A nanocomposite probe consisting of carbon quantum dots and phosphotungstic acid for fluorometric determination of chromate(VI) with improved selectivity

A fluorometric quenching assay is described for the determination of chromate(VI) by using a nanocomposite probe consisting of carbon quantum dots (CQDs) and phosphotungstic acid (HPW). The stable nanoprobe was synthesized via hydrothermal carbonization of glucose in the presence of HPW. HPW promotes the dehydration and carbonization and acts as an "electronic receptor". It blocks the radiative electron/hole recombination in the CQDs and leads to a product whose fluorescence (with excitation/emission peaks at 360/463 nm) is quenched. The CQD/HPW was characterized by transmission electron microscopy, FT-IR spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-vis absorption and fluorescence spectroscopy to characterize their surface morphology, functional groups and elemental composition, crystal structure and optical properties. The nanocomposite is nearly mono-disperse with an average particle diameter of 1.7 nm, and displays excitation wavelength-dependent and pH-dependent photoluminescence. Fluorescence drops on addition of chromate(VI) due to an inner filter effect. The ability of receiving electron for HPW can hinder the electron transfer from CQD/HPW to other metal ions, so the nanocomposite showed excellent selectivity towards chromate(VI). Fluorescence drops linearly with the concentration of chromate(VI) in the range from 2 to 80 μM, with a limit of detection of 0.16 μM. .