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Communication

DOI: 10.1002/marc.200500885

Summary: Nanofibrous membranes that possess reactive groups are fabricated by the electrospinning process from PANCAA solutions that contain MWCNTs. Field emission scanning electron microscopy is used to evaluate the morphology and diameter of the nanofibers. Potentials for applying these nanofibrous membranes to immobilize redox enzymes by covalent bonding are explored. It is envisaged that the electrospun nanofibrous membranes could provide a large specific area and the MWCNTs could donate/accept electrons for the immobilized redox enzymes. Results indicate that, after blending with MWCNTs, the diameter of the PANCAA nanofiber increases slightly. The PANCAA/ MWCNT nanofibrous membranes immobilize more enzymes than that without MWCNTs. Moreover, as the concentration of the MWCNTs increases, the activity of the immobilized catalase is enhanced by about 42%, which is mainly attributed to the promoted electron transfer through charge-transfer complexes and the p system of MWCNTs.

The covalent immobilization of redox enzymes, such as catalase, on a PANCAA/MWCNTs nanofiber.

Nanofibrous Membranes Containing Carbon Nanotubes: Electrospun for Redox Enzyme Immobilization Zhen-Gang Wang,1 Zhi-Kang Xu,*1 Ling-Shu Wan,1 Jian Wu,2 Christophe Innocent,3 Patrick Seta3 1

Institute of Polymer Science and Key Laboratory of Macromolecule Synthesis and Functionalization, Zhejiang University, Ministry of Education, Hangzhou 310027, P. R. China Fax: þ 86 571 8795 1773; E-mail: [email protected] 2 Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China 3 Institute of Europe´e des Membranes, UMR CNRS no.5635, 34293 Montpellier Cedex 05, France

Received: December 29, 2005; Accepted: February 2, 2006; DOI: 10.1002/marc.200500885 Keywords: carbon nanotubes; catalase; electrospinning; enzyme immobilization; enzymes; membranes; nanofibrous membranes; poly(acrylonitrile-co-acrylic acid)

Introduction In recent years, electrospinning has gained widespread attention since it is known to be an effective fabrication tool for preparing fibrous materials with diameters ranging from several micrometers down to tens of nanometers.[1–13] Among various polymers, acrylonitrile-based homopolymers and copolymers have most recently been fabricated into nanofibrous materials with reinforcing, superhydrophobic, and/or catalytic properties.[4a,5–13] In our previous work, novel nanofibrous membranes that possess reactive

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carboxy groups were fabricated from poly(acrylonitrile-comaleic acid) by the electrospinning process.[13] Lipase from Candida rugosa is covalently immobilized onto this membrane surface via the activation of carboxy groups. It is found that the enzyme loading and the activity retention of immobilized lipase on the nanofibrous membrane are much higher than those on the poly(acrylonitrile-co-maleic acid) hollow fiber membrane. Thus, the process described in our work presents a convenient approach to fabricate nanofibrous membranes with reactive groups for enzyme immobilization. These enzyme-immobilized nanofibrous

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membranes with high enzyme loading and bioactivity may have great potential in the fields of biocatalysts for polymer synthesis, in-situ formation of nanofiber reinforcement composites, biosensors, and biocatalysis/separation. However, for redox enzymes such as catalase, a direct electrontransfer path should be built between the immobilized enzyme and the nanofibrous membrane. Since their discovery by Ijima,[14] carbon nanotubes (CNTs), which consist of cylindrical graphite sheets with nanometer diameter, have attracted intensive attention for superb electrical conductivity, high chemical stability, and remarkable mechanical strength.[15] CNTs behave either as metals or as semiconductors, depending on their dimensions, electronic structure, and topological defects present on the tube surface.[16] Their unique electronic properties suggest that CNTs have the ability to promote the electrontransfer reactions of biomolecules in electrochemistry.[17] For example, multiwalled carbon nanotubes (MWCNTs) show good communication with redox proteins, including those where the redox center is embedded into the glycoprotein cell, such as glucose oxidase.[18] Therefore, this communication reports a simple approach to fabricate nanofibrous membranes from poly(acrylonitrileco-acrylic acid) (PANCAA) solutions containing MWCNTs. These nanofibrous membranes possess both reactive groups and MWCNTs for the immobilization of redox enzymes. As mentioned above, electrospinning is a nanofiber assembly technique to generate fibrous membranes with large specific surface areas stemmed from tiny fibers with diameters in the nanometer scale.[19] This large specific area could greatly increase the catalyzing ability of immobilized enzymes because the remarkable reduction of diffusional restriction for substrate and product transport,[20] as well as provide more room for enzyme immobilization. On the other hand, MWCNTs in the nanofibrous membrane might enhance the electron transfer efficiency between the immobilized enzyme and the membrane. Such PANCAA/MWCNTs nanofibrous membranes thus combine the major advantages of CNTs and the electrospinning technique. The activity of the immobilized redoxase is expected to be obviously enhanced.

Experimental Part Poly(acrylonitrile-co-acrylic acid) (PANCAA) with a molecular weight (Mv) of 8.32
104 g  mol1 was synthesized by a water phase precipitation copolymerization process. The molar content of acrylic acid in this copolymer was about 10%. MWCNTs were synthesized by a chemical vapor deposition (CVD) process and purchased from Nanotech Port (Shenzhen, China). Catalase (hydrogen peroxide oxidoreductase, EC1.11.1.6, from bovine liver) and 1-ethyl-3-(3dimethyllaminopropyl)carbodiimide (EDC, analytical grade) were obtained from Sigma. Coumassie brilliant blue (G250) for the Bradford protein assay was purchased from Urchem and BSA (bovine serum albumin, BP0081) from Sino-American Biotechnology. N-Hydroxysuccinimide (NHS) was of bioMacromol. Rapid Commun. 2006, 27, 516–521

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logical grade and hydrogen peroxide (30%) was of analytical grade. To purify and uniformly disperse MWCNTs in the copolymer matrix, MWCNTs were treated with a mixture of concentrated sulfuric and nitric acids (3:1, 98 and 70%, respectively) at 40 8C.[21a] Carboxy (–COOH) and hydroxy (–OH) groups were introduced onto the oxidized MWCNTs as identified by IR spectroscopy, similar to the others reported before.[8,21] After chemical etching, the surface-oxidized MWCNTs were well-dispersed in N,N0 -dimethylformamide (DMF) after sonication for 10 h. PANCAA was dissolved in the suspension of MWCNTs in DMF at 100 8C. Its concentration was always kept at 6 wt.-% in DMF. The concentration of the MWCNTs was varied with 0, 10, 20, and 30% of the mass of dissolved PANCAA. Electrospinning was carried out using a syringe with a 1.2 mm diameter spinneret at an applied electrical potential difference of 12 kV over a 15 cm gap between the spinneret and the collector. A microinfusion pump was set to deliver the solution at a flow rate of 1.0 mL  h1 using a 20 mL syringe. The collector was a flat support wrapped with conductive tinfoil. The electrospun fibrous membranes were dried in a vacuum oven at 60 8C for at least 5 h to remove residual solvent before used. Field-emission scanning electron microscopy (FEI, SIRION-100, USA) was applied to evaluate the morphology and diameter of neat or composite nanofibers. Before analysis, the samples were sputtered with gold using an Ion sputter JFC1100. Catalase was covalently immobilized onto the nanofibrous membranes with the EDC/NHS activation procedure, as described previously.[22] An appropriate amount of nanofibrous membrane was thoroughly washed with de-ionized water, and then rinsed with phosphate buffer solution (50
103 M, pH 7.0). After this, the pretreated membranes were submerged into an EDC/NHS solution (5.0 g  L1 in PBS buffer, 50
103 M, pH 7.0, the molar ratio of EDC to NHS ¼ 1:1) and were shaken gently for 6 h at room temperature. The activated membranes were taken out, washed several times with phosphate buffer (pH 7.0), and mixed with a catalase solution (0.20 mg  mL1 in PBS, pH 7.0). Enzyme immobilization was conducted at room temperature for 2 h. The resultant catalaseimmobilized membrane was washed with PBS (50
103 M, pH 7.0) until no protein was detected in the washings. Protein content in the solution was determined by the method of Bradford[23] using BSA as the protein standard, on a UV spectrophotometer (756PC, Shanghai Spectrum Instruments Co. LTD). The amount of immobilized enzyme protein was estimated by subtracting the amount of protein determined in the washings from the total amount of protein used in the immobilization procedure. Catalase activity was determined spectrophotometrically by the direct measurement of the decrease in the absorbance of hydrogen peroxide at 240 nm with a specific absorption coefficient of 0.03921 cm2  (mmol H2O2)1. A sample of 3 mL of the reaction mixture that contained 9.7
103 M substrate in 50
103 M phosphate buffer at pH 7.0 was pre-incubated at 25 8C for 10 min and the reaction was started by adding 0.25 mL of catalase solution (0.01 mg enzyme  mL1). The decrease in absorbance at 240 nm was recorded for 3 min. The ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. Scanning electron microscopic images of PANCAA/MWCNTs nanofibrous membranes with A) 0, B) 10, C) 20, and (D) 30 wt.-% MWCNTs.

rate of change in absorbance was calculated from the initial 2 min portion with the help of the absorbance vs. time curve. A 0.4 mg sample of the immobilized enzyme preparation was introduced into the assay mixture to initiate the reaction as above. After 2 min, the reaction was terminated by removing the nanofibrous membrane from the reaction mixture. The absorbance of the reaction mixture was determined and the immobilized catalase activity was calculated. One unit of activity was defined as the decomposition of 1 mmol hydrogen peroxide per min at 25 8C and pH 7.0. The activity of free catalase was given as mmol H2O2  (mg  enzyme)1  min1 and the immobilized catalase’s activity as mmol H2O2  (mg  immobilized enzyme)1  min1 at 25 8C and pH 7.0.

Results and Discussion PANCAA is selected here to blend with MWCNTs for electrospinning because it can provide reactive groups, i.e., –COOH, for enzyme immobilization through covalent bonding. In addition, the negatively charged functional

groups (–C N:) can form charge-transfer complexes with the surface-oxidized MWCNTs, which leads to elevated electrical conductivity for the composite nanofibrous membranes and enhances the interfacial interaction between the MWCNTs and the polymer chains.[8] The as-received MWCNTs are treated with a mixture of sulfuric and nitric acid (3:1, 98 and 70%, respectively) to introduce polar groups such as –COOH onto their surfaces. Typical results agree with previously published work.[21a] It is found that such a MWCNT suspension in DMF can be kept stable for several days after sonication. PANCAA/ MWCNT composite nanofibrous membranes are formed with a PANCAA solution that is ejected from a syringe spinneret under a driving force of high voltage. From Figure 1, it can be seen that the surface morphology of the composite nanofibrous membrane is smooth without MWCNTs and becomes rougher as the MWCNT concentration increases. This result indicates that there are some MWCNTs not embedded into the nanofiber matrix, i.e., the

Table 1. Effects of the MWCNT concentration on the fiber diameter of PANCAA/MWCNTs nanofibrous membrane and the immobilized catalase. MWCNT concentration wt.-% 0 10 20 30

Nanofiber diameter

Bound enzyme

Activity

Activity retention

nm

mg enzyme  (g fibers)1

mmol H2O2  (mg enzyme)1  min1

%

181.49  15.04 310.51  11.05 277.21  11.37 286.72  9.32

23.87  0.62 30.33  2.29 26.89  3.15 31.10  4.54

824.78  103.98 962.23  81.25 1088.57  99.08 1178.69  100.14

33.11 39.10 43.70 47.90

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protruding MWCNTs render the nanofiber surface relatively rougher.[8a] From Table 1, it is obvious that the nanofibers with MWCNTs show larger diameters than the pure PANCAA nanofibers. Many parameters could affect the electrospun nanofibers during the electrospinning process. In addition to operating and ambient conditions, the inherent properties of the solution play a significant role in controlling the nanofiber’s characters.[3] In the case here, the carboxy groups attached on the MWCNTs surfaces can interact with DMF and the –COOH groups (through hydrogen bonds) in PANCAA, which in turn increases the solution viscosity. Therefore, the larger diameter of the nanofibers mainly results from the higher viscosity of the spinning solution.[24] However, with the further increase of MWCNT concentration, the fiber diameters show no increase but reduce slightly. This might be attributed to the poorer dispersion of MWCNTs at higher concentrations. Catalase is chosen as a model redox enzyme to confirm the idea mentioned above. Generally speaking, fibers with larger diameters have lower specific surface areas, which will reduce the amount of bound enzyme. Furthermore, one could envisage that higher MWCNT concentrations lead to

lower PANCAA concentrations, followed by fewer reactive groups for catalase immobilization. However, in this case, the actual amount of loaded enzyme does not decrease as the MWCNT concentration increases, especially in comparison of the PANCAA/MWCNT composite nanofibrous membranes with the PANCAA membrane. One interpretation might be that the protruding surface-etched MWCNTs also provide reactive groups for the immobilization reaction with catalase. On the other hand, catalase immobilized on the MWCNT-containing nanofibrous membrane presents a higher activity than that on the PANCAA membrane. As the MWCNT concentration increases, the exhibited activity increases and the retention of activity of the immobilized enzyme increases from 33.11 to 47.90 and 42%, respectively. The mechanism of catalysis and the role of the MWCNTs are schematically indicated in Figure 2. Catalase is a tetrameric heme enzyme with four identical tetrahedrally arranged subunits.[25] The heme group consists of a protoporphyrin ring and a central Fe atom, i.e., ferriprotoporphyrin, where iron is usually in its stable oxidized resting state.[26a] The catalytic mechanism of catalase follows the equations (Reaction 1–3) in Figure 2. Catalase catalyses the disproportionation of hydrogen peroxide into

Figure 2. Schematic representation for the promoted electron transfer from hydrogen peroxide to the immobilized catalase through PANCAA/MWCNTs nanofiber. The inset is the speculated path for convenient electron transfer. MWCNTs herein act both as electrons acceptor and donor. The tetrameric catalase structure was adopted from Leowen et al.[26a] Macromol. Rapid Commun. 2006, 27, 516–521

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water and oxygen (Reaction 1). The reaction takes place in two-electron reactions with the first hydrogen peroxide molecule oxidizing the heme to compound I to generate the oxoferryl species and a porphyrin cation radical (Reaction 2). Two electrons then transfer from the second hydrogen peroxide to regenerate the resting enzyme while releasing water and a molecule of oxygen (Reaction 3).[26a] Reaction 2, in which H2O2 serves as a H donor for Compound I, proceeds exceedingly rapidly, and the peroxidative reaction proceeds relatively slowly.[25] Therefore, Reaction 3 dominates the total reactions. In electrode reactions, the electrode participates in the reaction by providing electrons[26b] and the CNTs play a role of mediator,[27] in which a direct electron-transfer path is built through the CNTs. In the case of the non-electrode fibrous membranes here, there might be a somewhat different transfer path, which is based on the PANCAA/MWCNT nanofiber showing distinct electrical conductivity along and normal to the spinning direction.[28] When electrons transfer from hydrogen peroxide to Compound I, channels through the charge-transfer complexes (mentioned previously) and nanotube p system (Figure 2b) might be formed, which promotes the oxidation of the second H2O2. This model may also interpret the effective enhancement of activity of immobilized catalase by the incorporation of MWCNTs. Since it is difficult for the electrons to transfer from H2O2 to Compound I (k2) directly, they could take the short cut (the channel) to the goal.

Conclusion In summary, it is shown that PANCAA/MWCNT nanofibrous membranes that possess reactive groups can be fabricated for catalase immobilization with the advantages of higher enzyme loading and activity. These composite nanofibrous membranes integrate the high specific surface area of nanomaterials with the metal-like electrical conductivity of MWCNTs. Moreover, an electron-transfer path could be formed through charge-transfer complexes and MWCNTs. It is believed that similar results will be obtained when other redox enzymes are immobilized.[29] The described method offers great promise to develop novel polymeric supports and corresponding biosensors from redox enzymes. Acknowledgements: Financial support from the National Natural Science Foundation of China (Grant no. 50273032) and the Programme Sino-Franc¸ais de Recherches Avance´es (No. PRA E03-05) are gratefully acknowledged.

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