Dual-mode colorimetric and fluorometric probe for ferric ion detection using N-doped carbon dots prepared via hydrothermal synthesis followed by microwave irradiation

Dual-mode colorimetry and fluorescence detection methods have gained more attentions in recent years. In this paper, a colorimetric and fluorescent dual-mode probe was constructed based on N-doped carbon dots (CDs) that was synthesized using hydrothermal heating followed by microwave irradiation. The CDs were purified using a polarity-based method followed by a size-based method. The CDs selectively form a purple-colored complex with Fe3+ ions accompanied by quenching of the fluorescence emission. The fluorescence emission of CDs was not affected by common metal cations and anions, but was only complexed and quenched by Fe3+ ions. Cyclic voltammetry and optical spectroscopy were used to measure the HUMO and LUMO levels of the CDs. The fluorescence quenching is due to the transfer of the photo-induced electrons from the excited state of the CDs to the LUMO of Fe3+ ions. The fluorescence lifetime of CDs in the presence and absence of Fe3+ was examined by time-correlated single-photon counting. These results along with changes in the absorption spectra suggest a static fluorescence quenching mechanism. The CDs were used as a dual-readout probe for the colorimetric and fluorescent detection of Fe3+ ions in real samples. The lower limit of detection obtained using colorimetry was 0.5 μM (S/N = 3), while that obtained using fluorimetry was 50 nM. The fluorescence intensity of the CDs is pH independent between 2 and 9, which increases the working range for practical application. Furthermore, the CDs in aqueous solution are photostable, thermostable, and stable under high ionic strength, which makes the probe robust and rugged.