Electrochemical Biosensing of L-DOPA Using Tyrosinase Immobilized on Carboxymethyl Starch-Graft-Polyaniline@MWCNTs Nanocomposite

The electrochemical behavior of the immobilized tyrosinase (Tyrase) on a modified glassy carbon electrode with carboxymethyl starch-graft-polyaniline/multi-walled carbon nanotubes nanocomposite (CMS-g-PANI@MWCNTs) was investigated. The molecular properties of CMS-g-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-g-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(s)) was calculated at 0.4 s(−1). 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(−1) cm(−2) and limit of detection (LOD) 25 and 30 μM, respectively. The Michaelis-Menten constant (K(m)) 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(−) and -OH groups in carboxymethyl starch, -NH(2) groups in polyaniline, and high surface-to-volume ratio and electrical conductivity of multi-walled carbon nanotubes in the CMS-g-PANI@MWCNTs nanocomposite cause good Tyrase immobilization on the surface of the electrode.