Fig. A) Schematic representation of the synthesis of a two-dimensional gold single crystal using chloroauric acid and amyloid nanofibril; B) An optical micrograph of a two-dimensional gold single crystal; C) The front of a two-dimensional gold single crystal/polyurethane composite film (top left ), back (bottom left) and optical photomicrographs (right); D) Sensing behavior of the composite membrane to stress/strain.

After hundreds of millions of years of natural evolution, many natural biocomposites with excellent high-strength and super-toughness have been formed in the natural world. The nano-structured units with two-dimensional geometric shapes (eg, leaf-like vermiculite in shells and plateau apatite in bones) play a key role in the performance of these materials, thus synthesizing two-dimensional geometry and properties. The slice structure has gradually become a research hotspot.

The two-dimensional quantum confinement effect on the electrons of the graphene sheet also shifts people's research interest to two-dimensional metal nanocrystals with anisotropic and quantum size effects. Two-dimensional gold nanocrystals, due to their shape, structure and the decisive influence of the external environment on their performance, a variety of new synthesis methods have emerged in recent years, and exhibited a wide range of optical, mechanical, electrical, catalytic, sensor and biomedical aspects Application potential. Although researchers have done a lot of work on the shape and size control synthesis of gold nanocrystals, the size of the two-dimensional gold nanocrystals currently synthesized is limited to the nanometer and micrometer scales.

Recently, under the leadership of researcher Li Zhaoxu, the bionic smart materials team of the Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, has made new research progress in soft fibrous nanomaterials. The research group used a unique protein self-assembly structure, amyloid nanofibrils, to synthesize two-dimensional single crystal gold flakes with a side length of nearly half a millimeter. The relevant research results have been published in Advanced Materials (Adv. Mater., 2015, 27, 1945-1950).

Due to the extremely high aspect ratio and excellent mechanical properties of amyloid nanofibrils, Li Zhaoxu has been using amyloid nanofibrils to construct functional materials (such as hydrogels, artificial bones, and graphene composite papers) since 2009 at the ETH Zurich. . Subsequently, he and his collaborators succeeded in preparing a composite film material based on nano-gold/amyloid fibrils that can be used in food packaging, cosmetics, and decorative applications. Recently, using a simple "one-step synthesis" method, they synthesized large-size two-dimensional gold single crystals with different thicknesses.

This two-dimensional gold single crystal has a surface area of ​​more than 104 square microns and a thickness of more than 100 nanometers. Due to the potential health problems of nanomaterials, the two-dimensional gold single crystal has more geometric dimensions than the nanometer scale, which is of great value for further expanding the application space of the gold single crystal. Like the cosmetics, watch and jewelry industries, gold has long been used as a premium packaging for foods such as desserts and drinks in the food industry. Due to its atomic-scale smooth surface, the two-dimensional gold single crystal is expected to serve as a substitute for gold foil, greatly reducing the actual amount of gold used.

This novel two-dimensional material that is visible to the naked eye can also be applied to optical and electronic nanodevices, nano-antennas, sensing, and imaging. In addition, by combining with polyurethanes, they prepared a unique flexible conductive film with strain-stress sensing behavior.

This study was supported by projects of the Ministry of Science and Technology, the National Natural Science Foundation of China and Qingdao City Science and Technology.

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