Decoupling the CO-Reduction Protocol to Generate Luminescent Au22(SR)18 Nanocluster

Abstract

Development of efficient synthetic strategies for highly luminescent gold nanoclusters (Au NCs) requires a good understanding of their synthesis process and environmental factors involved which could affect their tailorability. In a recent study, we reported a new type of luminescent Au NCs featuring with the aggregation-induced emission (AIE) characteristic. This AIE-type luminescent Au NC has a molecular formula of Au22(SR)18 (SR denotes thiolate ligand), and it shows strong red emission at ∼665 nm with a high quantum yield of ∼8%. However, the formation process and reaction parameters that may affect the synthesis of Au22(SR)18 were not understood because of the lack of experimental evidence. Here, we revisit the synthetic protocol, a two-step carbon monoxide (CO) reduction method, to further understand the formation process of Au22(SR)18. First, we systematically investigate several reaction conditions (e.g., the solution pH and the duration of each step) that could affect the yield of red-emitting Au22(SR)18 in the protocol. Second, we use the time-course measurements of the optical properties (UV–vis absorption and photoemission) of the reaction solution to study the formation process of Au22(SR)18, which allows us to identify several key NC intermediates and makes possible the reconstruction of the formation process of red-emitting Au22(SR)18. Upon the basis of our experimental data, we propose a two-stage process for the growth of Au22(SR)18: (1) the reduction of Au(I)-thiolate complexes to form Au NCs with a narrow size distribution, which are subsequently focused to Au18(SR)14; and (2) a pH-induced aggregation of short Au(I)-thiolate complexes on Au18(SR)14, which are finally converted to Au22(SR)18. Our study suggests that the pH-induced aggregation of Au(I)-thiolate complexes on the in situ formed thiolated Au NCs could be an effective way to generate luminescent Au NCs with the AIE characteristic. This principle can also be used to synthesize other AIE-type metal NCs with strong luminescence.