Carbon dots have drawn extensive attention in the detection of metal ions with good stability, excellent biocompatibility and low toxicity. Meanwhile, the quantum yield, response rate and the detection mechanism for Cu2+ ions are vital to their development and application. To obtain more selective and sensitive materials to detect Cu2+ ions, N-doped carbon dots (DN-CDs) were synthesized by a one-step hydrothermal method using citric acid as the carbon source and diethylenetriamine (DETA) as the nitrogen source. The obtained DN-CDs exhibited stable and intense blue light emission and special near-infrared up-conversion fluorescence at 820 nm, attributed to the effect of introducing N atoms into the structure of carbon dots. Due to the dynamic quenching of the DN-CDs by Cu2+ ions, the fluorescence intensity (λex = 820 nm) of DN-CDs was quantitatively decreased in the presence of Cu2+ ions. The DN-CDs had a rapid response within 3 min. The DN-CD system exhibited a linear relationship with a concentration range from 2.5 to 50 µM and low detection limit (LOD) of 42 nM. After careful investigation, an interesting conclusion was proposed: N-doped CDs with N/O = 1:1 or higher with relatively abundant N atoms prefer to detect Cu2+ ions while those with N/O = 1:2 or lower prefer to detect Fe3+ ions.
Fixed carbon source and different dopants are mainly used to study the effect of heteroatoms on the structure and properties of carbon dots (CDs). As reactants, some dopants with conjugated structure and high nitrogen content may have important contributions to the structure and properties of doped CDs in addition to providing heteroatoms. Herein, to study the effect of fixed dopant on the structure and properties of CDs, three different CDs were synthesized using nicotinamide (NAA) and three common α-hydroxy acids (4–5 carbon atoms), and the optimal conditions were determined by orthogonal experimentation. Transmission electron microscopic micrographs showed that the average size of CDs based on nicotinamide are relatively large, up to 19.40 nm. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy demonstrated that these CDs have graphite nitrogen and several functional group structures. Ultraviolet-visible absorption spectra, fluorescence emission spectra, and fluorescence lifetime illustrated that these CDs have similar emission centers (460–470 nm) and fluorescence processes. The influence of carbon source on the surface structure of CDs was determined by systematically analyzing the response of these CDs in different pH ranges. DFT calculations revealed the distribution characteristics of the electrons in the excited state at the HOMO and LUMO energy levels of CDs. All the above characterizations and calculations proved that NAA is a desirable dopant with an important contribution to the structure and properties of CDs.
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