One-Step Synthesis of Color-Tunable Carbon Dots in Silica Sol uSing Microwave and Their Applications

Carbon quantum dots possess several beneficial properties, such as water solubility, photostability, resistance to photobleaching, and biocompatibility. However, conventional carbon dots have hydrophilic surfaces which cause aggregation-induced emissions (AIE) in solid-state materials, resulting in low quantum yield. Since silane-functionalized carbon dots are protected by the silica shell of carbon dots, they are resistant to quenching. Most solid-state matrices containing high concentrations of silane-functionalized carbon dots have been synthesized via hydrothermal method or grafting methods. These processes often involve two steps: synthesizing the carbon dots and functionalizing them with silane compounds, which are often time-consuming and labor-intensive.
In this study, we suggest a one-step microwave-assisted method to create relatively fast color-tunable carbon dots embedded in silica sol. It is made with just two precursors, APTES ((3-aminopropyl)triethoxysilane) and CA (citric acid). The carbon dots with the N-doped graphitic structure are synthesized in an amorphous silica sol. The color and the concentration of carbon dots can be controlled depending on the time and power of microwave during synthesis. The lower the power and the longer the time, these carbon dots can be synthesized from blue to orange. This demonstrates that the size and concentration of carbon dots depend on the amount of energy and how long certain power is applied. In the mechanism of the formation, hydrolysis and condensation of the silane occurs first, followed by the formation of carbon dots. When carbon dots are formed above a threshold energy, the reaction of silane and the formation of carbon dots occur together. Therefore, the more rapidly it reaches a certain energy to form carbon dots, the thinner the silane shell of carbon dots is, resulting in the reduction of carbon dot size. Furthermore, the silica groups in the shell of carbon dots are hydrolyzed and condensed by water, leading to gelation. These features can be used to create solid-state materials, such as core-shell particles, surface coating, and films.
Among these, the monodisperse fluorescent particles can be used for various applications such as biosensing or anti-counterfeiting. The size of the particle is affected by the gelation rate of the sol and the rate depends on solvents, temperature, and stirrer speed. Especially, IPA (Iso Propyl Alcohol) could cause fast gelation of the sol. Therefore, the size distributions under different conditions are proposed and the size of the particles is measured by scanning electron microscope (SEM) and zeta sizer. Through these studies, we aim to demonstrate the formation mechanism, characteristics and structure of carbon dot-embedded silica sols, and their potential applications.