This is the first report of dopamine (DA) detection using a carbon nano-onion (CNO) and poly(diallyldimethylammonium chloride)(PDDA) composite.The film was deposited by a coating method on a glassy carbon electrode surface, applying a drop of solution containing the suspended CNOs and PDDA. The electrochemical properties of the composite in phosphate buffered saline (PBS) solution were examined and their ability to detect dopamine was verified. The results showed good selectivity and sensitivity for dopamine analysis. The CNOs/PDDA composite allows the determination of dopamine in a range between 5 x 10(-5) and 4 x 10(-3) mol L-1. in the presence of ascorbic (AA) and uric (UA) acids, and simultaneous assays all three molecules in solution. The modified electrode can also be used to determine the concentration of dopamine. Results were investigated by cyclic voltammetry, differential pulse voltammetry and square wave voltammetry. (C) 2012 Elsevier Ltd. All rights reserved.