Fe3O4 quantum dots anchored on functionalized graphene: A multimodal platform for sensing and remediation of Cr(VI)

Despite their extreme toxicity for human and the environment, Cr(VI) species are widely used in multiple industrial processes. This brings the need of potent strategies for detection at trace level and for remediation of contaminated effluents. In this article, we present a novel composite containing Fe3O4 quantum dots (QDs) immobilized on polydopamine-functionalized graphene obtained by a simple and cost-effective hydrothermal process. The high affinity of the composite for Cr(VI) ions resulted in an efficient chemical coupling with the Fe3O4 QDs, offering a powerful platform for multimodal reactivity. First, the composite acted both as a fluorescence as well as nanozymatic sensing platform for detecting toxic Cr(VI) ions in aqueous solution with a limit of detection (LOD) of 18 nM and 47 nM, respectively. In addition, the composite proved to be a very efficient catalyst for the visible-light photoreduction of Cr(VI) to non-toxic Cr(III), showing an outstanding reduction by 99% in 20 min under natural sunlight and a good recyclability. This study is the first of its kind where metal oxide QDs anchored functionalized graphene has been explored both as fluorescence and colorimetric sensing platform for Cr(VI) detection as well as an efficient photocatalyst for reduction of toxic Cr(VI) to non-toxic Cr(III) ions.