Owing to its excellent fluorescence stability, unique up-conversion fluorescence properties and biocompatibility, carbon-quantum dots (CQDs) have become a research hot spot in the field of carbon materials. This study used a series of different preparation strategies to synthesise CQDs such as microwave method, solvothermal method and hydrothermal method. The prepared quantum dots can be further applied to design and metal ion detection. It is worth noting that the CQDs prepared by the hydrothermal method are evenly distributed in the range of 2–3 nm without any agglomeration. Fluorescence quenching was performed by Fe³⁺, and the introduction of PO₄ ³⁻ ion could increase the fluorescence intensity of CQDs-Fe³⁺ system. Furthermore, CQDs were hydroxyl functionalised by treating with ethanol. The functionalised CQDs synthesised by solvothermal method greatly enhanced the fluorescence intensity.