CuAu bimetallic plasmonic-enhanced catalysts supported on alginate biohydrogels

Carbohydrate Polymers 297 (2022) 120021

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

CuAu bimetallic plasmonic-enhanced catalysts supported on
alginate biohydrogels
Oscar Ramírez a, b, c, Sebastian Bonardd b, c, C´esar Saldías a, Yadira Zambrano d, David
Díaz Díaz b, c, e, *, Angel Leiva a, *
a

Departamento de Química Física, Facultad de Química y Farmacia, Pontificia Universidad Cat´
olica de Chile, Macul, 7820436 Santiago, Chile
Departamento de Química Org´
anica, Universidad de la Laguna, Avda. Astrofísico Francisco S´
anchez 3, La Laguna, 38206 Tenerife, Spain
Instituto Universitario de Bio-Org´
anica Antonio Gonz´
alez, Universidad de la Laguna, Avda. Astrofísico Francisco S´
anchez 2, La Laguna, 38206 Tenerife, Spain
d
Departamento de Ingeniería Química y Bioprocesos, Facultad de Ingeniería, Pontificia Universidad Cat´
olica de Chile, Macul, 6904411 Santiago, Chile
e
Institut fỹr Organische Chemie, Universită
at Regensburg, Universită
atsstr. 31, 93053 Regensburg, Germany
b
c

A R T I C L E I N F O

A B S T R A C T

Keywords:
Bio-based hydrogel
Bimetallic nanoparticles
Surface plasmon resonance
Plasmonic catalysis

We describe the synthesis, characterization and catalytic properties of a series of hybrid materials composed of
inorganic plasmonic mono- and bimetallic nanoparticles supported on organic bio-based hydrogel beads. The
bimetallic materials showed a localized surface plasmon resonance in the visible region, with a maximum light
absorption correlated to the metal composition of the alloyed systems. Thermogravimetric analysis revealed a
total water content near to 90 % w/w, which was in good agreement with the free-volume calculated from μCT
scan reconstruction of lyophilized samples. Catalytic essays for the reduction of 4-nitrophenol demonstrated that
alginate beads loaded with bimetallic nanoparticles exhibit a 5.4-fold higher apparent kinetic constant (kapp)
than its monometallic counterparts. Additionally, taking advantage of the plasmonic properties given by the
nanoparticles is that the materials were tested as photocatalysts. The activity of the catalysts was enhanced by
near 2.2 times higher in comparison with its performance in dark conditions.

1. Introduction
Catalytic activity plays a crucial role in diverse aspects, such as the
conversion rate and selectivity of many chemical transformations.
Thereby, it has a high impact in many industrial processes including
petrochemistry, pharmaceuticals, environmental remediation and
hydrogen generation (Rodrigues et al., 2019). Consequently, a consid­
erable research activity has been focused on developing more efficient
catalytic materials with minimal energy consumption, waste generation

and able to perform chemical reactions under mild conditions (room
temperature and ambient pressure) with an environmentally responsible
perspective (Anastas & Eghbali, 2010).
In this context, nanosized materials are suitable candidates for se­
lective and efficient catalysis since they can be incorporated in diverse
types of chemical reactions4. Furthermore, nanomaterials as catalysts,
especially acting in heterogeneous supports, offers a high surface area,
tunable optical properties, high versatility and performance in varied
chemical environments, among other attributes. Specifically, unique
optical properties of hybrid nanomaterials can be harnessed for

photocatalytic applications. TiO2-based materials are an excellent
example of photocatalytic materials to achieve the degradation of
several contaminants from water sources and also to accomplish
hydrogen and oxygen evolution by water splitting reactions (Bagheri
et al., 2014; Carlucci et al., 2019; Chen et al., 2012; Dal Santo & Naldoni,
2018; Dey & Mehta, 2020; Oi et al., 2016). However, one of the main
drawbacks of these materials is its poor light absorption, derived by its
wide band gap, limiting the light absorption to the UV interval, which
corresponds to approximately 5 % of the solar light that reaches the
earth's surface. For this reason, the development of photocatalysts
having outstanding conditions to absorb and exhibit significant catalytic
activity under visible light radiation is highly desirable (Ismail & Bah­
nemann, 2014; Marcelino & Amorim, 2019).
Considering the above, metallic nanomaterials based on Au, Ag and
Cu enable catalysts to achieve this goal, mainly due to the optical
properties that arise by the quantum-confinement of these metals into
the nanometric scale. Specifically, the localized surface plasmon reso­
nance (LSPR) displayed by those metals is of great importance, phe­
nomenon that consists in the collective oscillation of conduction

* Corresponding authors.
E-mail addresses: (D.D. Díaz), (A. Leiva).
/>Received 10 July 2022; Received in revised form 17 August 2022; Accepted 19 August 2022
Available online 25 August 2022
0144-8617/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( />

O. Ramírez et al.

Carbohydrate Polymers 297 (2022) 120021

electrons induced by the oscillating electric field component of an
electromagnetic radiation (Kim et al., 2019). Consequently, nano­
materials presenting this property are recognized as plasmonic nano­
structures (Araujo et al., 2019) and are studied as appropriated materials
to carry out light-driven chemical reactions processes as plasmonic
catalytic systems (Ezendam et al., 2022). Additionally, well-controlled
optical properties of nanoparticles can be achieved by varying both
the size and shape of these nanomaterials, as well as the composition of
the nanomaterial (Huang & El-Sayed, 2010). This last can be achieved
by obtaining bimetallic nanostructures, which can render unique optical
properties and an innovative strategy to enhance the light absorption of
plasmonic materials.
Indeed, AuAg alloyed nanoparticles reported by Zhang et al. (2007)
showed to be a plasmonic material with a clear LSPR between the fre­
quency of the corresponding pure metals, showing a wide tunability of
its optical properties by varying and controlling the metallic composi­
tion of the nanoparticles of the respective alloy. Additionally, the
alloying process of metals to obtain bimetallic nanoparticles enables the
apparition of synergic effects on different catalytic processes. A

remarkable example is the case of NiPd bimetallic nanoparticles, system
able to catalyze a group of C–C cross-coupling reactions, with a notable
enhanced catalytic activity obtained by electronic charge transfer pro­
cess between the two metals in the nanostructured system (Rai et al.,
2016).
Although the benefits that can be derived from the use of nano­
structured catalysts, these kinds of materials tend to suffer from
agglomeration processes, which dramatically affects its performance in
catalysis. Consequently, stabilizing agents are usually required (Boyer
et al., 2010; Kvítek et al., 2008; Virkutyte & Varma, 2011), and poly­
meric materials have been broadly used to perform this task, enabling
the obtaining of nanocomposite materials (Gao et al., 2013; Kubacka
et al., 2009; Pastoriza-Santos et al., 2018), used in the form of homo­
polymers, block copolymers, graft polymers and dendrimers (Huang &
Yang, 2004; Martínez et al., 2019; Sakai & Alexandridis, 2004; Sunday
et al., 2012). Biopolymers, polymers that can be found in living organ­
isms, are highlighted due to their biocompatibility, adequate biode­
gradability, low toxicity and improved sustainability (Hasırcı et al.,
2001; Hong & Chen, 2017; Yadav et al., 2015). An exceptional
biopolymer example used for high-tech environmental applications is
sodium alginate, a biopolymer extracted from seaweeds, and due to its
wide availability of hydrophilic functional groups, are easily processed
in the form of hydrogels (Thakur et al., 2018). In addition to the above,
the chemical functional groups of alginate allow its employment as
adsorbent for many organic and inorganic species, property that has
been exploited for water treatment solutions (Gao et al., 2020).
Considering the current evidence, the aim of this work was the in situ
preparation of plasmonic bimetallic alloyed nanoparticles, composed by
a noble metal, such as gold, and an earth abundant metal, like copper,
supported on alginate hydrogel beads acting as a heterogeneous biobased catalyst. The bimetallic nanocomposites are expected to present

optical properties dependent on the composition of the system. We hope
that the combination of both metals into nanostructured systems is
going to be able to achieve enhanced catalytic performance in contrast
with its monometallic counterparts. Finally, considering the optical
properties of the plasmonic nanoparticles, the model reaction used to
test the catalytic performance of the systems, the reduction of 4-nitro­
phenol (4NP) into 4-aminophenol (4AP) using sodium borohydride,
was performed under white LED lights in order to study the performance
of the hybrid hydrogel as a plasmonic photocatalyst.

alginate (MW = 380,000 g/mol, G:M = 25:75), calcium (II) chloride
(CaCl2, 98 %, Merck), hydrazine monohydrate (N2H4xH2O, 64–65 %,
Sigma-Aldrich), 4-nitrophenol (4NP, Indicator grade, Sigma-Aldrich)
and NaBH4 (98 %, Merck). All reagents were used without further pu­
rifications. Water used in all experiments was Milli-Q grade (18.6 MΩ/
cm).
2.2. Catalyst preparation
2.2.1. Hydrogel beads synthesis
The synthesis of sodium alginate hydrogel beads was carried out
using a previously reported method (Asadi et al., 2018; Saha et al.,
2010). Typically, using a syringe connected to an infusion pump at 0.3
mL/min, 10 mL of a 3 % w/v sodium alginate solution was added
dropwise on 90 mL of a CaCl2 5 % w/v solution, (gelling medium) under
mild stirring. Once the alginate solution was entirely added, the pre­
pared hydrogel beads were kept in the gelling medium without stirring
for 1 h to allow the beads to be successfully formed. Next, the beads were
washed three times with water to remove the excess of calcium ions.
Finally, the hydrogels were stored in water for further use.
2.2.2. Synthesis of metallic and bimetallic nanoparticles on hydrogel beads
The synthesis of mono- and bimetallic nanoparticles was achieved

using a two-step method consisting of the adsorption, and subsequently
reduction of metal ions on the hydrogel beads. Firstly, 2 g of alginate
beads were immersed in 100 mL of water and aliquots from CuCl2x2H2O
(72.3 mM) and KAuCl4 (24.8 mM) aqueous solutions were added. The
amounts of both metal solutions were calculated to obtain beads with a
metallic loading of 5 mol%, with respect to the monomeric unit of
alginate. For the bimetallic nanoparticles, the copper:gold molar ratios
were set as 3:1, 1:1 and 1:3. The aliquots used in each batch are listed in
Table S1. Then, beads were left under continuous stirring for 48 h and,
subsequently, washed up to 3 times with 50 mL of water to remove nonadsorbed ions.
Secondly, the reduction of metal ions was carried out by adding 1 g of
the beads loaded with metal ions using a Schlenk tube containing 15 mL
of water. The system was consecutively sealed, purged with nitrogen for
20 min and finally stabilized at 60 ◦ C. To start the reduction reaction,
500 μL of a 20 % v/v hydrazine solution was added and kept under
constant stirring (500 rpm) for 2 h. During this process, noticeable color
changes were observed, going from pale blue to reddish tonalities in the
case of copper-rich systems, while the initially yellowish gold-rich sys­
tems adopted a purple coloration. Once the reaction was completed, the
hydrogels were removed by filtration, washed several times with water,
and stored in degassed water at 4 ◦ C.
2.2.3. Catalytic and photocatalytic performance for the reduction of 4nitrophenol
The reduction of 4-nitrophenol (4NP) into 4-aminophenol (4AP)
using sodium borohydride was used as a model reaction to evaluate the
catalytic performance of the obtained systems. The reaction was carried
out in a water-jacketed glass flask connected to a recirculate water bath
at 25 ± 0.1 ◦ C. In a typical reaction, 20 mg of beads containing mono- or
bimetallic nanoparticles were immersed in 10 mL of an aqueous solution
of 4NP (75 μM). Then, 950 μL of a freshly prepared NaBH4 (0.8 M) were
added to start the reaction. The progress of the reaction was followed by

UV–visible spectroscopy following the absorbance changes of the band
centered at 400 nm. On the other hand, the photocatalytic activity of
these systems was evaluated using the same above protocol but con­
ducting the experiments under constant light irradiation provided by
white cool LED strips (LUMILEDS). Note that, the reactor irradiation was
set up with connection to a power source using an electric potential and
current intensity of 18 V and 0.7 mA, respectively (Fig. S1), values that
were kept constant during the complete photocatalytic testing and
without considerable fluctuations, maintaining a light intensity of 16.4
mW/cm2 as measured by a lux meter.

2. Experimental section
2.1. Materials
Potassium gold (III) chloride (KAuCl4, 99.995 %, Sigma-Aldrich),
copper (II) chloride dihydrate (CuCl2x2H2O, 99.9 %, Merck), sodium
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Carbohydrate Polymers 297 (2022) 120021

2.2.4. Characterizations
Thermal stability of alginate hydrogel beads was characterized by
thermogravimetric analysis (TGA), performed on a TGA/SDTA851
Mettler-Toledo thermobalance. The thermograms were measured from
25 to 900 ◦ C at a heating rate of 10 ◦ C/min under nitrogen atmosphere.
Optical properties of the materials were studied by UV–visible spec­
troscopy, which were recorded in a cary-60 UV–vis spectrophotometer
from 200 to 800 nm. The chemical structure of alginate beads was

characterized by FT-IR; all spectra were collected using a PerkinElmer
UATR spectrometer by directly inserting lyophilized samples in the ATR
probe. The spectra were recorded between 4000 and 400 cm− 1, with a
resolution of 1 cm− 1. X-ray photoelectron spectroscopy (XPS) spectra
were collected measuring lyophilized samples using a SPECS equipped
with an PHOIBOS 150 analyzed, 1D-DLD detector, and a focus 500
monochromatic excitation source (X-ray Al Kα hυ = 1486.71 eV) using a
Flood gun to compensate charge effects. FE-SEM was measured on
lyophilized samples using a FEI QUANTA FEG 250 microscope equipped
with an Oxford X-Max50 Energy Dispersive X-ray spectroscopy (EDX)
analyzer. Image processing of FE-SEM images was carried out using
Image-J software measuring a minimal amount of 150 nanoparticles for
statistical analysis. Finally, X-ray microtomography (μCT) of wet and
lyophilized samples was performed using a Skyscan 1272 Model with
operating voltage and a current of 41 kV and 120 μA, respectively. Image
acquisition was optimized obtaining an image pixel of 2.5 μm. Each
sample was scanned over an interval of 0–180◦ with 0.2◦ of rotation
step, and almost 50 min of exposure time. No filter was used for the
scanning. The projection images were reconstructed with 1149 slices
using reconstruction software (Nrecon, Skyscan, Belgium). The critical
parameters related to the reconstruction process were post-alignment,
beam hardening correction 15 %, smoothing, and ring artifacts.
Finally, the images were processed and analyzed using CTAn Software.

clear color tendencies can be related to copper:gold compositions.
The presence of nanoparticles in the hydrogels was confirmed by FESEM analysis, presented in Fig. 2A. All obtained nanoentities, regardless
of the metal composition, exhibited spherical morphologies. In addition,
in terms of the average-size, the synthesized copper nanoparticles (24.9
± 1.09 nm) resulted to be bigger than the gold nanoparticles (12.35 ±
0.45 nm). This tendency was also evidenced in others bimetallic sys­

tems, where those having a higher gold composition showed smaller
sizes along with narrower size distributions (Fig. 2B). Micrographs and
histograms of the mono- and bimetallic nanoparticles can be found in
Fig. S4. This tendency in the nanostructures size over the evaluated
composition range could be related to the nucleation rate in which
copper and gold ions are reduced. A similar tendency was observed by
Shen et al. (2017), in which Cu and Au monometallic samples demon­
strated sizes between 13 and 5 nm, respectively, and CuAu nanoalloys
showed sizes in between the diameters of monometallic samples. This
phenomenon could be explained by the close relation with the reduction
kinetics of metal ions, in fact, Marcus theory (Marcus, 1993) regarding
electron transfer reactions that correlate the dependency between the
reduction rate of metallic ions with its reduction potential, in which high
reduction potentials enables fast reduction rates. Regarding this, gold
(III) species present higher reduction potentials in comparison with
copper (II) ions, thus the higher reduction rate of gold-richer nanoalloys
is favored, promoving a faster nucleation, which leads to the formation
of smaller nanoparticles.
Consequently, the different color tonalities displayed by alginate
beads after the reducing process should be related to variations on their
optical properties, directly related to the presence of mono- or bimetallic
nanoparticles. Regarding the above, beads containing gold and copper
nanoentities exhibited distinctive purple and red colors, respectively,
which should be associated to absorption bands within the visible range
due to plasmonic phenomena. On the other hand, the bimetallic nano­
particles presented colors with different shades depending on the loaded
content of copper and gold, which is in good agreement with previous
reports for copper-gold nanostructures (Min & Wang, 2020; Sytwu et al.,
2019; Valizade-Shahmirzadi & Pakizeh, 2018). UV–Visible spectra were
recorded for beads bearing mono- and bimetallic nanoparticles, aiming

to confirm the presence of plasmonic phenomena, particularly the
localized surface plasmon resonance (LSPR) (Fig. 2C). Both mono­
metallic systems showed typical LSPR bands associated to copper and
gold zero-valence nanoparticles, centered at 587 and 515 nm, respec­
tively. However, in those systems containing bimetallic structures, clear
changes in their absorption spectra were induced by varying the Cu:Au
compositions. In this sense, for all bimetallic samples, a single band
located between the frequencies of both monometallic systems was
visualized. Importantly, the maximum absorption of these bands dis­
played gradual shifts according to the composition of the metal nano­
particle (Fig. 2D). This would be indicative of a successful formation of
bimetallic nanoparticles (i.e. nanoalloys) over alginate beads. It is
important to mention, that the optical properties of the mono- and
bimetallic nanoparticles were found in the range of visible region of the
spectrum, allowing to test these materials as plasmonic photocatalysts
activated by visible light.
To gain insights into the interactions taking place between the
alginate structure and metal species, FTIR analyses were performed (Fig.
S5). In all spectra, the presence of the typical bands associated to the
alginate backbone was successfully identified. In this regard, the signal
– O asymmetrical and sym­
attributed to the –OH at 3353 cm− 1 and C–
metrical stretching bands at 1596 and 1404 cm− 1, are highlighted due to
their essential roles against ions and nanoparticle coordination.
In this context, Papageorgiou et al. (2010) reported a protocol to
evaluate the interactions existing between alginate and metal ions,
based on the chemical shifts experienced by carbonyl vibrational bands
before and after the adsorption of metal cations. Thereby, by measuring
–O
the difference between the symmetrical and asymmetrical C–

stretching bands (ΔνCO), relevant information about the coordination

3. Results and discussion
Alginate hydrogel beads were successfully obtained using an
aqueous CaCl2 solution as gelling medium. This process is mainly driven
by the excellent affinity existing between G-Block alginate carboxylate
groups and divalent cationic ions like calcium ions (Cao et al., 2020). In
this way, uniform ionically cross-linked alginate hydrogel beads with an
average diameter of 2.62 ± 0.03 mm were obtained (Fig. S2A). The
water content of the hydrogel beads was characterized by TGA. The
analysis shows a pronounced weight-loss stage centered at 100 ◦ C,
which should be attributed to the evaporation of water that forms part of
the hydrogel inner structure. This weight loss corresponds around 97 %
of the total mass of the alginate bead, which highlights the low amount
of polymer required to obtain the hydrogel beads (Fig. S2B).
Considering the ability of alginate chains to strongly interact with
metal cations, is that the hydrogels were used as an adsorbent of copper
(II) and gold (III) ions. In this way, the synthesis of mono- and bimetallic
nanoparticles was proposed via a two-step synthetic route which starts
with i) the adsorption of metal ions on alginate bead surfaces, followed
by their ii) chemical reduction. Camargo et al. (Rodrigues et al., 2018)
reported the influence of different reducing agents on the composition,
morphology, and size of a wide diversity of metal nanoparticles.
Particularly, the synthesis of cooper nanoparticles deserves special
attention due to its well-known tendency to be oxidized. Accordingly,
the use of hydrazine as a reducing agent for mono- and bimetallic CuAu
nanoparticles would preferentially allow the reduction of copper ions
into its zero-valence metallic state, in contrast to agents such as sodium
borohydride, that acts as both reductant agent and base, inducing
important pH changes in the reaction media and, thereby, driving to the

formation of copper oxides species instead of zero-valent copper
(Gawande et al., 2016) (Fig. S3). The synthetic route employed for the
preparation of hydrogels decorated with mono- and bimetallic nano­
structures is displayed in Fig. 1. It draws attention the noticeable color
change observed after both adsorption and reducing steps, in which
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Carbohydrate Polymers 297 (2022) 120021

Fig. 1. Schematic representation of pristine alginate beads, after the adsorption of gold ions and the further reduction to obtain mono and bimetallic nanoparticles.

Fig. 2. (A) FE-SEM micrograph of alginate hydrogel loaded with CuAu 1:1 nanoparticles. (B) Mean size distribution of the mono- and bimetallic nanoparticles, (C)
LSPR of copper-gold bimetallic nanoparticles normalized at 400 nm, and (D) the maximum absorption resonance frequency at each composition.

modes, and therefore, the interactions strength can be extracted. Ac­
cording to this, the ΔνCO value is categorized in three classes, which are
summarized in the following equations:
Δʋ (COO− )complex ≪Δʋ(COO− )Na

(1)

Δʋ (COO− )complex ≈ Δʋ(COO− )Na

(2)

Δʋ (COO− )complex ≫Δʋ(COO− )Na

(3)

Whereas after the adsorption of copper and gold ions, a slight shift was
found to lower values between 182 and 185 cm− 1 (values summarized in
the Table 1), suggesting a pseudo bridged coordination mode for the
case of the different metal combinations.
After the reduction step, there are no notable differences in the IR
spectra, indicating that the alginate groups remain practically un­
changed after this step. However, slight changes were founded on the
frequency of alginate functional groups, with a considerable decrease in
the wavenumber of O–H stretching band after the reduction step,
suggesting a considerable interaction of these groups with the nano­
particles surface. The mechanism by which these functional groups,
hydroxyls and carboxylates, acts during the stabilization of mono- and
bimetallic nanoparticles on these materials could be related to the
analogous systems obtained by NPs stabilization with polyols- (Dong

Eq. (1) is correlated to a bidentate chelating coordination, Eq. (2) to a
pseudo bridging coordination, while Eq. 3 is correlated to a unidentate
coordination mode. In our study the ΔνCO value calculated from the
FTIR spectrum of pristine sodium alginate was approximately 192 cm− 1.
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Carbohydrate Polymers 297 (2022) 120021

region showed three different peaks approximately at 929.4, 933.0, and
952.2 eV. The peaks at 933.0 and 952.2 eV are assigned to the metallic

copper peaks Cu 2p3/2 and Cu 2p1/2, with a spin-orbit split of 19.2 eV
which falls typically in the range for zero-valent copper atoms. The third
peak is shifted to lower binding energies, suggesting the presence of
electron-poorer copper atoms which might be interacting with the
electron-richer gold atoms described above, serving as a good indicator
to confirm properly that prepared bimetallic nanostructured are alloys.
Additionally, the Cu 2p region did not show a copper oxide peak, and a
clear lack of a signal approximately at 940 eV helping to confirm the
absence of oxidized copper species.
Thermal properties of the alginate beads containing metallic nano­
particles were analyzed to determine the thermal stability of the mate­
rials. The TGA and DTG profiles are showed in Fig. 5A and B. As
expected, alginate beads loaded with mono- and bimetallic nano­
particles presented a marked weight loss related to the water evapora­
tion stage, pretty similar to the behavior presented by pristine beads,
enabling the use of these materials in applications at temperatures below
water boiling.
Furthermore, aiming to gain insights regarding the inner structure of
these materials, μCT measurements were performed on an alginate bead
and its corresponding cryogel. As can be seen, Fig. 5C and D revealed a
smooth and continuous inner phase for the hydrogel making impossible
to distinguish between the polymer network and solvent. Conversely,
Fig. 5E and F displays the result achieved for the freeze-dried sample,
revealing a clear inner porous structure exhibiting a large number of
cavities. Interestingly, by comparing both μCT scans, calculations allow
to estimate a total free volume of approximately 11.8 mm3 corre­
sponding to around 90 % of the total volume of the sample. This would
be in good agreement with TGA results considering that this volume was
initially occupied by water. The rest of the volume of the bead corre­
sponds to scaffolds formed by fine walls of cross-linked alginate,

showing an average thickness near to 9.31 μm.
After concluding the characterization of obtained materials, their
performance as heterogeneous catalysts were evaluated using the cata­
lyzed reduction of 4NP as model reaction, which can be easily monitored
by UV–visible spectroscopy, the results are shown in Fig. 6A. The kinetic
data of the mono- and bimetallic nanoparticles was fitted to a pseudo
first-order kinetic model, using Eq. (4).

Table 1
Characteristic band wavenumber of the main functional groups in alginate
beads.

Ions

Nanoparticles

Alg-Na+
Alg-Cu2+
AlgCu2+Au3+
3:1
Cu2+Au3+
1:1
Cu2+Au3+
1:3
Alg-Au3+
Alg-Cu
Alg-CuAu 3:1
CuAu 1:1
CuAu 1:3
Au

ν OH

νasymmetric

νsymmetric
(CO− )

Δν
(CO− )

3353
3313
3314

1596
1594
1591

1404
1409
1409

192
185
182

3314

1591

1409

182

3313

1591

1409

182

3316
3275
3263
3266
3278
3280

1593
1591
1590
1591
1587
1590

1411
1415
1412

1415
1411
1413

182
176
178
176
176
177

(CO− )

et al., 2015; Varanda et al., 2019) and citrates (De Souza et al., 2019;
Ullah et al., 2017), groups.
The characterization of the surface morphology and composition of
the hydrogels was performed by FE-SEM and complemented with EDX
elemental mapping analysis (Fig. 3). The results suggesting a homoge­
neous distribution of both metals in all composition range, without any
appreciable presence of agglomerates or segregated structures of
metallic nanoparticles. The experimental metallic proportion in each
bimetallic nanocomposite is detailed in Table S2.
In order to confirm that bimetallic nanoparticles were obtained in
the form of metallic alloys, as inferred from the optical properties of the
materials, XPS analyses to determine the oxidation state of both metals
were carried out. As is shown in Fig. 4, the deconvoluted gold and
copper regions show the typical signals attributed to the Au 4d and Cu
2p peaks. In the Au region, there is a clear presence of two doublet
signals attributed to the 4d5/2 and 4d3/2. The first doublet appears
around 346.6 and 351.0 eV, respectively, indicating the presence of gold

atoms in zero-valent state (Sun et al., 2021). Additionally, a second less
intense pair of peaks, at 345.1 and 348.6 eV, respectively, shifted to
lower binding energies, decrease that might suggest the presence of
electron-richer gold atoms associated to the formation of CuAu alloys
(Cabello et al., 2019; Kim et al., 2003). On the other hand, the Cu 2p

LnAbst = LnAbs0 − kapp t

(4)

Fig. 3. FE-SEM and EDX mapping of both copper and gold atoms, accompanied by the EDX mapping of both metals per separate, bar scale 5 μm in all cases.
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Carbohydrate Polymers 297 (2022) 120021

Fig. 4. XPS deconvoluted spectra of (A) gold 4d region and (B) Cu 2p region of alginate beads with CuAu 1:3.

The Abst and Abs0 correspond to the absorbance of the 4-nitropheno­
late band at time t and at the beginning of the reaction, respectively, t is
the time, and kapp corresponds to the apparent kinetic constant, calcu­
lated from the slope value.
Interestingly, either in dark conditions or under light irradiation,
materials loaded with metallic nanoparticles were able to carry the re­
action in contrast to pristine alginate beads, with which no measurable
changes were observed for the 4NP band even after 2 h of experiment
(data not shown). These results confirm the role of alginate as mere
support for nanostructures and demonstrate the critical importance of

the nanoparticles to carry the reaction even in dark conditions (Strachan
et al., 2020). The catalytic assessment of these materials, in terms of kapp
and conversion values, begins by evaluating their performance under
dark conditions (red bars in Fig. 6B), the hydrogels loaded with mono
and bimetallic nanoparticles showed an active role during the reaction.
Regarding monometallic nanostructures, it can be seen that the system
bearing gold nanoparticles showed a higher catalytic activity than the
one based on copper nanoparticles. The maximum conversion values for
both systems were achieved after 90 min of reaction, being 75 % and 56
% for Au NP and Cu NP, respectively (Fig. 6C). Moreover, beads deco­
rated with Au NP also exhibited a higher kapp value against copper
nanoparticles, going from 0.018 to 0.013 min− 1, respectively (1.5 times
higher). Considering these results, it should be expected that alloys be­
tween both metals display an intermediate behavior between the ac­
tivity of these two materials, hopefully increasing from copper-rich to
gold-rich systems. Surprisingly, bimetallic systems presented a dra­
matic enhancement of their catalytic performance compared to mono­
metallic ones, supported by the notably higher conversion and kapp
values (Fig. 6C and D). All bimetallic systems achieved full conversion
(100 %) within 40 min, time at which pristine gold and copper systems
only showed conversions of around 44 % and 39 %, respectively.
Consequently, nanoalloys also exhibited notably higher kapp values of
0.108, 0.119 and 0.094 min− 1 for 3:1, 1:1 and 1:3 CuAu NP systems,
respectively, giving rise to a volcano-shaped trend when are compared
to Au NP and Cu NP. It has been reported that other bimetallic systems
showed an improvement on the activity ascribed to a synergic effect
achieved by the combination of two different metals (Qiu et al., 2020;
Rangasamy et al., 2021; Wang et al., 2018). Based on previous reports,
we believe that the synergy observed in our systems could be related to
electronic phenomena that improves the reduction of 4NP at the surface

of the nanoparticles similar to the one reported by Chu and Su (2014),
who explored the synergy in noble metal AuPt bimetallic nanoparticles
for the reduction of 4NP. In our case, based on previously reported
literature arguments, the enhanced catalytic activity could be attributed
to an electronic synergy between both metals, allowing a preferential

adsorption of 4-nitrophenolate and BH−4 species on neighboring surface
atoms of the catalyst. The results indicate that, due to synergic effects,
the kapp for the copper-richest alloy is 8.3 times higher than the
measured for the monometallic copper system and 5.9 times higher the
value of the sample containing AuNP. In addition, this sample also
exhibited a higher kinetic constant than the gold-richest alloy, being
surpassed only by the CuAu 1:1 sample. This result is very relevant from
an economic perspective, because demonstrates that a significant
amount of gold can be replaced by this earth-abundant and cheaper
metal, keeping full catalytic conversions and higher rates of reactions.
Motivated by the above results and the inherent plasmonic property of
both metals, the catalytic properties of these systems under visible light
irradiation were tested. The photocatalytic assays were performed
irradiating the reaction vessel with white LED strips, as a low-cost and
environmentally friendly alternative compared to other options such as
lasers, bulb lamps and arc lamps (Jo & Tayade, 2014). Surprisingly,
photocatalytic tests of all samples revealed a remarkable improvement
of their catalytic performances. Both monometallic systems displayed a
notorious increase in conversions and kapp values relative to dark con­
ditions. For example, under light irradiation, beads decorated with Au
NPs achieved a maximum conversion value of 93 % at 70 min of reac­
tion, while the one bearing Cu NPs reached a value of 70 % in 90 min,
Fig. 6D. It is worth mentioning that the spherical morphologies of
pristine alginate beads were well maintained after the reaction. In terms

of kapp, values of 0.017 and 0.042 min− 1 were calculated for samples
containing copper and gold nanoparticles, respectively, showing an
increment of 31 % and 128 % relative to their values obtained in dark
conditions. Results showed that under dark or light conditions, gold
nanoparticles displayed better performance than copper and, in addi­
tion, the boost of the catalytic response under light stimuli was more
pronounced also in Au NP-containing system. This could be ascribed to
the lower chemical stability that Cu NPs exhibit against, for example,
oxidation phenomena (Wang et al., 2020; Zhao et al., 2015). This issue
will be discussed later during recyclability tests. Notwithstanding the
above, the considerable light-driven catalytic improvement could be
attributed to plasmonic effects triggered in these nanostructures. Light
irradiation also allowed enhancing the catalytic response of bimetallic
systems, showing again better performance than their monometallic
counterparts. Interestingly, all irradiated CuAu nanoalloys reached full
conversion in less than 30 min and displayed higher kapp. Accordingly,
samples CuAu 3:1, 1:1 and 1:3 exhibited kapp values of 0.215, 0.129 and
0.157 min− 1, respectively, corresponding to enhancements of 99 %, 8 %
and 67 % relative to their values determined in absence of light. It is
worth noting the outstanding catalytic enhancement showed by the
sample CuAu 3:1, which kapp is the higher between all tested samples
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Carbohydrate Polymers 297 (2022) 120021

Fig. 5. Thermogravimetric and (B) DTG profile of alginate beads loaded with copper-gold nanoparticles. μCT scan analysis of (C and D) alginate hydrogel bead and
(E and F) its corresponding lyophilized aerogel.

being 12.7 and 5.1 times higher than the ones reported for irradiated
beads bearing CuNPs and AuNPs, respectively. Considering the obtained
results, the remarkable light enhancement suggests a faster electron
transfer on the surface of the particles. In this sense, the collective
oscillation of electrons caused by the light absorption allows charge
carriers to be quickly transferred to nitrophenol and borohydride spe­
cies, previously adsorbed on the nanoparticles. Previously, Barbosa
group (Barbosa et al., 2018) has proposed a plausible mechanism to
explain the light enhancement, based on the stimulation of reactive
electrons driven by the excitation of conduction electrons of plasmonic
nanoparticles, that enables a faster electron transfer from the plasmonic
nanoparticles to the lowest unoccupied orbital of 4-nitrophenol

molecules.
The high kapp value obtained by alginate beads loaded with CuAu 3:1
nanoparticles is highly remarkable. The performance exhibited by this
system was even comparable with other bimetallic structures, like the
case of Cu@Au bimetallic nanoparticles prepared by Matinise et al. and
stabilized by dendrimers (Matinise et al., 2022), showing a kapp of 0.318
min− 1 with average particle sizes of 4.7 ± 1.7 nm, notably lower
compared with our case, which might indicate the higher kapp value.
However, the format employed for that system difficult its reusability in
further testing. Taking advantage of the alginate beads used as support,
reusability studies of the catalyst were performed. In this sense, the
assays were carried out by performing the reduction reaction with the
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Carbohydrate Polymers 297 (2022) 120021

Fig. 6. (A) The advance of the reaction followed by the disappearance of the band at 400 nm using UV-spectroscopy adjusted to a pseudo first-order kinetic and (B)
apparent kinetic rate constant (kapp) for the reaction with and without light irradiation. Conversion profiles for 4NP reduction in (C) dark and (D) under light
irradiation.

catalyst, once the reaction was completed, the catalyst was removed,
washed, and reused in a new catalytic test. The process was repeated to
three consecutively cycles. The results are presented in Fig. 7A and B. As
can be seen, the catalyst is capable to complete the reaction with a
conversion near to a 100 % yield after each cycle. However, the kapp of
the reaction exhibited a notable decrease in activity after the first re­
action cycle, with no changes in further cycles.
In order to explain the decrease in activity of the catalyst, reusability
experiments were also performed using beads with monometallic
nanoparticles. The obtained results are shown in Fig. 7C, as normalized
percentage reduction of the kapp in order to achieve a more accurate
comparison, because both metals showed different kapp. It is evident that
the kapp of both metals decreases when they are tested in multiple cycles.
Firstly, alginate beads loaded with monometallic copper nanoparticles
showed a considerable reduction in their activity at the second cycle,
reaching less than 5 % of the initial kapp. On the other hand, gold
nanoparticles also showed a decrease of 48 %, similar to the result
exhibited by the bimetallic alloy. Considering this, a high contribution
in the reduction of activity observed in the bimetallic CuAu 3:1 nano­
particles might be related to the formation of copper oxide after the first
cycle, due to the basic pH induced by sodium borohydride in the
reduction reaction. To confirm this, the alginate beads loaded with
copper nanoparticles were characterized by UV–visible both before and

after the first cycle, Fig. 7D. A notable decrease on the LSPR of copper
nanoparticles accompanied by an increase of light absorption at wave­
lengths below 350 nm, probably related to the formation of copper oxide
nanoparticles (Berra et al., 2018) were obtained, which explains the
change on the activity of the catalyst. Despite this decrease in the kapp of
the alginate beads loaded with CuAu 3:1 nanoparticles, it is necessary to
highlight that the reduction in their activity produced a similar result to

that presented by the monometallic gold nanoparticles. Therefore, this
means that it is possible to obtain a similar behavior to that obtained
with monometallic gold nanoparticles, substituting much of the gold for
a cheaper and earth-abundant metal, like copper.
4. Conclusions
In this work, a simple method to in situ synthesize monometallic and
bimetallic nanoparticles was successfully carried out using a bio-based
hydrogel as a stabilizing and supporting agent. The obtained bime­
tallic nanoparticles presented interesting optical properties in the visible
interval, exactly between 515 and 587 nm related to LSPR absorption,
highly desired for photocatalytic applications. The obtained materials
presented a notable activity as catalysts for the reduction reaction of
4NP in light and dark conditions, presenting an interesting synergy as
both metals were alloyed into nanoparticles, in which CuAu 3:1 nano­
particles presented a catalytic enhancement activity around 5.4 times in
comparison with gold nanoparticles, probably due to an electronic
enhancement, suggesting an important synergic behavior by the alloy of
both metals. Additionally, the activity of the materials was boosted by
almost 2.2 times by irradiating with LEDs as a visible light source, taking
advantage of the plasmonic properties of the nanoparticles. Finally,
CuAu 3:1 bimetallic system demonstrated an adequate recyclability,
reaching quantitative conversion during 3 cycles and maintenance of its

kinetic constant considerably better than copper and comparable to the
behavior of gold nanoparticles. The reported results position Cu/Au
bimetallic nanoparticle systems on a biological support such as alginate
as attractive materials for use in light-enhanced catalytic reactions.

8


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Carbohydrate Polymers 297 (2022) 120021

Fig. 7. (A) Reusability conversion profiles and (B) pseudo first-order fitting accompanied by the kapp of each cycle using alginate beads loaded with CuAu 3:1
bimetallic nanoparticles for the reduction reaction of 4NP. (C) Reusability essays of alginate beads loaded with monometallic nanoparticles and (D) the optic
properties of alginate beads loaded with CuNPs before and after the reduction of 4NP.

CRediT authorship contribution statement

Appendix A. Supplementary data

Oscar Ramírez: Investigation, Conceptualization, Methodology,
Writing – original draft, Formal analysis, Funding acquisition. Sebas­
tian Bonardd: Supervision, Methodology, Writing – review & editing.
´sar Saldías: Writing – review & editing. Yadira Zambrano: Re­
Ce
sources, Formal analysis. David Díaz Díaz: Supervision, Project
administration, Writing – review & editing, Funding acquisition. Angel
Leiva: Supervision, Project administration, Writing – review & editing,
Funding acquisition.

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.carbpol.2022.120021.
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The authors declare that they have no known competing financial
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Data availability
Data will be made available on request.
Acknowledgments
O. Ramírez thanks Beca doctorado nacional ANID 21191002. S.
´n contract
Bonardd thanks MINECO for a Juan de la Cierva – Formacio
FJC2019-039515-I. C. Saldías thanks Fondecyt Project 1211022. A.
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