Electrochimica Acta 57 (2011) 132–138 Contents lists available at ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Electrochemical preparation of over-oxidized polypyrrole/multi-walled carbon nanotube composite on glassy carbon electrode and its application in epinephrine determination Saeed Shahrokhian a,b,∗ , Reyhaneh-Sadat Saberi b a Department of Chemistry, Sharif University of Technology, Tehran 11155-9516, Iran b Institute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran a r t i c l e i n f o Article history: Received 26 November 2010 Received in revised form 6 April 2011 Accepted 7 April 2011 Available online 23 April 2011 Keywords: Epinephrine Multi-walled carbon nanotube Over-oxidized polypyrrole Composite electrodes Modified electrodes a b s t r a c t A composite film constructed of surfactant doped over-oxidized polypyrrole and multi-walled carbon nanotube was prepared on the surface of glassy carbon electrode by the electro-polymerization method. Surface characterization of the modified electrode was performed by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectrometry. The investigations have been proved that the over-oxidation of the modifier film resulted in a porous thin layer that improves the interlayer diffusion mechanism for the electroactive species. On the other hand, the negative charge density on the surface of the electrode excludes the negative analytes (e.g. ascorbate and Fe(CN) 6 3-/4- ) and attracts the positive ones (e.g. dopamine and epinephrine). A remarkable enhancement in the microscopic area of the elec- trode resulted in a considerable increase of the peak current of epinephrine (EN) oxidation (∼35 times). Differential pulse voltammetry has been applied as a very sensitive analytical method for the determina- tion of sub-micromolar amounts of EN. Two linear ranges have been obtained for the EN concentration within the ranges of 0.1–8.0 and 10–100 M. The voltammetric detection limit of the modified electrode for EN (based on 3) was 40 nM. On the other hand, this sensor has an excellent selectivity and sensi- tivity for the EN determination in the presence of clinical interferences (e.g. uric acid, ascorbic acid, and the human serum contents). The recovery for the EN determination in pharmaceutical injections was obtained as 96.4% with a RSD of 4.8%, based on 5 replicate determinations. These excellent properties make the prepared sensor suitable for the analysis of the trace amounts of EN in pharmaceutical and clinical preparations. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction The high application potential of conducting polymers (CPs) in chemical and biological sensors is one of the main reasons for the intensive investigation and development of the studies on these materials [1]. In the case of polypyrrole (ppy), useful fea- tures considered include, its capacity to form adhesive coatings on different substrates, the possibility of growing in aqueous media, easiness of the chemical substitution to modify its properties, high porosity that enables fast kinetics ion exchange with the surround- ing medium, high electronic conductivity, high chemical stability, thickness controllability, ease of electrochemical polymerization, and good reversibility between its conducting and insulating. These ∗ Corresponding author at: Department of Chemistry, Sharif University of Tech- nology, Azadi Avenue, Tehran 11155-9516, Iran. Tel.: +98 21 66005718; fax: +98 21 66012983. E-mail address: shahrokhian@sharif.edu (S. Shahrokhian). characteristics make the ppy highly suitable for various electro- chemical applications, including voltammetric and potentiometric [2] amperometric and impedimetric [3,4] techniques for the deter- mination of various molecular species [5]. The ppy in its oxidized form (oppy) is a positively charged conducting polymer. Upon the over-oxidation process, it loses its conductivity and charge and its porosity improves. Characteriza- tion of these films has revealed that the over-oxidation resulted in addition of carbonyl and carboxyle groups to the structure of the conducting polymer. These groups attract the cationic species and reciprocally reject negatively charged ions, e.g. ascorbate anions [6]. Carbon nanotubes (CNTs) are considered as a novel form of car- bon materials in the last two past decades [7]. Recently, carbon nanotubes have also been incorporated into the electrochemical sensors. While they have many of the same properties as other types of carbon, they offer unique advantages including enhanced electronic properties, a large edge plane/basal plane ratio, and rapid kinetics of the electrode processes. Therefore, in comparison to the 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.029