In the first section of this study, a novel conductive nanocomposite film containing multiwall carbon nanotubes (MWCNTs) decorated with K 5 [PW 11 CoO 39 ]@MB has been modified on the surface of a glassy carbon electrod for simultaneous electrochemical determination of dopamine and uric acid. The nanocomposite film showed excellent electrocatalytic activity towards the oxidation of dopamine and uric acid in universal buffer (pH= 7.0). The presence of K 5 [PW 11 CoO 39 ]@MB in the nanocomposite film increases the sensitivity of the modified electrode. The surface morphology, structure, properties and electroanalytical behavior of the nanocomposite film was characterized by Fourier Transform Infrared Spectroscopy, X-ray Diffraction, Scanning electronic microscope and cyclic voltammetry. In addition, the electrocatalytic activity of the modified electrode towards dopamine and uric acid was systematically studied by cyclic voltammetry and differential pulse voltammetry techniques. Under the optimum conditions, the linear ranges for the detection of DA and UA were 5.0 × 10 ? 8 - 2.0 × 10 ? 5 M and 2.5 × 10 ? 8 - 2.5 × 10 ? 5 M with limits of detection (LOD, S/N = 3) of 1.5 × 10 ? 8 nM and 0.5× 10 ? 8 ?M, respectively. Furthermore, the modified electrode was applied to real sample analysis. In the second section of this study, electrochemical performance of the 3D-rGO@ Au based aptasensor for As (III) detection was studied. In this work, aptamerr conjugated gold nanoparticles (Apt/Au) were used and studied for electrochemical sensing of As (III) ions in solution. As (III) ion specifically interacts with Apt/3D-rGO@Au to form As (III)/Apt/3D-rGO@Au using electrochemical impedance spectroscopy. Under optimal conditions, the aptasensor has a good calibration range between 2.5×10 ? 15 - 2.0×10 ? 12 M As (III) and with a detection limit of 0.9×10 -15 M. The proposed aptasensor showed excellent selectivity, and acceptable reproducibility, with a good recovery for detection of Arsenic (???) in real water samples.