Colloidal Synthesis and Photoluminescence Properties of Near-IR-Responsive Ag8GeS6 Quantum Dots

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Near-infrared (NIR)-responsive quantum dots (QDs) hold immense promise for a range of applications, such as photovoltaics, photocatalysts, and biomedical imaging. Their unique optoelectronic properties, including tunable photoluminescence (PL) peaks and high PL quantum yields (QY), originate from the quantum confinement effect. In biomedical imaging, QDs offer superior photostability and enhanced biocompatibility compared to traditional organic fluorophores. Although binary QDs, such as CdTe, CdSe, PbS, and HgTe, have been extensively explored, their practical use is limited by the content of heavy metals with high toxicity. Here, we report the first synthesis of Ag8GeS6 QDs emitting a NIR PL peak at 920 nm and their potential as luminescent probes for in vivo bioimaging.Ge(gly)2(H2O)2 was prepared by the modified method described in previous study, as a germanium precursor resistant to air and moisture.[1] Precursor powders of Ge(gly)2(H2O)2, silver(I) diethyldithiocarbamate, and thiourea were dispersed in a mixture solvent containing 1-dodecanethiol and oleylamine, followed by the heat treatment at 423 K for 20 min. The obtained Ag8GeS6 QDs were surface-coated with a ZnS shell layer with different shell thicknesses.Spherical Ag8GeS6 QDs were formed with the average size of 4.2–4.6 nm. XRD diffraction patterns showed that the obtained QDs had an orthorhombic Ag8GeS6 crystal structure. The absorption spectra were broad and structureless, with an onset wavelength at around 850 nm. The energy gap was almost constant at 1.48-1.48 eV, regardless of the Ge/(Ag+Ge) ratio in the precursors. The Ag8GeS6 QDs prepared with Ge/(Ag+Ge) = 0.82 exhibited a broad PL peak at 920 nm with a PLQY of 11%. The surface coating with a ZnS shell enhanced the PLQY to 40–42%. The ligand exchange of the QDs with 3-mercaptopropionic acid (MPA) enabled uniform dispersion in an aqueous solution for the application to in vivo bioimaging. The resulting MPA-modified Ag8GeS6@ZnS QDs maintained similar size distribution but the PL QY was slightly decreased to ca. 23%. These QDs were successfully applied for in vivo bioimaging by injecting them into the back of mouse, in which a clear PL image was obtained and the detected PL intensity was proportional to the concentration of injected QDs.In conclusion, the present Ag8GeS6@ZnS QDs seem promising as a probe for bio-imaging, marking a significant advancement in the field and opening avenues for safer and more effective NIR-responsive QDs.[1] A.S.R. Chesman, et al., Chem. Mater, 26, 5726 (2014).