ORIGINAL PAPER Carbon dots doped by nitrogen and sulfur for dual-mode colorimetric and fluorometric determination of Fe 3+ and histidine and intracellular imaging of Fe 3+ in living cells Mojtaba Amiri 1 & Ali Mohammad Haji Shabani 1 & Shayessteh Dadfarnia 1 & Nader Shokoufi 2 & Behnam Hajipour-Verdom 3 & Sodeh Sadjadi 4 Received: 7 June 2020 /Accepted: 18 August 2020 # Springer-Verlag GmbH Austria, part of Springer Nature 2020 Abstract The first dual-modality highly intensive fluorescent and colorimetric nanoprobe for Fe 3+ 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 wave- length 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 Fe 3+ ions which are recovered upon the addition of histidine. The addition of Fe 3+ ions also induces a color change from yellow to red. Using colorimetric determination, Fe 3+ 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 Fe 3+ 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 Fe 3+ and histidine respectively. The relative standard deviations (n = 5) for Fe 3+ (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 Fe 3+ and 150 μmol L -1 of histidine were 3.2 and 5.6%, respectively. The developed fluorometric method was applied for the determination of Fe 3+ 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 Fe 3+ and histidine, respectively. Keywords Carbon dots doped by nitrogen and sulfur . Colorimetry . Fluorometry . Fe 3+ and histidine detection . Dual-mode nanoprobe Introduction Recently, the dual-mode detection strategy has gained consid- erable attention, as proved to be more efficient than the single- mode method. Up to now, several dual-mode optical assays have been developed, such as colorimetry/fluorescence [1] and fluorescence/surface-enhanced Raman scattering (SERS) [2], fluorescent/MRI [3], and magnetic resonance imaging [4]. However, colorimetric/fluorescent dual-mode assay has attained much more attention and becomes an extremely sen- sitive analytic tool in biomedical applications. This is because the dual-mode fluorescent and colorimetric probes provide the advantages of low detection limit, high selectivity, and sensi- tivity of fluorescence methods as well as the simplicity, avail- ability of equipment, and possibility of naked eyes monitoring of colorimetric methods. Based on the aforementioned Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00604-020-04512-3) contains supplementary material, which is available to authorized users. * Ali Mohammad Haji Shabani hshabani@yazd.ac.ir * Shayessteh Dadfarnia sdadfarnia@yazd.ac.ir 1 Department of Chemistry, Yazd University, Safaieh, P.O. Box 89195-741, Yazd, Iran 2 Analytical Instrumentation and Spectroscopy Laboratory, Faculty of Clean Technologies, Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14665-1513, Tehran, Iran 3 Department of Biophysics, Faculty of Biological Sciences, Tarbiat Modares University, P.O. Box 14115-154, Tehran, Iran 4 Radiation Application Research School, Nuclear Science and Technology Research Institute, P.O. Box 11365-3486, Tehran, Iran Microchimica Acta (2020) 187:562 https://doi.org/10.1007/s00604-020-04512-3