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Enzymatic Cascade Catalysis in a Nanofiltration Membrane: Engineering the Microenvironment by Synergism of Separation and Reaction Huiru Zhang, Hao Zhang, Jianquan Luo, and Yinhua Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05371 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019
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Enzymatic Cascade Catalysis in a Nanofiltration
2
Membrane: Engineering the Microenvironment
3
by Synergism of Separation and Reaction
4
Huiru Zhang, †,§ Hao Zhang,†,§ Jianquan Luo,*,†,§ Yinhua Wan†,§
5
†State
Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
6 7 8
§School
of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, PR China
9 10
ABSTRACT: Microenvironment plays an significant role in enzymatic catalysis,
11
which directly influences enzyme activity and stability. It is important to regulate the
12
enzyme microenvironment, especially for the liquid with unfavored properties (e.g. pH,
13
dissolved oxygen). In this work, we propose a methodology that can regulate pH and
14
substrate concentration for enzymatic catalysis by a biocatalytic membrane, which was
15
is composed of glucose oxidase (GOx) and horseradish peroxidase (HRP) co-
16
immobilized in a polyamide nanofiltration (NF) membrane (i.e. beneath the separation
17
layer). By virtue of the selective separation function of NF membrane and in-situ
18
production of organic acid/electron donor with GOx, a synergism effect of separation
19
and reaction in the liquid/solid interface was manipulated for engineering the
20
microenvironment of HRP to enhance its activity and stability for micro-pollutant
21
removal in water. The outcome of this work not only provides a new methodology to
22
precisely control enzymatic reaction, but also offers a smart membrane system to
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efficiently and steadily remove the micro-pollutants in portable water.
24
KEYWORDS:
25
microenvironment engineering
nanofiltration,
cascade
catalysis,
micro-pollutants,
enzymes,
26 27
1. INTRODUCTION
28
Due to human activity, there are increasing micro-pollutants such as
29
pharmaceuticals, endocrine disruptors and polycyclic aromatic hydrocarbons in water,
30
which arouse serious attentions because of their adverse effects on human body
31
Numerous strategies have been applied for micro-pollutants removal such as
32
adsorption3, membrane filtration4-5, advanced oxidation process6, and biodegradation7,
33
among which enzyme-based (e.g. laccase, peroxidase and tyrosinase) methods are more
34
attractive due to their high efficiency, eco-environmentally friendly, mild operation and
35
effective detoxification. However, the trade-off between catalytic efficiency and long-
36
term stability is a big issue that limits the application of enzymatic degradation of
37
micro-pollutants. For example, laccase as a multicopper redox enzyme, can catalyze
38
numerous recalcitrant micro-pollutants such as bisphenol A (BPA), tetracycline,
39
diclofenac8-12, but it was reported that the higher laccase activity resulted in the worse
40
reusability due to more polymerization products depositing close to laccase
41
products with poor solubility may form in the narrow channels connecting T2/T3 sites
42
of laccase that are crucial for electron delivery, which potentially block the further
43
entrance of dissolved oxygen into the channel, eventually causing a decrease of laccase
44
activity with time
14.
13.
1-2.
The
A strategy to break the trade-off between laccase activity and
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operation stability is to add mediators into reactants. Mediator is a kind of small-size
46
compounds able to generate soluble radicals which won’t cause the enzyme inactivation.
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It can act as a carrier of electrons between the enzyme and the substrate thereby
48
overcoming the steric hindrances between them15, as well as avoiding the accumulation
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of the oxidation products around the enzyme active sites and further
50
polymerization/deposition. However, mediators such as 1-hydroxybenzotriazole (HBT)
51
and 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) are toxic, which
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seem unsuitable for water treatment.
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Horseradish peroxidase (HRP), an enzyme widely used in the treatment of micro-
54
pollutants16-17 that needs hydrogen peroxide (H2O2) as an electron donor. HRP requires
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a constant supply of H2O2 to keep its activity, while excess H2O2 triggers the substrate
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inhibition and the change of enzyme conformation, lowering the activity of HRP18. It’s
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hard to precisely control a suitable H2O2 concentration in the reactants since the reaction
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efficiency is changing. Moreover, it is commonly known that potable water is at neutral
59
pH, while HRP has higher activity in acidic pH19. On the other hand, glucose oxidase
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(GOx) is a promising enzyme which oxidizes the β-D-glucose in the presence of oxygen
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for in-situ production of gluconic acid and H2O220-21. If HRP coexists with GOx, the
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produced gluconic acid lowers the pH of the microenvironment for HRP, while the
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other product (H2O2) can be used as the electron donor22. Therefore, the presence of
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GOx provides a possibility for the precise engineering of the HRP microenvironment,
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which is of great importance for achieving high catalytic activity and stability of HRP23.
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However, for free enzymes, there is a certain spatial distance between the two enzymes,
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which weakens the mass transfer between their active centers24, decreasing the
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efficiency of such micro-environment regulation.
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Co-immobilization of GOx and HRP in/on a matrix is a practical strategy that shortens
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the spatial distance between two enzymes and enhances enzyme stability as well as
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reusability. Polymeric capsules25, nanoparticles26, and supramolecular hydrogel27 are
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the most frequently-used matrixes for enzymes immobilization. It is well known that
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enzymes were normally entrapped in the matrixes to increase the loading amount and
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maintain the activity. Thereinto, concentration gradient diffusion is the major mass
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transfer mechanism of substrates, intermediates and products around the immobilized
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enzymes, and the high diffusion resistance inside the immobilization matrix greatly
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limits the catalysis efficiency28-29. Great efforts have been devoted to enhancing mass
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transfer efficiency for benign interactions between enzymes and substrates30-32. The
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porous membranes, served as a support for enzyme immobilization enable enzyme
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reuse, improve enzyme stability and promote enzymatic efficiency due to the co-effect
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of diffusion and convection33-34. Therefore, biocatalytic membranes with enzymes
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immobilized on/in membranes have attracted increasing attention in various areas. In
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the early stage of biocatalytic membrane development, the most commonly used matrix
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was microfiltration (MF) membrane, and such membrane was only acted as a carrier of
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enzyme immobilization35. Gradually, with the wide applications of ultrafiltration (UF)
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membranes, biocatalytic membranes with both separation (only for macromolecules)
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and catalysis functions appeared36. Nanofiltration (NF) membrane, the ultrathin skin
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layer of which adheres onto the supporting base by physical entanglement37, acting as
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both the support for enzymes and the selective barrier for small molecules, becomes a
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burgeoning tool to enhance the enzymatic reaction. Biocatalytic membrane based on
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NF membrane was mainly applied for micro-pollutants removal1, 38. Enzymes could be
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immobilized on either the skin layer or the supporting layer of the membrane. Normally,
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NF membrane is designed to fully or partly retain the substrates, and on the one hand,
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it can realize in-situ product removal which reduces product inhibition for macro-
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molecule hydrolysis39; on the other hand, it is able to alleviate enzymatic catalysis
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burden or regulate the substrate concentration involving the reaction if the enzymes are
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immobilized beneath the skin layer
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membrane could be used to manipulate the cascade catalysis behavior via controlling
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the involved substrate concentration.
13, 40.
Inspired by this, we speculated that NF
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Recently, mussel-inspired chemistry has attracted widespread interest in membrane
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technology. Dopamine (DA), as a well-known neurotransmitter, can self-
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polymerization to form a stable polydopamine (PDA) coating layer in alkaline solutions
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and adhere onto nearly all kinds of substrates by different interactions including
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electrostatic and hydrophobic absorptions, hydrogen bonds, π-π stacking and
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chelation41. It was found that the homogeneous polymerization and deposition of DA
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could be promoted by the addition of polyethyleneimine (PEI)42. The PDA/PEI co-
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deposition strategy was also widely used in enzyme immobilization, due to its
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simplicity, stability and biocompatibility. Additionally, PDA layer can also reduce the
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leakage of enzymes13 and improve the catalysis efficiency by adsorbing and enriching
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the substrates43.
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Inspired by these, we proposed a new, simple and effective method for multi-enzyme
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immobilization utilizing co-deposition of PDA/PEI in this work. As illustrated in Fig.
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1, a biocatalytic NF membrane was prepared via co-immobilization of GOx and HRP
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in a NF membrane, where a cascade catalysis was carried out for degrading micro-
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pollutant (BPA as an example). In detail, glucose partly passed through the NF
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membrane and was oxidized with GOx in the membrane, and the produced gluconic
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acid would decrease the microenvironment pH, which was supposed to improve the
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HRP-catalyzed oxidation of BPA with the generated H2O2. Here, we attempted to
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control the H2O2 concentration in the membrane by tuning the glucose permeation
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through the NF separation layer since H2O2 is very crucial for the activity and stability
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of the immobilized HRP. Theoretically, a solute retention of NF membrane is mainly
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determined by feed concentration and operating pressure (i.e. diffusion and
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convection)44. Thus, by manipulating the diffusion and convection transport of glucose
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across the membrane (via adjusting glucose concentration in the reactants and operating
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pressure), a high and stable BPA removal by the prepared biocatalytic membrane may
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be realized. For the first time, a synergism of separation and reaction accomplished by
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a biocatalytic NF membrane was proposed for engineering the microenvironment of
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enzymes, which would not only provide a new methodology to manipulate enzymatic
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reaction, but also offer a smart membrane system to efficiently remove the micro-
130
pollutants in portable water. In the real application, by simply adding biocompatible
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glucose which could trigger multi-enzyme reaction, the efficient and stable micro-
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pollutants removal could be achieved. In addition, a quick-response system will be
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constructed by regulating the glucose permeance though the NF membrane according
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to the micro-pollutants concentration in water.
135 136 137
Figure 1. Schematic of biocatalytic membrane preparation and cascade catalysis in the NF membrane for engineering microenvironment and removing micro-pollutants.
138 139
2. EXPERIMENTAL SECTION
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2.1. Materials. In this research, a dead-end filtration cell (Amicon 8050, Millipore,
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U.S.A) was utilized and the effective area of which is 13.4 cm2. Polyethyleneimine (PEI)
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(molecular weight (MW) ~ 600 Da) was supplied by Aladdin. Glucose oxidase (GOx)
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(EC 1.1.3.4, 160 kDa, 100 U mg-1) from Aspergillus niger and peroxidase from
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horseradish (HRP) (EC 1.11.1.7, 44 kDa, >300 U mg-1) were obtained from Aladdin
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(Shanghai, China). NF270 membrane (polyamide, molecular weight cutoff (MWCO)
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~200-300 Da) was supplied by DOW-Filmtech, which has a dense skin layer to partly
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retain micropollutant and glucose. Dopamine hydrochloride was obtained from Sigma-
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Aldrich (Shanghai, China). Bisphenol A (BPA, 96%) was supplied by J&K (Beijing,
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China).
D-Glucose
(198.17 Da) was purchased from Xilong Scientific Co., Ltd.
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(Guangdong, China). Peroxide (H2O2, 30%) was obtained from Beijing Chemical
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Works (Beijing, China). Fluorescein isothiocyanate (FITC, MW ~389.38 Da) was
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purchased from MedChemExpress Co., Ltd. (Shanghai, China). Rhodamine B
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isothiocyanate (RhB, MW ~479.02 Da) was bought from Aladdin (Shanghai, China).
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All chemicals used in experiments were of analytical grade.
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2.2. GOx/HRP co-immobilization in NF membrane. In this work, GOx and HRP
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immobilization was carried out using the reverse filtration method and dopamine/PEI
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co-deposition technique. The experimental details are presented in supporting
158
information.
159
The enzyme immobilization amount was determined using the mass balance equation
160
(the difference between protein in the original feed and the sum of protein in the
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remaining and washing solutions), and the protein content was measured via Bradford
162
method 40. The Bradford reagent and the sample were mixed at a ratio of 1:1, and the
163
absorbance was measured at 595 nm. The enzyme detection limit was 20 μgL−1.
164
2.3. Characterization. The fluorescence-labeled enzymes were used to explore the
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enzymes immobilization distribution in NF270 membrane, and the enzyme distribution
166
was analyzed by confocal laser scanning microscopy (CLSM) on a Leica SP8 STED
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3X system (Germany). FITC-labeled and RhB-labeled HRP were synthesized by the
168
method reported in literature45 (More details can be found in supporting information).
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The morphologies of the membranes were examined by a Scanning Electron
170
Microscope (SEM) (SU8020, Hitachi, Japan). Zeta potentials of different membranes
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were measured by the SurPASS Anton Paar analyzer at a pH range from 4 to 10.
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Contact angle measurements before and after enzyme immobilization were measured
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using a water drop shape system (OCA20, Dataphysics, Germany).
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2.4. BPA removal. The pristine and biocatalytic membranes were respectively placed
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into the dead-end filtration cell. 40 ml 10 mg·L-1 of BPA solution was poured into the
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filtration cell for BPA removal under 100 rpm agitation at a certain transmembrane
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pressure (2, 3, 4 bar). The experimental temperature was controlled at 20 ± 1℃ by
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placing the cell into water bath. After 10 ml of liquid passed though the membrane,
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another 10 ml of BPA solution was added to treat a total of 40 ml BPA solution. The
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BPA content in the permeate and retentate was measured using high-performance liquid
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chromatography, which is reported in our previous work1, 40.
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2.5. Synergism of separation and reaction. The cascade catalysis efficiency relied on
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the suitable H2O2 concentration supplied to HRP, which was directly related to the
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glucose concentration passing though the membrane. By utilizing the separation
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function of NF membrane, the glucose permeation through the membrane could be
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regulated via changing bulk concentration (1, 5, 50 mM) and operating pressure (2, 3,
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4 bar). In order to ascertain the optimal conditions to maximize the synergism of
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separation and reaction, the effect of H2O2 concentration (0.02-5 mM) on the stability
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of the immobilized HRP during BPA removal with reuse cycle was studied. Then, the
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in-situ generated H2O2 caused by glucose accessible to the immobilized GOx was also
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detected using the feed solution only containing glucose at different concentrations (1,
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5, 50 mM) and pressures (2, 3, 4 bar).
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2.5.1. Glucose retention. Glucose concentration was measured by using 3,5-
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dinitrosalicylic (DNS) method46-47. The retention of glucose (Rglu) by the pristine and
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biocatalytic membranes is calculated by following equation:
196
𝑅𝑔𝑙𝑢 = 1 ― 𝐶𝑓𝑔 × 100%
197
where Cpg and Cfg represented glucose concentrations in the permeate and feed,
198
respectively.
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2.5.2. Measurement of H2O2 concentration. H2O2 concentrations were determined by
200
visible spectrophotometry after color development with Ti(SO4)248-49. In the presence
201
of H2SO4, the H2O2 reacts with Ti(SO4)2 to form yellow substance which has an
202
adsorption maximum at 410 nm.
(
)
𝐶𝑝𝑔
(1)
Ti(SO4)2 + H2O2 + H2O→H4TiO5 + H2SO4
203
(2)
204
60 µl of H2SO4 (0.3 mol·L-1) and 200 µl of Ti(SO4)2 (0.3 mol·L-1) were added into 1.74
205
ml of samples, which were incubated for 5 min and then measured at 410 nm.
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2.5.3. Determination of substrate inhibition. The substrate inhibition of H2O2 on
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HRP was explored by measuring the BPA oxidation rate with various concentrations
208
of H2O2 in 10 ml of BPA solution (containing 1 mgL-1 HRP and 10 mgL-1 BPA). The
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H2O2 concentration ranges were 0.005-20 mM. The BPA concentration in solution after
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oxidation was measured by HPLC, and the BPA oxidation rate is calculated by
211
following equation:
212
V=
(𝐶𝑏 ― 𝐶𝑎)
(3)
𝑇
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where Cb and Ca represented BPA concentrations before and after oxidation in solution,
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respectively; T is the reaction time.
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2.6. Reusability and stability of biocatalytic membrane. The biocatalytic membrane
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reusability and stability were examined by testing the BPA removal for 7 days (20 ml
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for each day) and continuous 10 h (300 ml BPA solution was treated totally),
218
respectively. The operational conditions: 3 bar, 100 rpm, 21 oC, 10 mg·L-1 BPA and 1
219
mM glucose in deionized water. For the 7 days measurement, the membrane was
220
continuously washed under pressure each time until no BPA was detected in washing
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solution before stored at 4 ℃. The BPA removal efficiency is calculated as the BPA
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amount difference between original feed and permeate divided by the BPA amount in
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the original feed.
224 225
3. RESULTS AND DISCUSSION
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3.1. Characterization. Distribution of the immobilized GOx and HRP in the NF270
227
membrane was clarified by CLSM. The CLSM images of the biocatalytic membrane
228
obviously indicated that both GOx-FITC (green) and HRP-RhB (red) were well-mixed
229
and mainly anchored in the intermediate layer between skin layer and support layer (Fig.
230
2).
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231 232 233 234
Figure 2. Confocal laser scanning microscopy images of the biocatalytic membrane with immobilized GOx and HRP. (a) support layer; (b) skin layer; (c) 3D structure and (d) crosssection.
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The contact angles of the pristine and biocatalytic membranes are shown in Fig. 3.
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It was found that the contact angle of the support layer significantly decreased after
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enzyme immobilization and PDA/PEI coating while the hydrophilicity of the skin layer
238
did not change. Such hydrophilicity improvement on support layer was attributed to the
239
highly hydrophilic PDA/PEI coating layer, indicating that the coating was successful
240
and this might be beneficial to membrane permeability. SEM images and Zeta potential
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of the pristine and biocatalytic membranes (Figs. S1 and S2) also revealed that the
242
homogenous coating in the support layer was successfully obtained by co-deposition of
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PEI and PDA.
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Figure 3. Contact angle of pristine and biocatalytic membranes’ skin and support layers.
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3.2. Enzymatic cascade catalysis for BPA removal. We first compared BPA removal
247
efficiency by the pristine, PDA/PEI coated, and biocatalytic membranes. As shown in
248
Fig. 4a, the pristine NF membrane can retain more than 70% of BPA in the first cycle,
249
however, BPA molecules would adsorb and dissolve in the polyamide skin layer, and
250
then diffuse through the membrane, resulting in a continuous retention decrease with
251
filtration cycle ( it becomes below 40% in the fourth filtration cycle), which agrees well
252
with the previous results in literature40. Even though the BPA retention by the PDA/PEI
253
coated membrane increased a little thanks to the possible pore narrowing effect induced
254
by the coating layer, the BPA retention decline with reuse cycle was still observed.
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While for the biocatalytic membrane with two enzymes, the BPA retention almost kept
256
constant (close to 100%) with reuse cycle when a suitable glucose concentration was
257
added. Thus, enzyme catalysis plays a significant role to improve the efficiency and
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stability of BPA removal. During the biocatalytic membrane preparation, we explored
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the permeability variation of the membrane (Fig. 4b). The membrane permeability
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decreased remarkably after enzymes immobilization, which might be derived from the
261
protein fouling layer formed during the reverse filtration, increasing the filtration
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resistance of the membrane. After the PDA/PEI coating, the membrane permeability
263
recovered greatly because the hydrophilicity of the membrane support was improved
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(Fig. 3). Moreover, the membrane permeability further increased after membrane
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reversal due to the enzyme movement and leakage.
266 267
Figure 4(a) BPA removal efficiency by the pristine, PDA/PEI coated, biocatalytic membranes
268
when 10 gL-1 BPA solution containing 5 mM β-D-glucose was treated with an agitation speed of
269
100 rpm and an operation pressure of 3 bar for 4 cycles (10 ml per cycle). (b) Membrane
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permeability variations at different enzyme immobilization stages.
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The enzyme immobilization mechanisms included adsorption, entrapment and
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covalent bonding as discussed in our previous work, and it was also confirmed that
273
there was almost no enzyme leakage during the storage for 9 days
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loading in the membrane is about 29.85 µg cm-1 in this work, which was much higher
275
than in literature. On the other hand, under pressure-driven flow-through mode, the as-
276
prepared
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substrate/intermediates/products around the anchored enzymes, but the contact time
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between substrates and enzymes became short, thus the enzyme activity had to be high
membrane
would
facilitate
the
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mass
13, 40.
The enzyme
transfer
of
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enough to ensure a high micro-pollutant removal. It was reported that the cascade
280
catalysis by GOx and HRP followed these steps (Eqs. 4-7)50-51. In the first step, the
281
required gluconic acid and H2O2 are in-situ produced by GOx through oxidizing β-D-
282
glucose in the presence of oxygen:
283
β-D-glucose + O2
284
In the next step, HRP is activated by H2O2, leading to the generation of compound I
285
and H2O. Then, compound I can oxidize BPA via oxidoreduction and generate
286
compound II and free radicals. Finally, the produced compound II interacts with other
287
BPA to generate radicals and regenerated HRP.
288
HRP + H2O2
289
compound I +BPA
290
compound II +BPA
GOx
gluconic acid + H2O2
compound I + H2O compound II + BPA radical BPA radical + HRP
(4)
(5) (6) (7)
291
In order to clarify the effect mechanisms of the cascade catalysis on BPA removal
292
by the biocatalytic membrane, we then investigated the effect of pH and glucose
293
concentration on the BPA removal efficiency by free enzymes. The results revealed
294
that the acidic pH caused by gluconic acid could enhance the activity of HRP toward
295
BPA degradation by virtue of H2O2 although gluconic acid itself had a little inhibition
296
effect on HRP compared to hydrochloric acid at the same pH (Fig. S3). It also was
297
confirmed that for the free GOx and HRP, the BPA removal efficiency was closely
298
related with the available glucose concentration, which was eventually attributed to the
299
produced H2O2 concentration since the pH variation was similar at the glucose
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concentrations from 5 to 125 mM (Fig. S4a).
301
Though increasing glucose concentration resulted in higher BPA removal by free
302
enzymes for a long reaction time (Fig. S4b), the catalytic behavior in the biocatalytic
303
membrane (a very short reaction time in flow-through mode) may be different, and
304
moreover, the reusability of the biocatalytic membrane may be worse at high glucose
305
concentration because the excessive H2O2 in-situ generated by GOx would inhibit or
306
damage the immobilized HRP. Next we investigated the effect of the glucose
307
concentration in the reactants on the BPA removal by the biocatalytic membrane. As
308
shown in Fig. 5a, in the first flow-through cycle, the BPA removal efficiency reaches
309
around 100% with 5 mM and 50 mM glucose, while it is only 93% with 1 mM glucose,
310
indicating that a higher glucose concentration would promote HRP activity on BPA by
311
decreasing pH and offering more H2O2 for the microenvironment of the immobilized
312
HRP in the membrane (Fig. 5b). However, with increase of reuse cycle, the BPA
313
removal by the biocatalytic membrane declined gradually when the glucose
314
concentration was 5 and 50 mM, on the contrary, with 1 mM glucose, the biocatalytic
315
membrane kept 100% of BPA removal in the next three reuse cycles. This can be
316
explained by the inactivation effect and substrate inhibition of the excessive H2O2 on
317
HRP, and the latter was confirmed by the BPA oxidation rate variation along with H2O2
318
concentration via free HRP-induced catalysis (Fig. 5c). Furthermore, the optimal range
319
of H2O2 concentration was determined by adding H2O2 instead of glucose into BPA
320
solution (Fig. 5d), and the BPA removal efficiency by the biocatalytic membrane could
321
reach nearly 100% in four reuse cycle when the H2O2 concentration ranges from 0.05
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to 1 mM. It is worth noting that the BPA removal efficiency by the biocatalytic
323
membrane decreased with the reuse cycle when the H2O2 concentration was 0.02 mM,
324
which was attributed to the insufficient H2O2 concentration. In the initial filtration stage,
325
the BPA retention was high due to the adsorption effect (Fig. 4a) and the BPA in the
326
permeate could be fully oxidized under a low H2O2 concentration. However, with the
327
increase of reuse cycles, the BPA concentration in the permeate gradually increased
328
due to “adsorption saturation” of the membrane (Fig. 4a), and such low H2O2
329
concentration was insufficient to oxidize the flowing BPA. Besides the improper H2O2,
330
the undesirable pH (~ 7) would result in severer decline of the BPA removal efficiency
331
with increasing reuse cycle (Fig. S5).
332 333 334 335 336
Figure 5. Effect of glucose concentration on (a) BPA oxidation by cascade catalysis in membrane and (b) the permeate pH with reuse cycle. Effect of H2O2 concentration on (c) BPA oxidation rate catalyzed by free HRP and (d) BPA oxidation by immobilized HRP in the membrane with reuse cycle.
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337
3.3. Synergism of separation and reaction for engineering microenvironment. It
338
was verified that adding a suitable H2O2 concentration could achieve stable BPA
339
removal, but the pH of the real potable water is unfavored for HRP activity and the
340
H2O2 concentration in the membrane could not be adjusted by NF (H2O2 passes through
341
the membrane freely). Glucose is partly retained by the NF270 membrane, and the
342
glucose accessible to the immobilized enzymes can be regulated by both its bulk
343
concentration and permeate flux, which provides a simple and rapid way to control pH
344
and H2O2 concentration in the reaction zone. First, the biocatalytic membrane had a
345
little higher retention of glucose than the pristine NF270 due to the pore blocking effect
346
by the immobilized enzymes, and the glucose retention increased with permeate flux at
347
higher operating pressure owing to the enhanced convection of solvent. (Fig. S6).
348
Second, the permeate flux at higher glucose concentration was lower because the larger
349
osmotic pressure reduced the effective driving force across the membrane (Fig. 6a).
350
Third, the glucose permeance was affected by both its feed concentration and permeate
351
flux, which became larger at higher feed concentration and lower permeate flux (Fig.
352
6b). The above results regarding the glucose separation behavior could be used to
353
understand the BPA removal efficiency by the biocatalytic membrane at different
354
conditions. As shown in Fig. 6c-e, with 1 mM glucose, the BPA removal is less 95% in
355
the first flow-through cycle due to insufficient glucose passing through the membrane,
356
and although the contact time and glucose permeance are supposed to be higher at lower
357
pressure, the BPA removal at 2 bar is surprisingly lower than the others, implying that
358
the enzymatic reaction in the confined space may be different from in bulk solution. In
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the second reuse cycle, due to the glucose accumulation in the membrane with filtration
360
time, the BPA removal greatly increased to 100% except for the case at 4 bar possibly
361
because of its higher glucose retention and shorter contact time (Fig. 6c). In the third
362
and fourth cycle, the BPA removal at all operating pressure reached 100%. While with
363
5 mM glucose, the BPA removal efficiency kept at a stable level at 100% under various
364
operating pressure with reuse cycle because such glucose concentration produced a
365
suitable H2O2 concentration in the membrane, which was confirmed by the results in
366
Fig. 6f (0.25-1.0 mM). However, with 50 mM glucose, the BPA removal declined in
367
the second cycle and became worse in the fourth cycle, especially at lower pressure.
368
This was explained by a fact that an excessive H2O2 posed a negative effect on the HRP,
369
and even in the first cycle, the generated H2O2 was much higher than 1.0 mM (Fig. 6f).
370
In the fourth cycle, the BPA removal was a little higher at higher pressure because it
371
resulted in lower glucose permeance and shorter contact time alleviating the inhibition
372
effect of the excessive H2O2. Therefore, by tuning the glucose concentration in water
373
and operating pressure, the BPA removal by the biocatalytic membrane can be
374
manipulated via engineering the microenvironment (pH and H2O2 concentration)
375
around the immobilized enzymes in the membrane. As shown in Fig. 7, a super high
376
and stable BPA removal (>80%) by our biocatalytic membrane (the BPA removal by
377
the pristine NF270 was only 40%) is obtained in a long-term operation (continuous
378
running for 10 h or batch operation for 7 days). To the best of our knowledge, these are
379
the top results on the reusability and stability of BPA removal when compared with
380
other biocatalytic membranes with immobilized laccase reported in literatures (Table
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1). In detail, most biocatalytic membranes used for BPA removal were based on laccase
382
due to its high catalysis activity on different micropollutants. However, the laccase is
383
easily deactivated, resulting in its poor stability. In our previous work, the average BPA
384
removal efficiency decline of the laccase-loaded membranes ranged from 4% to 9% in
385
each cycle (10 mg L-1 BPA), while in present work, this number was only 1.36%,
386
showing much higher stability and reusability. Furthermore, the throughput of our
387
biocatalytic membrane was the highest among the results listed in Table 1,
388
demonstrating that this novel biocatalytic membrane could remove much more BPA in
389
a very short filtration time.
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390 391 392 393 394 395
Figure 6. (a-b) Effect of operating pressure on (a) permeate flux and (b) glucose permeance at different glucose concentrations. (c-e) Effect of operating pressure on cascade catalysis in the membrane at different glucose concentrations of (c) 1 mM; (d) 5 mM and (e) 50 mM with reuse cycle. (f) H2O2 concentration produced by immobilized GOx in the membrane at different glucose concentrations with reuse cycle (operating pressure=3 bar).
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396 397 398 399 400 401 402 Matrixes
Figure 7. BPA removal performance during (a) continuous filtration for 10 h and (b) long-term batch operation for 7 days. Experimental conditions: operating pressure = 3 bar, feed solution = 10 mg·L-1 BPA and 1mM glucose, the total treated volume was 300 mL for continuous filtration, the treated volume was 20 mL for each batch. Table 1 Comparison of reusability of biocatalytic membrane in BPA removal. a
Enzyme
BPA removal
Initial BPA concentration (mgL-1)
efficiency in the
BPA removal Reuse cycle
first cycle (%)
efficiency in the last cycle (%)
Average decline
Throughput
every cycle (%)
(Lm-2h-1)b
Ref.
CNTs-PVDF (MF)
laccase
4.57
96
4
91
1.25
10.00
52
TiO2 (sol-gel)-PVDF (MF)
laccase
34.24
92
4
84
2.00
20.00
53
TiO2 (NP)-PVDF (MF)
laccase
34.24
95
15
15
5.33
5.00
54
Cu-PVDF (MF)
laccase
10.00
99
5
57
8.68
10.00
55
PDA-NF270 (NF)
laccase
10.00
88
7
34
7.29
5.97
40
CuPDA-NF270 (NF)
laccase
2.00
87
9
77
1.11
2.49
13
PAN-MIL-101 (UF)
laccase
10.00
92
7
62
4.29
6.45
10
PDA/PEI-NF270 (NF)
GOx/HRP
10.00
89
7
80
1.36
21.94
This work
403 404
a
MF: microfiltration, UF: ultrafiltration, NF: nanofiltration;
b
Throughput was calculated by volume of feed
solution/membrane area/processing time.
405 406
4. CONCLUSIONS
407
A biocatalytic membrane for cascade catalysis was simply prepared by co-
408
immobilization of GOx and HRP in a NF membrane via reverse filtration and
409
subsequent co-deposition of PDA/PEI. When glucose in the reactants passed through
410
the biocatalytic membrane, it was catalyzed by GOx to produce gluconic acid and H2O2
411
for engineering microenvironment of the immobilized HRP, where the pH was
412
decreased to enhance the HRP activity, and BPA was degraded by HRP in the presence
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413
of H2O2. Such cascade catalysis in a NF membrane toward micro-pollutant removal
414
could be well tuned by changing the bulk glucose concentration and operating pressure.
415
The BPA removal by the biocatalytic membrane in a flow-through cycle achieved 100%
416
with 5 mM glucose and 10 mg·L-1 BPA in the deionized water operated at 2-4 bar,
417
which even kept above 80% in a continuous filtration for 10 h or batch operation for 7
418
days. This biocatalytic NF membrane is promising for efficient and stable micro-
419
pollutant removal in portable water (as domestic water dispensers or tap water filters),
420
and a smart membrane system with a micro-pollutant sensor may be constructed, where
421
the glucose permeance through the NF membrane can be regulated by adjusting glucose
422
concentration and operation pressure according to the micro-pollutant concentration in
423
water, ensuring a suitable microenvironment for the HRP activity and stability.
424 425
ACKNOWLEDGEMENTS
426
The financial supports are supplied by the Beijing Natural Science Foundation
427
(2192053) and Youth Innovation Promotion Association (2017069) of Chinese
428
Academy of Sciences.
429 430
ASSOCIATED CONTENT
431
Supporting Information. SEM images and Zeta potential of pristine and biocatalytic
432
membranes (both sides). Effect of pH and acids on BPA oxidation by free HRP. Effect
433
of glucose concentration on pH of the solution and BPA removal efficiency by cascade
434
catalysis with free enzymes. Effect of pH on BPA oxidation by cascade catalysis in the
435
biocatalytic membrane. Glucose retention as a function of permeate flux by the pristine
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436
and biocatalytic NF membranes at different glucose concentrations. This material is
437
available free of charge via the internet at http://pubs.acs.org.
438 439
AUTHOR INFORMATION
440
* Corresponding author: E-mail:
[email protected] (J. Luo)
441 442
NOTES
443
The authors declare no competing financial interest.
444 445
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