An evergreen blue. Spectroscopic properties of Egyptian blue from pyramids to Raphael, and beyond

Covalently Functionalized Egyptian Blue Nanosheets for Near-Infrared Bioimaging

Fluorophores emitting in the near-infrared (NIR) wavelength region present optimal characteristics for photonics and especially bioimaging. Unfortunately, only few NIR fluorescent materials are known, and even fewer are biocompatible. For this reason, the scientific interest in designing NIR fluorophores is very high. Egyptian Blue (CaCuSi₄O₁₀, EB) is an NIR fluorescent layered silicate that can be exfoliated into fluorescent nanosheets (EB-NS). So far, its surface chemistry has not been tailored, but this is crucial for colloidal stability and biological targeting. Here, we demonstrate covalent surface functionalization of EB nanosheets (EBfunc) via Si-H activation using hydrosilanes with variable functionalities. In the first part of this work, EB-NS are grafted with the visible fluorescent pyrene (Pyr) moieties to demonstrate conjugation by colocalization of the Vis/NIR fluorescence on the (single) EB-NS level. Next, the same grafting procedure was repeated and validated with carboxyl group (COOH)-containing hydrosilanes. These groups serve as a generic handle for further (bio)functionalization of the EB-NS surface. In this way, folic acid (FA) could be conjugated to EB-NS, allowing the targeting of folic acid receptor-expressing cancer cells. These results highlight the potential of this surface chemistry approach to modify EB-NS, enabling targeted NIR imaging for biomedical applications.

Preparation, properties and applications of near-infrared fluorescent silicate nanosheets

The layered silicates Egyptian Blue (CaCuSi₄O₁₀, EB), Han Blue (BaCuSi₄O₁₀, HB) and Han Purple (BaCuSi₂O₆, HP) are known as historic pigments, but they also possess novel optoelectronic properties with great potential for fundamental research and technology. They fluoresce in the near-infrared (NIR) range and can be exfoliated into two-dimensional (2D) nanomaterials (i.e. nanosheets, NS) which retain the photophysical properties of the bulk materials. These and other characteristics fuel the growing excitement of the scientific community about these materials. EB-, HB- and HP-NS have been used in various applications ranging from smart inks, energy storage, bioimaging, to phototherapy and more. In this review article, we report the fundamental properties of these low-dimensional silicate nanomaterials, discuss applications and outline perspectives for the future.