Carbon dot based fluorescent “on–off–on” assays for the determination of Au(iii) ions and biothiols†

Gold (Au) is extensively used in modern industry, which raises hard-to-ignore environmental problems due to its potential toxicity. Herein, highly luminescent carbon dots (CDs) are prepared from the carbon sources of o-phenylenediamine and hydroquinone, and applied for the selective quantification of Au³⁺. Significantly, fluorescence quenching is observed upon the addition of Au³⁺ in 2 min. The ions are reduced to gold shells which coat the surface of CDs, resulting in electron transfer and emission quenching. The degree of quenching varies linearly with an Au³⁺concentration from 0.1 to 7 μM. Biothiols are then introduced to interact with the gold shells. It is found that cysteine, glutathione, 3-mercaptopropionic acid and DL-dithiothreitol can be used to digest the shell, restoring the fluorescence emission. The “on–off–on” fluorescence biosensing strategy is challenged with environmental and biological samples with excellent results, demonstrating that this study provides a novel methodology to examine Au³⁺ and biothiol levels for potential practical applications.