Egyptian blue, Chinese blue, and related two-dimensional silicates: from antiquity to future technologies. Part A: general properties and historical uses

The ancient Egyptian blue pigment was developed over 5000 years ago and was used extensively for around four millennia until its use mysteriously declined dramatically during the Early Middle Ages. It recently attracted a lot of attention along with some related materials, leading to a fast-growing number of applications in fields, such as sensors, solar concentrators, energy-saving, and medicine. The new surge in interest began in 1996 with the discovery of their intense NIR photoluminescence that surprisingly can be triggered even by visible light. In 2013, the possibility of exfoliating them and producing NIR luminescent nanosheets was established, expanding the family of 2D nanomaterials. More recently, the discovery of their high antibacterial effects and biocompatibility, and very promising optical, electric and magnetic properties, has further boosted their applications. The characteristics of Egyptian blue are due to its main component: the very stable crystalline compound CaCuSi4O10. This tetragonal sheet silicate is the synthetic analogous of the rare cuprorivaite mineral. In Part A of this review, we summarize the historical uses and main properties (i.e., composition, structure, color, stability, luminescence, and biological activity) of cuprorivaite and related 2D silicates, i.e., BaCuSi4O10 (the main constituent of the ancient pigment Chinese Blue), BaCuSi2O6 (the main constituent of the ancient pigment Chinese Purple), SrCuSi4O10 (synthetic analogous of wesselsite) and BaFeSi4O10 (synthetic analogous of gillespite). The Part B of the review will focus on the modern rediscovery of these materials, their modern synthesis and exfoliation, and the innovative applications based on their properties.