Rapid colorimetric flow injection sensing of hypochlorite by functionalized graphene quantum dots

Abstract

Graphene quantum dots (GQDs) with oxygen-containing functional groups along their edges have been prepared by controlled thermalization of citric acid to develop a new spectrophotometric reagent for hypochlorite determination. 2, 4-Dinitrophenylhydrazine (DNPH) was reacted with hypochlorite to form 2, 4-dinitrophenyl diazonium cation (DNPDC) in acidic condition through a flow injection manifold and then coupled to GQDs to produce a colored product. Characterization of the appropriate materials were performed by applying different imaging and spectroscopic techniques. The results obtained from the CCD optimization approach revealed that all of the chemical reagents had a significant role individually while their interactions were not of any importance. Under optimum condition, Beer’s law was obeyed for hypochlorite determination at 423 nm in the range of 0.2–12.3 μg mL−1. The obtained detection limit of hypochlorite was 0.1 μg mL−1. Also, the intra-day experiments (n = 10 at 0.7 and 11.0 μg mL−1 of hypochlorite) showed recoveries of 100.2 and 99.8%, respectively and that of inter-day (n = 5 at 0.7 and 11.0 μg mL−1 of hypochlorite) presented, respectively, 99.3 and 99.5% recoveries. The sample throughput was about 100 h−1. Tap and pool water samples as well as bleach solutions were analyzed by this and other appropriate reference analytical methods. No statistical differences were observed between the results of the presented method and the results obtained by the reference methods. Beside the studies, a model structure of a typical functionalized GQD was optimized using density functional theory. The natural charges on the carbons along the edge of GQD were calculated, and the results indicated that the ortho positions with respect to the phenolic OH moieties are the most preferable sites for the electrophilic substitution reactions.