Application of response surface methodology and green carbon dots as reducing agents in speciation of iron

Herein, for the first time, we used a green synthetic approach, via the hydrothermal treatment of grape and onion without any functionalization, to produce reducing carbon dots (CDs). The method has the advantages of low cost, easy operation and being environmentally friendly. The as-synthesized grape and onion CDs were characterized by UV-Vis spectrophotometry, spectrofluorimetry, FTIR spectroscopy and transmission electron microscopy (TEM). Interestingly, it was found that the synthesized CDs could reduce Fe³⁺ to Fe²⁺. Based on this finding, a method based on complexation with 1,10-phenanthroline was introduced for determination of Fe³⁺ and total iron in water samples. A response surface methodology was employed to explore the factors influencing the response, i.e. concentration of 1,10-phenanthroline and concentration of as-synthesized CDs. The proposed method provides a simple and sensitive colorimetric approach to detect Fe³⁺ over a wide linear range of 4.6-160 μM with a low detection limit of 0.1 μM. Moreover, for the first time, the reducing strength of CDs was estimated by the well-known Prussian blue assay.