Single-layered graphene quantum dots with self-passivated layer from xylan for visual detection of trace chromium(Vl)

Single-layered graphene quantum dots (GQDs) are commonly prepared through bottom-up strategy by aromatic molecules or other carbon precursors with complicated procedure. Herein, single-layered GQDs were firstly fabricated by nonaromatic xylan only with the assistance of NaOH/urea under hydrothermal conditions. Due to the gases (ammonia and carbon oxide) released by decomposition of urea, GQDs were single-layered and hindered from interaction and aggregation for the protection of self-passivated layer. The synthesized GQDs showed a fluorescent quantum yield of 23.8%, self-passivated xylan layer on the surface of GQDs avoided ions-induced interference, and were only destroyed by strong oxidants like those containing Cr(VI). And there were the inner filter effect and static quenching for chelating interaction between Cr(VI) and nitrogen, oxygen functional group from GODs. These results entrusted GQDs with good selectivity and sensitivity toward Cr(VI). Furthermore, in order to realize on-site visual detection of Cr(VI), a microfluidic chip and a smartphone were used to construct portable detection platform. The linear detection range was from 3 μM to 75 μM and the limit of detection was 0.10 μM, which was lower than that of GQDs solution through fluorescence spectrophotometer. Therefore, this equipment provides a simple method to on-site monitor water environment.