In this study, the target carbon dots could be prepared controllably only by adjusting the applied voltage.

The acid oxidation method consists mainly of acid treatment of the carbon source to oxidize the functional groups on the surface of carbon source to generate fluorescence. The advantages are the simple preparation method and low requirements for the experimental equipment; The disadvantage is that the yield of preparing carbon dots is generally low, the separation is difficult, and the carbon dots generally require post-treatment passivation, poor control of particle size, a violent reaction process, harsh conditions, and many steps.

In 2018, the N-CQDs, S-CQD, and Se-CQDs obtained by Iannazzo et al. showed tunable photoluminescence performance, higher quantum yield (QY), and longer fluorescence lifetime than pure CQDs [25]. The experimental results show that heavily doped heteroatoms will affect the photoluminescence characteristics, which is positively correlated with the electronegativity of N, S, and Se. The active heteroatoms on the surface of the CQDs will adjust the electronic structure of the corresponding CQDs. Therefore, when used as an electrocatalyst, it will have good electrocatalytic activity. On the other hand, the heavily doped CQDs have the ability to coordinate with transition metal ions. N-CQDs, S-CQD, and Se-CQDs may also have the potential to absorb other metal ions, such as Fe³⁺, CO²⁺, and Ni²⁺, to form so-called monatomic catalysts.

The “bottom-up” method is used mainly to carbonize and assemble the molecular precursors (such as sugars, organic compounds, ethylenediamine, etc.) to synthesize carbon dots through combustion or heat treatment, and the methods used are mainly the hydrothermal method [26], microwave method [27], template method, etc.

The hydrothermal method is used to prepare carbon dots by mixing carbon source and solvent and heating in autoclave. It is worth mentioning that the hydrothermal method is the most commonly used method to prepare carbon dots at present. Reaction temperature and reaction time are the two most important parameters, which directly affect the optical properties and quantum yield of CDs. The advantages are that the source of raw materials is very wide, the reaction equipment is simple, and the reaction conditions are easy to control. Because the reaction is carried out in a closed reactor, the influence of impurities in the air is avoided, the prepared carbon dots are relatively uniform, and most of the carbon dots synthesized by the hydrothermal method have good water solubility. The disadvantage is that due to the limited volume of the hydrothermal kettle, the amount of carbon dots prepared at one time is limited, and the prepared carbon dots will produce more impurities, which makes it difficult to separate and purify the carbon dots; it is therefore not suitable for large-scale industrial production, and the size of the prepared CDs is difficult to control, which also limits the use of this method to a certain extent. In addition, due to the use of organic solvents, the solvothermal method is more dangerous than the hydrothermal method at high temperature, and the post-treatment of excessive organic solvents after reaction also needs special consideration.

Research Evolution: Peng et al. first reported the hydrothermal synthesis of fluorescent carbon dots in 2009 [28]. In this study, they first dehydrated carbohydrates with concentrated sulfuric acid to produce carbonaceous substances, then treated carbonaceous substances with nitric acid to oxidize them, and finally modified them with nitrogen-containing groups into carbon dots emitting blue fluorescence. However, the quantum yield of carbon dots synthesized by this method is not high, and the size distribution is uneven. Subsequently, the researchers found that in the process of hydrothermal synthesis of carbon dots, the size, surface chemical state, and luminescence wavelength of carbon dots can be achieved by adjusting the reaction conditions such as the type of carbon source and solvent. In 2013, Zhu et al. synthesized high fluorescence carbon dots using the hydrothermal method, and the proposed citric acid and ethylenediamine system is very classic in the field of carbon dots [29]. Citric acid and ethylenediamine undergo condensation reaction under the action of aqueous solvent to form polymer-like carbon dots, which are further carbonized to form carbon dots with an average size of 2–6 nm. The carbon dots prepared by this method have uniform size distribution, and their luminous effect is comparable to that of fluorescent dyes. In 2015, Jiang et al. used o-diphenylamine, p-Diphenylamine, and m-diphenylamine as carbon sources to synthesize carbon dots emitting red, green, and blue fluorescence in the ethanol solvent by the solvothermal method [30]. In this study, the isomers of phenylenediamine were used as carbon sources to react with the ethanol solvent. The mixed products were separated by silica gel column to obtain carbon dots with different emission wavelengths. In 2017, Raji et al. synthesized carbon dots using monk fruit as the carbon source using the hydrothermal method. The monk fruit was sealed in an autoclave, heated at 180 °C for 6 h, and the resulting solution was cooled to room temperature. The solution changed from light yellow to dark brown. The monk fruit was dehydrated, polymerized, and carbonized to finally form carbon dots with blue fluorescence [26].

Microwave heating is a method that uses a microwave to raise the temperature rapidly in a short time to polymerize and carbonize the reactant monomers to form carbon dots. It provides a new and rapid method for the synthesis of carbon dots. Experiments show that constant high-pressure reaction vessel is the best method to control the particle size distribution and photoluminescence properties of CDs [31]. The advantages are that it is convenient and fast, the preparation conditions are simple, raw materials are easily available, etc. The disadvantage is that the reaction process is unstable, the reaction temperature is difficult to control, and the fluorescence quantum yield is low.

Research evolution: In 2009, Zhu et al. synthesized carbon dots under microwave-assisted reaction conditions with polyethylene glycol and sugars as the carbon sources [32]. They put all the reactants in the microwave oven and set the power to 500 W and the reaction time to 2–10 min. It could be observed that the color of the reaction solution experienced a change of colorless to a yellow dark-brown. Finally, the blue fluorescent carbon dots were obtained through separation and purification. By changing the reaction time, the size, emission peak position and quantum yield of carbon dots can effectively be controlled. In 2012, Tang et al. synthesized carbon dots emitting deep UV fluorescence using glucose as the carbon source by microwave-assisted hydrothermal synthesis [33]. In 2015, Pan et al. used formamide and citric acid as the carbon sources and microwave-heated them at 160 °C for 60 min. After separation and purification, they obtained fluorescent carbon dots with full-spectrum emission [30]. In 2020, Li et al. used guanine and ethylenediamine as the carbon sources and deionized water as the solvent, heated it in a household microwave oven with a set power of 700 W for 10 min, and removed large impurities with a dialysis bag to obtain an aqueous solution of carbon dots [26]. The carbon dots prepared by this method have excellent water solubility, excellent stability at different pH, and extremely high ionic strength, and have been successfully used as fluorescent probes to detect Ag⁺.

The advantage of template synthesis of carbon dots is that the size of the carbon dots can be controlled by the size of template, and the aggregation of the carbon dots can be reduced in the process of synthesis. The prepared carbon dots have a uniform size and high fluorescence quantum yield; The disadvantage is that some templates are difficult to separate from the carbon dots, and the fluorescence performance of the carbon dots may be affected in the process of acid-base etching or heating to remove templates. The advantages are that the size of the carbon dots can be controlled by the size of the template, and the aggregation of the carbon dots can be reduced in the process of synthesis. The prepared carbon dots have a uniform size and high fluorescence quantum yield. The disadvantage is that some templates are difficult to separate from the carbon dots, and the fluorescence performance of the carbon dots may be affected in the process of acid-base etching or heating to remove the templates.

Research evolution: In 2012, Kwon et al. first synthesized fluorescent carbon dots with a size of 1.403 ± 0.148 nm using glucose as the carbon source by using the micro lotion template method [34]. Firstly, the water in the oil microemulsion was formed by mixing 10% glucose aqueous solution with 1-octanol in oil phase. Then, cetylammonium was added for heating and carbonization. Finally, cetylammonium modified fluorescent carbon dots were obtained. In this process, the number of carbon sources in each micelle can be limited by adjusting the concentration of micro lotion, and the size of carbon dots can be further controlled. In 2013, Yang et al. first proposed the method of soft hard template, using copolymer P123 as the soft template and ordered mesoporous silica as the hard template to prepare fluorescent carbon dots [35].