Silver-based quantum dots (QDs) such as Ag₂S, Ag₂Se, and Ag₂Te, which emit in the second near-infrared window (NIR-II, 900–1700 nm), have attracted great research interest due to their prominent optical properties and eco-friendly compositions. Over the past decade, the controllable synthesis, bandgap modulation, and fluorescence improvement of NIR-II Ag-based QDs have greatly promoted their practical applications. In this review, we summarize the development process and latest achievements of NIR-II Ag-based QDs, covering major synthesis techniques for fabricating NIR-II Ag-based QDs, general methods for improving their fluorescence properties and recent advances in the applications of NIR-II Ag-based QDs from bioimaging to optoelectronic devices. Finally, we discuss the challenges and prospects of NIR-II Ag-based QDs in their optical properties and applications. This review aims to present synthesis and modification strategies and future application prospects for NIR-II Ag-based QDs, providing guidance for the design and integration of fluorescent probes in NIR-II window.

Cation exchange (CE) has been emerged as a promising post-synthesis strategy of colloidal nanocrystals. However, it is unclear how the cation precursor affects the CE process and the final colloidal nanocrystals. Herein, we utilized two Zn-B Lewis acid-base adduct complexes (B = oleylamine (OAM) and methanol (MeOH)) as Zn precursors for CE with Ag₂S quantum dots (QDs). Our study revealed that the steric hindrance and complexing capabilities of Zn precursor significantly affect the CE kinetics. As a result, the Zn-doped Ag₂S (Zn:Ag₂S) and Ag₂S@ZnS core–shell QDs were successfully obtained with enormous enhancement of their photoluminescence (PL) intensities. Theoretical simulation showed that the Zn-OAM with higher desolvation energy and spatial hindrance tended to form doped Zn:Ag₂S QDs due to the inefficient cation exchange. Whereas the Zn-MeOH with lower exchange barrier promoted the conversion of Ag-S to Zn-S, thus forming Ag₂S@ZnS core–shell QDs. We anticipate that this finding will enrich the regulatory approaches of post-synthesis of colloidal nanocrystals with desirable properties.

Surface ligands of colloidal quantum dots (QDs) have a profound influence on their surface states, which has been verified in the studies of the effect of ligand head groups on the photoluminescence (PL) properties of QDs. However, the investigation of the ligand chain length is limited. Here, we systematically explored the effect of chain length on the Ag₂Se QDs by selecting three ligands, 1-octanethiol (OTT), 1-dodecanethiol (DDT), and 1-hexadecanethiol (HDT), with diverse chain lengths. We found that the PL intensity of Ag₂Se QDs increased with the decrease of the ligand chain length due to the enhanced passivation of surface defects emerging from the robust QD-ligand interface binding affinity and the weaker hydrophobic chain–chain interaction. Subsequently, AgAuSe QDs terminated with OTT were obtained by alloying parent OTT-Ag₂Se QDs with Au precursor with a record absolute PL quantum yield (PLQY) of 87.2% at 970 nm, facilitating ultrasensitive in vivo angiography imaging in a nude mouse model. We expect that our finding of the important role of the ligand chain length on the optical properties of QDs will be suggestive to the design and synthesis of high-quality QDs, and also look forward to the clinical applications of the ultra-bright AgAuSe QDs.