Label-Free Chemiluminescence Detection of Cysteine in Biological Fluids Based on Its Catalytic Effect on the Reduction of Gold Ions to Gold Nanoparticles by Luminol  
T. Choleva1,*, E. A. Akrivi2,3, N. Kourkoumelis2, D. L. Giokas1, A. G. Vlessidis1 and G. Z. Tsogas4 Laboratory of Analytical Chemistry, Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece Department of Medical Physics, School of Health Sciences, University of Ioannina, 45110 Ioannina, Greece Laboratory of Analytical Chemistry, School of Chemistry, Faculty of Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Correspondence: tacholeva@gmail.com  
Biothiols are naturally occurring biomolecules that play a significant role in a variety of biological procedures [1]. In the human organism, cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are the most abundant biothiols, and their concentrations are related to several clinical disorders and diseases [2]. The determination of biothiols is customarily based on the reactivity of their sulfhydryl group which enables the determination of the total concentration of biothiols but not their discrimination. In this work, we demonstrate a selective assay for the determination of l-cysteine, based on the enhancement of the chemiluminescence signal generated when luminol is oxidized by gold ions towards the formation of gold nanoparticles. By appropriately regulating the experimental conditions and the sequence of reactions it is possible to discriminate Cys from GSH and Hcy, enabling its selective determination in protein-free blood plasma samples. The analytical features of the method in terms of sensitivity, linear range, recoveries, and reproducibility were satisfactory suggesting that the method holds promise to the speciation of biothiols in biological fluids. ## References  
* _Tsogas_ et al. (2018)_ Tsogas, G.Z. ; Kappi, F.A. ; Vlessidis, A.G.; Giokas, D.L. Recent advances in nanomaterial probes for optical biothiol sensing: A review. _Anal. Lett._**2018**, _51_, 443-468. * _Kostara_ et al. (2018)_ Kostara, A.; Tsogas, G.Z. ; Vlessidis, A.G.; Giokas, D.L. Generic assay of sulfur-containing compounds based on kinetics inhibition of gold nanoparticle photochemical growth. _ACS Omega_**2018**, \(3\), 16831-16838. * _Kartashov 1,2,*, J. Barek1,2 and E. I. Korotkova2_
* _Department of Analytical Chemistry, Faculty of Science, UNESCO Laboratory of Environmental Electrochemistry, Charles University, Albertov 6, CZ 12843 Prague 2, Czech Republic
* _Department of Physical and Analytical Chemistry, National Research Tomsk Polytechnic University, 634050 Tomsk, Russia_
* _Correspondence: maks99661@gmail.com_  
Rivaroxaban is an oral anticoagulant from the group of direct factor Xa inhibitors. It is used to treat deep vein thrombosis and pulmonary embolism, as well as to prevent thrombi formation during atrial fibrillation and after hip or knee surgery [1]. Rivaroxaban (Figure 30) could be considered for thromboprophylaxis in high VTE risk COVID-19 outpatients [2]. To date, there is only one voltammetric technique intended for the determination of rivaroxaban [3], which shows the need to develop new voltammetric techniques and to study the electrochemical behavior of rivaroxaban in more detail. The aim of this work is to study the voltammetric characteristic of the electrochemical redox reaction rivaroxaban and to develop a new \"green\" voltammetric method for the determination of rivaroxaban in dosage forms using various types of carbon electrodes to minimize the processes of their passivation. The influence of pH on cathodic voltammetric signal of rivaroxaban was investigated using graphite working electrode. Ag |AgCl was used both as an auxiliary and a reference electrode. LSV technique was used at the scan rate 100 mV/s. The maximum cathodic peak was observed in an alkaline medium (pH 10) at a potential of \(-\)1.68 V and in an acidic medium (pH 4); a maximum anodic peak was found at a potential of 1.38 V. A calibration curve was constructed under optimal conditions.