Study on the Tunable Fluorescence Emission Mechanism of Nitrogen-Doped Carbon Dots and Ion Sensing Potential Applications

In this paper, a hydrothermal strategy is reported for the synthesis of nitrogen-doped carbon dots (NCD) which used citric acid (CA) and urea as reactants with different pristine molar ratios of carbon/nitrogen atoms (C/N). The fluorescence emission wavelength of the aqueous solutions of NCD can be tuned from 462 to 546 nm under a single and uniform UV light excitation (365 nm). Through many characterization analysis methods, it is demonstrated that graphitic N and C=O extended the effective conjugation length which reduced the energy gap between the highest occupied molecular orbital (HUMO) and the lowest unoccupied molecular orbital (LUMO). This is considered to be responsible for the redshift and tunability of the fluorescence emission of NCDs with different C/N molar ratios. This will be useful for designing a synthetic strategy of long-wavelength fluorescence emission carbon dots (CDs) in a non-benzene ring precursor system. As a potential fluorescence sensor, the as-prepared NCDs can specifically detect Fe 3+ ions and Al 3+ ions in aqueous solutions. Additionally, the NCDs not only are nontoxic but also exhibit large and stable fluorescence intensity, indicating that they have broad potential application in the fields of sensors and optoelectronic devices.