A carbon paste electrode (CPE) chemically modified with TiO2 nanoparticles and quinizarine (QZ) was used as a selective electrochemical sensor for the simultaneous determination of minor amounts of D-penicillamine CD-PA) and tryptophan (Trp). This modified electrode showed very efficient electrocatalytic activity for anodic oxidation of both D-PA and Trp. Substantial decreases of anodic overpotentials for both compounds made this analysis possible. Results of square wave voltammetry (SWV) using this modified electrode showed two well-resolved anodic waves for the oxidation of D-PA and Trp, which makes the simultaneous determination of both compounds possible. The peak potential for the oxidation of D-PA was lowered by at least 220 mV compared with that obtained for an unmodified CPE. In optimal conditions, linear ranges spanned a D-PA concentration from 0.8 mu M to 140.0 mu M and the detection limit was 0.76 mu M at a signal-to-noise ratio of 2. In addition, the sensor had good stability and reproducibility. (C) 2010 Elsevier B.V. All rights reserved.