Tryptophan (TRYP) is an essential amino acid that plays several key roles in different physiological processes, ranging from acting as a building block for protein synthesis to being involved in neurotransmission and cellular signaling.1 Humans can only obtain TRYP from food intake,1 and therefore this compound is frequently added to dietary supplements. Considering the potential toxic effects that arise from TRYP overdoses,2 an efficient quality control should be performed on products containing TRYP in their formulations. In this way, this work proposes an efficient voltammetric sensor as a cost-effective and feasible tool to detect TRYP in commercial supplements. The sensor was based on glassy carbon electrodes modified with poly(L-arginine) (GCE/p-ARG), which was electrochemically synthesized in situ from L-arginine monomers.3 The conditions for p-ARG electropolymerization were optimized through multivariate statistical approaches in order to enhance the electrode’s sensing capability. Cyclic voltammetry and electrochemical impedance spectroscopy analysis in the presence of [Fe(CN)6]3-/4- redox probe showed that p-ARG significantly improved the electron transfer at the electrode surface. TRYP was irreversibly oxidized on GCE/p-ARG displaying a peak at +0.73 V (vs. Ag/AgCl), which was more intense than the anodic peak showed by the bare GCE. This was due to the presence of p-ARG on the GCE surface, which enabled the occurrence of multiple interactions that favoured TRYP detection. The amino acid oxidation followed a diffusion-controlled mechanism and showed to be dependent on the supporting electrolyte pH. A linear relationship between the anodic peak current (obtained through square wave voltammetry) and the TRYP concentration was observed in the range from 1.0 to 7.0 μmol L-1. Limits of detection and quantification were found to be equal to 0.30 and 0.91 μmol L-1, respectively. The developed electroanalytical method was successfully applied to detect TRYP in dietary supplements, with good precision and accuracy. In conclusion, it can be stated that the GCE/p-ARG is a promising candidate for food and pharmaceutical industry to enable cheap, fast, and reliable quality control of TRYP-based commercial products. References M. Platten, E. A. A. Nollen, U. F. Röhrig, F. Fallarino, and C. A. Opitz, Nature Reviews Drug Discovery, 18, 379–401 (2019).Y. Xia, F. Zhao, and B. Zeng, Talanta, 206 (2020).J. Soleymani et al., Materials Science and Engineering C, 77, 790–802 (2017).