X-ray absorption spectroscopy examination of Cr, Co, and Cu binding on fluorescent carbon dots

Abstract

Fluorescent carbon dots (CDs) synthesized from tartaric acid and triethylenetetramine showed potential for the detection of Cr6+, Co2+ and Cu2+ in aqueous solution, with the most effective sensing being based on fluorescence quenching for Co2+. A good linear relationship between the concentration of Co2+ and the relative fluorescent intensity was obtained at 1.7–167.7 μM (R2 = 0.9993) with a limit of detection at 0.94 μM. X-ray absorption spectroscopy (XAS) was used to examine the absorbed transition metal ions on the CDs. Measurements were carried out at Beamline 8, Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand. Cr, Co, and Cu K-edges were measured in various standard compounds containing different oxidation states. An energy shift of 12.49 eV in the absorption of the Co K-edge was observed from elemental Co0 to Co2+, 3+, while shifts of 17.96 eV and 5.45 eV were observed for elemental Cr0 to Cr6+ and elemental Cu0 to Cu2+, respectively. A small decrease in edge energy of the Cr K-edge XAS spectra compared to standard compounds with the LCF results suggested two forms of Cr6+ found in Cr sorption on carbon dots while a mix of Cu1+ and Cu2+ was found in Cu sorption on carbon dots. Interestingly, an increase in edge energy of Co K-edge was observed for Co sorption on carbon dots compared to +2 oxidation state compounds, suggesting that metal species transform to a higher positive charge during binding to CDs. Cobalt species might participate in the formation of a chemical bond with functional groups existing on the surface of CDs. Fourier transform infrared (FTIR) spectra confirmed the results from XAS. © 2020 Elsevier Ltd