Graphene oxide/graphene quantum dots: A platform for probing ds-DNA-dimethoate interaction and dimethoate sensing

A nanocomposite of graphene oxide and graphene quantum dots (GO@GQDs) were used for the first time to probe the ds-DNA-dimethoate (DMT) interaction and detect the DMT at a trace level. Graphene quantum dots were electrodeposited onto a glassy carbon electrode (GCE), followed by casting a graphene oxide layer. This platform was employed to adsorb the ds-DNA to fabricate DNA biosensors (ds-DNA/GO@GQDs/GCE). Microscopic and electrochemical characterizations of the modified electrodes were performed. The GO/ GDQs/GCE platform exhibited remarkable electrocatalytic oxidation of DMT from acetate and phosphate buffers. The oxidation process of DMT was pH-dependent and irreversible, proceeding under the mixed adsorption and diffusion-controlled mechanism. The interaction of salmon sperm double-strand DNA (ss-dsDNA) with DMT was investigated using differential pulse voltammetry and UV/vis spectroscopy. Mixed binding modes of intercalation and the electrostatic were ascertained for the interaction of DMT to dsDNA. Such a result was confirmed from the voltammetric data of single-stranded DNA (ssDNA) and the ionic strength study. The UV-Vis study has also supported that finding. The indirect DMT detection is verified by the drop in DNA oxidation signals or increasing DPV peak currents conducted from [Fe(CN)6]3-/4- probe. The decrease in the DNA peak currents linearly correlated to DMT linear range of 10-15 M to 10-10 M with a limit of detection about 10-15 M. The same detection limit was achieved through the increasing the peak current of [Fe(CN)6]3-/4-at DMT linear range of 10-15 to 10-12M. The mechanism of DMT oxidation on GO/GQDs/GCE was proposed and confirmed by forming a self-assembled layer (SAM) of DMT onto the gold electrode (AuE). The surface DMT undergoes anodic oxidation with Ep close to the one recorded at GO/GQDs modified surface, referring that the amide group is only possible to oxidize, and the sulfur-containing groups are involved in SAM. The DMT-SAM/AuE would present additional evidence for the surface analysis of DMT-DNA interaction where a change in the DMT peak current upon adding DNA was recorded. ds-DNA-GO@GQDs/GCE sensor exhibits a rapid response, reproducibility, a low detection limit, and high selectivity. The present work may expand the use of GO/GQDs modifiers in the field of electrochemical sensors.