Humic Acids Affect the Detection of Metal Ions by Cyanobacteria Carbon Quantum Dots Differently

A "top-down" synthesis of carbon quantum dots (CQDs), novel fluorescent C materials from waste biomass, is both cost-effective and environmentally friendly. N-rich cyanobacteria are promising precursors to produce CQDs with high fluorescence (FL) intensity for the detection of metal ions. Herein, we synthesized cyanobacteria-based CQDs using a hydrothermal process and evidenced their high FL intensity and stability. The cyanobacteria-based CQDs showed powerful sensitivity for the specific detection of Fe3+ and Cr6+, which could be ascribed to (i) static FL quenching as a result of the interaction between -OH, -NH2, and -COOH groups with the metal ions, (ii) internal filtering effects between the CQDs and Fe3+ or Cr6+, and (iii) fluorescence resonance energy transfer between CQDs and Cr6+. Humic acids (HAs) coexisting led to an underestimation of Fe3+ but an overestimation of Cr6+ by the CQDs due to the different FL quenching mechanisms of the CQDs. HAs sorbed Fe3+ and wrapped the CQDs to form a barrier between them, inhibiting FL quenching of CQDs by Fe3+. As for Cr6+, HAs reduced Cr6+ and also led to FL quenching; the sorbed HAs on the CQDs acted as a carrier of electrons between Cr6+ and the CQDs, enhancing FL quenching of the CQDs. This study is the first work to evidence the interference of HAs in the detection of metal ions by CQDs derived from cyanobacteria, which would enlighten the application of CQDs in a natural aqueous environment.