Green synthesis and: In situ immobilization of gold nanoparticles and their application for the reduction of p -nitrophenol in continuous-flow mode

Article abstract of DOI:10.1039/c8ra10373a

A green and facile method has been developed for the preparation of in situ immobilized gold nanoparticles (AuNPs) using agarose as a reducing and stabilizing agent. The size of the synthesized AuNPs ranges between 10 and 100 nm, and their average size can be controlled by the concentrations of the agarose and gold salt. The agarose matrix as a mild and green reaction medium can provide a good dispersion environment for forming AuNPs, and the hydrogel can be well homogenized with polyacrylic macroporous microbeads as well, which can adsorb and stabilize the particles leading to the simultaneous synthesis and immobilization of AuNPs avoiding harmful inorganic compounds or organic solvents. The supported gold nanocatalyst was successfully applied as a catalyst in packed bed reactors for efficient NaBH₄-mediated reduction of p-nitrophenol in continuous-flow mode.

Full text of DOI:10.1039/c8ra10373a

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Green synthesis and in situ immobilization of gold
nanoparticles and their application for the
Cite this: RSC Adv., 2019, 9, 9193
reduction of p-nitrophenol in continuous-flow
mode
ab
Rozsa Szucs, Diana Balogh-Weiser,cd Evelin Santa-Bell,c Eszter Toth-Szeles,b
}
´
´
´
´
Tamas Varga,e Zoltan Konya,
Laszlo Poppe
and Istvan Lagzi
´
ef
cg
bh
*
´
´
´
´
´
A green and facile method has been developed for the preparation of in situ immobilized gold
nanoparticles (AuNPs) using agarose as a reducing and stabilizing agent. The size of the synthesized
AuNPs ranges between 10 and 100 nm, and their average size can be controlled by the
concentrations of the agarose and gold salt. The agarose matrix as a mild and green reaction medium
can provide a good dispersion environment for forming AuNPs, and the hydrogel can be well
homogenized with polyacrylic macroporous microbeads as well, which can adsorb and stabilize the
particles leading to the simultaneous synthesis and immobilization of AuNPs avoiding harmful
inorganic compounds or organic solvents. The supported gold nanocatalyst was successfully applied
as a catalyst in packed bed reactors for efficient NaBH4-mediated reduction of p-nitrophenol in
continuous-flow mode.
Received 18th December 2018
Accepted 2nd March 2019
DOI: 10.1039/c8ra10373a
rsc.li/rsc-advances
two main methods, either based on steric hindrance, therefore
using bulky protecting groups, or based on electrostatic repul-
sion between ligands bearing the same charge.10,11 Both stabi-
1. Introduction
Nowadays, gold nanoparticles (AuNPs) are being extensively
used as efficient redox catalyst materials.1–7 There has been an
explosion in the number of reactions for which AuNPs have
shown exceptional efficacy.8 Nanoparticles (NPs) are usually
thermodynamically unstable, and hence do not possess good
long-term stability, primarily as a result of the aggregation due
to van der Waals attraction.9 Therefore, establishing and
maintaining stability is of particular importance in order to use
NPs in catalysis. The long-term stabilization can be reached by
lization methods have their own advantages and drawbacks, the
steric stabilization is oen effective and sustainable, however,
the stabilizing agents can have a negative effect on the
morphology of the formed particles or on the accessibility of the
surface, leading to a decrease in catalytic activity. Stabilizing
throughout electrostatic interaction is rather straightforward,
as it can be ne-tuned by adjusting the concentration of the
ionic species.12 The possible use of AuNPs as catalysts has
attracted vast research interest, focusing on their unique
properties that can be linked to their particle size and shape.
As an alternative to classical batch methods, organic reac-
tions carried out in continuous-ow reactors have recently
attracted much attention in synthetic organic chemistry.13
These reaction systems can offer safer, more practical,
furthermore more sustainable means for the preparation of
valuable organic compounds. In such systems, the use of
immobilized catalysts can further improve the green aspect of
the reactions.
aMTA-BME Computer Driven Chemistry, Budapest University of Technology and
´
´
Economics, H-1111, Szent Gellert ter 4, Budapest, Hungary
bDepartment of Physics, Budapest University of Technology and Economics, 1111,
´
Budafoki ut 8, Budapest, Hungary
cDepartment of Organic Chemistry and Technology, Budapest University of Technology
´
´
and Economics, H-1111, Szent Gellert ter 4, Budapest, Hungary
dDepartment of Physical Chemistry and Materials Science, Budapest University of
´
Technology and Economics, H-1111, Budafoki ut 8, Budapest, Hungary
eDepartment of Applied and Environmental Chemistry, University of Szeged, H-6720,
´
´
Rerrich Bela ter 1, Szeged, Hungary
In this work, we present a novel green and one-step method
for the synthesis and immobilization of AuNPs in the agarose
matrix (AuNPs-AR). The combination of agarose gel and solid
macroporous polymer beads (AuNPs-AR-P) provides a stable
and easy-to-handle catalyst for the reduction of organic
compounds in continuous-ow mode using packed bed
reactor. To prove the concept of the applicability, we
fMTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, H-6720, Rerrich
´
´
Bela ter 1, Szeged, Hungary
gBiocatalysis and Biotransformation Research Centre, Faculty of Chemistry and
´
Chemical Engineering, Babe¸s-Bolyai University of Cluj-Napoca, Arany Janos Str. 11,
RO-400028 Cluj-Napoca, Romania
hMTA-BME Condensed Matter Research Group, Budapest University of Technology and
´
Economics, H-1111, Budafoki ut 8, Budapest, Hungary. E-mail: istvanlagzi@gmail.
com; Fax: +36 1463 3567; Tel: +36 1463 1341
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ꢀ
mixture at 90 C for 30 min with a continuous stirring, it was
allowed to cool down to room temperature (RT) to result in
a colored hydrogel.
2.2 Characterization of synthesized gold nanoparticles
UV-Vis spectra were recorded at RT using a UV-1600PC spec-
trophotometer (VWR). All transmission electron micrographs
were acquired via a FEI TECNAI G2 20 X-Twin high-resolution
transmission electron microscope (HRTEM) operated at 200
kV accelerating voltage.
2.3 The preparation of the catalyst mixture in batch mode
An agarose–gold solution mixture (10 mL: 1.0 m/v% agarose and
1.0 mM gold salt) was prepared as described above. Aer cool-
ing to RT, 400 mg of ethylamine-functionalized macroporous
polymer resins (ReliZymeTM EA 403; polymethyl methacrylate
˚
beads, particle size 150–300 mm, pore size 400–600 A, purchased
from Resindion S.r.l. Rome, Italy) were added to AuNPs-AR as
immobilization carrier. The mixture was homogenized by
mechanical stirring, then it was centrifuged for 10 minutes at
330 RCF (RT) to recover the formed AuNPs-AR-P beads. Aer
washing with ethanol, the beads were dried at room tempera-
ture. For reduction of nitro compounds 1a–e in batch mode,
AuNPs-AR-P (5 mg) was added to the mixture of nitro compound
(1a–e; 3 mM; 10 mL, in H2O in case of 1a, b, c, e and in H2O/
ethanol 9/1 v/v in case of 1d) with NaBH4 (2.0 M in triethylene
glycol dimethyl ether, 25 fold excess) and the reaction mixture
was shaken at room temperature. Samples were analyzed by
a UV-VIS spectrophotometer measuring the spectra from l ¼
200 to 600 nm.
Fig. 1 In situ synthesis and immobilization of AuNPs in agarose
combined with macroporous polymer beads (AuNPs-AR-P) and the
application of Au-AR-P as an immobilized catalyst in reduction of 1a–e
in batch mode and in a packed bed reactor for continuous-flow
reduction of p-nitrophenol (1a).
2.4 Preparation of cartridge for continuous-ow
experiments
The AuNPs-AR-P beads described in 2.3 were packed into
stainless steel CatCart™ columns (inner diameter: 4 mm; total
length: 70 mm; packed length: 65 mm; inner volume: 0.816 mL)
according to the lling process of ThalesNano Inc. The columns
were settled by silver metal (Sterlitech Silver Membrane from
Sigma-Aldrich, Z623237, pore size: 0.45 mm; pure metallic silver,
99.97% with no extractable or detectable contaminants) and
PTFE (Whatman® Sigma-Aldrich, WHA10411311, pore size:
0.45 mm) lter membranes. The sealing units were made of
PTFE (lling weight: 150 mg) and used in a ow reactor oper-
ated in continuous-ow mode.
performed the reduction of p-nitrophenol to p-aminophenol
by sodium borohydride (NaBH4) catalyzed by AuNPs-AR-P
system (Fig. 1).
2. Experimental section
2.1 Synthesis and immobilization of gold nanoparticles
using agarose
AuNPs were synthesized using agarose (Type I Agarose, Sigma-
Aldrich) and gold(III) chloride trihydrate (Sigma-Aldrich) at
3. Results and discussion
various concentrations of both reagents in water. A solution of In several wet synthesis methods for the production of AuNPs,
agarose (4.5 mL water, 0.025 g to 0.150 g agarose, leading to 0.5 the reducing and the capping/stabilizing agents are the same
and 3.0 m/v% concentrations calculated for the 5 mL overall (e.g., citrate,14 amino acids15 and gelatin16). In our approach, we
solution) was heated to 90 ꢀC and vigorously stirred in a heating used agarose as a reducing agent, and the agarose and the
magnetic stirrer at 400 rpm. When the solution of agarose formed hydrogel matrix stabilized the formed AuNPs. In most
became transparent (aer 15 min of heating), a solution of the studies published in the literature, the agarose gel has been
gold salt (0.5 mL, with concentration ranging between 0.10 and primarily used as a matrix for the pre-prepared AuNPs, which
10.0 mM, leading to nal concentrations between 0.01 and 1.0 were synthesized by others reducing agents (trimeric alanine
mM) was added to the stirred hot solution. Aer keeping the phosphine conjugate, sodium borohydride and sodium
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citrate).17,18 Only one study utilized agarose monomers as
a reducing agent and used those AuNPs further as a template to
produce a porous nano-hybrid material.19 No detailed investi-
gation has been provided on the effect of the agarose on the
formed AuNPs, and no example has been shown to use this
hybrid hydrogel for catalytic purposes.
The effect of the concentrations of gold salt (AuHCl4 ꢁ 3H2O)
and agarose was systematically studied on the synthesis of
AuNPs. Aer the synthesis process, the samples had various
distinct colors: red, purple, or blue depending on the concen-
trations of the reagents (Fig. 2). This indicates that the generated
AuNPs have various average sizes with various polydispersity.
Fig. 3 presents this fact in a more qualitative way showing the
observed wavelength of maximum absorption peak in regard to
the concentration of the reactants. At xed agarose concentra-
tions, the wavelength of the maximum absorption peak (lmax
)
shis toward higher wavelengths with the increasing concen-
tration of the gold precursor, indicating the formation of larger
AuNPs. Due to the surface plasmon resonance (SPR) of the metal
core of the particles, the wavelength of the maximum absorption
peak in the UV-Vis spectra of AuNPs correlates with the size of the
particles. Samples containing small (ꢂ5–10 nm) and mono-
disperse (<15%) AuNPs have lmax around 520 nm (having red
color), both bigger size and more polydispersity of the AuNPs
Fig. 3 Effect of the concentrations of the agarose and the gold salt on
the wavelength of maximum absorption (lmax) of the UV-Vis spectra of
the formed agarose gels containing AuNPs (AuNPs-AR).
generate greater lmax values (Fig. 3). Higher agarose concentra-
tion provides particles with a smaller size (lower wavelength of
the maximum absorption peak). Transmission electron micros-
copy (TEM) fully supports these ndings (Fig. 4), namely the
average size of the AuNPs (and the polydispersity of the particles)
can be ne-tuned by the initial concentrations of the agarose
(reducing agent) and the gold salt.20 Our result is in good accor-
dance with a nding that higher concentration of a gold salt
provides larger particles with higher polydispersity.21–23 Interest-
ingly, AuNPs-AR are stable for several weeks aer the synthesis.
Agarose gel is macroporous (pore diameter ꢂ100–200 nm) with
large inner surface area,24 which provides sites for nucleation and
growth.25,26
These sites immobilize particles in the gel matrix, therefore
preventing them from further aggregation. In other words, this
Fig. 4 The average size of the AuNPs, formed at agarose concen-
tration of 2.0 m/v% and various concentrations of gold salt, as deter-
Fig. 2 Effect of the agarose and Au precursor concentration on the mined by transmission electron microscopy (TEM). Insets show TEM
colour of the formed and in situ immobilised AuNPs in agarose matrix micrographs and size distributions of two selected samples indicated
(AuNPs-AR).
by red and blue colours.
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technique does not require the use of external capping (stabi-
lization) agents. Even though agarose gel matrix is an effective
agent for the ne-tunable synthesis and immobilization of
AuNPs, application of this system in batch mode and in packed
bed reactors in continuous-ow mode is unmanageable due to
its poor mechanical properties and signicant swelling. Thus,
macroporous polyacrylamide beads were applied as support for
the AuNPs entrapped in agarose gel.
The Au-catalyzed reduction of aromatic nitro compounds
1a–e has high potential, while they are widely used building
blocks for the synthesis of bioactive agents. The catalytic activity
of noble metal catalysts, can be determined easily by using UV-
Vis spectroscopy.7,27,28 To demonstrate the reductive efficiency of
our immobilized Au catalyst (AuNPs-AR-P) the nitro group of
substrate 1a–e was reduced in the presence of NaBH4 (Fig. 1).
First, we examined the blank reaction, in the absence of AuNPs-
AR-P, in each cases. Since no signicant reduced product 2a–e
was observed without Au-catalysis in any case upon mixing the
1a–e and the reducing agent for 24 h, it is safe to conclude that
the conversions observed in the presence of AuNPs-AR-P cata-
lyst are due to Au-catalysis (Table 1).
Based on our results in the reduction in batch mode, the
resulted stable and easy-to-handle catalyst (AuNPs-AR-P
synthesized using 1.0 m/v% agarose and 1.0 mM gold salt)
can be used as a lling material in packed bed columns. In the
continuous-ow experiments, the AuNPs-AR-P-mediated
reduction of p-nitrophenol (1a) to p-aminophenol (2a) by
sodium borohydride (NaBH4) used in 10-fold excess under
alkaline conditions was investigated as a model reaction. In this
set of experiments, the concentrations of substrate 1a and the
ow rate were systematically changed and conversions were
analyzed (Fig. 5). Conversion was determined as an average of
ve consecutive measurements aer 5 min intervals at the
steady state. The standard deviation of conversions being under
5.0% (1.6% in average) indicated the robustness of this system.
The effluents investigated in the wavelength region character-
istic for AuNPs (l ¼ 523 nm) indicated no leaching of the
catalyst in any of the experiments. By varying the concentration
and ow rate in the range indicated in Fig. 5, conversions
between 17% and 100% could be achieved.
Fig. 5 Catalytic efficiency of AuNPs-AR-P (1.0 m/v% agarose and
1.1 mM gold salt) characterized by conversion and space time yield of
the reduction of p-nitrophenol (p-NP) in continuous-flow mode at
different substrate concentrations and flow rates.
use of the p-nitrophenol/p-aminophenol system is popular to
demonstrate the catalytic efficiency of newly formed AuNPs,
recirculation of the AuNP catalysts from batch processes –
which were investigated in most cases,29,30 – remains a chal-
lenge. By using our ow system, the catalyst is readily reusable
aer the reaction just by washing the packed bed column with
water to remove the lingering reactants
As expected, the ow rate as well as the reactant concentra-
tion had signicant impact on the conversion of the p-nitro-
phenol reduction in continuous-ow experiments. Although the
Table 1 The catalytic efficiency (c: conversion, rbatch: productivity of
the catalyst) of AuNPs-AR-P in the reduction of 1a–e in batch mode
4. Conclusions
With AuNPs-AR-P
catalyst
Without AuNPs-AR-P
In summary, we presented a facile and green one-step method
for the production of AuNPs using agarose as a reducing and
stabilizing agent. The size of the AuNPs and the polydispersity
of the samples could be controlled by the concentrations of the
initial reagents. We used macroporous polymer beads to
support the AuNPs immobilized in the agarose gel enabling Au-
catalysis in a packed bed reactor. The catalytic efficiency of this
hybrid system was demonstrated in reduction of p-nitrophenol
by sodium borohydride carried out in continuous-ow mode.
Substrate 1
ca/%
cb/%
rbatch/U ꢁ gꢃ1
a
b
c
d
e
<0.1
<0.1
<0.1
<0.1
<0.1
56.4
53.0
10.7
11.3
56.5
332
312
63
67
332
a
b
c was calculated aer reaction time 24 h. c was calculated aer
reaction time 10 min.
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Conflicts of interest
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Acknowledgements
Authors acknowledge the nancial support of the National
Research, Development and Innovation Office of Hungary
(NN125752 and K120115) and the BME-Nanotechnology FIKP
grant of EMMI (BME FIKP-NAT).
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