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 novel NIR fluorophores is very high. Egyptian Blue (CaCuSi4O10, EB) is a 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. EB-NS were first grafted with the visible fluorescent pyrene (Pyr) moieties to prove conjugation by colocalization of the Vis/NIR fluorescence on the (single) EB-NS level. The same procedure was performed and validated with carboxyl group (COOH)-containing hydrosilanes. These groups can serve as generic handle for further (bio)functionalization of the EB-NS surface. Finally, folic acid (FA) was conjugated to these COOH-functionalized EB-NS to target folic acid receptor-expressing cancer cells. These results highlight the potential of this surface chemistry approach to modify EB-NS and enable targeted NIR imaging for biomedical applications.