ABSTRACT:A novel sensitive electrochemical sensor has been developed by modification of glassy carbon electrode (GCE) with multi-walled carbon nanotube (MWCNT), chitosan (CHIT) and zirconium oxide (ZrO 2 ) nanoparticles. The morphological characteristics of nanocomposite (MWCNT-CHIT-ZrO 2 or MCZ) were studied by scanning electron microscope and atomic force microscopy. The electrochemical behavior of serotonin at nanocomposite modified GCE (MCZ/GCE) was investigated in pH 6.5 Briton Robinson buffer solution using cyclic voltammetry and square wave voltammetry. MCZ/GCE showed an enhancement in the current response as compared to bare GCE and showed a linear response to serotonin in the range 800 to 5000 nM. The limit of detection was found to be 0.6 nM, which is lower than many other sensors reported for serotonin in literature. The modified sensor showed high sensitivity (9 nA/nM) and selectivity for serotonin. The proposed method was successfully employed for quantification of serotonin in human blood serum samples.