Band structure induced photodegradation mechanism of PEI-based carbon quantum dots for tetracycline and toxicity evaluation for these intermediates

Many kinds of carbon quantum dots (CQDs) are already released into the environment, and the factor that determine the photodegradation ability of these CQDs for antibiotics is still unclear. Herein, three PEI-based CQDs (CAPEI, HAPEI and BCNPEI) were prepared by hydrothermal method to clarify the structural factors that determine their photodegradation performance against tetracycline (TC) when exposed to visible light. Experimentally, BCNPEI photodegraded 62 % of TC, while CAPEI and HAPEI photodegraded TC only about 5–8 %. Sizes, functional groups, aromaticity and energy band structures of these CQDs have been analyzed, and the energy band structure has the best linear relationship with the photodegradation properties of CQDs on TC. The conduction band (CB) value of BCNPEI is − 0.623 V and the valence band (VB) value is 2.207 V, which are strong enough to produce hydroxyl radicals (·OH) and superoxide radicals (·O2-) under visible light exposure. And this has been verified with photoactive species capture experiments and ESR experiments. Thus, band structure is the factor that determine the photocatalytic performance of CQDs for TC. Furthermore, a possible photodegradation pathway of TC by CQD was proposed in a study of high-performance liquid chromatography-mass spectrometry (HPLC-MS) results. The photodegradation intermediates of TC were less toxic than TC, suggesting that the photodegradation of TC by CQD can reduce its environmental risk. In this study, valuable insights are provided into the behavior of CQDs exposed to the environment.