∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 18 J. Biol. Today's World. 2015 Jan; 4 (1): 18-28 J o u r n a l o f B i o l o g y a n d T o d a y ' s W o r l d ISSN 2322-3308 http://www.journalbio.com Received: 23 August 2014  Accepted: 12 November 2014 Research doi:10.15412/J.JBTW.01040104 Design a biosensor for measuring H 2 O 2 using modified carbon paste electrode with Single-wall Carbon Nanotubes and Catalase Saeed Rezaei-Zarchi 1 , Leila Mirzaei 2 , Samira Ghobadzadeh 3 , Masoud Negahdary 4 , Gholamreza Mazaheri 1 , Marziyeh Ajdary 5 , Fariba Pishbin 1* 1 Department of Biology, Payame Noor University, IR. Of IRAN 2 Albert Szent-Gyorgyi university, Hungary 3 Department of Medical Immunology, Faculty of Medical Science, Tarbiat Modares University, Tehran, Iran 4 Yazd Cardiovascular Research Center, Afshar Hospital, Shahid Sadoughi University of Medical Sciences and Health Services, Yazd, Iran 5 Young Researchers and Elite Club, Khorasgan Branch, Islamic Azad University, Isfahan, Iran *correspondence should be addressed to Fariba Pishbin, Department of Biology, Payame Noor University, IR. Of IRAN; Tell: +989132809943; Fax: +989132809943; Email: faribappp@yahoo.com. ABSTRACT In this study, carbon paste electrode (CPE) was modified by single-wall carbon nanotubes (SWNTs) and catalase enzyme (CAT). The modified electrode was used for determination of hydrogen peroxide (H 2 O 2 ). The SWNT was synthesized by catalytic chemical vapor deposition (CCVD) of methane on the Fe/Co/MgO catalyst and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-Ray Diffraction (XRD), Raman spectroscopy and UV-visible spectroscopy. The best experimental condition could result in carbon yield of 250% and high quality SWNTs with the controlled diameter of 0.9- 2.7 nm and length 5-30m and purity more than 90% and less than 3% amorphous carbon were obtained. Direct electrochemistry of catalase enzyme in CPE was easily obtained and a reversible pair of peaks, that are similar, were appeared from Fe (ΙΙ) and Fe (ΙΙΙ) with the formal potential (E  ) about +0.045 volts. This electrochemical process occurred in 0.1 M phosphate buffer solution (PBS) at PH=7. The designed sensor with high sensitivity and short response time (less than 8 second), and in the linearity of 3 to 370 M can be used to determine the concentration of H 2 O 2 . Moreover, this sensor has a very good stability. Key words: carbon paste electrode, single-wall carbon nanotubes, catalase enzyme, hydrogen peroxide Copyright © 2015 Saeed Rezaei-Zarchi et al. This is an open access article distributed under the Creative Commons Attribution License. 1. INTRODUCTION ince the discovery of carbon nanotubes (CNTs) by Iizima in 1991, interest to exploit these miniaturized entities for using in such areas as field emitters, batteries, nanotube actuators , probe tips, reinforced composites, nanoelectronics display devices, sensors and biosensors etc. has been strongly increased. CNTs can be created using chemical vapor deposition, carbon arc methods, Ball milling or laser evaporation. Single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT), as the two main variants, have a high tensile strength (Young’s modulus > 1 terapascal, making CNTs as stiff as diamond and flexible along the axis), ultra-light weight, and excellent chemical and thermal stability. They are known for having semi- and metallic- conductive properties (1). These features has led to many suggested applications in the biomedical field, including biosensors, drug and vaccine delivery. Both SWNTs and MWNTs are typically a few nanometers in diameter and several micrometers (10μm) to centimeters long. SWNT possesses a cylindrical nanostructure with high aspect ratio that is formed by a rolling up single graphite sheet called graphene into a tube form. All the carbons in graphene sheets are sp2 hybridized and have a delocalized π electron structure that is responsible for the extraordinary electronic qualities. CNT displays metallic, semiconducting and S