A NiAg-graphene quantum dot-graphene hybrid with high oxidase-like catalytic activity for sensitive colorimetric detection of malathion†

This paper reports the synthesis of a nickel-silver-graphene quantum dot-graphene hybrid. Histidine-functionalized graphene quantum dots (His-GQDs) were bonded to graphene oxide (GO) and then combined with Ni2+ and Ag+ to form a NiAg/His-GQD/GO complex, followed by thermal reduction under N-2. The resulting hybrid offers a well-defined three-dimensional structure. The NiAg nanoparticles are uniformly dispersed on the graphene sheets. The special architecture creates high oxidase-like catalytic activity towards the oxidation of 3,3 ',5,5 '-tetramethylbenzidine (TMB) into a blue product. Furthermore, the hybrid was used as a mimetic enzyme for colorimetric detection of malathion. In the detection, acetylcholinesterase catalyzes hydrolysis of acetylthiocholine to produce thiocholine. In the presence of malathion, the catalytic activity of acetylcholinesterase was effectively inhibited. This will lead to a reduced thiocholine concentration. As sulfhydryl in the thiocholine can combine with silver and nickel in NiAg/His-GQD/G to form Ag-S and Ni-S bonds, the presence of thiocholine results in a reduced oxidase-like catalytic activity of NiAg/His-GQD/G. With the increase of malathion, NiAg/His-GQD/G gives a higher oxidase-like catalytic activity. This causes a faster rate for oxidation of TMB, indicating an increased absorbance of the blue product. The result shows that the inhibition rate has a good linear relationship with the malathion concentration in the range of 10-120 ng mL(-1) and detection limit of 3.10 ng mL(-1) (S/N = 3). Meanwhile, the sensing platform was also applied to paper strips for rapid determination of malathion in vegetable samples by using a smartphone application (APP), which provides a promising platform for on-site pesticide detection.