An amperometric serotonin (5-hydroxytryptamine) sensor, based on an overoxidized polypyrrole (oPPy) modified electrode, is described. An oPPy composite film has been electrochemically synthesized through overoxidation of polypyrrole doped with a dodecylsulfonate micelle. Although the oPPy composite film was electroinactive, differential pulse voltammetry revealed that a glassy carbon (GC) electrode coated with the composite film had about four-fold higher sensitivity toward the neurotransmitter serotonin than a bare GC electrode with a linear response range of 1 to 100 mM (a correlation coefficient of 0.996). The oPPy modified electrode showed excellent reproducibility and stability for determination of serotonin at trace level concentrations (detection limit: 1 muM). (C) 2003 The Electrochemical Society.