International Journal of Biological Macromolecules 73 (2015) 39–44
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Short communication
In situ generation of silver nanoparticles within crosslinked 3D guar gum networks for catalytic reduction Yian Zheng a , Yongfeng Zhu a,b , Guangyan Tian a,b , Aiqin Wang a,∗ a Center of Eco-materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China b University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 September 2014 Received in revised form 20 October 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Guar gum Biopolymer Silver nanoparticles Green reduction 4-Nitrophenol
a b s t r a c t The direct use of guar gum (GG) as a green reducing agent for the facile production of highly stable silver nanoparticles (Ag NPs) within this biopolymer and subsequent crosslinking with borax to form crosslinked Ag@GG beads with a 3D-structured network are presented here. These crosslinked Ag@GG beads were characterized using UV–vis absorption spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy, and then tested as a solid-phase heterogenerous catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess borohydride. The results indicate that these crosslinked Ag@GG beads show excellent catalytic performance for the reduction of 4-NP within 20 min and can be readily used for 10 successive cycles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, metal nanoparticles (NPs) have been investigated extensively to understand their physical and chemical properties, due to their potential applications in emerging areas of nano-science and nano-technology. In the nano size regime, metal NPs have received special attention because of their characteristic optical, electronic and catalytic properties [1–4]. Generally, the preparation of metal NPs involves the reduction of metal ions with a suitable reducing agent, such as hydrazine, dimethyl formamide (DMF), and sodium borohydride (NaBH4 ). All of these are highly reactive chemicals and will pose potential environmental and biological risks. Following the principles of “green chemistry”, the primary challenges in this regard are the maximization of usage of environmentally friendly materials and adoption of sustainable processes in the generation of nano-sized metal particles. It is now well established that polymers are excellent host materials for the preparation of metal NPs and serve also as a surface-capping agent when those NPs are embedded or encapsulated in a polymer. Due to large reserves, biodegradability and eco-friendly “green” processing [5,6], the use of biopolymer such as starch [5], alginate [7,8], chitosan [9,10] and cellulose [11] in
∗ Corresponding author. Tel.: +86 931 4968118; fax: +86 931 8277088. E-mail address: [email protected] (A. Wang). http://dx.doi.org/10.1016/j.ijbiomac.2014.11.007 0141-8130/© 2014 Elsevier B.V. All rights reserved.
research and industry has significantly increased. The biopolymers can provide a size-confined micro-environment where the reduction of metal ions into NPs can be achieved by biopolymer itself via adsorption coupled reduction pathways [12,13], or by external assistance via chemically [9,14], photochemically [8], by heating [15–17], by laser ablation [18] or by high-energy radiation [19]. Some biopolymers (such as chitosan) not only show a better ability to stabilize the resulting metal NPs by anchoring them, but also act as the reducing agent for the surrounding metal ions [20], but this in situ reduction requires heating and controlled pH. Guar gum (GG) is an edible carbohydrate polymer extracted from the seeds of Cyamopsis tetragonoloba and is considered as a polysaccharide with one of the highest molecular weights of all naturally occurring water soluble polymers. It is a nonionic, branched-chain polymer, consisting of straight-chain mannose units joined by -d-(1-4) linkages having ␣-d-galactopyranose units attached to this linear chain by (1-6) linkages. Galactose and mannose are the repeating units in GG. Compared with native GG, sulfated or phosphorylated GG shows better antioxidant activities [21,22]. Grafting GG with acrylamide irradiated by microwaves can be used as a better drug delivery system in colon [23]. GG has a strong hydrogen bond forming tendency in water which makes it an excellent thickener and stabilizer [24]. GG has also a strong tendency to form gel in the presence of borax, an efficient crosslinker for polymers bearing hydroxyl groups [25]. These characteristics enable GG to entrap, protect and stabilize the synthesized metal
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NPs by acting as an excellent surface capping agent [26]. GG solution is stable over time, not prone to coagulation over a wide range of salinity and pH. Therefore, GG can be used to effectively improve stability and mobility of zerovalent iron NPs used for in situ remediation of groundwater [27,28]. Actually, GG has been testified as a suitable candidate to effectively stabilize iron NPs than starch and alginate [29,16]. Furthermore, the inherent biocompatibility and biodegradability in the presence of specific enzymes and microorganisms makes GG are potential alternatives used widely in a variety of applications in biotechnology and in environmental protection. Based on above backgrounds, we present a totally green approach toward the direct synthesis and stabilization of metallic Ag NPs in this study using GG as the reducing agent for the in situ production of Ag NPs as well as their stabilizer. Followed by granulation and crosslinking with borax, we further explore the possibility of the resulting Ag@GG beads as an eco-friendly heterogeneous catalyst for the conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). 4-NP is one of the most common organic pollutants in wastewater, while the resulting 4-AP is an important intermediate for the preparation of many analgesic and antipyretic drugs such as paracetamol, acetanilide, phenacetin, and so forth [30]. With the rapid development of nanotechnology in the past two decades, the borohydride reduction of 4-NP to 4-AP with the assistance of metal NPs has been considered to an effective approach and received increasing attention owing to its fast, simple and mild characteristics [31,32]. 2. Materials and methods 2.1. Materials Guar gum (GG, food grade, number-average molecular weight of 220,000) was purchased from Wuhan Tianyuan Biology Co. (Wuhan, China). Sodium borohydride, silver nitrate, 4-nitrophenol (4-NP), sodium tetraborate and acetone were of analytical grade and used as received. All solutions were prepared with distilled water.
a 1 cm path length, which was quickly placed in the cell holder of the spectrophotometer. The progress was monitored by recording the time-dependent absorbance with a UV-vis spectrophotometer to follow the evolution of the reaction in a scanning range of 250–550 nm at room temperature. 2.4. Reusability Typically, 10 mg crosslinked Ag@GG beads were added into 3 mL of mixed solution containing 10 mg/L 4-NP and 20 mM NaBH4 . The reaction was followed at set time intervals using a UV-vis spectrophotometer at the maximal absorbance of 400 nm for 4-NP in the presence of NaBH4 . Then, the crosslinked Ag@GG beads were filtered using 100-mesh stainless sieve, washed with 100 mL distilled water, and placed into another 3 mL of mixed solution containing 10 mg/L 4-NP and 20 mM NaBH4 . The consecutive time interval was 20 min for the reduction of 4-NP. 2.5. Characterization UV–vis absorption spectra of the liquid samples as well as samples for catalytic studies were recorded using a TU-1900 double-beam UV-vis spectrometer in the range 250–600 nm. XRD patterns were acquired on a Philip X’Pert Pro diffractometer using Cu-K␣ radiation of 1.5406 A˚ (40 kV, 30 mA). The surface morphologies of crosslinked Ag@GG beads before and after the catalytic reaction were observed by a field emission scanning electron microscope (FESEM, JSM-6701F, JEOL) after coating the samples with gold film. The transmission electron microscopy (TEM) image was performed on a TECNAI G2 TF20 transmission electron microscope. The samples were dispersed in absolute ethanol by sonication and then dropped onto a carbon-coated copper grid. FTIR spectra were recorded on a Thermo Nicolet NEXUS TM spectrophotometer using KBr pellets in the range of 400–4000 cm−1 , with a resolution of 4 cm−1 .
3. Results and discussion 2.2. Preparation of crosslinked GG or Ag@GG beads 3.1. Formation of Ag@GG beads A 100 mL beaker containing 50 mL distilled water was kept at room temperature with constant mechanical stirring (500 rpm), which was followed by addition of an appropriate amount of GG powder into the beaker. After 30 min, an accurately weighed amount of AgNO3 was added according to its final concentration in the solution (0, 5, 10, 20, 30 and 40 mmol/L), and then stirred at 500 rpm for another 30 min. After 24 h, the color of GG solution from yellow to brown was observed according to different silver concentration. By dropping this mixture with a syringe into a beaker containing 100 mL acetone, many beads can be formed and stayed for 2 h in the solution. Afterwards, these beads with an average size of 3.2 mm were collected from acetone by filtration, transferred to 100 mL 1 wt% borax aqueous solution and stayed for 4 h for enough crosslinking between GG and borate ions. After crosslinking, the average size of these beads shows a slight increase to 3.3 mm. Finally, these beads were collected from the solution, washed with acetone and dried at room temperature for further use. The dried beads show a reduced size to 50% of its original size. 2.3. Catalytic reduction In a typical experiment, an accurately weighed amount of 10 mg crosslinked GG or Ag@GG was added into 3 mL of mixed solution containing 10 mg/L 4-NP and 20 mmol/L NaBH4 in a quartz cell with
GG is a nonionic and branched-chain polymer with repeating units of galactose and mannose (Supplementary Fig. S1a). However, the incompatibility of GG with water miscible solvent, such as acetone in this study, is observed. The acetone can rapidly deprive the GG of water causing the GG’s precipitation. Therefore, when the GG solution was injected into acetone using a syringe, a large amount of beads immediately occur at the droplet surface, which are further crosslinked by borate ions (Supplementary Fig. S1b). It is observed that the shape of these beads can be adjusted by varying the concentration of GG solution (Supplementary Figs. S2 and S3), and the shape of these beads transforms from sphere to rod as a result of increasing viscosity (Supplementary Fig. S2). The granulation results indicate that the concentration range of 0.7–0.8% (0.75% was chosen in this study) is more appropriate for further mixing with Ag+ ions. Upon the addition of AgNO3 into GG solution, the semitransparent solution turned yellow gradually and darkened to brown with the time indicating the formation of Ag NPs (Supplementary Fig. S4), which is attributed to the increased amount of Ag NPs produced. Due to the presence of a large amount of hydroxyl groups, GG is expected to react with borate ions to form a crosslinked structure (Supplementary Fig. S1b) [33]. A great number of crosslinks between GG and borate ions is favored for securing those Ag NPs within the 3D networks against leakage.
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NPs. Before the catalytic reaction, the XRD pattern of crosslinked Ag@GG exhibits one characteristic peak at 2 = 38.1◦ indexed to the [1 1 1] plane of a face-centered cubic (fcc) lattice of Ag NPs, while the peaks belonging to other lattice planes cannot be well observed, which may be due to the presence of large amount of premature Ag lattice planes. There are several OH groups in GG that can not only bind with the Ag clusters, but also be crosslinked by borate ions. Information from FTIR spectra (Fig. 1c) suggests that for the stretching vibrations of C O(H), the band shape and intensity at 1021, 1081 and 1158 cm−1 have been changed after successful crosslinking by borate ions. Micrograph images (Fig. 2a and c) show that the Ag particles are embedded and encapsulated in the GG matrix in a scattered manner. The majority of the particles formed have sizes about 5 nm in diameter (Supplementary Fig. S6a). These particles are highly dispersed in the biopolymer material but with a broad distribution of the particle size. Some aggregates are also observed: the agglomeration of small NPs leads to the formation of large Ag particles. The small sized Ag particles are expected to show higher catalytic activity: the smaller the size of catalyst particle, the greater the catalytic activity. 3.3. Catalytic property
Fig. 1. (a) UV–vis spectra of GG solution with and without AgNO4 after 24 h. The concentration of GG is 0.075 wt%, and the concentration of silver is 2 mmol/L. (b) XRD patterns of crosslinked GG, Ag@GG before and after the catalytic reduction from the bottom. (c) FTIR spectra of GG powder, crosslinked GG and Ag@GG from the bottom.
3.2. Characterization The UV–vis absorption spectra of GG and Ag NPs-embedded GG solutions are shown in Fig. 1a and Supplementary Fig. S5. The GG solution shows no any peak in the visible region, while an absorption band closer to 450 nm is obtained for GG solution containing AgNO3 . Typically, Ag NPs show a distinct SPR (surface plasmon resonance) band between 390 and 420 nm [34,35]. This emerging absorption band at ∼450 nm with a significant red shift of SPR is associated with the changes in size, shape [36] and composition [37] of metal NPs. Generally, spectral shifts associated with SPR of metal NPs are influences to a greater extent by deviation from spherical geometry than by an increase in size [38,36]. The XRD spectra shown in Fig. 1b confirm the formation of crystalline Ag
The catalytic activity of the as-prepared Ag@GG beads is substantiated through 4-NP reduction in the presence of NaBH4 under atmospheric conditions. This reaction is a thermodynamically feasible but kinetically restricted process in the absence of a catalyst and has been used as one of the model reactions for evaluating the catalytic activity of various metal NPs. Upon the addition of crosslinked Ag@GG beads, a gradual decrease in absorbance at 400 nm is associated with the concomitant evolution at 300 nm (Fig. 3a and b), and the latter is ascribed to the absorption of 4-AP. Meanwhile, a fading and ultimate bleaching of the yellow-green color derived from 4-nitrophenolate ions in the aqueous solution and evolution of small bubbles of hydrogen gas surrounding the catalyst particles are observed, suggesting the occurrence of a reduction reaction from 4-NP to 4-AP. During this process, the produced hydrogen from NaBH4 can not only help to stir the solution, but also purge out the air to prevent the aerial oxidation of 4-AP. As a result, catalyst particles remain distributed in the reaction mixture during the reaction time, offering favorable conditions for a smooth reaction. However, when crosslinked GG beads are added into the aqueous solution of 4-NP in the presence of NaBH4 , the absorption peak at 400 nm remains unaltered for a long duration (Fig. 3c), and this excludes the possibility that the reduction reaction might be activated by crosslinked GG beads. When the catalytic reaction is complete, those premature Ag lattice planes have been improved, and accordingly, the Bragg’s reflections at 2 = 38.1◦ , 44.3◦ , 64.5◦ , and 77.4◦ corresponding respectively to [1 1 1], [2 0 0], [2 2 0] and [3 1 1] lattice planes appear (Fig. 1b), which are ascribed to the fcc structure of Ag NPs embedded in crosslinked GG networks. TEM evidence indicates that after the catalytic reduction, the size of Ag NPs is growing to about 8 nm in diameter (Fig. 2b and d, Supplementary Fig. S6b). Chen et al. observed that the cultivated Pichia pastoris (P. pastoris) GS115 showed the adsorption capacity for Pd(II) ions and during this process, an adsorption coupled reduction pathway occurred for achieving the Pd NPs attached onto the P. pastoris cell surface, with the conclusions that the slow reduction of Pd(II) ions with the P. pastoris cell allowed preferential nucleation over the cell surface, strengthening the interaction between Pd NPs and the cell surface, and accordingly, the Pd NPs/P. pastoris gave a better durability for the multi-cycle catalysis [39]. Furthermore, for some redox reactions, the rate of catalysis involving the growing metal NPs is higher than those involving fully grown NPs [40]. During the reaction
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Fig. 2. TEM (a–d) and SEM (e and f) images of Ag@GG before (a, c, e) and after (b, d, f) the catalytic reaction.
process, we have also observed that in the presence of NaBH4 , the color of Ag@GG is further darkened, which may be correlated to the growth of Ag NPs within the crosslinked GG networks. Here, it is important to note that the metal NPs loaded onto Ag@GG beads are not determined for that no free Ag+ ions are detected during the preparation process, i.e. all the Ag+ ions added into the solution are embedded within the crosslinked GG networks. In industrial applications, catalysis may be performed in homogeneous systems. However, in the case of expensive catalysts such as those involving precious and strategic metals, it is important to recover these metal catalysts at the end of the reactions. It is evident from Fig. 3d that the as-prepared Ag@GG beads are not only very effective for the catalytic reduction of aromatic nitro compounds, but also are recoverable and can be used for catalyzing the same reaction at least for ten cycles without significant loss in activity. From FTIR spectra (Supplementary Fig. S7), we cannot find any changes in the characteristic absorption bands before and after the catalytic reduction. This information suggests that the crosslinked Ag@GG beads are sufficiently stable during the catalytic reaction, demonstrating its potential application in catalysis. Based on above discussions, the schematic preparation of crosslinked GG and Ag@GG beads as well as their catalytic reduction for 4-NP are proposed as Fig. 4. When AgNO3 was added into the GG
solution, a clear yellow solution was obtained, which is due to the appearance of Ag clusters in the solution. Because of the presence of a long linear chain of mannose units and a side chain of galactose units, the anchored Ag clusters were prevented from growing further by steric hindrance, and thus, they were stabilized. When the droplets of the GG solution with or without Ag clusters were extruded through a syringe into acetone, many beads can be formed (stage a). Afterwards, these beads were transferred from acetone to 1 wt% borax solution for crosslinking between GG and borate ions, and accordingly, we can obtain the crosslinked GG or Ag@GG beads (stage b). For the latter, more Ag clusters will be embedded within the crosslinked GG networks. The resulting beads were collected, washed and dried for further catalytic reduction of 4-NP. During this process, the sizes of Ag particles will suffer from a growing stage, and these growing Ag particles may show a higher catalytic activity for 4-NP. Yin et al. [41] investigated the reductive process of ionic Au(III) to Au NPs by ubiquitous dissolved organic matter (DOM) in river water using X-ray photoelectron spectroscopy and revealed that phenolic, alcoholic, and aldehyde groups in DOM can be served as the reductive sites to facilitate the formation of Au NPs. For GG, an important property is its hydrogen bonding activity and this is generally attributed to the presence and behavior of numerous
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Fig. 3. (a) Successive UV–vis spectra for 4-NP reduction in solution. (b) The changes in A/A0 for 4-NP reduction at 400 nm and 300 nm in solution. (c) The changes in A/A0 for 4-NP reduction in the presence or absence of Ag@GG. (d) Catalytic kinetics of 4-NP for 10 successive reactions with the same batch of Ag@GG. Reaction conditions: 10 mg/L 4-NP, 20 mmol/L NaBH4 and 10 mg crosslinked GG or Ag@GG.
Fig. 4. Schematic illustration for the formation of crosslinked GG and Ag@GG beads as well as their catalytic reduction for 4-NP.
hydroxyl groups [42], which enables those Ag ions to be in situ reduced into Ag NPs in this study by acting as the green reducing agent. Another characteristic for GG is the crosslinking reaction involving borate ions (Supplementary Fig. S1), like in the case of poly(vinyl alcohol)/borate ion coordinates. Owing to efficient crosslinking reaction, we can obtain many large granular GG beads (Supplementary Fig. S3) with 3D structured networks, which can then be utilized as the catalyst to facilitate the reduction of 4-NP to 4-AP. Therefore, those Ag NPs cannot be escaped from that formed 3D network, and furthermore, these crosslinked GG beads can be collected directly after the catalytic reaction.
impurities into the products. Catalytic performances reveal that the as-prepared Ag@GG beads show excellent catalytic activity toward the reduction of 4-NP in the presence of NaBH4 . GG is commercially available, inexpensive and environmentally friendly green biopolymer, and this preparative procedure involved in this study is surprisingly simple. It can provide a facile and green approach toward manufacturing of metallic nanocomposites, antimicrobial materials, low-temperature catalysts, and other useful materials.
4. Conclusions
The authors are grateful for the support of the National Natural Science Foundation of China (Nos. 21107116, 21377135 and 21477135) and Science and Technology Achievement Transformation Foundation of Jiangsu Province (BA2011100).
With GG as the green reducing agent, crosslinked Ag@GG beads were in situ prepared in this study without introducing disturbing
Acknowledgements
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Short communication
In situ generation of silver nanoparticles within crosslinked 3D guar gum networks for catalytic reduction Yian Zheng a , Yongfeng Zhu a,b , Guangyan Tian a,b , Aiqin Wang a,∗ a Center of Eco-materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China b University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 September 2014 Received in revised form 20 October 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Guar gum Biopolymer Silver nanoparticles Green reduction 4-Nitrophenol
a b s t r a c t The direct use of guar gum (GG) as a green reducing agent for the facile production of highly stable silver nanoparticles (Ag NPs) within this biopolymer and subsequent crosslinking with borax to form crosslinked Ag@GG beads with a 3D-structured network are presented here. These crosslinked Ag@GG beads were characterized using UV–vis absorption spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy, and then tested as a solid-phase heterogenerous catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess borohydride. The results indicate that these crosslinked Ag@GG beads show excellent catalytic performance for the reduction of 4-NP within 20 min and can be readily used for 10 successive cycles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, metal nanoparticles (NPs) have been investigated extensively to understand their physical and chemical properties, due to their potential applications in emerging areas of nano-science and nano-technology. In the nano size regime, metal NPs have received special attention because of their characteristic optical, electronic and catalytic properties [1–4]. Generally, the preparation of metal NPs involves the reduction of metal ions with a suitable reducing agent, such as hydrazine, dimethyl formamide (DMF), and sodium borohydride (NaBH4 ). All of these are highly reactive chemicals and will pose potential environmental and biological risks. Following the principles of “green chemistry”, the primary challenges in this regard are the maximization of usage of environmentally friendly materials and adoption of sustainable processes in the generation of nano-sized metal particles. It is now well established that polymers are excellent host materials for the preparation of metal NPs and serve also as a surface-capping agent when those NPs are embedded or encapsulated in a polymer. Due to large reserves, biodegradability and eco-friendly “green” processing [5,6], the use of biopolymer such as starch [5], alginate [7,8], chitosan [9,10] and cellulose [11] in
∗ Corresponding author. Tel.: +86 931 4968118; fax: +86 931 8277088. E-mail address: [email protected] (A. Wang). http://dx.doi.org/10.1016/j.ijbiomac.2014.11.007 0141-8130/© 2014 Elsevier B.V. All rights reserved.
research and industry has significantly increased. The biopolymers can provide a size-confined micro-environment where the reduction of metal ions into NPs can be achieved by biopolymer itself via adsorption coupled reduction pathways [12,13], or by external assistance via chemically [9,14], photochemically [8], by heating [15–17], by laser ablation [18] or by high-energy radiation [19]. Some biopolymers (such as chitosan) not only show a better ability to stabilize the resulting metal NPs by anchoring them, but also act as the reducing agent for the surrounding metal ions [20], but this in situ reduction requires heating and controlled pH. Guar gum (GG) is an edible carbohydrate polymer extracted from the seeds of Cyamopsis tetragonoloba and is considered as a polysaccharide with one of the highest molecular weights of all naturally occurring water soluble polymers. It is a nonionic, branched-chain polymer, consisting of straight-chain mannose units joined by -d-(1-4) linkages having ␣-d-galactopyranose units attached to this linear chain by (1-6) linkages. Galactose and mannose are the repeating units in GG. Compared with native GG, sulfated or phosphorylated GG shows better antioxidant activities [21,22]. Grafting GG with acrylamide irradiated by microwaves can be used as a better drug delivery system in colon [23]. GG has a strong hydrogen bond forming tendency in water which makes it an excellent thickener and stabilizer [24]. GG has also a strong tendency to form gel in the presence of borax, an efficient crosslinker for polymers bearing hydroxyl groups [25]. These characteristics enable GG to entrap, protect and stabilize the synthesized metal
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NPs by acting as an excellent surface capping agent [26]. GG solution is stable over time, not prone to coagulation over a wide range of salinity and pH. Therefore, GG can be used to effectively improve stability and mobility of zerovalent iron NPs used for in situ remediation of groundwater [27,28]. Actually, GG has been testified as a suitable candidate to effectively stabilize iron NPs than starch and alginate [29,16]. Furthermore, the inherent biocompatibility and biodegradability in the presence of specific enzymes and microorganisms makes GG are potential alternatives used widely in a variety of applications in biotechnology and in environmental protection. Based on above backgrounds, we present a totally green approach toward the direct synthesis and stabilization of metallic Ag NPs in this study using GG as the reducing agent for the in situ production of Ag NPs as well as their stabilizer. Followed by granulation and crosslinking with borax, we further explore the possibility of the resulting Ag@GG beads as an eco-friendly heterogeneous catalyst for the conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). 4-NP is one of the most common organic pollutants in wastewater, while the resulting 4-AP is an important intermediate for the preparation of many analgesic and antipyretic drugs such as paracetamol, acetanilide, phenacetin, and so forth [30]. With the rapid development of nanotechnology in the past two decades, the borohydride reduction of 4-NP to 4-AP with the assistance of metal NPs has been considered to an effective approach and received increasing attention owing to its fast, simple and mild characteristics [31,32]. 2. Materials and methods 2.1. Materials Guar gum (GG, food grade, number-average molecular weight of 220,000) was purchased from Wuhan Tianyuan Biology Co. (Wuhan, China). Sodium borohydride, silver nitrate, 4-nitrophenol (4-NP), sodium tetraborate and acetone were of analytical grade and used as received. All solutions were prepared with distilled water.
a 1 cm path length, which was quickly placed in the cell holder of the spectrophotometer. The progress was monitored by recording the time-dependent absorbance with a UV-vis spectrophotometer to follow the evolution of the reaction in a scanning range of 250–550 nm at room temperature. 2.4. Reusability Typically, 10 mg crosslinked Ag@GG beads were added into 3 mL of mixed solution containing 10 mg/L 4-NP and 20 mM NaBH4 . The reaction was followed at set time intervals using a UV-vis spectrophotometer at the maximal absorbance of 400 nm for 4-NP in the presence of NaBH4 . Then, the crosslinked Ag@GG beads were filtered using 100-mesh stainless sieve, washed with 100 mL distilled water, and placed into another 3 mL of mixed solution containing 10 mg/L 4-NP and 20 mM NaBH4 . The consecutive time interval was 20 min for the reduction of 4-NP. 2.5. Characterization UV–vis absorption spectra of the liquid samples as well as samples for catalytic studies were recorded using a TU-1900 double-beam UV-vis spectrometer in the range 250–600 nm. XRD patterns were acquired on a Philip X’Pert Pro diffractometer using Cu-K␣ radiation of 1.5406 A˚ (40 kV, 30 mA). The surface morphologies of crosslinked Ag@GG beads before and after the catalytic reaction were observed by a field emission scanning electron microscope (FESEM, JSM-6701F, JEOL) after coating the samples with gold film. The transmission electron microscopy (TEM) image was performed on a TECNAI G2 TF20 transmission electron microscope. The samples were dispersed in absolute ethanol by sonication and then dropped onto a carbon-coated copper grid. FTIR spectra were recorded on a Thermo Nicolet NEXUS TM spectrophotometer using KBr pellets in the range of 400–4000 cm−1 , with a resolution of 4 cm−1 .
3. Results and discussion 2.2. Preparation of crosslinked GG or Ag@GG beads 3.1. Formation of Ag@GG beads A 100 mL beaker containing 50 mL distilled water was kept at room temperature with constant mechanical stirring (500 rpm), which was followed by addition of an appropriate amount of GG powder into the beaker. After 30 min, an accurately weighed amount of AgNO3 was added according to its final concentration in the solution (0, 5, 10, 20, 30 and 40 mmol/L), and then stirred at 500 rpm for another 30 min. After 24 h, the color of GG solution from yellow to brown was observed according to different silver concentration. By dropping this mixture with a syringe into a beaker containing 100 mL acetone, many beads can be formed and stayed for 2 h in the solution. Afterwards, these beads with an average size of 3.2 mm were collected from acetone by filtration, transferred to 100 mL 1 wt% borax aqueous solution and stayed for 4 h for enough crosslinking between GG and borate ions. After crosslinking, the average size of these beads shows a slight increase to 3.3 mm. Finally, these beads were collected from the solution, washed with acetone and dried at room temperature for further use. The dried beads show a reduced size to 50% of its original size. 2.3. Catalytic reduction In a typical experiment, an accurately weighed amount of 10 mg crosslinked GG or Ag@GG was added into 3 mL of mixed solution containing 10 mg/L 4-NP and 20 mmol/L NaBH4 in a quartz cell with
GG is a nonionic and branched-chain polymer with repeating units of galactose and mannose (Supplementary Fig. S1a). However, the incompatibility of GG with water miscible solvent, such as acetone in this study, is observed. The acetone can rapidly deprive the GG of water causing the GG’s precipitation. Therefore, when the GG solution was injected into acetone using a syringe, a large amount of beads immediately occur at the droplet surface, which are further crosslinked by borate ions (Supplementary Fig. S1b). It is observed that the shape of these beads can be adjusted by varying the concentration of GG solution (Supplementary Figs. S2 and S3), and the shape of these beads transforms from sphere to rod as a result of increasing viscosity (Supplementary Fig. S2). The granulation results indicate that the concentration range of 0.7–0.8% (0.75% was chosen in this study) is more appropriate for further mixing with Ag+ ions. Upon the addition of AgNO3 into GG solution, the semitransparent solution turned yellow gradually and darkened to brown with the time indicating the formation of Ag NPs (Supplementary Fig. S4), which is attributed to the increased amount of Ag NPs produced. Due to the presence of a large amount of hydroxyl groups, GG is expected to react with borate ions to form a crosslinked structure (Supplementary Fig. S1b) [33]. A great number of crosslinks between GG and borate ions is favored for securing those Ag NPs within the 3D networks against leakage.
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NPs. Before the catalytic reaction, the XRD pattern of crosslinked Ag@GG exhibits one characteristic peak at 2 = 38.1◦ indexed to the [1 1 1] plane of a face-centered cubic (fcc) lattice of Ag NPs, while the peaks belonging to other lattice planes cannot be well observed, which may be due to the presence of large amount of premature Ag lattice planes. There are several OH groups in GG that can not only bind with the Ag clusters, but also be crosslinked by borate ions. Information from FTIR spectra (Fig. 1c) suggests that for the stretching vibrations of C O(H), the band shape and intensity at 1021, 1081 and 1158 cm−1 have been changed after successful crosslinking by borate ions. Micrograph images (Fig. 2a and c) show that the Ag particles are embedded and encapsulated in the GG matrix in a scattered manner. The majority of the particles formed have sizes about 5 nm in diameter (Supplementary Fig. S6a). These particles are highly dispersed in the biopolymer material but with a broad distribution of the particle size. Some aggregates are also observed: the agglomeration of small NPs leads to the formation of large Ag particles. The small sized Ag particles are expected to show higher catalytic activity: the smaller the size of catalyst particle, the greater the catalytic activity. 3.3. Catalytic property
Fig. 1. (a) UV–vis spectra of GG solution with and without AgNO4 after 24 h. The concentration of GG is 0.075 wt%, and the concentration of silver is 2 mmol/L. (b) XRD patterns of crosslinked GG, Ag@GG before and after the catalytic reduction from the bottom. (c) FTIR spectra of GG powder, crosslinked GG and Ag@GG from the bottom.
3.2. Characterization The UV–vis absorption spectra of GG and Ag NPs-embedded GG solutions are shown in Fig. 1a and Supplementary Fig. S5. The GG solution shows no any peak in the visible region, while an absorption band closer to 450 nm is obtained for GG solution containing AgNO3 . Typically, Ag NPs show a distinct SPR (surface plasmon resonance) band between 390 and 420 nm [34,35]. This emerging absorption band at ∼450 nm with a significant red shift of SPR is associated with the changes in size, shape [36] and composition [37] of metal NPs. Generally, spectral shifts associated with SPR of metal NPs are influences to a greater extent by deviation from spherical geometry than by an increase in size [38,36]. The XRD spectra shown in Fig. 1b confirm the formation of crystalline Ag
The catalytic activity of the as-prepared Ag@GG beads is substantiated through 4-NP reduction in the presence of NaBH4 under atmospheric conditions. This reaction is a thermodynamically feasible but kinetically restricted process in the absence of a catalyst and has been used as one of the model reactions for evaluating the catalytic activity of various metal NPs. Upon the addition of crosslinked Ag@GG beads, a gradual decrease in absorbance at 400 nm is associated with the concomitant evolution at 300 nm (Fig. 3a and b), and the latter is ascribed to the absorption of 4-AP. Meanwhile, a fading and ultimate bleaching of the yellow-green color derived from 4-nitrophenolate ions in the aqueous solution and evolution of small bubbles of hydrogen gas surrounding the catalyst particles are observed, suggesting the occurrence of a reduction reaction from 4-NP to 4-AP. During this process, the produced hydrogen from NaBH4 can not only help to stir the solution, but also purge out the air to prevent the aerial oxidation of 4-AP. As a result, catalyst particles remain distributed in the reaction mixture during the reaction time, offering favorable conditions for a smooth reaction. However, when crosslinked GG beads are added into the aqueous solution of 4-NP in the presence of NaBH4 , the absorption peak at 400 nm remains unaltered for a long duration (Fig. 3c), and this excludes the possibility that the reduction reaction might be activated by crosslinked GG beads. When the catalytic reaction is complete, those premature Ag lattice planes have been improved, and accordingly, the Bragg’s reflections at 2 = 38.1◦ , 44.3◦ , 64.5◦ , and 77.4◦ corresponding respectively to [1 1 1], [2 0 0], [2 2 0] and [3 1 1] lattice planes appear (Fig. 1b), which are ascribed to the fcc structure of Ag NPs embedded in crosslinked GG networks. TEM evidence indicates that after the catalytic reduction, the size of Ag NPs is growing to about 8 nm in diameter (Fig. 2b and d, Supplementary Fig. S6b). Chen et al. observed that the cultivated Pichia pastoris (P. pastoris) GS115 showed the adsorption capacity for Pd(II) ions and during this process, an adsorption coupled reduction pathway occurred for achieving the Pd NPs attached onto the P. pastoris cell surface, with the conclusions that the slow reduction of Pd(II) ions with the P. pastoris cell allowed preferential nucleation over the cell surface, strengthening the interaction between Pd NPs and the cell surface, and accordingly, the Pd NPs/P. pastoris gave a better durability for the multi-cycle catalysis [39]. Furthermore, for some redox reactions, the rate of catalysis involving the growing metal NPs is higher than those involving fully grown NPs [40]. During the reaction
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Fig. 2. TEM (a–d) and SEM (e and f) images of Ag@GG before (a, c, e) and after (b, d, f) the catalytic reaction.
process, we have also observed that in the presence of NaBH4 , the color of Ag@GG is further darkened, which may be correlated to the growth of Ag NPs within the crosslinked GG networks. Here, it is important to note that the metal NPs loaded onto Ag@GG beads are not determined for that no free Ag+ ions are detected during the preparation process, i.e. all the Ag+ ions added into the solution are embedded within the crosslinked GG networks. In industrial applications, catalysis may be performed in homogeneous systems. However, in the case of expensive catalysts such as those involving precious and strategic metals, it is important to recover these metal catalysts at the end of the reactions. It is evident from Fig. 3d that the as-prepared Ag@GG beads are not only very effective for the catalytic reduction of aromatic nitro compounds, but also are recoverable and can be used for catalyzing the same reaction at least for ten cycles without significant loss in activity. From FTIR spectra (Supplementary Fig. S7), we cannot find any changes in the characteristic absorption bands before and after the catalytic reduction. This information suggests that the crosslinked Ag@GG beads are sufficiently stable during the catalytic reaction, demonstrating its potential application in catalysis. Based on above discussions, the schematic preparation of crosslinked GG and Ag@GG beads as well as their catalytic reduction for 4-NP are proposed as Fig. 4. When AgNO3 was added into the GG
solution, a clear yellow solution was obtained, which is due to the appearance of Ag clusters in the solution. Because of the presence of a long linear chain of mannose units and a side chain of galactose units, the anchored Ag clusters were prevented from growing further by steric hindrance, and thus, they were stabilized. When the droplets of the GG solution with or without Ag clusters were extruded through a syringe into acetone, many beads can be formed (stage a). Afterwards, these beads were transferred from acetone to 1 wt% borax solution for crosslinking between GG and borate ions, and accordingly, we can obtain the crosslinked GG or Ag@GG beads (stage b). For the latter, more Ag clusters will be embedded within the crosslinked GG networks. The resulting beads were collected, washed and dried for further catalytic reduction of 4-NP. During this process, the sizes of Ag particles will suffer from a growing stage, and these growing Ag particles may show a higher catalytic activity for 4-NP. Yin et al. [41] investigated the reductive process of ionic Au(III) to Au NPs by ubiquitous dissolved organic matter (DOM) in river water using X-ray photoelectron spectroscopy and revealed that phenolic, alcoholic, and aldehyde groups in DOM can be served as the reductive sites to facilitate the formation of Au NPs. For GG, an important property is its hydrogen bonding activity and this is generally attributed to the presence and behavior of numerous
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Fig. 3. (a) Successive UV–vis spectra for 4-NP reduction in solution. (b) The changes in A/A0 for 4-NP reduction at 400 nm and 300 nm in solution. (c) The changes in A/A0 for 4-NP reduction in the presence or absence of Ag@GG. (d) Catalytic kinetics of 4-NP for 10 successive reactions with the same batch of Ag@GG. Reaction conditions: 10 mg/L 4-NP, 20 mmol/L NaBH4 and 10 mg crosslinked GG or Ag@GG.
Fig. 4. Schematic illustration for the formation of crosslinked GG and Ag@GG beads as well as their catalytic reduction for 4-NP.
hydroxyl groups [42], which enables those Ag ions to be in situ reduced into Ag NPs in this study by acting as the green reducing agent. Another characteristic for GG is the crosslinking reaction involving borate ions (Supplementary Fig. S1), like in the case of poly(vinyl alcohol)/borate ion coordinates. Owing to efficient crosslinking reaction, we can obtain many large granular GG beads (Supplementary Fig. S3) with 3D structured networks, which can then be utilized as the catalyst to facilitate the reduction of 4-NP to 4-AP. Therefore, those Ag NPs cannot be escaped from that formed 3D network, and furthermore, these crosslinked GG beads can be collected directly after the catalytic reaction.
impurities into the products. Catalytic performances reveal that the as-prepared Ag@GG beads show excellent catalytic activity toward the reduction of 4-NP in the presence of NaBH4 . GG is commercially available, inexpensive and environmentally friendly green biopolymer, and this preparative procedure involved in this study is surprisingly simple. It can provide a facile and green approach toward manufacturing of metallic nanocomposites, antimicrobial materials, low-temperature catalysts, and other useful materials.
4. Conclusions
The authors are grateful for the support of the National Natural Science Foundation of China (Nos. 21107116, 21377135 and 21477135) and Science and Technology Achievement Transformation Foundation of Jiangsu Province (BA2011100).
With GG as the green reducing agent, crosslinked Ag@GG beads were in situ prepared in this study without introducing disturbing
Acknowledgements
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