Evaluation of Kynurenine Behaviour on the Glassy Carbon Electrode Modified By Nafion Layer
Authors: Mroczka Robert , Sadok Ilona , Staniszewska Magdalena 
Published: May 2020

Kynurenine is the first stable metabolite of tryptophan degradation formed via kynurenine pathway. Inflammation accompanying several disorders can activate kynurenine pathway, therefore increased level of kynurenine is associated with cancer, depression, schizophrenia, encephalopathy, HIV, dementia, amyotrophic lateral sclerosis, malaria, Alzheimer’s, and Huntington’s disease [1]. Kynurenine is an immuneregulatory molecule, which is further converted to other compounds showing biological activity, i.e. neuroprotective or neurotoxic effects on human cells. Studying properties of kynurenine is currently the important topic and might bring new information in context of biological processes [2].
The presented work examines the potential of the cation exchange membrane Nafion® (used i.e. in voltammetric analytical approaches) to interact with kynurenine. Nafion is the polymer (ionomer) consisted of tetrafluorethylene or Teflon® backbone with perfluorovinyl ether groups terminated with sulphonate groups. The Nafion allows positively charged species (molecules or cations) to pass through, and at the same time excludes negatively charged compounds [3]. No studies have been conducted on the electrochemical behavior of kynurenine at Nafion covered glassy carbon electrode. Our preliminary studies have shown that kynurenine exhibits efficient adsorption on the Nafion layer, previously deposited by drop coating on the glassy carbon substrate.
Voltammetric response of kynurenine strongly depends on the thickness of the Nafion layer. Thus, we attempted to evaluate the relationship between voltammetric signal of the molecule and morphology of the Nafion film deposited in different ways on the glassy carbon substrate (flat and rod electrode). For comparison several different approaches have been utilized for Nafion deposition, i.e. layer of varied thickness of Nafion were deposited on the rod carbon glassy electrode (CGE) or different withdrawal speed of the electrode from the solution of varied concentration of polymer was applied. The great benefit of this technique is possibility to control the thickness at a very high precision level on the whole electrode surface. The morphology surface electrode (roughness and waviness) both with flat and rod shape before and after Nafion deposition were evaluated by optical profilometer, mechanical profilometer, and atomic force microscopy. Moreover, Nation layer thickness on the rod GCE was determined by laser scan micrometer. Morphology characteristic of flat GCE was combined with chemical analysis at atomic and molecular level using complementary methods, such as: TOF-SIMS (time of flight secondary ion mass spectrometry) and XPS (X-ray photoelectron spectrometry). In addition, we propose new methodology for in situ molecular analysis of the rod CGE/Nafion after kynureine adsorption. It combines TOF-SIMS with cyclic voltammetry (CV). In this fashion CGE/Nafion was withdrawn at constant speed from electrolyte contained kynurenine, whilst CV measurements were conducted. When CV experiment was finished, samples were transferred to TOF-SIMS instruments and chemical analysis of different segments with area 100x100 um2 along length of rod were conducted. In that way molecular analysis of every segment surface CGE/Nafion corresponding to applied potential was performed.
The results presented here provide new insights in analytical electrochemistry in case of kynurenine behavior on Nafion layer.
[1] Y. Chen, G.J. Guillemin, Kynurenine Pathway Metabolites in Humans: Disease and Healthy States, Int J Tryptophan Res. 2009; 2: 1-19.
[2] I. Sadok, A. Gamian, M. Staniszewska, Chromatographic analysis of tryptophan metabolites, J Sep Sci. 2017; 40(15):3020-3045.
[3] R.K. Nagarale, G.S. Gohil, Vinod K. Shahi, Recent developments on ion-exchange membranes and electro-membrane processes, Adv. Colloid Interface Sci. 2006; 119: 97-130.