Comparison between hydrothermal and microwave-assisted synthesis of carbon dots from biowaste and chemical for heavy metal detection: A review

•Carbon dots synthesis methods (hydrothermal and microwave assisted) are compared.•Pretreated biowaste and pure chemicals can be used as carbon dots precursor.•Quenching mechanisms and application of carbon dots as sensor were summarized.•Carbon dots can be used to detect heavy metals, and other compounds in wastewater.•Carbon dots can be immobilized into solid substrate. Carbon dots (CDs) have gained scientific popularity due to their photoluminescence properties that are useful for the detection of hazardous materials. These safe and green nanomaterials can be synthesized from biowaste and chemicals using top-down or bottom-up methods. Hydrothermal and microwave-assisted synthesis methods are compared to reveal their simplicity and flexibility. CDs were hydrothermally synthesized between 180 °C and 200 °C within 2–12 h. The carbonation at a lower temperature could be offset by increasing the hydrothermal duration. Chemicals with higher purity resulted in higher quantum yield in general. The hydrothermal duration could be shortened to 1 h under microwave irradiation, but the quantum yield was not significantly improved even using a closed vessel under subcritical conditions. Additional nitrogen and sulfur dopants enhanced the optical and fluorescent properties, while the immobilization of CDs into a solid substrate facilitates the reuse. CDs could interact with heavy metals differently, resulting in quenching mechanisms such as static, dynamic, inner filter effect, and fluorescence resonance energy transfer. The photoluminescence changes allow the quantification of heavy metal concentrations such as mercury, copper, and lead concentration in water samples.