A novel, simple and sensitive sensor based on carbon paste electrode (CPE) modified by indolyldihydroxyquinone was developed for the electrochemical determination of tryptophan (Trp). Indolyldihydroxyquinone as a bio-based modifier was synthesized by electrochemical method and then incorporated in the carbon paste electrode as a modifier. The electrooxidation of tryptophan was studied on modified carbon paste electrode using cyclic voltammetry and differential pulse voltammetry as diagnostic techniques. It was found that the oxidation peak current of tryptophan on the modified carbon paste electrode was greatly improved compared to that obtained on the bare carbon paste electrode. Under optimum conditions the sensor provides two linear DPV responses in the range of 0.1-100 mu M and 100-380 mu M in phosphate buffer solution of pH 7 with a detection limit of 0.03 mu M. The proposed sensor was successfully applied for monitoring of Trp in serum and urine samples and satisfactory results were obtained. (C) 2016 Elsevier B.V. All rights reserved.