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Catalytic Membrane Reactor Immobilized with Alloy NanoparticlesLoaded Protein Fibrils for Continuous Reduction of 4-Nitrophenol Renliang Huang, Hongxiu Zhu, Rongxin Su, Wei Qi, and Zhimin He Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03431 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016
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Catalytic Membrane Reactor Immobilized with Alloy
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Nanoparticles-Loaded Protein Fibrils for Continuous Reduction of
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4-Nitrophenol
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Renliang Huang,†, ‖ Hongxiu Zhu,‡, ‖ Rongxin Su,*,‡,§,# Wei Qi,‡,§,# and Zhimin He‡
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†
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China
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‡
State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology,
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Tianjin University, Tianjin 300072, PR China
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§
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),Tianjin 300072,
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PR China
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#
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Tianjin 300072, PR China
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University,
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‖
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* Correspondence concerning this article should be addressed to R. Su at
[email protected] R. Huang and H. Zhu contributed equally to this work.
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ABSTRACT
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A catalytic membrane reactor, which contains a membrane matrix and a catalytic film of alloy
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nanoparticle-loaded β-lactoglobulin fibrils (NPs@β-LGF), was developed for the continuous-flow
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reduction of 4-nitrophenol (4-NP). The Cu-Ag and Cu-Ag-Au alloy NPs were synthesized using
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β-LGF as a scaffold and stabilizing agent. In this process, the Cu nanoclusters were formed in the
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initial stage and were able to promote the synthesis of Ag0, which acts as a reducing agent for the
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rapid formation of Au0. Furthermore, a catalytic membrane reactor was constructed by depositing
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the NPs@β-LGFs on a membrane matrix. The catalytic activity of the Cu-Ag-Au alloy NPs was
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higher than that of the Cu-Ag alloy NPs, using the reduction of 4-NP to 4-AP as a model reaction.
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The observed rate constant in the continuous-flow system is also higher than that in the batch
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system. In addition, these catalytic membrane reactors had good operating stability and antibacterial
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activity.
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KEYWORDS: catalytic membrane reactor, continuous-flow, alloy nanoparticles, β-lactoglobulin
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INTRODUCTION
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Membrane reactors (MRs), in which both the catalytic reaction and separation are performed in a
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single unit, have attracted considerable attention for their application in the fields of environmental
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engineering,1-3 energy production, 4, 5 and chemical engineering.6, 7 Generally, MRs can be classified
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as inert membrane reactors (IMRs) and catalytic membrane reactors (CMRs). The membrane in the
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IMRs is adjacent to the catalytic reaction zone containing the suspended or packed catalysts, while
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the membrane in the CMRs is either embedded with the catalysts or possesses catalytic activity
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itself.6 Regarding the combination of the reaction and separation, MRs are widely used in
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continuous-flow systems, which provide high operability, easy reuse of the catalysts, easy control of
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the reaction, automation, and scale up. However, the systems that are equipped with IMRs often
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suffer from the limited mass transport because the membrane pores are blocked by the suspended
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catalysts (e.g., enzymes, nanoparticles) or dead-end pores are present in the packed catalyst beads.
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Because the catalysts are embedded in the membranes, CMRs are enable to reduce this mass
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transport limitation and are attracting an increased amount of research interest for their industrial
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applications. It is worth nothing that the catalyst loading will inevitably influence the membranes’
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properties in many cases; therefore, it is essential to determine how to construct the catalytic
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membrane reactor to simultaneously achieve good catalytic and separation performance.
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Metal nanoparticles (NPs) are widely used as catalysts in environmental engineering due to their
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excellent catalytic performance, such as Au nanoparticles,8, 9 zerovalent iron,10-14 and Pt
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nanoparticles.15, 16 However, these nanocatalysts aggregate easily, reducing their activity, and thus
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are undesirable for industrial applications.17 Strategies to address this problem by immobilizing
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these nanoparticles on carriers, such as active carbon,18 metallic oxide,19, 20 protein crystals,21, 22 and 3
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mineral chabazite,23 have met with some success. The column reactors packed with these
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immobilized catalysts are generally applied to the continuous-flow systems, but they have a
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disadvantage in terms of the mass transport limitation mentioned above. Alternatively, a promising
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strategy is to construct CMRs via the integration of the metal nanoparticles within the membranes.
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For instance, Seto et al 24 recently reported a new catalytic membrane reactor immobilized with a
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palladium (Pd)-loaded polymer nanogel for continuous-flow Suzuki coupling reaction. The Pd
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nanoparticles were synthesized in situ on the membrane and exhibited high catalytic activity and
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long-term stability. In addition to the in situ growth, embedding the prepared metal NPs is also a
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good choice because it is easy to control the synthesis of the NPs. In this study, we attempt to
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develop an efficient approach for the synthesis of metal NPs and construct an NP-loaded catalytic
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membrane reactor.
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Currently, the fabrication of metal NPs is dominated by solution-based techniques using
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dissoluble metal salts. Inspired by the mineralization in biology, one of the most attractive
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directions for this research is the use of proteins and peptides as reducing and/or stabilizing agents
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to direct the synthesis of metal NPs with controlled shape, size, uniformity, and stability.25 Using
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the Au NPs as an example, various proteins, such as bovine serum albumin (BSA), α-amylase,
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trypsin, and lysozyme, have been utilized as templates, because they contain a large number of
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specific amino acid residues (e.g., cysteine, tyrosine, or arginine).25-28 However, the spherical
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structure, small size, and low mechanical strength of these proteins cannot match the demand of the
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CMRs. On the contrary, protein fibrils, which are regarded as pathogenic proteins in many
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neurodegenerative disorders,29 have very long contour lengths (>5 μm) and possess exceptional
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mechanical properties (Young’s modulus: 0.2-14 GPa).30 In previous studies, some protein fibrils
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were made from affordable protein sources in vitro and used to fabricate hybrid composite
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films,31-33 hybrid aerogels,34 etc. For example, Li et al 31 demonstrated that graphene and
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β-lactoglobulin (β-LG) fibrils can be combined to create a new class of biodegradable composite
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film with shape-memory and enzyme-sensing properties. The Young's modulus of the β-LG fibrils
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(β-LGFs) is in the range of 2-4 GPa and is comparable to the value of silk.35 Moreover, β-LG fibril
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is also a good template for the synthesis of inorganic nanomaterials. For instance, Mezzenga and
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co-workers synthesized the metal (gold, silver and palladium) nanoparticles,36 gold single
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crystals,37-39 TiO2 nanowires,40 and Fe3O4/β-LG fibrils (or nanoclusters) hybrids 41 for diverse
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applications. Due to its high strength and easy availability,42 the β-LG fibril is a promising
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candidate to serve as the catalyst scaffold for the construction of the metal NP-loaded CMRs.
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Recently, Bolisetty et al 43 reported a catalytic membrane containing gold or palladium
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nanoparticles immobilized on amyloid hybrid fibrils for continuous flow reaction. The metal NPs
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were synthesized on the surface of β-LGFs using NaBH4 as reducing agent. Here, we confirm these
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earlier findings on the efficiency of metal-β-LGF hybrids membrane for continuous flow catalysis
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and we further expand to metal alloys-β-LGF hybrids membranes. So far, fabrication of bimetallic
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NPs in membranes was reported by some groups; for example, Su and co-workers developed the in
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situ synthesis of Au-based bimetallic NPs in polyelectrolyte multilayers.44-47 In this study, we report
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a new approach for the rapid synthesis of Ag/Au-based alloy NPs on β-LG fibrils. The synthesis of
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metal alloys was carried out in one pot by the adsorption of metal ions (Cu2+, Ag+, AuCl4-) and in
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situ reduction (without using any additional reducing agent), in which Cu0 was preferentially
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formed and facilitated the synthesis of Ag0, that in turn promoted the production of Au0, leading to
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the formation of Cu-Ag and Cu-Ag-Au alloy NP-loaded β-LG fibrils (denoted as Cu-Ag@β-LGF
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and Cu-Ag-Au@β-LGF, respectively). Furthermore, a catalytic membrane reactor, in which a
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catalytic layer of alloy NP-loaded β-LG fibrils was attached to nylon membrane matrix, was
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developed for a continuous-flow system. Morphological and structural characterizations were
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performed to confirm the successful formation of the alloy NPs on the β-LG fibrils using several
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microscopy and spectroscopy techniques. The catalytic performance of the membrane reactor was
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evaluated using the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) as a model
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reaction.48-50 The flow rate dependence of the reactant solution, operating stability, and antibacterial
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activity of the membrane reactor were also investigated.
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EXPERIMENTAL SECTION
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Materials. β-lactoglobulin (>90%, β-LG) and Thioflavin T (>98%, ThT) were purchased from
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Sigma-Aldrich (Beijing, China). Hydrochloroauric acid (HAuCl4), 4-nitrophenol (99%, 4-NP) and
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sodium borohydride (NaBH4) were obtained from Aladdin Reagent Company (Shanghai, China).
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Sodium hydroxide, copper sulfate, silver nitrate, and other chemicals were analytical grade and
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obtained from commercial sources. Nylon membranes with an average pore size of 0.22 μm and the
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ultrafiltration centrifuge tubes with a molecular weight cut off of 3,000 Da were obtained from
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Millipore Corp. (Bedford, MA, USA).
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Preparation of the β-LG Fibrils. To purify β-LG, 50 mg of the β-LG powder was dissolved in 1
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mL of ultrapure water, and the pH of the resulting solution was adjusted to 9.0 with 1 M NaOH.
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After stirring at 4 °C for 1 h, the precipitate containing the insoluble protein was removed by
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centrifugation (15,000 rpm, 15 min). The supernatant was then washed three times with 0.1 M
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NaOH solution in a 3,000 Da ultrafiltration centrifuge tube (4 oC, 8,000 rpm, 15 min) to remove the
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metal ions. The final concentration of β-LG was adjusted to 40 mg mL-1, as determined by the 6
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absorption of the solution at 278 nm using the specific molar absorption coefficient 0.8272 L g-1
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cm-1. In a typical experiment, the pH of the purified β-LG solution was adjusted to 12 with 1 M
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NaOH and then incubated at 90 °C for 20 h to yield the β-LG fibrils.
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Synthesis of the Metal NP-Loaded β-LG Fibrils. In a typical experiment, 1.8 mL of CuSO4
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solution (20 mM), 2.4 mL of AgNO3 (10 mM), and 500 μL of the β-LG fibril solution were added to
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40.3 mL of ultrapure water. The pH of the resulting solution was adjusted to 12 with 2 M NaOH.
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After being purged with N2 for 10 min, the solution was incubated in a 90 °C water bath for 5 min.
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The formed Cu-Ag@β-LGF was collected and washed five times with ultrapure water in a
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ultrafiltration centrifuge tube (4 °C, 8,000 rpm, 15 min).
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Similarly, Cu-Ag-Au@β-LGF was also synthesized starting from a 45 mL precursor solution,
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which was prepared by adding 1.8 mL of a CuSO4 solution (20 mM), 2.4 mL of AgNO3 (10 mM),
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1.1 mL (or 2.5 mL) of HAuCl4 (20 mM), and 500 μL of the β-LGF solution into 39.2 mL (or 37.8
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mL) of ultrapure water. The other conditions were the same as those used for the synthesis of
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Cu-Ag@β-LGF.
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Fabrication of the NP-Loaded Membrane Reactors. A nylon membrane (d=1.3 cm, area=0.78
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cm2) with an average pore size of 0.22 μm was used as the membrane matrix for the construction of
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the membrane reactor (Figure S1). Two mL of the metal NP-loaded β-LG fibril (denoted as
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NPs@β-LGF) solution was filtered through the membrane at a flow rate of 200 μL min-1. The flow
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rate was controlled by a syringe pump (LSP01-2A, Longer Precision Pump Co., China) equipped
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with a 10 mL syringe. Subsequently, 7 mL of ultrapure water was flowed through the membrane at a
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flow rate of 300 μL min-1 to remove the free molecules. The NP-loaded membrane reactor was
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dried overnight at room temperature for further use.
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Assessment of the Catalytic Performance of the NP-Loaded Membranes. The reactant
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solution was prepared by adding 750 μL of the 4-NP stock solution (3 mM) into 7 mL of ultrapure
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water. N2 gas was then purged through the solution for 10 min to remove the dissolved O2.
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Subsequently, 1 mL of a freshly prepared NaBH4 solution (0.3 M) was injected under stirring. Five
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mL of the mixture was passed through the membrane reactor at 300 K and various flow rates
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(60-540 μL min-1) using a syringe pump (LSP01-2A, Longer Precision Pump Co., China). The
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effluent was continuously collected, and the concentrations of 4-NP in the effluent were determined
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using UV-vis absorption spectroscopy. The amount of product (4-AP) in the effluent was also
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analyzed with an Agilent 1200 HPLC system using an Agilent Eclipse XDB-C18 column (150
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mm×4.6 mm, 5 μm particle size). Each reaction was performed five times using five freshly
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prepared membranes.
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Given that the concentration of NaBH4 significantly exceeds that of 4-NP in the reaction system,
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the catalytic kinetics can be considered as pseudo-first-order, with respect to 4-NP. In these cases,
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the residence time (τ) and the kinetics of the reaction were defined by
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τ=πr2Lε/V
(1)
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ln(1-x)=-kobsτ
(2)
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where r, L, ε, V, x, and kobs are the membrane radius, thickness, membrane porosity, flow rate, conversion rate, and the observed rate constant, respectively. For comparison, the control experiments were also performed in a batch system using the
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NPs@β-LGF composites as catalysts. The reactant solution was prepared as described above and
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190 μL of the reaction solution was added to 96-well plates. Then, 10 μL of a dispersion of the
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NPs@β-LGF composites were added to initiate the reaction. In these cases, the conversion of 4-NP
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is given by
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–rt = –dCt/dt = kCt
(3)
where -rt is the conversion rate of 4-NP at time t, Ct is the concentration of 4-NP at time t, and k is the first-order rate constant.
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Antimicrobial Assays. A colony of ampicillin-resistant Escherichia coli (E. coli) (No. ATCC
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43886) was chosen as a model bacterium. The E. coli were cultured on solid LB (lysogeny broth)
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medium for 24 h at 37 °C. Subsequently, a fraction of the E. coli solution was diluted with 1 mL of
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water and 10 μL of a dispersion of β-LGF or NPs@β-LGF were added. The mixture was then
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incubated at 37 °C for 24 h. The OD600 values were used to represent the concentrations of E. coli.
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Characterizations. The morphology of the metal NPs, β-LG fibrils, and membranes were
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observed with a field emission scanning electron microscope (FESEM, S-4800, Hitachi
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High-Technologies CO., Japan) operated at 3 keV and a high resolution transmission electron
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microscope (HRTEM, Tecnai G2 F20, FEI, Holland) equipped with an EDS analyzer at an
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acceleration voltage of 200 kV. The UV-vis spectra were monitored on a TU-1810
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spectrophotometer (Persee Instruments Ltd., China) at wavelengths ranging from 200-600 nm.
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Fourier transform infrared spectroscopy (FTIR) spectra of β-LGFs and NP-loaded β-LGFs were
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recorded on a Nicolet-560 FTIR spectrometer (Nicolet Co., USA) with a KBr pellet method across
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the range of 400−4000 cm−1. The fluorescence emission spectra and 3D-fluorescence spectra were
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recorded on a fluorescence spectrophotometer (Varian Cary Eclipse, Agilent, USA). The ThT
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fluorescence analysis was performed to investigate the kinetics of fibrils formation with an
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excitation wavelength of 450 nm and emission wavelength of 480 nm. The Fourier-transform
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infrared spectroscopy (FTIR) spectra were recorded on a Nicolet-560 FTIR spectrometer (Nicolet
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Co., USA) from 400−4000 cm−1 using a KBr pellet method. A total of 16 scans were accumulated,
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with a resolution of 4 cm−1 for each spectrum. The crystal structure of NPs@β-LGF was analyzed
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by X-ray diffraction (XRD, D/max 2500, Rigaku, Japan) with a CuKα radiation source (λ=0.154056
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nm). The data were analyzed using the Jade 5 software package, which included a JCPDS powder
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diffraction database. The elemental analysis was performed using X-ray photoelectron spectroscopy
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(XPS, Axis Ultra DLD, Kratos Analytical, UK) with a monochromatic Al Kα source radiation at
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1486.69 eV and an acquisition time of 0.15 s. The amount of Cu, Ag, and Au loaded in the
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membranes was measured by inductively coupled plasma mass spectrometry (ICP-MS, IRIS
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Advantage, Thermo, USA) after the membranes were dissolved in nitrohydrochloric acid at 60 °C.
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RESULTS AND DISCUSSION
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Scheme 1 illustrates the strategy for the construction of the metal NP-loaded catalytic membrane
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reactor. In the proposed method, the β-LG fibrils were synthesized via the self-aggregation of the
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β-LG molecules. Subsequently, the dissoluble metal salts were added to the β-LGF solutions for the
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in situ growth of the metal NPs. The NPs@β-LGFs were then deposited onto a microfiltration
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membrane, and thus a catalytic membrane reactor was constructed for the continuous-flow reaction
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(Figure S1).
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Scheme 1. Schematic illustration of the construction of the metal NP-loaded catalytic
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membrane reactor.
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Preparation and Characterization of the Alloy Nanocatalysts. To prepare the β-LG fibrils, the
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β-LG solution was incubated at 90 °C for 20 h. As shown in Figure 1a, uniform β-LGFs measuring
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approximately 15 nm in diameter and at least 5 μm in length were obtained in this study. After the
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addition of CuSO4 into the β-LGF dispersion and incubation at 90 °C for 4 h, the color of the
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solutions visibly changed from clear to light yellow, suggesting that Cu0 had successfully formed on
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the β-LGFs (denoted as Cu@β-LGF, Figure S2). The HRTEM image shows that the Cu
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nanoclusters have an approximate diameter of 2.5 nm and a lattice plane distance (LPD) of 2.08 Å,
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which is indicative of the Cu (111) plane (Figure S3a). We further investigated the optical
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characteristics of Cu@β-LGF using UV-vis and photoluminescence spectroscopy. As shown in
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Figure 2a, an aqueous dispersion of Cu@β-LGF has an absorption peak at approximately 330 nm,
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without any accompanying SPR peak near 570 nm (the λmax of Cu), which is the characteristic
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absorption peak of Cu nanoparticles (Figure 2a). Meanwhile, the Cu@β-LGF dispersion exhibits
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purple luminescence (λem=405 nm) under an excitation wavelength of 330 nm. These results
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demonstrated that β-LGF has the capability to reduce Cu2+ into Cu0 nanoclusters. 11
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Figure 1. a-b) TEM images of (a) β-LGF and (b) Cu-Ag-Au1@β-LGF. c-d) HRTEM images of (c)
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the Cu-Ag alloy nanoparticles and (d) Cu-Ag-Au1 alloy nanoparticles.
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We further attempted to synthesize the Ag NPs with the assistance of β-LGFs; however, the yield
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of the Ag NPs was low, even after a 4 h incubation, as shown by the weak SPR peaks in the UV-vis
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spectra (Figure 2b, blue and black curves). This may be attributed to the weak binding and/or
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reducing ability of the amino acid residues for Ag+ ions, leading to the slow nucleation and growth
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of Ag0 crystals, particularly in the absence of chemical reductants like NaBH4 and sodium citrate.
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Compared with Ag+, Cu2+ has a stronger affinity for the amino acid residues and can be more easily
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reduced to Cu0 nanoclusters, although it has a lower standard electrode potential (Eo(Cu2+/Cu)
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=0.34V vs (Eo(Ag+/Ag)=0.79V). Therefore, we speculate that the addition of Cu2+ is able to
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promote the growth of the Ag NPs due to the reducing effect of Cu nanoclusters. As expected, when 12
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Cu2+ and Ag+ were added into the dispersion of β-LGFs, the color of the mixture changed from
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clear to light brown within approximately 5 min (Figure S2). The corresponding UV-vis spectrum
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exhibits a strong SPR peak at approximately 410 nm (Figure 2b, olive curve), suggesting the that
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Cu-Ag alloy NPs had formed. When the incubation time was increased to 4 h, a small red shift in
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the SPR peak from 410 to 412 nm was observed and the intensity was also slightly increased
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(Figure 2b, orange curve). The results indicate that the growth of the Cu-Ag NPs proceeds quickly
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in the initial stage and is nearly complete at 5 min. Additionally, the HRTEM image shows that the
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LPD values range from 2.11-2.22 Å (Figure 2c), likely corresponding to the Cu-Ag alloy (111)
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plane.
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Figure 2. UV-vis absorption spectra of aqueous suspensions of (a) β-LGF and Cu@β-LGF, (b)
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Ag@β-LGF and Cu-Ag@β-LGF, and (c-d) Cu-Ag-Au@β-LGF. The blue curve in (a) is the
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fluorescence emission spectrum of an aqueous suspension of Cu@β-LGF.
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Using the same procedure, the Cu-Ag-Au alloy NPs were synthesized with the assistance of
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β-LGFs and the addition of Cu2+, Ag+ and AuCl4- ions. As shown in Figure 1b, the NPs grew on the
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surface of β-LGF, which provides evidence to support the hypothesis that β-LGF acts as stabilizing
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agent and prevents the ions from aggregating. In this study, two alloy NPs were prepared by varying
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the molar ratio of the Ag and Au salts, one of which exhibited a single SPR peak at 465 nm and the
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other exhibited a peak at 512 nm (denoted as Cu-Ag-Au1@β-LGF and Cu-Ag-Au2@β-LGF,
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respectively, Figure 2c-d). After a 5 min incubation, the intensities of the SPR peaks were
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approximately 0.5, indicating that the rate of Cu-Ag-Au alloy NP growth is also very high in the
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initial stage. It is worth noting that the growth of the Cu-Au alloy NPs is slow in the absence of Ag+
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(Figure 2d, red curve). We expect that the Ag0, which is formed by the reduction of Ag+ by Cu0, can
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serve as a reducing agent and thus promotes the formation of Au0 (Eo(AuCl4-/Au )= 1.002 V).
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Figure 1d and Figure S3b show the HRTEM images of Cu-Ag-Au1@β-LGF and
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Cu-Ag-Au2@β-LGF, respectively. The LPD values for the Cu-Ag-Au alloy NPs ranged from
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2.2-2.3 Å, which is different from the classical values for the Cu (111) (2.08 Å), Ag (111) or Au
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(111) planes (2.35 Å). Additionally, an LPD of 1.3 Å was also observed in a HRTEM image (Figure
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1d), indicating the presence of the Cu (220) plane. As shown in Figure 1d, a notable icosahedron
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containing five twin structures was observed, which is one of the most popular planar defects in
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face center cubic (FCC) structures and results from the change in the volume of the Cu phases
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during the formation of the alloy NPs. In addition to the alloy NPs, the HRTEM images revealed the
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presence of Cu nanoclusters, with a size of less than 3 nm, while the Cu-Ag-Au alloy NPs have a
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size distribution of 3-13 nm (Figure S4).
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Figure 3. The fitted core-level XPS spectra of (a) Cu 2p, (b) Ag 3d and (c) Au 4f in the
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NPs@β-LGF.
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To confirm that the alloy formed, the XPS spectra of Cu@β-LGF, Cu-Ag@β-LGF, and
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Cu-Ag-Au@β-LGF were obtained (Figure 3). In the Cu 2p spectra, two peaks were observed at
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approximately 931.6 eV and 951.5 eV, which correspond to the Cu 2p3/2 band and Cu 2p1/2 band,
275
respectively, and indicate of the presence of Cu0 in all the samples. The Ag 3d core-level spectra
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show peaks at 367.9 and 373.2 eV, which is in good agreement with the values of Ag0, suggesting
277
that there was no Ag+ (for example, in the form of Ag2O) on the particles’ surfaces. In the Au 4f
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spectrum, two peaks centered at binding energies of 84.1 eV (Au 4f7/2) and 87.5 eV (Au 4f5/2)
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appeared, confirming the presence of Au0. Additionally, the TEM elemental line scan mapping
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(Figure S5) also demonstrated that Cu, Ag and Cu, Ag, Au were homogeneously distributed in the
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alloy NPs. The results of elemental analysis (Figure S5-S6) indicate the content of Ag in
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Cu-Ag-Au@β-LGF decreased with increasing incubation time from 5 min to 4h due to its
283
consumption in the reduction of AuCl4-. Additionally, it is worth noting that few Ag or Au
284
nanoparticles may be formed when the metal ions were bound to the free space of fibrils surface.
15
285 286
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Figure 4. The XRD patterns of the Cu-Ag@β-LGF and Cu-Ag-Au1@β-LGF composites.
The crystal structure of the NPs@β-LGF was further characterized by XRD. As shown in Figure
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4, Bragg reflections of the Cu-Ag phase are clearly observed at 2θ=38.8°, 44.4°, 64.6°, and 77.4°,
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which are between the values of the individual Cu crystal (JCPDS No. 65-9026) and Ag crystal
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(JCPDS No. 65-2871) and correspond to the (111), (200), (220), and (311) planes of the Cu-Ag
291
crystals, respectively. For the Cu-Ag-Au alloy NPs, Bragg reflection peaks appeared at 2θ=38.7°
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and 45.4°, which are between the values of the Ag-Au alloy (JCPDS No. 65-8424, 2θ=38.2° and
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44.4°) and Ag-Cu alloy (JCPDS No. 65-8608, 2θ=40.3° and 46.9°) and correspond to the (111) and
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(200) planes of the ternary alloy. The XRD results are consistent with the LPD values from the
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TEM analysis.
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Mechanistic Analysis of the Formation of NPs@β-LGF. According to a previous study, β-LG
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contains two tryptophan (Trp) and four tyrosine (Tyr) residues and has an emission wavelength (λem)
298
at 332 nm following excitation at 280 nm, while the λem of the β-LG fibrils formed in alkaline
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conditions was 350 nm (Figure 5a). This distinct red shift should be attributed to a change in the
300
tertiary structure of β-LG, which inhibits the non-radiative energy transfer between Tyr and Trp51.
301
The results indicate that the Trp and Tyr residues in the β-LGF are exposed to water from the 16
302
hydrophobic core, and thus are easily able to capture the metal ions. Moreover, the 3D-fluorescence
303
(λex=280 nm, λem=350 nm) of β-LGF was significantly quenched after the addition of the Cu2+ ion
304
(800 μM) (Figure 5a-b), which provides powerful evidence in support of the hypothesis that β-LGF
305
chelated Cu2+. Moreover, the FTIR spectra (Figure S7) show that the metal NP-loaded β-LGFs have
306 307
an increase in amide II absorption band at ∼1560 cm−1 (the N−H bending vibration and C−N
308
from C−O and C−N stretching vibrations, respectively, also appeared in the spectra of NP-loaded
309
β-LGFs. These results suggested that metal NPs probably be bound to the amino acid residues
310
containing N−H, C−O, and/or C−N groups, such as Trp and Tyr. Such observation is consistent with
311
that in the fluorescence analysis as described before.
stretching vibration) compared to β-LGFs. Two new peaks at 1110 cm-1 and 1190 cm-1, resulted
312 313
Figure 5. 3-D fluorescence spectra of β-LGF (a) before and (b) after the addition of 800 μM of Cu2+.
314
(c) Plots of lg((F0-F)/(F-Fsat)) versus lg([M]) for the fluorescence response to the different metal
17
315
316
ions. (d) The change in ThT fluorescence intensity proceeds with incubation at 90 °C.
We calculated the binding constant (Kb) and the numbers of binding sites (n) by monitoring the
317
fluorescence quenching of β-LGF at 350 nm in the presence of metal ions (Figure 5c). The
318
parameters were calculated using Eq (4) 27.
319
lg((F0-F)/(F-Fsat))=nlg[M]+nlgKb
(4)
320
where F0 and F represent the Tyr fluorescence of β-LGF before and after the addition of metal
321
ions, respectively; Fsat represents the Tyr fluorescence of β-LGF saturated with metal ions, and [M]
322
is the concentration of the metal ions.
323
As shown in Table 1, the Kb of Cu2+ to β-LGF is 3 orders of magnitude higher than that of Ag+.
324
Therefore, the growth of the Ag NPs in the absence of Cu2+ is very slow, as discussed above.
325
Additionally, the value of n for Cu2+ is also larger than that for Ag+. The high binding affinity and
326
binding sites for Cu2+ ions allow them to preferentially bind β-LGF and be further reduced to Cu0
327
nanoclusters. The AuCl4- ions have a high electrode potential; however, the low concentration of
328
AuCl4- ions around β-LGF derived from the electrostatic repulsion results in the slow growth rate of
329
Au0 in the absence of the Cu2+ and Ag+ ions.
330
Table 1. Numbers of binding sites (n), binding constant (Kb) of β-LGF with different metal ions,
331
and their standard potential. Metal ions
n
Kb
Standard potential
Cu2+
2.53
1487.3
Eo(Cu2+/Cu) = 0.34 V
Ag+
0.688
1.28
Eo(Ag+/Ag) = 0.79 V
AuCl4-
1.25
3565.8
Eo(AuCl4-/Au )= 1.002 V
332
18
333
According to previous results, a proposed mechanism is shown in Figure S8. At pH 12, which is
334
much higher than the pI value (~5.2) of β-LG, a large number of amino acid residues in β-LGF
335
possesses negative static charge. Upon the addition of the metal ions, the Cu2+ ions were
336
preferentially bound to β-LGF and the Cu0 cluster was formed due to the high binding affinity.
337
Subsequently, Cu0 can reduce the Ag+ ions into Ag0 on the surface of the Cu0 cluster, while the Ag0
338
can more easily reduce the AuCl4- into Au0 in the presence of Cl- ions compared to Cu0 due to the
339
bigger potential difference (Eo(AgCl/Ag)=0.22V, Eo(Cu2+/Cu)=0.34V, Eo(AuCl4-/Au)=1.002V),
340
leading to a new kind of Cu-Ag-Au ternary nucleus. The reaction was shown below. Additionally,
341
the addition of Ag+ ions probably increases the concentration of AuCl4- ions around β-LGF because
342
of the screening of negative charge. Therefore, the high reducing ability of Ag in the presence of Cl-
343
ions and the increased AuCl4- concentration around β-LGF facilitate the synthesis of
344
Cu-Ag-Au@β-LGF.
345
β-LGFre+ Cu2+
346
Cu0+2Ag+
347
3Ag0+AuCl4-
348
β-LGFox+ Cu0
Cu2++2Ag0,
(5)
ΔEo=0.45
3AgCl+Cl-+Au0,
ΔEo=0.78
(7)
This Cu0-promoted synthesis of the alloy NPs was also demonstrated in other protein systems. As
349
shown in Figure S9, the yield of the Cu-Ag NPs is much higher than that of the Ag NPs in the
350
absence of Cu2+ ions in the different protein solutions, including lysozyme, BSA, insulin, and
351
ovalbumin.
352
(6)
In addition, the results of the ThT fluorescence assay indicate that the metal NPs can also
353
promote the growth of β-LGFs (Figure 5d). In the control experiment, there was no obvious change
354
in the fluorescence intensity of β-LGFs as the incubation time increased. However, the fluorescence 19
355
intensity of the Cu-Ag NPs was significantly increased, suggesting that the growth of β-LGFs
356
occurred by aggregation of the β-LG molecules. Similarly, the other NPs can also promote this type
357
of fibrillation (Figure 5d).
358
Catalytic Reduction of 4-NP in the Membrane Reactor. Figure 6 shows the SEM images of
359
the nylon membrane matrix and the catalytic membranes. After the attachment of the NPs@β-LGF
360
catalytic layer with a thickness of ~8 μm (Figure S10), the pore size of the membrane surface
361
decreased from ~0.22 μm to 0.04-0.12 μm. The contents of the metal elements in the catalytic
362
membranes were determined by ICP-MS. As summarized in Table 2, the Cu-Ag@β-LGF membrane
363
contains 45.31 μg mg-1 Cu and 46.16 μg mg-1 Ag, while the Cu-Ag-Au membranes are mainly
364
composed of Cu and Au elements.
365
Table 2 The contents of the Cu, Ag and Au elements (μg mg-1) in the Cu-Ag@β-LGF, Cu-Ag-Au1@
366
β-LGF and Cu-Ag-Au2@β-LGF membranes, respectively. Cu-Ag@β-LGF
Cu-Ag-Au1@β-LGF
Cu-Ag-Au2@β-LGF
Elements
Membrane
Membrane
Membrane
Cu
45.31
34.7
33.23
Ag
46.16
0.48
0.51
Au
-
112.66
200.74
20
367 368
Figure 6. SEM images of the (a) nylon membrane matrix, (b) Cu-Ag@β-LGF membrane, (c) the
369
Cu-Ag-Au1@β-LGF membrane, and (d) the Cu-Ag-Au2@β-LGF membrane.
370
4-NP is a common pollutant that is generated from the production of pesticides, herbicides,
371
insecticides, etc. Nevertheless, its corresponding derivative, 4-AP, is in great demand in a large
372
number of industrial applications. In this study, the reduction of 4-NP to 4-AP in the presence of
373
NaBH4 was selected as a target reaction to quantitatively evaluate the catalytic performance of the
374
NPs@β-LGF composites and membrane reactors. As shown in Figure 7a, the intensity of the
375
UV-vis absorption peak at 400 nm quickly decreased due to the consumption of the substrate (4-NP)
376
during the reaction process. Figure 7b shows the plots of ln(C0/Ct) versus the reaction time for the
377
reduction of 4-NP. The C0/Ct values were obtained from the relative intensity of the absorption at
378
400 nm, because it is proportional to the concentration of 4-NP. The linear relationship between 21
379
ln(C0/Ct) and reaction time (t) confirms the pseudo-first-order kinetics. The rate constants (k) of the
380
catalytic reaction were 0.29 min−1, 1.56 min−1, and 2.30 min−1 for the Cu-Ag@β-LGF,
381
Cu-Ag-Au1@β-LGF, Cu-Ag-Au2@β-LGF composites, respectively. The catalytic activity is much
382
higher than that of Ag NPs (0.175 min-1) and Au NPs (0.89 min-1) alone, as demonstrated in our
383
previous studies21, 48.
384
We investigated the effect of flow rates on the conversion of 4-NP in the catalytic membrane
385
reactors. As shown in Figure 8, with increasing flow rate, the conversion of 4-NP decreased, due to
386
the decreased residence time. We further test the Cu-Ag-Au2@β-LGF catalytic membrane reactor
387
with the lake water (from a lake located in Tianjin University) containing 4-NP (note: 4-NP was
388
added into lake water in lab). At a flow rate of 60 μL min-1, the conversion of 4-NP in lake water is
389
approximately 86.3%, which is very close to the value (~89%) in the ultrapure water (Figure 8).
390
Additionally, compared with the Cu-Ag@β-LGF CMR, the Cu-Ag-Au@β-LGF CMR has better
391
catalytic performance, indicating a significant effect of the concentration of Au on the reaction rate.
392
The enhanced catalytic performance when Au is added to the catalytic system should be attributed
393
to the high adsorption constant of 4-nitrophenol on Au surface.52 The results are also consistent with
394
those from the NPs@β-LGF composites, as mentioned above. According to Eq.(1) and (2), the
395
observed rate constants (kobs) of Cu-Ag@β-LGF, Cu-Ag-Au1@β-LGF and Cu-Ag-Au2@β-LGF
396
CMRs were calculated as 9.33 min-1, 17.1 min-1, and 20.83 min-1, respectively, as shown in Figure
397
7c, which are significantly higher than that of the NPs@β-LGF composites in batch systems.
22
398 399
Figure 7. (a) Time-dependent UV-vis absorption at 400 nm in the presence of the NPs@β-LGF
400
composites. (b) The rate constant (k) for the conversion of 4-NP using 10 μL of the NPs@β-LGF
401
composites as catalysts. c) The observed rate constant (kobs) for the conversion of 4-NP by flowing
402
the reactant solution through the catalytic membranes.
403
23
404 405
Figure 8. The effect of flow rates on the conversion of 4-NP in the catalytic membrane reactors.
406
The operating stability of the NPs@β-LGF membrane reactor was also investigated by the
407
continuous conversion of 4-NP for 8 cycles (5 mL per cycle). The change in the conversion ratio of
408
4-NP in the continuous-flow system is shown in Figure 9. The membrane reactor retained its
409
catalytic activity over eight cycles, which is indicative of good operating stability. The cycle
410
performance of these catalytic membranes (>97% productivity after eight cycles) is comparable to
411
or even better than those of metal NPs immobilized on the other carriers, such as eggshell
412
membrane (92% productivity after eight cycles),48 and magnetite-polymer hybrid (~80%
413
productivity after nine cycles).53 After the operation of the catalytic membrane, the presence of
414
alloy NPs on the membrane was confirmed by SEM (Figure 6b-d, inset figures). In addition, the
415
fibrous structures of the NPs@β-LGF composite film were maintained. According to the ICP-MS
416
results, leakage of the alloy NPs was not observed.
24
417 418
Figure 9. The operating stability in the conversion of 4-NP using the NPs@β-LGF membrane
419
reactors at a flow rate of 180 μL/min.
420
Antimicrobial Assays. Membrane fouling from microorganisms is a ubiquitous and problematic
421
phenomenon that can reduce the lifetime of the membrane reactors. In the present study, the
422
antimicrobial activity of the NPs@β-LGF composite was evaluated. As shown in Figure 10, the
423
Cu-Ag@β-LGF, which contains a high content of Ag, exhibited higher antimicrobial activity
424
towards E. coli than the other NPs@β-LG composites. The two samples of Cu-Ag-Au@β-LGF have
425
a similar content of Ag, but the antimicrobial activity of Cu-Ag-Au1@β-LGF is higher than the
426
other composite, likely due to its higher content of Ag on the outer surface of the NPs.
25
427 428
Figure 10. The content of E. coli in the presence of β-LGF or the different NPs@β-LGF composites
429
after incubation at 37°C for 24 h. The OD600 value was used to represent the concentrations of E.
430
coli. The E. coli content in the β-LGF membrane (used as a control) was normalized to 100%.
431
In this work, we have successfully developed a facile and one-step synthesis of Cu-Ag and
432
Cu-Ag-Au alloy nanoparticles using β-LGFs as reducing and stabilizing agents. The addition of
433
Cu2+ ions into a dispersion of β-LGFs produced Cu0 nanoclusters on the fibrils’ surfaces, which
434
promoted the synthesis of Ag0 due to its reducing ability. Meanwhile, the formed Ag0 is also able to
435
speed up the formation of Au0, leading to the rapid synthesis of the Cu-Ag-Au alloy NPs.
436
Furthermore, a NPs@β-LGF composite film was fabricated on a nylon membrane matrix and
437
applied to a catalytic membrane reactor for the reduction of 4-NP to 4-AP. The observed rate
438
constant in the continuous-flow system is higher than that in the batch system. In the
439
continuous-flow reaction, the catalytic activity of Cu-Ag-Au alloy NP-loaded membrane reactor
440
was also higher than that of the Cu-Ag alloy NP-loaded membrane. In addition, the catalytic
441
membrane reactor exhibited good operating stability over eight cycles. During this process, it was
26
442
not necessary to recover the catalyst from the reactants/products solution, and the alloy NP-loaded
443
membrane was reusable. The results agree with earlier report by Bolisetty et al.43 on the feasibility
444
of metal-betalactoglobulin hybrids membranes for continuous flow catalysis. Additionally, the
445
NPs@β-LGF composite also possessed antibacterial activity toward E. coli, likely due to the
446
presence of Ag0 in the alloy nanoparticles. This strategy is expected to be applicable to the
447
construction of various catalytic membrane reactors. The development of this type catalytic
448
membrane reactor for continuous-flow reactions also has industrial significance.
449
AUTHOR INFORMATION
450
Corresponding Author
451
*E-mail:
[email protected] (R. S.)
452
Tel: +86 22 27407799. Fax: +86 22 27407599.
453
Notes
454
The authors declare no competing financial interest.
455
ACKNOWLEDGMENTS
456
This work was supported by the Ministry of Science and Technology of China (No.
457
2012YQ090194), the Natural Science Foundation of China (Nos. 51473115, 31071509 and
458
21306134).
459
ASSOCIATED CONTENT
460
Supporting Information
461
Photos of nylon membrane matrix, NPs@β-LGF loaded catalytic membrane, and continuous-flow
462
catalytic membrane reactor system, photos of the dispersions of β-LGF and NPs@β-LGF before
463
and after incubation, HRTEM images of Cu@β-LGF and Cu-Ag-Au2@β-LGF, size distribution of
27
464
Cu-Ag-Au1@β-LGF, element line scan mapping for Cu-Ag@β-LGF and Cu-Ag-Au1@β-LGF, EDX
465
spectra of Cu-Ag@β-LGF, Cu-Ag-Au1@β-LGF, and Cu-Ag-Au2@β-LGF, FTIR spectra of the
466
β-LGF, Cu@β-LGF, Cu-Ag@β-LGF, and Cu-Ag-Au1@β-LGF, the proposed mechanism for the
467
formation of Cu-Ag-Au@β-LGF, the intensity of UV-vis absorption of different NPs@protein
468
systems, the thickness of (a) Cu-Ag@β-LGF and (b) Cu-Ag-Au1@β-LGF composite film. This
469
material is available free of charge via the Internet at http://pubs.acs.org/.
470
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