PDF《sensors》

1991 and the subsequent report of the synthesis of large quantities by Ebbesen and coworkers [11] in 1992, carbon nanotubes (CNT) have been the major subject of numerous experimental and theoretical investigations [12]. Because of their novel structural and electronic properties, their high chemical stability, and their extremely high mechanical strength and modulus [12], CNT have found a wide range of potential applications from structural materials [13,14] to nanoelectronic components [15]. Specific applications of CNT include their use as a high sensitivity microbalance [16], gas detector [17,18], catalyst support [19,20], electron source in field emission mode for display [21], tiny tweezers for nanoscale manipulation [22] and probe tips for scanning probe microscopy [23]. Theoretical calculations have shown that, depending on its symmetry and diameter, CNT can be metallic or semiconducting [24,25]. The subtle electronic properties suggest CNT have the ability to promote electron transfer in electrochemical reactions when used as an electrode, representing a new application of CNT. The ability of CNT-modified electrodes to promote electron transfer reactions has been documented in connection to important biomolecules [26-34]. We have studied the direct electrochemical oxidation of dopamine and NAD(P)H [32,34] at a CNT- modified electrode. CNT can reduce the overpotential of electrochemical oxidation of NAD(P)H for

400 to

600 mV. Our goal is to explore new application of CNT as a new electrode material in facilitating the direct electron transfer between biomolecules and electrode. In this work, we report the direct electron transfer of hemoglobin (Hb), horseradish peroxidase (HRP) and glucose oxidase (GOx), which was immobilized onto the surface of CNT, respectively. The surface of CNT was covered with a layer of surfactant molecule, cetyltrimethylammonium bromide (CTAB, a cationic surfactant). The cyclic voltammetric results indicated that the direct electron transfer between the redox active center of immobilized Hb, HRP and GOx , respectively, and the surface of electrode occurred effectively. 2. Experimental Sensors 2005,

5 222 2.1 Chemicals Hemoglobin (Hb, from Bovine Red Cells, Worthington Biochemical Corporation), horseradish peroxidase (HRP, EC 1.11.1.7, RZ >

3,

250 U/mg, Sigma), glucose oxidase (GOx, EC 1.1.3.4, from Aspergillus niger,

111 U/mg, Nanjing Sunshine Biotechnology Ltd., Nanjing, China), flavin adenine dinucleotide (FAD, disodium salt, 96%, Sigma) and hexaammineruthenium (III) chloride (99%, Strem Chemicals) were used as received. Nafion (10% in methanol with equivalent weight of about 1100) was obtained from Aldrich and was diluted to 5% with H2O before use. Multi-wall carbon nanotube (CNT, 95%) was purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China). All other chemicals were of analytical grade. All the solutions were prepared with doubly distilled water. 0.1 M phosphate buffer solutions (PBS, pH 6.9), which were made up from Na2HPO4 and NaH2PO4, were always employed as supporting electrolyte except that the pH-dependent experiments were carried out in PBS with various pH values. 2.2 Immobilization of enzymes and proteins on the surface of CNT The GC electrode (4-mm in diameter) was polished sequentially with metallographic abrasive paper (No. 6), slurries of 0.3 and 0.05-?m alumina to a mirror finish. After rinsed with doubly distilled water, it was sonicated with absolute ethanol and then with doubly distilled water for about

1 min, respectively. CNT (1 mg) was dispersed in