Reported in 2004, carbon dots (CDs) have been widely used in various fields due to their excellent optical properties. However, the mechanism of their fluorescence modulation is still a controversial issue, which also seriously affects the further development of carbon dots. In this paper, m‐hydroxybenzaldehyde is used as a raw material to obtain multicolor luminescent CDs by pyrolysis under different reaction conditions, thereby revealing the forbidden band tuning and formation mechanism of CDs. Different acid–base conditions lead to different reaction paths of the precursors, forming molecular fluorophores with different conjugated structures, which aggregate to eventually form CDs and further enhance the photoluminescence of the system by inhibiting the movement of the fluorescent centers.

Carbon dots (CDs) are widely used nanomaterials that not only exhibit good biocompatibility and photostability, but also benefit from a simple preparation process and easy functionalization, making them promising for broad applications in the fields of heavy metal ion detection and optoelectronic devices. Based on the excellent optical properties of CDs and the current situation of increasing energy shortages, this paper selects the natural polyphenolic compound tannic acid (TA) found in biomass materials as the carbon source and innovatively adopts a simple and convenient solvent-free pyrolysis method without auxiliary reagents or solvents. The CDs with good water solubility and certain fluorescence properties were directly prepared under the condition of high temperature, and the obtained CDs exhibited blue fluorescence, and a high QY of 35.4% was obtained at 300 °C. The analysis and results demonstrate the selectivity of these CDs for the detection of various metal ion solutions. In particular, these CDs are sensitive to Ni²⁺ and can be used as fluorescent sensors for the efficient and sustainable detection of Ni²⁺, whereas previous sensors were often specific to Fe³⁺ and Hg²⁺. Thus, a new sensing technique has been developed for the detection of Ni²⁺ to achieve more sensitive and rapid detection.

Red, green, and blue CDs were successfully prepared by a solvothermal method using gallic acid and o -phthaladehyde as raw materials.