Metallic nanoparticle-graphene quantum dot nanocomposites for the electrochemical detection of methyl parathion.

Master of Science in Chemistry. University of KwaZulu-Natal, Durban, 2017.This work reports on the detection of methyl parathion (MP) on electrodes constructed or decorated with graphene based nanomaterials. The sensing nanomaterials used include graphene quantum dots (GQDs), metal oxide [(MO): Cu2O, NiO, Al2O3 and MnO2] and metal hydroxide [(MOH): Cu(OH)2, Ni(OH)2, Al(OH)3 and Mn(OH)2] nanoparticles (NPs). The nanomaterials were synthesized using prescribed procedures. Characterisation of the materials was achieved using various techniques including transmission electron microscope (TEM) and scanning electron microscope (SEM). The MO NPs sizes were determined by TEM to be between 31 nm and 70 nm whereas the MO NPs sizes were found to be smaller with particle sizes ranging between 20 nm and 55 nm. The nanocomposites of GQDs/MO and GQDs/MOH were synthesized and characterised using SEM. SEM revealed that the MO and MOH nanoparticles were well-dispersed on the surface as well as within the graphene quantum dot sheets. The following electrodes were prepared: a graphene quantum dot paste electrode (GQDPE) decorated with either MO or MOH NPs and a glassy carbon electrode (GCE) modified with either GQDs/MO or GQDs/MOH nanocomposites. The electrochemical characterisation of these electrodes revealed that faster electron transfer kinetics occurred at the GQDPE. The ability of the modified electrodes to electrochemically detect MP was evaluated using cyclic voltammetry and the results revealed that the modified GQDPE did not exhibit any electrocatalytic performance. However, the GCE modified with the GQDs/MOH nanocomposite showed the best catalytic activity with lower detection limits compared to GQDs/MO modified electrodes. Electrochemical characterisation further revealed that amongst all of the metal based nanocomposites, GQDs/Cu(OH)2 exhibited the best catalytic activity with the highest sensitivity towards MP. This work demonstrated for the first time that these sensing nanomaterials have a favourable catalytic behaviour, ideal for the detection of organophosphate pesticides