Synthesis of Environmentally Friendly Heavy-Metal-Free AgInS₂/ZnS Quantum Dots Using Microwave Heating

Luminescent semiconductor nanocrystals, also known as quantum dots (QDs), have had a major impact on applications of optical techniques in the life sciences, such as biosensing and bioimaging. Their unique optical properties, such as broad absorption bands with high molar extinction coefficients, together with their narrow size-dependent photoluminescence (PL) bands, high PL quantum yields (QYs), and a high photostability, are unique advantages for QDs as optical reporters in immunoassays, theranostics, in vitro/in vivo imaging, and flow cytometry. This also laid the foundation for numerous applications of optical multiplexing and barcoding. The most popular QD element compositions are based on heavy-metal elements, such as cadmium or lead, which pose a high toxic risk and are meanwhile restricted in Europe by REACH regulation. Increasing environmental concerns and the growing interest in and need for nanomaterials accessible by sustainable synthesis routes triggered the search for more eco-friendly QDs in the last years. Promising heavy-metal free candidates are ternary AgInS₂ QDs, which can be synthesized with high-quality optical properties using aqueous synthesis methods. However, the use of conventional heating mantles or oil baths to heat the reaction mixture to the desired reaction temperature can lead to significant batch to batch variations caused by an inhomogeneous heat diffusion influencing the nucleation and growth of the QDs. A solution to this problem is the use of microwave-assisted heating. This enables a fast and homogeneous heat distribution throughout the entire reaction vessel, resulting in high-quality nanomaterials prepared with high batch-to-batch reproducibility. In this chapter, we will describe the synthesis steps for the preparation of AgInS₂/ZnS QDs using microwave-assisted heating. The influence of the stabilizing ligands, variation of precursor concentrations, and the pH of the reaction solution will be detailed in the note section.