| Summary: | The electrochemical behavior of the immobilized tyrosinase (Tyrase) on a modified glassy carbon electrode with carboxymethyl starch-<i>graft</i>-polyaniline/multi-walled carbon nanotubes nanocomposite (CMS-<i>g</i>-PANI@MWCNTs) was investigated. The molecular properties of CMS-<i>g</i>-PANI@MWCNTs nanocomposite and its morphological characterization were examined by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). A simple drop-casting method was employed to immobilize Tyrase on the CMS-<i>g</i>-PANI@MWCNTs nanocomposite. In the cyclic voltammogram (CV), a pair of redox peaks were observed at the potentials of +0.25 to −0.1 V and E°’ was equal to 0.1 V and the apparent rate constant of electron transfer (K<sub>s</sub>) was calculated at 0.4 s<sup>−1</sup>. Using differential pulse voltammetry (DPV), the sensitivity and selectivity of the biosensor were investigated. The biosensor exhibits linearity towards catechol and L-dopa in the concentration range of 5–100 and 10–300 μM with a sensitivity of 2.4 and 1.11 μA μΜ<sup>−1</sup> cm<sup>−2</sup> and limit of detection (LOD) 25 and 30 μM, respectively. The Michaelis-Menten constant (K<sub>m</sub>) was calculated at 42 μΜ for catechol and 86 μΜ for L-dopa. After 28 working days, the biosensor provided good repeatability and selectivity, and maintained 67% of its stability. The existence of -COO<sup>−</sup> and -OH groups in carboxymethyl starch, -NH<sub>2</sub> groups in polyaniline, and high surface-to-volume ratio and electrical conductivity of multi-walled carbon nanotubes in the CMS-<i>g</i>-PANI@MWCNTs nanocomposite cause good Tyrase immobilization on the surface of the electrode.
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