Nitrogen- and sulfur-doped carbon dots as peroxidase mimetics: colorimetric determination of hydrogen peroxide and glutathione, and fluorimetric determination of lead(II)

Fluorescent carbon dots co-doped with nitrogen and sulfur (N/S CDs) were prepared and found to display viable peroxidase mimicking activity. They have a blue fluorescence (with excitation/emission maxima at 340/456 nm) with a quantum yield of 35%. The N/S CDs catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂, and this leads to the appearance of a blue solution with a absorption maximum at 654 nm. A colorimetric method was developed for the determination of H₂O₂ that has a 1.75 μM detection limit and a linear response in the 10⁻⁵ to 10⁻⁴ M concentration range. The method can be extended to the enzymatic determination of glutathione with a 0.26 μM detection limit and a working range from 0.20 to 100 μM. In addition, the CDs respond to lead(II) which is a quencher of the blue fluorescence at 456 nm, with a detection limit of 11 μM and a working range up to 100 μM. Simultaneously, the color changes can be visually detected with absorbance signal changes from 10 to 100 μM with limit of 3.9 μM. A multiple detection system was worked out that allows monitoring of H₂O₂ and glutathione successively, and of lead(II).

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