Chemosensors 2022, 10, 240. https://doi.org/10.3390/chemosensors10070240 www.mdpi.com/journal/chemosensors Article Voltammetric Electrochemical Behavior of Carbon Paste Electrode Containing Intrinsic Silver for Determination of Cysteine Maša Buljac 1, *, Denis Krivić 2 , Ivana Škugor Rončević 3 , Nives Vladislavić 3 and Marijo Buzuk 3 1 Department of Environmental Chemistry, Faculty of Chemistry and Technology, University of Split, 21000 Split, Croatia 2 Division of Biophysics, Gottfried Schatz Research Center, Medical University of Graz, 8010 Graz, Austria; denis.krivic@medunigraz.at 3 Department of General and Inorganic Chemistry, Faculty of Chemistry and Technology, University of Split, 21000 Split, Croatia; skugor@ktf-split.hr (I.Š.R.); nives@ktf-split.hr (N.V.); buzuk@ktf-split.hr (M.B.) * Correspondence: masa@ktf-split.hr; Tel.: +385-21-329-479 Abstract: In this paper, the electrochemical behavior of cysteine is described, using carbon paste electrodes (CPEs) modified with ternary silver-copper sulfide containing intrinsic silver at two pH values (pH 3 and 5). Experiments have revealed that presence of cysteine has a large impact on the electrochemical behavior of modified CPEs. Observed phenomena take place in solution, as well as at the surface of the modified CPEs, and can be applied for electroanalytical purposes. Based on the electrochemical behavior observed in the examined system, differential pulse voltammetry (DPV) was selected as an electroanalytical method for determination of cysteine. The effects of the various parameters on the electroanalytical signal, such as the amount of electroactive material, electroana- lytical parameters, pH etc., were investigated using differential pulse voltammograms. The results indicated that electrochemical signal characterized with well-defined cathodic peak at 0.055 V vs. Ag/AgCl (3 M) in acetic buffer solution at pH 5 can be used for indirect electrochemical determina- tion of cysteine. The optimization procedure revealed that the most sensitive and stabile electrode was that containing 5% modifier. The DPV response of the electrode, in the presence of cysteine, showed two different linear concentration ranges of 0.1 to 2.5 μM, and 5.6 to 28 μM. The explanation of the origin of two linear ranges is proposed. The lower concentration range was characterized by remarkable sensitivity of the 11.78 μA μM –1 , owing to the chosen indirect method of determination. The calculated limit of detection (LOD), as well as limit of quantification (LOQ) were 0.032 and 0.081 μM, respectively. The influence of interfering agents on the electroanalytical response was exam- ined, and low or no interference on the DPVs was observed. The proposed method was validated and applied for the determination of cysteine in pharmaceutical preparations with satisfactory re- coveries in the range of 97 to 101.7%. Keywords: electroanalysis; cysteine; differential pulse voltammetry; carbon paste electrodes; ternary sulphides; intrinsic silver 1. Introduction Thiols, as components of various proteins and simple molecules, play an important role in many biological and environmental systems. Biological thiols such as cysteine, glu- tathione and homocysteine have attracted special attention because they are widely dis- tributed in the environment, as intermediates important in the anaerobic decomposition of organic matter in sediments and soil. These three particular thiols share a similar struc- ture, which has been a major obstacle for researching their physiological functions [1]. Citation: Buljac, M.; Krivić, D.; Rončević, I.Š.; Vladislavić, N.; Buzuk, M. Voltammetric Electrochemical Behavior of Carbon Paste Electrode Containing Intrinsic Silver for Determination of Cysteine. Chemosensors 2022, 10, 240. https://doi.org/10.3390/ chemosensors10070240 Academic Editor: Xudong Wang and Hongshang Peng Received: 26 May 2022 Accepted: 23 June 2022 Published: 24 June 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).