Fig. 4. (a) Preparation of CQDs and N-CQDs from petroleum coke through the chemical ablation method (this figure has been adapted/reproduced from ref. 51 with permission from Elsevier, Copyright 2014). (b) Electrochemical synthesis of CQDs from simple organics. In simple conditions, one Pt sheet was used as the anode, the other one as the cathode and the simple organics as the carbon source. Under a suitable DC control, the organics were broken into CQDs (this figure has been adapted/reproduced from ref. 53 with permission from Wiley-VCH, Copyright 2014). (c) Synthesis of CQDs through the laser ablation method (this figure has been adapted/reproduced from ref. 92 with permission from Elsevier, Copyright 2018).