Electrochemical characterization of a glassy carbon electrode modified with microbial succinoglycan monomers and multi-wall carbon nanotubes for the detection of quercetin in an aqueous electrolyte

The glassy carbon electrode (GCE) was modified with multi-wall carbon nanotubes (MWNTs) and succinoglycan monomers (M1, M2 and M3) by dropping small volume of a solution mixture on the GCE surface and the modified electrode was electrochemically characterized for monitoring quercetin that is dispersed in an aqueous electrolyte solution. Quercetin, as an antioxidant, has attracted researcher's much attention to the determination of its concentration in a variety of environment, because it has many biological activities such as cardiovascular protection, anticancer activity, and antiallergy activity. Bare GCE and MWNTs-modified GCE do not show any redox peak of quercetin in a potential range of 0-0.5 V vs. SCE. Three different succinoglycan monomers M1, M2, and M3 were separately immobilized on the GCE surface together with the MWNTs to prepare three different kinds of sensing electrodes, for example, (MWNTs + M1)/GCE, (MWNTs + M2)/GCE, and (MWNTs + M3)/GCE. The GCE modified with MWNTs and beta-cyclodextrin (beta-CD) complex (MWNTs + beta-CD/GCE) was also prepared for comparing its electrochemical behavior with those of the (MWNTs + M1)/GCE, the (MWNTs + M2)/GCE, and the (MWNTs + M3)/GCE. Results showed that the reduction and the oxidation peaks around 350 and 400 mV vs. SCE were observed with the (MWNTs + beta-CD)/GCE, the (MWNTs + M2)/GCE, and the (MWNTs + M3)/GCE. Likely to the (MWNTs + beta-CD)/GCE, cyclic voltammograms of the (MWNTs + M3)/GCE showed pH-dependent characteristics. Anodic peaks shown around 400 mV vs. SCE in the cyclic voltammograms of the (MWNTs + M2)/GCE and the (MWNTs + M3)/GCE are due to the oxidation of quercetin that is captured by the immobilized M2 or M3. But, because M1 does not have a succinyl group as a substituent, the (MWNTs + M1)/GCE does not show any redox peak of the cyclic voltammogram in the given potential range. Succinyl groups in succinoglycan monomers are to be associated with the conjugation of MWNTs and water-dispersed quercetin molecules. Square wave voltammograms and calibration curve of the (MWNTs + M3)/GCE were shown in the range of 2.36-59 mu M of quercetin and about sixteen times of higher sensitivity was observed compared with the measured sensitivity of the (MWNTs + beta-CD)/GCE. Electrode kinetic parameters, V-max and K-m of M3 in an aqueous medium obtained from the Lineweaver-Burk plots were 15 mu A and 7.82 mu M, respectively. @ 2008 Elsevier B.V. All rights reserved.