups. Such recycling behavior of
Au-CF was quite similar to that of Pd0 nanoparticles supported
by CF in our earlier report.9
Fig. 9 Conversion of 4-nitrophenol in twenty successive uses of Au-
EGCG0.1-CF and Au-CF as catalysts.
Conclusions
In conclusion, EGCG-grafted CF can be used as a support for
preparing stable AuNPs. The EGCG acts as both reductant and
stabilizer in the process of preparation, so that no additional
reducing agents or treatments are needed. This process is
performed in aqueous solution and is a green approach. The
particle size and distribution of AuNPs can be easily controlled
by tuning the grafting degree of EGCG on CF surface. These
supported AuNPs exhibited a good catalytic activity and high
reusability for the reduction of 4-nitrophenol in aqueous phase.
This preparation strategy of AuNPs appears very simple, green
and cost-effective, and exhibits great potential for practical
applications. Further works to explore the application of the
AuNPs system in other fields are currently ongoing in our
laboratory.
It is known that reusability is the main advantage of using
heterogeneous catalyst rather than homogeneous catalyst for
industrial application. Although several catalytic studies of 4-
NP reduction using AuNP catalysts have been reported in the
literature, there exist only a few reports where the AuNP catalysts
were successfully recovered for consecutive reuses.3d,3e,19b,19e,19h
To check the reusability of Au-EGCG-CF catalyst, the solid
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Acknowledgements
We acknowledge the financial supports provided by the Key
Program of National Science Foundation of China (20536030),
National Natural Science Foundation of China (20776090) and
A Foundation for the Author of National Excellent Doctor
Dissertation of P. R. China (FANEDD200762). We also give
thanks to Test Center of Sichuan University for the help of the
TEM (Dr Ming Liu) and XRD tests.
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©