Enhanced fluorescence of Zn-doped carbon quantum dots using zinc citrate chelate as precursor for fluorescent sensor applications

The Zn2+ doped Carbon Quantum Dots (Zn-CQDs) were synthesized with zinc citrate chelate as the precursor by a hydrothermal method. The evaluation results indicated that Zn2+ was successfully embedded in CQDs. Most of Zn2+ ions were combined with carboxyl groups on the surface of carbon dots to form Zn-O bonds and some Zn2+ ions were embedded in the interstice points of CQDs. The remaining Zn2+ were reduced to zinc atoms and adsorbed on the surface of CQDs. With the increase in Zn2+ concentration, the size of synthesized Zn-CQDs increased. When zinc ion concentration increased to 9%, the average size of Zn-CQDs grew up to 8 nm, but the size of undoped CQDs was just 1.5 nm. When Zn2+ concentration increased from 1% to 9%, the fluorescence intensity increased, while reaching above 9%, the fluorescence intensity showed a downward trend. When Zn2+ concentration was 7%, the fluorescence quantum yield of Zn-CQDs increased to 48%, but the fluorescence quantum yield of undoped CQDs was only 37%. With the increase in Zn2+ concentration, the PL spectrum of Zn-CQDs showed a red shift phenomenon. In addition, Zn-CQDs showed superior fluorescent sensing capability to Fe3+ and Hg2+, presenting greater fluorescence quenching effect on Fe3+ and Hg2+ than CA-CQDs. Therefore, Zn-CQDs demonstrated excellent fluorescence properties and can be applied in fluorescent sensor, biological imaging, optoelectronics, and catalysis fields.