Preparation of N-doped carbon quantum dots and their applications in the selective sensing of Mercury (II)

This master’s thesis presents a comprehensive study on the optimal design and synthesis of nitrogen-doped carbon quantum dots (N-CQDs) from small molecule carbon and nitrogen precursors and their application as fluorescence sensor for the detection of heavy metal ions. By employing the Box-Behnken design (BBD), the optimal synthetic condition for hydrothermal method was obtained, which led to the achievement of the high quantum yield of 51.7% for N-CQDs. The as-prepared N-CQDs are with brownish-yellow color and showed a bright blue light irradiation. To stabilize the N-CQDs, immobilization of N-CQDs onto a glutaraldehyde cross-linked chitosan was then performed to prepare the N-CQDs@GACTS hydrogel film for the selective sensing of Hg2+ ion. FTIR and XPS analyses revealed that N-CQDs were embedded into the GACTS matrix mainly through weak hydrogen bond or electrostatic attraction. Among the three tested heavy metal ions (Cd²⁺, Hg²⁺ and Pb²⁺), the N-CQDs@GACTS hydrogel film exhibited remarkable sensing sensitivity and selectivity of Hg²⁺. The excellent selectivity could be attributed to a stronger interaction between the hydrogel film and Hg²⁺ ion. Due to the strong oxidizing ability and chelating power, Hg²⁺ can be more readily combined with polar groups on the surface of N-CQDs@GACTS hydrogel to form new complexes by either chelation or electrostatic attraction, which provokes an effective electron transfer for the fluorescence quenching of the N-CQDs@GACTS hydrogel. The prepared N-CQDs@GACTS hydrogel demonstrates enhanced practicality in terms of fast response, sensitivity, selectivity, and economical pricing. It has great potential for practical applications in selectively detecting Hg²⁺ from either drinking water or wastewater