High-Yield Synthesis of AIE-Type Au₂₂(SG)₁₈ Nanoclusters through Precursor Engineering and Its pH-Dependent Size Transformation

In recent years, ultrasmall metal nanoclusters (NCs) with aggregation-induced emission (AIE) characteristics have drawn enormous attention due to their strong photoluminescence (PL) and potential applications in many fields, such as bioimaging, diagnostics, therapeutics, and sustainable fuel production. Among all the reported AIE-type metal NCs, the red-emitting Au₂₂(SR)₁₈ NCs (where SR is a thiolate ligand) with a precise molecular formula remain as a promising candidate. Common synthetic strategies for Au₂₂(SR)₁₈ NCs (e.g., CO-reduction method) not only have relatively low synthesis yield, but also form the byproduct of Au₁₈(SR)₁₄. In addition, the lack of systematic understanding of the reaction mechanism and nontrivial postsynthesis steps greatly hinders its large-scale production. To address these issues, herein, we report a modified CO-reduction protocol to improve the yield of AIE-type Au₂₂ NCs in solution. By deconvoluting the original CO-reduction protocol into two stages, the importance of each stage can be studied in detail. By optimizing the key parameters of each stage (e.g., incubation time and temperature), the yield of Au₂₂ NCs can be significantly improved (to ∼73%). In addition, we also demonstrated the role of precursors in tailoring the size of the final NC products of the modified CO-reduction protocol. By fine-tuning the incubation time of Stage I and keeping the rest of the protocol unchanged, we can obtain either Au₁₈ or Au₂₂ NCs as the final product. Moreover, an interesting pH-dependent size transformation from Au₂₂ to Au₁₈ NCs was also observed regardless of the reaction temperature. The observed synthetic improvement and the postsynthetic size transformation can show impact on the synthesis of other AIE-type metal NCs with different sizes and compositions.