Bis-Schiff base linkage makes gold nanoclusters brighter

We develop an efficient strategy to increase the photoluminescence quantum yield of water-soluble gold clusters at the single-cluster level via formation of bis-Schiff base linkages, and its mechanism is deeply investigated.

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Over the past few decades, metal nanoclusters (NCs) have emerged as promising functional materials and have attracted considerable attention owing to their unique structural, optical, electrical, magnetic, and catalytic properties. Among these properties, visible to near-infrared photoluminescence (PL) is one of the most appealing due to the extensive use of PL-based techniques in chemical sensing, biological imaging, and light-emitting devices. However, the relatively weak luminescence of metal NCs has been a non-negligible obstacle to their applicability. As a result, boosting the luminescence efficiency of metal NCs has become a continuous and interesting topic in the cluster community.

It has been widely accepted that the luminescence property of metal NCs is closely associated with the metal-ligand motifs on the NC surface, and the restriction of intramolecular motion (RIM) of these surface-bound complexes is an efficient way to enhance the emission efficiency of metal NCs. In our work, a dialdehyde-mediated intracluster cross-linking strategy was developed to enhance the luminescence of metal NC in the aqueous solution by activation of the RIM process at the single-cluster level (Fig. 1a). The influence of interactions between Au(I)-SG (SG: L-glutathione) motifs and dialdehydes on the cluster luminescence was investigated by employing Au22(SG)18 NCs as a model NC, leveraging on their good structural flexibility and amino groups-enriched surface (Fig. 1b). It has been shown that among all the selected dialdehydes, 2,6-pyridinedicarboxaldehyde (PDA) has the highest luminescence enhancement ability for Au22(SG)18 NCs (Fig. 1c). The bis-Schiff base linkages formed between PDA and SG ligands can significantly suppress the flexibility of the Au(I)-SG motifs on the surface of Au22(SG)18 NCs, leading to a remarkable decrease in nonradiative decay rate and an obvious increase in the radiative decay rate. Accordingly, enhanced photoemission from PDA-Au22(SG)18 NCs with a luminescence QY as high as 48% was achieved. Furthermore, this investigative approach for improving the luminescence efficiency can be extended to other luminescent gold NCs with a core-shell structure. Our results also show that the content of surface-bound Au(I)-SG complexes has a significant impact on the PDA-induced luminescence enhancement, and a high ratio of Au(I)-SG will be beneficial in increasing the photoluminescence intensity of gold NC (Fig. 1d).

Fig. 1 a Schematic of the correlation of emission intensity with the binding PDA. b UV-vis absorption, photoexcitation, and photoemission spectra of the purified Au22(SG)18 NCs. Inset: Digital photos of the PAGE gels of gold NCs under visible (left) and UV (right) light. Bands 1-3 correspond to Au22(SG)18, Au18(SG)14, and Au15(SG)13, respectively. c Comparison of the luminescence intensity enhancement effects of different dialdehydes on Au22(SG)18 NCs. d Effect of the content of surface-bound Au(I)-SG complexes on the PDA-induced luminescence enhancement of gold NCs.

This study offers an in-depth understanding of the structure-luminescence relationship of metal NCs and provides a promising way for the rational design of highly emissive metal NCs for their subsequent use in biomedical imaging, optical sensing, clinical diagnosis, and light-emitting displays. For more details on this work, please read the full paper in Nature Communications at https://www.nature.com/articles/s41467-022-30760-3.

Hao-Hua Deng

Associate Professor, Fujian Medical University