The supramolecular chemistry of gold and l-cysteine: Formation of photoluminescent, orange-emitting assemblies with multilayer structure

• HAuCl4 and l-cysteine form photoluminescent products spontaneously.
• The products are nanoparticles with a multilayer superlattice structure.
• Formation is monitored via a combination of PL spectroscopy and time-resolved SAXS.

The protein mediated approach is a common method for the synthesis of photoluminescent gold quantum clusters (GQCs), where proteins, acting as reducing and stabilizing agents, react with gold salts through cysteine residues. For the better understanding of the phenomenon, the aqueous phase reaction of HAuCl4 and l-cysteine has been investigated at the supramolecular level by various experimental techniques and molecular mechanics simulations. We have observed the formation of a novel photoluminescent product, (AuCys)nβ, which shows emission in the orange region of the spectrum. Small- and wide-angle X-ray scattering (SWAXS) measurements have revealed the presence of nanosized lamellae, which have an internal multilayer superlattice structure with a characteristic periodic distance of 1.3 nm. Based on the results, the layers are built up by zigzag shaped (AuCys)n polymer chains connected through aurophilic bonds. The aurophilic network is stabilized via salt bridges and hydrogen bonds, which are also responsible for the interlayer interactions. Here, the evolution of the multilayer structure has been monitored by the combined application of photoluminescence spectroscopy and time-resolved SAXS. It has been concluded that there is a strong correlation between the emission and the scattering intensity, which suggests that the two- and three-dimensional aggregation of the building blocks to form sheets and multilayers are simultaneous processes. Furthermore, we have revealed that the formation and behavior of (AuCys)nβ show significant differences to that of Au-l-glutathione compounds described earlier despite the similarity of l-cysteine and l-glutathione. These results evidence that l-cysteine and gold species form building blocks that can be applied expansively in supramolecular and cluster chemistry.

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