A glassy carbon electrode modified with carbon quantum dots and polyalizarin yellow R dyes for enhanced electrocatalytic oxidation and nanomolar detection of L-cysteine Nader Amini a,b , Mojtaba Shamsipur a , Mohammad Bagher Gholivand a, ⁎, Ali Barati a a Department of Analytical Chemistry, Razi University, Kermanshah, Iran b Kurdistan Environmental Health Research Center, Kurdistan University of Medical Sciences, Sanandaj, Iran abstract article info Article history: Received 20 June 2016 Received in revised form 22 October 2016 Accepted 6 November 2016 Available online 09 November 2016 A novel electrochemical sensor for the detection of L-cysteine was proposed based on immobilizing poly (alizarin yellow R)/carbon quantum dots film on glassy carbon electrode. Hydrothermal treatment was used to prepare carbon quantum dots. Transmission electron microscopy and Fourier transform infrared spectroscopy were used for characterization of carbon quantum dots. Electrochemical impedance spectroscopy, cyclic voltammetry and amperommetry were utilized to confirm the successful stepwise assembly procedure of the sensor. The elec- trocatalytic behaviors of the sensor were also investigated by cyclic voltammetry and amperommetry. Results showed that poly (alizarin yellow R)/carbon dots exhibited a remarkable electrocatalytic activity for the oxida- tion of L-cysteine under optimal conditions. The electrocatalytic response of the sensor was proportional to the L-cysteine concentration in the range of (0.3 to 3.6 μM) and (3.9 to 7.2 μM) with a limit of detection and sensitivity of 90 nM and 0.482 μA/μM, respectively. The modified electrode demonstrated many advantages such as simple preparation, high sensitivity, low detection of limit, excellent catalytic activity at physiological pH values, short response time, and remarkable antifouling property toward L-cysteine and its oxidation product. © 2016 Published by Elsevier B.V. Keywords: L-Cysteine Carbon dots Alizarin yellow R FTIR Amperometry 1. Introduction Cysteine is a typical thiol compound and classified as a hydrophilic non-essential amino acid even if essential for humans. Because of high reactivity of the thiol group, cysteine is an important structural and functional component of many proteins and enzymes [1–2]. Moreover, cysteine or compounds containing cysteine derivatives are widely used in many pharmaceutical, cosmetic and food products [3]. Thus, de- termination protocols of cysteine compounds have been developed for both clinical and industrial purposes. Among existing methods for cys- teine detection, chromatographic (especially HPLC) and spectroscopic methods are the most abundant assays. However, chromatographic separation based methods present some disadvantages as the derivati- zation and extraction stage of the procedure can increase analysis time and the total cost of the assay. In addition, due to a lack of strong chromophore in cysteine structure, its determination by absorbance measurements is very difficult. Therefore, spectrophotometric detection is based on derivatization with cromogenic reagents [4]. However, a fast protocol with minimal sample pretreatment is needed for the determination of thiols. Electrochemical techniques can be nominated as the most favored methods for the determination of thiols. The elec- trochemical analysis of thiols at the surface of conventional electrodes (e.g., glassy carbon and gold electrodes) encounters such drawbacks as a sluggish electrochemical process and requirement of high overpotentials [5–7]. To overcome these drawbacks many strategies have been employed. One of these strategies is modification of the sur- face of the conventional electrodes with different materials such as a polymeric layer [8], composite materials [9], metallic complexes [10], nanoparticles [11] and organic compounds [12]. For examples, polypyr- role and graphene quantum dots @ Prussian Blue hybrid film [13], nickel oxide nanoparticles on N-doped reduced graphene oxide [14], polyoxometalates and Au@2Ag core–shell nanoparticles [15] have been utilized for determination of L-cysteine. L-Cysteine (L-CySH) as a thiol, due to its crucial role in biological systems, has been the subject of many electrochemical studies investigating both mechanism and de- tection aspects [16]. Quantum dots (QDs), also called semiconductor nanocrystals, are a class of nanoparticles containing group II–VI elements or group III–V el- ements, with diameter between 1 and 10 nm. Due to their size-tunable, chemically functionalizable surface, electronic property and catalytic effect, they are widely used as modified material in electrochemical sensors [17–18]. Among quantum dots, carbon dots (CDs) is the most Microchemical Journal 131 (2017) 9–14 ⁎ Corresponding author. E-mail addresses: naderamini95@yahoo.com (N. Amini), mbgholivand@yahoo.com (M.B. Gholivand). http://dx.doi.org/10.1016/j.microc.2016.11.004 0026-265X/© 2016 Published by Elsevier B.V. Contents lists available at ScienceDirect Microchemical Journal journal homepage: www.elsevier.com/locate/microc