Contents lists available at ScienceDirect Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios Label-free electrochemical DNA biosensor for guanine and adenine by ds- DNA/poly(L-cysteine)/Fe 3 O 4 nanoparticles-graphene oxide nanocomposite modified electrode Majid Arvand ⁎ , Mona Sanayeei, Shiva Hemmati Electroanalytical Chemistry Laboratory, Faculty of Chemistry, University of Guilan, Namjoo Street, P.O. Box: 1914-41335, Rasht, Iran ARTICLE INFO Keywords: DNA biosensor Guanine Adenine Nanomaterials Electroanalytical technique ABSTRACT In this study, we aim to design a simple and effective electrochemical DNA biosensor based on a carbon paste electrode modified with ds-DNA/poly(L-cysteine)/Fe 3 O 4 nanoparticles-graphene oxide (ds-DNA/p(L-Cys)/Fe 3 O 4 NPs-GO/CPE) for sensitive detection of adenine (A) and guanine (G). The electrocatalytic oxidation of A and G on the electrode was explored by differential pulse voltammetry (DPV) and cyclic voltammetry (CV). This sensor shows separated and well-defined peaks for A and G, by which one can determine these biological bases in- dividually or simultaneously. The ds-DNA/p(L-Cys)/Fe 3 O 4 NPs-GO/CPE exhibited an increase in peak currents and the electron transfer kinetics and decrease in the overpotential for the oxidation reaction of A and G. Under the optimal conditions a linear relationship is figured out between the peak current and the analytes' con- centrations on a range of 0.01–30.0 μM and 0.01–25.0 μM for simultaneous determination of A and G, with detection limits of 3.48 and 1.59 nM, respectively. As well as, individually determination is resulted two linear concentration ranges of 0.01–30.0 μM for A and 0.01–25.0 μM for G with detection limits of 3.90 and 1.58 nM for A and G, respectively. The proposed biosensor exhibited some advantages in terms of simplicity, rapidity, high sensitivity, good reproducibility and long-term stability. Furthermore, the measurements of thermally denatured single-stranded DNA were carried out and the value of (G + C)/(A + T) of DNA was calculated as about 0.77 for various DNA samples. This study also ascertained that the proposed biosensor can be profitable to evaluate DNA bases damage. 1. Introduction In recent years, there has been an increase in the use of nucleic acids as a tool in the recognition of many compounds by using deoxyr- ibonucleic acid (DNA) as a surface-modification element in electro- chemical biosensors (Rezaei et al., 2016). DNA has gained increasing attention as well as their promise in the field of electrochemistry, which plays a vital role to transfer genetic information from one descendant to another (Zeng et al., 2013). Therefore, sequence-specific detection of DNA has been reported important for medical diagnosis of genetic diseases, mutations, molecular biology and forensic science (Esmaeili et al., 2016). The technology of DNA biosensors has been developed to measure the occurrence of DNA hybridization via the matching of complementary bases; adenine-thymine (T) and guanine-cytosine (C) (Tabrizi and Shamsipur, 2015). The electrochemical DNA biosensor is one of the most attractive methods because of their low cost, high sensitivity, rapid response rate, and good selectivity as well as their capacity for instrument miniaturization (Huang et al., 2014). To date, various types of DNA electrochemical biosensors, using a broad range of nanomaterials such as metal nanoparticles and carbon nanomaterials have been utilized to increase performance of DNA biosensors (Peng et al., 2015). Guanine and adenine are two of the four important bases present in DNA structure and play fundamental roles in the life process (Ensafi et al., 2014). They have effects on coronary and cerebral circulation, control of blood flow, prevention of cardiac arrhythmias, inhibition of neurotransmitter release and modulation of adenylate cyclase activity (Huang et al., 2013). The abnormal changes of the bases in organisms suggest the deficiency and mutation of the immunity system and may indicate the presence of various diseases such as cancer, AIDS, myo- cardial cellular energy status, disease progress and therapy responses (Chen et al., 2007; Huang et al., 2011). A mutation is a permanent change in the DNA. A mutation can arise spontaneously without ap- parent cause, or in response to radiation, ultraviolet light, certain chemicals, or viruses. Some mutations involve the substitution of one base pair for another and one or more base pairs can be added to, or http://dx.doi.org/10.1016/j.bios.2017.11.002 Received 8 September 2017; Received in revised form 30 October 2017; Accepted 1 November 2017 ⁎ Corresponding author. E-mail address: arvand@guilan.ac.ir (M. Arvand). Biosensors and Bioelectronics 102 (2018) 70–79 0956-5663/ © 2017 Elsevier B.V. All rights reserved. MARK