Insight into Microwave-Assisted Growth of Water-Soluble Ag2S Nanocrystals

Understanding and controlling reaction mechanisms are essential for the rational design of synthesis protocols for colloidal nanoparticles. In the case of luminescent nanocrystals and semiconductor quantum dots, understanding the correlation between structural and optical properties further enables their optimization for specific applications. Silver sulfide (Ag2S) nanocrystals are promising bioimaging probes due to their tunable photoluminescence emission in the NIR-I and NIR-II ranges and the absence of toxicity of bulk Ag2S. We developed an efficient microwave-assisted aqueous synthesis method using glutathione (GSH) as both the sulfur source and the capping agent for the production of directly water-soluble nanocrystals with potential for bioimaging applications. The nucleation and growth of these crystals are investigated using synchrotron-based X-ray absorption and X-ray photoelectron spectroscopies, which do not rely on either a minimum particle size or their crystallinity. We reveal the local structure around the silver atoms and the surface properties of the nanocrystals and relate them back to their optical properties, size, and crystallinity. This comprehensive and multiscale study reveals the different growth mechanisms taking place as a function of the Ag/GSH precursor ratio. In particular, a switch from a reaction-controlled growth regime to aggregative growth occurs in the case of a low precursor ratio, leading to an abrupt shift in the emission wavelength.