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• Successful covalent immobilization of MAO-A enzyme on multi-walled carbon nanotubes. • Improvement of pH, thermal stability and kinetics of immobilized MAO-A enzyme. • Improved operational and storage stability of immobilized MAO-A enzyme. • The biocomposite exhibited a low detection limit for serotonin: 2 × 10−7 M. Serotonin is a potential biomarker for diverse neuropsychiatric and neurodevelopmental disorders. In this study a novel biocomposite based on the enzyme monoamine oxidase-A (MAO-A) immobilized by covalent binding on multi-walled carbon nanotubes (MWCNT) has been developed for detection of serotonin. Immobilized MAO-A (MWCNT/MAO-A) retained 94.3% of its catalytic activity, showing good thermal and pH stability at temperatures from 35 to 40 °C and pH from 7–8.5. A decrease of the apparent Michaelis-Menten constant was observed compared to free MAO-A indicating an improvement of its affinity towards its substrate. Evaluation of MWCNT/MAO-A by scanning electrochemical microscopy (SECM) demonstrated that the immobilized enzyme maintains its activity. The biocomposite was employed for successfull detection of serotonin through quantification of the enzymatically produced hydrogen peroxide (H 2 O 2). A MWCNT/MAOA modified glassy carbon electrode (GCE) was characterized by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Electrochemical measurements were performed in simulated body fluid (SBF), obtaining a linear response of 5.67 × 10-7 M - 2.26 × 10-6 M, with a low detection limit of 2 × 10-7 M (35.6 ng mL-1;S/N = 3) and a quantification limit of 6.73 × 10-7 M (118.7 ng mL-1;S/N = 3); values that are in range with serotonin levels found in different neurological disorders. The biocomposite have the potential to be employed in future development of biosensing devices for serotonin detection. [ABSTRACT FROM AUTHOR]