Electrochemical oxidation of adenine using platinum electrodes modified with carbon nanotubes Florina Pogacean a , Alexandru R. Biris a , Maria Coros a,n , Fumiya Watanabe b , Alexandru S. Biris b , Simona Clichici c , Adriana Filip c , Stela Pruneanu a,n a National Institute for Research and Development of Isotopic and Molecular Technologies, Donath Street, No. 65-103, RO-400293 Cluj-Napoca, Romania b Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204, USA c Department of Physiology, University of Medicine and Pharmacy, Cluj-Napoca, Romania HIGHLIGHTS  Radio Frequency-catalytic Chemical Vapor Deposition was used for CNTs synthesis.  The electro-catalytic characteristics of the carbon nanotubes were tested.  MW and DW carbon nanotubes are more suitable for adenine electrochemical detection. article info Article history: Received 2 October 2013 Accepted 14 January 2014 Available online 23 January 2014 Keywords: Carbon nanotube Adenine oxidation Electrochemical impedance spectroscopy Modified electrode abstract Single- (SWNTs), double- (DWNTs), and multi-walled (MWNTs) carbon nanotubes were synthesized by controlled Radio Frequency-catalytic Chemical Vapor Deposition (RF-cCVD). Their morphological and structural characteristics were identified using Transmission Electron Microscopy (TEM/HRTEM) and X-ray powder diffraction (XRD). Next, three platinum electrodes with identical amounts of each nanotube material (denoted Pt-SW, Pt-DW, and Pt-MW) were modified in order to test the electro-catalytic characteristics of the carbon nanotubes and further used for the electrochemical oxidation of adenine. The signal recorded with the Pt-SW electrode was very poor, due to the predominantly semiconducting properties of these nanotubes. In contrast, the signal recorded with Pt-DW or Pt-MW was well-defined, with the peak potentials at 1.07 and 1.01 V vs Ag/AgCl, respectively. In both cases, the detection limit (DL) for adenine was found to be 3  10 6 M. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Adenine is an important organic molecule found in nucleic acids, which participates in many biological processes. Abnormal changes in this purine base are related to deficiencies in the immune system; therefore, its concentration level is considered important for the diagnosis of cancer, disease progress, and therapy responses [1]. It is thus crucial to discover highly sensitive methodologies for the determination of this base in DNA. Various approaches have been used to detect adenine: electro- phoresis [2], flow injection chemiluminescence [3], and high performance liquid chromatography [4]. The latter two methods are highly selective and sensitive, but they are also time- consuming and costly. In contrast, electrochemical techniques are more attractive due to their low cost [5]. Direct oxidation of adenine at bare electrodes (metallic or glassy carbon) is difficult to achieve, due to its strong adsorption on the active area, thus leading to low sensitivity and selectivity [6]. For these reasons, several modified electrodes have been designed to detect adenine: ionic liquid–CNT/gold nanoparticle (AuNP) complex composite film-coated electrodes [7], MWNT–ionic liquid film-modified car- bon paste electrodes [8], graphene–COOH modified glassy carbon electrodes [9], graphene-modified carbon ionic liquid electrodes [10], TiO 2 –graphene nanocomposites [11], or graphene sheets having various amounts of gold nanoparticles embedded [12]. Carbon nanotubes (CNTs) were discovered in 1991 and have attracted significant scientific interest due to their unique structural morphologies as well as mechanical, chemical, and optoelectronic properties, which include a remarkably high tensile strength, excellent electrical conductivity, and good chemical stability [13]. These unique properties make CNTs an appropriate material for chemical and biological sensors [14], batteries [15], and nanoelec- tronic devices [16]. CNTs consist of sp 2 hybridized carbon atoms, arranged in a cylindrical nanostructure. Single-walled carbon nano- tubes (SWNTs) exhibit excellent mechanical and thermal properties and are usually found in bundles composed of tens to hundreds of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2014.01.016 n Corresponding authors. Tel.: þ40 264 584 037; fax: þ40 264 420 042. E-mail addresses: maria.coros@itim-cj.ro (M. Coros), stela.pruneanu@itim-cj.ro (S. Pruneanu). Physica E 59 (2014) 181–185