Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Trace level detection of guanine and adenine and evaluation of damage to DNA using electro-synthesised ZnS@CdS core-shell quantum dots decorated graphene oxide nanocomposite Majid Arvand ⁎ , Zahra Khoshkholgh, Shiva Hemmati Electroanalytical Chemistry Laboratory, Faculty of Chemistry, University of Guilan, Namjoo Street, P.O. Box: 1914–41335, Rasht, Iran ARTICLE INFO Keywords: Quantum dots ZnS@CdS Electrodeposition Guanine Adenine Electrochemical sensor ABSTRACT Quantum dots owing to their unique properties have been the subject of extensive investigations in different areas of science and technology in the past two decades. In this study, preparation of the electrochemical sensor for the selective and sensitive determination of guanine (G) and adenine (A), two of the most important com- ponents of DNA and RNA, were investigated by modifying a glassy carbon electrode (GCE) with ZnS@CdS quantum dots and graphene oxide (GO). This sensor shows separated and well-defined peaks for A and G, by which one can determine these biological bases individually or simultaneously. The novel electrode showed linear responses towards G and A over the concentration range of 0.01 μM to 50 μM. Detection limit was de- termined as 1.45 and 1.81 nM according to signal to noise ratio (S/N = 3). 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. The proposed electrochemical sensor exhibited some ad- vantages in terms of simplicity, rapidity, high sensitivity, good reproducibility and long-term stability. The relative standard deviation of the proposed system, obtained using A and G standards, were 2.7% and 1.5% for intra-day and 4.3% and 4.1% for inter-day precisions, respectively. The results demonstrated that the proposed electrochemical sensor not only provides a novel and sensitive approach to detecting purine bases, but also ascertained that the proposed electrochemical sensor can be profitable to evaluate DNA bases damage. 1. Introduction In recent years, the emerging fields of nanomaterials have spurred up enormous interest in electrochemical sensors and biosensors [1]. Among them, quantum dots (QDs) have attracted worldwide attention due to their novel properties such as unique electrochemical and lu- minescence properties [2,3], high resistance, large absorption and narrow emission spectra [4]. QDs are nanocrystals of semiconductor materials whose electrons and holes are quantum-limited in all three spatial dimensions [5]. QDs have been associated to many biomolecules and used in several biomedical applications such as cancer diagnosis, photodynamic therapy, cell labeling and biosensors [6–9]. On the other hand, QDs have a high surface area, due to their reduced size, which results in a large number of surface defects. Thus, to reduce the non- radioactive recombination, the coating of the nanocrystals usually with a second semiconductor layer of wider band gap can be used to passi- vate the surface, originating a core/shell structure, such as CdS/ZnS [10]. The surface modification of a wide band gap semiconducting shell around a narrow band gap core can modify the charge, functionality, and reactivity of the materials and consequently enhance the functional properties due to localization of the electron-hole pairs [11,12]. How- ever, with a QD interlayer, detection could already be achieved at low working potentials. The selection of the electrode material is so im- portant in the structure of the electrochemical sensor and has same advantages in terms of high electrochemical reactivity, good mechan- ical rigidity, lower cost and the fluency the modification. The modified electrodes by the incorporation of QDs promote direct electron transfer between the biomolecules and the electrode surface. The most popular kinds of QDs are including CdS, CdTe, CdSe, ZnSe and ZnS. The synthesis of CdS and ZnS semiconductors has been the focus of recent scientific research due to their important nonlinear optical properties, quantum size effect and other important physical and electrochemical properties [6]. Electrodeposition is a noteworthy method for the synthesis of semiconductor thin films and nanos- tructures, because it is comfortable, cheap, rapid, simple and parameter controllable [13]. A variety of semiconductors have been constructed with electrodeposition, in particular cadmium based compounds and CdS as one of the most semiconductors has been employed for https://doi.org/10.1016/j.jelechem.2018.04.010 Received 6 January 2018; Received in revised form 29 March 2018; Accepted 3 April 2018 ⁎ Corresponding author. E-mail address: arvand@guilan.ac.ir (M. Arvand). Journal of Electroanalytical Chemistry 817 (2018) 149–159 Available online 05 April 2018 1572-6657/ © 2018 Elsevier B.V. All rights reserved. T