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.
Carbohydrate Polymers 297 (2022) 120021 Contents lists available at ScienceDirect 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 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 % 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: ddiazdiaz@ull.edu.es (D.D Díaz), aleivac@uc.cl (A Leiva) https://doi.org/10.1016/j.carbpol.2022.120021 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 (http://creativecommons.org/licenses/bync-nd/4.0/) 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 % w/v sodium alginate solution was added dropwise on 90 mL of a CaCl2 % 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 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, 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 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 times with 50 mL of water to remove nonadsorbed ions Secondly, the reduction of metal ions was carried out by adding 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 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 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 ◦ 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 Experimental section 2.1 Materials Potassium gold (III) chloride (KAuCl4, 99.995 %, Sigma-Aldrich), copper (II) chloride dihydrate (CuCl2x2H2O, 99.9 %, Merck), sodium O Ramírez et al 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 cm− 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 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 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− 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 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 It draws attention the noticeable color change observed after both adsorption and reducing steps, in which O Ramírez et al Carbohydrate Polymers 297 (2022) 120021 Fig Schematic representation of pristine alginate beads, after the adsorption of gold ions and the further reduction to obtain mono and bimetallic nanoparticles Fig (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− (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 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− O Ramírez et al 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 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 FE-SEM and EDX mapping of both copper and gold atoms, accompanied by the EDX mapping of both metals per separate, bar scale μm in all cases O Ramírez et al Carbohydrate Polymers 297 (2022) 120021 Fig 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 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 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− 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 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− 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 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 %, % 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 O Ramírez et al Carbohydrate Polymers 297 (2022) 120021 Fig 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− 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 O Ramírez et al Carbohydrate Polymers 297 (2022) 120021 Fig (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 % 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 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 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 O Ramírez et al Carbohydrate Polymers 297 (2022) 120021 Fig (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 References Anastas, P., & Eghbali, N (2010) Green chemistry: Principles and practice Chemical Society Reviews, 39(1), 301–312 Araujo, T P., Quiroz, J., Barbosa, E C., & Camargo, P H (2019) Understanding plasmonic catalysis with controlled nanomaterials based on catalytic and plasmonic metals Current Opinion in Colloid Interface Science, 39, 110–122 Asadi, S., Eris, S., & Azizian, S (2018) Alginate-based hydrogel beads as a biocompatible and efficient adsorbent for dye removal from aqueous solutions ACS Omega, 3(11), 15140–15148 Bagheri, S., Muhd Julkapli, N., & Bee Abd Hamid, S (2014) Titanium dioxide as a catalyst support in heterogeneous catalysis The Scientific World Journal, 2014 Barbosa, E C., Fiorio, J L., Mou, T., Wang, B., Rossi, L M., & Camargo, P H (2018) Reaction pathway dependence in plasmonic catalysis: Hydrogenation as a model molecular transformation Chemistry–AEuropean Journal, 24(47), 12330–12339 Berra, D., Laouini, S., Benhaoua, B., Ouahrani, M., Berrani, D., & Rahal, A (2018) Green synthesis of copper oxide nanoparticles by Pheonix dactylifera L leaves extract Digest Journal of Nanomaterials Biostructures, 13(4), 1231–1238 Boyer, C., Whittaker, M R., Bulmus, V., Liu, J., & Davis, T P (2010) The design and utility of polymer-stabilized iron-oxide nanoparticles for nanomedicine applications NPG Asia Materials, 2(1), 23–30 Cabello, G., Nwoko, K C., Marco, J F., S´ anchez-Arenillas, M., M´endez-Torres, A M., Feldmann, J., & Smith, T A (2019) Cu@ Au self-assembled nanoparticles as SERSactive substrates for (bio) molecular sensing Journal of Alloys Compounds, 791, 184–192 Cao, L., Lu, W., Mata, A., Nishinari, K., & Fang, Y (2020) Egg-box model-based gelation of alginate and pectin: A review Carbohydrate Polymers, 242, Article 116389 Carlucci, C., Degennaro, L., & Luisi, R (2019) Titanium dioxide as a catalyst in biodiesel production Catalysts, 9(1), 75 Chen, H., Nanayakkara, C E., & Grassian, V H (2012) Titanium dioxide photocatalysis in atmospheric chemistry Chemical Reviews, 112(11), 5919–5948 Chu, C., & Su, Z (2014) Facile synthesis of AuPt alloy nanoparticles in polyelectrolyte multilayers with enhanced catalytic activity for reduction of 4-nitrophenol Langmuir, 30(50), 15345–15350 Dal Santo, V., & Naldoni, A (2018) Titanium dioxide photocatalysis MDPI Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper 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 Leiva thanks to FONDECYT 1211124 and FONDAP 15110019 projects for the financial support of the research D D Díaz thanks financial support from the Spanish Government (PID2019-105391GB-C21/AEI/ 10.13039/501100011033) and NANOtec, INTech, Cabildo de Tenerife and ULL for laboratory facilities O Ramírez et al Carbohydrate Polymers 297 (2022) 120021 Qiu, Y.-P., Shi, Q., Zhou, L.-L., Chen, M.-H., Chen, C., Tang, P.-P., & Wang, P (2020) NiPt nanoparticles anchored onto hierarchical nanoporous N-doped carbon as an efficient catalyst for hydrogen generation from hydrazine monohydrate ACS Applied Materials Interfaces, 12(16), 18617–18624 Rai, R K., Gupta, K., Tyagi, D., Mahata, A., Behrens, S., Yang, X., & Singh, S K (2016) Access to highly active Ni–Pd bimetallic nanoparticle catalysts for C-C coupling reactions Catalysis Science Technology, 6(14), 5567–5579 Rangasamy, R., Lakshmi, K., & Selvaraj, M (2021) Synthesis of ultrafine AuPd bimetallic nanoparticles using a magnetite-cored poly (propyleneimine) dendrimer template and its sustainable catalysis of the Suzuki coupling reaction New Journal of Chemistry, 45(31), 14227–14235 Rodrigues, T S., Zhao, M., Yang, T H., Gilroy, K D., da Silva, A G., Camargo, P H., & Xia, Y (2018) Synthesis of colloidal metal nanocrystals: A comprehensive review on the reductants Chemistry–AEuropean Journal, 24(64), 16944–16963 Rodrigues, T S., da Silva, A G., & Camargo, P H (2019) Nanocatalysis by noble metal nanoparticles: Controlled synthesis for the optimization and understanding of activities Journal of Materials Chemistry A, 7(11), 5857–5874 Saha, S., Pal, A., Kundu, S., Basu, S., & Pal, T (2010) Photochemical green synthesis of calcium-alginate-stabilized Ag and Au nanoparticles and their catalytic application to 4-nitrophenol reduction Langmuir, 26(4), 2885–2893 Sakai, T., & Alexandridis, P (2004) Single-step synthesis and stabilization of metal nanoparticles in aqueous pluronic block copolymer solutions at ambient temperature Langmuir, 20(20), 8426–8430 Shen, L., Zhou, X., Wang, A., Yin, H., Yin, H., & Cui, W (2017) Hydrothermal conversion of high-concentrated glycerol to lactic acid catalyzed by bimetallic CuAu x (x= 0.01–0.04) nanoparticles and their reaction kinetics RSC Advances, 7(49), 30725–30739 Strachan, J., Barnett, C., Masters, A F., & Maschmeyer, T (2020) 4-Nitrophenol reduction: Probing the putative mechanism of the model reaction ACS Catalysis, 10 (10), 5516–5521 Sun, Y., Qi, T., Jin, Y., Liang, L., & Zhao, J (2021) A signal-on fluorescent aptasensor based on gold nanoparticles for kanamycin detection RSC Advances, 11(17), 10054–10060 Sunday, D., Ilavsky, J., & Green, D L (2012) A phase diagram for polymer-grafted nanoparticles in homopolymer matrices Macromolecules, 45(9), 4007–4011 Sytwu, K., Vadai, M., & Dionne, J A (2019) Bimetallic nanostructures: Combining plasmonic and catalytic metals for photocatalysis Advances in Physics: X, 4(1), 1619480 Thakur, S., Sharma, B., Verma, A., Chaudhary, J., Tamulevicius, S., & Thakur, V K (2018) Recent progress in sodium alginate based sustainable hydrogels for environmental applications Journal of Cleaner Production, 198, 143–159 Ullah, I., Khan, K., Sohail, M., Ullah, K., Ullah, A., & Shaheen, S (2017) Synthesis, structural characterization and catalytic application of citrate-stabilized monometallic and bimetallic palladium@ copper nanoparticles in microbial antiactivities International Journal of Nanomedicine, 12, 8735–8747 Valizade-Shahmirzadi, N., & Pakizeh, T (2018) Optical characterization of broad plasmon resonances of Pd/Pt nanoparticles Materials Research Express, 5(4), Article 045038 Varanda, L C., Souza, C G., Moraes, D A., Neves, H R., Souza, J B., Silva, M F., & Beck, W (2019) Size and shape-controlled nanomaterials based on modified polyol and thermal decomposition approaches A brief review Anais da Academia Brasileira de Ciˆencias, 91, Article e20181180 Virkutyte, J., & Varma, R S (2011) Green synthesis of metal nanoparticles: Biodegradable polymers and enzymes in stabilization and surface functionalization Chemical Science, 2(5), 837–846 Wang, Q., Fu, F., Yang, S., Martinez Moro, M., Moya, S., Ramirez, M.d L A., & Astruc, D (2018) Dramatic synergy in CoPt nanocatalysts stabilized by “Click” dendrimers for evolution of hydrogen from hydrolysis of ammonia borane ACS Catalysis, 9(2), 1110–1119 Wang, R., Liu, H., Wang, X., Li, X., Gu, X., & Zheng, Z (2020) Plasmon-enhanced furfural hydrogenation catalyzed by stable carbon-coated copper nanoparticles driven from metal–organic frameworks Catalysis Science Technology, 10(19), 6483–6494 Yadav, P., Yadav, H., Shah, V G., Shah, G., & Dhaka, G (2015) Biomedical biopolymers, their origin and evolution in biomedical sciences: A systematic review Journal of Clinical Diagnostic Research: JCDR, 9(9), ZE21–ZE25 Zhang, Q., Lee, J Y., Yang, J., Boothroyd, C., & Zhang, J (2007) Size and composition tunable Ag–Au alloy nanoparticles by replacement reactions Nanotechnology, 18 (24), Article 245605 Zhao, P., Feng, X., Huang, D., Yang, G., & Astruc, D (2015) Basic concepts and recent advances in nitrophenol reduction by gold-and other transition metal nanoparticles Coordination Chemistry Reviews, 287, 114–136 De Souza, C D., Nogueira, B R., & Rostelato, M E C (2019) Review of the methodologies used in the synthesis gold nanoparticles by chemical reduction Journal of Alloys Compounds, 798, 714–740 Dey, S., & Mehta, N S (2020) Synthesis and applications of titanium oxide catalysts for lower temperature CO oxidation Current Research in Green Sustainable Chemistry, 3, Article 100022 Dong, H., Chen, Y.-C., & Feldmann, C (2015) Polyol synthesis of nanoparticles: Status and options regarding metals, oxides, chalcogenides, and non-metal elements Green Chemistry, 17(8), 4107–4132 Ezendam, S., Herran, M., Nan, L., Gruber, C., Kang, Y., Gră obmeyer, F., & Cort es, E (2022) Hybrid plasmonic nanomaterials for hydrogen generation and carbon dioxide reduction ACS Energy Letters, 7(2), 778–815 Gao, B., Rozin, M J., & Tao, A R (2013) Plasmonic nanocomposites: Polymer-guided strategies for assembling metal nanoparticles Nanoscale, 5(13), 5677–5691 Gao, X., Guo, C., Hao, J., Zhao, Z., Long, H., & Li, M (2020) Adsorption of heavy metal ions by sodium alginate based adsorbent-a review and new perspectives International Journal of Biological Macromolecules, 164, 4423–4434 Gawande, M B., Goswami, A., Felpin, F.-X., Asefa, T., Huang, X., Silva, R., & Varma, R S (2016) Cu and Cu-based nanoparticles: Synthesis and applications in catalysis Chemical Reviews, 116(6), 3722–3811 Hasırcı, V., Lewandrowski, K., Gresser, J., Wise, D., & Trantolo, D (2001) Versatility of biodegradable biopolymers: Degradability and an in vivo application Journal of Biotechnology, 86(2), 135–150 Hong, M., & Chen, E Y.-X (2017) Chemically recyclable polymers: A circular economy approach to sustainability Green Chemistry, 19(16), 3692–3706 Huang, X., & El-Sayed, M A (2010) Gold nanoparticles: Optical properties and implementations in cancer diagnosis and photothermal therapy Journal of Advanced Research, 1(1), 13–28 Huang, H., & Yang, X (2004) Synthesis of polysaccharide-stabilized gold and silver nanoparticles: A green method Carbohydrate Research, 339(15), 2627–2631 Ismail, A A., & Bahnemann, D W (2014) Photochemical splitting of water for hydrogen production by photocatalysis: A review Solar Energy Materials Solar Cells, 128, 85–101 Jo, W.-K., & Tayade, R J (2014) New generation energy-efficient light source for photocatalysis: LEDs for environmental applications Industrial Engineering Chemistry Research, 53(6), 2073–2084 Kim, M.-J., Na, H.-J., Lee, K C., Yoo, E A., & Lee, M (2003) Preparation and characterization of Au–Ag and Au–Cu alloy nanoparticles in chloroform Journal of Materials Chemistry, 13(7), 1789–1792 Kim, M., Lee, J H., & Nam, J M (2019) Plasmonic photothermal nanoparticles for biomedical applications Advanced Science, 6(17), Article 1900471 Kubacka, A., Cerrada, M L., Serrano, C., Fernandez-Garcia, M., Ferrer, M., & Fern´ andezGarcia, M (2009) Plasmonic nanoparticle/polymer nanocomposites with enhanced photocatalytic antimicrobial properties The Journal of Physical Chemistry C, 113(21), 9182–9190 Kvítek, L., Pan´ aˇcek, A., Soukupova, J., Kol´ aˇr, M., Veˇceˇrov´ a, R., Prucek, R., & Zboˇril, R (2008) Effect of surfactants and polymers on stability and antibacterial activity of silver nanoparticles (NPs) The Journal of Physical Chemistry C, 112(15), 5825–5834 Marcelino, R B., & Amorim, C C (2019) Towards visible-light photocatalysis for environmental applications: Band-gap engineering versus photons absorption—A review Environmental Science Pollution Research, 26(5), 4155–4170 Marcus, R A (1993) Electron transfer reactions in chemistry: Theory and experiment (Nobel lecture) Angewandte Chemie International Edition in English, 32(8), 1111–1121 Martínez, N P., Inostroza-Rivera, R., Dur´ an, B., Molero, L., Bonardd, S., Ramírez, O., & Saldías, C (2019) Exploring the effect of the irradiation time on photosensitized dendrimer-based nanoaggregates for potential applications in light-driven water photoreduction Nanomaterials, 9(9), 1316 Matinise, N., Khutlane, J T., & Malgas-Enus, R (2022) The effect of magic number phosphine stabilised mono-and bimetallic Au, Cu and Au-Cu nanoparticles as catalysts in the reduction of 4-nitrophenol–A kinetic study Nano-Structures & NanoObjects, 29, Article 100814 Min, Y., & Wang, Y (2020) Manipulating bimetallic nanostructures with tunable localized surface plasmon resonance and their applications for sensing Frontiers in Chemistry, 8, 411 Oi, L E., Choo, M.-Y., Lee, H V., Ong, H C., Abd Hamid, S B., & Juan, J C (2016) Recent advances of titanium dioxide (TiO 2) for green organic synthesis RSC Advances, 6(110), 108741–108754 Papageorgiou, S K., Kouvelos, E P., Favvas, E P., Sapalidis, A A., Romanos, G E., & Katsaros, F K (2010) Metal–carboxylate interactions in metal–alginate complexes studied with FTIR spectroscopy Carbohydrate Research, 345(4), 469–473 Pastoriza-Santos, I., Kinnear, C., P´ erez-Juste, J., Mulvaney, P., & Liz-Marz´ an, L M (2018) Plasmonic polymer nanocomposites Nature Reviews Materials, 3(10), 375–391 10 ... 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... Schematic representation of pristine alginate beads, after the adsorption of gold ions and the further reduction to obtain mono and bimetallic nanoparticles Fig (A) FE-SEM micrograph of alginate hydrogel... syringe connected to an infusion pump at 0.3 mL/min, 10 mL of a % w/v sodium alginate solution was added dropwise on 90 mL of a CaCl2 % w/v solution, (gelling medium) under mild stirring Once the alginate