Carbon dots doped by nitrogen and sulfur for dual-mode colorimetric and fluorometric determination of Fe3+ and histidine and intracellular imaging of Fe3+ in living cells

The first dual-modality highly intensive fluorescent and colorimetric nanoprobe for Fe3+ ions and histidine is reported. The carbon dots doped by nitrogen and sulfur (N,S-CDs) prepared by the one-pot hydrothermal method have an excitation/emission wavelength of 320/420 nm with 56% quantum yield. N,S-CDs exhibit strong visible fluorescence with high stability at pH ~ 7.0. The fluorescence intensity of the N,S-CDs is quenched in the presence of Fe3+ ions which are recovered upon the addition of histidine. The addition of Fe3+ ions also induces a color change from yellow to red. Using colorimetric determination, Fe3+ and histidine exhibited linearity in the range 75–675 and 100–375 μmol L−1, respectively, while with fluorometric determinations the dynamic range was 0.1–275 and 0.1–3 μmol L−1 for Fe3+ and histidine, respectively. The limits of detection were 19 nmol L−1 and 0.03 μmol L−1 using fluorometry and 20 μmol L−1 and 24.2 μmol L−1 using colorimetry, for Fe3+ and histidine respectively. The relative standard deviations (n = 5) for Fe3+ (10 μmol L−1) and histidine (1 μmol L−1) using fluorometry were 4.6 and 7.3% and using colorimetry at 100 μmol L−1 of Fe3+ and 150 μmol L−1 of histidine were 3.2 and 5.6%, respectively. The developed fluorometric method was applied for the determination of Fe3+ and histidine in various foods and biological fluid samples as well as intracellular imaging of iron. The accuracy of the method for iron determination was confirmed by the analysis of certified reference materials (wheat flour, tomato leaves, and whole milk powder) and quality control materials (whole milk powder, serum, and urine), whereas for histidine, the accuracy was determined by recovery experiment and independent analysis. Good recovery values in ranges of 92–96% and 94–98% were achieved for Fe3+ and histidine, respectively.