The most efficient and cost-effective method for synthesizing cadmium selenide CdSe quantum dots with controlled size and shape using green chemistry principles is the aqueous synthesis method. This method is environmentally friendly, as it uses water as the solvent and minimizes the use of hazardous chemicals.Here is a general procedure for synthesizing CdSe quantum dots using the aqueous synthesis method:1. Prepare the precursors: Prepare aqueous solutions of cadmium salt e.g., cadmium chloride or cadmium acetate and sodium selenosulfate. Sodium selenosulfate can be synthesized by dissolving selenium powder in a solution of sodium sulfite.2. Stabilizing agents: Add stabilizing agents such as thioglycolic acid TGA or 3-mercaptopropionic acid MPA to the cadmium salt solution. These agents help control the size and shape of the quantum dots by preventing aggregation and promoting uniform growth.3. pH adjustment: Adjust the pH of the cadmium salt solution to around 9-11 using a base like sodium hydroxide. This step is crucial for controlling the size and shape of the quantum dots.4. Mixing the precursors: Slowly add the sodium selenosulfate solution to the cadmium salt solution under constant stirring. The reaction between the two precursors will result in the formation of CdSe quantum dots.5. Temperature control: The reaction temperature should be maintained between 50-100C. Higher temperatures can lead to faster nucleation and growth of the quantum dots, while lower temperatures can result in smaller and more uniform quantum dots.6. Reaction time: Allow the reaction to proceed for a specific time, typically between 30 minutes to a few hours, depending on the desired size and shape of the quantum dots. Longer reaction times can lead to larger quantum dots.7. Purification: After the reaction is complete, the CdSe quantum dots can be purified by centrifugation and washing with water and ethanol to remove any unreacted precursors and byproducts.8. Characterization: The synthesized CdSe quantum dots can be characterized using techniques such as UV-Vis absorption spectroscopy, photoluminescence spectroscopy, and transmission electron microscopy TEM to confirm their size, shape, and optical properties.By optimizing the reaction conditions, such as precursor concentrations, stabilizing agent concentration, pH, temperature, and reaction time, it is possible to synthesize CdSe quantum dots with controlled size and shape using the aqueous synthesis method, adhering to green chemistry principles.