Determination of D-L kynurenine in human plasma by electrochemical measurement with multi-wall carbon nanotube-modified carbon paste electrode

This study presents the design and electrochemical characterization of a highly sensitive and selective sensor for the determination of D−/L- kynurenine (D−/L- KYN), based on a carbon paste electrode (CPE) modified with multi-walled carbon nanotubes (MWCNTs). The electrocatalytic oxidation of D−/L-KYN at the MWCNT-modified CPE was systematically investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The Structural and morphological characteristics of the MWCNTs were characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The combined characterization results confirmed the successful functionalization, morphological integrity, and crystalline structure of the MWCNTs, supporting their suitability as an efficient electrode modifier. At physiological pH (7.0) in phosphate buffer solution (PBS), D−/L-KYN exhibited a well-defined oxidation peak at +0.85  V (vs. Ag/AgCl). The electrochemical behavior of the analyte followed a diffusion-controlled mechanism involving a two-electron/two-proton transfer process. Key analytical parameters including solution pH and scan rate were optimized to enhance sensor performance. The fabricated sensor exhibited a linear concentration range of 0.06–40.0 μM, with an impressive limit of detection 3 nM (S/N = 3). Furthermore, the sensor demonstrated excellent reproducibility and repeatability, with relative standard deviations (RSDs) of 3.8% and 3.3%, respectively. The developed platform was successfully applied to the quantification of D−/L-KYN in spiked human plasma samples, demonstrating its potential for clinical and biomedical applications.